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 B4O4 subcores are noted for their catalytic activity in water oxidation (OER). We synthesized a series of ternary spinel oxides, AB2O4, derived from LiMn2O4 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: Zn2+ > Mg2+ > 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 [Mn4O4]4+ core relative to LiMn2O4 (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 (LiMn2–xCoxO4, 0 ≤ x ≤ 1.5) both symmetrizes the [Mn4–xCoxO4] 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 AMn2O4 spinels, those starting with Mn(III) in the resting lattice (prior to oxidation) result in longer, weaker Mn–O bonds for this eg1 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.