Interfacial aspects in the development of polymer electrolyte fuel cells

...-__ l!!B

‘2&J

SOUD STATE

ELSEVIER

IONKS

Solid State Ionics 94 (1997) 249-257

Interfacial aspects in the development of polymer electrolyte fuel cells Giinther G. Scherer Paul

Scherrer

Institur,

Elektrochemie,

CH-5232

Villigen

PSI,

Switzerland

Abstract A brief overview is given of the fuel cell families attracting interest today. Problems in the optimization of interfaces in fuel cells, in particular in Polymer Electrolyte Fuel Cells (PEFCs) are discussed next. The influence of the parameters of low precious metal loading, fuel purity, oxygen solubility, and water distribution in the solid polymer electrolyte on the performance of cells are briefly described. Finally, some current problems in the development of Direct Methanol Fuel Cells are presented. Keywords:

Polymer

electrolyte

fuel cell; Interface; Gas diffusion electrode; Etectrocatalysis

1. Introduction Recently interest in fuel cells has grown tremendously, primarily due to environmental concerns. More than a hundred Phosphoric Acid Fuel Cells (PAFCs) units of nominal power (200 kW,) have been installed in recent years, resulting in the accumulation of more than lo6 hours of operation and demonstration of PAFC technology on the verge of commercialization [l]. In 1996, several prototype demonstrations of great technical importance took place, among them Daimler-Benz’s NECAR II (New Electric Car) [2], powered by Polymer Electrolyte Fuel Cells (PEFCs, or sometimes Proton Exchange Membrane Fuel Cells, PEMFCs), and M-C Power Corporation’s, USA, installation of a 2 MW Molten Carbonate Fuel Cell unit (MCFC) in California [3]. 0167-2738/97/$17.00 PII

01997

SO167-2738(96)00616-9

Elsevier Science B.V. All rights reserved

Nevertheless, the development of first generation technologies for most fuel cell types requires further intensive materials research to develop improved and less expensive components and engineering research for stack development, including system integration aspects, to improve lifetime and to reduce cost. Therefore, many opportunities for fundamental work in the various areas of fuel cell research will continue over the long term. It was the intention of the organizers of this meeting to bring together scientists from the two communities concerned with the liquid as well as the solid inter-facial electrochemistry so that they could discuss areas of mutual interest. In accordance with this intention, this publication is of a more tutorial nature. After a brief overview of the fuel cell families attracting interest today, this short contribution focuses in very general terms on some problems

250

G.G. Scherer I Solid State Ionics 94 (1997) 249-257

arising in the development of an optimal electrode/ electrolyte interface for fuel cells which utilize a solid polymer electrolyte.

2. The fuel cell families

The important fuel cell families

of actual interest

The idea of a gaseous voltaic battery or fuel cell dates back to Grove, who in 1839 described the first hydrogen/oxygen fuel cell consisting of platinized platinum electrodes immersed in sulphuric acid [4]. Generally speaking, a fuel cell is an electrochemical device that continuously converts the chemical energy of a fuel (and oxidant) directly into electrical energy as shown schematically in Fig. 1. Heat is generated as a byproduct. The fuel cell process has the major advantage of not being Carnot-limited, thus allowing a theoretical efficiency higher than that of a heat engine. Fuels can include for example, H,, N,H,, NH,, CH,OH, coal gas, or hydrocarbons. In the case of pure hydrogen, a fuel cell acts as a local zero emission converter. The oxidant for terrestrial applications is air, and in exceptional cases pure oxygen is utilized. Historically, fuel cells are classified by the nature of the electrolyte and/or by the temperature of operation. Thus, one separates fuel cells into alkaline or acidic, or low temperature (up to lOO”C), medium temperature (up to 2OO”C), or high temperature (up to 1000°C). Currently, interest focuses on the fuel cell families depicted schematically in Fig. 2 [5,6]. In general terms, the nature of the oxidant as well as of the fuel sets restrictions on the operating conditions of the various fuel cell types. PEFCs, PAFCs, and Direct Methanol Fuel Cells (DMFCs or sometimes also DM-PEFCs) are acidic fuel cells that can

Fig. 1. Scheme of energy conversion

in a fuel cell.

