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Electrolyte Membranes for Fuel Cells: Synthesis, Characterization and Degradation Analysis Soma Banerjee1, Kamal K. Kar1,2,* and Malay K. Das3 1

Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur-208016, India; 2Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India; 3Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India Received: March 12, 2014; Accepted: August 27, 2014; Revised: October 1, 2014

Abstract: Fuel cells are the energy conversion devices that can convert the chemical energy of fuel into electrical energy efficiently and, more importantly, in a greener way by utilizing air as an oxidant. Solid polymeric electrolyte membrane is one of the major parts of the fuel cell device, which selectively allows the passage of protons while blocking the flow of electrons through it and makes the device functional. Till date, Nafion® is the most successful candidate in this field but it loses its proton conductivity at elevated temperature due to the loss of water molecules. In addition to the functional requirement of membranes, the durability and stability are also prime concerns for the electrolyte membranes. To develop alternative suitable membranes at lower cost different systems have been investigated time to time as fuel cell membranes. In this review, patents and research articles on various approaches that have been practiced to fabricate the alternative membranes, their characterization, stability, durability, degradation mechanism along with the life cycle estimation will be discussed to acquire a complete overview about the material design and fabrication of a perfect membrane electrode assembly.

Keywords: Fuel cell, ionic conductivity, Nafion®, poly ether ether ketone, polymer electrolyte membrane, proton conductivity. 1. INTRODUCTION In the current century, with increasing crisis of nonrenewable energy resources, exploration of alternative energy is of great importance to fulfill our daily energy requirements. To overcome this problem, several efforts have been under taken to use renewable energy, which can provide energy at high efficiency with minimum emission of pollutants to the environment. In this context, the fuel cells are of prime interest. These are the energy efficient devices that convert the chemical energy of fuel into electrical energy. The output can be continuous as long as fuel and oxidants are supplied. The major parts of fuel cells are cathode, anode and solid electrolyte. At anode, the catalyst oxidizes fuel and converts it into ions with the simultaneous release of electrons. The electrolytes only allow ions to pass through it and block every electron. Depending on the nature of electrolyte and operating temperature, the fuel cells can be of several types such as (a) polymer electrolyte membrane fuel cell (PEMFC), (b) alkali fuel cell (AFC), (c) phosphoric acid fuel cell (PAFC), (d) molten carbonate fuel cell (MCFC), (e) solid oxide fuel cell (SOFC), etc. The classification of different types of fuel cell systems is given in Table 1. Among the previously mentioned fuel cell systems, the polymer electrolyte membrane fuel cell (PEMFC) utilizes *Address correspondence to this author at the Advanced Nanoengineering Materials Laboratory, Material Science Programme, Indian Institute of Technology Kanpur, Kanpur-208016, India; Tel: 0512-2597687; Fax: 05122597408; E-mail: [email protected] 1874-4648/14 $100.00+.00

solid polymeric membrane as electrolyte and generally operates in the temperature range of 70 to 100°C. In this type of fuel cell, hydrogen is supplied as fuel at the anode. The catalyst oxidizes hydrogen into hydrogen ions, which pass through the solid electrolyte. Membrane only allows the passage of protons through it and the electrons flow through the external circuit generating an electrical current. At cathode, the protons react with oxygen to produce energy and water as harmless byproduct. Assembly of cathode, anode and solid electrolyte membrane is called as the membrane electrode assembly (MEA), which is schematically represented in Fig. (1). As a basic requirement, the PEM should possess the following combination of properties for automobile applications [1, 2] viz. •

High ionic conductivity (0.07 and 0.1Scm-1 at room temperature and 120°C, respectively)



Operating temperature (120°C)



Area specific resistance (0.02 cm2)



Durability (minimum 5000 hours)



Low fuel crossover (2mAcm-2)



Low cost (20US$/m2)

Nafion® is the most successful and commercialized candidate as membrane material for PEMFC. It contains Teflon® like backbone with a sulfonic acid pendent group attached to it as shown in Fig. (2). Nafion®, because of its polytetra© 2014 Bentham Science Publishers

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Table 1.

Banerjee et al.

Types of Fuel Cells.

Type

Temperature (°C)

Fuel

Electrolyte

Mobile Ion

PEM

70-110

H2, CH 3OH

Solid polymeric membrane

(H2O)nH+

AFC

100-250

H2

Aqueous KOH

OH-

PAFC

150-250

H2

H3PO 4

H+

MCFC

500-700

Hydrocarbons, CO

(Na,K)2CO 3

CO3 2-

SOFC

700-1000

Hydrocarbons, CO

(Zr,Y)O 2-d

O2-

Fig. (1). Reaction mechanism of PEMFC.

F2 C

F2 C

F2 C

F C

x O

y F2 C

F C

O

m

F2 C

n

SO3H

CF3 Nafion® 117 m=1, n=2, x= 5-13.5, y=1000 Flemion m=0, 1; n=1-5 Aciplex m=0, 3; n=2-5, x=1.5-14 Fig. (2). Structure of Nafion®.

fluoroethylene (PTFE) like backbone, has high mechanical strength as well as high thermal and chemical stability. The hydrophilic group present in the structure provides high proton conductivity at fully humidified condition but the conductivity falls drastically at higher temperature. The proton conductivity of membranes depends on the water content within the membrane. Water acts as vehicle for the mobility of proton through the solid electrolyte. With increase in temperature and reduction of relative humidity, the loss of water molecules takes place and thereby the conductivity is lowered down.

According to the structure property relationship, Nafion® has a nanophase structure with hydrophilic and hydrophobic segments [3, 4]. Proton conduction occurs through the sulfonate ions and the presence of water favors the transport of proton. Thus, the mobility of ions is dependent on the amount of water present in it. At the elevated temperature, with the loss of water molecules, the conduction pathway is broken and thus a decrement in the performance is quite common phenomena. As reported by Gubler and Scherer in order to perform better as membrane material, it should have

Electrolyte Membranes for Fuel Cells

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Fig. (3a). Phase separated structure of Nafion® and (3b) Structural arrangement for nanophase separated structure.

a nanophase separation as shown in Fig. (3a) [5]. Structurally this can be possible by blending of two polymers, where one acts like an ionomer and other provides mechanical strength to the membrane. The second possible route can be the formation of a graft polymer, in which a second polymer chain bearing a hydrophilic group is grafted into the main chain of a hydrophobic polymer. Third approach, is to use block copolymer containing repeated block of hydrophilic and hydrophobic segment. Schematically these three technical approaches are represented in Fig. (3b). At low humid condition and higher temperature, Nafion® suffers from several problems. The present day research is focused on the preparation of some alternative cheap polymeric membranes, which can serve better under these conditions. In this review, different types of materials studied by several research groups and efforts to develop a suitable highly conductive, durable membrane are discussed. The major reasons for the reduction of lifecycle of the polymer electrolyte membrane along with the possible degradation mechanism and preventive measures to enhance the service life have also been included. 2. TRANSPORT PHENOMENA IN MEMBRANE The transport of proton through the membrane is still a matter of debate. The most widely accepted ones are proton hoping or Grotthuss mechanism and vehicular mechanism. The presence of water in the membrane plays a dominant role to decide its conductivity. As the water content within the membrane increases, the hydrophilic domain grows in size and forms water clusters. The water clusters are finally connected with each other through water channel and provide transport pathways for the protons. Zawodzinksi et al. have reported the role of water in the Nafion® membrane [6]. According to this report, the activity of water vapor goes to unity when the number of water molecules per sulfonic acid group reaches to 14 out of which, about 3-5 water molecules hydrate the sulfonic acid group and 9-11 of them remain outside the hydrated shell. When seven more water molecules are attached to the Nafion®, a secondary phase of water with the sulfonic acid moiety is formed. This free water is

like bulk water and capable to form hydrogen bonding and thereby providing the suitable pathways for proton hopping. According to the proton hoping mechanism of conduction, the proton hops from one hydrated ionic side to the other by formation of clusters. This transport occurs via the formation of hydronium ions as depicted in Fig. (4). The hydrogen ions generated during the reaction combine with the water molecules forming hydronium ion, which then hops with other hydronium ion during the process of conduction [7]. In diffusion or vehicular mechanism, water acts like a vehicle. In this case, because of the electrochemical potential difference, the hydrated proton diffuses through the aqueous medium as shown in Fig. (5). The free volume present within the polymer chain allows proton transport [7, 8]. The proton transport occurs via a single hydronium ion than that of the hydrogen-bonded network of water molecules as in case of the Grotthuss or proton hopping mechanism. This mechanism governs under low humid condition. At lower relative humidity, lesser numbers of water molecules are available for the formation of hydrogen bonded network structure. With increase in the available water, the diffusion preferably occurs through the Grotthuss pathway. 3. FACTORS AFFECTING THE PERFORMANCE OF PEMFC 3.1. Hydration Level The power generation in PEMFC is directly related to the proton conductivity of membrane. According to the mechanism of proton transport through the membrane the mobility of proton is greatly dependent on the level of hydration. The protons transport through the hydrated channels. Thus, a considerable amount of water should be present for the higher ionic conductivity. However, very high amount of water is not desirable since it will affect the strength of membrane due to the excessive swelling and flooding at the cathode side resulting in the subsequent lowering of oxidation reaction i.e. the kinetics of proton conduction in Nafion® based membranes. This phenomenon is known as electro osmotic drag effect and is quantitatively represented by the

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Fig. (4). Proton hoping mechanism.

Fig. (5). Vehicular mechanism.

electro osmotic drag coefficient, which is the number of water molecules transported per proton [9]. Moreover, the electro osmotic drag is dependent on the amount of water present in the membrane but are independent on the chemical nature of membrane. The relative humidity of gases used for the reaction can also affect the cell performance. As the relative humidity of reactant gases increases, the resistance of cell is reduced considerably thereby exhibiting an increase in overall performance of the fuel cell system. 3.2. Thickness Another important parameter in this respect is the thickness of membrane. Since the conductivity is reciprocal of the resistivity, therefore lower the resistivity higher will be the conductivity. Hence, decreasing the membrane thickness can be a valuable approach towards the reduction of resistivity. Thinner membranes promote the rapid hydration due to the reduction in resistance offered by the membranes in water penetration. Susai et al. have demonstrated that the fuel cell membrane could function at lower humidity if thinner membranes are used since it can enhance the dehydration rate at elevated temperature and thus generate the larger concentration gradient of water [10]. Furthermore, decreasing the thickness will also lower the material cost, but at the same time, the strength of the membranes has to be taken in to

consideration during the selection of membrane thickness for durability and mechanical stability of membrane at the time of running conditions. Therefore, a judicial selection of the membrane thickness is essential at which the membrane can achieve optimum proton conductivity by fulfilling the other required properties. An ideal approach to achieve a balance between the thickness and functional properties is to increase the charge density in the polymeric membrane and control the spatial distribution of functional groups responsible for the proton conduction. The charge density could be improved by fabricating the polymeric membrane in the form of thin asymmetric composite film using solvent and nonsolvent mixture. Polymeric membrane of polyphenylene oxide or partially sulfonated polystyrene prepared in chloroform (solvent) and methanol (non-solvent) mixture has exhibited increased charge density and thus the improved proton conductivity [11]. 3.3. Microstructure Spatial distribution of functional groups (acidic groups) i.e. the chemical microstructure has also been observed to play a crucial role in achieving the high proton conductivity. Kreuer and Portale have examined the microstructure of Nafion® by small angle X-ray scattering obtained in the large volume fraction of water (7-56 vol%) [12]. In this study

Electrolyte Membranes for Fuel Cells

Nafion® in its salt form and at highly hydrated condition is investigated for the water domain by using parallel cylinder model. The study suggests that the Nafion® and other polyelectrolytes commonly contains a thin film of water, which acts as positively charged glue (due to the presence of protonic charge carriers) and binds the negatively charged polymer backbone by the electrostatic attraction forces. These electrostatic interactions originated from thin water film provide stability to the morphology of ionomers and hence control proton conduction ability of PEMs. The evaluation of microstructure of the ionomers reveals a phase separated morphology as explained earlier [13]. The transport behavior of SPEEK differs from Nafion® due to the difference in microstructure and the acid dissociation ability (pKa) of sulfonic acid groups. In SPEEK the hydrophilic and hydrophobic channels are quite narrower and less interconnected due to the larger separation between the acidic -SO3 H groups. However, the Nafion® is characterized by wide interconnected channels, more separation between the hydrophilic and hydrophobic regions and smaller separation between the acidic -SO3H groups. These microstructural characteristics render the Nafion® to exhibit greater proton conductivity value than the SPEEK. Similarly the conductivity of partially sulfonated poly([vinylidene-difluoride-cochlorotrifluoroethylene]-g-styrene[P(VDF-co-CTFE)-g-SPS] graft copolymer and poly([vinylidene-difluoride-co-hexafluoropropylene]-b-styrene[P(VDF-co-HFP)-b-SPS] block copolymer based polymer electrolyte membranes has also been observed to depend on the chemical microstructure of polymers [14]. 3.4. Cell Temperature The ionic conductivity of membrane can be enhanced with increasing the fuel cell temperature. At higher temperature, an enhanced performance is observed due to the increase in ionic conductivity and decrease in mass transport resistance of the solid electrolyte. However, the operating temperature cannot be increased beyond a certain limit since the membrane will start degrading after a certain temperature. In addition, depending on the cell temperature, the hydration state of membrane varies which in turn changes the current densities in the external circuit. At higher temperature, water produced at the cathode side cannot sufficiently humidify the anode and causes membrane drying, which leads to the lowering current densities in external circuit. Similarly, when cell temperature is optimum then sufficient water is produced that can hydrate from cathode to anode by back diffusion mechanism and consequently the current densities in the external circuit is significantly increased. Moreover the fuel cell performance is greatly influenced by the desorption rate of water from the membrane surface [1517]. As the water desorption rate is directly proportional to the temperature therefore the fuel cell cannot be operated beyond some critical temperature if a high proton conductivity is desired. 3.5. Operating pressure By increasing pressure at the cathode side, one can increase water content in the membrane and diffusivity of the reactant gases since the increased pressure reduces the mass

