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

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 Pt-based materials at the HT-PEMFC cathode, we propose an Fe, Co, and N-functionalised catalyst with a nanocarbon backbone consisting of a carbon nanotube (CNT) and silicon oxycarbide (SiOC) blend. SiOC is a subclass of polymer-derived carbon (PDC). The CoFe–N–CNT/PDC catalyst is prepared using 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, a voltage loss of 36 mV was observed after a 3-hour HT-GDE test at a constant current density of 100 mA cm⁻² for the CoFe–N–PDC catalyst, while Fe–N–C (Pajarito Powder, LLC) showed a corresponding value of 80 mV. A maximum power density (P_max) with CoFe–N–CNT/PDC in the HT-GDE of 187 mW cm⁻² was obtained, outperforming the P_max of 153 mW cm⁻² for Fe–N–C. During the HT-PEMFC single-cell tests, Fe–N–C with a P_max of 134 mW cm⁻² surpassed the CoFe–N–CNT/PDC and CoFe–N–CNT cathodes (P_max 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 and an optimised nanocarbon backbone.