Application Of Transition Metal Sulfides In Electrocatalysis

People’s increasing demand for energy has led to the rapid consumption of fossil fuels and brought about the outbreak of environmental problems.The pressure from energy and the environment drives people to seek new energy and new ways of energy storage and transformation.Among many types of energy storage and conversion,energy conversion and storage through electrocatalysis has been recognized as one of the feasible and efficient ways.The four major reactions related to water in electrocatalysis,including hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,oxygen reduction reaction（ORR）and hydrogen oxidation reaction（HOR）,are widely used in electrolysis cells,fuel cells,metal Air batteries have become a research hotspot in recent years.Currently,noble metals such as IrO₂/RuO₂ and Pt-based catalysts are considered to have the best activities for OER,HER,and ORR,respectively.However,the high cost and poor stability of these precious metals hinder their large-scale commercial applications.In recent years,researchers have been searching for efficient non-precious metal bifunctional catalysts.For example,various materials such as transition metal sulfides,phosphides,borides,and carbides have been extensively studied.Transition metal sulfides exhibit excellent catalytic activity as monofunctional or multifunctional catalysts,which can be attributed to the high thermodynamic stability and rich valence distribution of metal sulfides.Generally,atoms that are not fully coordinated at the M-S edge have high catalytic activity,which is the result of improved catalytic activity.However,metal sulfides still have some deficiencies,such as poor electrical conductivity and easy aggregation.Based on this,we have done the following work:1.Ni₃S₂-CoMoS_x was grown on the surface of Ni₃S₂-CoMoS_x by using the three-dimensional porous structure of nickel mesh to obtain Ni₃S₂-CoMoS_x/NF composite material with nickel foam（NF）as the substrate.In this work,CoMo-MOF was first prepared and co-sulfided with NF as a precursor.During the vulcanization process,needle-like Ni₃S₂ grows uniformly on the NF surface.The CoMo-MOF precursor further formed a CoMoS_x dodecahedral structure dispersed in acicular Ni₃S₂ Electrocatalytic tests found that there was an obvious synergistic effect between CoMoS_x,Ni₃S₂ and NF,resulting in a more excellent bifunctional catalytic activity.The overpotentials of Ni₃S₂-CoMoS_x/NF catalysts are only 234 mV and 90 mV at a current density of 10 mA cm^-2.In the total water splitting test,using Ni₃S₂-CoMoS_x/NF as the cathode/anode two-stage catalyst,a low voltage of 1.52 V was obtained at a current density of 10 mA cm^-2,and it was stable for more than 65 hours.2.Firstly,the capsule-shaped NiCo-PVP precursor was synthesized and vulcanized.During the vulcanization process,NiCo₂NS with hollow capsule structure was obtained by the self-sacrificial templating method.In this experiment,NiCo-PVP precursor reacted with urea to first obtain NiConitride.During the subsequent vulcanization,S is doped into the nitride and replaces part of the N.The doping of S in NiCo₂NS may introduce a large number of defects and vacancies,thereby increasing the number of catalytically active sites.The ORR/OER performance of the finally synthesized NiCo₂NS material is greatly enhanced due to its excellent electrical conductivity,large specific surface area,and a large number of active sites introduced by S doping.At 10 mA cm^-2,NiCo₂NS exhibits an overpotential of 154 mV during OER and a half-wave potential of 0.76 V during ORR.When NiCo₂NS is applied to ZABs as an air electrode catalyst,1200 stable charge-discharge cycles can be performed.The flexible battery assembled with NiCo₂NS as catalyst also showed excellent performance,capable of 60 stable charge-discharge cycles at a current density of 2 mA cm^-2.The flexible battery exhibits excellent flexibility and can maintain good charge/discharge stability when the battery is bent at different angles during cycling.3.We grew CuCo-LDH nanosheets on the surface of NF and further sulfided them.During the vulcanization process of NF,a tapered Ni₃S₂ grows on the surface,which can well disperse the vulcanized LDH and avoid the reduction of active sites caused by the easy agglomeration of transition metal sulfides.Secondly,the sulfided LDH is coated on Ni₃S₂ to form a two-phase interface.The diffusion mechanism between the two phases makes it easier to form catalytic sites at the interface,enhances the synergistic effect of the interaction between the interfaces,and enhances the stability of the material.CuCoS_x-Ni₃S₂ is formed without changing the 3D framework structure of NF.CuCoS_x nanosheets are cross-grown on the tapered Ni₃S₂ to form a two-phase interface,where a large number of defects and oxygen vacancies are formed,providing more active sites for electrocatalysis.CuCoS_x-Ni₃S₂/NF nanomaterials showed excellent performance in HER and OER,with overpotentials of only 158 mV and 130 mV at current density of 10 mA cm^-2,and in the total water splitting test,CuCoS_x-Ni₃S₂/NF acts as an anode-cathode catalyst,which can reach a current density of 10 mA cm^-2 at a voltage of only 1.50 V,and can maintain it stably for more than 80 hours.