Construction Of Cationic Doped Transition Metal-based Electrocatalysts And Their Catalytic Performance For Oxygen Evolution

In order to cope with the environmental crisis caused by carbon emissions and environmental pollution,China’s 13th Five-Year Plan explicitly proposed that renewable energy accounts for 15%and 20%of primary energy consumption by 2020and 2030 respectively.Therefore,renewable energy represented by solar energy,water energy and wind energy has become a major strategic measure for the global energy transformation and the realization of the goal of addressing climate change.Water electrolysis technology produces high-purity hydrogen using electricity generated from renewable energy,thus avoiding environmental pollution caused by traditional fossil fules.Electrolytic water splitting can be divided into two half reactions of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.OER involves the transfer of four electrons and requires high overpotential,which greatly limits the development of electrolytic water technology.Therefore,the development of OER catalysts with high efficiency and high stabalbility is essental for the development of the upstream industry of hydrogen energy.In recent years,transition metal-based compounds have been shown high electrocatalytic activity for OER.Fe-Co-Ni-based catalysts have shown great potential for OER,because of their environmental friendliness,abundant reserves and thermal stability.Addtionaly,the combination of iron,cobalt and nickel and the doping of foreign metals can effectively adjust the electronic structure,optimize the surface energy and reaction kinetics,and theoretically improve the electrocatalytic activity greatly.To improve the ralatively lower catalytic performance of single metal compound,Co Se/Fe Se bimetal catalyst was designed and synthesized by high temperature organic phase method,in which the molar ratios of Co and Fe also were adjusted.Although no electron interaction between Co and Fe was observed in the nanocomposites,the presence of Fe Se could significantly influence the electrochemical oxidation of Co Se.Further electrochemical studies revealed that catalyst with the optimal metal ratio of Co Fe_0.7Se_1.7 showed an ultra-low initial overpotential of 1.511V vs.RHE and a small Tafel slope of 43.90m V dec^-1.The electro-catalytic performance of Co Fe_0.7Se_1.7 at a high current density of 100 m A cm^-2 is higher than that of commmercial Ru O₂.The improved catalytic performance of Co Se/Fe Se is resulted from the synergistic effects of Fe and Co.Although transition metal-based selenides have been widely studied for OER catalysis,copper-based selenides have rarely been reported.Therefore,a nano-composite of Cu₂Se@Co Se was synthesized using a colloidal method.This synthetic approach was capable for the synthesis of Cu₂Se,Co Se and Cu₂Se@Co Se.The nano-composite of Cu₂Se@Co Se exhibited high electrocatalytic OER activity,with low overpotential of 251 m V at 10 m A·cm^-2 and 320 m V at 100 m A·cm^-2,and even beat the Ru O₂ at high current density(i.e.>80 m A·cm^-2).Long-term electrolysis test and post-electrolysis experimental analyses indicated that Co OOH/Cu（OH）₂ on Cu₂Se@Co Se played an important role in OER electrocatalysis,which were readily oxidized from Cu⁺in Cu₂Se and Co²⁺in Co Se before the onset potential of OH oxidation.The Co OOH/Cu（OH）₂ served as both catalytic material and protective layer of Cu₂Se@Co Se,thus leading to enhanced OER catalytic activity and stability.It is believed that Cu₂Se@Co Se can be a potential candidate for fabricating sustainable OER electrocatalytic electrode.To clarify the mechanism of improved OER performance caused by different metal ion doping on the same selenides.Ni-Fe co-doped orthotropic Co Se₂ mesoporous microspheres were designed and synthesized by colloidal synthetic method and their electrocatalytic properties were systematically studied.The results show that both Ni doping and Fe doping can significantly improve the catalytic activity of Co Se₂,but in different ways.Electrocatalytic characterization and density functional theory calculation show that Ni doping does not change the Co active site and OER pathway,but reduces the energy barrier of rate limitied step by modifying the electronic state around the Co active site.In contrast,Fe doping changes the active site from Co to Fe,and the rate-limiting step is moved to the O-OH formation stage,thus optimizing the reaction kinetics.These findings were further confirmed by Ni-Fe co-doping Co Se₂with different Ni and Fe contents.The optimized Ni_0.04Fe_0.16Co_0.8Se₂ showed OER activity,with an overpotential of 230 m V under 10 m A cm^-2 and a Tafel slope of 39 m V dec^-1.To improve OER performance of Ni Fe LDH,Ta doped Ni Fe LDH with OER high catalytic activity and stability was synthesized by one-step hydrothermal method.Ta doping leads to lattice expansion and electron structure change in LDH as well as significant electron transfer from Fe to Ta.DFT calculations further verified that the modulated electronic structure among Ni,Fe and Ta and the modified eg orbital of Ta induced by charge transfer are beneficial for the adsorption of OH species on Ta site in Ta-doped Ni Fe LDH,increasing the intrinsic metallic property of Ni Fe LDH.Consequently,the Ta site has lower overpotential compared with other sites on Ni Fe LDH including the pristine oxygen vacancies,which can improve the electrocatalytic activity for OER.Furthermore,the optimized Ta-Ni Fe LDH（0.5:6:1.5）exhibited a superior OER activity in contrast to bimetallic LDH,with a low overpotential of 260m V at the current density of 50 m A·cm^-2 and a small Tafel slope of 58.95 m V·dec^-1.This work provides the theoretical basis for the enhancement of electrochemical OER activity by doping LDH with high-valence state foreign metal.