Fabrication Of Nickel-based Catalysts For Hydrogen Production From Alkaline Water Electrolysis

Electrocatalytic water splitting for hydrogen generation powered by renewable energy source has been recognized as an eco-friendly sustainable approach to effectively address the environmental crisis and the excessive consumption of conventional energy sources.However,the practical application of water electrolysis is severely hampered by high-cost and scarcity of precious metal materials and high-energy consumption.Therefore,designing efficient and robust non-precious metal materials and developing energy-saving electrolytic configuration are highly desired.Owing to their moderate affinities towards the oxygen-containing intermediates（*OH,*O and*OOH）during the OER process,nickel-based catalysts which are prospective to achieve the activities of Ru/Ir-based catalysts,have drawn extensive attention.Whilst considerable research progress has been obtained,their OER activities and stability still face challenges.Furthermore,the HER activities of nickel-based catalysts are suffering from the sluggish water dissociation kinetics and stronger Ni-H bonding.Motivated by the above concerns,Co Ni N@Ni Fe LDH,V-Fe Ni₃N/Ni₃N,VO_x/Ni/Ni₃N,and Ni-Mo-N are fabricated via interfacial engineering and doping.It is expected to achieve an efficiently stable electrochemical water-hydrogen conversion with low-energy consumption.（1）The efficiency of water electrolysis suffers from the sluggish kinetics and high overpotential of OER.Although Ni Fe LDH is regarded as a promising OER catalyst,its poor conductivity result in the limitations of HER performance.Therefore,a hybrid nano-architecture of Co Ni N@Ni Fe LDH as a bifunctional electrocatalyst for overall water splitting（OWS）was fabricated via in-situ electrochemically assembling hierarchical Ni Fe LDH nanosheets on the surface of Co Ni N.Co Ni N@Ni Fe LDH showed excellent activities for OER with the overpotentials of 227 and 291 m V to deliver the current densities of 10 and 100 m A cm^-2,respectively.Moreover,when Co Ni N@Ni Fe LDH was used as anode and cathode,respectively,an alkaline electrolyzer required a cell voltage of 1.63 V for reaching 10 m A cm^-2.In addition,the excellent stability with 100 h towards OWS outperforms most transition metal-based bifunctional electrocatalysts.The superior performance is attributed to the abundant active sites,the strong electronic interaction,and the corresponding metal（oxy）hydroxides species generated by in situ surface reconstruction and phase transformation.（2）V-doped bimetallic nitrides heterostructure Fe Ni₃N/Ni₃N was synthesized via a hydrothermal-nitridation protocol and used as electrocatalysts for water and urea electrolysis.V-Fe Ni₃N/Ni₃N electrode exhibited superior HER and OER activities.Moreover,when V-Fe Ni₃N/Ni₃N electrode was used as anode and cathode,respectively,a two-electrode device required a voltage of 1.54 V to reach a current density of 10 m A cm^-2 with outstanding stability over 120 h of continuous electrolysis,surpassing most previously reported electrocatalysts.The superior performance is predominantly attributed to the synergistic effects of V-doping and interfacial engineering,which could enhance water adsorption and modulate the adsorption/desorption of intermediate species,thereby boosting the kinetic processes of HER and OER.Besides,a urea-assisted two-electrode electrolyzer was designed,a cell voltage for reaching the same current density was 80 m V lower than that of conventional water electrolysis.（3）A heterostructure catalyst VO_x/Ni/Ni₃N grown in on carbon cloth was successfully synthesized by introducing vanadium oxides and regulating nitridation temperature.The catalyst showed remarkable activities and long-term durability with a lower overpotential of 46 m V to obtain 10 m A cm^-2 for HER and the potentials of 1.38V and 44 m V at 100 m A cm^-2 towards urea oxidation reaction（UOR）and hydrazine oxidation reaction（Hz OR）,respectively.Such excellent activities were derived from the modification of the Ni/Ni₃N electronic structure by vanadium oxides and the abundant active sites formed at the amorphous/crystalline interface.Furthermore,when VO_x/Ni/Ni₃N is used as the cathode and anode of a urea/hydrazine-assisted electrolyzer,respectively,cell voltages of 1.40 V and 92 m V were required to drive a current density of 10 m A cm^-2.（4）The sluggish kinetics of anodic water oxidation severely inhibits electrocatalytic water splitting for hydrogen generation.Moreover,the economic value of the anodic product（O₂）is lower.Replacing the OER process with the thermodynamically favorable electrooxidation reaction of biomass derivatives can simultaneously generate H₂ at the cathode and high value-added chemicals at the anode.Herein,a heterostructure Ni-Mo-N bifunctional catalyst was synthesized for HER and5-hydroxymethylfurfural oxidation（HMFOR）.Benefiting from the abundant interfaces,fast electron transfer capability and intrinsic activity,it exhibited superior alkaline HER and HMFOR activities and stabilities.A lower overpotential of 61 m V was required at10 m A cm^-2 for HER.For HMFOR,when the Faraday efficiency was 83.3%,a nearly complete conversion of HMF（99.9%）was obtained with a yield of 99.6%for 2,5-furandicarboxylic acid（FDCA）at 1.45 V（vs RHE）.Moreover,an electrolyzer employing the Ni-Mo-N as a bifunctional catalyst towards the reduction of water to H₂at the cathode and the oxidation of HMF to FDCA at the anode required only a low cell voltage of 1.435 V to drive 10 m A cm^-2.