Synthesis Of Transition-metal Catalysts And Study Of Hydrogen Evolution Reaction In Electrocatalysis

For current society,it is urgent to get rid of the environmental pollution and energy crisis brought by tranditional fossil fuels.Hydrogen,a clean energy with feature of sustainability,storability,is a promising substitute for fossile fules.Electrochemical water splitting is a friendly and efficient way for hydrogen production.Moreover,noble metal catalysts are the most active materials for water splitting;however,the high cost and scarcity limited its wide applications.Transition-metal catalysts with low cost,abundant and better chemical activity,appeal to researchers.To make the reactivity of transition-metal catalysts compatable to noble metals,it is necessary to manipulate the structure and chemical composition of materials.NiFe LDH is the most active transition-metal catalyst toward OER in alkaline media.However,its HER activity in the same solution is not satisfactory,which is bad for overall water splitting.In addition,transition-metal phosphides are the most active catalysts toward HER,especially in acid solution.But its activity toward HER in alkaline solution remains poor compared to noble metal catalysts.To accommodate the practical applications,the activity of NiFe LDH and phosphides toward HER in alkaline media should be improved.In recent years,defect engineering has become an important strategy to improve the HER performance of transition-metal catalysts owing to the higher reaction kinetics.The flexible convertion of Ce betweent Ce³⁺and Ce⁴⁺in different chemical environment gives a possibility for high electron/ion conductivity,and production of more oxygen vacancie.As a result,CeO₂ often emerges as a reaction promoter in electrocatalytic process.Here,composite materials of NiFe LDH/CeO_x and NiCo₂P/PFV_O-CeO_x have been prepared via hydrothermal,electrodeposition and annealing method.Defect engineering has been introduced in composite materials to improvie the HER performance of NiFe LDH and NiCo₂P in alkaline solution.The main research contents are shown as following:1)Oxygen vacancy has been introduced via electrodepositing CeO_x nanoparticles on NiFe LDH nanosheets.The influences of electrodeposite time on the amount of oxygen vacancies and the influences of oxygen vacancies on the activity of NF@NiFe LDH/CeO_x toward HER in alkaline solution are investigated;The formation mechanism of oxygen vacancies at the interface of NiFe LDH/CeO_x is investigated by density functional theory?DFT?calculation.In addition,the improved factors are also researched through electrochemical tests,which shew that the different of electron potential of NiFe LDH and CeO₂ lead to the emerge of oxygen vacancies.The amount of oxygen vacancies of NF@NiFe LDH/CeO_x after 10 min electrodeposition is the highest and the catalysts activity toward HER is the best.The overpotential of NF@NiFe LDH/CeO_x is 154 mV at the current density of 10 mA cm^-2,along with better stability.2)Catalysts NiCo₂P/PFV_O-CeO_xwith P atoms filling oxygen vacancies of CeO_x have been fabricated via electrodepositing CeO_x on NiCo₂-OH nanocones followed by phisophide at 300°C.The influences of solide-state P source amount in annealing process on the phase of NiCo₂P,the morphology,structure and filling amount of NiCo₂P/PFV_O-CeO_x are investigated.The influence of P atoms filler content on the HER performance of NiCo₂P/PFV_O-CeO_x is also researched through electrochemical tests.Results shew that there are few connections in the amount of NaH₂PO₂·H₂O with the phase of NiCo₂P,the morphology of NiCo₂P/PFV_O-CeO_x.However,the more NaH₂PO₂·H₂O were used,the more amount of P atoms filled in the oxygen vacancies of CeO_x.When 0.3 g NaH₂PO₂·H₂O were used,the catalyst of NiCo₂P^0.3/P^0.3FV_O-CeO_x possesses the best reactivity toward HER in 1.0 M KOH solution.A current density of10 mA cm^-2 is driven by 33.6 mV,and the Tafel slope is 61.24 mV dec^-1,with better long cycle durability.