Controlled Preparation Of Ni-based Oxides/Phosphides For Electrocatalytic Water Splitting And Urea Electrolysis

It is urgent to seek new energy with high-efficient,environmentally friendly and recyclable due to the energy shortage and environmental pollution restricting the development of human society.Hydrogen production from electrocatalytic water splitting is regarded as a friendly,efficient and potential technology for storing sustainable energy sources,which consists of two half-reactions:cathodic hydrogen evolution reaction（HER）and anodic oxygen evolution reaction（OER）.The kinetics of both electrochemical reactions under typical operation conditions is inherently slow,which necessitates the use of catalysts with high activity and stability to reduce the overall electricity consumption.Especially,the OER involves more complex 4e^-process with slow sluggish kinetics.To reduce the energy consumption of hydrogen production,urea oxidation reaction（UOR）is expected to replace the anodic OER from water splitting because of the far-lower theoretical thermodynamic potential（0.37 V）,compared with that of OER with 1.23 V.Furthermore,the effective treatment of urea wastewater can reduce environmental pollution.Although urea is thermodynamically more oxidized,the process involves 6e^-transfer with slow kinetics.Therefore,it is of great signicificance to develop a class of multifunctional electrocatalysts with high efficiency in catalyzing UOR and HER to purify urea wastewater and produce hydrogen,which is conducive to the development of the field of overall water splitting.For the most potential nickel based materials with problems,such as less active sites of surface activation,strong adsorption of intermediates and poor stability,a series of Ni-based oxides/phosphides with low overpotentials and high durability are designed and prepared.The performance of these electrocatalysts on the water splitting and urea electrolysis is systematically investigated.In this thesis,we have carried out the following researches:（1）Porous nickel（p-Ni）with an abundant porous structure was firstly electrodeposited on Ni substrate by controlling electrodeposition conditions.Then,a large number of Ni（OH）₂ nanosheets were generated on the p-Ni surface by hydrothermal self-oxidation,which can further increase the surface area of the catalyst to expose more active sites.Finally,Ni（OH）_x/p-Ni（5 m A）was obtained by in-situ anodic oxidition.The results reveal that the in-situ anodic oxidition could effectively promote the structural reconstruction of the catalyst compared with Ni（OH）₂/p-Ni,achieving the pre-oxidation process from Ni²⁺to Ni³⁺.Meanwhile,the generated abundant metal-oxygen bond（Ni-O）facilitates the oxidation rate of electrode,thus promoting the overall water splitting and urea electrolysis.The Ni（OH）_x/p-Ni（5 m A）as a tri-functional electrocatalyst shows excellent electrocatalytic performance in HER,OER and UOR,with a potential of 0.218,1.609and 1.38 V at a current density of 100 m A cm~2.It is thus configured for overall water splitting and urea electrolysis with excellent performance.（2）Although the Ni（OH）₂/p-Ni with rich interface can effectively enhance the catalytic performance of the electrode,the intrinsic activity of the monometallic Ni-based catalyst is low.The formed hydroxides lead to the decrease of conductivity of the electrode.Therefore,based on the above study,Ru-Ni O/p-Ni was prepared by ion-exchange and calcined in muffle furnace.The results show that there is a strong interaction between Ru and Ni after incorporation of Ru,which optimized the charge state of Ru-Ni O/p-Ni and thus enabled the electrode to achieve a rapid reaction in the electrocatalytic reaction.Meanwhile,the presence of Ru can generated abundant Ru-O that facilitates the electrode oxidation rate,thus promoting the overall water splitting and urea electrolysis.The Ru-Ni O/p-Ni shows excellent electrocatalytic performance in HER,OER and UOR,with a potential of 0.127,1.56 and 1.39 V at a current density of 100 m A cm~2.Meanwhile,it can be configured for overall water splitting and urea electrolysis with excellent performance.（3）Compared to electrodeposited p-Ni,nickel foam（NF）with 3D strucutrehas strong bonding and mechanical strength.Therefore,the Ni（OH）₂ nanosheets were directly formed in-situ on the surface of NF by hydrothermal self-oxidation in this section.The Ce was incorporated into the Ni（OH）₂ by ion exchange method,followed by phosphorization to form the Ce-Ni₂P/NF.The presence of Ce will lead to the lattice distortion of Ce-Ni₂P/NF and generates rich defects due to the larger ionic radius of Ce³⁺.In addition,the formation of a large number of oxygen vacancies and"Ce³⁺/Ce⁴⁺redox pairs"accelerate the charge transfer during the electrocatalytic process and provide an effective buffering space for the pre-oxidation process from Ni²⁺to Ni³⁺.The Ce-Ni₂P/NF shows excellent electrocatalytic performance in HER,OER and UOR,with a potential of 0.26,1.73,1.47 V at a current density of 100 m A cm~2.It can be configured for overall water splitting and real urine electrolysis with excellent performance.It is thus beneficial for electrolytic cells for overall water splitting and coupled urea electrolysis,providing a promising material candidate for wastewater treatment and clean energy production.