Time-resolved Kinetic Study Of Water Oxidation Mechanisms On Cobalt-based Oxide Catalysts

The oxygen evolution reaction（OER）is a crucial reaction that provides protons and electrons,playing a key role in both natural and artificial photosynthesis.Understanding the OER mechanisms is vital for the rational design and optimization of OER catalysts.However,since the complexity of the multi–electron and multi–proton transfer processes involved in the OER catalytic cycle,revealing the intermediates and reaction mechanisms are challenging.Cobalt–based oxides have proven to be highly valuable for water oxidation due to their excellent OER performance both theoretically and experimentally.In this dissertation,we explored the sequential oxidation kinetics in OER cycle on cobalt oxide catalysts by constructing a dye molecule–cobalt oxide water oxidation system and using transient absorption spectroscopy on reaction timescale（milliseconds to seconds）.The following research achievements were obtained:（1）We revealed the sequential oxidation kinetics mechanism for catalyzing water oxidation on Co₃O₄ nanoparticles,and proposed a multi–center sequential redox mechanism,in which both surface Co²⁺and Co³⁺sites participate in the water oxidation catalytic cycle.The surface Co²⁺sites are first oxidized to Co³⁺intermediates to initiate the catalytic water oxidation cycle,then the Co³⁺intermediates and surface Co³⁺sites are oxidized to Co⁴⁺intermediates,and finally oxygen is released through the reduction of Co⁴⁺intermediates.The kinetic features of the fast generation and slow consumption of both Co³⁺and Co⁴⁺intermediates in the sequential redox process are demonstrated,resulting in the critical role of Co⁴⁺intermediates in the water oxidation performance.（2）The effect of proton participation on OER activity was investigated on Co₃O₄nanoparticles.It was clarified that high-valence cobalt intermediates on the surface of Co₃O₄ nanoparticles are involved in proton transfer in the redox process.Two tuning mechanism of proton acceptor concentration（solution pH）on the redox kinetics was found,and the tuning mechanism of proton transfer on OER activity was also revealed.One is that changing the concentration of the proton acceptor in the BO₃^3––type buffer will not change the OER mechanism and kinetics of proton–coupled electron transfer but will modulate the coverage of surface intermediates by the limitation of proton acceptor concentration.This tuning mechanism may be originated from the large difference in the kinetics between the transformation of Co³⁺and Co⁴⁺intermediates on the surface of Co₃O₄ nanoparticles.The other is that changing the concentration of the proton acceptor in the B₄O₇^2––type buffer directly changes the kinetics and mechanism of proton–coupled electron transfer,leading to the activity change.（3）To further study the tuning mechanism for multi–redox kinetics in OER cycle,a series of amorphous CoO_x nanoparticles with various particles sizes were prepared.The tuning mechanisms of both valence change mechanisms of high-valence cobalt intermediates as well as water oxidation performance were investigated by modulating surface hole concentration.The stabilizing effect of Co⁴⁺intermediates by multi–center structures is proposed.By studying the kinetics of intermediate valence change of amorphous CoO_x nanoparticles with different sizes,the size effect on tuning mechanism of the intermediate valence change kinetics and oxygen evolution activity was proposed.