Design,Synthesis And Application Of High Efficiency Transition Metal Phosphide Electrocatalyst

With the decrease of fossil energy reserves and the increasing environmental problems,there is an urgent need to develop a clean and sustainable energy source instead of traditional fossil energy.The hydrogen energy has the advantages of high combustion calorific value,no pollution of products and abundant raw materials.The hydrogen energy has been regarded as one of the best alternative sources of fossil energy.Electrocatalytic splitting of water is an efficient and feasible clean hydrogen production method.However,due to the slow kinetics of producing hydrogen,it requires high overpotential than the theoretical voltage,leading to high energy consumption.Therefore,the use of appropriate catalysts in the electrolytic water process reduces overpotential and improves the energy conversion efficiency.At present,precious metals,such as Pt and RuO2/IrO2,are widely recognized as the fastest reaction kinetics and high current density electrocatalysts for water splitting.However,the high cost and scarcity hinder the commercial application of these noble metals.Therefore,exploitation of inexpensive,stable,feedstock-rich non-precious metal electrocatalysts with excellent water splitting properties is challenging and meaningful for the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).In past years,some transition-metals and their compounds,such as oxides,nitrides,sulfides,phosphides and so on,have been used to replace noble metals for electrochemical water splitting.Specially,the transition-metal phosphides(TMPs),such as the similar structure to the hydrogenase,abundant raw materials,good electrical conductivity and so on,are widely used as alternatives to noble metals for HER or OER.Based on the analysis of the research status of non-noble metal catalyst electrolytic water at home and abroad,this paper designed,prepared and explores efficient,inexpensive and stable transition metal phosphide catalyst for hydrogen evolution reaction and oxygen evolution reaction.The catalytic activity and the number of active sites of transition metal phosphide catalysts can be regulated by effective strategies to obtain efficient electrolytic water efficiency,which can greatly promote the development of electrolytic electrochemical technology.The specific research contents and summary are as follows:(1)Three-dimensional flower tungsten phosphide(WP2)catalyst(WP2/NF)was grown in situ on nickel foam(NF)by hydrothermal and phosphating methods.The electrochemical performance test shows that the current densities of the 3D nanostructured WP2 NW/NF electrode achieved 10 mA cm-2 at lower overpotential of 130 and 311 mV for hydrogen evolution reaction and oxygen evolution reaction,and the corresponding Tafel slopes are 94 and 67 mV dec-1,respectively.The prepared catalyst serves as the cathode and anode,respectively,and the potential for total water dissolution is 1.65 V.The stability of the catalyst after 5000 cycles is evaluated by after 5000 cycles test,and the results show the overpotential growth rates of hydrogen evolution and oxygen evolution are 5.3%and 8.6%,respectively.The morphology and crystal structure of the 3D nanostructured catalyst maintained good stability after 5000 cycles and thus its catalytic properties also maintained good stability.According to electrochemical impedance spectroscopy analysis,the 3D flower-like WP2/NF has a smaller impedance,indicating that the material exhibits faster charge transfer kinetics.Besides,since this 3D nanostructured electrocatalysts(WP2 NW/NF)can expose the abundant active sites,and promote electrolyte osmosis to shorten the electron transfer path,the WP2 NW/NF electrode shows excellent electrocatalytic performance.(2)A self-supporting Ni2P/PANI hybrid electrocatalysts supported on the Ni foam was successfully prepared by hydrothermal,low temperature phosphating and polymerization(PANI/Ni2P-NF).The nitrogen atom in the amino group on polyaniline has a lone pair of electrons,which can effectively trap hydrogen protons(H+)from the hydrated ion to form a protonated amino group,which makes the hydrogen protons(H+)have a higher positive charge density on the electrode surface,resulting in proton hydrogen(H+)fast kinetics,low overpotential and superior catalytic performance for HER.The electrochemical performance shows that in 0.5 H2SO4,the overpotential of hydrogen evolution only needs 83 V to obtain a current density of 10 mA Cm-2,and the Tafel slope is 69 mV dec-1.After 5000 cycles of CV cycles,the overpotential of the catalyst only increases by 4.6%.Under constant potential,the long-term timing is 100000 s,and the current density line is almost unchanged,indicating that the catalyst has good stability.Impedance spectroscopy shows that the catalyst has a small impedance of 4.1 ?,and the PANI/Ni2P-NF composite catalyst has good conductivity and fast kinetics.