Construction Of Phosphate Based Material And Their Electrocatalytic Activities For Water Splitting

The development and utilization of clean energy is an important strategy to solve the problem of fossil fuel energy shortage and environmental pollution.Hydrogen energy,as a zero-carbon emission energy,is recognized as one of the cleanest energy sources.How to effectively and sustainably produce hydrogen is the primary problem to be solved in the future.Among many hydrogen production methods,electrolytic water method is the most environmentally friendly,effective and purest method.The ideal electrocatalyst should have the advantages of high catalytic activity,good stability,low cost and abundant resources.Pt-based materials exhibit the best catalytic activity in electrolytic water hydrogen evolution reaction,and RuO2 and IrO2 have the best oxygen evolution performance,but due to the high price and scarce reserves,precious metal based catalyst can not be used in large-scale practical production.Therefore,researchers are committed to designing and preparing cheap and efficient electrocatalysts to reduce the cost of electrolytic water technology.Transition metal phosphates have excellent energy conversion and storage capacity,rich reserves and low price,which have become a hotspot in the field of electrocatalysis.In this thesis,the effects of preparation method and surface modification on the catalytic activity of ferric phosphate for water splitting process are studied and ferric phosphate with excellent catalytic activity and high stability was successfully prepared.The main research contents are as follows:First,a needle-like ferric phosphate nanocone(FePO4/NF)on nickel foam substrate was prepared by two-phase solvothermal method.Then,the Ag-FePO4/NF material was prepared by deposition of silver nanoparticles on the surface of the nanocone by photoreduction deposition.The experimental results show that the electrocatalytic activity of FePO4/NF was enhanced by the surface modification of with trace amount of Ag.For Ag-FePO4/NF electrode,the overpotential for hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)is 117 and 193 mV at 10 mA·cm-2 in 1 M alkaline solution,respectively.Compared to that of FePO4/NF electrode,the overpotential is reduced by 44%for HER and 22%for OER.Moreover,in the process of OER,the prepared Ag-FePO4/NF electrode exhibits excellent catalytic activity and high stability even at the high current density of 100 mA·cm-2.The overpotential of OER was only 249 mV,and there was no obvious change in potential in 100 h stability test at 100 mA·cm-2.The actually decomposition voltage at 10 mA·cm-2 was 1.66 V when the prepared Ag-FePO4/NF sample was both served as the anode and cathode.This work confirmed that the prepared Ag-FePO4/NF is a potential bifunctional electrocatalyst for overall water splitting.The high specific surface area and open structure of ferric phosphate nanocone is the key factor for the excellent catalytic activity.The open structure is favorable for the electrolyte to penetrate into the catalyst,and the tip of the conical structure has poor adhesion to the bubble,which is beneficial to the bubble release.The modification of Ag nanoparticles improves the overall conductivity of ferric phosphate catalyst,shortening the electron transport path and generating more active sites.In view of the complex preparation process,easy to agglomeration,uneven growth and other defects of phosphate,ferric phosphate was prepared by electrodeposition method on a nickel foam substrate.The effect of electrodeposition conditions on the catalytic activity for oxygen evolution process was investigated.The results showed that the sample prepared by cyclic voltammetry with 12 cycles presents the best electrocatalytic activity for OER,and the overpotential was only 275 mV at the current density of 100 mA·cm-2 in 1 M alkaline solution.The prepared ferric phosphate is rich in hydroxy group,which is ferric hydroxyphosphate.It has a loose and porous nanoparticle cluster structure with a high specific surface area.In addition,FeCoPi(OH)/NF electrodes with different compositions were prepared by adjusting the moler ratio of Fe to Co salts in the electrolyte.The effect of the moler ratio of Fe to Co on the electrocatalytic activity for OER process was investigated.The results show that the single FePi(OH)/NF electrode has the highest catalytic activity.The porous structure of FePi(OH)/NF electrode and the uniform growth of the catalyst on the nickel foam by electrodeposition are the key factors for its excellent catalytic performance.The electrodeposition method has short preparation time and the prepared catalysts present high repeatability.It is a feasible method for the preparation of transition metal phosphate catalysts with low cost,high efficiency and good stability.