PRODUCTION OF A QUALIFIED POLYMER ELECTROLYTE FUEL CELL MEMBRANE ELECTRODE ASSEMBLY FOR EMERGING COMMERCIAL APPLICATIONS

Transcription

1 PRODUCTION OF A QUALIFIED POLYMER ELECTROLYTE FUEL CELL MEMBRANE ELECTRODE ASSEMBLY FOR EMERGING COMMERCIAL APPLICATIONS Simon Cleghorn W. L. Gore & Associates, Inc. 201 Airport Road Elkton, Maryland As Published in: 2000 Fuel Cell Seminar in Abstracts, page 35-39, November 2000, Portland, OR, USA INTRODUCTION Technological advancements in the polymer electrolyte membrane fuel cells (PEMFCs) made during the 1990s, and the increasing interest in potentially more efficient and environmentally friendly energy solutions have led this technology to the brink of commercialization. Instrumental in enabling this development has been the availability of advanced high performance PRIMEA membrane electrode assemblies (MEAs) from W. L. Gore and Associates (Gore) [1,2]. Early product development at Gore focused on manufacture of high performance MEA products capable of demonstrating power density targets necessary for market penetration of PEMFCs. Successive products from Gore since 1995 have been demonstrated in fuel cell systems in portable [2-6], stationary [7], and automotive [8,9] applications used in both hydrogen and reformate fueled systems. Many of these fuel cell stacks and systems are now being prepared for commercialization. Typical fuel cell performance data for PRIMEA Series 5510 and 5561 MEAs has been previously reported [1] for a variety of operating conditions. Over the last 1-2 years, MEA development at Gore has been directed at sustaining high power density requirements for commercial life; the most demanding systems life requirements may exceed 40,000 hours of continuous stable performance for stationary systems. This paper focuses on an experimental approach that was used as a guide in development of more durable MEA products for stationary applications from Gore, and also provides useful information for designing fuel cells for maximum MEA life. EXPERIMENTAL In order to understand MEA integrity and performance decay in operating fuel cells, the effects of reliability, stability and durability must be defined, separated and understood. An appreciation of these effects focuses our study of performance decay and engineering of more durable products. Reliability MEA reliability failure is typically defined as either inability of the MEA to operate in a stack or single cell at start-up, or a short-term (less than 100 hours) membrane failure. MEA reliability may be associated with the use of defective MEAs, or the result of poor cell or stack design or assembly, causing MEA shorting, puncturing or burn-through. MEA reliability problems are addressed with attention to MEA manufacturing quality, proper handling, and effective cell or stack design, and are not the topic of this presentation.

2 Emerging Commercial Applications Durability MEA durability has been defined as the ability of an MEA to resist permanent change in performance over time. This is typically associated with irreversible material changes (i.e. membrane failure or catalyst sintering). Therefore, decay in MEA power density as a result of poor MEA durability is not recoverable or reversible. Stability Fuel cell stability is the recoverable function of voltage or current density decay. MEA performance decay resulting from cell instability is typically the result of non-steady state behavior, and therefore, by definition is fully recoverable. MEA stability is associated with the MEA s sensitivity to operating conditions (i.e. water management) or reversible material changes. Experimental Approach An experimental approach used at Gore to study MEA performance decay is to use designed experiments in fuel cell life testing, in combination with other ex-situ protocols. To illustrate this, we describe here a series of designed experiments, that consisted of approximately 80 fuel cell life test experiments with a total of over 90,000 hours of testing. Experiments were operated continuously for 1,000 2,000 hours; stability and durability decay rates were extracted. The overall cell performance decay rates measured during uninterrupted operation (between polarization curves) contain the additive effects of MEA stability and durability. Durability performance decay rates were measured by comparing polarization curves as a function of time. A stability decay rate can be calculated by subtracting the un-recovered performance decay rate (i.e. the portion that was not recovered from performing polarization curves) from the total measured decay rate. It is important to recognize recovery methods used in this type of analysis should be limited to those possible in real fuel cell systems. Single-cell, fuel cell hardware with an active area of 25 cm 2 was chosen for investigations of MEA life. A single-cell design was chosen to provide a uniform and controlled environment for MEA testing, where the effects of fuel cell test parameters can be individually studied with respect to performance decay. The MEA can be exposed to conditions that may simulate an environment it may experience in a fuel cell stack. By simulating the most aggressive stack environment expected for the MEA, accelerated or predictive life testing can be performed at a sub scale. RESULTS and DISCUSSION Designed Experiment 1 (DE1) Fuel cell operating conditions The aim of DE1 was to determine the effects of fuel cell operating conditions (cell temperature, pressure, cell operating current density including duty cycle, reactant inlet relative humidity, gas diffusion media type and cell configuration / assembly) on performance decay and to identify accelerated MEA testing conditions. The variables chosen for this experiment reflect parameters that may be readily modified when designing fuel cell stacks, and therefore, the results should be considered as a guide when engineering a system for maximum MEA life. The strongest factors influencing MEA performance durability (listed in Table 1) were determined to be: cell operating temperature and pressure, inlet reactant humidification, and cell design uniformity. Within DE1, cell design uniformity factors were limited to reactant flow configuration (Figure 1 - accelerated conditions shown) and active area compression. Both of these factors strongly influenced MEA durability: many other cell design variables are also expected to be important, such gas diffusion media uniformity, current distribution, hydration, and reactant distribution, but have not yet been fully investigated. Although the factors current density