Fig. 2. The important

fuel cell families of current interest.

utilize air as oxidant. AFCs must operate on pure oxygen so as to avoid carbonization of the alkaline electrolyte. PEFCs, DMFCs, and AFCs operate at temperatures up to lOO”C, PAFCs normally at 200°C. PAFCs tolerate fuels with CO levels in the range of several hundred ppm, in contrast to PEFCs with platinum-based anodes, which require high purity hydrogen. Operation of PEFCs with CO-containing hydrogen produced from a reforming process, as well as DMFCs fed with gaseous or liquid methanol, require anode catalysts with high CO-tolerance (see below). Generally speaking, the fuel purity problem is closely related to fuel processing, an issue as important as the development of the fuel cell itself. Weight and volume as well as start-up time of the whole fuel processing/fuel cell system are of paramount importance [7], especially in transport applications. Aspects of the high temperature fuel cells MCFC and SOFC are described in the contribution of Steele in this issue [8]. The basic design of a fuel cell, an ionically electrolyte and separator conducting layer sandwiched between two electronically conducting gas diffusion electrodes (the fuel anode and the oxidant cathode), is also shown schematically in Fig. 2. Typically, under open circuit conditions HZ/air fuel cells exhibit a cell voltage of around 1 V and less than 1 V under current flow. Higher voltages, necessary for any application, result from stacking

G.G. Scherer I Solid State Ionics 94 (1997) 249-257

individual cells in a bipolar arrangement. High specific power (2 350 W/kg) and high power density ( 2 350 W/l) are required for transport applications, which necessitates lightweight and compact stack materials and design [9].

3. The importance of the electrode/electrolyte

interface In the early stages, Grove discovered one of the major obstacles in designing an efficient fuel cell: It is necessary to have a high interfacial area of the three-phase boundary between gas, electrolyte (ionic conductor), and electrode (electronic conductor) (Fig. 3). He expressed this prerequisite in his second publication by stating that a fuel cell needs a notable surface of action [lo]. Even today the design and preparation of three-phase boundaries of high interfacial area is one of the major challenges for fuel cell research. With regard to this three-phase boundary, an aqueous electrolyte has an advantage in comparison to a ceramic one, e.g., in SOFCs, in that the reactant gases are soluble in the aqueous medium (Fig. 3). This notable surface of action is achieved by making use of a porous gas diffusion electrode (gde) that fulfils two essential prerequisites: A high electrochemically active surface area and a possible mass flow perpendicular to the electrode/electrolyte plane.

Fig. 3. Scheme of the porous gas diffusion electrode/electrolyte interface illustrating the three-phase txxmdary.

251

As an illustration a cross-section of a commercial gde (typically 400 km thick), originally developed for PAFCs and today widely utilized also in PEFCs, is shown in Fig. 4 [ 111. A carbon cloth or carbon paper serves as support (middle) for the active layer (ca. 50 j.rm, right side) and the wet-proofing gas diffusion layer (left side). The active layer is composed either of platinum black or highly dispersed platinum particles (2 to 5 nm) deposited onto carbon particles (Pt/C electrodes); the precious metal is necessary for corrosion resistance due to contact with the acidic electrolyte. The interphase is tailored by selective impregnation of the three layers with PTFE particles, which act as a hydrophobizing as well as a bonding agent between the carbon particles. The design of an optimal interface is strongly dependent on the pore structure of the active layer of the gas diffusion electrode. According to Fig. 3, the liquid electrolyte has to penetrate into the pores and to wet the pores so that a thin layer of electrolyte covers the pore wall (low contact angle). This electrolyte film should be as thin as possible, so that a short diffusion path for the reactant gases exists. A high solubility of the reactant gases in the electrolyte film is also favourable. Further, the pores of the support and the wet-proof layer have to allow mass flow of reactants (fuel and oxidant) to and of products (liquid water, at temperatures below 100°C) from the wetted pore of the active layer, where the electrochemical reactions take place. This requires a balance of hydrophilic (carbon surface) and hydrophobic pores (PTFE), achieved by selective impregnation of the different layers with PTFE-particles and subsequent processing (heat treatment, rolling, etc.). The optimal interfacial design for anode and cathode may be different, although today in many cases the same electrode type is used for both electrodes. The optimization of interfaces for low temperature fuel cells has always depended on the availability of special materials to control the porosity and wetting behaviour of the respective pores by the electrolyte. For example, one of the breakthroughs in the development of AFCs was the work of Bacon at a time when the chemically stable hydrophobizing agent PTFE was not yet available [12]. He managed interface control with a dual-layer, dual-porosity

252

G.G.

