Synthesis And Catalytic Performance Of Element-Doped Transition Metal (Borate) Phosphates

Transition metal（borate）phosphates,due to their low price,green environmental protection,high stability,unique structure and performance,have potential application value in efficient energy conversion and energy storage.Phosphate,borate or their borophosphate polyanion formed by co-apex can coordinate with transition metal to easily form an open skeleton structure,and these anions act as proton acceptors and can accelerate the transfer of protons.In addition,they can also stabilize the p H environment during the reaction,which is essential for the oxygen evolution reaction（OER）process.Therefore,the development of a high-activity and low-cost transition metal（borate）phosphate electrocatalyst to replace traditional noble metal catalysts has potential application prospects for electrocatalytic water splitting.However,the conductivity and activity of transition metal（borate）phosphates insufficient bits limit their electrocatalytic performance.There is an urgent need to develop new methods to improve their catalytic activity.In this paper,three kinds of transition metal（borate）phosphate materials with different morphologies and structures were synthesized by hydrothermal method,and the electronic structure was adjusted by element doping to improve furtherly the electrocatalytic activity of the materials.The focus is on the OER process of electrocatalytic water splitting of electrocatalysts under alkaline conditions,and further exploration of its application in energy storage fields such as zinc-air batteries and supercapacitors.The main research contents and results of this paper are as follows:（1）We successfully synthesized a regular hexagonal prism type transition metal borophosphate crystal material by hydrothermal method,studied the effect of doping different metal cations（Fe,Ni,Zn）on the electrocatalytic performance,and optimized the different metal cation doping ratio.In 1 M KOH electrolyte,when the Co/Fe molar ratio is 3:1（Na C₃FBPO）,it exhibits excellent electrochemical OER performance.Only 289 mV overpotential can reach a current density of 10 mA/cm²,and it can be stable for 16 hours at this current density,and has a low Tafel slope(84.25 mV·dec^-1).Combined with the characterization of the material,the excellent electrocatalytic performance of the Na C₃FBPO catalyst is mainly attributed to the three-dimensional structure of the 12-membered ring channel that is conducive to proton/electron transport;in addition,the synergistic effect between the bimetallic Co and Fe in the Na C₃FBPO catalyst is more beneficial to promote the charge transfer in the OER process,thereby improving the catalytic activity.（2）We prepared two-dimensional transition metal-based phosphate（NH₄Co PO₄·H₂O）and isomorphic bimetallic NH₄Co_1-xFe_xPO₄·H₂O by hydrothermal method.And through a simple and environmentally friendly in-situ pyrolysis method,realize the doping of N-atoms into the phosphate system using the NH₄⁺contained in the compound.By optimizing the calcination temperature and the Co/Fe ratio,it is determined that the calcination under a nitrogen atmosphere at 350 degrees and a Co/Fe molar ratio of 4:1 have excellent OER performance.At a current density of 10m A/cm²,it has an OER low overpotential of 278 mV and a Tafel slope of 58.92 mV·dec^-1.At the same time,the catalyst has good catalytic activity for ORR（oxygen reduction reaction）,half-wave potential of ORR is 0.74 V.Based on the high-efficiency and long-lasting dual-functional activity of the catalyst,we use it as an electrode material in zinc-air batteries.The prepared device has high power density(74.6 mW cm^-2),specific capacity(750 mAh g _Zn^-1)and long-term stability(over 30 hours under 10 mA cm^-2).Our research provides a new design idea to prepare heteroatom-doped electrocatalytic materials with mesoporous structure,which have good electrocatalytic activity and zinc-air battery performance.（3）According to the above in-situ N-doping method,the Co-based phosphate precursor NH₄Co₃（HPO₄）₂（H₂PO₄）F₂ with NH₄⁺cation and F^-anion was calcined at different temperatures to prepared a N,F co-doped mesoporous monometallic cobalt phosphate with oxygen vacancies,which was used as a dual-function electrode for oxygen release reaction（OER）and supercapacitors.The best N,F co-doped mesoporous monometallic cobalt phosphate drived a current density of 10 mA cm^-2 on a glassy carbon electrode with an overpotential of 276 mV and a small Tafel slope(57.11 mV dec^-1).In addition,shows a long-term stability(at least 35 hours at a current density of 10 mA cm^-2).Furthermore,the optimal material applied in supercapacitor displays specific capacitance of 206.3 F g^-1 at 1A·g^-1 and maintains cycling stability with 80%after 3000 cycles.The excellent electrochemical properties should be attributed to N,F-co doping into this Co-based phosphate effectively modulates its electronic structure;in addition,its amorphous structure provides more active sites;moreover,its mesoporous structure should be beneficial to the mass transfer and electrolyte diffusion.