Unraveling the valence state and phase transformation of iron-based electrocatalysts towards oxygen reduction reaction

Abstract The commercialization of fuel cells relies largely upon the development of highly active non-precious metal electrocatalysts. Insufficient understanding of the reaction mechanism of Fe-based electrocatalysts towards oxygen reduction reaction (ORR) limits the further improving of its catalytic performance. Herein the valence state and phase transformation of three Fe-based electrocatalysts comprising different valences of iron during the ORR in alkaline medium are investigated by in situ synchrotron radiation X-ray in direct borohydride fuel cell. There is a dynamic structure associated with the ‘Fe2+/3+’ redox transition confirmed by in situ X-ray absorption near edge structure tests in Fe-PPy-C and FeS-PPy-C. The metallic Fe, FeS, and Fe2O3 of the three Fe-based electrocatalyst are all converted to FeOOH during ORR. The local structure of Fe atoms and valence transition barrier contribute to most of the ORR activities of the three Fe-based electrocatalysts.