Scherer

I Solid State tonics

94 (1997)

249-257

Fig. 4. Electron micrograph of ELAT-type E-TEK gas diffusion electrode.

electrode, made out of Ni powders of different grain size. The later availability of PTFE opened up new possibilities to improved gde-designs.

4. The polymer

electrolyte fuel cell

The idea of using a proton-conducting ion exchange membrane as Solid Polymer Electrolyte in a fuel cell was first demonstrated by Grubb [13]. Later on SPE became a trade name for the technology (fuel cells, electrolyzers, etc.) developed by General Electric. Due to the harsh environment prevailing in

these cells, the lifetime of PEFCs was limited by the stability of the mainly hydrocarbon-based membranes available at that time [ 141. A real breakthrough was the development of perfluorinated cation exchange membranes (Nafion, DuPont, USA) by Grot, which extended the lifetime to several thousands of hours at operation temperatures below 100°C [ 151. The required properties of solid polymer electrolyte membranes have been described elsewhere [ 161. Generally speaking, the proton-conducting polymer membrane, typically 50 to 200 m thick, combines the separator function of the solid polymer

G.G. Scherer

I Solid Stare lonics

film with the properties of an aqueous-like trolyte, contained within the phase-separated mer nanomorphology [ 171.

5. The electrochemically PEFCs

active interfaces

with

low platinum

249-2.57

elecpoly-

catalyst

in

thin polymer film

As stated above, proton-conducting polymer membranes combine the advantage of a solid polymer as gas barrier and the favourable conductivity of an aqueous-like electrolyte. The formation of an efficient three-dimensional reaction zone in contact with a gde is difficult to realize, however, because the membrane does not penetrate into the electrode pores. An additional problem is that an as low as possible precious metal loading ( < 1 mg/cm’) with highest utilization is required for cost reasons. Both requirements together necessitate a highly efficient dispersion of nm size precious metal particles, supported on carbon (Pt/C), in contact with the solid electrolyte. 5.1. Electrodes

94 (1997)

loading

Modeling an spe interface, Chernyshov suggested first that incorporation of solid polymer electrolyte particles into the active layer, thus extending the electrode-electrolyte interface into the third dimension, should improve the electrochemical polarization behaviour [ 181. This was realized to some extend by pressing the gdes onto the surface of the hydrated membrane at a temperature above its glass transition, thereby bringing about the penetration of electrocatalyst particles into the membrane surface. A solubilized form of the perfluorinated polymer membranes [ 191 offered the possibility to impregnate the active layer of gdes and thereby increase the electrochemically available platinum surface, as shown schematically in Fig. 5 [20,21]. Platinum particles dispersed on carbon, which have not been in contact with the solid electrolyte previously, become electrochemically active due to the thin film of solid electrolyte now covering their surface. Combining both methods results in fuel cell polarizations curves much superior to those obtained with non-treated electrodes. For example, using electrodes 0.4 mg Pt/C leads to an increase in current density

Fig. 5. Scheme of impregnated pore m PEFC

of 10 to more than 200 mA/cm’ at a cell voltage of 600 mV under the same operating conditions (HZ / O,, 1 atm). Cyclic voltammetry in the hydrogen underpotential deposition range [22] provides evidence of the pronounced benefits of Pt/C-electrodes compared to Pt-black electrodes. A tenfold increase in the electrochemically active area is observed for Pt/C electrodes (Table I), whereas the difference in the area of the blank and the impregnated platinum black electrode is only a few percent. This again demonstrates the importance of forming an optimal threephase boundary in the case of low platinum loadings and with carbon as support material. A new approach to preparing electrodes of low platinum loading ( = 0.1 mg/cm.‘) involves casting thin films. typically with a thickness of a few km, from a suspension of electrocatalyst particles in solubilized membrane material onto an inert support and subsequently hot-pressing the dried film directly

Table

I

Effect of Nation impregnation on the electrochemuxl

active area

of Pt/C

based gas

(0.6

mg/cm’)

and Wblack

diffusion electrodes (ELAT-type.

