Indentifing Active Species Of NiCo₂O₄ Nanorod Arrays For Electrocatalytic Water Oxidation

With the aggravation of energy crisis and environmental issue,exploring sustainable clean energy has become an urgent task for humanity.Electrocatalytic water splitting is a promising approach to obtain hydrogen energy because of its simple process and high efficiency.Generally,electrocatalytic oxygen evolution reaction(OER)is a strong oxidation process that usually drives the self-oxidation of electrocatalytic materials,leading to structural transformation of catalytic materials into real active species.Recently,the influence of photothermal effect on OER activity has attracted wide attention.However,everyone mainly understands the role of temperature from a kinetic perspective,and there is still little research on the regulation of electrocatalytic active species by temperature.In this thesis,self-supported Ni Co₂O₄spinel oxide nanorod arrays were selected as the model material.In situ characterization methods such as electrochemical in situ Raman spectroscopy and online X-ray absorption spectroscopy(XAS)were used to study the active species of Ni Co₂O₄in the oxygen evolution reaction.Addtionally,the effect of temperature on the active species was discussed.The main results are as follows:At room temperature(25?),the active species of Ni Co₂O₄spinel oxide for OER is Ni Co₂O₄itself,rather than Ni(Co)oxyhydroxides that are usually observed for most materials.When the temperature of the electrolyte(1 M KOH)is increased to45°C,the active species of Ni Co₂O₄is converted into Ni(Co)oxyhydroxides.Impressively,the combined data of in situ Raman and online X-ray absorption spectroscopy(XAS)revealed that the temperature-regulated transformation of active species is reversible,and the reversible transformation can be repeated through consecutively varying the temperature.The active species of Ni(Co)oxyhydroxides can be converted back into Ni Co₂O₄spinel oxide when the temperature decreases back to 25°C.And,the results of density functional theory calculations show that the rate-limiting steps are different in the temperature-induced change of active species in Ni Co₂O₄,and Ni OOH may own lower overpotential for OER than Ni Co₂O₄.Therefore,this work not only provides a new understanding for the active species in the oxygen evolution reaction process of materials,but also develops a unique method for adjusting active species for designing other high-efficiency electrocatalysts.