Uncovering The Impact Of Oxygen Defects On Oxygen Evolution Reaction Activity Of Transition Metal Oxides

Developing high performance electro-catalysts for oxygen evolution reaction?OER?are critical for many electrochemical energy conversion systems.Low-cost transition metal oxides?TMO?are widely investigated as promising OER catalysts.Their performances,however,are still inferior to those of the currently used noble metal catalysts.In recent years,oxygen defects were reported to critically impact the electro-catalytic activity of TMO.Despite many successful examples,it is still challenging to introduce oxygen defects into TMO controllably,and the role of oxygen defect in determining the OER performance is still not well understood.The objective of this thesis is to understand the role of oxygen vacancies in determining OER performance of TMOs,and to apply defects engineering for achieving high performance OER electro-catalysts.Transition metal oxides thin film grown by pulsed laser deposition was used as the model systems.Oxygen vacancies were introduced to TMOs by Ar plasma treatments or Ar thermal treatments.The impacts of oxygen vacancies on OER activity for TMOs were investigated by the combination of electrochemical test and surface characterization techniques.Simultaneously,oxygen defect engineering was applied to prepare practical OER electro-catalysts composed of Sr Co_0.8Fe_0.2O_3-?perovskite powder and carbon fiber paper.Firstly,we systematically investigate the correlation between the concentration of oxygen vacancies and OER activity by using a Ni O thin film model system grown by pulsed laser deposition.Oxygen vacancies are introduced into Ni O thin film by Ar plasma treatments with different power?50 W,100 W,and 150 W?.The OER activities of Ni O thin films are found to be significantly enhanced after plasma treatment.The Ni O thin film subjected to 50 W Ar plasma treatment shows 5.8 times higher current density at 1.7 V vs.reversible hydrogen electrode?RHE?than that for the pristine film.The improvement in electrochemical OER performance is attributed to the change of Ni valance states and surface adsorption properties,as confirmed by X-ray photoelectron spectroscopy and contact angle measurements.Our results demonstrate that plasma treatment as promising method for rational design of high performance electro-catalysts through oxygen defect engineering.Besides Ar plasma treatments,Ar thermal treatments at medium temperatures were applied to introduce a small quantity of oxygen vacancies into La_0.7Sr_0.3Co O_3-?perovskite thin film.The La_0.7Sr_0.3Co O_3-?perovskite thin film after 300°C thermal treatment showed enhanced OER activity while maintaining the surface morphology.In addition,oxygen vacancies were introduced into composite consists of Sr Co_0.8Fe_0.2O_3-?perovskite powder and carbon fiber paper by electro-chemical reduction.The composite subjected to-1.0 V?vs.Ag/Ag Cl?treatment for 10 min showed the best OER activity,and the potential can stay stable for 12 h at a current density of 10 m A/cm².