On solid polymer fuel cells

On solid polymer fuel cells

237 J. Electroanal. Chem., 357 (1993) 237-250 Elsevier Sequoia S.A., Lausanne JEC02591 Review On solid polymer fuel cells * Hari P. Dhar BCS Techno...

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237

J. Electroanal. Chem., 357 (1993) 237-250 Elsevier Sequoia S.A., Lausanne

JEC02591 Review

On solid polymer fuel cells * Hari P. Dhar BCS Technology Inc., 111 East 27th Street, Suite 212 Bryan, TX 77803 (USA) (Received 8 October 1992; in revised form 13 October 1992)

Abstract

The solid polymer fuel cell is one of the most convenient sources of clean energy. All-solid components, relatively mild operational conditions and high power density make the fuel cell attractive for many applications as a portable power source. The widespread uses of solid polymer and other types of fuel cells are expected to lead to the beginning of the "hydrogen economy" in contrast with the present-day largely fossil-fuel-based economy. This paper reviews the state of the development of solid polymer single fuel cells and multicell stacks. The development work has led to the decrease of platinum loadings of fuel cell electrodes from 5 to 0.5 mg cm -2, and, in recent work on ultralow platinum loading electrodes, to 0.05 mg cm -2. The effects of kinetic parameters, temperature and pressure on fuel cell performance, and also possible future research areas that need emphasis, have been discussed.

INTRODUCTION

The author is pleased to make this contribution to the special edition of this journal on the occasion of Professor Bockris's 70th birthday, and to pay tribute to his efforts in making hydrogen a prime energy source. The use of hydrogen as a fuel can be considered as the ultimate in the generation of clean energy. The source of hydrogen in the form of water is unlimited. Bockris [1-5] has advocated the use of hydrogen as a fuel in the development of world technology. An economy based on the use of h y d r o g e n - - t h e hydrogen e c o n o m y - - r a t h e r than the current fossil-fuel-based or oil-based economy would enable environmental pollution of the Earth to be avoided. Bockris and Reddy [6] have pointed out that in the closing

* Dedicated to Professor John Bockris on the occasion of his 70th birthday. 0022-0728/93/$06.00 © 1993 - Elsevier Sequoia S.A. All rights reserved

238 years of the nineteenth century energy generation took the wrong path in the direction of the inefficient thermal conversion of fuel to usable energy. The first fuel cell was described by Grove in 1842. Ostwald called for the replacement of heat engines by electrochemical energy converters in 1894. The Earth Conference of 1992 in Rio de Janeiro was an extraordinary meeting of more than 100 world leaders and 30 000 other participants to deal with the environmental pollution that has increased rapidly in most cities. Even more alarming are the possible atmospheric changes that loom ahead because of ozone depletion and build-up of greenhouse gases. Many plants and animal species that shared the planet with man have disappeared in the last several decades. The fuel cell is bound to be one of the alternative power sources of the future. It provides clean non-polluting energy, producing water and heat as the byproducts. It is expected that the commercialization of fuel cells, and their use in homes, offices, hospitals, shopping complexes, automobiles and space missions will usher in the beginning of the hydrogen economy envisioned by Professor Bockris. In this paper we shall deal specifically with the development of solid polymer fuel cells. As the name implies, these fuel cells use a solid polymer as the electrolyte. The solid polymer needs to be ion conducting, specifically proton conducting, for the cell reaction to occur. The protons produced at the hydrogen electrode (the anode) migrate through the electrolyte to the oxygen electrode (the cathode). The protons then combine with oxygen along with electrons transported through an external load from the anode to the cathode to produce water. For proton migration to occur through the polymer electrolyte, the latter is prepared in the form of a proton exchange membrane. The solid polymer fuel cell (SPFC) is thus also known as SPE ® or the PEM fuel cell. The aim of this paper is to summarize advances made in the SPFC technology in the last decade. Although considerable progress has been made in fuel cell technology over the last few decades, fuel cells are considered an emerging technology and are poised to make significant market access to meet demands as portable power sources. A number of articles of a general nature in fuel cell areas have been published [7-11]. The development of SPFCs up to the middle of the 1960s has been reviewed by Niedrach and Grubb [12], Magnet [13], and Liebhavsky and Cairns [14]. Appleby and Yeager [15] and Appleby and Foulkes [16] have reviewed SPFC technology with respect to its early development, membrane structure and functioning. The early fuel cell development work, beginning in 1955, was conducted at the General Electric laboratory. Later, in 1984, the General Electric technology was acquired by United Technologies Corporation as Hamilton Standard Electrochem. Inc. The SPFC was chosen by NASA as the auxiliary power source for Gemini space flights in the 1960s. Later, the alkaline fuel cell was chosen as the power source in the Apollo space program. The reason for this decision was based on the understanding that at that time SPFCs were comparatively resistive, and that the NASA power density requirement would be better served by the alkaline fuel cell. The choice of the alkaline fuel cell by NASA delayed further development of the SPFC

