Tailoring MOF structure via iron decoration to enhance ORR in alkaline polymer electrolyte membrane fuel cells

Abstract

Fe-N-C catalysts were synthesized by combining a Zn-based zeolitic imidazolate framework (ZIF-8) structure, adopted as a nitrogen-carbon template, with an iron salt and conductive carbon support followed by a thermal treatment. The effect of three different pyrolysis temperatures (700, 900, and 1000 °C) on Zn removal from ZIF-8 was investigated to enhance the formation of Fe-based moieties in the Nx-C groups during carbonization. Electrochemical characterization using a rotating ring disk electrode in an alkaline electrolyte demonstrated that ORR activity increased as the pyrolysis temperature increased. This trend can be ascribed to a more effective Zn removal and formation of high-active iron- and nitrogen-based catalytic sites, as pointed out by the Fe-N-C materials' chemical surface analysis after the pyrolysis step. The sample Fe-N-C-1000 demonstrated a remarkable ORR activity, even higher than Pt/C taken as reference.When subjected to accelerated stress tests, the Fe-N-C-1000 sample displayed higher performance durability over a long cycling duration (30,000 cycles) compared to Pt/C taken as control. Tests in the AEMFC fed with H2 showed that the performance of the Fe-N-C-1000 catalyst was competitive (OCV = 0.98 vs. 1.05 V, 149 vs. 148 mW cm−2) compared to the state-of-the-art Pt/C electrode, using a FUMASEP® FAA-3-50 membrane. The material found an application also in alkaline direct methanol fuel cell (ADMFC) fed with methanol solutions at high concentrations (up to 10 M) due to a high methanol tolerance, as pointed out by rotating disk electrode experiments.