Preparation And Anodic Electro-catalytic Properties Of Transition Metal-based Compounds

Urea-assisted electrochemical water splitting via urea oxidation reaction(UOR)and preparation of efficient OER catalysts could decrease the potential and improve the efficiency of hydrogen production.Up to now,nickel-based metal and alloy,nickel-based hydro/oxide and composites,et al.used as UOR catalysts still encounter the problems of high over-potential,less stability,and low urea electrolysis efficiency,which restrict the practical applications of afore mentioned catalysts.Besides,some controversies about the catalytic activity sites for different catalysts during UOR process are still under debating.The catalytic reaction mechanism of nickel-based catalysts for UOR still needs to be further studied.CoFe catalysts are also confronted with a large over-potential for OER,and the effect of Fe incorporated in CoFe catalysts for OER catalytic activity still needs further study.In order to prepare catalysts with excellent UOR and OER performance,we designed three types of nickel-based catalysts and CoFe catalyst as follows:Ni(OH)₂and MnO₂ hybrids,nickel-based sulfide(NiS_x),and Cu doped NiS.Furthermore,the dynamic mechanism for UOR in perspective of the metal itself oxidation state as well as the changes of oxidation state during UOR process was studied.Meanwhile,we also design and prepare CoFe compounds for OER,and illustrate the role of Fe in influencing OER catalytic activity on CoFe compounds.CFP-Ni(OH)₂/MnO₂ catalysts with the controllable compositions were prepared by coupling MnO₂ nanoparticles with Ni(OH)₂ clusters on the surface of carbon fiber paper(CFP)via ultrasound and electro-deposition.The role of Mn in CFP-Ni(OH)₂/MnO₂ catalysts were systematically investigated in perspective of the changing in the Ni oxidation state and oxidation equivalents during UOR process.The aim of this work is to reveal the relationship between the metal oxidation state and the UOR reaction dynamics.About 100%and 65%of the Ni ion population of CFP-Ni(OH)₂get oxidized during OER and UOR processes,respectively,which is significantly more than observed in CFP-Ni Mn_2.4(about 80%and 35%).It could be concluded that the activity of UOR in CFP-Ni(OH)₂/MnO₂ catalysts is negatively corrected with the capacity of accumulating Ni and Mn oxidation equivalents.The addition of Mn could suppress the capacity of Ni oxidation equivalents,and improve the UOR performance.NiS_x nanoparticles could be synthesized through a one-step solvothermal strategy at the mild condition.The crystallinity,compositions,and oxidation state of Ni could be adjusted by annealing at different temperatures.NiS_x nanoparticles with the lowest crystallinity and the highest Ni oxidation state exhibited the best catalytic activity for UOR.The self-supported NiS_x@NF catalyst which is prepared by in-situ formation of NiS_x on the surface of the nickel foam(NF)could be used in urea-assisted water electrolysis for hydrogen production.The hydrogen production system could provide a current density of 10 m A cm^-2at a cell voltage of 1.52 V,and sustain remarkable stability for 20 h.Cu-NiS@CFP catalyst was synthesized via one-step solvothermal strategy to obtain Cu doped NiS catalysts on CFP.The active site of Cu-NiS@CFP for UOR is Ni OOH,which is formed during the electro-chemical process.The catalytic activity of UOR was improved by Cu doping.Cu-NiS@CFP exhibited better UOR performance than NiS@CFP.The potential for Cu-NiS@CFP to drive a current density of 10 mA cm^-2 was 1.38 V vs.RHE,which was 20 m V lower than that of NiS@CFP.The steady state Tafel slope was 32.4 m V dec^-1,which is 29.0 m V dec^-1smaller than that of NiS@CFP.Moreover,Cu-NiS@CFP exhibited excellent efficiency of urea electrolysis with the highest efficiency of 93.2%and stability for 20h at current density of 10 m A cm^-2 without obvious change.CoFe compounds(Fe x%-Co CH@CM)with controllable Fe content were prepared by one-step hydrotheromal method to synthesize Fe incorporated Co(CO₃)_0.5(OH)·0.11H₂O(Co CH)on Cu mesh(CM).Incorporating moderate amounts of Fe in Fe x%-Co CH@CM could enhance the OER catalytic activity.Fe20%-Co CH@CM exhibited the best OER performance with over-potentials of 350m V at the current density of 10 mA cm^-2.