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until the mid 1980s, when Ballard Power Systems Inc. [17,18] in Canada received assistance from the Canadian Department of National Defense to develop the SPFC for the growing military power needs of the Canadian Government. The SPFC is very attractive from the point of view that all components in the fuel cell are solid and that its operational conditions are relatively mild: the temperature range is 50-90°C and the pressure is 3-6 atm (304-608 kPa). Thus the SPFC system would be an excellent candidate for commercial viability. RESEARCH AND DEVELOPMENT W O R K ON SINGLE CELLS

Optimization of fuel cell operation Systematic investigations of the SPFC system began at the Los Alamos National Laboratory (LANL) [19-23], with funding from the US Department of Energy. The program started with an attempt to decrease loading of electrodes from the initial 5 mg cm-2-0.5 mg cm -2, and to optimize conditions for making the membrane-electrode assembly. Fuel ceils were built with electrodes of area 5 cm 2. Following the early work at LANL, the researchers at the Center for Electrochemical Systems and Hydrogen Research, Texas A & M University, carried out optimization work with a view to attaining high power density in solid polymer fuel cells [24,25]. One of the serious challenges in SPFC technology is to bring the platinum catalyst dispersed on the carbon particles in the gas diffusion electrode into contact with the solid polymer electrolyte. Raistrick [19] has shown that the extent of the three-dimensional reaction zone of the catalyst in the solid electrolyte can be increased, and the performance of an SPFC can be improved significantly. In this approach, prior to making the membrane-electrode assembly, the electrodes are brushed with a dilute solution of the electrolyte. The amount of electrolyte deposited is about 0.3-0.6 mg cm -2 in the dry state. Despite the improvement of the three-dimensional reaction zone, the efficiency of catalyst utilization remained very 1ow--10%-20% [21] compared with almost 100% efficiency with a liquid electrolyte such as phosphoric acid [26]. The process of making a membrane-electrode assembly consists of the following steps. The active sides of the electrodes are impregnated with a 5% Nation* solution. The solvent is allowed to evaporate in an oven at 70°C. The optimum content of Nation in the dry state is about 0.6 mg cm-2. The membrane is cleaned in a hot (80-90°C) 5 wt.% solution of hydrogen peroxide for about 1 h. This process oxidizes any absorbed organic impurities. The membrane is then washed several times with hot water to remove hydrogen peroxide. This washing is followed by cleaning the membrane in 0.5 M H2SO 4 to remove any metallic impurities. The final washing is also done with hot water to remove absorbed sulfuric acid. The next step is to combine the two electrodes with the membrane electrolyte. Two impregnated electrodes, one on each side of the membrane, are placed

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between two stainless steel plates and inserted between two platens of a hot press at 100°C. This temperature allows time for drying the assembly slowly. The temperature is then raised to 125-135°C, the glass transition temperature of Nation. The membrane-electrode assembly is hot pressed at 1000 psig (690 N cm -2) for about 1 min. The assembly is taken out of the press and put in a cell test fixture. A schematic diagram of the cell assembly is given in Fig. 2 of ref. 24. Management of heat and water are critical problem areas in SPFC technology. The fuel cell becomes non-operative if the membrane electrolyte becomes dry and proton transfers between electrodes cannot occur. In contrast, excessive water in the fuel cell can lead to electrode flooding. Thus an optimum amount of water is needed for fuel cell operation. A number of methods and cell designs for maintaining heat and water balance in fuel cell stacks have been described in the patent literature [27-35]. Srinivasan and coworkers [20,22] have described some methods for maintaining water balance in small laboratory-scale fuel cells. In one approach, the reactant gases, prior to entering the cell, are humidified in stainless steel bottles containing water at temperatures 5-15°C above the cell temperature. The higher temperature of humidification ensures that the reactant gases carry enough water in the fuel cell. The optimum conditions for humidification were found to be a temperature 5°C higher than the cell temperature for oxygen or air, and a temperature 10-15°C higher than the cell temperature for hydrogen. The lower humidification temperature of oxygen reflects the fact that the oxygen side of the cell produces water; therefore it requires less humidification. In another approach, each humidification chamber contains a composite of Nation membrane and porous titanium sheet that separates the water and gas flow compartments. The water that permeates through the membrane humidifies the reactant gases. The fuel cell performance is characterized by potential-current plots, which, in turn, can be analyzed by the following relationship [24]: dV//dj

