Design Of Oxygen Evolution Catalysts Used For Water Splitting In Alkaline Media And The Structure-Property Relationship

With the rapid development of science and technology,fossil energy shortage and environmental pollution are the necessary challenges in the rapid development of modern society.Hydrogen energy has a high calorific value and no by-product emissions,which will not cause any pollution to the natural environment.The best solution is to convert new energy sources such as solar energy,wind energy,and tidal energy into electric energy,and then use water electrolysis method to prepare hydrogen,thereby realizing the energy conversion and storage.In addition,the produced oxygen also has great use value in other fields.The mechanism of water splitting is straightforward.Moreover,the corresponding electrolyzer system is much easier to realize large-scale application because of its simple structure of the device.The catalysts can accelerate the reaction rate,and reduce energy consumption during the process of water splitting.The overpotentials of the catalysts used for hydrogen evolution have been greatly reduced.However,due to the four-step proton coupling involved in oxygen evolution process,which is kinetically sluggish,the overall reaction rate of water splitting is severely restricted.Although the water electrolysis technology has been commercialized,the catalysts used are mainly the palladium group,and other precious metal-based materials to reduce the overpotential.Taking into account the reserves and economic costs of precious metal catalysts,it is urgent to develop efficient,and low-cost catalysts for reducing the cost of the production hydrogen from water electrolysis,therefore promoting the rapid development of hydrogen technology.In this paper,the activity and stability of catalyst are taken as an evaluation index.The relationships between the catalyst structure and catalytic properties have been clearly elucidated according to the different catalyst systems as the research objectives.This mechanism would help to build a universal law for designing high-efficiency catalysts.This paper mainly analyzes the following three different catalyst systems:1.Single-atom catalysts(SACs)require the matched substrate material to provide the optimal coordination environment for single atoms and significantly reduce the utilization amounts of catalysts by maximizing the atom-efficiency.Compared with the iridium single atoms anchored on the nickel hydroxide iron substrate(Ir1/NFH),the single-atom catalyst achieves a low loading and high dispersion density Ir single atoms on nickel-iron sulfide nanosheet arrays substrate(Ir1/NFS)by a two-step electrochemical method.The separation of substrate construction and Ir SAC deposition makes the distribution of Ir atoms only on the outer surface of the substrate,rather than inside the electrode,therefore maximizing the utilization of precious metals.The suitable chemical environment of Ir single-atom on the surface of NFS by Ir-S-M bond(M stands for Ni or Fe)efficiently reduces the kinetic energy barrier to form*OOH group from*O group,and thus accelerates the OER process.2.It is highly advantageous that the structure and component of metal organic frame materials(MOFs)and their derivatives are flexibly regulated.By introducing polar H2O molecules and 2-methyl methamphetamine to coordinate with cobalt ions,there are significant changes in the structure and morphology of nanorod-shaped ZIF.Moreover,the nanorods could be self-assembled by hydrogen-bond.Compared with the typical polyhedral ZIF-67,the nanorod-shaped ZIF is easily transformed into α-Co(OH)2 after OH-exchange in alkaline environment.Subsequently,the α-Co(OH)2 is further reconstituted to CoOOH in the process of electroactivation.The CoOOH as active component improves electrocatalytic oxygen evolution property.3.Single-molecule cataly sts can provide a stable coordination environment for metal active centers,but the activity of oxygen evolution is poor.Through a "bottom-up"approach,FePc,which is easy to self-assemble into nanorods,is dispersed on the MoS2 nanosheet surface in the form of single molecule(MD-FePc/MoS2).The MDFePc/MoS2 is dispersed on porous nickel electrode surface.After that,the MoS2 nanosheets are oxidized and dissolved in solution by in situ electrochemical activation.At the same time,the Ni electrode is found to form a layer of nickel oxyhydroxide(NiOOH)which further intensifies the immobilization of molecularly dispersed FePc(MD-FePc)on the electrode surface.The electronic structure of Fe center FePc molecular is regulated by the axial Fe-O bonds with the surface of NiOOH substrate,improving the electrocatalytic oxygen evolution property of molecular catalyst.Different characterization methods were used to confirm that the Fe center in FePc molecular experienced reversible changes from O-Fe-N4 to O-Fe-N4-O▼(▼ indicates the different intermediate states during OER process)under different applied potential.This scheme provides a unique model system for the study of nickel-iron-based catalysts.