Preparation Of Non-noble Transition Metal Nanomaterials And Their Applications For Electrocatalytic Water Splitting

To cope with the rapid growth of energy consumption,such as fossil fuel burning and oil over-exploitation,and alleviate the associated environmental issues,renewable and clean energy sources have attracted immense research interest recently.Among these renewable and clean energy sources,hydrogen energy presents the promising alternative to these traditional fossil fuels that can partially support demand of world for clean energy.The electrolytic process of water contains two half-reactions,（hydrogen evolution reaction（HER）,2H⁺（aq）+2e^-→H₂（g））and（oxygen evolution reaction（OER）,2H₂O（l）→4e^-+4H⁺（aq）+O₂（g））.However,because of the inevitable dynamic overpotentials of HER and OER,the electrolysis efficiency is severely limited.At present,although noble metals such as Pt,IrO₂ and RuO₂ have good catalytic activity for HER or OER,the low reserve of precious metals on the earth and their high cost limit their wide application.Therefore,exploring inexpensive,active,and stable non-precious metal catalyst with good electrocatalytic performance could be the focus for further study.Transition metal phosphides（TMPs）and transition metal selenides（TMSes）are well-known HER catalysts due to their moderate affinity to hydrogen and appropriate adsorption of the H atom and desorption of hydrogen.In addition,the formation of metal oxo/hydroxo species on the catalyst surface which can create a heterostructure show high efficient catalytic performance for OER.However,the property of electron transition of TMPs and TMSes is worse than that of carbon materials and metal materials,which limits some steps taken place in chemical reaction,resulting bad performance.Therefore,seeking an effective method to improve the conductivity may contribute to better performance.In this work,the compounds of Co_0.5Ni_0.5P/nitrogen-doped carbon/nickel foam(Co_0.5Ni_0.5P/NC/NF)and different ratios of nickel cobalt selenide/nitrogen-doped carbon layer（CoNiSe/NC-x）were synthesized by simple and environmentally-friendly methods,and then performance of composite materials for water splitting was also researched in detail.The content of research includes the following two parts:1.Co_0.5Ni_0.5P/NC/NF was synthesized by hydrothermal process and calcination.Other samples with different mass ratios of Co and Ni（3:0,2:1,1:1,1:2 and 0:3）were also synthesized in the similar way to optimize electrocatalytic performance.The results of the characterizations indicate that nickel foam with the property of good conductivity could improve the capacity of electron transition.In addition,hydrogen and oxygen could be released through abundant channel in time,which contributes to stability of catalysis.As expected,overpotentials for HER and OER are just 90 and 230mV vs.RHE in 1 M KOH.For overall water splitting,with the potential of 1.49 V vs.RHE,the current density could reach 20 mA cm^-2.2.Different samples of mass ratios（3:0,2:1,1:1,1:2,3:0）of Co and Ni were synthesized by hydrothermal process and calcination for the research about how the catalysis with varying content of Ni effect the conductivity of materials.The samples with/without the encapsulation of N-doped carbon layer could be used to study how carbon layer effect conductivity.Characterization results reveal that although CoSe could catalyze HER and OER with good activity,electron transition capacity is poor and could be oxidized easily.With theoptimum doping of Ni,electron transition capacity could be improved effectively and encapsulation of N-doped carbon layer could defend catalysis from oxidation resulting in good stability.In 1.0 M KOH,CoNiSe/NC-3（1:1）delivers the current density of 10 mA cm^-2 at overpotentials of 100and 270 mV vs.RHE for HER and OER respectively.When used for overall water splitting,current density of 20 mA cm^-2 could be got with the potential of 1.65 V vs.RHE.