= -b//j

-

Roh m

(1)

where b and Rohm are the Tafel slope and the electrolyte resistance respectively. At low current densities, the first term in eqn. (1) predominates and is reflected in the high slope of the V-j plot. At higher current densities, the second term becomes important and is responsible for the linear region of the V-j plot. The value of Ronm in the linear region of the V-j plot is indicative of the level of performance of the fuel cell. The minimum value of Roh m in the solid polymer fuel cell measurement is reported to be about 0.1 I~ cm -2 [24]. At a still higher current density beyond the linear region, the mass transport of the reactant to the fuel cell becomes important and a rapid fall of potential with increasing current occurs.

Fuel cell electrodes The electrodes are typical gas diffusion electrodes. The backing is a porous carbon cloth with a hydrophobic coating. The front of the electrode contains a

241 catalyzed layer of Pt-on-C with a balanced proportion of a hydrophobic agent. Ticianelli and coworkers [20-23] used several methods to incorporate changes in the electrode structure and to have electrodes with platinum layers at the front surface. (i) Electrodes were prepared with 20 and 40 wt.% Pt-on-C, but the same platinum loading Was maintained as in the conventional electrodes with 10 wt.% Pt-on-C. The thickness of the active layer was thus decreased in electrodes with a higher Pt : C ratio. (ii) A thin layer (0.05 mg cm -2) of platinum was sputter-deposited onto standard electrodes containing 10 wt.% Pt-on-C (see also ref. 36). The additional platinum deposit corresponded to a thickness of about 10 nm on a smooth surface. (iii) Other methods involved chemical and electrochemical deposition of Pt at the front surface of an electrode. A current density of 1 A cm -2 was achieved at a cell potential of 0.6 V and a temperature of 80°C, and at pressures of 3 atm (304 kPa) and 5 atm (506 kPa) for hydrogen and oxygen respectively. The electrolyte was Nation 117 of thickness 175 ~m. With air as the reactant at the cathode, the cell performance corresponding to the above conditions was 0.54 V. The electrodes had platinum loadings of 0.45 mg cm -2. Ticianelli and coworkers [20-23] also determined the electrochemically active surface areas in fuel cell electrodes by the cyclic voltammetric technique (see also ref. 37) and found that only about 10%-20% of the platinum was electrochemically active in the fuel cell reaction.

Ultralow platinum loading electrodes Attempts to make very low platinum loading electrodes continue. Wilson and Gottesfeld [38] developed a novel method of making the membrane-electrode assembly by depositing the catalyst layer of Pt-on-C mixture directly onto the membrane electrolyte. A suspension of Pt-on-C catalyst is made in a solution of Nation that is in the Na ÷ form. Both sides of the membrane are catalyzed with the suspension. In the next step, this membrane is transformed to the H + form. An uncatalyzed gas diffusion electrode is then placed on each side of the catalyzed membrane. The next step is to put the assembly in the fuel cell hardware. Electrodes with platinum loadings of 0.1-0.2 mg cm -2 were prepared. The power output was about 0.9 W cm -2 at 0.6 V, 80°C and pressures of 3 atm (304 kPa) for hydrogen and 5 atm (506 kPa) for oxygen. The electrolyte was a Nation 117 membrane. The corresponding power output with the Dow membrane was about 1.2 W cm -2. The latest work by these authors [39] has shown that a significant performance deterioration occurred in fuel cells prepared with their ultralow loading electrodes. At 0.5 V, the current density fell from 900 mA cm-2 to 600 mA cm-2 during a 2000 h life-test. Taylor and coworkers [40,41] have reported the preparation of gas diffusion electrodes with platinum loadings of 0.05 mg cm -2. The preparation method

