Construction Of Carbon-based Non-Precious Metal Electrodes For Electrocatalytic Water Splitting

Electrocatalytic water splitting is one of the key technologies that promises to construct a clean and sustainable hydrogen energy framework in the future.However,this technology has the problem of low energy conversion efficiency,the fundamental reason is the energy consumption caused by the overpotential of electrode.The development of high-activity and low-cost electrocatalysts is the key to solve this problem.At present,the most promising large-scale water splitting systems are the proton exchange membrane electrolytic water system and alkaline electrolytic water system,the required reaction environment of which is acidic and alkaline,respectively.Considering that the electrocatalytic oxygen evolution reaction in acidic environment and the electrocatalytic hydrogen evolution reaction in alkaline environment will go through the extra step of hydrolysis,the key to the development of proton exchange membrane electrolytic water splitting system and alkaline electrolytic water splitting system is to develop high-performance acidic electrocatalytic anode materials and alkaline electrocatalytic cathode materials.However,such electrocatalysts are still concentrated in precious metal based catalysts,which is not conducive to the large-scale application of water splitting technology.Recently,carbon-based materials have gotten widespread attention due to the advantages of low cost and good conductivity,but how to achieve high-efficiency oxygen evolution in acidic environment and hydrogen evolution in alkaline environment to meet the needs of industrialization is still a huge challenge.Aiming at promoting the industrialization of water splitting,this thesis selected low-cost carbon-based materials with good conductivity as the research basis for solving the difficulties of proton exchange membrane electrolytic water splitting process and alkaline electrolytic water splitting process,and realizing the development of high-performance electrolytic water catalytic materials by the optimization of geometry structure and the regulation of electronic structure.In this thesis,in order to simplify the synthesis process of high-performance catalytic materials,a self-supporting three-dimensional(3D)graphene array structure graphite substrate material was designed and developed firstly.The catalyst can be used directly as an electrode material,optimizing the process by which traditional powder catalysts require secondary preparation.In order to improve the electrocatalytic oxygen evolution performance of carbon-based materials in acidic environments,the electronic structure of carbon materials is optimized by the way of oxygen doping,and non-metallic carbon material(OEEG)of oxygen-rich functiona groups was designed and developed on the basis of graphite substrate materials of 3D graphene array structure.Through oxygen heteroatom doping to improve its natural activity,OEEG catalyst showed high catalytic activity in electrocatalytic oxygen evolution.In acid,it only required ?10=334 mV,the Tafel slope was 136 mV dec-1,and it can run continuously and steadily for more than 10 hours.At the same time,the work also confirmed for the first time that COOH functional group is the main active site of OEEG material as electrocatalytic oxygen evolution reaction under acidic environment.Further,aiming at the problem of low efficiency of electrocatalytic oxygen evolution under acidic condition,a 3D graphene array structure graphite substrate in situ growth nitrogen doped carbon material(NF/EG)with reticulated structure was designed and developed.This reticulated structure optimized the morphology,increase the surface area,and thus exposesed more active sites.NF/EG catalyst showed excellent elecrtocatalytic activity in electrocatalytic oxygen evolution both under acidic and alkaline condition.In acid,it only required ?10=368 mV,the Tafel slope was 163 mV dec-1,and it can run continuously and steadily for more than 10 hours.At the same time,the work confirmed that pyridinic nitrogen and graphitic nitrogen are the main active sites of NF/EG material as electrocatalytic oxygen evolution reaction both under acidic and alkaline environment,and confirmed that nitrogen-carbon(N-C)coordination layer has good acid resistance.To further improve the activity,introducing transitional metal atoms into N-C layer,a new electrocatalytic anode material,which anchored transitional metal and nitrogen coordination in 3D nitrogen-carbon conductivity matrix(TMNx/NF/EG),was designed and developed.On the one hand,this transitional metal and nitrogen coordination structure optimized the electronic structure and improved the electrocatalytic activity.On the other hand,transitional metal atoms anchored on an acid-resistant N-C layer,avoided the lattice oxygen evolution of metals and improved the stability.The TMNx/NF/EG catalyst showed excellent elecrtocatalytic activity and good stability in electrocatalytic oxygen evolution both under acidic condition.In acid,the optimized catalyst(FeN_x/NF/EG)only required ?10=269 mV,the Tafel slope was 129 mV dec-1,and it can run continuously and steadily for more than 24 hours.At the same time,the work also confirmed that FeN₄ is the main active site of FeN_x/NF/EG material as electrocatalytic oxygen evolution reaction under acidic environment.In addition,using this catalyst and Pt/C to construct an overall water splitting system,it only required 1.60 V to achieve the current density of 10 mA cm-2,and can run continuously and steadily for more than 10 hours.In view of the low efficiency of electrocatalytic hydrogen evolution reaction under alkaline conditions,a novel hybrid electrocatalyst comprising atomically dispersed nickel-nitrogen coordination species anchored on EG with embedded nickel nanoparticles(Ni NP |Ni-N-C/EG)was designed and developed.In alkaline,it only required ?10=147 mV,the Tafel slope was 114 mV dec-1,and it can run continuously and steadily for more than 24 hours.At the same time,the work also confirmed that high activity comes from the synergy between nickel nanoparticles and nickel-nitrogen coordination species,and nickel-nitrogen coordination species are as the main active site,nickel nanoparticles accelerate the hydrolysis process.In addition,using this catalyst as binfunctional electrode to construct an overall water splitting system,it only required 1.58 V to achieve the current density of 10 mA cm-2 and can run continuously and steadily for more than 10 hours.In this thesis,the fabricated OEEG,NF/EG,and FeN_x/NF/EG electrocatalytic oxygen evolution anode materials improve the current situation of proton exchange membrane electrolytic water splitting system;the fabricated Ni NP|Ni-N-C/EG electrocatalytic hydrogen evolution cathode materials improve the current situation of alkaline electrolytic water splitting system.This work provides new research ideas and theoretical guidance for the commercialization of electrocatalytic oxygen evolution anode materials under acid condition and electrocatalytic hydrogen evolution cathode materials under alkaline condition.