Understanding the Influence of Fe-N-C Cathode Catalyst Structure on Their Performance and Durability in High Performing Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) are a potentially very low-cost alternative to traditional proton exchange membrane fuel cells (PEMFCs) 1,2 – as long as their promise to offer high performance and durability with no platinum group metal (PGM) catalysts is realized. From the perspective of performance and durabiltiy, AEMFCs are doing well quite well with some studies even able to achieve peak power densities of 3.5 W/cm 2 3 with H 2 /O 2 gas feeds and more than 2000 hours of continuous operation with less than 5% voltage decay 4 . However, these were achieved very high areal PGM loadings (anode + cathode) of 1.0-1.3 mgPGM/cm 2 . Recently, the U.S. Department of Energy (DOE) set some challenging activity targets for AEMFCs 5 . The near-term (2021-2023) targets all require a PGM loading £ 0.2 mg/cm 2 . The 2024 target is ⩽ 0.125 mg/cm 2 and the final milestone of entirely PGM-free AEMFCs is slated for 2030. To get there, there is a need to develop PGM-free catalysts for both the anode and cathode 6 . At the anode, there are very few options for PGM-free catalysts. This means that the intermediate-term cell development should focus on using PGM-free cathodes and low-PGM anodes. Fe-N-C catalysts have emerged as the most promising candidates for the oxygen reduction reaction at the AEMFC cathode. However, though this family of materials has been of interest for several years, reported catalysts in the literature show quite different activity (in ex-situ experiments) and performance (in operating cells), despite reporting similar synthesis routes and precursors 2 . One reason behind this poor performance could be the catalyst's overall structure and the dispersion of catalytically-active single-atom reaction sites. In this study, multiple Fe-N-C catalysts were prepared and investigated. The main goal of the preparation was to prepare Fe-N-C cathodes with different levels of atomic dispersion and overall catalyst microstructure. After synthesis, catalysts were physically characterized using a suite of tools including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution aberration-corrected scanning transmission electron microscopy (STEM). The electrochemical behavior of the catalysts was also studied – both ex-situ in a three electrode cell as well as in operating AEMFCs. From these results, our team was able to find important links between the catalyst structure and its in-cell performance, providing guidance for the design of future materials. Of particular note, AEMFCs were constructed with a Fe-N-C cathode and PtRu/C anode that were able to reach peak power densities over 2 W cm 2 with H 2 /O 2 reacting gases and stable operation for more than 100 h. These cells were also able to achieve an iR-corrected current density at 0.9 V as high as 124 mA/cm 2 , exceeding the U.S. Department of Energy target of 44 mA/cm 2 . In an alternate configuration, the Fe-N-C cathode was paired with a low-loading PtRu/C anode electrode to create AEMFCs with a total PGM loading of only 0.125 mg Pt‐Ru cm −2 (0.08 mg Pt /cm 2 ). That configuration achieved 10.4 W mg PGM −1 (16.25 W/mg Pt 1 ), which exceeds the U.S. Department of Energy 2022 milestone for AEMFC initial performance for the first time. References: Adabi Firouzjaie, H. & Mustain, W. E. Catalytic Advantages, Challenges and Priorities in Alkaline Membrane Fuel Cells. ACS Catal. (2019) doi:10.1021/acscatal.9b03892. Santori, P. G., Speck, F. D., Cherevko, S., Firouzjaie, H. A., Peng, X., Mustain, W. E. & Jaouen, F. High Performance FeNC and Mn-oxide/FeNC Layers for AEMFC Cathodes. J. Electrochem. Soc. 167 , 134505 (2020). Mandal, M., Huang, G., Hassan, N. U., Peng, X., Gu, T., Brooks-Starks, A. H., Bahar, B., Mustain, W. E. & Kohl, P. A. The Importance of Water Transport in High Conductivity and High-Power Alkaline Fuel Cells. J. Electrochem. Soc. 167 , (2020). Ul Hassan, N., Mandal, M., Huang, G., Firouzjaie, H. A., Kohl, P. A. & Mustain, W. E. Achieving High-Performance and 2000 h Stability in Anion Exchange Membrane Fuel Cells by Manipulating Ionomer Properties and Electrode Optimization. Adv. Energy Mater. n/a , 2001986 (2020). Thompson, S. T., Peterson, D., Ho, D. & Papageorgopoulos, D. Perspective—The Next Decade of AEMFCs: Near-Term Targets to Accelerate Applied R&D. J. Electrochem. Soc. 167 , 84514 (2020). Omasta, T. J., Zhang, Y., Park, A. M., Peng, X., Pivovar, B., Varcoe, J. R. & Mustain, W. E. Strategies for reducing the PGM loading in high power AEMFC anodes. J. Electrochem. Soc. 165 , F710–F717 (2018).