ISSN 09655441, Petroleum Chemistry, 2011, Vol. 51, No. 7, pp. 480–491. © Pleiades Publishing, Ltd., 2011.
Membranes for Low and Medium Temperature Fuel Cells. StateoftheArt and New Trends1 Gérald Pourcelly Director of the “European Membrane Institute”, UMR 5635 CNRS, ENSCM, UM2, University of Montpellier 2, CC 047, Place Eugé5ne Bataillon, 34095 Montpellier Cdx 5, France CNRS Research Grouping: “Fuel Cells & Systems” PACS 3339 email:
[email protected]montp2.fr Received October 11, 2010
Abstract—Proton exchange membrane fuel cells are considered as a key issue against oil rarefaction and green house gas emissions. But some problems prevent from the wide use of this type of fuel cells in modern technologies. A lot of attempts were made in order to increase working temperature of membranes up to 120– 150°C and to decrease the dependence of their conductivity on humidity. In this review novel results con cerning new types of membrane syntheses, approaches to the membrane modification and on the membrane use in low and medium temperature fuel cells are considered. DOI: 10.1134/S0965544111070103 1
1. INTRODUCTION
Fuel cells are electrochemical devices with high energy conversion efficiency, minimized pollutant emission and other advanced features. Proton exchange membrane fuel cells (PEMFC, see Fig. 1 for general knowledge) are considered as a key issue against oil rarefaction and green house gas emissions [1]. Although some companies have produced vehicles driven by PEMFCs, they have to face problems of water management, carbon monoxide poisoning of fuel cell catalysts, membrane behaviour and costs. High temperature PEMFCs have been proposed to solve problems of catalyst poisoning by CO and fuel cell electrode flooding, as well as to improve fuel cell efficiency, reduce the amounts of noble metal catalyst and avoid reactant humidification [2]. Most of PEMFC research efforts aim to increase performances (yield efficiency, power density, reduc tion of catalyst content, durability), to increase mechanical, thermal and electrochemical stabilities, and to decrease mass, volume and costs. Operating at an increased temperature (120–150°C) causes greater challenges for PEMFC [3]. Novel materials that can give high performance and high durability under such conditions are prerequesite for high temperature PEMFC, among which alternative electrolyte mem branes that can work at high temperature (120– 150°C) and low relative humidity (RH = 25–50%) are one of the most important. Many current research efforts are therefore devoted to the development of alternative electrolyte membranes [3], including non fluorinated hydrocarbon polymers [4], inorganic 1 The article is published in the original.
polymer composite [5], anhydrous proton conducting polymers [6] such as PBI/H3PO4 [7, 8] or Nafion®/H3PO4 [9]. In this short review, based on remaining bottle necks for low temperature PEMFC, we focus on the material challenges for “hightemperature” PEMFC, mainly membranes. Among the dynamic characteristics of a Mem braneElectrodeAssembly (MEA), the VoltageCur rent Response (VCR, Fig. 2) reveals three regions. The first is governed by the electrode catalysis (reduction of the charge transfer overvoltage), the 2nd by the membrane behaviour (reduction of the ohmic losses), the 3rd by the cell design (improvement of fluid management). The obtention of an optimized MEA needs R&D works in these three parts. For most PEMFCs, the proton exchange membranes are cur rently based on perfluorosulphonic acid (PFSA) poly mers, e.g. Nafion® [10]. This membrane material exhibits high conductivity, excellent chemical stability, mechanical strength and flexibility. However, its oper ating range lays only in a highly hydrated state and therefore is limited to temperatures up to around 80°C under ambient pressure in order to maintain a high water content in the membrane. If the Nafion® mem brane still remains the reference despite a dehydration over 90°C and a relative high methanol crossover, new proton conducting polymers have been developed on the basis of both new polymers and new designs. Several challenges for the PEMFC power technology are associated with low operating temperature [11]. Fuel processors, such as hydrogen storage tanks and hydrocarbon or alcohol reformers with subsequent CO removers are voluminous, heavy and costly. Water
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Fig. 1. Scheme of the PEMFC structure and how it works.
