In-Operando Observation of Geometrical-Site-Dependent Activity of Spinel Co3O4 for Water Oxidation
Authors: Wang Hsin-YI , Hung Sung-Fu , Chen Han-Yi , Chan Ting-Shan , Liu Bin 
Published: February 2020

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.

Figure 1