Preparation Of Iron-Based Transition Metal Catalyst And Study On Overall Water Splitting

Hydrogen production from electrolysis water using electric energy and chemical energy conversion is the most ideal,green and pollution-free hydrogen production technology.The overall water splitting consists of two half reactions,oxygen evolution reaction（OER）at the anode and hydrogen evolution reaction（HER）at the cathode,which control the reaction efficiency of water splitting.At present,the efficient electrocatalysts are still precious metals,such as the oxides of iridium and ruthenium of OER（Ir O₂,Ru O₂）and platinum of HER（Pt）,etc.The scarcity of these precious metals hinders the widespread use of water splitting.Iron-based transition metal catalyst with low cost and easy preparation will generate MOOH as the intrinsic catalytic active site in OER reaction process to reduce reaction overpotential,while appropriate hydrogen adsorption energy in HER reaction process is also a major advantage of Iron-based transition metal catalysts.Therefore,Iron-based transition metal catalyst has great application prospect as water electrolysis material.However,the stability and activity of these catalysts are still a certain gap compared with precious metals,so the material should be adjusted appropriately to improve the catalytic activity and stability of the catalysts.The main research results of this thesis are as follows:（1）Developing low-cost and efficient electrocatalysts is critical to accelerate the kinetics of oxygen evolution reaction（OER）for hydrogen production.Here,we prove that NH₄F as structure-directing agent can replace the imidazole ligand in ZIF-67 and Fe ions can take the opportunity to incorporate into the ZIF structure,and finally obtain a novel open-framework bimetallic fluoride with diperovskicture(（NH₄）₃Fe_xCo_1-xF₆).Compared with ZIF-67,the novel fluoride has a porous hollow and an indistinguishable network structure.This unique open-framework and morphology can provide less charge transfer resistance.Density functional theory calculation verifies that the open-framework diperovskite structure with Co ion coordination is the best catalytic activity.This study provides a new idea for the synthesis of novel Co Fe-based fluoride.（2）Electrochemically splitting water to produce clean hydrogen fuel requires high-performance electrocatalysts.We designed and fabricated self-supporting hybrid-metal hydroxyl fluoride[Co_xFe_y（OH）F]nanosheet arrays on the nickel foam（NF）through simple one-step hydrothermal method.The experimental results demonstrate that the value of x and y in Co_xFe_y（OH）F can regulate the ratio value of M²⁺/M³⁺and control the morphology of nanosheets,thus providing more active sites for overall water splitting.The DFT calculation further discloses that the different ratio of Co and Fe can change the potential determining step,which reduced the energy barrier of electrocatalytic reactions.Especially,the Co_0.21Fe_0.28（OH）F has lower charge transfer resistance,abundant active sites,and larger electrochemical surface area.The Co_0.21Fe_0.28（OH）F is also simultaneously used as cathode and anode to form a two-electrode cell for overall water splitting,which requires a voltage of 1.53 V at 10 m A cm^-2 in alkaline electrolyte.Furthermore,the structure and morphology of Co_0.21Fe_0.28（OH）F is reconstructed after OER reaction which makes it show better OER catalytic performance.（3）In this study,nickel foam was used as the Ni source to convert nickel into nickel hydroxide by electrodeposition method.Then two-dimensional nanosheets were grown on nickel foam by hydrothermal method,and then oxides/hydroxides of nickel-iron were translated into sulfide of nickel-iron.The results showed that the final product had two-dimensional lamellar structure with large specific surface area and excellent water splitting performance.