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...