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Compositionally versatile, nonstoichiometric, mixed ionic–electronic conducting metal oxides of the form A n +1 B n O 3 n +1 ( n = 1 → ∞; A = rare-earth-/alkaline-earth-metal cation; B = transition-metal (TM) cation) remain a highly attractive class of electrocatalysts for catalyzing the energy-intensive oxygen evolution reaction (OER). The current design strategies for describing their OER activities are largely derived assuming a static, unchanged view of their surfaces, despite reports of dynamic structural changes to 3d TM-based perovskites during OER. Herein, through variations in the A- and B-site compositions of A n +1 B n O 3 n +1 oxides ( n = 1 (A 2 BO 4 ) or n = ∞ (ABO 3 ); A = La, Sr, Ca; B = Mn, Fe, Co, Ni), we show that, in the absence of electrolyte impurities, surface restructuring is universally the source of high OER activity in these oxides and is dependent on the initial oxide composition. Oxide surface restructuring is induced by irreversible A-site cation dissolution, resulting in in situ formation of a TM oxyhydroxide shell on top of the parent oxide core that serves as the active surface for OER. The rate of surface restructuring is found to depend on (i) composition of A-site cations, with alkaline-earth-metal cations dominating lanthanide cation dissolution, (ii) oxide crystal phase, with n = 1 A 2 BO 4 oxides exhibiting higher rates of A-site dissolution in comparison to n = ∞ ABO 3 perovskites, (iii) lattice strain in the oxide induced by mixed rare-earth- and alkaline-earth-metal cations in the A-site, and (iv) oxide reducibility. Among the in situ generated 3d TM oxyhydroxide structures from A n +1 B n O 3 n +1 oxides, Co-based structures are characterized by superior OER activity and stability, even in comparison to as-synthesized Co-oxyhydroxide, pointing to the generation of high active surface area structures through oxide restructuring. These insights are critical toward the development of revised design criteria to include surface dynamics for effectively describing the OER activity of nonstoichiometric mixed-metal oxides.

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Efficient electrochemical transformations of molecular oxygen (oxygen reduction and evolution) for energy conversion/storage rely largely on the effective design of heterogeneous electrocatalysts. Tuning the electrocatalytic properties of materials by controlling the electronic structure of active sites is a promising but challenging approach. Structural and compositional flexibilities of nonstochiometric mixed metal oxides present unique opportunities toward this goal, as the reactivity of their metal cationic centers can be modified via ligand and charge transfer modes. Herein, theoretical calculations combined with experiments demonstrate that highly catalytically active 4d/5d transition metal cations for oxygen reduction can be generated by tuning the distinct intrinsic oxophilicity of 3d and 4d/5d metal cations within a perovskite structure. Tailoring the perovskite composition is shown to switch catalytically poor Rh in supported catalysts to highly catalytic active Rh cationic centers within a perovskite framework (LaNi1–x Rh x O3, x ≤ 0.01). These findings open up opportunities for extrapolating the function of such catalytic systems to other targeted chemistries.

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