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transport resistance. Hence, the output voltage as well as the cell performance can be enhanced by increasing the pressure at cathode side. Moreover, the cell performance of PEMFC can be improved further as the increased pressure at cathode side favors the oxygen reduction reaction. An increase in the back pressure improves the cell performance due to the increase in the exchange current and thereby facilitates the mass transfer process. However, the excessive back pressure increases the problems associated with hydrogen crossover. Apart from these parameters the stoichiometry flow ratio (i.e. ratio of amount of oxygen or hydrogen to the amount that is required for electrochemical reaction) in the feed (reactants) also plays a key role on the cell performance as the electrochemical reactions are solely controlled by the availability of reactants. The enhanced cell performance with increasing the stoichiometric flow ratio of gases at anode is reported by Tohidi et al. [18]. 4. MODIFIED NAFION® MEMBRANES Since the Nafion® loses its performance at elevated temperature, several attempts have been taken to make it usable at higher temperature. The balance of water in the operating device is an important criterion to keep the device functional with high efficiency. To maintain the balance of water in membrane, swelling with non-aqueous low volatile solvents is tried. Phosphoric acid (PA) is a low volatile acid and it solvates the hydrophilic group within Nafion®. Because of its low volatility, it can be useful up to temperature of 200°C. However, after a certain period of operation, the failure occurs at the anode side of MEA and thus it is not a quite successful approach. Che et al. have investigated the phosphoric acid doped Nafion®, which shows a conductivity of 0.05Scm-1 at a temperature of 150°C [19]. Nafion® 117 can also be impregnated with phosphotungstic acid (PTA) but at higher temperature it evaporates. In addition to PTA, heterocycles such as imidazoles, pyrazole containing proton donor (NH) and acceptor (N) can also be an interesting candidate. The balance of water within the membrane can be maintained in several ways, such as•

An external supply of water source to maintain humidification



Utilizing the water produced during the course of reaction



Utilisation of the capillary forces by back diffusion

Hygroscopic oxides such as SiO2, TiO2, etc. can absorb moisture from atmosphere. The water absorption property of Nafion® is found to be enhanced by incorporation of these oxides. An increase in water absorption of 43 wt % is observed for 3wt% SiO2 particle (particle size ~7 nm) filled Nafion® [20]. Nafion® is also modified with tetraethyl orthosilicate (TEOS) [21, 22]. Among the inorganic solid proton conductors, zirconium phosphates (ZrP), heteropolyacids, metal hydrogen sulphate are well explored for high temperature fuel cell applications [23]. Hydrogen sulfates, MHXO4, where M stands for large alkali species such as Rb, Cs, etc. and X stands for S, P, etc. are also used as solid inorganic conductor of protons. Cesium hydrogen sulfate shows proton conductivity of about 10-2Scm-1 at 141°C, which is quite close to Nafion®. Addition of ZrP can enhance

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Banerjee et al.

the proton conductivity of Nafion membrane because of its good water retention ability and better ionic mobility on the ZrP surface [23]. The addition of hydrophilic additive is advantageous since it promotes the hydrogen bonding of available water and thus the evaporation of water can be lowered down [24]. Heteropolyacids, another group of compounds, are used in this respect. In their crystalline form, they contain hydrated water molecules. These additional water molecules assist in the conduction mechanism and hence positively contribute to the proton conductivities. Self-humidifying membranes with Nafion® modification have also been tried. Nafion® is modified by forming a composite membrane with ZrP and platinum (Pt). The reaction of hydrogen and oxygen produces water on the surface of Pt particles. Thus, the water generated in situ will humidify the membrane. ZrP is added in order to enhance the proton conductivity at the elevated temperature and to impart good water retention capability and improved mobility of protons at the surface of it [25]. Other attempts also have been tried by adding platinum nanoparticles and hygroscopic solids such as SiO2 or TiO 2 [20, 26]. In another research work, sandwich type of membrane is prepared by sputter coating of platinum particles on both sides of the perfluorosulfonylfluoride copolymer resins [27, 28]. Laponite (Lp) or montmorillonite (MMT) shows Table 2.

hydrophilic nature, thus they are used for the modification of Nafion® membranes for PEMFC applications. The major advantage of addition of these materials is that, they can prevent the loss of hydrated water at low humid condition and higher operating temperatures. Another modified Nafion® membrane is reported by incorporation of solid inorganic fillers. SiO2-P2O5 is added as conducting filler in the PFSA matrix. The Si-OH and P-OH are generated as intermediate in the reaction mixture from Si(OEt)4 and PO(Me)3. The presence of Si-OH and P-OH groups facilitates the water retention capabilities of membrane at higher temperature and thereby maintains its desired properties [29]. The property comparison of different kinds of modified Nafion® membranes is shown in Table 2 [30-48]. In another study, Na2Ti3O7 nanotubes containing composite Nafion® membrane have been prepared by dispersing nanotube with the aid of ultrasonication. In the subsequent step of fabrication method, Nafion® is first dissolved in DMF solvent and then Na2Ti3O7 nanotubes prepared in the same solvent are mixed together with the help of an ultrasonicator to achieve proper filler dispersion. The Na2Ti3O7 serves as the moisture retentive material in the polymer matrix. Therefore, the membrane thus prepared exhibits excellent ionic

Property Comparison of Modified Nafion® Membrane. Types of Membranes

(Water uptake %) [Swelling %] { IEC Mequivg-1 }

Conductivity (Scm-1)

Conditions

References

Nafion® 117

(21) {0.9 }

9.510-2

25°C

[30-32]

90°C

[33,34]

18 ± 1°C, 33 ± 5 % RH

[35]

®

Nafion 115

(26)

®

310

-2

-2

Nafion 211

--

1.55710

Nafion® 212

--

1.34710-2

18 ± 1°C, 33±5 % RH

[35]

Nafion®/TiO 2

(29) {0.93}

0.15-0.18

85°C,100 % RH

[36]

--

[37,38]

®

-2

Nafion /SiO2

(34)

1.0710

Nafion®/ZrO 2

(24)

210-2

--

[39]

Nafion®/WO 3

(37)

102

100°C

[38]

100°C,140 % RH

[38]

92 % RH

[40,41]

®

Nafion /SiO2/PWA ®

(38)

2.610

2

Nafion /Zirconium phosphate

--

Pt/ SiO 2/Nafion® PTFE

(54) [6.7]

--

--

[42]

(38.6)

0.01

70°C,100% RH

[43]

0.06

25°C

[25]

(30)

1.4101

25°C

[44]

--

0.1

160°C, 100% RH

[45]

18 ± 1°C, 33 ± 5 % RH

[35]

®

Nafion /Mordenite Pt/ ZrP/Nafion

®

Nafion®/Zeolite ®

Nafion /imidazole ®

2.510

2

-2

Nafion 212/MMT (5%)

--

0.16810

Nafion®/H3 PO4

--

510-2

150°C

[46]

Nafion / di-isopropyl phosphate

--

0.4

25ºC

[47]

--

> 0.3

90ºC

[48]

®

®

Nafion / PMoA + SiO 2

Electrolyte Membranes for Fuel Cells

conductivity and low methanol permeability and easy processability [49]. Ionic liquids incorporated in Nafion® show good conductivity and thermal stability at the elevated temperature under low humid conditions. They show ionic conductivity of 10 mScm-1 under dehydrated condition [50]. Nafion® has also been polymerized in situ with furfural alcohol to form Nafion®-polyfurfuryl alcohol composites exhibiting low methanol crossover at a concentration of 3.9 to 8% [51]. Several approaches are made to improve the strength of Nafion® membranes either by incorporating reinforcement or by imparting crosslinking. In order to lower the resistance, the thickness of membrane can be reduced further. With a decrease in the thickness of membrane, the resistance falls down and an increase in conductivity is observed. However, this decreases the strength of membrane significantly. Thus, another attempt has been taken to reinforce perfluorosulfonic acid (PFSA) membranes with the micro fibril of Teflon®. A good combination of conductivity and mechanical strength is observed at a thickness range of 5-30 micron [52, 53]. Nafion® membranes have been also modified with sulfonated polysulfone (SPSU). This new membrane is obtained by sandwiching two Nafion® layers in to the sulfonated polysulfone. This sandwiched membrane is advantageous due to the locking of excessive swelling by the hydrophobic Nafion® layers at the outer side. Thus, water-soluble SPSU do not suffer from leaching problem and can be used up to 120°C [54]. Another approach to lower the excessive swelling tendency of membrane and to improve the proton selectivity is to incorporate the crosslinking in the polymer matrix. Proton selectivity of Nafion® can be enhanced by crosslinking reaction. Fluorine containing polyimide (FPI) and Nafion® 212 has been solution casted and crosslinked to form reinforced membranes. The crosslinked membrane shows good dimensional stability, mechanical strength and good conductivity. With an increase in the FPI content, the dimensional stability of membranes is found to be increased but a subsequent lowering of conductivity is observed. The crosslinked membranes show proton conductivity in the range of 2.010-2 to 8.910-2 Scm-1 at a temperature of 30 to 100°C for various FPI contents [55]. Poly(vinyl pyrrolidone) (PVP) has been reported to be crosslinked with Nafion® to improve the proton conductivity and low methanol permeability. The addition of PVP modifies the Nafion® chains by crosslink formation, which in turn provides better selectivity than the native Nafion® membrane. About 38% increments in ionic conductivity along with 5% reduced methanol permeability is obtained [56]. Thus, these membranes can be useful for direct methanol fuel cell (DMFC) applications. Pore filling is another concept used for Nafion® modification, where one of the polymer materials is used as matrix and another polymer fills the pore within it. PTFE® or Teflon® is used as the substrate and acrylic acid, vinyl sulfonic acid copolymer serves the role of proton conducting matrix [57]. In order to improve the degradation behavior of the perfluorinated sulfonic acid membranes, sodium stannate is used as an additive. This improves the life of membrane. In addition, it decreases the voltage reduction in the fuel cell operating at a temperature of 95°C and relative humidity of

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50%. The loss of the fluoride ions is claimed to be lowered at the optimized system conditions [58]. 5. ALTERNATIVE MEMBRANES Presently, the preparation of some good membrane alternative to Nafion® is of prime concern to the researchers. Keeping in mind the challenges associated with the already present Nafion® membrane, various approaches are tried to prepare an alternative membrane at a lower cost, which can perform better under low humid conditions and high operating temperatures. The most common approaches are the following •

Functionalized aromatic polymeric membranes



Acid-base complexes



Crosslinked membranes



Hydrocarbon membranes



Composite membranes



Blend membranes



Grafted membranes



Ionic liquid doped membranes

5.1. Functionalized Polymeric Membranes Aromatic polymeric backbones have high thermal and chemical stability. They can perform well at the high operating temperature and chemically harsh environment. Aromatic backbones are easy to modify with electrophilic and nucleophilic substitution. In polymer electrolyte membrane, the aromatic polymeric backbones are functionalized with various functional groups with the aim of increasing its hydrophilicity and acidity [59, 60]. Smitha et al. have reviewed the different types of fuel cell membrane by classifying them into four categories viz. perfluorinated membranes, nonfluorinated hydrocarbons, aromatic (sulfonated polyarylenes) and acid based complexes [61]. The membranes are reviewed for their conductivity, thermal stability and other functional properties. Nafion®, sulfonated poly(4phenoxybenzoyl-1,4-phenylene) and phosphoric acid doped PBI emerges as the most promising and suitable polymers in the first, third and fourth categories, respectively for fuel cell applications. The proton transport mechanism is discussed in view to enhance the performance of membranes. With this objective, the common functionalization approaches of different polymers like poly ether ketones, poly ether ether ketones (PEEK), poly ether ketone ketone (PEKK), polysulfones, polyimides, polyesters are demonstrated. The review reveals that sulfonation and phosphonation have been widely used functionalization strategies for the modification of polymers. 5.1.1. Sulfonation With the aim to enhance the hydophilicity and thereby conductivity, the sulfonation of polymeric backbone is performed. Sulfonation of PEEK modifies its chemical structure. The chemical structures of PEEK and sulfonated poly ether ether ketone (SPEEK) are shown schematically in Fig. (6). Sulfonation of PEEK increases its acidity as well as ion exchange capacity and makes it soluble in common organic

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C

O

O

O

n

PEEK

C

O

O

O SO3H

n

SPEEK

Fig. (6). Structure of poly ether ether ketone (PEEK) and sulfonated poly ether ether ketone (SPEEK).