E-TEK.

(4

mg/cm’) USA)

Platinum

Nafion

Catalyst utilization

(mglcm’)

(mglcm’)

I%)

0.6 on C

_

0.6 on C

0.65

4, black

_

54

4. black

0.6

h0

3.4 34

254

G.G. Scherer

/ Solid State Ionics 94 (19973 249-257

onto the membrane surface [23]. A conventional gas diffusion layer can next be attached to the thin active layer.

to pure platinum. Hydroxyl groups (-OH) form on the surface of ruthenium in the respective potential range and they facilitate the oxidation of CO which is blocking platinum surface sites. It has been demonstrated that Pto.,Ruo,s can tolerate up to approximately 100 ppm CO, when the electrode preparation method is optimized [30]. The second strategy is the development of fuel purification systems that reduce the CO-content of the fuel to levels tolerable to platinum electrodes ( < 10 ppm). One of the purification methods currently under investigation uses thin Pd/Ag membranes, which show high permeation rates and selectivity for hydrogen [3 11. The application of any purification method depends also on system integration factors and system costs. It is possible that neither one of these two strategies will be successful by itself and a combination of purification system and CO-tolerant electrocatalyst has to be considered.

5.2. Fuel purity requirements Any application of a PEFC is highly dependent on its fuel supply. Platinum based electrodes require pure hydrogen since the operating temperature of PEFCs is below 100°C. Any other fuel, e.g., hydrogen from methanol steam reforming or partial oxidation, contains up to a few percent of CO, which adsorbs on platinum and blocks active sites for hydrogen oxidation at these temperatures [24-271. CO-concentration as low as 10 ppm can be detrimental to the current-voltage behaviour, as shown in Fig. 6 for a single cell with a Pt-anode [24]. Currently, there exist two strategies to circumvent this problem: Firstly, the utilization of a CO-tolerant anode catalyst. Platinum-ruthenium has been investigated in many laboratories (e.g., 128,291) as an alternative

I

1000,

5.3. Oxygen permeation membrane

According to the pictorial model shown in Fig. 5, the gases have to permeate through the thin impregnated solid electrolyte layer to react at the platinum surface. Permeation of oxygen in waterswollen perfluorinated membranes has been studied at the interface of a platinum micro-electrode in contact with membranes of different equivalent weights (EW) [32,33]. Chronoamperometry facilitated a separation of the oxygen permeability into its solubility and diffusivity components. As is evident from the results shown in Table 2, solubility is favoured by a higher EW and respectively higher content of the perfluorinated backbone phase, while diffusivity is favoured by a higher water content

, Siemens

AG

0 200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

currentdensity[Ak&]

Fig. 6. Influence of CO concentration (ppm) in H, on performance of fuel cell (data Dr. Waidhas, Siemens AG).

Table 2 Solubility,

diffusion

Membrane/ material

coefficient

and permeation

Equivalent weight

of 0,

in different

Water uptake (%)

in the electrolyte

materials

at 50°C and 5 bar 0,

Diff. coeff. (cm*/s) X IO6

Solubility (mol/cm3)

[32]

X 10’

Permeability (M/cm) X IO’*

(g/eq) AcipIex”

1004s

Nafion@ 115 Nation” 120 PTFE 1 M H,SO, a Value measured

at

loo0

43

3.1

1100 1200

37 27

3.0 1.8 0.35 31

1 bar, calculated for 5 bar assumong a linear pressure law.

