Design,Synthesis And Electrocatalytic Properties Of Transition Metal Composite Electrocatalysts Towards Water Splitting

Hydrogen is a clean and efficient secondary energy carrier,and its coupled application with primary renewable energy is expected to fundamentally solve the global problems such as energy shortages and environmental pollution.Splitting-water using electrical energy generated from primary green energy sources such as solar energy,wind energy,and tidal energy,while providing a sustainable hydrogen production method,offers a feasible solution for the effective use of renewable energy.Crucial to enabling to this vision is to develop high-efficient water electrolysis technology.Water splitting involves two half reactions,namely hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.The precious metal Pt and Ir-based,Ru-based oxides are well known for their excellent HER and OER activities,respectively.However,due to scarce resources and high cost,these precious metal catalysts are difficult to be widely used in technology.For decades,researchers from various countries have been actively committed to the development of efficient and low-cost HER/OER catalysts,and have made positive progress in creating new materials,developing preparation technologies,and deepening their understanding of the mechanism.But the catalytic performance of existing catalysts is still far below the practical requirements.Therefore,the development of high-performance,low-cost HER/OER electrocatalysts to reduce the reaction energy consumption is still a key issue that needs to be resolved in the process of promoting the practical application of water electrolysis technology.Based on the current status of that research,this thesis work selects the precious metals Pt,non-precious transition metal phosphides and borides as research objects,focusing on the scientific/technical issues such as design and preparation of the efficient,low-cost water-splitting catalysts and the correlation of catalyst phase composition-structure-performance.The research has made major progress as follows.?1?Under the guidance of the calculation results of density functional theory,a supported Pt/Ni O@Ni/NF nano-composite electrocatalyst capable of efficiently catalyzing alkaline HER was designed and synthesized by employing a combination of synergistic catalysis,surface element modification and nanostructuring strategies.First,a Ni O@Ni film supported on nickel foam were sequentially prepared by electrodeposition and electrochemical etching methods,and then tiny Pt nanoclusters/nanoparticles were introduced by electrostatic adsorption and electrochemical reduction methods.The target catalyst prepared by this method has a Pt content as low as?0.1 mg cm^-2,but its alkaline HER catalytic activity is even better than that of commercial Pt/C catalysts,and it has good durability.This is benefied from the successful construction of a large number of Pt/Ni O synergistic catalytic active sites and the good conductivity of catalyst materials.From the perspective of improving the utilization efficiency of precious metals,this work has prepared a HER catalyst with high catalytic performance and low material cost by a simple method,which provides a feasible solution for the development of cost-effective electrocatalysts.?2?A 3D hierarchical nanostructured Ni2-_xMo_xP/Ni Mo O₄-_y/NF electrocatalyst supported on nickel foam was prepared using hydrothermal and subsequent phosphating methods,in which the Ni2-_xMo_xP was dispersed on the oxygen-defective Ni Mo O₄-_y nanosheet substrate in the form of tiny nanoparticles.DFT calculations show that Ni Mo O₄-_y can effectively promote the dissociation of water and the nearby Ni2-_xMo_xP facilitates the adsorption and desorption of hydrogen atoms.The combination of the two species can synergistically catalyze alkaline HER.The hierarchical structure of the catalyst facilitates the exposure of abundance of active sites and improves the mass transfer kinetics of reactant/product.In addition,the introduction of a large number of oxygen defects in the substrate oxide lattice can effectively improve the conductivity of the catalyst.As a consequence of the above factors,the Ni2-_xMo_xP/Ni Mo O₄-_y/NF electrocatalyst exhibits excellent HER activity and stability in alkaline media.It only requires an overpotential of?36 m V to achieve a current density of 10m A cm^-2 and its comprehensive performance ranks among the advanced level of similar catalysts.This work used a simple synthesis method to simultaneously achieve improved intrinsic activity,increased number of active sites and modifed accessibility,and improved catalyst conductivity,highlighting the importance of multi-element catalyst design and the feasibility of method realization.?3?An amorphous Co B catalyst supported on nickel foam was prepared by a simple electroless plating method,and its performance for alkaline water electrolysis was studied.Combining the use of the techniques such as structural characterization,spectral analysis and chemical state analysis,it was revealed that the surface phase change of the Co B catalyst in the alkaline OER process resulted in the active Co OOH,thus exhibiting good OER activity.In the other hand,it was found that Co B catalyst also has alkaline HER activity.The possible catalytic mechanisms are as follows.First,Co/B atoms undergo electron transfer to form electron-rich metal Co active sites.Second,the surface of the Co B catalyst is partially oxidized to form Co?OH?₂ under HER conditions.The combination of these two species forms synergistic catalytic active site for the alkaline HER.Water electrolysis cell using Co B/NF catalyst couple as both cathode and anode requires a voltage of 1.69 V to afford a current density of 10 m A cm^-2.This work revealed the potential of cheap transition metal borides as bifunctional electrocatalysts for alkaline water splitting.The above research work has designed and synthesized several new types of water-splitting catalysts by combining multiple modification strategies and simple preparation methods.While expanding the research scope of electrocatalytic materials and enhancing the catalytic activity of alkaline HER/OER,it has enriched/deepened the understanding of the internal correlation of phase composition-structure-performance of water-splitting catalysts,and provides an experimental and theoretical basis for the development of efficient and low-cost water-splitting catalysts.