242 consisted in depositing a solution of a solid polymer electrolyte on an uncatalyzed electrode and then depositing platinum electrochemically on the electrode. Halfcell studies of oxygen reduction showed that these ultralow loading electrodes performed as well as the conventional electrodes with a loading of 0.5 mg cm -2. The success of these electrodes was explained in terms of the deposited platinum's being in ionic contact with the solid polymer electrolyte and also in electronic contact with the carbon support of the electrode. Some chemical techniques for platinizing the surface of Nation have been described [42-44]. Porous platinum layers down to 0.4 mg cm -2 could be deposited within a distance of about 0.5 I~m from the surface [43,44]. However, the usefulness of this type of electrode in fuel cell measurements has yet to be tested.

Different membrane electrolytes At the time of writing (September 1992) the only membrane that is commercially available in the USA is the Nation membrane of thickness 175 ~m marketed by E.I. DuPont. Some experimental membranes from a number of companies have been made available to researchers in SPFC technology. In the USA, the Dow Chemical Company has produced a 100 t~m thick membrane that has a higher water retention capability and specific conductivity than those of Nation 117. These desirable properties of the Dow membrane are due to the fact that it contains more sulfonic acid groups per molecule. The Dow membrane can be represented by the molecular unit CF 2~ CFOCF2CF 2SO 3H and the Nation membrane by CF 2 ~ C F O C F 2CFOCF 2CF2SO3H /

CF 3 Because of the enhanced conductivity and smaller thickness of the Dow membrane, it exhibits a superior performance in SPFCs. Srinivasan et al. [24] have shown that a power output of 1.4 W cm-2 was achieved at 0.7 V under experimental conditions of 95°C and 5 atm (506 kPa) pressure. Hydrogen and oxygen were the cell reactants. Murphy et al. [45] have shown that a power output of 2.5 W cm -2 was achieved at 0.45 V, 95°C and 12 atm (1216 kPa) pressure. Another experimental membrane has been made available by the Asahi Chemical Company of Japan. This membrane has similar properties to the Dow membrane. Recently DuPont has also produced a lower-equivalent version of Nation that has characteristics of other low equivalent weight solid polymer membranes.

Kinetic parameters To consider the effects of the kinetic parameters T and P on fuel cell performance, it is necessary to consider the cell reactions involved: reduction of

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oxygen at the cathode and oxidation of hydrogen at the anode. The fuel cell terminal voltage is given by V~= V~- ( - Va)

(2)

where Vc and V~ are the half-cell potentials of the cathode and anode respectively. Expressing the half-cell potentials in terms of the electrode reversible potentials and overpotentials, and assuming that the mass transfer overpotential is negligible, eqn. (2) can be written in the following form: Vt = Er(c+a) + 'r/c+a

(3)

Therefore the temperature and pressure effects on eqn. (3) have to be considered.

Temperature effect To consider the temperature effect, eqn. (3) can be written as Vt = E r + (b log j o - b log j) - RohmJ

(4)

where E r is the cell reversible potential and RohmJ is the voltage due to ohmic resistance. It is assumed that the hydrogen oxidation has negligible overpotential compared with that of oxygen reduction and that the mass transfer limitations are negligible in the useful range of polarization of the fuel cell. Srinivasan et al. [20] and Ticianelli et al. [22] evaluated the values of (E r + b log J0), b and Rohm by nonlinear least-squares fits to experimental points at different temperatures. The conclusions are that higher temperatures result in lower values of b and Rohm, higher values of i 0 and a better performance of the fuel cell.

Pressure effect It may be necessary to operate a fuel cell at a high pressure of both reactants. The pressure may be different for each reactant. The oxygen may be replaced by air, and hydrogen may be replaced by re-formed gases. Under the latter conditions, cell operation under pressure becomes mandatory. The re-formed hydrogen may contain a diluent such as CO2, and a catalyst poisoning agent such as CO. The present author, together with other workers [26,46], has previously carried out elaborate studies of the voltage loss due to the presence of a diluent in the reactant, and the poisoning loss due to the presence of CO in hydrogen as related to a phosphoric acid fuel cell. Similar diluent and poisoning losses can be expected from an SPFC. Since the CO poisoning loss increases dramatically with decreasing temperature, a small amount of CO in the anode gas may be intolerable. Gottesfeld and coworkers [47,48] have shown a way of minimizing CO poisoning in the SPFC. Srinivasan et al. [20] considered a Tafel type variation of fuel cell potential with pressure. Recently Parthasarathy et al. [49] have carried out similar treatments of pressure effect data on oxygen reduction potentials at the Pt-Nafion interface. No justification for the use of a Tafel type relationship of potential variation with

