Preparation Of Graphene/Nickel Phosphide Composite And Their Assembly Of Asymmetric Supercapacitor

Supercapacitor has many advantages,such as rapid charging and discharging rate,high charging efficiency,long cycle life,etc.These make it become a hotspot in new energy field,and possess a wide application prospects.Graphene is a kind of new electrode material for supercapacitors,and characterized by large theory surface area,good electrical conductivity and thermal stability.However,there are two major problems in the process of preparation.The one is stack phenomenon occured between graphene sheets due to the effect of van der Waals interactions,which gives rise to the greatly decreased of effective electric double layer area;The other is the weakening of hydrophilic,due to the reduce of oxygen-containing functional groups on the graphene sheets surface.All of these are not conducive to giving full play of the electrochemical properties of graphene materials.Hence,in this these,we synthesized fold graphene and graphene/nickel phosphate composite material.Therefore,the conductivity and specific capacity of electrode materials are improved through controlling the content of oxygen-containing functional groups on the graphene sheets surface and the content of grapheme in composite material.The research contents were illustrated in detail as follow:Fristly,using natural graphite as raw material,fold graphene material was prepared through removing manganese dioxide from manganese dioxide graphene oxide composite after heat treatment,which was synthesized by improved Hummers method.The fold graphene material synthesised by this method contains large amounts of oxygen-containing functional groups,not only increasing the hydrophilic surface area as well as electrically active surface area,but also providing a constraint capacitance by Faraday reaction.What is more,the properties of electrode materials are declined with the rise of heat treatment temperature.This is mainly due to the large loss of oxygen-containing functional groups on the graphene sheets,leading to the capacitance provided by Faraday reaction decreases.The results show that the G-300 electrode displays a high specific capacitance of 335 F/g in 6 mol/L KOH electrolyte.when the scan rate increases to 500 mV/s,its specific capacity is 241 F/g.In addition,the specific capacitance remains 94.58%after 10000 times at 200 mV/s.Secondly,graphene/nickel phosphate composite material was synthesised by means of low temperature thermal reduction method.The addition of graphene not only provides a superhighway for the transmission of the ions and electrons to improving the conductivity,also plays a role of support and steady,thereby inhibiting the accumulation of Ni2P nanoparticles and volume expansion during cycling process.The composite material displays high specific capacity and excellent stability.In order to further improving the electrical conductivity of the composite material,hydroiodic acid reduction method was used.The results show that the RGO/Ni2P-aq-10 electrode displays a high specific capacitance of 2504 F/g in 2 mol/L KOH electrolyte.And after reduced by hydroiodic acid for 40 mins,RGO/Ni2P-aq₁0-HI-40 electrode displays a high specific capacitance of 2735 F/g in 2 mol/L KOH electrolyte.Moreover,when the scan rate increases to50 mV/s,its specific capacity remains 2136 F/g,showing that RGO/Ni2P-aq-10-HI-40 possesses excellent electrochemical property.In addition,the RGO/Ni2P-aq-10//G-300 asymmetric supercapacitor was assembled using RGO/Ni2P-aq-10 and G-300 material as positive electrode and negative electrode,respectively.This asymmetric supercapacitor exhibits high energy density of 69 Wh/kg at power density of 310 W/kg,and 26.7 Wh/kg at 12030 W/kg.