Research On The Preparation And Properties Of Several Transition Metal-Based Oxygen Evolution Reaction And Oxygen Reduction Reaction Electrocatalysts

Energy shortages,environmental pollution and climate change are three of the most serious problems facing humanity in the 21st century.The development of new clean renewable energy sources is considered to be an effective solution to these problems.Due to the advantages of high energy conversion efficiency and convenient application,electrochemical energy storage and conversion technology has received extensive attention in the world in recent years.In many electrochemical devices such as fuel cells,metal-air batteries,electrolyzed water systems,etc.,oxygen evolution reaction?OER?and oxygen reduction reaction?ORR?are limited by their slow reaction kinetics,and there is an urgent need for efficient catalysts to accelerate the reaction process.The most advanced OER and ORR catalysts are precious metals and their oxides,but the large-scale applications are limited due to the high price and low reserves of them.Therefore,the development of new non-precious metal OER and ORR catalysts has become an important topic in electrochemical energy science and technology.On account of the above background,we have developed several high-efficiency transition metal-based OER and ORR catalysts based on the rational design of materials.The main research of this thesis can be as follows:1.Study on the double perovskite OER and ORR electrocatalysts with complex spin configurationWe studied the OER and ORR properties of double perovskite electrocatalysts La₂CoMnO_6-? and La₂NiMnO_6-?.The experimental results show that they are effective OER and ORR catalysts.For the La₂CoMnO_6-? sample annealed at 600 0C,its ORR half-wave potential is as low as 0.75 V versus reversible hydrogen electrode?RHE?in the alkaline media.Subsequently,we studied the correlation between the electrocatalytic activities of La₂CoMnO_6-? and La₂NiMnO_6-? and the e_g orbital occupation numbers,it is found that when the e_g electrons of two kinds of transition-metal ions are all taken into account,the e_g-filling rule proposed by Shao-Horn and co-workers is valid in La₂CoMnO_6-?.However,in the case of La₂NiMnO_6-?,since the samples prepared in different conditions all have unity average e_g occupation,the e_g-filling rule should be modified.We propose to describe the catalytic activity of La₂NiMnO_6-? using an average deviation from unity e_g occupation.The modified e_g fill rule can be used to design perovskite catalysts with unity average e_g occupation.2.Two-dimensional metal-organic framework-derived bimetallic phosphide/carbon composite material as OER electrocatalystsUltrafine cobalt-iron bimetallic phosphide nanoparticles embedded in carbon nanosheets are synthesized using two-dimensional?2D?metal-organic frameworks?MOFs?as the precursor.The 2D morphology of the carbon matrix as well as the ultrafine characteristics of COi-xFexP nanoparticles are beneficial to improve the OER electrocatalytic performance.By optimizing the molar ratio of Co/Fe atoms in MOFs,we prepared a series of CO_i-xFe_xP/C catalysts.Among them,Co_0.7Fe_0.3P/C exhibits the best OER performance.In an alkaline electrolyte,the overpotential is as low as 270 mV versus RHE when the current density is 10 mA cm^-2 and it has an ultralow Tafel slope of 27 mV dec^-1.Moderate iron doping preserves the cataiytically active sites and enhances the ability of the surface of CO_1-xFe_xP nanoparticles to be oxidized,thus enhances the OER activity.Our finding paves the way for rational design of the morphology and chemical composition of OER catalysts.3.2D bimetallic phosphide ultrathin nanosheets as OER electrocatalystsWe studied a 2D bimetallic phosphide(CO_1-xFe_xP)ultrathin nanosheet as a novel OER electrocatalyst.2D iron-doped?CO₃?_0.5?OH?·0.11 H₂O was prepared as a precursor by hydrothermal method,and then 2D CO_1-xFe_xP ultrathin nanosheets were obtained by fast-heating and phosphating.The 2D morphology of the nanosheets and the synergistic effect between different transition metal elements are beneficial to improve the catalytic performance of OER.By optimizing the doping ratio of Fe atoms,Co_0.8Fe_0.2P nanosheets exhibited the best OER catalytic performance.In an alkaline electrolyte,it reached an overpotential of 270 mV versus RHE at a current density of 10 mA cm^-2 and has a Tafel slope of 50 mV dec^-1.Moderate iron doping improves the de_gree of oxidation of the CoP nanosheet surface and maintains the conductive and chemically stabilizing host,thereby improving OER performance.Our findings demonstrate the enormous potential of rationally designing new non-layered 2D nanomaterial OER catalysts.