11 10 21 37 5.0

=

41 30 49 13 155 n

G.G. Scherer

I Solid State lonics 94 (1997) 249-257

(swelling caused by ionic content) of the membrane. These results have to be further explored with respect to impregnation of gdes, particularly for the cathode where reduction of oxygen occurs at rather high overpotential. 5.4. Effects of membrane

humidijcation

Specific proton-conductivity is a linear function of membrane hydration [34]. During fuel cell operation water is transported across the membrane from the anode to the cathode side due to the electro-osmotic drag of H+. This causes a depletion of water at the anode, which has to be balanced by backdiffusion from the cathode due to the built-up concentration gradient. Models predict that for a given membrane thickness the water gradient across the membrane should increase with higher current densities and also increase at constant current density with increasing membrane thickness [35,36]. Both have been verified experimentally by demonstrating the non-linear membrane resistance of a 200 p,rn thick Nafion 117 membrane in the current density range of 0 to 800 mA/cm”. It was found that the resistance increased by 20% over this range [37]. In contrast for two thinner membranes, Nafion 112 (60 km) and Nafion 115 ( 150 km), the resistance over the same current density range remained constant in the first case and increased by 13% for the thicker membrane [38]. These results also have consequences for the interpretation of current-voltage plots due to the fact that the membrane resistance, at least for higher thicknesses, can no longer be assumed to be linear, as has been done in the literature. The effect of the water depletion on the kinetics of the hydrogen oxidation reaction also has to be studied in more detail in the future.

6. PE-DMFC,

strategies

for improvement

A future alternative to a hydrogen fuelled PEFC is the methanol fuelled PEFC or PE-DMFC. Due to the simpler transport, distribution, and storage of CH,OH (liquid fuel at room temperature) as compared to hydrogen, a polymer electrolyte fuel cell directly fed by gaseous or liquid methanol circumvents the problem of processing methanol into pure hydrogen, as discussed above. This concept is par-

255

titularly interesting for transport applications, where the total weight and volume of the whole system consisting of fuel storage, fuel processing and fuel cell components should be as low as possible in order to compete with gasoline-based internal combustion engines. From a thermodynamic point of view, the oxidation of methanol to CO, and water (En = 1210 mV) is comparable to the oxidation of hydrogen to water (E” = 1230 mV). Unfortunately, methanol oxidation on platinum is hindered by the adsorption of C,intermediates which block catalytic sites, so that the reversible potential is never observed. This causes a rather high anode overpotential loss at current densities as low as 100 to 200 mA/cm’, thus yielding much lower power densities as compared to those obtained with the hydrogen fuelled cell [39,40). Currently, three major problems exist in the development of PE-DMFC with improved performance and lower cost: Anode electrocatalyst: As in the case of COcontaminated HZ, Pt,,,Ru,,, has been shown to be a viable electrocatalyst. The mechanism of methanol oxidation on PtRu has been investigated by numerous groups (e.g., [41-461). These investigations found a rather complicated interdependence of optimal methanol feed stoichiometry and PtRu surface composition. Generally speaking, Pt,, 5R~0s has been found to be an optimal composition with regard to the kinetics of the methanol oxidation. Methanol cross-over through perfluorosulphonic acid solid electrolytes: The solubility parameters of water and methanol are similar, hence, solvent uptake of the Nafion membranes for both solvents are similar. In mixtures of both, the swelling is even increased [47], further facilitating the transport of methanol within the polymer matrix (48,491. At low methanol concentration (1 M, lean feed) the concentration gradient provides the essential driving force for the transport of methanol across the membrane [SO]. Transport by electro-osmosis due to current flow is negligible. At higher concentration ( > 2 M) methanol cross-over due the electro-osmotic drag of H+ becomes significant. Alternative membranes with lower methanol cross-over have to be developed in the future so that fuel utilization can be increased. Methanol-tolerant oxygen reduction catalysts: A further consequence of the methanol cross-over is the

256

C.G.

