Study On Preparation And Energy Storage Performance Of Nickel Phosphates And Its Composites

Using fossil fuels has caused great pollution to the environment for many years.The development of eco-friendly new energy sources to replace the non-renewable energy sources is a hot topic for many researchers in the current century.Green and clean energy storage devices are very important roles in energy storage and utilization.Supercapacitors with high power density,greener,fast charge-discharge and long cycle life are being considered recently for new energy vehicles and many other areas.Electrode material is the impotent part of supercapacitors.The excessive metal Ni is one of the best elements for electrode materials,which has richness in natural resources,high energy storage capacity,easy to store and transport,inexpensive and so on.Phosphates with unique microstructures are easy to prepare amorphous compounds and nanomaterials,which is very suitable as an electrode material.This work focuses on the preparation of nickel phosphates and analysis of their energy storage performance.The specific research contents are as follows:?1?Using?NH₄?₂HPO₄ and Ni?NO₃?₂?6H₂O as raw precursors,amorphous NiHPO₄?3H₂O was successfully synthesized at room temperature by a simple precipitation method.It was demonstrated that NiHPO₄?3H₂O was like blocks which were filled with nanospheres and nanopores.Meanwhile,the diffused path of ions had been shortened and the utilization of active materials had been improved.In a three-electrode system,the excellent capacitance of NiHPO₄?3H₂O reached up to 2191 F?g^-1 at 0.5 A?g^-1 in 2 M KOH?The cycle performance was illustrated by the asymmetric supercapacitor which used NiHPO₄?3H₂O as the positive material and activated carbon as the negative material.The specific capacitance maintained 101%of the initial capacitance(81.2 mAh?g^-1)after 500cycles.And its energy density and power density were as high as 105 Wh?kg^-1 and 215W?kg^-1,respectively.?2?Ni₃?PO₄?₂?8H₂O was successfully prepared by hydrothermal method using NH₄H₂PO₄?Ni?NO₃?₂?6H₂O and H₂NCONH₂.Using the controlled variable method-the input different ratios of H₂NCONH₂,different hydrothermal temperatures and different hydrothermal reaction time,the microstructure and electrochemical performance of the obtained compounds were analyzed.The evolution mechanism of the Ni₃?PO₄?₂?8H₂O nanosheets is nanosheet-stack-split-nanospheres.According to the analysis of electrochemical performance,the product via the molar ratio of 1:1:1,the temperature of 150?,and the reaction time of 2 h has the best performance and the specific capacitance is up to 1564 F?g^-1.The supercapacitor with a specific capacity of 46.9 mAh?g^-1 was assembled by Ni₃?PO₄?₂?8H₂O as the positive electrode material and activated carbon as the negative.After 1,000 cycles,the specific capacitance is still maintained at 65%,indicating it has a good cycle performance.?3?Ni₃?PO₄?₂/rGO composite was successfully prepared by a two-step method of hydrothermal synthesis and calcination.During the hydrothermal reaction,the Ni₃?PO₄?₂?8H₂O nanospheres added to the surface of GO,which effectively reduced the agglomeration of the Ni₃?PO₄?₂?8H₂O nanospheres,by calcining in an Ar atmosphere at400?,the GO with low conductivity was transformed into rGO with high conductivity.Meanwhile,the Ni₃?PO₄?₂?8H₂O was transformed into Ni₃?PO₄?₂,which improved the effective application of nickel phosphate.The specific capacity of the Ni₃?PO₄?₂/rGO is 895F?g^-1,and the specific capacity contributed by Ni₃?PO₄?₂ is 2486 F?g^-1.The supercapacitor with the Ni₃?PO₄?₂/rGO as the cathode material had lower resistance and good cycle stability.