Polymer electrolyte membrane fuel cells (PEMFCs) are considered as one of the most promising alternatives to internal combustion engines (ICEs). In a fuel cell electric vehicle (FCEV), the technology provides high energy conversion efficiency and power density at relatively low operating temperatures, with zero or low greenhouse gases emissions. Materials and systems research and development over the last decade have significantly improved the output power and lifetime of the PEMFC stacks. For wide adoption of PEMFCs to replace ICEs, continuing improvements in durability and cost are required.

Cell voltage polarity reversal can result in irreversible damage of materials in the PEMFC membrane electrode assembly (MEA), gas diffusion layer (GDL), and even the bipolar plates [1], especially in fuel starvation caused cell reversal events. A number of circumstances can lead to fuel starvation, such as ice formation and severe water flooding which block the anode reactant channels, a sudden change in reactant demand due to load change, and improper reactant control during fuel cell start-up [2]. In fuel starvation caused cell reversal events, carbon oxidation (MEA material degradation) or oxygen evolution reaction (OER, i.e., water electrolysis) will take place to sustain the current generated from unaffected neighbor cells in the fuel cell stack. To avoid the detrimental oxidation of MEA materials, the water electrolysis process is preferred to be promoted by incorporating OER catalysts into the anode catalyst layer.

Ruthenium and iridium based oxides are state-of-art PEM water electrolyzer (PEMWE) catalysts. In spite of their different anode layer design and operating conditions, the OER activity and stability of these catalysts have been investigated as the reversal tolerant catalyst (RTC) in PEMFCs [3]. Ru based catalysts provide high OER catalytic activity and increase the fuel cell reversal tolerance. However, they are prone to degradation due to insufficient stability in the PEMFC environment and operating condition. In PEMWE, the catalyst is expected to be stable at potentials higher than 1.4V, on the other hand, in PEMFC, the OER catalyst in the anode is required to be robust to potential swings between 0V to 1V (vs. SHE), because of the fuel cell start-up and shutdown processes. Considering the structure of the anode compartment, the OER catalyst is in direct contact with the electrode substrate in PEMWE, whereas in PEMFC, carbon supported platinum (Pt/C) catalyst of the hydrogen oxidation reaction (HOR) is also incorporated in the anode layer. The presence of Pt/C catalyst adds further complication toward the development of reversal tolerant anode since the carbon corrosion resistance is also required to be considered. Figure 1.a reveals an example in which using the same OER catalyst with similar loading, the cell reversal tolerance for two anode HOR catalyst designs can be significantly different.

In this study, the impact of OER catalyst activity and stability on fuel cell reversal tolerance and durability is explored. Also, the strategic pathways to improve both the reversal tolerance and durability of OER catalysts in PEMFC are discussed. It has been demonstrated that these approaches can provide remarkable reversal tolerance without a significant trade-off in the performance and durability of the fuel cell. Figure 1.b shows that by catalyst treatment, both cell reversal tolerance and durability of the anode layer have been improved.

References:

[1] J. Wu, X. Z. Yuan, J. J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, W. Merida, J. Power Sources 184 (2008) 104–119.

[2] A. Taniguchi, T. Akita, K. Yasuda, and Y. Miyazaki, J. Power Sources 130 (2004), 42–49.

[3] E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T. J. Schmidt, Catal. Sci. Technol. 2014, 4 (11), 3800–3821.

[4] P. He, T. Cheng, R. Bashyam, A. Young and S. Knights; ECS Transactions, 33 (2010) 1273–1279.