solvents. At a degree of sulfonation < 30%, it is soluble in conc. sulfuric acid. Above 30% the polymer becomes soluble in hot organic solvents such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), etc. With increasing the degree of sulphonation the polymer become increasingly soluble and finally at 100% degree of sulfonation (DS) it becomes completely soluble in hot water [59]. Sulfonation is an electrophilic substitution reaction in which one of the hydrogens of aromatic ring is substituted with sulfonic acid groups to enhance the hygroscopicity of material. With increasing the degree of sulfonation, the polymer such as PEEK is separated into two environments, namely hydrophilic and hydrophobic. According to the structural requirement, the phase separated structure of Nafion® is the best studied material in this field and is responsible in selective conduction mechanism. In this context, the sulfonation of polymer backbone is performed to introduce the phase separation in polymer backbone. The first and most easy route of sulfonation is the direct introduction of sulfonate group by treating it with the sulfonating agents. The reagents that can be used are chlorosulfonic acid, concentrated sulfuric acid, sulfur trioxide and acetyl sulfate. The choice of the reagent is reported to be dependent on the type of the material to be functionalized [62]. In addition to the choice of reagent, the orientation of sulfonic acid group into the polymeric backbone is also an important point. Surface modifying macromolecule (SMM) is a type of blending method based on the surface segregation approach. SMM when added in to a polymer matrix modifies the bulk properties, morphology of the polymer and migrates to surface. It is added in very small amount according to the requirement (usually less than 5%) to modify the surface of polymer. A report has indicated two type of SMM with hydrophobic and hydrophilic surface modifiers [63]. The SMM are the polyurethane based polymeric materials having different pendent groups according to the polarity. For the hydrophobic one, the side group is a fluoro telomer and for the hydrophilic one it contains poly(ethylene glycol) or poly(propylene glycol) as side chain. The SMM blended SPEEK membranes are observed to be more conductive than sulfonated PEEK with conductivity value as high as 6.410-3 Scm-1 [63]. Although,

direct sulfonation is an easy method but the control over the degree of sulfonation is quite difficult. Thus, other approaches are tried to have better control over the reaction sequence. •

Grafting of sulfonic acid bearing group in to another polymeric backbone



Synthesis of polymer from monomer containing functional group



Three step process of lithiation, sulfonation followed by oxidation

The grafting of polymers is done with the aim of achieving a dual combination of chemical characteristics in the same material viz. hydrophilicity and hydrophobicity. The two polymers can be grafted prior to the addition of the functional group or one of them can be functionalized first and then can be grafted in to the other polymeric backbone. Sulfonation of polybenzimidazole (PBI) is carried out by grafting of benzylsulfonate. At 75% of benzylsulfonate grafting, the membrane gives proton conductivity similar to that of the commercial Nafion® membranes [64]. The use of these membranes is restricted due to their brittleness with the loss of water molecules. The best way to induce functionalization is to start with a precursor having desired functionality. Recently, some of the researchers have tried this approach successfully. A new approach is also tried to prepare functionalized PEEK by direct polycondensation reaction with the aim of controlled degree of functionalization [65]. Lithiation followed by hydrolysis is another common approach of producing sulfonated polymers [66] as illustrated in Fig. (7). In this method, at first the polymer is treated with butyl lithium to introduce labile Li group that is further replaced by the sulfonic acid group by reacting with sulfonating agents. This approach is also tried for introduction of functional groups such as phosphonic acid or carboxylic group, etc. Huge research is already established to sulfonate different types of polymers in a number of ways to prepare the membranes of desired functional properties. In an invention, functional membranes with good mechanical properties and

Electrolyte Membranes for Fuel Cells

Recent Patents on Materials Science 2014, Vol. 7, No. 3

O O

O BuLi

S

9

O

S

O

O Li SO2

O O

S O

Oxidation

SO2Li

O O

S O SO3H

Fig. (7). Sulfonation of poly(sulfones).

high ionic conductivities are prepared from sulfonated polyarylene ether copolymers [67]. In various studies, the monomers of polyarylene ether copolymer are polycondensed in presence of different solvents. The sulfonic acid group is introduced by reacting the polymer with different acids such as conc. sulfuric acid, ClSO3H, fuming SO3, fuming sulphuric acid triethyl phosphates at different temperatures. The membranes have been claimed to exhibit good thermal and chemical stability, processability, high dimensional stability with desirable conductivity values. SPEEK with metal organic framework is also investigated [68]. In the SPEEK, finely divided metal organic framework (MOF) is added. The so prepared SPEEK exhibits a degree of functionalization of 50% and performs better than the Nafion®. The MOF is prepared with porous solids and then crosslinked by phosphonates, terephthalic acid and carboxylate or nitrogen donor complexes. In order to achieve good properties, the loading is kept within 3 to 20wt %. The membrane is suitable to be used over 100°C [68]. Meredith et al. have developed an organic and inorganic composite PEM based on fluoropolymer. The fluoropolymer is homo or copolymer of polyvinylidene fluoride containing sulfonated or phosphonated groups [69]. The membrane consists of a polymer electrolyte and different types metal oxides nanoparticles (silica, zirconia, alumina, titania, etc.), nanoclays, phosphates, metal acids, sulfonated nanotubes, zeolites, carbon nanotubes, graphenes, silsesquioxanes, etc. The nanofillers are functionalized with MOF, transition metal sulfide, sulfonic acid containing groups, etc. An enhanced proton conductivity and mechanical property is reported for the nanoparticle filled polymer electrolytes. HPA are the conductive solids, which can provide additional pathways for proton conduction. The incorporation of them into the SPEEK provides desirable proton conductivity with retention of adequate mechanical property. Fillers such as tungstophosphoric acid (TPA), its sodium salt (Na-TPA) and mo-

lybdophosphoric acid (MPA) are commonly used. The conductivities of TPA and Na-TPA at room temperature (RT) are 1.910-1 and 10-2Scm-1, respectively. The addition of TPA in SPEEK can lead to proton conductivity of 10-2Scm-1 at RT and 10-1Scm-1 above 100°C [70]. Lee et al. have investigated the composite PEM containing solid acid [71]. The solid acid used in this study is an ion conducting compound with one hydroxyl group substituted with an organic moiety bearing a terminal cation exchange group in it. The solid acid with a calixarene or calixresorcinarene core is incorporated as conducting material in the polymer matrix. The loading of solid acid varied from 0.1 to 40 parts per 100 parts of the polymeric material, which include sulfonated poly(ether ether ketone) (SPEEK), polyimide, polybenzimidazole, sulfonated poly(ether ether sulfone) (SPEES), etc. The polymer electrolyte membrane developed herein exhibits low methanol cross-over and high ionic conductivity [71]. PEEK, as described earlier, is sulfonated in order to enhance its hydrophilicity and acidity. Although, at a higher degree of sulfonation the ionic conduction shows higher value but, the swelling and strength of the membrane has to be taken into consideration simultaneously. In order to overcome the problem of swelling, highly sulfonated PEEK is blended with various polymers such as poly(amide imide) (PAI), poly(vinyl pyrrolidone) (PVP), poly(ether imide) (PEI), and poly(ether sulfone) (PES). The blending of SPEEK and PAI shows a better selectivity of 3.46  104Sscm-3 for 70/30 blend of SPEEK/PAI, which is comparable to that of commercial Nafion® [72]. Polysulfones or PES is of great interest because of their inherent swelling resistance, high durability along with high ionic conductivity. A combination of rigid structural unit with high hydrophobicity and flexible structural unit of PES can impart a good balance between the ionic conductivity and swelling properties [73]. Ion conducting sulfonated polym-

10 Recent Patents on Materials Science 2014, Vol. 7, No. 3

eric materials is also prepared from direct polymerization of sulfonated monomers. Polyimides and polysulfones are polymerized in this way. Heteropolyacids are also added in these sulfonated polymers to prepare a nanocomposite membrane. The membrane has improved thermal stability, enhanced conductivity above 100°C with reduced water uptake [74]. Polysulfone based polymer electrolyte membranes with sulfonic acid group directly attached to the main chain of polymer backbone are also investigated. The poly arylene ether based polymers are also known to exhibit good combination of properties such as heat and chemical stability, lower water absorption, easy processability and desirable conductivity when they contain sulfonic acid groups in the main polymeric backbone. The membranes containing the conductive fillers such as SiO2, TiO2, inorganic phosphoric acid, sulfonated SiO2, sulfonated ZrO2 and sulfonated ZrP are also studied [75]. A PEMFC membrane made of hyper branched polymeric chain with partial sulfonation of the chain ends has been studied [76]. In this membrane, hyper branched bismaleimide polymer is present about 15-10wt% as base materials and the major part contains 85-90% by weight of sulfonated tetra fluoro ethylene copolymer. The membrane has shown good dimensional stability and high conductivity even at low relative humidity. PEMFC based on functionalized fluoro resin and Nafion® resins containing SO3 or -SO2F groups are studied by Gao et al. [77]. The functionalized fluoro resin contains -SO2 M group, where M stands for the metal ion. Although, the incorporation of sulfonic acid group in polymeric backbone enhances the conductivity depending on the degree of substitution, but the membranes start to lose its properties in 200 to 400°C due to the desulfonation. Thus, the developments of thermally stable polymeric membrane with good ionic conductivity are being tried by several approaches. Introduction of alkylsulfonic group into the backbone can enhance the thermal stability depending on the length of alkyl group [78]. 5.1.2. Phosphonation Keeping in mind the problem of desulfonation of the sulfonated polymers at high temperature, the phosphonation of polymers are tried to develop a membrane, which can perform well under low humidity and high temperature by eliminating the problem associated with sulphonation. Thus, the introduction of phosphonated group is quite advantageous due to the fact that, they can provide an excellent combination of properties such as stability and ionic conductivity. Phosphonated polymers have low water uptake and higher hydrogen bonding at the similar degree of functionalization than the corresponding sulfonated one. The presence of C-P bonds enhances their hydrolytic and thermal stabilities. These membranes are more suitable than the sulfonated ones under low humid conditions. A high degree of phosphonation is needed in order to have a large number of hydrogen bonding to provide the channels for proton transportation. Although, these membranes can be quite efficient at anhydrous condition and high temperature, but their synthesis is still very limited due to the difficulty of incorporation of phosphonated groups in the aromatic backbone.

Banerjee et al.

Among the phosphonated polymers, polysulfones have been explored extensively. Phosphonation degree as high as 150% have been achieved by the researchers without formation of crosslinked product. At this high level of functionalization, conductivity reaches to 12mScm-1 under completely hydrated condition [79]. The methanol permeability of these membranes is also found to be comparable to that of the commercial Nafion® membrane. These membranes are prepared by chloromethylation, phosphonation and hydrolysis of polysulfones. In another approach, physical doping of phosphoric acid group in the sulfonated poly ether ether ketone is tried. These composite membranes are claimed to be useful up to 160°C under dry condition. A proton conductivity of 210-2Scm-1 is reported at 160°C [19]. A PBI based MEA with acid doping less than 200mole% is developed for the fuel cell devices [80]. The MEA is fabricated from the assembly of cathode, anode and proton conducting polymeric material. The anode and cathode are doped with 5-30 and 10-50mmol/cm3 of phosphoric and alkyl phosphonic acid, respectively. The polymeric membranes are made up of crosslinked poly(2,5-benzimidazole) synthesized by the polymerization of benzoxazine-based monomer and a crosslinkable compound such as polybenzoxazole, polybenzimidazole, polyimide, etc. The MEA shows enhanced fuel cell performance at a doping level less than 200mole%. The performance of fuel cell can be easily tailored by controlling the amount of acid doping at cathode and anode side. The fuel cells are also claimed to be suitable under anhydrous condition and high temperature. Crosslinked phosphonated poly N-phenyl acrylamide membranes are also studied in this respect to obtain good oxidative stability and ionic conductivity. Proton conductivity of 8.810-2Scm-1 is reported at 80°C and 95% RH, but the conductivity falls to 4.710-5 Scm-1 at 30% RH with lowering in the hydration level [81]. Dimitrov et al. have reported a graft polymer electrolyte based on phosphonated poly(pentafluorostyrene) (PFS) and polysulfone [82]. The graft polymers are synthesized in a multi-step process. Initially the commercial polysulfone is modified with azide pendent groups. The PFS is functionalized with alkyne groups by radical polymerization. In the subsequent step of synthesis, PSF is grafted in the polysulfone backbone via click chemistry and finally the grafted polymer is subjected to post phosphonation to yield phosphonated PFS graft polymer electrolyte membrane. The PEM is characterized by optimum mechanical properties, thermal stability, and ionic conductivity (83mScm-1 at 120°C). In another study, Dimitrov et al. have fabricated polymer electrolyte membrane containing pendent phosphonated groups of different chain length [83]. The PFS is synthesized by radical polymerization and subsequently grafted to PSU backbone by click reaction. The grafted polymer is finally phosphonated by treating it with tris(tri methylsilyl) phosphite. The polymer electrolyte membrane containing 14 mole% phosphonated side chains exhibit high thermal stability and proton conductivity of 134mS cm-1 at 100°C under hydrated condition. The dimensional stability of the graft copolymer is further improved by blending it with pyridine modified PSU. The new acid base blend membranes exhibit slightly lower proton conductivity with improved thermal stability than the pristine graft copolymer membrane. Hybrid organic inorganic polymer electrolyte membrane comprising

