Spinel Co₃O₄, comprising one Co²⁺ ion in the tetrahedral site (Co²⁺_Td) and the other two Co³⁺ ions in the octahedral site (Co³⁺_Oh), has been widely explored as promising oxygen evoltuion reaction (OER) catalyst, and several research works have proposed that the electrochemical OER performance of spinel Co₃O₄ is geometry site dependent. However, without any conclusive evidence, the roles of two geometrical cobalt ions, expecially under applied bias, have remained elusive. Here, we investigate the geometrical-site-dependent OER activity of Co₃O₄ by substituting Co²⁺_Td and Co³⁺_Oh with inactive Zn²⁺ and Al³⁺, respectively. Following a thorough in-operando analysis by electrochemical impedance spectroscopy (EIS) and X-ray absorption spectroscopy (XAS), our results indicate Co²⁺_Td and Co³⁺_Oh differ from each other in surface-kinetics and electrochemical reactivity towards OER, and more importantly, Co²⁺_Td is the vital species in Co₃O₄ for electrochemical water oxidation.

We use a simple ion-substitution strategy to individually study the role of Co²⁺_Td and Co³⁺_Oh for OER, and our results indicate Co³⁺_Oh predominated catalyst performs a poor activity, yet Co²⁺_Td predominated catalyst still performs well. The OER activities of pristine and substituted cobalt oxides were first evaluated by cyclic voltammetry (CV) and linear scanning voltammetry (LSV) with corresponding calculated Tofel slope. Surface kinetic properties of OER can be revealed by the Tofel plots obtained from the normalized polarization curves. The general OER mechanism in alkaline solution on the metal site starts with a proton-coupled electron transfer from a surface-bound aquo species followed by an O-O bond formation. Among different samples tested, ZnCo₂O₄ shows a Tafel slope of 113 mV/dec, indicating that the OER process in ZnCo₂O₄ is rate-limited at the first stage where the surface of catalyst is strongly bonded with –OH groups (reaction order = 1 with respect to OH^– species with featured Tafel slope of 120 mV/dec). The Tafel slope for CoAl₂O₄ (~56 mV/dec) and Co₃O₄ (~69 mV/dec) are much smaller than that for ZnCo₂O₄ and close to a featured Tafel slope of 60 mV/dec, suggesting a different rate-determining step. The OER reactions of CoAl₂O₄ and Co₃O₄ are controlled by the equilibrium state between the –OH adsorption and O-O formation in an intermediate coverage regime of –OH groups on the active sites.

Due to the fact that the chemical environment of Co₃O₄ is dynamically changeable during electrocatalysis with applied bias, to gain in-depth information of underlying controlling factors for water oxidation, in-operando EIS analysis is applied here to gain in-depth information of underlying controlling factors. The Bode phase plots generalized from in-operando EIS reveal Co³⁺_Oh in Co₃O₄ is responsible for surface DLC, while Co²⁺_Td is responsible for main water oxidation reaction as an non-homogeneous charge distribution caused by surface oxidized species (e.g., Co³⁺→ Co⁴⁺) is observed on the low frequency range. This finding is identical to the two featured Tafel slopes, 120 mV/dec and 60 mV/dec, in terms of two different surface kinetics as revealed on Co³⁺_Oh predominated catalyst and Co²⁺_Td predominated catalyst, respectively.

To further probe the real-time variation of chemical environment on the catalyst during OER, In-operando XAS is carried out in a home-made in-operando cell. It is revealed Co²⁺_Td with initially low oxidation state is able to release electrons under applied bias, which facilitates the interaction with oxygen intermediates on the catalyst surface. Meanwhile, the interatomic distance between center Cobalt and oxygen is greadaully reduced as indicatd in our K-edge EXAFS spectra for Co²⁺_Td predominated Sample (CoAl₂O₄), while, we can not observe such kind of variation for Co³⁺_Oh predominated Sample (ZnCo₂O₄). This electron-releasing and oxygen-adopting process on Co²⁺_Td predominated CoAl₂O₄ suggests the formation of cobalt oxyhydroxide (CoOOH) can takes please with the assistant from the oxidation process of Co²⁺Td, which acts as the main active species for water oxidation on Co₃O₄. 

With using the exquisite ion-substitution approach and in-operando analysis, the role of Co²⁺_Td and Co³⁺_Oh in spinel Co₃O₄ during electrochemical oxygen evolution reaction can be individually indentified. Our work highlights the vital role of Co²⁺_Td for Co₃O₄ toward water oxidation especially under applied bias, and emphasizes the importance of in-operando investigations on electrocatalysis for instantaneously probing the real-time electrochemical kinetics and surface reactions.