Microstructure Regulation And Electrocatalytic Water Splitting Performance Of Oxide-Based Nanocomposites

Due to the fast depletion of traditional fossil fuels and the increasing green house effect caused by the excess emission of CO₂,the search for renewable energies as alternatives becomes more urgent.Among them,hydrogen is one of the most promosing candidates for large-scale application owing to its easier accessibility and carbon-neutral properties.Electrocatalytic water splitting is the key technique for hydrogen production,which can be separated into hydrogen evolution reaction（HER）at the cathode and oxygen evolution reaction（OER）at the anode.Choosing proper catalysts for cathode and anode to reduce the overpotential for electrocatalytic water splitting is an effective strategy to minimize the energy consumption.However,the catalysts used currently are still the effective but also high-cost noble metal materials.The development of more active and stable catalysts based on earth-abundant transition metal（such as Co,Ni,Fe and so on）to reduce the capital cost for electrocatalytic water splitting can further promote its large-scale application.In this dissertation,the researches are aimed to advance the techniques for electrocatalytic water splitting,and find out more practical methods to solve the dilemmas for the HER and OER.Besed on the general understanding on specific characteristics of individual transition metal materials,more active electrocatalysts are designed by constructing nanocomposites.Furthermore,the mechanisms of interface effects and the synertistic effects between two different phases in nanocomposite are investigated.The working environment for electrocatalytic water splitting devices can be in base or in acid.During the alkaline HER,water dissociation is needed,and the catalytic performance of most catalysts is limited due to the lack of proper adsorption sites for hydroxyl group.Thus,Mo^δ+-NiO（δ=4,5 and 6）is introduced with non-nobel metal Ni to enhance the water dissociation ability during a firstly reduction then annealing synthsis procedure.The structural characteristics of Mo^δ+-NiO/Ni（MNN）are confirmed by the transition electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）and X-ray adsorption spectroscopy（XAS）.Further electrochemical characterization reaveal MNN-3（16%）show the best alkalne HER performance.The overpotential to reach current density of 10 m A cm^-2 is only 50 m V,with Tafel slope of 84 m V dec^-1.The bi-active-site mechanism during alkaline HER are indentified after the comparison among the samples with different oxide/metal ratios.According to simulation results from the Density Functional Theory（DFT）,Mo^δ+can modulate the H binding ability of NiO and promote its water dissociation ability,thus Mo^δ+-NiO/Ni show much higher alkaline HER performance.In addition,metallic Ni is also an alkaline OER catalyst.Compared to 2e^-HER process,the 4e^-OER process is a thermaldynamically unfavorable and kinetically slow reaction.Moreover,most of the oxides that are active towards OER show poor conductivity,which significantly limites their catalytic performance.In this dissertation,Ni_xFe_2-xO₄,which has poor conductivity but many types of active sites are combined with Ni,which has good conductivity but limited active sites,to design a Ni_xFe_2-xO₄/Ni nanocomposite through a wet chemical reduction followed by solvothermal treatment.Confirmed by the TEM,XPS and Raman characterizations,the oxide/metal ratio in Ni_xFe_2-xO₄/Ni nanocomposite can be well-tuned by changing the Fe content in the precusors.The sample prepared with 15%Fe has the optimized oxide/metal ratio and close contact interface,thus delivers the best alkaline OER performance,with small overpotential of 225 m V to reach current density of 10 m A cm^-2 and Tafel slope of 44m V dec^-1.In comparison to alkaline environment,the highly corrosive condiction in acid greatly increases the complexity for catalyst design.Especially,most of the non-noble transition metal oxides show much large overpotential toward acidic OER.Among them,Co₃O₄ shows good catalytic activity and stability toward acidic OER,while the overpotential is still larger than that in base,and the understanding on the intermediates and reaction mechanism in acid is also very limited.The introdcutin of CeO₂ with Co₃O₄can get Co₃O₄/CeO₂ nanocomposite,which reduces the overpotential by～85 m V.The electronic modification from CeO₂ would alter the local bonding environment of Co₃O₄and endow it with much flexible local structure.Combinig the decent eletrochemcial characterizations such as kinetic isotope effect（KIE）,p H-and temperature dependence analyses,with spectroscopy characterizations including XAS and in situ Raman,the enhancement mechanism in Co₃O₄/CeO₂ are systematically studied.The changes in local structure due to the electronic modification can regulates the redox properties of Co₃O₄,making the Co³⁺in Co₃O₄/CeO₂ nanocomposite to be easier oxidized into the active Co⁴⁺,without charge accumulation process that leads to lattice contraction.The strategy developed herein that imporves the acidic OER performance by regulating the redox properties provides insight into the design of more efficient OER catalysts in future.