Construction Of Ni,Co And Fe Based Compound Electrode Materials By Dealloying And Vapor Phosphorization/Sulfuration And Studying Their Catalytic Performances

With the rapid development of world economy,energy consumption is increasing day by day,and environmental pollution is becoming more and more serious at the same time.Therefore,the exploitation of clean,safe,efficient and economic renewable energy is the direction of scientific research in the future.In recent years,many environment-friendly energy storage carriers,such as electrolysis of water for hydrogen production,fuel cells and metal-air batteries,have been developed rapidly.In order to improve the reaction rate involved in the internal process,a high-efficient and stable catalyst is necessary.At present,the best catalyst materials are mainly precious metals,but their high prices and limited reserves make them impossible to be popular.In this paper,by means of vacuum melting,rapid solidification technology,de-alloying and phosphating/sulfuring treatment,efficient and cheap electrocatalysts are designed to reduce the energy barrier in the process of hydrogen evolution,oxygen evolution or oxygen reduction reaction,so as to improve the efficiency of self-powered electrolysis of water.The main research contents are as follows.?1?Al₈₅Ni₁₅ binary alloy is prepared by vacuum induction melting furnace.The sacrificial component Al can be dissolved?de-alloyed?in NaOH solution with a certain concentration.By adjusting the de-alloying time?20,40,60 min?,self-supported nanoporous Ni?Al?precursor with different pore sizes could be obtained.The porous product is placed in phosphorous vapor for high temperature annealing,and finally the nanosheet structures are grown on the surface of porous channels.X-ray diffraction?XRD?,scanning electron microscope?SEM?and electrochemical test results show that when the de-alloying time is 40 min,the formed nanoporous Ni?Al?is mainly composed of Al₃Ni phase,and is the most sensitive to phosphorous vapor at high temperature.Therefore,nanosheet structures with large specific surface areas could be in-situ constructed.The P-np-Ni?Al?-40 electrode only needs 105 mV overpotential when HER current density reaches 10 mA cm^-2 in acidic environment,and it also possesses excellent stability and durability.It can be seen that the porous framework and nanosheet structure,obtained by dealloying and vapor phosphorization,have significant effect on hydrogen evolution activity of self-supported electrode materials.?2?The vacuum melting and rapid solidification?melt spinning?technology are adopted to introduce Co elements with different contents into the Al-Ni alloy system.After completed de-alloying and phosphating treatment,the P-Ni_xCo_15-x?x=5,7.5,10?powder electrodes are obtained.Through SEM and transmission electron microscope?TEM?,it can be found that the P-Ni₅Co₁₀ presents leaf-like structure and has a secondary nano-structure inside,which could dramatically increase the specific surface area of electrode material.The electrochemical results measured in alkaline electrolyte indicate that P-Ni₅Co₁₀ possesses better electrocatalytic activities for both OER(?₁₀=420 mV)and ORR(E_1/2=0.7 V).In addition,the catalyst also has methanol tolerance and is a superior OER-ORR bifunctional electrode material.?3?Based on the above research,we have improved it.Firstly,the Al₉₀Co₁₀binary alloy ingot is prepared,and the nanoporous Co?Al?precursor is obtained by slicing,grinding and de-alloying.Then,it has been placed in sulfur vapor for high temperature annealing.Finally,the nanowire structures are in-situ grown on the surfaces of porous channels.XRD results reveal that the nanoporous Co?Al?precursor is mainly composed of Al₉Co₂ intermetallic compound phase,but only CoS₂ phase can be detected after sulfidizing,indicating that the Al component has in-situ replaced partial Co sites in the form of doping.By means of SEM,X-ray photoelectron spectroscopy?XPS?and inductively coupled plasma spectrometer?ICP?,it can be found that the most Al elements in Al₉Co₂ phase could be in-situ doped into CoS₂ lattice and promote the dense growth of nanowires on the pore surface,which greatly increase the specific surface area of the electrode material.The electrochemical properties of catalysts are evaluated in acidic media.Thereinto,the self-supported Al-CoS₂ electrode only needs 86 mV overpotential when HER current density reaches 10 mA cm^-2,and it also possesses excellent stability and durability.According to the simulation of HER process by density functional theory?DFT?calculation,Al doping can not only promote the activity of Co site,but also stimulate the activity of inert S site,thereby significantly reducing the Gibbs free energy of hydrogen adsorption(?G_H*),contributing to the large amount of hydrogen production.?4?In order to further improve the electrochemical properties and application range of materials,Fe element is introduced into Al-Co alloy system,and the S-Co_xFe_10-x?x=2.5,5,7.5?composite catalysts are synthesized through completed de-alloying and sulfuration.By means of XRD and SEM,it's found that?CoFe??S₂?₂ sulfide electrode with nano-flake structure could be acquired at x=5?namely S-Co₅Fe₅?.The electrochemical test results manifest that S-Co₅Fe₅possesses the best catalytic activity for both HER(?₁₀=161 mV)and OER(?₁₀=300 mV).Taking S-Co₅Fe₅ as both anode and cathode catalysts for overall water splitting,it only needs 1.35 V to start whole system reaction and 1.62 V to afford the current density of 10 mA cm^-2.Furthermore,this catalyst has eminent ORR performance(E_1/2=0.79 V),methanol resistance and long-term stability.Using it as cathodic catalyst of Zn-air battery,the open-circuit voltage is 1.46 V and power density is as high as 179 mW cm^-2.Two batteries in series can not only successfully light the LED,but also realize self-powered electrolysis of water for hydrogen and oxygen production.Finally,the Zn-Air?S-Co₅Fe₅?battery as a secondary battery could afford a discharge voltage of 1.28 V and a charge voltage of 2.05 V with a voltage gap of 0.77 V.After 250 charging-discharging cycles,the potential gap only increases by 0.03 V,proving a high cycling stability for charging-discharging.To sum up,S-Co₅Fe₅ is an excellent trifunctional?HER-OER-ORR?electrocatalyst.