Electrolyte Membranes for Fuel Cells

of grafted phosphonic acid groups is studied by Li et al. [84]. The graft membranes are synthesized by sol-gel approach. The grafting of phosphonic acid group in to the polymer backbone is carried out by Si-C bond formation using diethyl-4-(diethoxy(methyl)silyl)-1,1-difluorobutylphosphonate and diethoxyphosphoryl-ethyltriethoxysilane as precursor materials. The graft membranes are thermally stable up to 220°C. The PEM exhibits proton conductivity of 6.210-2Scm-1 at 100°C under fully hydrated condition. The hybrid membranes are also characterized by considerably high proton conductivity under low humidity that makes them a suitable alternative for the high temperature fuel cell devices. Uensal et al. have fabricated a polymer electrolyte membrane functionalized with sulfonic and phosphonic acid groups [85]. The polymer is synthesized by copolymerization of hydrophilic monomers containing sulfonic and phosphonic acid functional groups and hydrophobic monomers. The synthesized copolymer may be random, graft or block conformation. The polymeric membranes are composed of at least 50wt% of sulfonic and phosphonic acid based copolymer. The hydrophobic monmomers are vinyl ether monomers, vinyl esters, 1-alkenes, branched alkenes, acetylene monomers, etc. The sulfonated and phosphonated copolymers are further crosslinked by thermal, chemical, and photochemical routes. The MEA exhibits excellent power density. The current density measured in the membrane is reported to be at least 0.05A/cm2. In an invention, Fritsch et al. have studied the PEM based on polyarylene consisting sulphonic and phosphonic acid functional groups in its polymeric backbone [86]. These types of membranes are proved to be beneficial for the fuel cell devices as uniform and homogeneous distribution of functional groups results in high oxidative and hydrolytic stability in the membranes. Moreover, the membrane exhibits high proton conductivity along with good mechanical strength. The incorporation of phosphonic acid groups in particular enables the membrane to remains operative at high temperature and provides long service life. Nanocomposite PEM made of hydrophilic fibrous nanoparticles and fluorine based proton conducting polymer is studied by Sung et al. [87]. The fibrous nanoparticles used in this study are composed of cellulose with functional groups like hydroxyl, acetyl, phosphate, etc. The fluorine based polymer also contains proton conducting functional groups such as phosphonic, sulfonic, carboxylic, phosphoric acid groups and their derivatives. The PEM are characterized by improved mechanical properties originated from the uniform distribution of cellulosic nanofibres in the fluoropolymer matrix. The PEM exhibits substantial proton conductivity that is maintained for prolonged time due to the combined effect of proton conducting fluoropolymer and hydrophilic fibrous nanomaterials. The PEM also provides a durable fuel cell device with minimal gas permeation. Recently, organic proton conducting molecule containing phosphonic acid group have attracted tremendous attention in the field of fuel cell technology. Jiménez-García et al. have synthesized (p, p-terphenyl-4,4-diyl)bisphosphonic acid, 1,3,5-tris(p-phosphonatophenyl)benzene, 1,3,5-tris[4phosphonato-2-biphenyl] -ylbenzene, 1,2,4,5-tetrakis(p-phosphonatophenyl)benzene, 1,3,5,7-tetraquis(p-phosphonatophenyl)adamantane, hexakis(p-phosphonatophenyl)benzene

Recent Patents on Materials Science 2014, Vol. 7, No. 3

11

and characterized for proton conductivity [88]. These types of organic molecules contain carbon-rich non-planar hydrophobic core and defined number of hydrophilic phosphonic acid group at the periphery. The self-assembly of molecules forms a column like supramolecular structure with continuous phosphonic acid phase outside the column, which creates phase separated structure responsible for proton conduction. The crystallinity of polymer affects the proton conductivity of membrane. Proton conductivity as high as 2.810-3Scm-1 at 150°C and 21% RH has been achieved in the molecule containing 6 phosphonic acid groups at the periphery. However, highest the proton conductivity at room temperature to 80°C is shown by the 3 phosphonic acid group containing molecule. 5.2. Acid-Base Complexes Acid-base complexes are formed by the reaction of polymer containing basic sites such as amide, imide, ether, alcohol, etc. and an acid such as sulfuric acid or phosphoric acid. Figure (8) shows some of the examples of acidic and basic polymers. In presence of basic polymer, acids undergo protonic dissociation by acid-base pair interaction and form hydrogen bonding between each other. The acidic proton (i.e. the proton of -SO3H) of Nafion® and sulfonated polyether ether ketone get dissociated more easily in presence of basic -NH2 group of the polypyrrole [89, 90]. These types of systems are good alternative to Nafion® because they can retain their conductivity even under low humid conditions and their conductivity is independent of hydration level. The main advantages of these membranes are that, they can be a good performer even at the elevated temperature and anhydrous condition. The conductivity of acid-base complexes is dependent on the temperature and doping level. At a doping level of 450% and 165°C, the conductivity of PBI membrane is 4.6  10-2Scm-1, which reaches to 0.13Scm-1 at a doping level of 1600% [91]. PBI doped with phosphoric acid is in recent trend [92]. Highly doped PBI membranes are prepared by sol-gel process. PBI itself is non-conducting in nature, but it has a good combination of properties such as thermal, chemical and hydrolytic stabilities. They are incorporated as a base in the acid moieties to enhance the dissociation of acid molecules, which give rise to a better conductivity and stability at the higher temperature. These membranes can be prepared by sol-gel process at a high doping level of 85%. PBI doped phosphoric acid membranes are reported to be used at temperature as high as 160 to 200°C. Bjerrum and coworkers have also studied the H3PO4-doped PBI system [93]. These membranes can show conductivity of 6.8  10 2 Scm-1 at 200°C and hydration level of 5%. Han et al. have studied the composite polymer electrolyte based on PBI for fuel cell device application [94]. The composite PEM consists of metal grafted porous structure in a PBI polymer matrix doped with phosphoric acid. The content of metal grafted porous structure is 0.1-30% by weight of base polymer. The porous structure is made of metals such as aluminum, copper, iron, nickel, and their combinations. The composite PEMs are characterized by improved thermal stability due to the presence of porous metal structure. Moreover, the porous structure provides an additional benefit by preventing the leaching of dopant. Hence, the proton conductivity of PEM is enhanced with phosphoric acid doping. Li et al. have

12 Recent Patents on Materials Science 2014, Vol. 7, No. 3

Banerjee et al. O

H3 C

O

CH3 C

S A

O

O SO3H

O

O

B

C n O

SO3H H N

N H

H3C

C

NH2

CH3

O

O

NH2

S D

O

O

Fig. (8). Structure of acidic polymers (A and B) and basic polymers (C and D).

developed a soluble and thermally stable phenylindane-PBI membrane for fuel cell applications [95]. A comparative evaluation is made under similar experimental conditions by synthesizing phenylindane-PBI and meta-PBI in presence of polyphosphoric acid, which acts as the solvent and dehydrating agent for the polymerization. The introduction of functional group is proved to be advantageous in improving the solubility of PBI in polar aprotic solvents. The membranes are characterized by proton conductivity as high as 0.061 Scm-1 at 180°C and a phosphoric acid (PA) doping level of 10.0 per mole of PBI repeat unit. The doped phenylindanePBI membranes exhibit comparable fuel cell performance as meta-PBI based MEA and claimed to be useful for high temperature fuel cell application. PBI based PEM are studied by Kim et al. [96]. The polymer electrolyte membrane containing benzoxazole units is synthesized by polycondensation of 3,3-dihydroxybenzidine, terephthalic acid, and 3,3diaminobenzidine in presence of polyphosphoric acid. The PEM is fabricated by the solution casting of polycondensation product on the support material followed by in situ phosphoric acid doping by keeping the membrane in atmospheric condition. The membrane exhibits enhanced mechanical properties and high proton conductivities and is claimed to be superior than the conventional phosphoric acid doped PBI with respect to high temperature fuel cell applications. Calundann et al. have developed a novel polyazole PEM and evaluated its performance for fuel cell devices [97]. The polymer is synthesized first by mixing tetramino compound (2,3,5,6-tetraminopyridine, 3,3,4,4-tetraminobenzophenone, etc.) with aromatic di-carboxylic acid monomers (e.g. isophthalic, phthalic acid, etc.) or esters (alkyl, aryl esters of tetracarboxylic acids) in presence of phosphoric acid to form a solution or a dispersion of the above compounds. The said solution or dispersion is heated subsequently at 350°C to

yield the polyazole polymer (such as polybenzimidazole, polyoxadiazoles, polythiadiazoles, polyimidazoles, etc.). The MEA is fabricated by applying the polymer on the support material. Precautions are taken to avoid the formation of polyphosphoric acid during heat treatment. The PEM exhibits proton conductivity value of 0.1 S cm-1 at 120°C and improved power compared to doped membranes. Jones and Rozière have reviewed the progress in the functionalized PBI membranes for the fuel cell applications [98]. The proton conductivity of PBI based PEMs are improved by several approaches viz. grafting of acidic functional group such as sulfonic acid on the PBI backbone, complexation with acids such as phosphoric and sulfonic acids, doping of PBI with inorganic and organic bases, etc. The introduction of protogenic groups into the polymer backbone are the common strategies for the improvement of proton conductivity of polymer electrolytes. The enhancement in ionic conductivity of the polymer electrolytes are due to the increase in the water up take capacity as well as density of the labile protons in polymer matrix. 5.3. Crosslinked Membranes Functionalization is found to be useful to induce hydrophilicity and to achieve high conductivity, but with increase in hydrophilicity of the membranes, the dimensional stability is lowered down and the strength of membranes under fully hydrated condition becomes questionable. Appropriate balance of conductivity as well as dimensional stability is achieved by preparing crosslinked membranes. Crosslinking imparts mechanical strength to the membranes but at the same time the membranes becomes less flexible and if the extent of crosslinking is more, the conductivity can be decreased also. Thus, crosslinking is an effective tool, which

Electrolyte Membranes for Fuel Cells

Recent Patents on Materials Science 2014, Vol. 7, No. 3

13

Fig. (9). In situ synthesis and sequential synthesis of semi-interpenetrating polymer network.

can be used deliberately to get suitable balanced of functional properties. Crosslinked membranes undergo lesser swelling and they are more stable mechanically and dimensionally and can prevent the leakage from membranes. Crosslinked polymeric membranes can be prepared by two ways, namely in situ and sequential synthesis as indicated in Fig. (9). Again, structurally they can be of two types, either both polymers are crosslinked or only one polymer is crosslinked and the other remains as the linear polymer. In the in situ synthesis route, all reactants are mixed before the start of reactions whereas in sequential synthesis, polymerization of one polymer is performed first and then the second network is formed in the presence of already formed network. Poly vinyl alcohol (PVA) is the common material crosslinked with other polymers such as styrene sulfonic acid-co-maleic acid [99] and sulfonated poly(arylene ether ketone) having pendent carboxylic acid group to prepare membranes for PEMFC. PVA is chosen very frequently due to its easy film formation ability, hydrophilicity and excellent solubility. It can be easily crosslinked with carboxylic acid groups by the application of heat. It can also be a valuable performer in DMFC because of its low methanol permeability. In a typical procedure, the first polymeric pair is crosslinked by the external crosslinking agents and the second polymeric pair is crosslinked further by the selfcrosslinking reaction involving -OH group of PVA and COOH group of poly (arylene ether ketone) induced by heat treatment [100]. Sulfonated poly(phthalazinone ether sulfone ketone) and poly acrylic acid semi-interpenetrating polymeric network membranes are also prepared. Conductivity of the membrane is reported to be 1.882  10-2Scm-1, which is 1.2 times of that of commercial Nafion® membrane and 3.9 times of the sulfonated poly (phthalazinone ether sulfone ketone) native membrane [101]. The enhancement in water uptake properties of the membrane is due to the presence of the carboxylic group in the poly acrylic acid. In another invention, Guerra has demonstrated the preparation of a durable crosslinked polymer electrolyte membrane by introducing pendent sulfonyl halide group into the

non-perfluorinated polymer or polymer mixture containing both non-perfluorinated and sulfonyl halide group bearing polymer by direct fluorination [102]. Hamrock et al. have reported the fabrication of crosslinked polymer electrolyte membrane by using ultraviolet radiation [103]. They have also utilized highly fluorinated fluoropolymer synthesized from tetrafluoro-ethylene monomer containing -SO2 X (where X is F, Cl, Br, OH or -O-M+ ; M+ is a monovalent cation) and -Y (may be Cl, Br and I) as pendent groups. The crosslinking in the polymeric structure can be also induced by electron beam (e-beam) irradiation. In a typical study, a perfluorinated fluoropolymer containing crosslinkable functional groups (as given by the typical formula -S02X, where X stands for F, Cl, Br, OH) is crosslinked by exposure under e-beam [104]. In this study, the inventors have prepared membranes with various thicknesses and then the crosslinks are initiated by 4 Mrad of e-beam radiation. Highly fluorinated crosslinked polymer with a pendant sulfonic acid group and a trivalent crosslink group is prepared by Grootaert [105]. The polymer electrolyte membrane containing perfluorinated polymer backbone and two pendant functional groups is fabricated by four steps method. The first group is composed of sulfonyl halide and the other is a nitrile group. This nitrile group in the subsequent step converts the polymer to the crosslinked structure and the sulfonyl group thereafter is converted to sulfonic acid group to provide ionically conductive PEM. In another study, Pintauro and Tang have reported about a cation exchange membrane based on the crosslinkable sulfonated polyphosphazene and non-crosslinked polyphosphazenes [106]. In this invention, the poly [bis (3-methylphenoxy) phosphazene] is used as the matrix polymer and then it is blended with other sulfonated polymers for the preparation of membrane material. The said membranes are claimed to have a high ionic conductivity and reduced methanol uptake. 5.4. Hydrocarbon Membranes Hydrocarbons are chosen as the backbone material for the PEMFC applications due to their low cost and ease of achieving the tailored properties according to the requirement by simple chemical modification. Examples of some common polymers are represented in Fig. (10). By introduc-

14 Recent Patents on Materials Science 2014, Vol. 7, No. 3

Banerjee et al.