E, V 1.2
Eeq = 1.23 V Charge transfer overvoltage
1.0
Ohmic losses ReJ
0.8
Mass transfer overvoltage
0.7
0.6 0.4 0.2 0
0 0.25 0.4 0.5 0.75 P0 = 0.175 W/cm2 P/P0 = 1.6 j/A cm−2 Catalysts of Membrane electrodes
0.9 1 P/P0 = 3.6
1.25
1.5 P/P0 = 6 Cell design
Fig. 2. VoltageCurrentResponse of a MembraneElectrodeAssembly.
management involves appropriate humidification of fuel and oxidant, airflow rate and power load regula tion. Temperature cooling is more critical for larger stacks, and the heat produced is of low value. PEMFC operating at higher temperatures (120–150°C) has been recognized as a promising solution to meet these challenges. So, from 2001, researches have been focused on this increase of the operating temperature (120–150°C) with a 25–50% relative humidity. 2. INCREASE THE OPERATING T OF PEMFC: THE NEW CHALLENGES The theoretical analysis and experimental investi gations have shown that working at high temperature (120–150°C) can provide the following advantages for PEMFC [12–14]. PETROLEUM CHEMISTRY
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Electrode reaction kinetics: the overall electrochem ical kinetics of a PEMFC is mainly determined by the slow oxygen reduction reaction (ORR [3]). Its accounts for the major overvoltage loss of PEMFC and remains a major focus of PEMFC researches [3, 15]. The reaction kinetics of hydrogen oxidation and ORR are both enhanced at high temperature [16]. CO tolerance: trace CO in hydrogen feed gas drasti cally depresses the performance of PEMFC due to the strong adsorption of CO on Pt electrocatalysts [17]. The adsorption of CO on Pt is weakened at high tem perature and CO tolerance is enhanced (10–20 ppm at 80°C, 1000 ppm at 130°C, 30000 ppm at 200°C). Heat management: a PEMFC operating at 80°C with an efficiency of 40–50% produces a large amount of heat that has to be removed in order to maintain the working temperature [18]. The rate of heat transport is
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Water production
Diffusion H2 humidified H+ transport (Dehydration due to electroosmotic drag)
Hydraulic Permeability
Anode Electrode
Membrane
O2 (humidified) Water accumulation
Cathode Electrode
Fig. 3. Modes of water transport in an operating H2/O2 PEMFC (from [3]).
proportional to the temperature difference between PEMFC and environment. For a PEMFC working at low temperature (≤80°C), the heat rejection rate of the conventional automotive radiators is not sufficient to reject continuous full power waste heat [19]. Increase of PEMFC working temperature to >120°C will allow use the cooling system of present internal combustion engines. Water management: PEMFC working at tempera ture ≤80°C under atmospheric pressure involves a dualphase system (liquid and vapor water). When the humidification is too high, water condenses and the electrodes are flooded (mainly the cathode), which makes water management difficult [20]. If a PEMFC is running at atmospheric pressure and at T > 100°C, only single water vapour exists [11], and therefore, the transport of water in the membrane, catalyst layers and diffusion layers is easier to balance. Nonplatinum catalysts: with the increase of tem perature, the electrode reaction kinetics is strongly enhanced, making possible the use of nonplatinum catalysts, reducing PEMFC cost. The previous advantages of “hightemperature” PEMFC are therefore very attractive and lots of researches are presently devoted to this challenge. 3. HIGH TEMPERATURE PROTON EXCHANGE MEMBRANES Most of the researches dealing with “hightemper ature” proton conducting membrane have their origin in the water management of a PEMFC, which is nec essary to simplify the lowT operation and enable a highT operation. The water balance in a PEMFC involves the following mechanisms: (i) water supply from the fuel and oxidant (humidification); (ii) water production at the cathode (current density); (iii) water drag from the anode to the cathode (current density, humidity, T); (iv) backdiffusion of water from the
