Tailoring MOF structure via iron decoration to enhance ORR in alkaline polymer electrolyte membrane fuel cells
Williane da Silva Freitas, Alessandra D’Epifanio, Carmelo Lo Vecchio, Irene Gatto, Vincenzo Baglio, Valerio C.A. Ficca, E. Placidi, Barbara Mecheri
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.