Electrocatalytic Behavior And Microscopic Reaction Mechanism Of Nickel-based Electrocatalysts For Watersplitting

Sustainable and renewable energy sources and related technologies are receiving increasing attention.Hydrogen has proven its potential as an ideal energy carrier towards the sustainable energy economy because of its high gravimetric energy density and zero emission during consumption.Watersplitting is currently the most effective and green large-scale hydrogen production technology.The development of low-cost,high-efficiency and high-stability non-noble metal-based electrocatalysts is an urgent problem to be solved in the current research.This dissertation selectes nickle-based electrocatalysts as research object.Based on the relationship between catalytic activity,surface reactivity and reaction path,the surface and interface regulation strategy of nickel-based electrocatalysts are explored.Combined with theoretical calculations and experiments,the catalytic activity and stability of nickle-based electrocatalysts are optimized by interfacial synergy,hierarchical morphology and doping.The theoretical guided surface and interface engineering strategies provide in-deepth understanding of catalytic reaction mechanism and design guide.The main contents are as following:Firstly,the common interfacial synergy effect of alkaline hydrogen evolution is extensively achieved.The hybrid Ni（OH）₂/MoS₂ electrocatalyst exhibits enhanced HER catalytic performance with an onset overpotential of 20 mV and a Tafel slope of 60 mV/dec.Furthermore,the combining of electrochemical tests and Density Functional Theory calculations shed more light on the synergy effect of Ni（OH）₂/MoS₂ interface.The charge transfer from Ni（OH）₂ to the MoS₂ leads to a more optimal hydrogen binding free energies value on S-edge of MoS₂.And the interfacial synergistic effect regulates the reaction path,reduces the energy barriers both of the initial water dissociation step.Secondly,the relationship between surface adsorption thermodynamics and reaction kinetics of alkaline hydrogen evolution is represented.It is found that the balance of H^*adsorption and OH^*adsorption is required for highly efficent alkaline hydrogen evolution.Then,the surface reactivities of common nickel-based electrocatalysts are investigated.The results show that the composite structure of low-valent nickel-based materials and molybdenum-based nitride may be potential HER electrocatalysts.Based on this prediction,the Ni-Ni_0.2Mo_0.8N heterostructure with good intrinsic electrical conductivity and high specific surface area has been synthesized.This heterostructure exhibits an extremely excellent alkaline HER activity and it only requires an overpotential of 87 mV to deliver a cathodic current density of 500 mA/cm².Furthermore,theoretical calculations and microkinetic analysis reveal the hydrogen evolution pathway and mechanism of the heterostructure.In addition,by combining with Ni/Ni_0.2Mo_0.8N composites with NiFe-LDH,an alkaline electrolyzers is assembled and exhibites highly effecient water splitting activity and stability.Thirdly,the doping method based on the above-mentioned dual-descriptors is carried out,which emphasizes the importance of hydroxyl adsorption for alkaline hydrogen evolution.Theoretical results show the unfavorable hydrogen adsorption on the the surface of Ni₃N.High valance-state doping of Mo,W and V can effectively reduce the center of the d-band of Ni₃N,leading to improved hydrogen adsorption strength.At the same time,Mo doping can maintain suitable hydroxyl adsorption strength,while W and V doping leads to unfavorable hydroxyl adsorption.Furthermore,the undoped and doped Ni₃N are successfully synthesized.The electrochemical test results are in good agreement with the theoretical expectation.The Mo-Ni₃N exhibits excellent alkaline HER electrocatalytic activity with an overpotential of 12 mV at 10 mA/cm²,a Tafel slope of 64 mV/dec and good stability,while the W doped and V doped lead to sluggish kinetics.Further microkinetic analysis results are consistent with theoretical expectations,and W and V doping introduce a large Volmer step activation energy,resulting in sluggish kinetics.At last,high-efficiency and high-stability water-splitting electrocatalysts is easily prepared at room temperature.The NiCuMo alloy is synthesized by simple electrodeposition method.The introduction of Cu can reduce the amount of Mo and improve the electrocatalytic activity of the NiMo alloy from both the microstructure and the intrinsic activity.The NiCuMo alloy exhibits an excellent alkaline HER electrocatalytic activity with an overpotential of 43 mV at 50 mA/cm² and a Tafel slope of 53 mV/dec.Besides,the NiCuMo alloy can stably work for more than 100 h at a large current of 500 mA/cm².Further DFT calculations and microkinetic analysis reveal the reaction pathway and reaction mechanism.The NiCuMo alloy reaches the balance of hydrogen adsorption and hydroxyl adsorption,leading to satisfactory reaction kinetics.In addition,the Ni₉₃Ce₇O_xH_y is also synthesized via electrodeposition method,and it exhibits excellent oxygen evolution activity.By combining with NiCuMo alloy,an alkaline electrolyzer is assembled and exhibites highly effecient water splitting activity and stability.The operating current density of 500 mA/cm²can be achieved at a cell voltage of 1.64 V,and maintains over 100 h.This work has built high-efficiency,high-stability,low-cost Ni-based water splitting electrocatalysts and proves that nickel-based electrocatalysts have application potential in water splitting.