Fe, Co, and ZIF-8 co-doped catalysts with carbon nanotube and SiOC composite backbone for high-temperature PEM fuel cell cathode

Abstract

The development of high-temperature proton exchange membrane fuel cell (HT-PEMFC) technology is crucial for implementing the hydrogen economy and decarbonising (heavy) transport and aviation sectors. To replace the Pt-based materials at the HT-PEMFC cathode, we propose a Fe, Co, and N-functionalised catalyst with a nanocarbon backbone consisting of carbon nanotube (CNT) and silicon oxycarbide (SiOC) blend. The CoFe-N-CNT/PDC catalyst is prepared using the Zeolitic imidazolate framework-8 (ZIF-8) as a nitrogen source via pyrolysis, followed by an acid-leaching step. Preliminary testing of oxygen reduction reaction (ORR) activity was conducted comparatively using a rotating ring-disc electrode (RRDE) method at room temperature in 0.5 M H₃PO₄ and with a high-temperature gas diffusion electrode (HT-GDE) half-cell setup at 160 °C in conc. H₃PO₄. Comparative stability testing under RRDE and HT-GDE conditions showed the superior durability of PDC in the catalyst backbone. In more detail, the voltage loss of 36 mV was observed after a 3-hour HT-GDE test at a constant 100 mA cm^-2 for CoFe-N-PDC catalyst, while Fe-N-C (Pajarito Powder, LLC) showed the corresponding value of 80 mV. The maximum power density (P_max) with CoFe-N-CNT/PDC in HT-GDE of 187 mW cm^-2 was obtained, outperforming the Pmax of 153 mW cm^-2 for Fe-N-C. During the HT-PEMFC single-cell tests, the Fe-N-C with Pmax of 134 mW cm^-2 surpassed the CoFe-N-CNT/PDC and CoFe-N-CNT cathodes (Pmax of 112-124 mW cm^-2). The high activity towards the ORR of the CoFe-N-CNT/PDC was attributed to the presence of Fe-, Co-, and N-based active sites, as well as an optimised nanocarbon backbone.