Selective Cathode Catalysts for Mixed-reactant ... - Wiley Online Library

DOI: 10.1002/fuce.200600007

ORIGINAL RESEARCH PAPER

Selective Cathode Catalysts for Mixedreactant Alkaline Alcohol Fuel Cells H. Meng1,3, M. Wu1,2, X. X. Hu3, M. Nie1,2, Z. D. Wei2, and P. K. Shen1* 1

2 3

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China College of Chemical Engineering, Chongqing University, Chongqing, 400044, China College of Mechanical Engineering, Zhejiang University of Technology, Hangzhou, 310014, China

Received March 31, 2006; accepted September 21, 2006

Abstract A highly selective Ag-W2C/C electrocatalyst for oxygen reduction is developed for potential application in mixedreactant alcohol fuel cells. The catalysts are prepared by an intermittent microwave heating (IMH) method. Both the W2C/C and Ag-W2C/C show catalytic activity for oxygen reduction in alkaline media. The introduction of W2C into the Ag/C results in an enhanced activity that is evidenced by a positive movement in the onset potential and an

1 Introduction

example, one of the differences between the acidic direct methanol fuel cell and the alkaline direct methanol fuel cell is that under acidic conditions water is produced through oxygen reduction at the cathode; while under alkaline conditions OH– ions diffuse through the membrane and form water on the anode. The reverse electro osmotic drag reduces methanol crossover from the fuel side to the air side. Load H2O, CO2

O2 + H2O

OHCathode

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Keywords: Anion Exchange Membrane Fuel Cell, Direct Alcohol Fuel Cell, Mixed-Reactant, Oxygen Reduction, Selective Catalyst

Anode

The liquid alkaline fuel cell (AFC) was the first fuel cell technology put into practical service and that made the generation of electricity from hydrogen feasible [1, 2]. AFCs have an intrinsic advantage over proton exchange membrane fuel cells (PEMFCs) in terms of both cathode kinetics and ohmic polarization. The inherently faster kinetics of the oxygen reduction reaction (ORR) in an alkaline fuel cell allows the use of non-noble metal electrocatalysts, which contributes directly to lower short-term costs but also has environmental benefits. The main contribution to cell resistance is the ionic resistivity of the electrolyte. AFCs have a lower electrolyte resistivity due to the use of liquid electrolyte. The simplicity of the electrolyte used in AFCs provides a distinct advantage compared to PEMFCs. The major operating constraint for liquid AFCs is carbonation and the formation of a precipitate is also significant [3]. When using alcohol as the fuel, alcohol crossover from the anode to the cathode poisons the catalyst. The use of a membrane or circulating the electrolyte can minimize this problem [4] but the use of a catalyst that cannot oxidize the alcohol would be the best solution. Recently, a new concept fuel cell, using an anion exchange membrane and selective catalysts, has been proposed [5–7]. A schematic diagram of an anion exchange membrane fuel cell (AEMFC) is shown in Figure 1. Using methanol as an

increased current density at the same conditions. The novelty of the Ag-W2C/C catalyst is the high selectivity for oxygen reduction in the presence of alcohol.

AEM H2 or Alcohol

Fig. 1 Schematic of an anion exchange membrane fuel cell.

–

[*] Corresponding author, [email protected]

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

447

Anode: CH3OH + 6OH–

→

Cathode: 3/2O2 + 3H2O + 6e– Overall: CH3OH + 3/2O2

5H2O + CO2 + 6e– →

6OH–

→ 2H2O + CO2

E° = –0.81 V

E° = 0.40 V E° = 1.21 V

(1) (2) (3)

where E° represents the standard electrode potential of an electrochemical reaction. The open circuit potential for a single direct methanol alkaline fuel cell (DMAFC) should be close to this value if polarization is negligible. An anion exchange membrane can be used instead of a conventional liquid electrolyte in an AFC to avoid the problems of leakage, carbonation, precipitation of carbonate salts, and to prevent the gas electrode from flooding and increasing the volumetric energy density. However, it is still possible that CO2 from the alcohol oxidation and air may dissolve in the aqueous solution and/or membrane to form H2CO3. The H2CO3 in aqueous solution further dissociates to HCO3– and CO32–. The equilibrium concentration of CO32–/HCO3– is less than 0.07%. Therefore, the influence is small. Moreover, the absence of cations (K+, Na+) eliminates the formation of precipitate. It is conceivable that, due on the research and development of both anode and cathode selective catalysts and an anion exchange membrane as a solid electrolyte, a mixed-reactant fuel cell could be assembled. The fuel in conventional fuel cells is completely separated from the oxygen by a bulky bipolar plate. In mixed-reactant fuel cells, highly selective anode and cathode electrocatalysts are used and a mixture of aqueous fuel and oxygen can be fed directly. Fuel cells with selective catalysts would eliminate the need for bipolar flow-field plates, which would cause a huge reduction in the volume and cost of a fuel cell stack. Priestnall and co-workers reviewed the 50-year history of the mixed-reactant literature [8]. Scott and coworkers reported the performance of fuel cells using FeTMPP/C, CoTMPP/C, FeCoTMPP/C, and RuSe/C as the selective cathode catalysts, while keeping PtRu/C as the anode catalyst [9]. Precious metal chalcogenides and chevrel-phases and organic-metal complexes could be the two categories of active electrocatalysts used in acidic media [10]. Beck and co-workers recently reported Bi2Pt2-yIryO7 pyrochlores that were used as methanol tolerant oxygen reduction electrocatalysts [11]. The authors’ previous work on this new concept fuel cell has centered on the development of the selective electrocatalysts [12, 13]. This paper relates to the further investigation of Pt-free selective electrocatalysts.