SO3H O

O

C

n

O SO3H

SPEEK H2 C

O

H C n

SO3H n

S

C

m

O

SO3H Sulfonated poly(phenylene sulfide)

Poly(styrene sulfonic acid)

n Sulfonated PPBP

Fig. (10). Structures of hydrocarbon membranes.

tion of polar side groups, the polymeric materials can be modified for enhanced hydrophilicity and higher proton conductivity since the water acts as vehicle for proton conduction. Another important reason for using hydrocarbon backbone is that, these materials are recyclable and their decomposition can be controlled by proper structural design. Thus, these membranes are also under focus as an alternative to the commercial Nafion® membranes. A hydrocarbon based multilayer PEM is developed by Nishii et al. [107]. This PEM is composed of sulfonated polyimide or polyarylene as the main component and a surface layer laminated with base material. The PEM contains two chemically different polymeric materials, one consists of a hydroxyl groups and the other a sulfonic or phosphoric acid group. The second polymer remains bonded in to the cross-linked structure of first polymer matrix (cross-linked by glutaraldehyde, suberoyl chloride, and terephthalaldehyde). Chemically, the first polymers is a vinyl resin (polyvinyl alcohol or ethylene-vinyl alcohol) or a polysaccharide (chitin, chitosan, cellulose) and the second polymer is a soluble sulfonated poly(ether sulfone), polystyrene sulfonic acid, sulfonated poly(ether ether ketone), polyvinyl sulfonic acid, etc. The MEA provides improved mechanical strength due to the better contact with the electrodes. Moreover, the multilayer PEM provides an opportunity of tailoring the properties of MEA by adopting the diverse design strategies for different single layer. The hydrocarbon PEM is also characterized by enhanced power generation capability due to the delayed ageing of the catalyst layer. 5.5. Composite Membranes Composite membranes are in general a combination of polymeric matrix and filler or additive. The polymeric matrix may also be functionalized material. They are prepared with the aim of enhancing proton conductivity and sometimes, thermal stability. An example of organic-inorganic composite membrane is given by Auer et al. [108]. They have studied the SPEEK membranes filled with TiO2 and

silicone oil at different temperatures and relative humidities. The membrane shows good water retention properties at around 120 to 140°C, good mechanical and thermal properties along with better chemical stability in oxidizing environment. In another study, organic-inorganic composite membranes composed of an organic material of sulfonated polymer or phosphoric acid doped polymer, and a water absorbing material with a large surface area is studied by Duan et al. [109]. The polymers such as Nafion®, SPEEK, S-PPO, SPES, S-PPBP and PBI doped H3PO4 are used in this study. Composite membranes can also be prepared by the addition of inorganic materials such as titania, alumina, boron phosphate, tungstophosphoric acid, etc. The polymers that are used in the preparation of composite membranes are sulfonated polysulfone (SPSU), SPEEK, poly(ethylene oxide) (PEO) [110]. Composite membranes containing silica, titania, zirconia, clay, and zeolite have been reviewed for different functional properties of the Nafion, SPEEK, PES, PBI, PVA, acrylates and fluorinated polymer based electrolyte membranes in particular with special emphasize on the filler morphology, shape, and chemical modification [111]. Composite membrane of 1-butyl-3-methylimidazolium (BMIM) and SPEEK doped with phosphoric acid (PA) are also studied and their properties have been presented in Table 3. This membrane is effective up to 160°C under anhydrous condition. Proton conductivity of 2.010-2 Scm-1 is obtained at a ratio of SPEEK: BMIM: PA= 1:0.6:9 [19]. Zeolite beta and aluminosilicates are added in sulfonated PEEK in order to improve the conductivity of polymer. At 68% degree of sulfonation and 60°C, the conductivity of SPEEK is found to be 0.06 Scm-1 and for the zeolite beta filled SPEEK composite membrane the conductivity is increased further to 0.13 Scm-1 [112]. Sulfonated PEEK and polyaniline composite membrane is also studied. Methanol permeability of these membranes is about four times lower than that of Nafion® 117 membrane [113]. A humidity preventing film of an oxide powder having self-moisture retention properties is formed on the PEM. In this process, an organic film is coated with

Electrolyte Membranes for Fuel Cells

the oxide solution on both sides or on a single side. The function of oxide particles is to provide the water retention capacity to the membrane and the membrane itself serves the role of good ion exchanger. The hydrated PEM can also be fabricated by the proper designing of the fuel cell system [114]. A new design containing a reservoir between the stack of fuel cell and cathode supply has also been studied. The system is designed in such a way that the water discharged from the fuel cell can be accumulated and collected to maintain the water level in the fuel cell stack during the operation of device. Sol-gel polymerization has been used for the preparation of hybrid inorganic organic polymer membranes using a silane compound for PEMFC applications. The proton donor groups of organic moiety are phosphonic acid group and its derivatives. As the inorganic phase, the metal cations such as Mg, Ca, Sr, Ba, Al, Fe are present [115]. A PEM of inorganic and organic material is studied by Jinping et al. [116]. A solution of silica sol and an organic polymer containing OH group is taken in the solution form and are mixed together to form a sol of silica. The membrane formed after baking shows high chemical stability, high conductivity, and moisture absorption capability without any need of external humidification. Hybrid composite membranes are also studied with the aim of improving the properties of SPEEK by the addition of functionalized polymers such as polysulfones. Sgreccia et al. have reported a hybrid membrane composed of SPEEK and silylated poly phenyl sulfone in the weight ratio of 93:7 [117]. The membrane exhibits a significant lowering in swelling and a good improvement in mechanical properties. Other composite membranes, made of SPEEK and PANI with heteropolyacids as a dopant, have also been studied [118]. The swelling resistance and the methanol permeability of the membrane are better due to the hydrogen bonding between the PANI and SPEEK. The proton conductivity of membranes is further enhanced by the incorporation of heteropolyacids. An excellent proton conductivity of 10-2 Scm-1 and halved methanol permeability of the membrane in comparison to the Nafion® is claimed at a particular composition of the above three materials. The performance of perfluorosulfonic composite membranes has also been evaluated by Liu et al. [119]. The metal ions such as Ce/Mn ions are totally exchanged with the sulfonic groups or sulfuryl groups in the perfluorosulfonic resins. The evenly distributed metal ions provide good mechanical strength, conductivity and optimum fuel cell performance. Krishnan et al. have developed a PEM containing an acid functionalized fluoro polymer and an additive [120]. The acidic functional groups selected in this study include sulfonic acid, phosphonic acid, sulfonylimide and their combinations. The fluorinated cycloaliphatic compounds are incorporated as the additives and are selected from the group of cis-perfluorodecalin, transperfluorodecalin, perfluorohydro- phenanthrene, etc. The content of the additive is varied from 0.5 to 10 weight %. The PEMs are characterized by improved fuel cell performance under anhydrous condition, high power density, improved heat and water management properties. Moreover, the fuel cell devices are of simple design with smaller stack size and also require minimum humidification. An MEA containing peroxide decomposer catalyst immobilized on a clathrate compound (zirconium phosphate) or a layered

Recent Patents on Materials Science 2014, Vol. 7, No. 3

15

compound (support material) is studied with the aim to fabricate durable fuel cell devices [121]. The device is fabricated by a PEM sandwiched between two electrodes, a gas diffusion layer, and a gas sealing material surrounding the MEA. The support material contains functional groups such as phosphoric acid, carboxylic acid, and phosphonic acid. The polymer electrolyte is made of a fluoropolymer with sulfonic acid functional groups reinforced with a reinforcing material. The fluoropolymers contain a fluorocarbon (-CF2-, - CFC1-), chlorocarbon (-CCl2-), and other chemical structures (-O-, -S-, -C(=O)-, -N(R)-; R, an alkyl group) with partial or completely fluorinated backbone. The immobilization of peroxide decomposer catalyst on the support helps to prevent its detachment from the surface and thereby the chances of salt formation by reacting with electrolyte are greatly reduced. Therefore, the deterioration of proton conductivity due to the consumption of catalyst is minimized and hence the device exhibits durable cell performance. Chen et al. have studied a composite membrane made of poly styrene sulfonated (PSS) and ion exchange resin [122]. The composite membrane is reported to show better properties in terms of swelling behavior, chemical stability and mechanical strength than the original PSS membranes. The degradation study of membranes also suggests that the composite membrane is better performer than the PSS membrane. Iyuke et al. have studied a membrane electrode assembly made of sulfonated poly styrene butadiene rubber and carbon nanostructures [123]. The sulfonation of poly styrene butadiene rubber is carried out by dissolving the polymer in a solvent followed by reacting with sulfur containing material such as chlorosulphonic acid. The sulfonated rubber is then reacted with the carbon material and finally solution casted to obtain the polymer electrolyte membrane. The carbon nanostructure chosen for the study is a carbon nanoball. The MEAs are fabricated by sandwiching the polymer electrolyte membrane in between two non-metallic carbon or metallic electrodes in a hot press. The incorporation of nanoballs improves the water up take, solvent up take, thermal stability of the membranes. Moreover, the nanoball containing membranes are characterized by reduced methanol cross over and improved proton conductivity as compared to the virgin membranes. The stability of composite membranes is enhanced by the addition of catalyst C, Ag, Pd, Ru, etc. to assist H2 O 2 decomposition [124]. The membrane is claimed to have good combination of properties with excellent durability at a low cost. Hande et al. have studied a nanocomposite membrane prepared from crosslinked SPEEK and an organo clay compound [125]. The developed nanocomposite has exhibited good oxidative stability, reduced swelling behavior, enhanced thermal stability and mechanical robustness. In an invention, a MEA comprising of perfluorinated polymer electrolyte membrane and cerium oxide filler is fabricated [126]. The cerium oxide used in this study is either CeO2 or Ce2 O3. The fillers are well dispersed in the perfluorinated polymer electrolyte containing acidic functional groups with chemical formula –O-CF2 -CF2-CF2 CF2-SO3 H or -O-CF2-CF(CF3 )-O-CF2 -CF2 -SO3H. The filler content is varied from 0.01 to 5 weight % of the total membrane weight. The addition of cerium oxide enhances the oxidative stability of membrane electrode assembly. In an-

16 Recent Patents on Materials Science 2014, Vol. 7, No. 3

other invention, Takeshita has developed a PEM containing porous polytetrafluoroethylene (PTFE) reinforcement and radical scavenger [127]. The radical scavenger is immobilized within the porous membrane and hence reduces the leaching tendency of former. The radical scavengers used in this study are CeO2, Ru, Ag, RuO2 , Fe-porphine, etc. The membrane electrode assembly (MEA) is fabricated by sandwiching polymer electrolyte membrane between two electrodes. The polymer electrolyte membranes are of good chemical resistance and mechanical strength due to the presence of porous PTFE polymer. The fuel cells are characterized by excellent durability, high power output that makes them suitable for practical fuel cell devices. Table 3 [128-157]. 5.6. Blend membranes The blend membranes are prepared with the aim of obtaining a nanophase-separated structure in which one polymer containing hydrophilic group acts as an ionomer and the other polymer provides the mechanical stability. To obtain the synergistic properties of two different polymeric materials, blend systems are introduced. Sulfonated poly ether ketone ketone (SPEKK) and poly ether sulfone blend membrane is reported by Swier et al. in which SPEKK is acting as the ionomer and the second polymer is playing the role of mechanical strength improver [158]. Here the SPEKK gives a good conductivity and flexibility and at the same time, the second component of the blend retains the desired strength at high level of hydration. In another study, Cai et al. have prepared a class of blend membranes of SPEEK with different degree of sulfonation and SPES at different compositions [159]. The easily prepared membranes exhibits excellent ionic conductivity, high strength and stability at low manufacturing cost. Another blend membrane is studied by blending poly(phenylene oxide) (PPO) and block copolymer of poly(styrene-b-vinylbenzylphosphonic acid). The membrane shows an improvement in stability, strength and conductivity with an increase in block copolymer content. A conductivity as high as 2.8510-2 Scm-1 is reported at 140°C for 50 wt% block copolymer containing membrane material. The membrane is proposed to be useful up to 350°C as analyzed by thermogravimetric analysis (TGA) [160]. Blend of sulfonated poly ether sulfone with sulfonated polybenzimidazole is studied by Lim and coworkers [157] with the aim to develop good dimensionally and thermally stable PEM. The polymers with controlled functionalization are synthesized from the respective monomers. The membranes obtained are of good flexibility and thermal stability up to 400°C. The ionic crosslinks between the two polymers reduce the swelling tendency, water uptake and thereby impart good dimensional stability. Blend membranes can also be a suitable candidate for high temperature PEMFC applications. An invention claims the fabrication of a membrane with the balance of all useful properties such as strength, stability, conductivity at higher temperatures [161]. In this study, Jin and Olaf have used the blends of three materials. The blend is prepared with a sulfonated polymer, PBI and an electro deficient compound acting as the anion accepter in the system. PBI acts as the solvent under anhydrous condition and the sulfonated polymer, having an electron rich moiety, serves as the Lewis

Banerjee et al.

base. As the anion receptor, triperfluorophenylene boron compound is used. The membrane works well above 100°C under anhydrous condition. In another invention, a number of blend membranes based on PBI and other thermoplastic polymers are fabricated for the high temperature fuel cell membrane [162]. In addition to the blend membrane, the study also reveals about the novel process of development of membrane electrode assembly and gas diffusion electrode for the fuel cell device. High thermal resistance, ionic conductivity and mechanical strength are the key features observed for those blend membranes. On the supporting substrate, carbon based metal catalysts are casted with the help of a binder consisting of PBI and blend of PBI with other thermoplastic polymers. The fuel cell made thereof can be operated at a temperature as high as 200°C. Aromatic polyethers are also blended with phosphoric acid doped pyridine. The membranes are reported to be useful for high temperature fuel cell applications with high thermal stability up to 400°C. These materials can show conductivity in the order of 10-2 Scm-1 at 130°C. The membranes prepared by these materials have better film formation ability, good miscibility and exhibit good proton conductivity. Thus, the pyridine based blends of polyether can be a suitable alternative of commercial Nafion® membranes [163]. 5.7. Grafted Membranes The grafting of polymers is performed with the aim of getting phase-separated morphology. It is incorporated in to the polymeric backbone by irradiating one polymer backbone to generate active free radicals. The free radicals then react at the unsaturation part of the other polymeric backbone and grafted polymer is formed. The functionalization of the grafted polymers is induced thereafter. In another way, one polymer can be functionalized first and after that, it can be grafted on other polymer backbone. Styrenated polymers are most commonly chemically grafted because of their ability to undergo easy free radical polymerization. Mechanically robust PEM fuel cell membrane for high temperature applications are prepared by grafting sulfonated styrenes into PEEK with a percentage grafting of 50%. The grafting is done by the phenoxy radicals in the PEEK backbone. The mechanical strength of these membranes is shown to be thrice than that of the commercial Nafion® membranes under similar conditions. X-Ray scattering confirms the solvent induced crystallization at amorphous phase of PEEK [164]. Poly tetrafluoro ethylene is also being grafted with polystyrene. After grafting of polystyrene in to the Teflon® backbone, it is sulfonated to enhance the hydrophilicity of grafted membranes. The desulfonation reaction are found to be started after 200°C with the completion at about 400°C. The TGA study indicates a three-step decomposition procedure starting with the loss of water, removal of sulfonic acid group and finally, the degradation of polymeric backbone [165]. A thermally stable and fairly conductive membrane is prepared by grafting of imidazole on to the alkoxysilane. The membranes are doped with phosphoric acid to increase conductivity. A conductivity of 3.210-3Scm-1 is obtained at 110°C in dry state. At higher relative humidity, the conductivity reaches to 4.310-2Scm-1 clearly indicating the influence of water on the proton conductivity. These membranes

Electrolyte Membranes for Fuel Cells

Table 3.

