Fabrication Of Three Dimensional Nickel Based Hvdroxide For Efficient Electrocatalvtic Oxygen Evolution

With the development of human society,the energy and environmental crisis are becoming more and more serious.As a promising technology,electrocatalytic water splitting to produce hydrogen and oxygen is expected to slove the above-mentioned problems and achieve the large-scale utilization of hydrogen energy.However,limited by the high energy consumption,electrocatalytic water splitting is currently difficult to satisfy the need of large-scale application.Compared with the hydrogen production reaction(HER)occurring at the cathode,the oxygen production reaction(OER)occurring at the anode,is more complicated and requires a four-electron transfer process.By reducing the overpotential of OER,the overall energy consumption of the reaction can be effectively reduced.At present,the widely used OER electrocatalysts are mainly commercial noble-metal iridium based catalysts.However,the rare reserves of elements and high price restrict their wide range of applications.Non-noble metal based catalysts were expected to replace the noble-metal-based catalysts due to their low cost and favorable stability.Based on this,a series of high-performance non-noble metal-based OER electrocatalysts were prepared in this thesis,and their corresponding structure-activity relationships as well as the changes of surface morphology and structure during electrocatalytic water splitting process were studied.The main contents are as follows:(1)Nitrogen doped α-Ni(OH)2 nanoparticles were in situ synthesized on nickel foam by gas phase hydrothermal method.The introduction of nitrogen significantly increased the electrochemical active surface area of the catalyst and reduced the charge resistance at the reaction interface.Besides,nitrogen doping endowed the catalyst with the enhanced gas-repellent property which made the generated oxygen escaped the surface of the catalyst more quickly and thus accelerated the mass transport during the electrocatalysis.The as-prepared catalyst exhibited excellent OER catalytic performance under alkaline solution,which showed a low OER overpotential of 170 mV and 670 mV at 10 mA cm-2 and 1000 mA cm-2 respectively,as well as the Tafel slope of 159.4 mV dec-1.Moreover,the catalyst presented an excellent stability in the chronopotentiometric measurement at 500 mA cm-2 after a long period of 10 h test.(2)Iron was further introduced into nitrogen-doped α-Ni(OH)2 through chemical impregnation.The introduction of iron led to new catalytical active sites,and optimized the electronic structure of catalyst which thus resulted in the enhanced the electrocatalytic activity of OER.Through the precise adjustment of reaction temperature,reaction time and concentration of Fe(NO3)3,the preparation conditions and structures of the catalysts were optimized.The catalyst showed an overpotential of only 146 and 362 mV at the current density of 10 mA cm-2 and 1000 mA cm-2 respectively,as well as a Tafel slope of 66.4 mV dec-1.Besides,after a long period chronopotentiometric measurement of 16 h at current density of 500 mA cm-2 and 1000 mA cm-2,no significant catalytic performance decay was observed.(3)By comparing the morphology of the catalyst before and after OER process,the anodic oxidation on the surface of catalyst could be clearly observed.Under high oxidative potential,the Ni(OH)2 on the surface of the nitrogen-doped Ni(OH)2 electrode was gradually oxidized to NiOOH,and during OER process the active sites of catalyst were NiOOH.For iron and nitrogen co-doped Ni(OH)2 materials,Ni(OH)2 was also oxidized to NiOOH,and the iron was oxidized to high-charged iron ions,indicating that the active sites of the catalyst are the anodized components.