Preparation Of Carbon-based Mul Ticomponent Bifunctional Electrocatalysts And Their Application In Overall Water Splitting

Hydrogen production by electrolysis of water with renewable energy sources,such as solar and wind power,is considered as the most promising hydrogen production technology.Hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）play an important role in the water electrolysis.They are all involved in the complex multi-electron transfer process,making the sluggish kinetics and high energy barrier.Therefore,high-efficiency electrocatalysts are needed to lower the energy barrier and accelerate the reaction.At present,noble metal platinum catalysts are the state-of-the-art HER catalysts,while the iridium-based oxides show high activity for OER.However,their scarcity and high cost greatly prevent the large-scale market applications.In order to settle the above-mentioned problems,a series of carbon-based non-noble metal multicomponent bifunctional electrocatalysts composed of Fe,Ni,W and N,P co-doped carbon are synthesized.The microstructures as well as the bifunctional HER-OER performance of the as-synthesized catalysts are preliminarily studied,then the roles of every component of the catalyst played in the HER and OER processes are analyzed.At last,the relationship between the structure and catalytic performance is established,which will provide the experimental and theoretical support for constructing highly active and low-cost bifunctional electrocatalysts by using the synergistic effect of multi-components.The main achievements are summarized briefly as follows:（1）Preparation of Fe_xNi_4-xP_y/N,P-CNTs composite electrocatalyst and its catalytic performance for HER and OER.The Fe_xNi_4-xP_y/N,P-CNTs catalyst is synthesized by phosphating the precursor obtained by annealing the mixture of melamine and Fe Cl₃,Ni Cl₂.The Fe_xNi_4-xP_y/N,P-CNTs composite catalyst is composed of Fe₃Ni,Fe_3.2Ni_0.8P₄and N,P co-doped carbon nanotubes,in which Fe₃Ni/Fe_3.2Ni_0.8P₄ nanoparticles are encapsulated in the carbon nanotubes or supported on the walls of carbon nanotubes.The effects of annealing temperature and the molar ratio of Fe to Ni on the catalytic temperature of 850℃and Ni/Fe’s molar ratio 1:1 shows the best catalytic performance.In 1.0 M KOH,a low overpotential of 266 m V is required for Fe_xNi_4-xP_y/N,P-CNTs composite catalyst to achieve the benchmark current density of 10 m A cm^-2 and the corresponding Tafel slope is 37 m V dev^-1,preceding that of commercial Ir O₂.Besides,the as-prepared catalyst also exhibits superior activity(η₁₀=269 m V)and stability for HER.When used the Fe_xNi_4-xP_y/N,P-CNTs to assemble the two-electrode water-splitting cell only requires a voltage of 1.73 V to deliver the current density of 10 m A cm^-2.The excellent performance could be attributed to the synergy of 10 m A cm^-2.The superb performance could be attributed to the synergy of surface area,good conductivity,multi-component active sites,and the anchoring effect N,P-CNTs for Fe_xNi_4-xP_y nanoparticles.（2）Preparation of Ni C/WC/W₂Fe₄N@NPC composite electrocatalyst and its catalytic performance for HER and OER.The Ni C/WC/W₂Fe₄N@NPC catalyst is successfully synthesized by a facile pyrolysis-phosphorization process.The effects of annealing temperature and the dosage of W source on the catalytic performance are investigated.The experimental results show that catalyst prepared at an annealing temperature of 750℃and the dosage of WCl₆ of 150 mg shows the best catalytic performance.The Ni C/WC/W₂Fe₄N@NPC exhibits both outstanding HER and OER performance in alkaline solution.For the OER,a low overpotential of 280 m V is required for Ni C/WC/W₂Fe₄N@NPC to achieve the current density of 10 m A cm^-2.The corresponding Tafel slope of 100.9 m V dec^-1,indicating the fast reaction kinetics.After continuous testing at 1.51 V（vs RHE）for 24 h,the activity of OER is well maintained.For the HER,the Ni C/WC/W₂Fe₄N@NPC needs overpotential of 294 m V at 10 m A cm^-2,the corresponding Tafel slopes of is 125 m A dev^-1.No apparent activity attenuation can be found after continuous testing at-0.294 V（vs RHE）for 24 h.The outstanding bifunctional electrocatalytic performance can be attributed to two possible reasons:1)Multicomponent active sites for both OER and HER;2)High conductivity tuned by the carbon materials.Moreover,the carbide and nitride nanoparticles are well dispersed on the N,P co-doped carbon substrate,preventing the agglomeration of metal nanoparticles during the electrochemical processes and thus,resulting in excellent electrochemical stability.