Tuning the electrocatalytic water oxidation properties of AB₂O₄ spinel nanocrystals: A (Li, Mg, Zn) and B (Mn, Co) site variants of LiMn₂O₄

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 (LiMn_2-xCo_xO₄, 0 ≤ x ≤ 1.5) both symmetrizes the [Mn_4-xCo_xO₄] 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 e_g¹ 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 andOOH) to the B site.