Fe-N-C Catalysts Synthesized on Conductive Non-Carbon Support Materials for Improved PEMFC Durability

Abstract

Global energy demand will be continuously rising in the foreseeable future. To mitigate the results of climate change it is necessary to reduce the emittance of green house gasses. One very promising approach is to switch to hydrogen as energy carrier and use proton membrane fuel cells (PEMFCs) as the primary energy generation technology. Here, hydrogen and oxygen/air are used to directly generate electrical energy with only water as byproduct. Both reactions need to happen efficiently at the same time. The oxygen reduction reaction (ORR), however, is a very sluggish four-electron-reaction and needs efficient catalysts.The state-of-the-art catalyst is platinum but that is a very scarce and expensive metal. Hence, it is important to switch to precious-metal-group (PGM) free catalysts. In recent years, bio inspired iron-based Fe-N-C catalysts have been shown to approach ORR activities that are competitive with those of platinum, even though they need a higher catalyst loading than Pt to reach the desired current densities.1-3 Those catalysts feature single Fe atoms surrounded by four N atoms in graphitic carbon. The drawback of those materials is their lower stability compared to PGM materials. In a nutshell, the carbon matrix oxidizes during operation in PEMFCs, changing the morphology of the catalytic center and rendering it less active, or inactive, depending on the carbon oxidation extent. Demetallation phenomena are also possible, leaving a N4 cavity without metal cation center. It was shown recently by Wu et al.4 that it is possible to raise durability by adding an additional carbon layer on the catalyst. One alternative approach to reduce those degradations is to interface Fe-N4 sites with a carbon-free support. The targeted support material must be immune to corrosion in acid and withstand the typical ORR electrochemical potentials. It also has to be electrically conductive to facilitate electron transfers, both locally and on long scale.In this work, conductive ceramic metal oxide materials were investigated as possible alternative catalyst supports. Those materials are resistant to acid and can withstand prolonged potentials at operating conditions in PEMFCs. The conductivity is usually raised by doping, since many of those ceramics are semi-conductors. However, during the synthesis of the Fe-N-C catalysts, high temperatures are generally applied. Such treatments can be detrimental to the conductivity of the ceramics due to metal aggregation (in case of doped materials) and may also lead to morphological changes. A robust method to synthesize Fe-N-C catalysts is by mixing the separate Fe, N and C precursors and using optimized thermal treatment at high temperatures. They can be further pyrolyzed with ammonia to introduce defects and basicity into the structure, which leads to higher catalytic activity but even lower stability in acid medium. In this study, the effect of the pyrolysis conditions on conductive oxide materials was investigated with various characterization methods, e.g. x-ray diffraction, scanning electron microscopy and N2 sorption isotherms. This gives insights into the changes that happen during the thermal treatment and can be used to find optimal synthetic conditions to keep most desired parameters of the ceramics preserved or only minimally lowered. Several different approaches were used to combine Fe-N-C catalysts with conductive ceramic metal oxide materials. The resulting materials were characterized and tested with rotating disc electrode techniques and in PEM fuel cells for their ORR-activity and durability.(1) F. Jaouen et al., Nature Catalysis, 2022, 5, 311–323(2) H. A. Gasteiger et al., Science, 2009, 324, 48−49(3) L. Elbaz et al., ACS Applied Energy Materials 2022, 5 (7), 7997-8003(4) G. Wu et al., Nature Energy, 2022, 7, 652–663