have been published elsewhere [12, 14]. The tungsten carbide nanocrystals and metal composite electrocatalysts were then prepared by the in situ reduction of the metal salt on a W2C/C matrix. For the preparation of the Ag-W2C/C catalyst, AgNO3 solution (0.773 g AgNO3 in 10 ml H2O and 5 ml isopropanol) was mixed with the as-prepared W2C/C powder (0.943 g) at a W2C:Ag mass ratio of 1:1. The mixture was treated by the IMH method to form Ag-W2C/C composite electrocatalysts. For electrode preparation, typically, 15 mg of electrocatalyst were added to 1ml of isopropanol. The mixture was then treated ultrasonically for 30 min. The mixture was placed on the top surface of a carbon rod at the desired electrocatalyst loading. A drop of a 0.5 wt.-% Nafion® suspension (DuPont, USA) was added to the top surface to prevent damage to the electrocatalyst layer. Electrochemical measurements were carried out on a VoltaLab 80 Universal Electrochemical Laboratory (Radiometer Analytical Company, France). A standard three-electrode cell with separate anode and cathode compartments was used. Platinum foil and Hg/HgO electrodes were used as the counter and reference electrodes, respectively. All the electrochemical measurements were carried out in 1 mol dm–3 KOH solution at 25 °C. The chemicals were of analytical grade and were used as received. All the solutions were freshly prepared with distilled-deionized water.

3 Results and Discussion Figure 2 shows the results for oxygen reduction at different electrodes in alkaline solution. These are the first results showing that tungsten carbide is active for oxygen reduction, Figure 2 [12]. However, the overpotential was higher than that of Ag and Pt. The addition of tungsten carbide to the Ag/C gave a similar performance to Pt/C. Obviously the use of Ag instead of Pt will significantly reduce the catalyst cost and consequently the fuel cell cost. 0 -5 -2

The reactions are as follows:

i / mA cm


Meng et al.: Selective Cathode Catalysts for Mixed-reactant Alkaline Alcohol Fuel Cells

-10 80ug W 2 C 80ug Pt 80ug Ag 80ug W 2 C +80ug Ag

-15 -20

2 Experimental Nanocrystalline W2C on carbon (Vulcan XC-72R, Cabot Corp., USA) was prepared by a solid-state reaction using an intermittent microwave heating (IMH) process. The details of the preparation and characterization of the W2C/C material

448


-400 -300 -200 -100

0

100 200 300

E / mV vs. Hg/HgO Fig. 2 Linear potential sweep curves of different catalysts for oxygen reduction in oxygen saturated 1 mol dm–3 KOH solution at 25 °C. Scan rate: 1 mV s–1.

www.fuelcells.wiley-vch.de

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60

Ag on W2C/C catalysts were prepared by an IMH method. Both the W2C/C and Ag-W2C/C catalysts showed catalytic activity for oxygen reduction in alkaline media. The introduction of W2C into the Ag/C resulted in an enhanced activity, which was evidenced by the positive movement in the onset potential and the increased current density under the same conditions. The novelty of the Ag-W2C/C catalyst is the high selectivity for oxygen reduction in the presence of alcohol. Such catalysts could potentially be used for mixed-reactant anion exchange membrane fuel cells [6–8]. The use of highly selective catalysts means that bipolar plate-free compact fuel cells, like the strip-cell, can be fabricated [7, 15]. There are 60

a

Pt/C

40

b

40

Pt/C

i / mA.cm

-2

-2

i / mA.cm

4 Conclusion

20

0

20

0

Ag-W2C/C -20 -1.2

-0.8

-0.4

0.0

E / V vs. Hg/HgO

Ag-W2C/C 0.4

-20 -1.2

-0.8

-0.4

0.0

0.4

E / V vs. Hg/HgO

Fig. 3 Cyclic voltammograms of Ag-W2C/C and Pt/C in 1 mol dm–3 KOH/1 mol dm–3 CH3OH solution. (a) with and (b) without O2, scan rate: 50 mV s–1.

