| Hydrogen,with its clean,high calorific value,is considered the ultimate energy source of the future.Water splitting is the main hydrogen production technology in the future.One of the most important use of hydrogen energy is the development of hydrogen fuel cells.Hydrogen fuel cell is the most promising power generation technology in the 21st century.Hydrogen production technology and air cathode construction technology are two key technologies of fuel cell.However,the slow kinetics of cathodic oxygen reduction reaction and water splitting(cathodic hydrogen evolution and anodic oxygen evolution)restrict the development of hydrogen fuel cell.Therefore,it is urgent to explore high-performance catalyst to speed up the reaction.Noble metal-based catalysts exhibit excellent electrocatalytic properties,but their scarcity and high cost restrict their large-scale commercial application in this energy system.Thus,it is of great significance and urgency to develop non-noble metal catalysts for water splitting and oxygen reduction.In this paper,it is aim to explore the design of non-metallic catalysts and the corresponding catalytic enhancement mechanism for water splitting and oxygen reduction at the atomic and electronic scales by combining theoretical simulation and experimental characterization,so as to provide guidance for the development of new ideas of catalyst design and new process of electrode construction.The main research contents are as follows:(1)From the perspective of atomic level catalysis and edge catalysis,based on the Gibbs free energy calculation results,the C3N4 quantum dots@Graphene composite(CNQDs@G)was designed and chosen the target material toward hydrogen evolution reaction.Then the"0 D-2 D"CNQDs@G structure was prepared by mechanical exfoliation and the following hybrid process of hydrothermal and heat treatment.The as-prepared CNQDs@G exhibits the expected electrocatalytic hydrogen evolution performance,with an overpotential of 0.11V at 10 m A·cm-2.What's more,based on theoretical calculations and electrochemical characterizations,it was found that the outstanding electrocatalytic performance of CNQDs toward hydrogen evolution reaction mainly originates from the bisynergistic effect:the synergistic effect of graphene and CNQDs;Synergistic catalysis of edge site and molecular sieve structure of CNQDs.(2)In order to investigate the electrocatalytic activity of integrated dual-transition metal oxide/hydroxide electrode toward oxygen evolution reaction,we constructed a stainless-steel mesh supporting three-dimensional cross-linked Ni(Fe)OxHy nanosheet integrated oxygen evolution electrode(SSNNi)by corroding the stainless-steel mesh in NH4HCO3 and Ni Cl2 solutions with hydrothermal treatment.The three-dimensional crosslinked structure of the electrode provides a large effective exposure area,increases the reaction site,and facilitates electrolyte infiltration and mass transfer,which lead to an excellent electrocatalytic performance with low overpotential of 0.23 V at 20 mA·cm-2,a small Tafel slope of 37 mV·dec-1,especially high current density(E=1.6 V vs.RHE,j=400mA cm-2).Meanwhile,as a typical self-supporting electrode,the electrode possesses outstanding structural and performance stability,due the feature of binder-free and its robust stainless-steel mesh skeleton.In addition,the preparation process of SSNNi electrode is simple,controllable,and economical.These advantages make it with the potential to develop into an OER industrial catalyst.(3)Based on these features of M-N4 toward oxygen reduction reaction:the adsorption free energies of the intermediates are linearly correlated,and the adsorption of intermediates is related to the bonding-antibonding states of M-N,we put forward a new design conceive:using electron-donating element doping control the antibonding state of M-N bond of M-N4,and then optimize the adsorption strength of intermediates and active center,promote the oxygen reduction reaction.As proof-of-concept studies,it was found that the B-doped Ni-N4 systems(Ni-N4-B)possesses a stronger antibonding state than Ni-N4 structure,exhibiting a superior ORR activity.Especially,the Ni-N4-2B-2 nanoribbon shows a lower theoretical overpotential(0.35 V).The two-fold coordinated B doping can induce the transformation from bonding state to antibonding state of Ni-N bond accompanied by the optimization of multiple descriptors towards ORR including a higher charge and spin density,a larger d-band center value,a narrower d-band width and a weaker coordination strength of Ni-N,thus greatly enhancing electrocatalytic activity.(4)Combined with theoretical calculations and experimental research,to investigate the role of nitrogen defects in g-C3Nx/G and surface functional groups in electrocatalytic oxygen reduction,aiming to reveal the catalytic mechanism of oxygen reduction of g-C3N4-based materials in the experiment,and to develop new ideas for the design of non-metallic oxygen reduction catalysts.Theoretical studies have found that when(CN2)V is present in g-C3Nx/G,that is,(CN2)v-CN/G structure exhibits excellent ORR activity with a hybrid mechanism in a low theoretical overpotential of 0.37 V.The enhanced oxygen reduction catalytic activity of(CN2)v-CN/G structure is mainly due to the synergistic effect between the(CN2)v defects and the OH functional groups,which results in an appropriate spin/charge density of the adjacent carbon atom and enhances the electron localization.Based on the theoretical results,we constructed the structure of g-C3Nx/G in one step by alkali-assisted method.Electrochemical studies have shown that this structure indeed has more outstanding oxygen reduction activity than P-CN/G with fewer nitrogen defects,with a very positive onset potential of 0.98 V vs.RHE and a number of transferred electrons as high as 3.98.This work not only revealed the source of the oxygen reduction activity of the g-C3N4-based materials in the experiment,but also developed a new catalytic enhancement mechanism to enhance oxygen reduction performance by the synergistic effect of defects and functional groups.It not only provides guidance for the design of carbon-based non-metallic oxygen reduction catalysts,but also has reference significance for the modification of other types of catalysts. |
PDF Full Text Request