cathode to the anode (gradient concentration, capil lary forces etc.) (Fig. 3). Great efforts have been and are being made to syn thesize proton conducting membranes and other materials operating at temperatures above 100°C [3, 4, 6, 11, 12, 21]. Membranes under development can be classified according to the following: (i) modified per fluorosulphonic acid (PFSA) membranes [3, 11, 13, 14, 28–30]; (ii) alternative membranes based on par tially fluorinated and aromatic hydrocarbon polymers [22, 23]; (iii) inorganicorganic composites [5, 24, 25], (iv) acidbase polymer membranes [26, 27]; typi cally a basic polymer doped with a nonvolatile inor ganic acid or blended with a polymeric acid. 3.1. Modified PFSA Membranes Development of PFSA membranes and their appli cations have been extensively reviewed. For operation at temperature above 100°C, modifications of the PFSA membranes have also been widely investigated. One major drawback of PFSA membranes is their low conductivity and their poor performance under low humidification and at elevated temperature (>90°C) due to the water loss (Fig. 4). The conductivity of Nafion® 115 drops significantly with a decrease in water activity at elevated T (80–140°C). Efforts have been made to modify the PFSA mem branes for operating at “high temperature”: (i) by replacing water by nonaqueous and low volatile media; (ii) reduction of the membrane thickness; (iii) impregnation of the membrane with hygroscopic oxide nanoparticles; (iv) impregnating of the mem brane by solid inorganic proton conductors. 3.1.1. Replacing water by nonaqueous solvents. High ionic conductivity of Nafion® membranes can also be achieved in other solvents than pure water, such as waterorganic mixtures, alcohols, organic acids and aprotic dipolar solvents [31]. The first attempt (Savinell (1994) [33], incorporated phosphoric acid in PETROLEUM CHEMISTRY
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Conductivity (S/cm) 2.5E01 2.0E01
140°C 100°C
1.5E01 1.0E01 120°C 80°C 5.0E02 0E+00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7 0.8 0.9 1.0 Water activity (Pw/Psat)
Fig. 4. Conductivity of Nafion® 115 as a funcdon of water activity at various temperatures according to [28].
Nafion® and obtained a conductivity of 0.05 S cm–1 at 150°C. Using 1H and 31P NMR spectroscopy, Was mus et al. [9] showed that only weak interactions between the sulfonic acid groups of the ionomer and the phosphoric acid molecules exist. Evidence was obtained for a relatively rapid exchange of protons, on the NMR time scale, between –SO3H, H3PO4 and residual water. Phosphoric acid acts as a Brönsted base and solvates the proton from the strong sulfonic acid group. The idea of impregnation has been extended to ionic liquids as recent papers envisaged the high con ductivities of Protic Ionic Liquids (PILs) as well as their potential for use in highT PEMFCs [34–36]. Using methanesulfonate as anion and triethylammo nium cation in a Nafion® as a host polymer resulted in high conductivity at 130°C approaching that of Nafion® at 80°C and 98% relative humidity [29]. Another approach is a new hybrid inorganicorganic nanocomposite proton conducting membrane based on Nafion®, SiO2 and thriethylammonium trifluo romethane sulfonate as a protic ionic liquid [30]. This protocol is promising in order to obtain waterfree materials for PEMFC membranes. Nevertherless, fur ther research efforts are necessary to obtain mem branes which meet the requirements for high T. The idea has been extended to impregnation of Nafion® membranes with other acids (phosphotungstic acid solution in acetic acid) [37]. Another interesting group of solvents with potential to replace water is the heterocycles such as imidazole, pyrazole or benzimidazole, containing both proton donor (NH) and acceptor (N) (Fig. 5). Waterfree Nafion® 117 membranes swollen in imidazole and imidazolium salt (trifluoroacetate and trifluoromethane sulfonate) exhibited conductivities of about 5 × 10–3 S cm–1 at 100°C [38]. The difficulties PETROLEUM CHEMISTRY
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may arise from (i) the immobilization of the liquids, especially in the presence of water and (ii) adsorption of the solvent on the catalyst surface. 3.1.2. Reinforced and thin PFSA membranes. Thin ner membranes reduce ohmic resistance but also improve water management during PEMFC opera tions [39]. One challenge for developing thinner mem branes is to overcome the reduction of mechanical strength, especially when swelling at high T. Rein forcement by porous PTFE sheet [40] or by micro PTFE fibril [41] make this possible. Thus, the thick ness of PFSA membranes was reduced down to 10– 30 μm, keeping good conducting and mechanical properties [42]. 