Solvent Refluxing Synthesis Of Phosphates And The Investigation Of Electrochemical Performances

In recent years,low dimensional transition metal phosphate materials with strong stability,high electrochemical activity and cheap raw materials,have shown excellent performance in the fields of supercapacitors and water splitting?especially oxygen evolution reaction?.However,the synthesis methods of low-dimensional materials,especially two-dimensional?2D?phosphate materials,are mostly hydrothermal method,solvothermal method and liquid phase stripping method.Among those methods,the high temperature and pressure environment in the hydrothermal and solvothermal processes have more stringent requirements on equipment;the liquid phase stripping process consumes time and energy,and the yield is low.These constraints undoubtedly bring great challenges to the production and application of the 2D phosphate materials.This work is to explore a simple,efficient and common method for the synthesis of low-dimensional materials,especially 2D phosphate materials.The effects of morphology,transition metal types and Co/Ni ratio on the capacitance and OER activity of phosphate materials were systematically studied,which lay a foundation for the performance improvement and application of the phosphate materials.1.Firstly,the most common iron phosphate functional material was selected as the target material,and an organic solvent reflux method was used to synthesize its low-dimensional structure?especially 2D?material.Using diethylene glycol as the solvent,nanosheets?thickness:13-15 nm?,micron-sized sheets and octahedron iron phosphate could be controlled synthesis by controlling the amount of raw materials added.Those also provided an experimental basis for the synthesis of other low-dimensional morphology transition metal phosphates materials.2.Secondly,based on the experimental conditions of nanosheets iron phosphate materials,a series of micro/nano-morphological transition metal phosphate materials were prepared by replaced the metal sources with different valence states and kind.For example,two-dimensional morphology Ni HPO₄,BiPO₄;nano-particle La PO₄,CePO₄,Zr₃?PO₄?₄ and micro rod-shaped Co₇?PO₄?₂?HPO₄?₄.3.The capacitance results of the above synthesized phosphate materials show that:a)By comparing the three morphologies of iron phosphate materials,the two-dimensional morphology can increase the specific surface area,increase the number of surface active sites,reduce the solution and mass transfer resistance;b)Different transition metal phosphates all behave as pseudo-capacitive electrode materials,their mass transfer process are controlled by OH^-,and their pseudo-capacitor properties are significantly different because of the different redox properties of transition metal elements;c)Appropriate Co/Ni ratio can achieve the comprehensive improvement of the capacitance and rate capability of phosphate materials.Among them,when the Co/Ni is 3/2,cobalt and nickel phosphate composite?CNPO-40?show the best capacitance property,the assembled new asymmetric all phosphate supercapacitor are assembled using CNPO-40 and bismuth phosphate,has a high energy density of 87.34Wh kg^-1 at a power density of 766.3 W kg^-1,which is higher than most reported asymmetric supercapacitor devices;4.The OER results of the above synthesized phosphate materials show that:a)By comparing the three morphologies of iron phosphate materials,two-dimensional morphology can significantly increase the number of OER active sites,enhance the mass transferability in the catalytic process;b)The OER activity of phosphate materials with different transition metal types is quite different;c)Appropriate Co/Ni ratio can improve the oxygen evolution overpotential,oxygen evolution rate,electrochemically active area,and conversion frequency.Among them,when the Co/Ni is 4/1,the overpotential of the cobalt and nickel phosphate composite?CNPO-20?is only 0.385 V(10 mA cm^-2),which is better than commercial precious metal catalysts.To sum up,in this solvent reflux method,the raw materials are common and easy to obtain,the instrument has no special requirements and the process has low energy consumption?200?,4 h?,showing an efficient method for synthesizing low-dimensional morphological phosphates.In addition,the low dimensionality of the morphology,the choice of transition metal type and the appropriate metal ratio are important ways to improve the supercapacitor and OER catalytic activity of phosphate electrode materials.