244

pressure was given. Therefore it is necessary to examine the theoretical basis of potential variation under pressurized conditions. From eqn. (3) it is apparent that the pressure effect on cell potential will be dependent on the reversible potentials and activation overpotentials of both electrodes. Considering a four-electron transfer process for oxygen reduction O z + 4H++ 4e-

~ 2H20

the gain in the reversible potential due to a change in the 0 2 pressure is obtained from the Nernst equation, and is given by AEr, c =

( RT/4F)

ln(P~2/P6z )

(5)

Similarly, for hydrogen oxidation H2

~ 2H++ 2e-

the gain in the reversible potential is given by AEr, a = (RT/ZF)

ln(P~2/P~2 )

(6)

Therefore the total voltage gain from the changes in the reversible potentials will be AE = AEr, c + AEr, a

(7)

For the special case when the pressure of both reactants is the same,

A E = ( 3 R T / 4 F ) ln( P " / e ' )

(8)

The above treatment is applicable at the reversible potential for each half-cell reaction. The oxygen reduction process may not be a straightforward four-electron reduction. Oxygen can undergo a two-electron reduction process, or both two- and four-electron processes. In these cases, the above equations should be modified accordingly. Also, the reversible potential is not measurable in practice and is usually substituted by the open-circuit voltage of the cell. The open-circuit voltage is a mixed potential that includes such effects'as corrosion, catalyst poisoning etc. Therefore eqn. (7) in practice leads only approximately to the open-circuit voltage gain due to pressure changes. The effects on overpotentials now have to be considered. The activation overpotential is given by the relation

~7 = - (RT/c~F) In(j/j0)

(9)

The pressure effect on -q arises because of the pressure dependence of the exchange current density J0- An expression for J0 for oxygen reduction can be obtained from the rate equation for oxygen reduction [50,51]: j = kPo2[H + ]3/2 exp( - a V F / R T )

(10)

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Substituting the value of V from eqn. (3), we obtain

j = kPo2[H+]3/2 e x p [ - a ( E r + r l ) F / R T ]

(11)

Therefore

Jo = kPo2[H+ ]3/2 exp( - a E r F / R T )

(12)

Equation (12) follows from the fact that J0 is the reversible current at ~ = 0. An explicit expression for J0 can be obtained for the four-electron reduction of oxygen by substituting the corresponding Nernst equation in eqn. (12). Further, the Tafel slope for oxygen reduction in the Nation electrolyte is 0.06 V [52]. This corresponds to a value of unity for the transfer coefficient a. Then J0 = k ' p 5 / 4 [ H + ]5/2[H 2O] -1/2

(13)

Substituting eqn. (13) into eqn. (9), and again assuming a = 1, one can obtain an expression for the pressure voltage gain due to changes in overpotential:

Art = ( 5 R T / 4 F ) In( P " / P ' )

(14)

An expression similar to eqn. (14) is also needed for hydrogen oxidation. One needs to know the type of dependence of J0 for hydrogen oxidation on pressure. Such an expression is not readily available. The overpotential for hydrogen oxidation is small: about 4 mV per 100 mA cm -2 of current in hot phosphoric acid [46], and about 2.5 mV per 100 mA cm -2 of current in a solid polymer electrolyte [53]. Thus the low field approximation of the Butler-Volmer equation describes the hydrogen oxidation reaction. Also, with the Tafel-Volmer reaction sequence (Tafeh H 2 + 2M ~ 2MH; Volmer: 2MH ~ 2M + 2 H + + 2 e - ) for hydrogen oxidation, where the Tafel reaction is the rate-controlling step, an equation derived by Vogel et al. [54] showed that J0 was dependent on the hydrogen concentration in the electrolyte and thus indirectly on the hydrogen pressure. The above considerations suggest that any voltage gain due to changes in overpotential with pressure would be small. Therefore the pressure voltage gain due to changes in overpotential for hydrogen oxidation will be omitted. The total voltage gain under polarized condition will be the sum of eqns. (8) and (14). Thus

av= (2RT/F) ln(e"/e')