3 Emerging Commercial Applications (including duty cycle) and gas diffusion media type were controlled variables in DE1, their response was complex and requires more complete discussion than possible here. Cell Temperature Operating Pressure Reactant humidification Cell Design and Configuration Lower operating temperature extends MEA life (however, higher temperature may be favored for improved electrode kinetics) Low operating pressure (preferred ambient pressure) MEA life is extended when it is operated fully humidified Stack and cell designed for uniformity: uniform cell compression and reactant flow configuration Table 1. Summary of factors shown to be statistically important, and therefore recommended fuel cell operating conditions preferred when designing for MEA life Counter-flow Cell 800 ma cm -2 (V) Co-flow Failure Failure Time (hours) Figure 1. A comparison of MEA durability at accelerated life test conditions in counter and co-flow cell configurations. When operating fuel cells with PRIMEA Series 5510 MEAs under the conditions advised by Table 1 (with the exception of co-flow reactants), over 10,000 hours of performance was recorded with extremely low decay rates. However, operating this same MEA at elevated temperature and pressure resulted in increased rates of irreversible voltage decay, and under some reactant inlet humidification and cell design conditions may lead to rapid membrane failures. There has been an acute need at Gore for reliable fuel cell life tests capable of accelerating key MEA degradation mechanisms, if new MEA product introductions are to be capable of exceeding the required design life of 40,000 hours. These observations have resulted in the adoption of the most aggressive conditions observed in DE1 as an accelerated MEA life test. An MEA life correlation between accelerated tests and those performed at more realistic operating conditions for stationary applications [10] provides a very powerful tool in product development at Gore. Within this accelerated procedure the relative effects of current density, cell operating temperature and pressure, reactant composition and saturation, cell design and assembly, and gas diffusion

4 Emerging Commercial Applications media on different MEA decay mechanisms have been evaluated to ensure no new or unrealistic degradation mechanisms are introduced. Fundamental decay mechanisms Accelerated and realistic fuel cell experiments performed in DE1, and most insightfully the associated diagnostic experiments, resulted in identification of several key MEA decay mechanisms, attributed to anode, cathode and membrane. Typical in-cell diagnostic measurements included fuel cell polarization, voltammetry to measure catalyst electrochemical area, CO stripping, electrode capacitance and hydrogen cross-over, and AC impedance spectroscopy. The study of decay mechanisms has been a major focus of MEA research at Gore, with development of in-situ and ex-situ materials and MEA testing targeted at accelerating very specific degradation mechanisms. The results of these fundamental efforts are incorporated into the development of new more durable and stable MEA technologies capable of meeting the expectations of current stationary fuel cell applications. For example, experimental evidence has confirmed that the strong and stable microporous eptfe support of the GORE-SELECT membrane provides substantial benefit in membrane durability. Designed Experiment 2 verification of new product improvements Initial verification of improved MEA products incorporating decay resistant technologies was performed in accelerated fuel cell testing. No performance decay was observed in approximately 2000 hours of accelerated life testing in co flow cell configurations (Figure 2). It is important to consider that improved MEA durability has been achieved without increasing catalyst loadings or increasing membrane thickness over that of standard PRIMEA Series 5510 MEAs. MEAs in these experiments contained a total platinum content of 0.7 mg cm -2 and a GORE SELECT membrane with a thickness of 25 microns. Further verification of these improvements is now underway at realistic stationary conditions [10]; to date, greater than 7,000 hours of durable performance has been observed New Technology Cell 800 macm -2 (V) PRIMEA 5510 MEA (Experiment terminated at 550 hours due to excessive decay) Time (hours) Figure 2. A comparison of a PRIMEA Series 5510 MEA and a new more durable MEA in an accelerated life test at constant current density, 800 ma cm -2 in a co-flow cell configuration.

