Study On Enhancement Of Performance Of Ni-based Electrocatalyst By Negative/Cationic Doping

The direct conversion of electrical energy into chemicals has profound implications for the development of society,especially in terms of energy.Now,we are looking for a good alternative energy carrier and carbon neutral reducing agent,in which we have found a way to use renewable energy to drive the splitting of water to produce H₂.Hydrogen energy has high energy density and conversion efficiency,which can replace traditional fossil energy.The water splitting process includes two half-reactions:the cathodic hydrogen evolution reaction（HER）and anodic oxygen evolution reaction（OER）.Owing to the unfavorable thermodynamics and sluggish kinetics of both the HER and OER,a highly efficient electrocatalyst is required to reduce the overpotential and accelerate the reaction rate.Meanwhile,by virtue of alternative anodic reactions,such as the urea oxidation reaction（UOR）,it is also possible to largely decrease the overpotentials,taking advantage of the low thermodynamic potential（0.37 V vs.RHE）.However,similar to the OER,the kinetics of the UOR is slow due to multielectron transfer.Although advanced materials design strategies have been applied to synthesize efficient UOR electrocatalysts,effective non-precious metal catalysts are still lacking,and the reaction overpotential is as high as 300-400 m V.Therefore,the development of highly efficient non-precious metal UOR electrocatalysts and the study of their reaction mechanisms are of great significance.In this paper,on the basis of existing studies,the catalytic activity of Ni base was further improved by adding active sites and improving its intrinsic activity.One of the most effective methods is to perform negative/cationic doping,which is a promising strategy that can radically increase activity,change electronic structure,and accelerate reaction kinetics.The OER and HER properties of Co-Ni₅P₄-Ni Co OH were found to be optimized after Co doping and phosphorylation.On this basis,Mo-Ni Co P@Ni Co P/Ni_XCo_YPO₂ was prepared,and the HER performance was improved after Mo doping and phosphorylation,but it did not contribute to the OER performance improvement.Finally,W doping was introduced to enhance the OER performance,and it was found that W doping not only enhanced the HER performance,but also greatly enhanced the OER performance.The main research results of this paper are as follows:（1）A series of M-doped-Ni₅P₄/M-doped Ni（OH）₂（M=Fe,Co,Cu,Cr）hierarchical materials with abundant oxygen vacancies was constructed for the first time by hydrothermal and partial phosphating methods.The Co-doped-Ni₅P₄/Co-doped-Ni（OH）₂（Co-Ni₅P₄-Ni Co OH）exhibited superior performance in HER,OER and urea oxidation reaction（UOR）.Moreover,the electrode couple is fitted with two Co-Ni₅P₄-Ni Co OH electrodes to drive the current density of 10 m A·cm^-2;the necessary cell voltage was 1.57 V in 1.0 M KOH with 0.5 M urea for urea electrolysis and water electrolysis required a 1.6 V cell voltage in 1.0 M KOH electrolyte,which is one of the best catalytic activities reported so far.The experimental results suggest that the co-action of Co-doping and oxygen vacancies increases the specific surface area of the material,enhances the electronic conductivity and promotes the exposure of more active sites,thus improving the water splitting and urea electrolysis performances of the catalyst.Density functional theory analysis suggests that Co-Ni₅P₄-Ni Co OH displays optimal adsorption energy of water and electrical conductivity,thus optimizing the adsorption/desorption of intermediates.（2）We firstly constructed a novel Mo-Ni Co P@Ni Co P/Ni_XCo_YH₂PO₂ core/shell nanorod heterostructure by hydrothermal and two-step phosphating on nickel foam（NF）.It is worth noting that Mo-doping could availably regulate the electronic structure of Ni Co P,resulting in the increased exposure of the active center and the increased inherent activity of each site.Furthermore,a strategy of improving catalyst activity was proposed,that is the Ni Co P nanorod core and Mo-Ni Co P/Ni_XCo_YH₂PO₂ nanorod shell was constructed by the two phosphating reactions to come into being mixed transition-metal phosphides（TMPs）,thus improving the synergistic catalytic effect of the material.In addition,the water and urea electrolysis apparatus was installed from two MNCP@NCP/Ni_XCo_YH₂PO₂ electrodes to actuate a current density of 10 m A·cm^-2,the necessary cell voltage was merely 1.348 V in 1.0M KOH with 0.5 M urea for urea electrolysis,while the higher 1.522 V of cell voltage was required in 1.0 M KOH for water electrolysis,which is one of the best catalytic activities reported so far.Experimental results show that the oxyhydroxide is the real active site during urea electrolysis process.Density functional theory calculation shows that the doping of Mo and Co increase the water adsorption energy and conductivity of the oxy-hydroxide material,so the water splitting performance of the catalyst is improved.（3）In this study,the structural engineering of complex WNi M-WNi LDH（M=Se,S,P）was firstly developed by in situ growth on Ni foam for use in overall water splitting and urea oxidation reaction.These WNi M-WNi LDH（M=Se,S,P）catalyst exhibits outstanding electrocatalytic performance in HER,OER and UOR,respectively.Only overpotential of 64m V of OER is required for WNi S-WNi LDH and 126 m V of HER is required for WNi P-WNi LDH to achieve 10 m A·cm^-2.The WNi Se-WNi LDH material display particularly outstanding performance of UOR with requiring potential of 1.25 V to drive 10 m A·cm^-2.Moreover,the optimized WNi S-WNi LDH as an anode and WNi P-WNi LDH as a cathode can achieve an 10m A·cm^-2 at a low cell voltage of 1.45 V in 1 M KOH solution for overall water splitting.The density functional theory calculation shows that the introduction of the Ni P₂ and WP material greatly reduces the Gibbs free energy of hydrogen adsorption of the material.