Designing And Water Electrolysis Properties Of Transition Metal Based Self-supported Electrodes

With the depletion of fossil fuels and the growing environmental crisis,the prospect of hydrogen energy from water electrolysis is expected.Although platinum-based materials exhibit superior hydrogen evolution activity,the high cost and scarcity of Pt largely suppress their widespread application.So,design of high-active,inexpensive transition metal based hydrogen-evolution catalysts plays a vital role in the development of sustainable hydrogen energy system.Powder catalysts often require time-consuming film coating process with the help of extra polymeric binders,which will reduce electrical conductivity and stability as well as increase mass transfer resistance.However,the self-supported electrode often has a large specific surface area,rapid electrolyte diffusion and gas release capacity,high stability,and it is easy to construct a hierarchical three-dimensional structure.On this basis,this thesis focuses on the design and preparation of transition metal compound based self-supported electrodes,and explores their potential applications in the field of electrocatalytic hydrogen evolution.The research optimizes the transition metal based self-supported electrolyzed water electrodes from three aspects:catalytic activity,stability and cost.The research contents in this thesis include the following parts:1.To expose active sites and increase catalytic activity,a nickel phosphide nanowire array/nickel foam?Ni-P NA/NF?self-supporting electrode was prepared by the direct phosphorization treatment of commercial Ni foam at low temperature.Its morphology and structural components were analyzed in detail.The three-dimensional conductive Ni skeleton combined with nickel phosphide nanowires array ensures the large specific surface area,rapid electrolyte diffusion path and good mechanical stability of Ni-P NA/NF electrode.When used as a bifunctional water splitting electrode,the Ni-P NA/NF exhibits outstanding electrocatalytic activity with a low cell voltage of 1.69 V to drive current density of 10 mA cm^-2,and maintains its high catalytic activity at least 20 h.In addition,the method in this chapter is also suitable for preparing other phosphide self-supported electrode.2.The above-prepared three-dimensional array structure fully exposes the active sites,but phosphide nanowires have poor conductivity in the vertical direction and are not conducive to charge transfer.In this work,carbon tubes array is first prepared by using ZnO as template.Then,NiMo alloy nanosheets are grown on carbon tubes by hydrothermal method and hydrogen reduction to obtain a hierarchical carbon fiber cloth/carbon tubes/NiMo alloy nanosheets?CF@CT-NiMo NS?self-supported electrode.Benefiting from the catbon tubes array with increased exposure and accessibility of active sites,improved vectorial electron transport capability and enhanced release of gaseous products,the CF@CT-NiMo NS electrode exhibits outstanding electrocatalytic activity with very low overpotentials of 41 mV and 261 mV to drive a current density of 20 mA cm^-2 toward hydrogen-evolution and oxygen-evolution reaction,respectively.Furthermore,the CF@CT-NiMo NS can be used as a high-performance bifunctional water splitting electrode.3.The active materials in abovementioned electrodes are directly exposed to the electrolyte and are easily eroded.In order to improve the stability of catalyst,carbon-coated structure is designed in this work.Using ionic liquids as an ink,a series of transition metal phosphide?TMP?nanoparticles encapsulated in N,P co-doped carbon supported on carbon fiber cloth?CF@NPC-TMP,TM=Mo,Co,Ni,Fe?electrodes are prepared on a large scale using inkjet printing technology.The in situ carbon nanolayers,derived from ionic liquids,not only prevent MoP nanoparticles from sintering at high temperature to expose abundant catalytically active sites but also allow rapid electron transport to the catalytically active sites during hydrogen-evolution reaction.Furthermore,carbon nanolayers can effectively prevent the TMP nanoparticles from corroding to enhance their stability.The as-prepared CF@NPC-TMP catalysts were highly active toward HER in both acidic and alkaline media.Remarkably,the CF@NPC-MoP catalyst presented the highest current density at the same potential and required overpotentials of only 87 and 71 mV to drive a current density of 10 mA cm^-2 in 0.5 M H₂SO₄ and 1.0 M KOH,respectively.In addition,this electrode can continuously operate at a constant current density of 10 mA cm^-2 for 20 h with negligible rise in operating potential.4.To avoid the use of expensive carbon cloth and reduce the cost of the catalyst,In this work,we propose a very simple,straightforward and cost-effective method to prepare self-supported biocarbon fiber?BCF?cloth decorated with molybdenum carbide nanoparticles?BCF/Mo₂C?electrode,and explore its catalytic performance in hydrogen-evolution reaction.The Mo₂C nanoparticles,as active material for hydrogen-evolution reaction,not only enhance the wettability but also improve the electrical conductivity of as-prepared biocarbon fiber cloth.When directly used as an integrated three-dimensional water splitting cathode,the BCF/Mo₂C electrode exhibits outstanding electrocatalytic activity with very low overpotentials of 115 mV and 88 mV to drive a current density of 20 mA cm^-2 in acidic and alkaline media,respectively.In addition,it can continuously work for 50 h with little decrease in the cathodic current density in both acidic and alkaline media.Furthermore,tungsten carbide-and vanadium carbide-based self-supported electrodes can be prepared by using the method in this chapter.