3.1.3. Composites with hygroscopic oxides. One of the ways to achieve low humidity and highT opera tion of PFSA membranes is to recast Nafion® with mixed hygroscopic oxides (such as SiO2 and TiO2). As a result of the water adsorption on the oxide surface, the back diffusion of the water produced at the cath ode is enhanced and the water electroosmotic drag from anode to cathode is reduced. Crystallized ZrO2 particles (6 nm) can thus be obtained in situ from a Nafion® solution through a solgel process according to Fig. 6 [43]. The formed Nafion®zirconia hybrid membrane shows enhanced water retention ability compared to recasted pure Nafion® membrane at higher temperature. The inorganic nanoparticles have to be able to fill into the hydrophilic cluster (~5 nm) in order to improve thermal stability and proton conduc tivity at high— T. 3.1.4. Composites with solid inorganic proton con ductors. Inclusion of hygroscopic oxide particles in PFSA membranes is for water retention. Bifunctional particles, both hydrophilic and proton conducting can also be incorporated. An exhaustive review was pub lished by Herring [5]. There are a variety of synthetic
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H H⎯O–
H
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amphoteric character
HN + NH
NH
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H O
H
H
N
H
N
NH
formation of hydrogen bonds
H H
2
O
H
H⎯O– +
H
+
O
H 2 N
H
autoprotolysis
N − N + HN + NH
NH
Fig. 5. Similar behaviour of imidazole and water versus proton.
Water content, % 2.5 Nafion dispersion in IPA/water drying redissovling in NMP
Desired concentration of Nation in NMP
TBZ in nbutanol
NafionZrO2
2.0 1.5
Nafion mixing
Nation/TBZ sol hydrolyzing
Hydrid Nation zirconia dispersion
1.0 0.5
recasting
Nationzirconia hybrid membrane
0
20
40 60 80 Relative humidity, %
Fig. 6. Left side: Protocol of Nafion®zirconia hybrid membrane. Right side: water uptakes at 100°C (According to [43]).
methods available for the formation of composite membranes which can be roughly divided into 4 ways: Incorporation of preformed insoluble particles in the pores of cast or extruded polymers; Adsorption of a soluble inorganic moiety into the preformed polymer from solution; Addition of preformed inorganic particles to a solution of polymer and casting and annealing of the composite membrane to give an insoluble film; In situ formation of the inorganic particle from suitable precursors in the polymer casting solution fol lowed by casting and a suitable heat treatment.
Proton conduction in ionomers and proton con duction mechanisms in pure conducting components have been extensively reviewed [44]. Thus, the highest values of proton conductivity κ (>0.1 S cm–1) are found in the decreasing order: the mineral acids, e.g. HCl, with H3PO4 having high κ up to around 200°C, followed by the solid super acids PFSA (e.g. Nafion®), HeteropolyAcids (HPAs) or sulfonated zirconium phosphonates, although most of these do not have high κ values above 100°C, CsHSO4 and related compounds above the superprotonic phase change. Most of the literature is therefore devoted to inorganic solid proton conductors with (i) zirconium phosphates [28], (ii) heteropolyacids, which are gen PETROLEUM CHEMISTRY
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erally clusters of tungsten, molybdenum belonging to a large class of inorganic oxides called the polyoxo metallates (POMs) [45], (iii) metal hydrogen sulphate [46]. Composite membranes with inorganic solid proton conductors have also been designed. PFSA composite membranes with both heteropolyacids and zirconium phosphate aim at improving the hydration character istics and raising the operational T [47, 48]. 3.2. Alternative Sulfonated Polymer Membranes and Their Composites Sulfonated polymer membranes are alternative to PFSA, due to their lower cost. 3.2.1. Types of polymers and their sulfonation. Basic polymers should have high chemical and thermal sta bility. To this end, two main groups of polymers have been investigated, the first containing inorganic ele ments such as fluorine in fluoropolymers and silicon in polysiloxanes, the second is aromatic polymers with phenylene backbones. Sulfonated polystyrenes were investigated in the 1960s and were the first generation of polymer electro lytes for fuel cells [49]. However, in this type of poly mer membrane, the tertiary C–H bonds in the styrene chains are sensitive to oxidation by oxygen and hydro gen peroxide. The bond strength for C–F is about 485 kJ mol–1 and that of C–H bonds is ranging 350– 435 kJ mol–1. Polymers containing C–F bonds there fore have high chemical and thermal stability. Partially fluorinated membranes have also been investigated: on a base of Poly(tetrafluoroethylenehexafluoropropy lene, FEP) films, by Scherer’s group [50, 51] and Polyvinylidene fluoride, (PVDF) by Sundholm’s group [52, 53]. The combination of the PVDF proper ties with