Surface Orbital Tuning Of Ni/Fe-Nitrogen Coordination Catalysts For Electrocatalytic Water Splitting

By considering the situation of current world that contend with global energy crisis and serious environmental pollution,electrolytic water drived by renewable electric energy converted from wind and solar energy is undoubtedly an ideal way to produce hydrogen.The process of water electrolysis consists of hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).However,the multi-electron transfer process involved in the reaction limits the reaction kinetics of electrocatalytic decomposition of water.Moreover,the reaction rate is closely related to some properties such as reactant adsorption,intermediate electron transfer,intermediates adsorption,and product desorption on the catalyst surface.So to overcome the energy barrier of the reaction kinetics process,and increasing the conversion rate from intermediates to products,additional higher overpotential is required.The adsorption essence between the catalyst surface and the adsorbed species is that the electron orbitals of the adsorbed sites are properly coupled with those of the adsorbed species.The transition metal nitride catalysts can be considered as a candidate for replacing the traditional noble metal catalysts due to their comparable activity to traditional noble metal catalysts to a certain extent.Furthermore,owing to their low price and abundant reserves,they can promote the large-scale industrial application of electrocatalysts.The purpose of this paper is to develop a new mechanism and method of surface electron orbital regulation by regulating the physical and chemical structure of transition metal catalyst through heterogeneous atomic doping,interface engineering and vacancy engineering.Besides,the single atom structure reconstruction and coordination regulation of materials in the process of catalytic reaction was studied to deeply understand the catalytic mechanism of active sites.At the same time,another target of this paper is to explore the correlation between the local coordination structure and modulation means of transition metal catalysts by using fine structure characterization technology,electrochemical measurement and first-principles calculation.Therefore,the structure-activity relationship between surface electronic configuration and catalytic activity is systematically revealed.The specific contents and achievements are as follows:1.The capability of manipulating the interfacial electronic coupling is the key to achieving on-demand functionalities of catalysts.It is demonstrated that the electronic coupling of Fe2N with high conductivity and comparable activity to traditional noble metal catalyst can be effectively regulated for HER catalysis by vacancy-mediated orbital steering after hydrogen annealing treatment.X-ray photoelectron spectroscopy(XPS)and X-ray absorption fine structure(XAFS)analysis reveals that the electronic and coordination states of Fe2N can be well manipulated by nitrogen vacancies.Theoretical studies further indicate that the nitrogen vacancy can uniquely steer the orbital orientation of the active sites to tailor the electronic coupling and thus benefit the surface adsorption capability.More importantly,real-time XAFS studies reveal the electronic state evolution of Fe sites during HER catalysis,which could be correlated to the hydronium ion(H3O+)adsorption behavior during catalysis.This work could shed light on the understanding of the catalytic mechanism and further innovate the catalyst design for HER catalysis and beyond.2.The high unoccupied d band energy of Ni₃N basically results in weak orbital coupling with water molecule,consequently leading to slow water dissociation kinetics.Cr doping can downshift the unoccupied d orbitals and strengthen the interfacial orbital coupling to boost the water dissociation kinetics.Refined structural analysis and density functional theory(DFT)calculation further confirm the downshifted d band energy can strengthen the orbital coupling between the unpaired electrons in O 2p and the unoccupied state of Ni 3d,which thus facilitates the water adsorption and dissociation.Moreover,the in-situ segregated nickel metal sites can also facilitate the final H*desorption from the catalyst surface and achieve a synergistic effect for alkaline HER catalysis.This work offers a powerful platform to achieve on-demand catalytic surface by manipulating the orbital coupling between the active sites and the related reactant species.3.Investigating the problems of unclear catalytic sites and unclear catalytic mechanism caused by structural remodeling during the OER catalytic process.The mechanism of catalyst reconstruction during the alkaline OER process was studied by synthesized Ni single atom supported on nitrogen doped carbon.The changes in Ni content during the OER process were detected by the inductively coupled plasma-atomic emission spectrometry(ICP-AES)method,electrochemical test,and solution corrosion.The results prove that the sample is reconstructed during the alkaline OER process and moreover,the reconstructed sample is not stable when immersed in the acid solution.The XPS,XAFS and electron energy loss spectroscopy(EELS)characterizations showed that the reconstructed active center is combined with oxygen atoms,which exhibites excellent alkaline OER catalytic activity and stability.This work provides a new perspective to understanding the mechanism of alkaline OER catalysis,and to guidance for the rational design of OER catalyst and beyond.