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 (E 1/2) 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 (ΔE 1/2 = 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.

Redox‐Inert Fe3+ Ions in Octahedral Sites of Co‐Fe Spinel Oxides with Enhanced Oxygen Catalytic Activity for Rechargeable Zinc–Air Batteries

“…Moreover,N-doped carbon nanotubes (NCNTs) can act as an ideal conductive substrate for metal oxides,o wing to its high electrical conductivity, large surface area, and intrinsic flexibility. [16] Upon coupling active hybrid spinel to the carbon material, the obtained Co 2 FeO 4 /NCNTs shows excellent catalytic performance that lend promise to its utilization as abifunctional electrocatalyst in zinc-air batteries.…”

Redox‐Inert Fe3+ Ions in Octahedral Sites of Co‐Fe Spinel Oxides with Enhanced Oxygen Catalytic Activity for Rechargeable Zinc–Air Batteries

“…Co−O bond length changes caused by replacement of Co by Fe can also affect the covalency of the bond. Moreover, N‐doped carbon nanotubes (NCNTs) can act as an ideal conductive substrate for metal oxides, owing to its high electrical conductivity, large surface area, and intrinsic flexibility . Upon coupling active hybrid spinel to the carbon material, the obtained Co 2 FeO 4 /NCNTs shows excellent catalytic performance that lend promise to its utilization as a bifunctional electrocatalyst in zinc–air batteries.…”

Advanced Characterization Techniques for Identifying the Key Active Sites of Gas‐Involved Electrocatalysts

“…According to the previous studies, the dopants are able to disturb the chemical bonding on the surface of the host oxides, and the adjacent atoms or the dopants themselves are regarded as active sties. [ 29b,128 ] Based on the above perspectives, numerous researchers have paid their attention to designing optimal electrocatalysts through doping metal heteroatom doping. For instance, Mo/W‐doped Fe 2 O 3 was prepared and HAADF‐STEM, XANES, in situ FTIR combined with the computer simulations were used to identify the active sites of the as‐synthesized materials for selective catalytic reduction of NO x to unravel the reason for the enhanced activity.…”

“…[ 129 ] Different from traditional views, the results reveal that the real electrocatalytic active sites of the doped Fe 2 O 3 materials are not the dopant atoms themselves but the adjacent Fe atoms, whose electronic structure was regulated by the dopant atoms, thus contributing to the enhanced Lewis acidic properties of the electrocatalysts. Otherwise, Mn‐doped cobalt ferrite spinel nanocrystal Mn 0.8 (CoFe 2 ) 0.73 O 4 (MCF‐0.8) was designed for catalyzing ORR, [ 29a ] and the structure was revealed by HAADF‐STEM (Figure 13g–l). The calculated d‐spacings (4.9 and 3.0 Å) are consistent with theoretical value of 4.85 Å (111) and 2.97 Å (20), confirming the cubic spinel structure of MCF‐0.8.…”

“…have attracted growing attention, which can be applied to monitor and even directly observe the catalytically active structures of the electrocatalysts. Specifically, a large number of microscopic, [ 29 ] spectroscopic, [ 30 ] and electrochemical [ 31 ] methods either in situ/operando or ex situ pattern, particularly at atomic resolution, have been developed in this area over the past decades. As shown in Figure , the rapid development of characterization technology from ex situ to in situ/operation has made the obtained information more comprehensive with the smaller scale and the finer structure.…”