Scherer

I Solid

Stute

mixed potential which develops at the platinum cathode between the oxygen reduction and the methanol oxidation reaction. Therefore, a lower cathode potential and a lower overall cell voltage is observed. High1 y selective reduction oxygen catalysts are required to circumvent the formation of a mixed potential. It has been found recently that transition metal chalcogenides of the type (Ru, _,Mo,),SeO, exhibit high selectivity towards oxygen reduction in an acidic aqueous electrolyte in the presence of small organic molecules like methanol [51 J. This deserves further attention to explore the potential of these novel compounds for their application in DMFCs.

lonics

94 (1997)

249-257

[3] A.J. Skok, A.J. Leo and T.P. O’Shea, Program and Abstracts of the 1996 Fuel Cell Seminar, Nov. 17-20, 1996, Orlando, FL, p. 16. [4] W.R. Grove, Phil. Mag., Ser. 3, 14 (1839) 127. [S] K. Kordesch and G. Simader, Fuel Cells and Their Applications (VCH Verlagsgesellschaft mbH, Weinhein, Germany, 1996). ]6] A.J. Appleby and F.R. Foulkes, Fuel Cell Handbook Nostrand Reinhold, New York, 1989).

(Van

[7] R. Kumar and S. Ahmed, Proc. I st Int. Symp. New Materials for Fuel Cell Systems, July 9-13, 1995, Montreal, Canada, eds. 0. Savadogo et al. (Editions de I’Ecole Polytechnique de Montreal, 1995) p. 224. [8] B.C. Steele, Solid State Ionics 94 (lY97) 239 (this issue). [9] RV. Helmholt, R. Homung and M. Waidhas, 12. PSITagessymposium Elektrochemische Energiespeicherung, 11.9.1996, CH-5232 Villigen PSI, Switzerland. [IO] W.R. Grove, Phil. Mag.. Ser. 3. 21 (1842) 417. [I I] E-TEK, Inc., Natick, MA, USA.

7. Conclusion Today PEFC-technology is being demonstrated in applications in a multi-kW power range. Nevertheless further intensive research for many aspects of this technology is required to improve lifetime and to reduce cost in order to bring about a broad penetration of this ‘old idea for electrochemical energy conversion’ into various markets. As has been shown in this short contribution, a number of highly interesting interfacial problems still do exist which offer many opportunities to carry out fundamental and applied electrochemical research for this technology in the future.

Acknowledgments The author thanks Drs. Hans-Peter Brack and Otto Haas for comments on the manuscript and Dr. Felix Biichi for providing an electron micrograph. The continuous support of fuel cell research carried out at PSI by the Swiss Federal Office of Energy (BEW, Bern) is gratefully acknowledged.

References [I] M.C. Williams, Program and Abstracts of the 1996 Fuel Cell Seminar, Nov. 17-20, 1996, Orlando, FL, p. I. [2] NECAR II - Driving without Emissions (Daimler-Benz AG, Kommunikation (KOM), D-70546 Stuttgart, Germany).

[I21 F.T. Bacon, BEAMA J. 6 (1954) 122. [I31 W.T. Grubb, US Patent 2913511. [14] A.P. Fickett, Proc. Symposium on Battery Separators, Columbus Section of the Electrochemical Society, 1970, p. 354. [ 151 W.R. Grot, Chem. Ing. Techn. 44 ( 1992) 167; 47 ( 1995) 617. [ 16) G.G. Scherer. Ber. Bunsenges. Phys. Chem. 94 (1990) 1008. ] 171 K.D. Kreuer. Th. Dippel, W. Meyer and J. Maier, Mat. Res. Sot. Symp. Proc. 293 (1993) 273. 1181 S.F. Chemyshov, J. Res. Inst. Catal. Hokkaido Univ. 30 (1982) 179. [ 191 W.R. Grot, European Patent 0 066 369. [20] S. Srinivasan, E.A. Ticianelli, C.R. Derouin and A. Redondo, J. Power Sources 22 (1988) 359. [21] I.D. Raistrick, US Patent 4 876 115. [22] G.G. Scherer and M. Staub, to be published. [23] M. Wilson and S. Gottesfeld. J. Appl. Electrochem. 22 (1992) 1. [24] M. Hachkar, T. Nappom, J-M. Leger, B. Beden and C. Lamy, Electrochim. Acta 41 (1996) 2721. [25] H. Grime, G. Luft, K. Mund and M. Waidhas, Program and Abstracts of the 1994 Fuel Cell Seminar, Nov. 28-Dec. I, 1994, San Diego, California, p. 474. [26] V.M. Schmidt, P. Brockerhoff, B. Hohlein, R. Menzer and U. Stimming, J. Power Sources 49 ( 1994) 299. [27] T.R. Ralph, G.A. Hards, D. Thompsett and J.M. Gascoyne, Program and Abstracts of the 1994 Fuel Cell Seminar, Nov. 28-Dec. I, 1994, San Diego, CA, p. 199. [28] H.A. Gasteiger, N. Markovic, P.N. Ross and E. Cairns, J. Phys. Chem. 98 ( 1994) 617. [29] R. Ianniello, V.M. Schmidt, U. Stimming, J. Stumper and A. Wallau, Electrochim. Acta 39 (1994) 1863. [30] M. Iwase and S. Kawatsu, Electrochem. Sot. Proc. 95-23 (1995) 12. [31] Information available from: Johnson Matthey Noble Metals, Orchard Road, Royston, Herts., SGS 5HE, UK. [32] F.N. BiJchi and S. Srinivasan, Paul Scherrer Institut Annual Report 1994/Annex V, General Energy Technology, p. 20.

