A Strategy for Increasing the Efficiency of the Oxygen Reduction Reaction in Mn-Doped Cobalt Ferrites.

Abstract

Alkaline fuel cells have drawn increasing attention as next-generation energy-conversion devices for electrical vehicles, since high pH enables the use of non-precious-metal catalysts. Herein, we report on a family of rationally designed Mn-doped cobalt ferrite (MCF) spinel nanocrystals, with an optimal composition Mn0.8(CoFe2)0.73O4 (MCF-0.8), that are effective electrocatalysts for the oxygen reduction reaction. MCF-0.8 exhibits a half-wave potential ( E1/2) of 0.89 V vs RHE in 1 M NaOH, only 0.02 V less than that of commercial Pt/C under identical testing conditions and, to the best of our knowledge, one of the highest recorded values in the literature. Moreover, MCF-0.8 exhibits remarkable durability (Δ E1/2 = 0.014 V) after 10 000 electrochemical cycles. In situ X-ray absorption spectroscopy (XAS) reveals that the superior performance of the trimetallic MCF-0.8 originates from the synergistic catalytic effect of Mn and Co, while Fe helps preserve the spinel structure during cycling. We employed in situ XAS to track the evolution of the oxidation states and the metal-oxygen distances not only under constant applied potentials (steady state) but also during dynamic cyclic voltammetry (CV) (nonsteady state). The periodic conversion between Mn(III, IV)/Co(III) and Mn(II, III)/Co(II) as well as the essentially constant oxidation state of Fe during the CV suggests collaboration efforts among Mn, Co, and Fe. Mn and Co serve as the synergistic coactive sites to catalyze the oxygen reduction, apparently resulting in the observed high activity, while Fe works to maintain the integrity of the spinel structure, likely contributing to the remarkable durability of the catalyst. These findings provide a mechanistic understanding of the electrocatalytic processes of trimetallic oxides under real-time fuel cell operating conditions. This approach provides a new strategy to design high-performance non-precious-metal electrocatalysts for alkaline fuel cells.