Recent Patents on Materials Science 2014, Vol. 7, No. 3

17

Property Comparison of Different Alternative Membranes. Types of Membranes

(Water Uptake %) [Swelling %] { IEC mequivg-1 }

Conductivity (Scm-1)

Conditions

References

PDPAA/ CYMEL

6.72

8.810-2

95% RH

[81]

(52)

1210

-3

100°C, 100% RH, DP 150%

[79]

SPEEK/ BMIM/H 3PO 4

--

2.0102

160°C, SPEEK:BMIM: H3 PO4= 1:0.6:9

[19]

Sulphonated gatone PEEK

--

0.04-0.235102

DS 70-80%

[128]

Phosphonated poly(arylene ether sulfone)

Reaction temp. 45-50°C SPEEKK

{0.62-0.95}

0.7-4102

25°C

[129]

SPPEK

(50.6){2.04}

1.06102

100°C

[130]

25°C

[131]

90°C

[33]

25°C, 100% RH

[132]

100% RH

[133-135]

100% RH

[136]

SPEEK

{0.7 - 1.5}[13-54]

2 - 7 10

-3

SPEEK/OMMT (10%)

--

8.810

SPEEK/Zirconium phosphate

--

1102

Sulfonated polyimide

{1.26}

2

0.4-210

2

2

Disulfonated biphenol based poly(arylene ether sulfone) copolymer

{1.3}

310

Sulfonated poly (4-phenoxybenzoyl 1,4phenylene)

{2.0}

1102

100% RH

[137]

Poly(oxa-p-phenylene-3,3-phthalido-pphenylene-oxa-pphenylene- oxy-pphenylene) SPEEK-WC

{0.76}

1.7102

100% RH

[138]

Sulfonamide functionalized Poly phosphazenes

{0.99}

6102

80°C, 100% RH

[139]

Sulfonated (styrene/ ethylenebutylene/styrene)

{1.78}

4102

100% RH

[140]

SPEEK/HPA

--

101

Above 100°C

[70]

SPEEK/ BPO 4

--

5101

160°C, fully hydrated

[141]

80°C, 98% RH

[142]

160°C, 100% RH

[143]

>150°C, 80% RH

[144]

140°C

[145]

39°C

[146]

SPSF/PAA

--

210

2

PBI/ SiWA+SiO 2

--

2.210

PVDF/CsHSO 4

--

102

Polysilsesquioxanes/ PWA

-3

2

--

310

Polyvinyl alcohol

--

10

-6

Polyvinyl acid-ZrP-Silicotungstic acid (10%)

(204) {0.902}[90]

310-3

60°C, 60% RH

[147]

ODA-STA-TPA-90

(65) {2.22}

1.582101

Water uptake at 25°C and conductivity at

[148]

80°C, 100% RH ODA-STA-IPA-90 ODA-STA-TFTPA-90

(67) {2.15} (55) {2.10}

1.42310 1.40410

1 1

do

[148]

do

[148]

18 Recent Patents on Materials Science 2014, Vol. 7, No. 3

Banerjee et al.

Table (3) contd….

Types of Membranes

(Water Uptake %) [Swelling %] { IEC mequivg-1 }

Conductivity (Scm-1)

Conditions

References

ODA-STA-TFIPA-90

(50) {2.13}

1.096101

do

[148]

ODA-STA-GA-90

(42) {2.01}

1.656101

do

[148]

ODA-STA-SEA-90

(42) {1.85}

9.97102

do

[148]

do

[148]

do

[148]

do

[148]

do

[148]

120°C

[149]

ODA-STA-SUA-90

(38) {1.90}

1.26210

1 1

ODA-STA-HFGA-90

(36) {2.02}

1.14510

ODA-STA-PFSEA-90

(34) {1.91}

1.254101

ODA-STA-PFSUA-90

(37) {1.90}

1.10610 2

PEO/ tungsten acid

--

Poly(ethylene-co-tetrafluoroethylene) grafted with poly(styrene sulfonic acid)

{3.22 }

1.9101

100% RH

[150]

Poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) grafted with poly(styrene sulfonic acid)+10% DVB)

--

4102

100% RH

[151]

Poly(vinylidene fluoride) grafted with poly(styrene sulfonic acid)

{1.83 }

5102

100% RH

[152, 153]

Poly(fluorinated arylene ether)s /Zirconium phosphate sulfonated

(85.6)

1.63102

25°C

[154]

Sulfonated PEEK- aminated polysulfone blend

{1.58 }

3102

100 % RH

[155]

Sulfonated PEEK- poly benzimidazole blend

{1.26 }

3102

100 % RH

[155]

Phosphoric acid-doped sulfonated polysulfone

{0.81 }

0.02-0.2

100 % RH

[156]

PES- polybenzimidazole blend

--

7.2102

100 % RH, 25°C

[157]

also show a good thermal stability and are stable up to 250°C [166]. In another study, a PEM based on organic polymeric materials is developed for fuel cell membrane applications [167]. The polymers are subjected to the radiation treatment to generate the free radicals prior to grafting to initiate the crosslinking in the polymeric structure and then it is grafted with the vinyl sulfonic or vinyl phosphonic acid moiety. The above membrane exhibits a combination of excellent thermal and chemical properties. Bollepalli has made a membrane using a sulfonating polymer containing heteroatom, which is grafted with the carbon black, graphite and other form of carbon as carbonaceous materials [168]. In the typical procedure, a heteroatom containing monomer capable of oxidative polymerization is grafted with the carbon materials and then the monomer or polymer is further sulfonated to produce a conductive carbon containing polymeric material. 5.8. Ionic Liquid Doped Membranes With the aim of developing high temperature fuel cell, number of approaches are being employed in the recent days. The ionic liquid is one of the suitable candidates in this field.

10

1

These are molten salts at the room temperature consisting of bulky organic cation and inorganic anion. These are of special kinds due to their ability to undergo numerous structural changes. These materials have suitable combination of properties such as thermal and chemical stability, low melting point, high ionic conductivity, low volatility with moderate viscosity, etc. Ionic liquids have ions such as imidazolium, pyrrolidium as cations and bis (fluorosulfonyl) imide, hexafluorophosphates, etc. as anions [169]. Ionic liquids can be used as a suitable alternative to water in Nafion®. A gel type composite membrane with the ability to operate at about 120°C is prepared using phosphoric acid as the solvent, PBI as the polymeric matrix and 1-propyl-3methylimidazolium dihydrogen phosphate as the ionic liquid. The membranes exhibit high conductivity of 2.010-3 Scm-1 at 150°C and anhydrous conditions. The ionic liquid serves number of roles in the membrane such as flexibilizer, retender of water in the membrane and plasticizer for PBI matrix [170]. Another work based on PBI as the polymeric matrix and 1-hexyl-3-methylimidazolium trifluoromethanesulfonate as the ionic liquid has been reported by Wang and

Electrolyte Membranes for Fuel Cells

Hsu [171]. The membrane shows ionic conductivity as high as 1.610-2Scm-1 under anhydrous condition and 250°C. These membranes are thermally stable up to 300°C and can be a suitable material for high temperature fuel cell applications. Sulfonated polyimide doped with n-methyl imidazolium tetrafluoroborate ionic liquid has also been studied for high temperature fuel cell applications [172]. Conductivity of these membranes can reach up to 5.5910-2 Scm-1 at 180°C. The interaction between the sulfonic acid group of sulfonated polyimide and the ionic liquid not only provides good thermal stability up to 200-250°C, but also good mechanical strength of 0.91 GPa at 300°C. SPEEK is also tried with ionic liquid to obtain high proton conductivity and thermal stability [173]. These membranes with conductivity of 8.310-3 Scm-1 at 170°C under anhydrous conditions can be preferably used up to 340°C. The thermal stability of the membrane is improved by the addition of ionic liquids due to the complex formation with charged imidazolium ring. Phase separated structure forms the ionic domains connected by small ionic channels. The membranes are quite interesting with the limitation of leaching of ionic liquid from the membrane. Thus, immobilization of them can be further approached in this field. Ionic liquids are also incorporated within the pores of a porous polymer structure. A novel proton exchange membrane and a fuel cell device is fabricated wherein the polymeric membranes are designed in such a way that, it can have some internal voids in it to retain the ionic liquid [174]. The hydrophobic ionic liquid is tried to be filled in the pores of polymer matrix. The ionic liquid containing a cation and an anion is chosen, where the cations are of the type pyridinium, pyhdazinium, pyrimidinium, etc. while non-Lewis acids serves as anions. In another case [175], a solid ionic membrane is developed by the incorporation of ionic liquid in the acidic polymer matrix containing sulfonic and/or phosphonic functional groups. In the typical procedure, at first the polymer is dissolved in the organic solvent and then the ionic liquid is added to it. The mixture is then casted over a porous PTFE frame. Removal of solvent by evaporation yields the desired proton conducting membranes. The ionic liquid because of its chemical nature can be bound to the polymer up to 30 to 90wt% of the polymer matrix. 6. CHARACTERIZATION OF MEMBRANES 6.1. Performance Study 6.1.1. Proton Conductivity The proton conductivity is the most important parameter for the PEMFC since the performance is largely dependent on its conductivity. It is determined by impedance spectroscopy using solatron gas phase analyzer. Prior to testing, the samples are completely moistened in water since the conductivity is largely dependent on the humidity level and testing temperature. Test samples of 1.51.5cm2 size are cut from the casted membrane and placed over the stainless steel electrode of the conductivity cell. The proton conductivity, in general, depends on the concentration of acidic sites inside the hydrophilic domain and the capability of acidic group to undergo dissociation in the water, which is present to facili-

Recent Patents on Materials Science 2014, Vol. 7, No. 3

19

tate the proton transport [79]. The proton conductivity of phosphonated membranes increases with the increase in degree of phosphonation and maximum conductivity of 12mScm-1 is reported with degree of phosphonation of 1.5% [79]. The samples can be tested in the room temperature or at the higher temperature to obtain the conductivity variation with the temperature and relative humidity. Eqn. 1 expresses the conductivity as follows  = L/(RS)

(1) -1

where,  is the protonic conductivity in Scm , R is the membrane resistance in ohm, L is the thickness in cm and S is the area of the membrane in cm2. The ionic conductivity is a thermally activated process and is governed by the Arrhenius law below the glass transition temperature of the materials. It is given by the Eqn. 2 as follows  = (A/T) exp{-Ea/(RT)}

(2)

where, A is the constant related to number of charge carriers, Ea is the activation energy, R is the ideal gas constant, and T is the temperature. Above the glass transition temperature, the conductivity is governed by the Vogel-Tamman-Fulcher equation as shown in Eqn. 3  = (A/T) exp{-B/(T-T0)}

(3)

where, B is the activation energy relating the segmental motion of the polymeric chains, and T0 is the equilibrium glass transition temperature. This equation also includes the dependence of proton conductivity on the side chain motion of the polymeric segments [7]. Figure 11 shows the variation of conductivity with temperature for the various polymer electrolyte membranes [176, 177]. The APTES/PDMS/POCl3 (APP) and Nafion 112 membranes initially present at 40°C and completely hydrated condition (i.e.100% RH) are subsequently heated to 130°C and the conductivity is measured as a function of temperature [176]. The study reveals that the conductivity of the APP membranes are retained at high temperature and low humid condition due to the presence of the thermally stable phosphorous groups in the membrane. On contrary, the Nafion® membranes undergoes severe loss of conductivity above 100°C due to the loss of water molecules. However, the conductivity of the zeolite membranes are found to be significant at high temperature [177]. The zeolite being a porous filler retain the water molecule within its pore. Hence the polymer electrolyte membrane consisting zeolite can retain the water molecules for the longer period of time enabling the fuel cell to exibit steady performance at high temperature. The conductivity of the NaA zeolite (Industrias Quımicas del Ebro, commercial zeolite) and ETS-10 (SiO4 tetrahedra and TiO6 octahedra) is observed to increase up to 120 and 150°C, respectively. 6.1.2. Water up Take and Swelling Ratio Since the proton conductivity of PEMFC is strongly dependent on the water content of membrane, the measurement of water uptake is quite useful for the prediction of the performance of these membranes. The presence of water although facilitates the transport of proton through the mem-

20 Recent Patents on Materials Science 2014, Vol. 7, No. 3

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Fig. (11). Variation of conductivity with temperature for Nafion 212, APP membranes [176] and Zeolite-polymer composite membranes [177].