(15)

An example of a calculation for voltage gain is given below for P" = 4 atm, P ' = 1 atm, T = 300 K, R = 8.31 J and F = 96540 C = 96540 A s, AV= 0.072 V. The above analyses have shown that the voltage gain resulting from pressure changes should be the same at all currents. However, it has been found to increase with increasing cell current [21,55]. An explanation for such a disagreement is not known at this time. Rho et al. [56] have investigated mass transport effects in solid polymer fuel cells in the presence of diluents (He and N 2) in oxygen, and have found that the mass transport overpotential for oxygen reduction was less with an 0 2 + He

246 mixture than that with 0 2 + N 2. The diluent species block the passage of oxidant to the electrode-electrolyte interface. Since helium is a smaller molecule it offers less hindrance to oxygen, and thus accounts for less mass transport overpotential.

Dependence of fuel cell performance on electrode structure Ticianelli et al. [23] attempted to correlate the performance of SPFCs with the morphological characteristics of the electrodes. These workers used transmission electron microscopy, scanning electron microscopy and Rutherford back scattering spectroscopy to observe platinum particles on an electrode and the bonding of the membrane to the electrode, and to determine the platinum loading of the electrode. They concluded that the use of electrodes with thin catalyst layers, electrodes made with a high Pt : C weight ratio and electrodes with platinum localized near the front surface increased the fuel cell performance.

Solid polymer fuel cell stacks A few companies have published information on building SPFC stacks and their performances, although fuel cell work is in progress in many laboratories in the USA, Canada, Japan and other countries around the world. Ballard Power Systems Inc. [17,18] in Canada has advanced the state of SPFC technology and has demonstrated substantial increases in cell power density. Ballard has also demonstrated fuel cell operation with air as the oxidant. At 0.625 V, a current density of 1 A cm -2 was achieved with H E + air reactants under a pressure of 3 atm (304 kPa) for each reactant. The electrolyte was the Dow membrane. In addition, Ballard has also demonstrated a methanol + air SPFC system. It consists of a methanol reformer, an oxidizer to remove CO from the re-formed gas stream, an air compressor and control systems for the re-former and the fuel cell system. The company is also involved in a program to demonstrate the operation of a transit bus using a 105 kW SPFC system. In the USA, a number of companies are developing SPFC stacks. The Dow Chemical Company and DuPont are developing membranes as well as SPFCs. General Motors Corporation, in collaboration with Ballard Power Systems, Dow Chemical and LANL, is involved in a program of developing SPFC stacks for electric vehicle applications which is sponsored by the US Department of Energy [57]. Initially, a 10 kW stack will be assembled and tested for its performance in the General Motors program.

Regenerative solid polymer fuel cell The SPFC would be useful in many Earth and space applications. However, the fuel cell would serve better if it were regenerative, i.e. it could perform both as a fuel cell and an electrolyzer. This versatility is desirable because, conveniently, a solar array could provide power to electrolyze water during the sunlit portion of

