Study On Cobalt-Based Bimetallic Phosphating Compounds Materials:Design,Synthesis And Water Splitting

Increasingly serious global energy crisis and climate change are two serious problems facing modern society.In order to reduce the dependence on fossil fuels and greenhouse gas emissions,it is urgent to develop sustainable energy conversion technology and clean energy carrier.Hydrogen(H2),as a carbon-free green energy carrier with high specific capacity,is considered as one of the most promising alternatives for future energy.At present,large-scale production of hydrogen mainly relies on steam reforming reaction of fossil resources,which further aggravates the depletion of fossil fuels and the greenhouse effect.Compared with steam reforming hydrogen production,electrolytic water hydrogen production technology takes water as reactant,has the advantages of no release of greenhouse gas hydrogen production efficiency,is widely regarded as a feasible and sustainable method of hydrogen production.Transition metal phosphating is considered to be an excellent electrolytic water catalyst due to its excellent properties.CoP has attracted considerable attention due to its excellent electrical conductivity,multi-redox properties and hybrid D-orbitals.Unfortunately,due to the slow adsorption and dissociation kinetics of CoP to water,the performance of CoP in the complete hydrolysis of water under alkaline conditions is not satisfactory.In order to solve the above problems,the structure with high hydrophilic and high specific surface area was constructed by doping transition metal elements(Cu,Fe),thus increasing the exposure of the active site of the catalyst,reducing the dissociation energy barrier of water,optimizing the adsorption of intermediates and improving the catalytic efficiency of the catalyst.The main contents of this paper are as followsIn this thesis,using carbon cloth as the support,two types of cobalt-based bimetallic bifunctional catalysts were designed and prepared for the study of water electrolysis performance,as follows:(1)Based on carbon cloth,Cu-doped CoP nanoflowers were prepared by hydrothermal copper doping and gas phosphating at low temperature.The prepared CoxCu1-xP was characterized by XRD,SEM and TEM by adjusting the proportion of copper and determining the optimal copper doping amount.The hydrophilicity test of the catalysts with different morphs confirmed that the flower-like structure has superhydrophilic properties,and the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)activities were tested in 1 M KOH.The experimental results show that when the copper doping amount is 4.7%,the performance of Co0.95Cu0.05P is the best,the overpotential of HER and OER of Co0.95Cu0.05P was 42 mV and 227 mV at the 10 mA cm-2,respectively,and the voltage of total solution water was 1.52 V,which exceeded that of commercial Pt/C and RuO2.Further theoretical calculation shows that Cu doping optimizes the electronic structure of Co and P,reduces the dissociation energy barrier of water,and makes the Gibbs free energy of hydrogen adsorption close to 0 eV.In the OER process,the reaction energy barrier is reduced.After Cu doping,the center of d band moves up,which accelerates the transfer of protons and enhances the adsorption of water.Meanwhile,the density of states increases and the conductivity of catalyst is improved,thus improving the catalytic efficiency of catalyst(2)On the basis of the work in the previous chapter,the element Fe,which is less electronegative than Cu,was selected to optimize CoP electronic structure.The Co-Fe double layer hydroxides were synthesized on carbon cloth by hydrothermal method,and then phosphating at low temperature was further used to prepare CoxFe1-xP nanostructures.By adjusting the proportion of Fe,the optimal amount of Fe doping was determined electrochemical tests were carried out on the prepared CoxFe1-xP.In the solution of 1 M KOH,Co0.75Fe0.25P had the best HER and OER activities.At the current density of 10 mA cm-2,the overpotential of HER and OER was only 52 mV and 260 mV,respectively.The corresponding tafel slope is 52.1 mV dec-1 and 65 mV dec1.Co0.75Fe0.25P is used as the anode and cathode for the total solution water test,and the voltage is only 1.54 VIn conclusion,by designing and synthesizing the structure of nanoflower-like arrays with three-dimensional branches,larger active sites can be exposed and the mass transfer efficiency can be accelerated.Further optimization of the electronic state of the catalyst’s surface by metal doping enhances the activation of water.The two synergistically enhance the catalytic activity and stability of the catalyst and improve the water splitting efficiency.The design concept of this paper can be extended to the preparation of other metal compounds,which has guiding significance for the development of devices applied to energy conversion and storage.