the conductive properties of sulfonated poly styrene gives both high water uptake and conductivity, but for low T [53]. Another type of Tresistant polymers of interest contains the chemical bond of Si–O (445 kJ mol–1). Si–O networks are formed at high T (ceramics) but can also be developed at low T in organic or aqueous solutions. So, organic groups can be bonded to the sil ica matrix to give organicmodified silicates (ORMO SIL®), organicmodified ceramics (ORMOCER®), or organic modified silicate electrolyte (ORMOLYTE®). Attempts have also been made to develop protonconducting membranes for PEMFC by using arylsulfonic anions [56] or alkylsulfonic anions grafted to the benzyl groups [54, 55]. These structures exhibit a proton conductivity of 0.01 S cm–1 at room T and a thermal stability up to 120°C. Aromatic hydrocarbons present a large group of polymers of low cost and commercially available. Polymers consisting entirely of linked benzene rings such as polypphenylene are resistant to oxidation. Polyphenylene sulphide and polyphenylene oxide have high melting points with good thermal and oxida PETROLEUM CHEMISTRY
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tive stability above 200°C. Aromatic polymers con taining ether finks have also been widely investigated such as polyetherether ketones. Polybenzimidazoles are another family of high performance polymers. To create proton conductivity charged units can be introduced into the polymer structure. This can be done by chemical modification of the polymers (post functionalization) through the introduction of an anion, typically sulfonate (−SO3− ). This sulfonation can be performed (i) by direct sulfonation in concen trated sulphuric acid or chlorosulfonic acid [57], sul phur trioxide or its complex with triethylphosphate [58]; (ii)by lithiationsulfonationoxidation [59]; (iii) by chemically grafting a group containing a sulfonic acid onto a polymer [59]; (iv) by graft copolymeriza tion using high radiation followed by sulfonation of the aromatic components [51]; (v) by synthesis from monomers bearing sulfonic groups [60]. 3.2.2. Characterization of membranes related to highT operation. Operating at “highT ” conditions (120–150°C) supposes a good knowledge of water retention, conductivity thermal and chemical stability. Generally, PFSA membranes and sulfonated aro matic polymers have different microstructures. The water filled channels in sulfonated polyaryls (sul fonated PEEK) are narrow compared to those in hydrated perfluorosulfonic polymers (Nafion®). They are less separated and more branched with more deadend spaces [61]. For PFSA membranes, the water content within the membrane is balanced by the extreme hydrophilicity of the sulfonic ion exchange groups. In the presence of water, only the hydrophilic domain of the nanostructure is hydrated to maintain the proton conductivity, while the hydrophobic domains provide the mechanical strength. The water uptake of PFSA membranes is high but very sensitive to relative humidity. In the case of sulfonated hydro carbon polymers, the hydrocarbon backbones are less hydrophobic and the sulfonic acid exchange groups are less acidic and polar. The water molecules are therefore distributed within all the nanostructure [62]. Sulfonation of polyphenylene sulphide leads to a pro ton conductivity of 0.01 S cm–1 in the range 30– 180°C. 3.3. InorganicOrganic Composite Membranes In addition to PFSA, sulfonated hydrocarbon polymers can be used as a host matrix for preparing inorganic/organic composites for high operating T. The goals of the development of such membranes are (i) improvement of the selfhumidification of the membrane at the anode side by the dispersion of an hydrophilic inorganic component; (ii) reduction of the electroosmotic drag and the dryingout of the membrane at the anode side; (iii) suppression of the methanol crossover in Direct Methanol Fuel Cells; (iv) improvement of the mechanical strength of the
486
− − + + + − + −
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− + + −
− +
−
−
+
−
−
+
+
−
− +
+ − + + − − + + + + −+ + − + − − + − + − + − − + + + − + − − + + − + − − − + − + − + + − + − + − − + − Crosslink between Poly(VDFcoHFP) and + Hydrated silica and terpolymer hidrophobic part of proton
− SO3− groups
Mesoporous silica
the terpolymer Hydrophilic part of the erpolymer
Fig. 7. Schematic illustration of the hybrid SiO2 ⎯ SO3H/terpolymer/poly(VDFcoHFP) copolymer mem brane (the silica domains exhibit a lamellar mesostructure with d = 10 nm, based on SAXS studies) (According to [66]).
