Electrocatalytic Performance Of Nickel-based Two-dimensional Nanostructures

Hydrogen(H2)is a renewable clean energy source with a variety of energy release methods,including the energy released by the combustion process,the mechanical energy of the thermal energy conversion in the heat engines,and the electricity directly converted in the fuel cell.In recent years,the development of solar energy conversion technology has stimulated the revolution of energy conversion and storage,especially solar-driven electrocatalytic water splitting to achieve efficient and large scale H2 production.However,the practical application of electrocatalytic water splitting to produce hydrogen is still limited by some bottleneck factors.Firstly,the theoretical voltage of water splitting can reach 1.23 V,but the voltage required for practical operating environment is higher,which mainly originates from the slow kinetics of anodic oxygen evolution reaction(OER),which leads to low energy conversion efficiency and high overpotential and higher energy consumption in the whole electrolysis reaction.Secondly,oxygen(O2)is the anode product in the process of water splitting,which has low economic value and is more easily obtained from air.Finally,If the O2 generated at the anode is not well isolated from the H2 generated at the cathode,it is very easy to explode.Hence,it is a key to optimizing the anodic reaction to establish a more efficient or/and more valuable water electrolysis system.In addition,researchers have successively developed a series of two-dimensional(2D)materials,such as transition metal oxides,sulfides and other layered materials.These materials have attracted widespread attention from researchers due to their high specific surface area and highly active edge atoms.In view of the above problems,this paper proposes the strategies for catalytic water splitting based on 2D materials with high catalytic activity as follows:(1)Polyethylenimine modified nickel phosphide nanosheets:Interfacial proton boosts hydrogen evolution reaction.Based on the interface engineering strategy,multi-branched polyethyleneimine(PEI)modified layered NiP2(PEI@NiP2-CC)was prepared due to the strong Ni-N bond interaction.PEI molecules are firmly anchored on the surface of the NiP2.In the acidic and neutral environments,the protonation of the-NH2 groups of PEI molecules increase the proton concentration at the catalyst/electrolyte interface.The topgallant molecular weight(M.W.)modified PEI@NiP2-CC nanohybrids processed only 44 mV and 100 mV overpotentials at a current density of 10 mA cm-2,which made great progress for HER activity comparing to the the unmodified NiP2-CC nanohybrids.This part of work provides a solid basis for the correlation of surface modification with catalytic activity.(2)Direct growth of holey Fe3O4 doped Ni(OH)2 sheets on nickel foam for oxygen evolution reaction,A mixed-cyanogel hydrolysis method makes it possible to synthesize porous and ultrathin Fe3O4 doped nickel hydroxide nanosheets(abbreviated as Ni(OH)2-Fe H-STs)with only about 1 nm thickness.The2D layered Ni(OH)2-Fe H-STs with extremely high surface area,abundant unsaturated coordination atoms and a large number of nanopores become the right hand for the electrocatalytic reaction.Meanwhile,the introduction of Fe increases the electrical conductivity of the material and regulates the electronic structure of Ni.Due to the special structural characteristics of 2D materials and the synergistic effect between Fe/Ni atoms,the Ni(OH)2-Fe H-STs with the optimal iron content exhibited excellent OER activity in 1 M KOH solution,far exceeding the commercial RuO2 nanoparticles.In addition,Ni(OH)2-Fe H-STs can be loaded on nickel foam(NF)substrate with further enhanced activity compared to the material without NF,with an overpotential as low as 200 mV at 10 mA cm-2 and a Tafel slope of 56 mv dec-1.The one-step synthesis of porous atomic thickness 2D materials provides new ideas for both energy storage and electrocatalysis.(3)Atomically thick Ni(OH)2 nanomeshes for urea electrooxidation.A strategy of a small organic molecule oxidation reaction urea oxidation reaction(UOR)as an alternative for OER to reduce the voltage required for overall water splitting was proposed.The one-step method for the direct synthesis of atomically thick nickel hydroxide nanomeshes(Ni(OH)2-NMs)can overcome the drawbacks of post-etching.Various physical characterizations indicate that the synthesized Ni(OH)2-NMs process the thickness of 1.7 nm,high surface area,abundant nanopores structure,and a large number of low coordination number surface/edge atoms.Research has revealed Ni(OH)2-NMs exhibited better electrocatalytic performance for urea oxidation than pore-free Ni(OH)2 nanoparticles with lower onset oxidation potential,faster reaction kinetics and higher quality activity.This strategy further reduces the voltage of water electrolysis,greatly improves the production efficiency of H2 and energy utilization,which expands a new orientation for electrocatalytic water splitting.(4)Benzylamine oxidation boosted electrochemical water-splitting:hydrogen and benzonitrile co-production at ultra-thin Ni2P nanomeshes grown on NF.Ultrathin Ni2P nanomeshes on NF substrate(denoted as Ni2P-UNMs/NF)applied to catalyze benzylamine oxidation reaction(BOR)elevating electrochemical water splitting.Ni2P-UNMs/NF with only 1.9 nm thickness was are successfully achieved through phosphidation treatment using ultra-thin Ni(OH)2 nanomeshes on NF substrate(denoted as Ni(OH)2-UNMs/NF)as reaction precursor.Ni2P-UNMs/NF nanocomposites show the outstanding activity for both hydrogen evolution reaction(HER)and benzylamine oxidation reaction(BOR)in alkaline electrolyte.Importantly,Ni2P-UNMs/NF nanocomposites can directly act as a bifunctional electrocatalyst for the electrochemical water-splitting in two-electrode system in the presence of benzylamine,which only requires an electrolysis voltage of 1.41 V to achieve the 10 mA cm-2 in alkaline electrolyte,accompanying with H2 production at cathode and value-added benzonitrile production at anode.Obviously,the BOR boosted electrochemical water-splitting provides an energy-saving and cost-competitive H2 production method.It provides a basis for the transition metal-based 2D materials to assistant water electrolysis by small molecular oxidation,and provides a new idea for the wide application of transition metal-based substituted noble metal-based catalysts in water electrolysis.