®

brane by forming a path of proton conduction but at the same time the presence of higher amount of water leads to the excessive swelling of the membranes and hence the dimensional stability and mechanical strength of these membranes has to be taken under consideration. The water up take by the membranes is determined by taking the weight difference of the dried membrane and wet membrane as expressed in Eqn. 4. The membranes are first dried to completely remove the water and then these are immersed in deionised water to make them fully hydrated. Water Uptake= [(Wwet-Wdry)/Wdry]100

(4)

where, Wwet indicates the weight of membranes after soaking in water for a sufficient period of time and Wdry is the weight corresponding to the dried membrane. Similarly, the change in thickness of the membranes after immersion in water is also determined and the % increase is given in Eqn. 5: Swelling Ratio = [(Tw-Td) /Td] 100

(5)

where, Tw and Td are the thickness of wet and dry membranes, respectively. Lambda is another parameter relating the conductivity and water content of the membranes in terms of ion exchange capacity (IEC) and is given by the Eqn. 6 as follows. = [(W wet-Wdry)/(18IECWdry)]

(6)

where, IEC indicates the ion exchange capacity of the membrane determined by titration method. The lambda is an estimation of the number of moles of water molecule per acidic groups present. It is also expressed as the ratio of partial pressure of water to the saturation partial pressure of the water. Thus, the water up take, IEC and the elastic modulus of the membrane is dependent on this parameter. Figure 12 shows the water up take behavior of several polymer electrolyte membranes with temperature [33,178,179]. The segmental motion of the polymeric chains is enhanced with the increase in temperature enabling the membrane to accommodate more water molecules within its

Fig. (12). Variation of water uptake with temperature for Nafion® 117, crosslinked sulfonated poly ether ether ketone membrane [178], sulfonated poly ether ether ketone membranes [179] and organic inorganic composite membranes [33].

pore by the swelling of polymer. Hence, in general the water up take of PEMs is increased considerably due to the extensive penetration of water molecules within the polymeric membranes. The crosslinking imparts improved dimensional stability by creating chemical interlink between the polymer chains and hence restricts the free movement. Therefore, the crosslinked SPEEK membranes (CSPEEK-4) shows much lower water up take value than that of the Nafion® 117 membrane [178]. Similar observation on the water uptake value of SPEEK and crosslinked SPEEK (SPEEK-1, SPEEK-2, SPEEK-3, SPEEK-4) based membranes have been observed at different temperatures [179]. The crosslinking density progressively increases from SPEEK-1 to SPEEK-4. Hence, SPEEK 4 membrane is characterized by the lowest water up take capacity among all these membranes as more the crosslinked density lowest is the chain mobility. On contrary, SPEEK membranes shows the highest water up take capacity. SPEEK/OMMT composite membranes are also studied for water up take capacity as a function of temperature [33]. The composite membranes are characterized by constant water up take with temperature up to 80°C. This may be attributed to the association of hydrophilic functional groups of the SPEEK and the MMT layers that in turn prevent excessive swelling of the composite membranes. 6.1.3. Ion Exchange Capacity (IEC) Since the conductivity has a direct correlation with the IEC of membrane, therefore the IEC is of prime importance in this respect. The conductivity of PEM is increased with an increase in the IEC. However, the dimensional stability and mechanical strength also has to be taken in to consideration. The IEC is determined by the titrimetric method [17]. The membranes are first treated with NaCl in order to replace the H+ ion from the membrane and to convert in to corresponding sodium salts. The H+ ions released in the

Electrolyte Membranes for Fuel Cells

Recent Patents on Materials Science 2014, Vol. 7, No. 3

21

medium are then back titrated with NaOH solution. The IEC in mmolg-1 is given by the Eqn. 7 as follows. IEC= (Consumed NaOH Molarity of NaOH)/Wdry

(7)

6.1.4. Membrane Electrode Assembly (MEA) Testing MEA is the most significant part of the PEMFC. A typical MEA consists of polymeric membrane, cathode and anode material. The performance of a single fuel cell is largely dependent on the proper functionality of MEA. By the help of bipolar plate MEAs are connected in series to achieve the desired workable voltage in the fuel cell system. The efficiency of a fuel cell is expressed in terms of a polarization curve giving the variation of voltage with respect to the current typically called as I-V curve. A typical cell voltage vs. current density variation curve is shown in Fig. (13) for different materials. The curve starts at a lower voltage than the open circuit voltage. The second portion of the curve shows a rapid initial fall, and then as the current density is further increased, the curve continues with a slow fall and lastly at very large current density, another rapid fall in the voltage is encountered. The first rapid fall at low current density is known as the activation loss. The activation losses are encountered due to the activation energy required to start the electrochemical reactions. These types of losses are more prominent in case of low temperature fuel cell and are called as activation over potential. The slow fall proportional to the current, known as Ohmic loss, occurs due to the resistance to flow of the electrons. The Ohmic losses can be lowered by a proper design of the bipolar plate and most importantly, by increasing the conductivity of electrolyte. The final rapid fall is called mass transport loss or concentration loss, which occurs due to the smaller diffusivity of the oxygen than the hydrogen through the electrodes, thus the rate of the reaction is lowered down due to the insufficient supply of oxygen than hydrogen. It may be mentioned that, the efficiency or the power density of fuel cell can be properly monitored by minimizing the sources of the above losses and proper designing of MEA.

Fig. (13). Cell voltage vs. current density curves for Nafion® recast and Nafion recast/ZrP (containing 36 wt% zirconium phosphate) [40]; Nafion 115, Nafion/SiO2, Nafion/SiO2/PWA, Nafion/TiO2, and Nafion/WO3 membranes [38]; Nafion/ZrO2 [39]; Pt– SiO2/Nafion/PTFE and NRE-212 membrane [42].

6.2. Durability Study 6.2.1. Thermal Stability The thermal stability of proton exchange membrane is an important parameter for high temperature applications. It is determined by thermo gravimetric analysis (TGA). At first, the degradation of pendant groups takes place followed by the degradation of polymeric backbone. Various approaches are taken by the researchers to enhance the thermal stability. By incorporating solid inorganics such as zirconia, silica, etc. considerable improvement in the thermal stability is noticed [180]. The thermal stability of different PEMs is depicted in Fig. (14). SPEEK membrane [181] undergoes two step weight loss, first at around 300-400°C due to the decomposition of the acidic functional group and the second at 550°C for the main chain degradation. The thermal stability of Nafion® and Nafion®/laponite nanocomposite membranes are also studied in the literature [182]. The nanocomposite membranes containing 3wt% laponite exhibits improved thermal stability than that of pristine Nafion® membrane due to the strong interaction between the

Fig. (14). Thermal stability of SPEEK [181]; Nafion® and Nafion®/laponite [182]; Nafion®-MO2 (M = Zr, Si, Ti) [183]; PBI and phosphoric acid doped membranes [184].

filler and polymer matrix. Nafion®/metal oxide nanocomposite are studied by Jalani et al. [183]. The nanocomposite membranes containing different types of metal oxide exhibit improved thermal stability up to 310°C with weight loss of only 10%. For SiO2 and ZrO2 filled nanocomposite membranes the degradation take place at 470°C and 360°C. The addition of the TiO2 does not show any remarkable improvement in the thermal stability of the Nafion®. The thermal stability of the PBI and phosphoric acid doped membranes are also demonstrated [184]. This membrane is very much stable in nature and shows no degradation up to 400°C temperature. However, for doped membrane about 13% weight loss is observed in this temperature range due to

22 Recent Patents on Materials Science 2014, Vol. 7, No. 3

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the dehydration of phosphoric acid present within the membrane. 6.2.2. Stability in Oxidative Environment As the proton exchange membrane is one of the important components of the PEMFC system and its performance with durability is essential criteria for the long service life of the fuel cell system. The durability of membranes falls with time due to the degradation of membrane by the radical generation. The free radicals generated in the medium lead to the rapid chain breakage resulting in deterioration of the materials properties. The oxidative stability of the membranes is determined in the Fenton’s reagent having a composition of 30ppm FeSO4 and 30% H2O2. The results are expressed in terms of weight loss with the immersion time. Stability of the membranes is also estimated in acidic medium such as HCl to find the effect of acidic medium on the proton conductivity of membranes. The oxidative stability of PEMs is represented in Fig. (15). Jheng et al. [185] have studied the oxidative stability of PBI, asymmetric PBI (Asym PBI 150) and crosslinked asymmetric PBI (Asym cPBI 150) membranes in 3% H2O2 solution at 80°C. The crosslinked membranes exhibit only about 3.9% weight loss after 120 hours. According to the study, the oxidative stability of membranes is very much dependent on the exposed surface area of films. The Asym PBI 150 membrane shows more weight loss than that of PBI membranes due to the high surface area of porous Asym PBI 150 membrane. Oxidative stability of membranes are improved by crosslinking the polymers [186, 187]. Linear PBI exhibits much higher weight loss compared to ionically [186] and covalently [187] crosslinked PBI as shown in the figure. The linear PBI membrane breaks after half an hour immersion in Fenton’s reagent and after 20 hours 15% weight loss is evidenced. However, Nafion® exhibits very high oxidative stability as only 1% weight loss is observed during the same immersion time. 6.3. Life Cycle Estimation The development of PEMFC membranes is focused on the performance, cost and durability issues. The durability issue is important in several applications, where the long term stability of the PEMFC is required. The cost of PEMFC is lowered by the use of thinner membranes, which potentially enhances the chances of earlier membrane degradation. The degradation of the membranes can be categorized in different classes such as mechanical, thermal, chemical and physical degradations. For the better and longer performances of the PEMFC, the solid electrolyte membrane must be durable, good proton conductor and should maintain minimal permeability of the reactant gases through it.

Fig. (15). Oxidative stability of PBI, asymmetric PBI (Asym PBI 150) and crosslinked asymmetric PBI (Asym cPBI 150) [185]; linear PBI, ionically [186] and covalently [187] crosslinked PBI membranes.

of reactions starting with the Pt catalyst particles. The Pt catalyst particles react with the hydrogen leading to the formation of hydrogen radicals, which in the subsequent step combines with the oxygen molecules to form peroxide radicals [188]. According to other study, the radicals are formed during the oxygen reduction reaction at the cathode [189]. The radicals can also be generated due to the decomposition of H2O2 during the O2 crossover from cathode to anode [190]. The presence of the impurity metal ions such as Fe2+ induces the radical generation. The radicals formed in the MEA attack the polymeric membrane either at the branched sides or at the - carbons of the polymeric chain depending on the type of membranes. The presence of hydrogen bearing groups is also susceptible to the radical attack by means of the hydrogen abstraction. Linden et al. have proposed the radical attack to the polymeric chain involving the initiation and propagation steps as indicated by the scheme I [191]. The catalyst surface may also play vital role for the stability of the membranes [192]. Various radical formation pathways are proposed time to time by the researchers, but the exact pathways has yet to be established. PH + OH  P + H2O P + O2  PO2  PO2 + PH  POOH + P PH + HO2  P + H2O2 PH + HO2  H + HO2 + P

(Scheme I)

6.3.1. Chemical Degradation

Degradation Mechanism of Nafion

The chemical degradation causes severe damage to the solid electrolyte membranes, which is a serious problem against the proper functioning of the fuel cell systems. The solid membrane is subjected to the chemically harsh conditions both at the cathode and anode sides of MEA. The membrane degradation takes place by means of the radical attack, which lowers the lifetime of PEMFC. The radical attack on the membranes takes place by the peroxide formation. The formation of the peroxide takes place by a number

The failure of PFSA type membranes are mainly accounted for the chemical degradation. The degradation of commercial Nafion® occurs at the main chain as well as at the side chains. The degradation of Nafion® by the radical attack takes place by two parallel steps. The carboxylate end group, which are formed inevitably during the polymer manufacturing process induces the chemical degradation process of the PFSA membrane due to its susceptibility in radical attack and the other attack occurs at the pendent side

®

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23

chain. The carboxylate group once generated in the reaction medium degrades the PFSA completely unless it is consumed totally. The process is thus governed by unzipping reaction mechanism. Failure starts at the -CHF2 and CF2COOH groups [193]. The degradation rate of the PFSA membranes is found to be enhanced greatly when -COOH group is present [194]. With the loss of the water molecules from the PFSA, the performance of Nafion® deteriorate drastically due to the dependence of the conductivity with the level of hydration. Chemically, the changes are predicted to be reversible but the rehydration of membrane does not become effective due to the shrinkage of hydrophilic part of the PFSA [188]. Huang et al. have reported that the crystallinity of the Nafion® changes with drying during operation [195]. The ion channels are broader and open in humid condition, but in the dried state the channels shrink considerably. Much work has been done on the effect of hydration over the proton conductivity of Nafion® [196-203]. According to these studies, the dissociation capabilities of the Nafion® are dependent on the number of water molecules present around the sulfonic acid side chains of PFSA. As the number of water molecules increases, the dissociation of sulfonic acid group is increased accordingly.

degradation and becomes dried, brittle and fragile. For the Nafion® membranes the change in the chemical structure is observed only above 150°C. Above 200°C, the sulfonate groups are detached from the polymeric backbone. From the TGA and FTIR of the membranes, Samms et al. have revealed that when Nafion® is subjected to thermal degradation in an inert atmosphere mainly loss of water molecules occur from the membrane along with the liberation of some sulfur dioxide and carbon dioxide in the temperature range of 35 to 280°C [210]. Up to 355°C, the sulfur dioxide and carbon dioxide content increases in the evolved gases. Wilkie et al. have reported that the thermal degradation of the Nafion® is associated with the breakup of the carbon-sulfur bond leading to the formation of the sulfur dioxides (SO2) and OH radicals [211]. Deng et al. have observed that Nafion® and Nafion® nanocomposites show minimal evolution of SO2 up to 400°C and afterwards at temperature around 480 to 615°C the content of SO2 increases [212]. The nanocomposite shows higher temperature resistance than the pristine Nafion® up to 450°C and above which the nanocomposite decomposes faster than the Nafion®. The most critical temperature range is 300-400°C since the membrane loses its conductivity in this range.

