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

Abstract

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/cm2 3 with H2/O2 gas feeds and more than 2000 hours of continuous operation with less than 5% voltage decay4. However, these were achieved very high areal PGM loadings (anode + cathode) of 1.0-1.3 mgPGM/cm2.Recently, the U.S. Department of Energy (DOE) set some challenging activity targets for AEMFCs5. The near-term (2021-2023) targets all require a PGM loading £ 0.2 mg/cm2. The 2024 target is ⩽ 0.125 mg/cm2 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 cathode6. 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 precursors2. 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 cm2 with H2/O2 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/cm2, exceeding the U.S. Department of Energy target of 44 mA/cm2. 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 mgPt‐Ru cm−2 (0.08 mgPt/cm2 ). That configuration achieved 10.4 W mgPGM −1 (16.25 W/mgPt 1), which exceeds the U.S. Department of Energy 2022 milestone for AEMFC initial performance for the first time.