Electrochemical Performance Of CuMOF-derived CuFe Prussian Blue Analogue Materials For Supercapacitors

As a new type of environmental energy storage device,supercapacitor has attracted much attention due to the advantages of high power density and long-cycle life,but its shortcomings such as low energy density and high cost limit the large-scale application.Supercapacitors with neutral aqueous electrolytes possess the features of safety,inexpensive and high rate performance.Especially,an asymmetrical cell consisting of an electric double-layer capacitor（EDLC）-type anode and a battery-type cathode can provide high operation voltage（ca.more than 2.0 V）,which endows aqueous supercapacitor with high energy density.There is an urgent demand to explore and develop a variety of cathode materials with high energy density,low cost,and long-cycle life.Currently,Prussian blue analogues（PBAs）have attracted much attention owing to their high capacity and remarkable cycle stability.CuFe Prussian blue analogue,which is also named as copper hexacyanoferrate（CuHCF）,allows fast diffusion and sufficient accommodation of alkali metal ions,making it an attractive electrode material.Generally,CuHCF is prepared by co-precipitation method.The precipitation reaction of[Fe（CN）₆]^3-and Cu²⁺ions is so fast that the growth of CuHCF crystals is uncontrollable.Herein,we proposed an effective two-step synthesis approach that HKUST-1 was used as template to prepare CuHCF in a controllable way.Furthermore,the CuHCF materials were utilized to assemble a supercapacitor cell,in which activated carbon was chosen as anode material.The detailed experimental contents are as follows:The precursor of HKUST-1 with regular octahedron structure was firstly synthesized hydrothermally using PVP as a surfactant,and then the HKUST-1 was converted into CuHCF.The material was characterized by means of XRD,SEM,TEM,FT-IR and nitrogen adsorption/desorption.The effect of PVP on CuHCF’s morphology and electrochemical properties was also investigated.The results showed that the optimal hydrothermal synthesis temperature was 120℃.The particle size of HKUST-1 could be controlled to 3μm with 0.1 g of PVP when the molar amount of 1,3,5-benzene tricarboxylic acid was 2 mmol.The characterization showed that the HKUST-1 derived CuHCF exhibited better performance.SEM and TEM images showed that the CuHCF was composed by fine particles,and the nitrogen adsorption and desorption isotherm proved its microporous structure with a pore size of about0.55 nm.The electrochemical performance of CuHCF was tested by cyclic voltammetry,galvanostaticcharge/discharge and AC impedance methods.The CuHCF electrode exhibited a specific capacity of 254.1 F g^-1 in the potential range of 0.1-1V at a current density of 1 A g^-1.According to the CV curve and the XPS results,the energy storage mechanism could be interpreted that the Cu²⁺/Cu⁺redox couple was activated during the processes of charge and discharge.Thus,the CuHCF electrode underwent the redox reactions of Fe³⁺/Fe²⁺and Cu²⁺/Cu⁺couples for charge storage.The obtained CuHCF was used as the positive electrode to assemble an asymmetric supercapacitor along with the activated carbon（AC）as the negative electrode.The hybrid cell CuHCF//AC delivered a specific capacitance of 45.7 F g^-1 at a current density of 1A g^-1,provided an energy density of 25.4 Wh kg^-1 at a power density of 1066.4 W kg^-1 and retained 83%capacitance after 2000 cycles.For improving its overall performance,building composite materials could be used as a common method.Meanwhile,multi-walled carbon nanotubes and MnO₂ nanosheets were employed to fabricate novel composite materials of CuHCF/CNT and CuHCF/MnO₂.The CuHCF/CNT material showed a specific capacitance of 209.0 F g^-1 with excellent rate performance.Furthermore,the CuHCF/MnO₂ hybrid material was fabricated.Because of the synergistic effect of CuHCF and MnO₂ nanosheets,the CuHCF/MnO₂ electrode exhibited a maximum specific capacity of 260.9 F g^-1 at a current density of 1 A g^-1,which was higher than the specific capacitance of CuHCF and MnO₂ alone.Furthermore,Ni-doped CuHCF and Zn-doped CuHCF materials were prepared with the goal of improving the electrochemical performance.No significant increase in specific capacity was observed on Cu Ni HCF and Cu Zn HCF materials.