Tuning the Electrocatalytic Water Oxidation Properties of AB₂O₄ Spinel Nanocrystals: A (Li, Mg, Zn) and B (Mn, Co) Site Variants of LiMn₂O₄

Abstract

Transition metal oxides containing cubic B 4 O 4 subcores are noted for their catalytic activity in water oxidation (OER). We synthesized a series of ternary spinel oxides, AB 2 O 4, derived from LiMn 2 O 4 by either replacement at the tetrahedral A site or Co substitution at the octahedral B site and measured their electrocatalytic OER activity. Atomic emission and powder X-ray diffraction confirmed spinel structure type and purity. Weak activation of the OER occurs upon A-site substitution: Zn 2+ > Mg 2+ > A-vacancy > Li + = 0. Zn and Mg substitution is accompanied by (1) B-site conversion of Mn(IV) to Mn(III), resulting in expansion and higher symmetry of the [Mn 4 O 4 ] 4+ core relative to LiMn 2 O 4 (inducing greater flexibility of the core and lower reorganization barrier to distortions), and (2) the electrochemical oxidation potential for Mn(III)/IV) increases by 0.15–0.2 V, producing a stronger driving force for water oxidation. Progressive replacement of Mn(III/IV) by Co(III) at the B site (LiMn 2– x Co x O 4, 0 ≤ x ≤ 1.5) both symmetrizes the [Mn 4– x Co x O 4 ] core and increases the oxidation potential for Co(III/IV), resulting in the highest OER activity within the spinel structure type. These observations point to two predictors of OER catalysis: (1) Among AMn 2 O 4 spinels, those starting with Mn(III) in the resting lattice (prior to oxidation) result in longer, weaker Mn–O bonds for this e g 1 antibonding electronic configuration, yielding greater core flexibility and a higher oxidation potential to Mn(IV), and (2) a linear free energy relationship exists between the electrocatalytic rate and the binding affinity of the substrate oxygen (*OH and *OOH) to the B site.