Synthesis And Electrochemical Properties Of Transition Metal Terephthalates

Supercapacitor?SCs?is a new type of electrochemical energy storage device with energy density between conventional capacitors and batteries.Its energy is stored by rapid ion adsorption/desorption at the interface of the electrode material and electrolyte or by a reversible faraday redox reaction.SCs have the advantages of high specific capacitance,high power density,long cycle life,low maintenance cost,rapid charge and discharge,wide range of operating temperature,etc.,Thus,SCs will have broad application and development prospects in the future energy storage field.However,the main reason that restricts the application of SCs is the low energy density.Therefore,the preparation of SCs with high power density and high energy density is one of the hot spots in the field of chemical energy storage.The performance of SCs is mainly dependent on electrode materials.Searching for new electrode materials is one of the key tasks in developing high-performance supercapacitors.Therefore,in this paper,a simple one-step hydrothermal method was used to grow terephthalates in situ on the nickel foam substrate,and the terephthalates were directly used as electrode materials in SCs.The specific research contents are as follows:?1?Using terephthalic acid and ferrous chloride as raw material,Ferrous hydroxide terephthalate,Fe?OH??C₈H₅O₄?,is first prepared by an in-situ hydrothermal growth method on nickel foam,which can be directly used as the positive electrodes for SCs with superior pseudocapacitive behaviors.It displays high specific capacitance of 5.25 F cm^-2(656.3 F g^-1)at current density of 10 mA cm^-2(1.25 A g^-1).It shows a capaxitance retention of 78.2%after 5000 cycles even at a very high current density of50 mA cm^-2.The assembled asymmetric supercapacitor devices make Fe?OH??Tp?as anode electrode materials,activated carbon electrodes are negative.The electrochemical testing results shows the voltage window to expand to 1.5 V,and offered the energy density is 16.68 Wh kg^-1 at the power density of 375 W kg^-1.After5000 cycles,the capacitance retention rate is 84.7%at the current dencity of 20 mA cm^-2.and the coulomb efficiency is 96%indicating that the electrochemical reaction stability and reversibility of the device are good.The above data show that the Fe?OH??Tp?electrode outstanding electorchemichal performance could be a promising meterial for SCs.?2?As a layered hydroxyl derivative,cobalt terephthalate dihydroxide,Co₂?OH?₂?C₈H₄O₄?,is successfully prepared in-situ on nickel foam through a simple hydrothermal route.The scanning electron microscopy results reveal that the products are composed of massive double-blade-lath crystals grown along a-axis.The as-obtained Ni foam-supported Co₂?OH?₂?Tp?is directly used as the positive electrode of supercapacitors without further treatment.The resulting Co₂?OH?₂?Tp?electrode exhibits ultrahigh charge storage capacity with the specific capacitance of 18.71 F cm^-2(2521 F g^-1)at a current density of 10 mA cm^-2(0.5 A g^-1),and good cycling stability with the capacitance retention of 82.4%over 5000 cycles at a current density of 50 mA cm^-2.In combination with activated carbon as the negative electrode in 3 M KOH aqueous electrolyte,the constructed asymmetric SCs delivers a high energy density of30.62 Wh kg^-1 at a power density of 221 W kg^-1 with a voltage window of 1.5 V.And the device had a larger specific capacitance of 98 F g^-1(3.33 F cm^-2)at the current density of 0.29 A g^-1(10 mA cm^-2).The capacitance retention rate of the device is 88.1%over 5000 cycles at the current density 50 mA cm^-2,which indicates the asymmetric SCs has a good cycle stability.These encouraging electrochemical performances enable the developed terephthalate-based cobalt hydroxide to be a promising electrode material for SCs applications.