Transition Metal Phosphide Based Nanomaterials As Active Electrocatalysts For Electrooxidation Reaction

Hydrogen has long been regarded as clean energy carrier for sustainable but intermittent energy storage and value-added feedstock for modern chemical manufacture.Water electrolysis is an innovative approach towards hydrogen production without carbon emission,yet its industrial application still fails to be realized owing to the low electricity conversion efficiency.Related to cathodic hydrogen evolution reaction(HER),anodic oxygen evolution reaction(OER)undergoes a four-electron transfer process with sluggish reaction kinetics,typically determining the overall efficiency.Current state-of-the-art OER electrocatalysts are noble-metal-based oxide such as RuO2 and IrO2,which still require an overpotential of>300 mV to reach the benchmark current density of 10 mA cm-2.Moreover,their high-cost and low-durability nature largely hinder the exploration for substantial large-scale implementations.In this regard,it is of high urgency to seek the OER electrocatalyst alternatives with low cost,high activity and good durability.Give the tunable electronic states,3d transition-metal-based materials have received increasing attention as OER electrocatalysts,especially in alkaline electrolyte.Among the materials,transition metal phosphide is a promising class of candidates due to their approximately zero-valent metallic feature with high electronic conductivity.Based on the above considerations,this thesis takes transition metal phosphide as the research object,and synthesized a series of high-performance transition metal phosphide electrochemical catalysts.The purpose of our research is to improve the energy conversion efficiency of water electrolysis for hydrogen production and realize the green and sustainable electro-oxidative synthesis of organic molecules.In addition,basic research on the reaction mechanism is performed.The main results are summarized as follows:1.We prepared FeNiPx 3D self-supporting electrode material as an efficient,stable and low-cost catalyst for oxygen evolution reaction in alkaline media.At the same time,the mechanism of catalyst activity improvement was explored by combining electrochemical tests,material structure characterization and theoretical research.The results indicate that the high activity of the catalyst is attributed to the synergistic effect of Fe-Ni,FeNiOxHy formed by in-situ electro-oxidation,POx active species,and 3D self-supporting structure.2.We report a co-phosphorization approach to construct a VPO4-Ni2P heterostructure on nickel foam for OER with strongly chemical binding,wherein phosphate acts as electronic modifier for Ni2P electrocatalyst.Profiting from the interfacial interaction,it is uncovered that electron shifts from Ni2P to VPO4 to render valence increment in Ni species.Such an electronic manipulation rationalizes the chemical affinities of various oxygen intermediates in OER pathway,giving a substantially reduced energy barrier.As a result,the advanced VPO4-Ni2P heterostructure only requires an overpotential of 289 mV to deliver a high current density of 350 mA cm-2 for OER in alkaline electrolyte,together with a Tafel slope as low as 28 mV dec-1.This work brings fresh insights into interfacial engineering for advanced electrocatalyst design.3.In this work,we show a more thermodynamically and kinetically favorable reaction,electrochemical oxidative dehydrogenation(EODH)of benzylamine to replace the conventional OER,catalyzed by a cobalt cyclotetraphosphate(Co2P4O12)nanorods catalyst grown on nickel foam.This anodic reaction lowers the electricity input of 317 mV toward the desired current density of 100 mA cm-2 compared with OER,together with a highly selective benzonitrile product of more than 97%.More specifically,when coupling it with cathodic hydrogen evolution reaction(HER),the proposed HER‖benzylamine-EODH configuration only requires a cell voltage of 1.47 V@100 mA cm-2,exhibiting an energy-saving up to 17%relative to conventional water splitting,as well as the near unit selectivity toward cathodic H2 and anodic benzonitrile products.