Electrocatalytic Water Splitting Over Transition Metal Organic Framework Derived Materials

The sharp reduction of fossil energy and the increasing environmental pollution are still major problems to be solved in the 21st century.The future development of society needs the reform and optimization of energy structure.Hydrogen energy,as one of the secondary energy in many renewable clean energy,has great potential to replace fossil energy as a new energy carrier.Electrocatalytic decomposition of water as an ideal large-scale hydrogen production method has been widely studied,among which the most widely studied and most mature is the decomposition of alkaline water,but there are still problems of high hydrogen production overpotential and poor stability.In the research of many water decomposition catalyst systems,direct growth of catalytic materials on conductive substrates such as Nickel foam?NF?has attracted wide attention.In these catalytic materials,metal organic frameworks?MOFs?materials,especially Co,Ni and other transition metal based MOFs materials,have attracted attention due to their porous and large specific surface area.However,the active sites and catalytic mechanism of these materials in the process of electrocatalysis are not very clear.In this paper,cobalt and nickel based MOFs materials were directly grown on the foamed nickel substrate by hydrothermal method.They were applied to hydrogen evolution reaction and oxygen evolution reaction respectively.The effect of heat treatment and electrochemical oxidation on the catalytic decomposition of water by MOFs was systematically studied.The details are as follows:?1?Co based MOFs carbonated materials was used for hydrogen evolution reaction.In this paper,Co₂?OH?₂BDC was grown on the surface of nickel foam by one-step hydrothermal method,and then transformed into nitrogen doped carbon coated cobalt core shell structure?Co@NC?after heat treatment in NH₃atmosphere.Then,the cobalt hydroxide and cobalt hydroxide nanowire mixture?Co O_xH_y?were formed on the electrode surface by electrochemical activation.The catalyst showed a low activation energy close to that of Pt,the Tafel slope characterizing the reaction kinetics and the poisoning test of adding SCN^-to the solution,all of which showed that Co O_xH_ycould significantly reduce the dissociation barrier of water and accelerate the process of hydrolysis to proton?Volmer step?.In addition,the electrode has a large electrochemical active area,which means more catalytic active sites;at the same time,the electrode shows good hydrogen evolution stability,indicating that the stability of the catalyst after heat treatment and electrochemical activation has been improved.The results show that this kind of MOFs material is a good precursor of catalytic material.The derived catalyst has the advantages of many active sites and no agglomeration.It has a potential application prospect in hydrogen production by water decomposition.?2?Study on the oxygen evolution performance of Ni based MOFs after electrochemical oxidation.In the electrocatalytic decomposition of water,the anodic oxygen evolution is a relatively complex four electron coupling process,involving a number of oxygen producing intermediates?OH,O,OOH?,which requires a higher external bias than the hydrogen evolution reaction,and is the rate determining step of the whole water decomposition reaction.In order to improve the overall efficiency of water decomposition,it is necessary to study oxygen evolution reaction catalyst.Based on the study of Co based MOFs for hydrogen evolution reaction,Ni based MOFs material Ni?BDC?is directly grown on the surface of foamed nickel by one-step hydrothermal method.Through the in-situ transformation of electrochemical oxidation to form the hydroxyl oxide of high valent nickel,the high valent nickel metal is easy to combine hydroxyl?OH^-?in the process of water oxidation,thus promoting the conversion of oxygen producing intermediates and accelerating the kinetics of water oxidation;The comparison shows that the corresponding molar amount of electron transfer of oxidation package of linear voltammetric curve and the molar amount of catalyst involved in the catalytic reaction are in one order of magnitude,indicate that the oxidation and collapse of Ni?BDC?leads to the formation of a whole with carbon confinement and linkage,which has the characteristics of bulk catalysis,so it has more catalytic active sites.At the same time,the electrode after electrochemical activation shows high stability,with continuous oxygen production of more than 160 h at the current density of 10 m A cm^-2,and the morphology and structure of the catalyst remain good.This experiment further indicates the potential application of MOFs in the field of electrocatalytic decomposition of water.