Degradation Mechanisms and Durability Improvement Strategies of Fe-N-C Catalysts for Oxygen Reduction Reaction
Among the many non-precious metal catalysts that have been reported so far,M-N-C catalysts based on transition metal-nitrogen-carbon structure are considered as the most promising candidates to replace Pt-based catalysts for oxygen reduction reaction.Compared with other M-N-C catalysts,Fe-N-C catalysts exhibit the highest ORR activity in acidic environments due to the suitable adsorption energy of oxygen-containing intermediates and thermodynamically favorable 4e pathway.However,the practical application of this catalyst is still limited by the challenge of insufficient stability under the high voltage and strong acidic conditions of PEMFC.Thus,the preparation of stable and efficient Fe-N-C catalysts still faces many challenges.In this review,we systematically summarize the common synthesis methods of Fe-N-C catalysts,including spatial confinement method and template-assisted strategy,outline the half-cell and single-cell test methods used to evaluate the catalyst stability,and analyze the reasons for the discrepancies between the two test results.In order to design highly stable catalysts,a clear knowledge and understanding of the degradation mechanism is required,so we describe four possible degradation mechanisms for Fe-N-C catalysts:demetallization,carbon oxidation,protonation,and microporous water flooding,subsequently we propose some specific strategies to enhance the stability of Fe-N-C catalysts.Finally,the future development direction of Fe-N-C catalysts is discussed in this review.It is hoped that the comprehensive and in-depth study of Fe-N-C catalysts will guide the design and development of highly stable Fe-N-C catalysts for the application of PEMFC.
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