(Invited) Diagnostic Methods Utilized in Accelerated Stress Testing of Polymer Electrolyte Membrane Fuel Cells

The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are the two major barriers to the commercialization of these systems for stationary and transportation power applications.¹ The cost and durability of fuel cells are inter-related as more often than not lower cost components are less durable than their higher cost counterparts. Given the long expected life times of these fuel cell systems in the field (> 5500 hours for transportation and > 60,000 hours for stationary applications), there is a pressing need to develop Accelerated Stress Tests (ASTs).² The ASTs should be representative of degradation in the field and are expected to provide estimates of fuel cell lifetime using short experiments. The U.S. DOE Fuel Cell Tech Team has recommended various ASTs for PEMFC components.^3,4 While these ASTs are good at evaluating the relative durability of the various materials used in PEMFCs, there is little information available on acceleration factors of these ASTs for various applications. In this paper we will present various diagnostic techniques that are utilized in conjunction with these ASTs to better evaluate the durability of fuel cell components and their effect on fuel cell performance.

Diagnostic techniques used to evaluate electrocatalyst durability include, performance curves, electrochemical surface area, mass activity, electrode capacitance, AC impedance, evolved CO₂ from the cathode, catalyst particle size, and electrode thickness. Figure 1 illustrates the performance of PEMFCs subjected to two different support ASTs; a 1.2V hold and a 1 to 1.5 V potential cycle @ 500 mV/sec. The observed degradation rate is 150 – 200 times faster during the potential cycling compared to the potential hold. This is consistent with the 100 times faster increase in Pt particle size observed during the potential cycling experiment (Figure 2). In addition to the Pt particle size increase and associated kinetic losses, there are clear mass transport losses associated with these ASTs. This is illustrated in Figure 3 where the lower frequency mass transport resistance dramatically increases during the AST especially for carbons that corrode faster.

Diagnostic techniques used to evaluate membrane durability include performance curves, hydrogen cross over, high frequency impedance, shorting resistance, fluoride emission, membrane thickness, membrane morphology and membrane mechanical strength. The importance of these diagnostic techniques in the development of a new membrane AST will be discussed.

References

1. R. Borup, et al., Chemical Reviews, V. 107, No. 10, 3904-3951 (2007).

2. S. Zhang, X. Yuan, H. Wang, W. Merida, H. Zhu, J. Shen, S. Wu and J. Zhang, Int. J. Hydrogen Energy, V 34, 388-404 (2009).

3. DOE Cell Component AST and polarization curve Protocols for PEM Fuel Cells (Electrocatalysts, Supports, Membranes and MEAs), Revised December 16, 2010.

4. N. L. Garland, T.G. Benjamin, J. P. Kopasz, ECS Trans., V. 11 No. 1, 923 (2007).

Acknowledgements

The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and the Technology Development Manager: Nancy Garland. The authors also wish to acknowledge Ion Power, Inc. for supplying the MEAs and SGL Carbon for the GDLs used in this study.