membrane without reduction of the proton conduc tivity; (v) improvement of the thermal stability; (vi) enhancement of the proton conductivity when solid inorganic proton conductors are used. 3.4. AcidBase Polymer Membranes Acidbase complexation represents an effective approach to development of protonconducting membranes through three ways: (i) basic polymers can be doped with amphoteric acid acting as a donor and an acceptor in proton transfer and therefore allowing the proton migration: (ii) H3PO4doped PBI polymer in which proton hopping from one N–H site to phos phoric acid anions contributes significantly to the conductivity; (iii) organic acidbase blends developed by Kerres [27, 63]. 3.5. New Routes… 3.5.1. Crosslinked terpolymers via a solgel strat egy. To overcome the issues of polymer mechanical stability and swelling, a new class of polymer electro lyte membrane based on a continuous thermostable, nonconductive, organic polymer matrix mixed with a protonconductive, sulfonated mesostructured silica network has been recently investigated [64, 65]. This approach aims to reproduce the behaviour of PFSA membranes where hydrophobic and hydrophilic
regions coexist. The hydrophilic regions contain the ionic groups and are supported by the functionalized mesostructured silica network while the hydrophobic regions contain the fluorobackbone of the polymer. The volumic fraction of the membrane supporting the proton conduction represents only 40% of the total volume of the hybrid organic/inorganic membrane. Thus in situ solgel growth of an acidfunctionalized inorganic network in a nonporogenic organic matrix has been recently presented [66] (Fig. 7). The improvement in the water uptake is related to the surface hydroxyl groups and/or the –SO3H groups in the hybrid interfacial region, which can strongly attract water molecules through hydrogen bonding. These organic/inorganic membranes exhibit proton conductivity values of 43 mS cm–1 at 65°C under 100% RH and a conductivity value of 12 mS cm–1 at 120°C. 3.5.2. Inert polymer matrix—proton conducting hybrid inorganic particles. New composite membranes have been prepared by inserting polystyrenesulfonic acidgrafted silica particles into an inert polymer matrix of poly(vinylidene fluoridecohexafluoropro pylene), PVDFHFP [67]. A percolation threshold of 30% filler particles within the polymer matrix was obtained, and power density of 1 W cm–2 were recorded at 70°C using nonhydrated gas feeds which indicates that this composite membrane is able to self humidity. Figure 8 shows the route for the preparation of grafted silica nanoparticles. Figure 9 exhibits single cell performances of the hybrid membranes which are higher than that of the Nafion® 112 membrane. 4. NEW CHARACTERIZATION METHODS, IN SITU, OPERANDO? As shown before, the Achilles heel of water medi ated ion conductors is that their ion conduction varies widely with water content and environmental condi tions of the membrane. One of the key challenges in the design of proton exchange membranes is to retain high conductivity at low water content [68]. The most powerful method to quantify the membrane water content in a running fuel cell is the smallangle neu tron scattering (SANS). Up to now, SANS spectra were recorded without any driving force applied to the membrane. As the water management in a PEMFC is the contribution of several dynamic operations (see Fig. 3), a special cell has been designed to record SANS spectra during PEMFC running (Fig. 10) [69]. The data analysis leads to the determination of water concentration profiles across the membrane that could be used to validate mass transfer model and to predict the best operating conditions of the PEMFC (Fig. 11). Another tool for measuring the local concentration gradients of water (or/and methanol) within a proton conducting membrane is the in situ confocalRaman measurement [70]. Figure 11 exhibits these profiles PETROLEUM CHEMISTRY
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MEMBRANES FOR LOW AND MEDIUM TEMPERATURE FUEL CELLS Cl
Si
Cl
Si O
Si OH Toluene, reflux 4h
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Si
487
Si OH
Si O
CPMS
Initiatorgrafted silicaparticles
A390
−
− −
−
−
−
−
−
−
−
CuCl, 2,2'dipyridyl
(2)
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+ SO3Na
H2O/MeOH(3/1) 403C, 20h
−
− −
Initiatorgrafted silicaparticles
−
− −
−
− −
−
− A390gPSSNa Fig. 8. Synthetic route from the preparation of protonconducting silica nanoparticles. (According to [67]).