(3)A self-supporting PANI/CoNiP composite catalysts were successfully synthesized by hydrothermal,phosphating and polymerization(PANI/CoNiP-NF).The electrochemical performance shows that the as-synthesized nanostructured PANI/CoNiP-NF electrocatalyst exhibits outstanding catalytic activity in 0.5 H2SO4,achieving 10 mA cm-2current density at a lower overpotential of 61 mV,and the Tafel slope is 80 mV dec-1.After 5000 cycles of CV cycles,the overpotential of the catalyst only increases by 4.9%.Under constant potential,the long-term timing is 864000s,the current density line is almost unchanged,and the morphology,crystal structure and valence state of the catalyst are maintained well,indicating that the catalyst has good stability.Impedance spectroscopy shows that the catalyst has a smaller impedance of 3.7 ?,indicating that the PANI/CoNiP-NF composite catalyst exhibits good conductivity and faster hydrogen adsorption kinetics.Density functional theory calculations proved that the introduction of a third metal atom optimizes the electronic structure of the catalyst,so that the d electron center of the catalyst is far away from the Fermi level,resulting in weakening of the interaction between the catalyst surface and hydrogen atoms,and the PANI/CoNiP-NF composite catalyst The adsorption energy between hydrogen atoms is close to zero,and it has good catalytic performance.In addition,the amino nitrogen atom of the polyaniline in the PANI/CoNiP-NF composite catalyst has a lone pair of electrons,which effectively captures the hydrogen protons(H+)in the electrolyte to form protonated amino groups,which makes the positive charge density of the catalyst surface higher than that of hydrated ions,which makes protons It is easier to accept electrons from the cathode,thereby reducing the overpotential.(4)A kind of 3D N-doped CoP nanowires array catalyst in-suit grown on carbon cloth(CC)was successfully prepared by in-situ phosphating strategy(N-CoP/CC).Urea is used to decompose NH3 at high temperature as a nitrogen source,and non-metallic nitrogen atoms with greater electronegativity are introduced into CoP to optimize the electronic structure of CoP.Electrochemical tests have shown that hydrogen evolution at 0.5 M H2SO4 and 1 M KOH requires overpotentials of 27 and 64 mV to reach a current density of 10 mA cm-2,and the corresponding Tafel slopes are 58 and 67 mV dec-1;1 M KOH oxygen evolution,the overpotential required to reach a current density of 10 mA cm-2 is 247 mV,and the Tafel slope is 55 mV dec-1.The prepared N-CoP/CC catalyst is used as the cathode and anode respectively,and the electric potential required to electrolyze water is 1.47 V,which is lower than commercial platinum carbon(Pt/C)and ruthenium dioxide(RuO2),which are divided into cathode and anode for water electrolysis.After 5000 CV cycles,hydrogen evolution at 0.5 M H2SO4 and 1 M KOH increased the overpotential of the catalyst by 1.3%and 7.8%,respectively;and oxygen evolution at 1 M KOH increased the overpotential of the catalyst by 4.2%.Under constant potential,the current density line is almost unchanged at 100000 s,and the morphology and crystal structure of the catalyst are maintained well,indicating that the catalyst has good stability.Impedance spectroscopy shows that the catalyst has a small impedance of 3.6 ?,and the material exhibits faster charge transfer kinetics.Theoretical calculations prove that the nitrogen atom is doped with CoP and the d electron center is far away from the Fermi energy,which weakens the interaction between the surface of the catalyst and the hydrogen atom,promotes the balance of hydrogen adsorption and desorption of the catalyst,and has good catalytic performance.(5)Ni2P nanoparticles anchored on N,P-codoped porous carbon(Ni2P@N,P-C)hybrids catalysts are successfully synthesized by the self-assembly strategy.The use of phytic acid with high phosphorus content is rich in PO4 groups,and the PO4 groups will be complexed with metal ions,and will be pyrolyzed to prepare phosphides at an appropriate reaction temperature.At the same time,phosphorus and nitrogen dope with carbon to coat the catalyst.The electrochemical performance shows that in a 1 M KOH solution,to obtain a current density of 10 mA cm-2,the hydrogen evolution and oxygen evolution overpotentials are required to be 98 and 300 mV,respectively.The prepared catalyst serves as the cathode and anode,respectively,and the potential for total water dissolution is 1.72 V.It has good stability.After 3000 cycles,the polarization potential does not drop significantly,and the morphology of the catalyst does not change significantly.Impedance spectroscopy analysis shows that the material exhibits faster charge transfer kinetics.Nitrogen and phosphorus non-metallic elements doped carbon to activate the electronic structure of carbon and improve catalytic activity.The carbon-based material is coated with Ni2P to enhance the conductivity of carbon,resulting in good performance of the catalyst.