5 Emerging Commercial Applications While durability experiments running periodic fuel cell diagnostics have been the main source of fuel cell data and certainly invaluable in detecting MEA decay mechanisms at Gore, they may disguise effects of long-term stability. Therefore, complementary stability experiments are also underway using test stations capable of establishing very accurately controlled environments without interruption or fuel cell perturbation. Results will be presented that demonstrate the longterm stability of new products within a wide stable window of operation. Full-scale, fuel cell stack field verification of Gore single cell testing results are continually performed as a crucial part of new MEA product development. In the field an MEA is likely to experience a much wider range of operating environments, under which it must be stable and durable. The designed experiment single cell data can therefore also be used as an excellent predictive tool in determining MEA life in these less uniform systems. CONCLUSIONS A major focus of MEA research and development at Gore over the last 1-2 years has resulted in a unique understanding of the factors and mechanisms influencing MEA life in polymer electrolyte fuel cell systems. The designed experiment approach to fuel cell life testing combined with other ex-situ techniques has resulted in a unique understanding of MEA degradation mechanisms. Experimentally it was determined that MEA operating conditions and fuel cell stack design also have a very strong influence on product integrity and performance decay, therefore, operating conditions have been identified which allow for maximum MEA life in fuel cells. Product development at Gore has advanced rapidly as a result of the identification of accelerated test methods and their correlation to realistic fuel cell operation conditions. Development at Gore has now resulted in more durable and stable PRIMEA Series MEAs, targeted at stationary applications. REFERENCES (1) B. Bahar, C. Cavalca, S.J.C. Cleghorn, J. Kolde, D. Lane, M. Murthy and G. Rusch, J. New Mat. Electrochem. Syst., 2 (1999) 179. (2) B. Bahar, C. Cavalca, S.J.C. Cleghorn, J. Kolde, P. Hertel, M. Murthy and G. Rusch, Portable Power Fuel cell Proceeding, Ed. F.N. Buchi, Lucerne, 1999, p 223. (3) P. Koschany, International Conference with Exhibition Fuel Cell 2000 Proceedings, Ed. L. Blomen, Lucerne, 2000, p 61. (4) M. S. Wilson, D. Decaro, J. K. Neutzler, C. Zawodzinski, and S. Gottesfeld, in Abstracts and Program of the Fuel Cell Seminar p331, Orlando, FL, Nov (5) J. Roser, G. Zettisch, J. Scholta, L Jorissen and J. Garche. International Conference with Exhibition Fuel Cell 2000 Proceedings, Ed. L. Blomen, Lucerne, 2000, p 75. (6) M. Daugherty, D. Habermen, N. Stetson,, S. Ibrahim, O Lokken, D. Daunn, M. Cherniack and C. Salter, Portable Power Fuel cell Proceeding, Ed F.N. Buchi, Lucerne, 1999, p 69. (7) M. L. Wald, Article published in The New York Times National, 17 June (8) P. A. Lehman and C. E. Chamberlin, in Abstracts and Program of the Fuel Cell Seminar, p714, Palm Springs, CA., (9) F. Barbir, J. Neutzler, W. Pierce, and R. Wynne, in Abstracts and Program of the Fuel Cell Seminar, p718, Palm Springs, CA., (10) T. Isono, S. Suzuki, M. Kaneko, Y. Akiyama, Y. Miyake, I. Yonezu, J. Power Sources, 86 (2000) 269. ACKNOWLEDGEMENTS The author acknowledges the contribution of a large technical team at Gore for the data presented. GORE-SELECT and PRIMEA are registered trademarks of W. L. Gore & Associates, Inc.