1.0
Loading 40 wt % Loading 50 wt % Loading 60 wt % Nafion 112
0.8
1.2 Power density, W/cm2
0.9
1.0
0.7 Voltage, V
0.8 0.6
0.6
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0.2 0.1 0
0
0.5
1.0
1.5 2.0 2.5 3.0 3.5 Current density, A/cm2
0.2 0 4.0
4.5
Fig. 9. Single cell PEMFC performances of hybrid membranes as function of filler loading and the Nafion® 112 membrane at 70°C with nonhydrated gas feeds (H2/O2; 2 bars) (From [67]).
obtained from the cell described in Fig. 12. This spe cific microfiuidic cell allows the study of molecules and solvent migration within ion exchange mem branes through the acquisition of Raman spectra dur ing the dynamic transport. The membrane is placed horizontally in the cell, the surface of the sample being perpendicular to the laser beam optical axis direction. PETROLEUM CHEMISTRY
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The last works are carried out in a new fuel cell transparent to the Raman beam (Fig. 13) (IEM Mont pellier with the collaboration of CEALiten (Greno ble)) (unpublished data). The goal of such experiments is to reach the same characterization results with the Raman method
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Fig. 10. Fuel cell transparent to neutrons (From [69]).
I, cm−1 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
0.4 A
0
1.1 A
0.2 A 0.3 A 0.1 A
0.04 A
Fig. 11. Series of SANS spectra obtained from a Nafion® 117 membrane using highly porous gas distribution. The membrane is first dried and then the current is increased step by step from 0 to 1.1 A (From [69]).
Optical axis
outle
outle t (sol. 2) t (sol. 1)
0 µm
z x
Me mbr
y
Fo
cal
ane
pla
inlet (sol. inlet 2) (sol. 1)
ne
Fig. 12. Schematic sketch of the microfiuidic cell designed for in situ Raman spectroscopy (From [70]). PETROLEUM CHEMISTRY
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MEMBRANES FOR LOW AND MEDIUM TEMPERATURE FUEL CELLS Cathodic inlet
Optic pathway for Raman excitation and detection
Anodic inlet
O2
H2
489
Gas inlet and electric contacts
lase r
GDL Electrode Anodic outlet H2 + H2O
Cathodic outlet O2/Air + H2O
Gas distribution channel
Membrane Current collector
Fig. 13. Fuel Cell transparent to the Raman beam. General view and details.
instead of the accurate but heavy SANS. The first results are promising but needs further experiments. 5. CONCLUSION A literature review on high T PEMFCs reveals that highT membrane development accounts for ~90% of the papers. A review of the patents literature reveals a large number on membrane technologies and their associated fabrication methods. Despite significant efforts into membranes for highT/low humidity operation, the status of this field is far from satisfac tory. Membranes generally dehydrate under these conditions, decreasing the performance of the fuel cell. A membrane which transports protons using media not including water is highly desirable. The used of Ncontaining heterocycles, but also other issues must be addressed. The membrane still remains the key of the PEMFC development. ACKNOWLEDGMENTS The author is gratefull to all the partners of the CNRS 3339 Research Grouping “Fuel Cells & Sys tems”, Topic “LowT Fuel Cells”. REFERENCES 1. W. Vielstich, A. Lamm, and H. Gateiger, in Handbook of Fuel Cells: Fundamentals, Technology, Applications, Eds. by W. Vielstich, A. Lamm, and H. Gateiger, (John Wiley, 2004). 2. S. Reichman, A. Ulus, and E. Peled, “PTFEBased Solid Polymer Electrolyte for High Temperature Fuel Cell Applications,” J. Electrochemical Soc. 154, 327– 333 (2007). 3. J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D.P. Wilkin son, Z.S. Liu, and S. Holdcroft, “High Temperature PEMFC,” J. Power Sources (London, 2006), p. 160. PETROLEUM CHEMISTRY
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