Structural Regulation Of Ruthenium/Iridium Oxides For Efficiently Electrocatalytic Water Oxidation In Acidic Media

Hydrogen production by proton exchange membranes（PEM）water electrolysis is considered a promising technology for hydrogen production due to its high current density,high hydrogen purity,and better adaptation to volatile renewable energy,such as solar and wind.However,the kinetics of oxygen evolution reaction（OER）at the anode of water electrolysis is slow,which largely determines the efficiency of PEM water electrolysis.Therefore,it is urgent to develop efficient electrocatalysts to reduce the reaction energy barrier and promote the OER reaction.Currently,IrO₂ is the only catalyst used in the devices of commercial PEM water electrolysis,however,the high price and low earth reserves limit the large-scale application of Ir,which hinders the commercialization process of PEM water electrolysis technology.Therefore,reducing the usage of Ir and improving the intrinsic activity of Ir-based electrocatalysts as well as exploring alternative catalysts are very important for the development of PEM water electrolysis but full of challenges.The price of Ru is much cheaper than Ir,meanwhile,the OER activity of RuO₂ is higher than that of IrO₂,but the stability is poor than IrO₂.In this dissertation,aiming at the problems of Ru/Ir oxides in acidic OER,the strategies of the construction of heterojunction,defect designing,heteroatomic doping,and morphological designing were developed to regulate the structure of Ru-and Ir-based oxides.The catalytic activity and stability of the prepared series of catalysts for OER in acidic media were evaluated.The internal relationship between structure and OER performance was preliminarily established,which provides a theoretical basis and practical support for the design of high performance and low cost OER electrocatalysts in acid media.The main points of this dissertation are summarized as follows:（1）Construction of Ru/RuO₂-Co₃O₄ heterojunction catalyst and its electrocatalytic performance toward oxygen evolution reaction（OER）in acidic medium.A Ru/RuO₂-Co₃O₄ heterojunction catalyst was synthesized by pyrolysis of the Ru³⁺modified ZIF-67precursor under air atmosphere.The carbon layer generated by the pyrolysis of ZIF-67framework was coated on the Ru/RuO₂-Co₃O₄,which enhances not only the electrical conductivity,but also the acid corrosion resistance of the composites.The deeply structural characterizations involved in X-ray photoelectron energy spectroscopy（XPS）and X-ray absorption spectroscopy（XAS）indicate that there are interface charge transfers between Co and Ru species.Co₃O₄,as an electron donor,supplies electrons to adjacent RuO₂,which reduces the valence state of Ru and weakens the covalence of Ru-O.This behavior will help to inhibit the dissolution of Ru during the OER process and thus improve stability.At the same time,the charge transfer between Co and Ru will promote the intrinsic catalytic activity of Co base components.Benefiting from the synergy of these aspects,the required overpotential of Ru/RuO₂-Co₃O₄ is only 226 m V to deliver the current density is 10 m A cm^-2 in 0.1 mol L^-1 HCl O₄ electrolyte,Ru/RuO₂-Co₃O₄ shows excellent stability with an increase in overpotential of less than 0.22 V after continuous testing for 19 h,which is much better than those of commercial RuO₂ and most of recently reported catalysts.（2）Synthesis of single atom Zn doped RuO₂ electrocatalyst and its electrocatalytic performance toward oxygen evolution reaction（OER）in acidic medium.A single-atom Zn-doped RuO₂（SA Zn-RuO₂）with enriched oxygen vacancies was developed by in situ thermal decomposition of Ru Zn-MOF derivatives.The catalyst showes excellent catalytic activity of OER in acidic solution.SA Zn-RuO₂ exhibits excellent catalytic activity with a lower overpotential of 213 m V at the current density of 10 m A cm^-2 and a 6-fold improvement in durability than commercial RuO₂.As demonstrated by results of X-ray absorption spectroscopy and electrochemical measurements,the Zn single-atom doping regulates the charge distribution of RuO₂ and lowers the valence state of Ru,which inhibits the over-oxidation of Ru in the OER process.Moreover,the newly formed Zn-O-Ru local structure effectively stabilizes the surface Ru atoms,synergistically improving long-term stability.Besides,the introduction of oxygen vacancies reduces the adsorption strength of reaction intermediates and thus enhances the OER activity.（3）Synthesis of Nd-doped Ir/IrO₂ hollow nanospheres and their electrocatalytic performance for OER in acidic solution.An Nd-Ir/IrO₂ hollow catalyst was prepared by the method of liquid phase combined with low-temperature solid phase synthesis using a carbon sphere as a template.As confirmed by the electrochemical measurements,the Nd-Ir/IrO₂ only requires potential of 1.489 V vs.RHE to achieve the current density of 10 m A cm^-2 in 0.1 mol L^-1 HCl O₄ electrolyte and exhibits a low increase of 50 m V in overpotential after continuous V-t testing for 23 h,which both superior to Ir/IrO₂ and commercial IrO₂.Advanced characterization results including of X-ray photoelectron spectroscopy（XPS）,X-ray near-edge structure spectroscopy（XANES）and X-ray extended fine structure spectroscopy（EXAFS）reveal that the electronic structure of Ir is regulated by Nd doping and the valence state of Ir is lowered.Meanwhile,there are many oxygen vacancies produced by Nd doping,which synergistically contribute to the enhancement of the catalytic activity and stability of Nd-Ir/IrO₂ catalyst for OER in acidic electrolyte.Moreover,the hollow structure not only helps to expose more active sites,but also improves the utilization rate of noble metal and cut down the cost of catalyst.