247 the orbit of a space vehicle or a space station and the same unit could then function as a fuel cell to generate power from hydrogen and oxygen during the dark portion of the orbit. In the design of a regenerative fuel cell, two approaches can be adopted. (i) The fuel cell and electrolysis units are separate. This approach was used in a design study by Chang et al. [58]. (ii) Only one unit functions as a fuel cell and an electrolyzer. Such a fuel cell is known as a unitized regenerative fuel cell. In the present state of the technology, an SPFC can only provide power and cannot function as an electrolyzer. In fact, if by chance the unit is polarized to split water, the fuel cell fails. The present author has investigated a unitized concept for a regenerative SPFC [59]. The cell design closely resembled the design of a single-unit SPFC. Pt-on-C and Pt + Ir-on-C gas diffusion electrodes were used on the oxygen side, and Pt-on-C electrodes were used on the hydrogen side. Electrolytes were Nation and Dow membranes. Fuel cells were built using the electrodes and membranes discussed above. The cells were run to obtain fuel cell and electrolysis data. Data for a maximum of five regenerative cycles were obtained. The current-potential data in the regenerative electrolysis cycles were characterized by a gradual decay with time. However, the fuel cell data were very stable. The decline of performance with cycling is believed to be related to a combination of factors: entrapped gases at the membrane-electrode interface, wetting of the electrode backing and some loosening of the electrode from the membrane. However, visual observations of the membraneelectrode assemblies showed that they were in very good condition, and no electrode corrosion was found. FUTURE RESEARCHAREAS The SPFC has milder operational conditions compared with other types of fuel cells; the technology is already sufficiently advanced to go into the commercial phase. However, for widespread commercial applications, the fuel cell needs to be further simplified in the following areas: high power density, even milder operational conditions with excellent stability, less electrolyte and ohmic voltage loss, less catalyst and high efficiency of catalyst utilization. The management of temperature and water in SPFCs are other areas that also require further research. Platinum alloy catalysts containing chromium, cobalt and nickel show promise of attaining higher power density in SPFCs. Mukerjee et al. [60] have shown that these catalysts perform better than platinum alone as cathodes in the SPFCs. The long-term performance characteristics of Pt + Cr alloy and platinum catalysts were observed to be similar. However, a general problem with the alloy catalysts is the possible dissolution/migration of the more oxidizable component across the membrane to the anode side, and the subsequent poisoning of the anode. The Pt + Cr alloy showed the greatest performance gain of 25 mV at 100 mA cm-2.

248 The Pt + Co and Pt + Ni alloys exhibited lower increments of 15 m V compared with a platinum electrode with the same catalyst loading of 0.4 mg c m - 2 . At the present time under most operational conditions, particularly those of extreme t e m p e r a t u r e and pressure (90°C, 1013 kPa), the electrode backing suffers from some degree of wetting that results in mass-transfer-limiting conditions in the reactants. The result is the deterioration of fuel cell performance. An obvious reason for this is the rather extreme conditions of humidification of the m e m b r a n e electrolyte. If the electrolyte is humidified by moisture carried by the reactant gases, water comes into direct contact with the electrode backing. Alternative processes for m e m b r a n e humidification would be desirable. The less stringent the fuel cell operating conditions, the more appealing and reliable will the fuel cell be as a power source. Thus efforts should be made to simplify cell design and to make operating conditions less stringent than they are at present. The amount of solid electrolyte used in the fuel cell is related to the cell's ohmic voltage loss. Less electrolyte would mean less ohmic loss and better fuel cell performance. Thus efforts should also be made to find the optimum amount of electrolyte for the fuel cell. The efficiency of platinum utilization in the present state-of-the-art SPFC is exceedingly low: 1 0 % - 2 0 % of the amount of platinum in the electrode. The potential gain in increasing this efficiency is enormous. The fuel cell could then be built with a high power density, high efficiency and a smaller amount of platinum catalyst. ACKNOWLEDGEMENTS Financial support during the preparation of this p a p e r was provided by the N A S A funded Center for Space Power, Texas A & M University, College Station, T X 77843. REFERENCES 1 J.O'M. Bockris in O.J. Murphy, S. Srinivasan and B.E. Conway (Eds.), Electrochemistry in Transition--From 20th to 21st Century,,Plenum, New York, 1992, pp. 304-307. 2 J.O'M. Bockris, Int. J. Hydrogen Energy, 13 (1989) 489-521. 3 J.O'M. Bockris, Environment, 13 (1971) 51. 4 J.O'M. Bockris, The Solar-Hydrogen Alternative, Halstead Press, New York, 1975. 5 J.O'M. Bockris, Energy Options, Halstead Press, New York, 1980. 6 J.O'M. Bockris and A.K.N. Reddy, Modern Electrochemistry, Vol. 2, Plenum, New York, 1973, pp. 1379-1382. 7 O. Lindstrom, Chemtech, 18 (1988) 409-497. 8 M. Warshay and P.R. Prokopius, J. Power Sources, 29 (1990) 193-200. 9 K. Trimble and R. Woods, J. Power Sources, 29 (1990) 37-45. 10 N. Itoh, IEEE Spectrum, 27 (1990) 40-43. 11 S. Srinivasan, J. Electrochemical Soc., 136 (1989) 41C-48C. 12 L.W. Niedrach and W.T. Grubb in W.T. Mitchell Jr. (Ed.), Fuel Cells, Academic Press, New York, 1963, p. 253.

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