Improved chemical water oxidation with Zn in the tetrahedral site of spinel-type ZnCo2O4 nanostructure

Spinel oxides with the composition of A^IIB^III₂O₄ (A and B are metal ions) represent an important class of anode material for water splitting to replace the currently used noble-metal catalysts. Although spinel electrocatalysts have widely been investigated for electrochemical water oxidation, the role of octahedral and tetrahedral sites influencing catalytic performance has been a topic of discussion for a long time and still under debate. Lately, this issue has been addressed by substituting redox-inert cation to the tetrahedral sites of cobalt spinels and comparing the electrochemical activity between them. However, rapid surface structural transformation of the catalysts under operating electrochemical conditions makes it difficult to infer the exact contribution of tetrahedral and octahedral sites for water oxidation. Herein, for the first time, we utilize the oxidant-driven water oxidation approach to reveal the responsible active sites using two spinel-type nanostructures, Zn^IICo₂^IIIO₄ and Co^IICo₂^IIIO₄ (Co₃O₄), synthesized by using a single-source precursor approach. Strikingly, a superior O₂ production rate (0.98 mmol_O2 mol_Co^?1 s^?1) following first-order reaction kinetics was achieved for ZnCo₂O₄ in the presence of Ce^IV as sacrificial electron acceptor compared to Co₃O₄ spinel (0.29 mmol_O2 mol_Co^?1 s^?1). The structural and morphological stability of the ZnCo₂O₄ and Co₃O₄ post water oxidation catalysis confirms that the catalytic activity is strictly controlled by the geometry and electronic structure of the active site of the spinel structure. The higher performance of ZnCo₂O₄ over Co₃O₄ further indicates that the presence of Co^II is not essential for catalytic water oxidation. The presence of redox inert Zn^II at the tetrahedral site of ZnCo₂O₄ can facilitate the stabilization of a high-valent Co^IV intermediate via oxidation of Co^III (situated at the octahedral site), and this intermediate can be regarded as the active species for water oxidation catalyst along with structural defects caused by surface Zn leaching.