G.G. Scherer

I Solid State Ionics Y4 (1997) ?49-2.~7

[33] F.N. Biichi, M. Wakinzoe and S. Srinivasan, J. Electrochem. Sot. 143 (1996) 927. [34] T.A. Zawodzinski Jr., C. Derouin, S. Radzinski, R.J. Sherman, V.T. Smith, T.E. Springer and S. Gottesfeld, J. Electrothem. Sot. 140 (1993) 1041. [3S] T.R. Nguyen and R.E. White, J. Electrochem. Sot. 140 (1993) 2178. [36] T.F. Fuller and J. Newman, J. Electrochem. Sot. 140 ( 1993) 1218. [37] F.N. Biichi and G.G. Scherer, J. Electroanal. Chem. 404 ( 1996) 37. [38] M. Rota, F.N. Biichi and G.G. Scherer, to be published. 1391 X. Ren, M.S. Wilson and S. Gottesfeld, Electrochem. Sot. Proc. 95-23, eds. S. Gottesfeld et al. (Electrochem. Sot.. Pennington, NJ, 1995) p. 252. [40] S.R. Narayanan. A. Kindler, B. Jeffries-Nakamura, W. Chun. H. Frank, M. Smart. S. Surampudi and G. Halpert, Electrothem. Sot. Proc. 9523. eds. S. Gottesfeld et al. (Electrothem. Sot., Pennington. NJ, 1995) p. 261. 141 1 K. Wang, H.A. Gasteiger, N.M. Markovic and P.N. Ross Jr., Electrochim. Acta 41 (1996) 2587. [42] A.S. Aprico. P. Creti. N. Giordano, V. Antonuci, P.L. Antonucci and A. Chuvilin, J. Appl. Electrochem. 26 (1996) 950.

257

[43] A.S. Lin, A.D. Kowalak and W.E. O’Grady. J. Power Sources 58 ( 1996) 67. [44] T. Frelink, W. Visscher, A.P. Cox and J.A.R. Veen, Ber. Bunsenges. Phys. Chem. 100 (1996) 599. [4S] A.B. Anderson. E. Grantscharova and S. Seong. J. Electrothem. Sot. 143 (1996) 2075. [46] V.M. Schmidt, R. lanniello, H.F. OetJen. H. Reger and U. Stimming, Electrochem. Sot. Proc. 95-23. eds. S. Gottesfeld et al. (Electrochem. Sot.. Pennington. NJ. lYY5) p. 266. [47] R.S. Yeo, Polymer 21 (1980) 432. [48] M.K. Ravikumar and S.K. Shukla. J. Electrochem. Sot. 143 (1996) 2601. [49) P.S. Kauranen and E. Skou, J. Appl. Electrochem. 26 ( IYYh) 909. [SO] X. Ren. T.A. Zawodzinski Jr. F. Uribe, H. Dai and S. Gottesfeld. Electrochem. Sot. Proc. Y5-23, eds. S. Gottesfeld et al. (Electrochem. Sot., Pennington. NJ. lYY5) p, 284. 151 ] N. Alonso-Vante. Hahn-Meitner-ln\titut. Berlm. Germany. personal communication.