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The onset potential and the magnitude of the current for oxygen reduction are almost the same with or without methanol. Tafel plots are shown in Figure 4a′. They clearly show that the onset potential for oxygen reduction on the Ag-W2C/C catalyst is 0.055 V vs. Hg/HgO. No shift occurs in the presence of methanol. However, the Pt/C catalyst is stable only at very low methanol concentrations. The reaction is gradually dominated by the oxidation of methanol at higher concentrations. Further observations, from the Tafel plots, found that the onset potential on the Pt/C electrode is 0.055 V in the absence of methanol and moved to –0.5 V in the presence of 1 mol dm–3 methanol, where the reaction is methanol oxidation rather than oxygen reduction (Figure 4b′). The competitive adsorption of oxygen and methanol on the surface of Pt results in a mixed potential. The electrode reaction is dominated by methanol oxidation at higher methanol concentrations and is limited by dissolved oxygen, since Pt is active towards both oxygen and methanol. This is the reason why the crossover of alcohol from the anode to the cathode reduces fuel cell performance for traditional DAFCs (both anode and cathode use Pt-based catalysts). The catalysts presented in this work showed high selectivity for oxygen reduction in the presence of alcohol even at significantly high concentrations.

RRDE measurements revealed that the reduction of oxygen on tungsten carbide occurs via a two-step pathway (the formation of a H2O2 intermediate, which is immediately reduced at the ring electrode) [14]. Peroxide was detected on the ring electrode, as soon as oxygen reduction started on the disk electrode. Alternatively, the reduction of oxygen on AgW2C/C is a four-electron process. Current was barely detected on the ring electrode until the potential moved to the hydrogen region. The results indicate that the combination of Ag and tungsten carbide enhances the activity for oxygen reduction in terms of the onset potential and current density. The tungsten carbide acted as a catalyst promoter to enhance the catalytic activity and at the same time hiding its pathway to produce the peroxide. This is significant since the formation of peroxide reduces the electrical efficiency and oxygen utilization. The attraction of these novel catalysts is that they show the characteristic selectivity for oxygen reduction. The effect of the presence of alcohol during the process of oxygen reduction on electrode performance was assessed by considering the effect of alcohol in direct alcohol fuel cells (DAFCs). Figure 3 shows cyclic voltammograms of the Ag-W2C/C and Pt/C electrodes in 1 mol dm–3 KOH/1 mol dm–3 CH3OH solution in the presence and absence of O2. It is clear that the Ag-W2C/C catalyst is inert towards methanol oxidation. In the presence of oxygen, there is an obvious peak during the reverse part of the cycle due to oxygen reduction (Figure 3a). On the other hand, Pt/C is a good catalyst for methanol oxidation. The increase in the current density in the case of the co-existence of methanol and oxygen is probably caused by a reduction in the concentration polarization due to the stirring effect of the bubbling oxygen. The effect of the alcohol on electrode performance was further examined in oxygen-saturated solution, containing different concentrations of methanol, ethanol, isopropanol, and glycerol. Figure 4 presents typical results for oxygen reduction in methanol-containing solution. Oxygen reduction on the Ag-W2C/C catalyst is stable in the methanol-containing solution up to a methanol concentration of 1 mol dm–3.

2

0

a

0

-2

log(-i / mA cm )

-2

i / mA cm

a'

1

-5 0M 0.01M 0.05M 0.1M 0.2M 0.5M 1M

-10 -15

-200 -150 -100 -50

0

-1 0M 0.01M 0.05M 0.1M 0.2M 0.5M 1M

-2 -3 -4

-20

-5

50 100

E / mV vs. Hg/HgO

-300 -200 -100

0

100

200

E / mV vs. Hg/HgO

30

10 0 -10

2

b'

1 -2

-2

0M 0.01M 0.05M 0.1M 0.2M 0.5M 1M

log(-i / mA cm )

b

20

i / mA cm



-20

0 -1 0M 0.2M 0.5M 1M

-2 -3

-200

-100

0

100

200

E / mV vs. Hg/HgO

-600 -450 -300 -150

0

150 300

E / mV vs. Hg/HgO

Fig. 4 Comparison of the polarization curves for the ORR on (a) Ag-W2C/C and (b) Pt/C in O2 saturated 1 mol dm–3 KOH solution containing different concentrations of methanol, as indicated in the figure at 25 °C, scan rate: 1 mV s–1. (a′) and (b′) are the Tafel plots under the same conditions.

consequent remarkable increases in the energy density and reductions in cost. Moreover, the absence of a metal cation hydroxide, by using an anion exchange membrane, is vital for the elimination of carbonate precipitation at the catalyst surface.

Acknowledgments The authors thank the NSF of China (20476108, 20476109), the NSF of Guangdong Province (04105500), the Guangdong Science and Technology Key Project (2005A11001002, 2005A11004001), and the Guangzhou Science and Technology Key Project (200623-C7031) for support.

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