6.3.2. Mechanical Degradation

6.3.4. Physical Degradation

The membranes suffer from mechanical degradation leading to the formation of perforations, pinholes and cracks. The presence of foreign particles can generate the stress concentration zones in the materials resulting in the formation of perforations. The generation of pinholes or perforations in the membranes increases the crossover of the reactant gases as a result the voltage in the MEA drops. The major causes of the mechanical degradation are associated with the improper fabrication of the MEA, non-uniform compression and in-plane tension or compression. The lowering of water content in the MEA can make the membranes brittle and fragile, which in turn enhances the mechanical degradation of membranes. During the fabrication of membrane, a uniform distribution of pressure on the membranes is required to reduce the chances of mechanical failure. Knights et al. [204] and Endoh et al. [205] have proposed that the mechanical failure of membranes occurs when subjected to the conditions like high temperatures, low relative humidity and high pressure. It is also established that the current density enhances by 50% with the reduction of the water content at the anode side [193, 206]. It has been shown that the penetration of catalyst particle in the membrane can generate local stress concentration zones in the solid electrolyte [190, 195]. Several efforts have been taken to enhance the mechanical strength of membranes by reinforcing it with other materials. The mechanical properties of the membranes are tried to be enhanced by reinforcing with porous polyethylene or PTFE [207, 208]. Incorporation of these above materials improves the dimensional stability of membrane and the shrinkage stress of membrane is also lowered down. The membranes are claimed to have longer service life at elevated temperature and low water content. They also exhibit lesser reduction in open circuit voltage (OCV) [208, 209].

In addition to the chemical, mechanical and thermal degradation, the solid electrolyte membrane also suffers from the physical decomposition. Under general operating conditions, the MEA is held with a compressive stress by the bipolar plates. This compressive stress produces creep or time dependent strain in the MEA. The creep in the membrane imposes permanent thinning and pin holing. Yu et al. have reported that the insufficient amount of water in the membrane can cause physical degradation as well [213]. The stress concentration zones suffer from more thinning. During the working condition of PEMFC, the generation of micro cracks is another common problem. The micro cracks appear at those areas, where the stress concentration is more localized. Other possible areas for crack generation are the boundary regions. Satterfield et al. have reported the creep response behavior of Nafion® 115 membranes [214]. At the beginning of tension or stretching process, the membranes having higher water content show higher creep rate. The creep rate falls down gradually with time for the wetted samples. For the dry samples the opposite phenomena is observed. In case of hydrated Nafion®, membranes show low creep rate and a long time is required to degrade the membrane material. At the same time, at higher temperature the mechanical creep occurs at faster rate than the usual condition. Blackwell and Mauritz have studied the creep behavior of the sulfonated poly(styrene-b-ethylene/butylene-bstyrene) (s-SEBS) membrane, and observed that, at a low sulfonation degree (< 6%) the creep resistance offered by the membranes is more due to the hydrogen bonding between the sulfonated group of membrane [215]. The resistance to creep is lowered down as the degree of sulfonation is increased further up to a certain level due to the creation of the continuous EB domain or plasticization of the sulfonated polystyrene domains by the residual water bound to the sulfonic acid groups. Reports show that pre-boiled Nafion® membranes have higher water retention, water/gas transport and ionic conductivity in comparison to the dried membranes

6.3.3. Thermal Degradation Due to the loss of water content of the PEMFC with the increase in temperature, the membrane suffers from severe

24 Recent Patents on Materials Science 2014, Vol. 7, No. 3

[216-218]. The membrane forms and breaks the ionic clusters due to the hydrothermal phenomena. Other study on Nafion® membranes reveals the effect of heat treatment on the morphological changes of Nafion®. Yeo and Yeager have studied three different types of membranes namely membranes without any heat treatment called the expanded form, the membranes heated at 80°C named as normal form and the membrane heated at 105°C called as shrunken form [219]. Tricoli et al. [220] and Sone et al. [221] have revealed that the heat treatment decrease the ionic conductivity of Nafion® membrane due to the changes in structure. Wei et al. have studied the structural relaxation of the membranes under dehydrated condition [222]. Thus, it can be taken into account that the processing history of material can contribute significantly in the degradation behaviour of membranes. 6.4. Prevention of Degradation Much efforts have been taken to prevent the degradation of the solid polymeric electrolyte membranes. Radical scavengers can lower the radical concentration and thereby lower the rate of chemical degradation of the membranes. Another way to reduce the degradation of the polymeric chain is to incorporate peroxide decomposer. Peroxide decomposer breaks every radical generated in situ and thus the degradation can be minimized. The reduction of the metal contamination from different sources can also be fruitful. Other possible routes can be •

water retention in the membrane



introducing mechanical reinforcement



reduction of gas permeability



use of a sacrificial material, which can form protective coating to prevent the direct attack on the membrane

6.5. Characterization of Membrane Degradation The lifetime of PEMFC is commonly measured by performing the accelerated fuel cell test since it provides good information on the durability of membranes at a lower testing time. The major accelerated testing conditions, at which the fuel cell durability is checked, include high temperature, open circuit voltage (OCV) and lesser humidity. At elevated temperature and dehumidified conditions, membrane undergoes degradation at a faster rate than the normal operating condition, and also suffers from the pin holing and crack generation [223, 224]. The degradation behaviors of the membranes are also characterized by the fluoride release rate, hydrogen crossover and OCV studies. The crossover is studied by purging nitrogen at the cathode side of MEA. When the fuel (hydrogen gas) is passed at the cathode, it is oxidized into H + and carried away through the membrane to the other side of the MEA, where it is reduced. Thus, the current generated in the cell is highly dependent on the crossover rate of the hydrogen [188]. The crossover depends on the several factors such as humidity, gas pressure and operating temperature of the fuel cell. The degradation behavior of membranes is determined by the fluoride release rate. The water are generally collected from the electrodes and the fluoride content in these samples are checked time to time by using ion chromatography (Dionex DX 500) equipped with conductivity detector [208]. The degradation

Banerjee et al.

of PFSA membranes generates fluoride ions, sulfate and small polymeric segments [225]. OCV is another way for identification of membrane failure but it is less informative than hydrogen crossover rate. It determines the present status of membrane. For the membranes to work effectively the voltage decay should be minimum. The effect of generated hydrogen peroxide on the membrane life time is studied under low humid condition combined with OCV [224, 226]. Under reduced humidity and OCV conditions, the membrane life is considerably reduced [192, 225, 227-229]. The life cycle of the fuel cell devices is also estimated in various studies with the aim to prevent the premature failure as well as to improve the longterm performance of the fuel cell devices with minor maintenance [230, 231]. Lee et al. have evaluated the life cycle of fuel cell devices by electrochemical impedance spectroscopy measurement [232]. The life span of fuel cell devices are evaluated by continuous examining the experimental parameters over a time period of 2200 hours. The degradation of membrane is evaluated by constantly monitoring the changes in charge transfer resistance and capacitance. These parameters ultimately give the idea about the extent of cathodic reduction reaction and thus the degradation of fuel cell. The measurement of cathode time constant from the AC impedance analysis proposes a suitable method for the constant monitoring of membrane degradation and hence the actual condition of the fuel cell devices during the operation. From the probability density function versus life span curve of the phosphoric acid doped polybenzoxazine membrane, the mean life span calculated to be 7609 hours (~317 days). However, the average life span may vary from one system to the other depending on the composition of the polymeric membrane, experimental conditions as well as MEA configurations. In the recent days, the degradation characteristic of PEMFC under accelerated conditions is becoming more attractive since it provides idea about the fuel cell life at lower testing time. 3M Company has pointed out some strategies for the life time prediction of the membrane under accelerated conditions but more detailed investigation yet to be carried out to acquire complete information about the membrane failure. 7. CURRENT & FUTURE DEVELOPMENTS Fuel cells are the emerging field of research in the present decade. The efficiency of PEMFC is highly dependent on the nature and functional properties of the solid polymeric membrane. Nafion® is the most successful and commercialized membrane till date but it is costly and losses its performance at the elevated temperature. Hence, researches are going on to improve the performance of Nafion® either by modification or by introducing suitable alternative to Nafion® membrane. As an alternative approach, incorporation of the hydrophilic groups such as sulfonic acid group is common and widely explored. The sulfonated polymers undergo desulfonation quite easily at the elevated temperature, hence they cannot be used at the higher operating temperature. In order to overcome this problem, other hydrophilic group such as phosphonic acid group is incorporated, which is found to be useful in terms of durability as well as proton conductivity. The blending of two polymeric materials is carried out to achieve phase separated morphology by means

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of suitable combination of hydrophilic and hydrophobic regions. Grafting of one polymeric material in to another polymeric backbone can also induce phase separated morphology. Ionic liquids are also incorporated in to the polymeric matrix with the aim of high temperature applications. These liquids are molten salts at room temperature, which can improve the stability of the membranes and lead to good proton conductivity. Thus, a large number of approaches are taken to find out an alternative proton exchange membrane with properties comparable or better than that of the commercial Nafion® at much lower cost. In addition to the structural and property requirements to serve as a suitable alternative to the commercial Nafion® membranes, the membranes should also be durable under working conditions. Life cycle estimation of the fuel cell membranes are thus important criteria to keep the device operational for a long time with high efficiency. Degradation can occur by means of mechanical, thermal, physical or chemical way. Extensive studies are going on to assess the performance of the membranes to fabricate highly efficient and durable devices. In this review, it is revealed that the sulfonic acid modified and acid-base electrolyte membranes are matured enough as emerging alternative membranes with respect to the commercialized, most acceptable and established Nafion® membranes due to their easy processing, high proton conductivity, and low fuel cross over. However, researches on these polymers are still going on with much emphasis on the transport and conduction mechanism and microstructure to further improve the performance of fuel cell membranes. Microstructure has been the most crucial in achieving higher proton conductivity as proper distribution and connectivity of the conduction channels provides favorable pathways for proton transport. Hence, in order to develop efficient fuel cells, membranes should be designed in such a way that the conduction channels are inter connected together to ease the proton transport through the membranes.

DS

=

Degree of Sulfonation

DVB

=

Divinylbenzene

FPI

=

Fluorinated Poly (Imide)

GA

=

Glutaric Acid

HFGA

=

Hexafluoroglutaric acid

HPA

=

Heteropolyacids

IEC

=

Ion Exchange Capacity

IPA

=

Isophthalic Acid

Lp

=

Laponite

MCFC

=

Molten Carbonate Fuel Cell

MEA

=

Membrane Electrode Assembly

MMT

=

Montmorillonite

MPA

=

Molybdo Phosphoric Acid

ODA

=

4,4-oxydianiline

OMMT

=

Organic Montmorillonite

PA

=

Phosphoric Acid

PAA

=

Phosphatoantimonic Acid

PAFC

=

Phosphoric Acid Fuel Cell

PAI

=

Poly(Amide imide)

PAMPS

=

Poly(2-acrylamido-2-methyl-1propanesulfonic acid)

PBI

=

Poly(Benzimidazoles)

PDPAA

=

Phosphonated poly(Nphenylacrylamide)

PEI

=

Poly (Ether imide)

PEM

=

Polymer Electrolyte Membrane

CONFLICT OF INTEREST

PEMFC

=

Proton Exchange Membrane Fuel Cell

The authors state that the content of the review article does not have conflict of interest.

PEO

=

Poly(Ethylene oxide)

PES

=

Poly(Ether sulfone)

PFSEA

=

Perfluorsebacic acid

PFSUA

=

Perfluorosuberic acid

ACKNOWLEDGEMENTS The authors acknowledge Department of Science and Technology, Government of India for granting the fellowship.

PMoA

=

H3PMo12O40.nH2O

PPBP

=

Poly (4-phenoxybenzoyl-1,4-phenylene)

ABBREVIATIONS

PPO

=

Poly(phenylene oxide)

AFC

=

Alkali Fuel Cell

PTFE

=

Poly (Tetra Fluoro Ethylene)

BMIM

=

1-butyl-3-Methylimidazolium

PTA & PWA =

Phosphotungstic Acid

BPO4

=

Boron Phosphate

PVA

=

Poly (Vinyl Alcohol)

CsHSO4

=

Cesium Hydrogen Sulphate

PVP

=

Poly(Vinyl Pyrrolidone)

CYMEL

=

Hexamethoxymethylmelamine

PSS

=

Poly Styrene Sulfonate

DMF

=

Dimethyl Formamide

PSSA

=

Polystyrene Sulfonic Acid

DMFC

=

Direct Methanol Fuel Cell

RH

=

Relative Humidity

DMSO

=

Dimethyl Sulfoxide

RT

=

Room Temperature

DP

=

Degree of Phosphonation

SEA

=

Sebacic Acid

26 Recent Patents on Materials Science 2014, Vol. 7, No. 3

Banerjee et al.

sSEBS

=

Sulfonated poly(Styrene-bethylene/butylene-b-styrene)

[13]

SiWA

=

Silicotungstic Acid

[14]

SMM

=

Surface Modifying Macromolecules

SOFC

=

Solid Oxide Fuel Cell

SPC

=

Sulfonated Polycarbonate

SPEEK

=

Sulfonated Poly(Ether Ether Ketone)

SPEEKK

=

Sulfonated Poly(Ether Ether Ketone Ketone)

SPEKK

=

Sulfonated Poly(Ether Ketone Ketone)

SPPEK

=

Sulfonated Poly(Phthalazinone Ether Ketone)

SPSU & SPSF =

Sulfonated Poly Sulfone

STA

=

2-sulfoterephthalic Acid Monosodium Salt

SUA

=

Suberic Acid

TEOS

=

Tetraethyl Orthosilicate

TFIPA

=

Tetrafluoroisophthalic Acid

TFTPA

=

Tetrafluoroterephthalic Acid

TGA

=

Thermogravimetric Analysis

TPA

=

Terephthalic Acid

ZrP

=

Zirconium Phosphate

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