Study of pH value effect on synthesizing UIO-66 and carbonized UIO-66 as active material for solid-state supercapacitors

Metal organic framework (MOF) with high surface area but low conductivity has been widely applied for energy storage. The Zr-based MOF, UIO-66, with well-defined octahedron structure presents high energy storage ability. Synthesizing environment plays important roles on morphology, crystallinity and conductivity of active material. It is the first time to study pH value effects for synthesizing UIO-66 on physical and electrochemical performances. Acetic acid with controlling agent and capping agent roles is the key factor to determine morphology of UIO-66. Carbonization is applied to enhance conductivity of UIO-66, and acid treatment is to remove metal residue for creating more pores. The largest specific capacitance (CF) of 117.7 F/g is achieved for carbonized UIO-66 electrode prepared using pH 4 and acid treatment (CMOFAT4), while UIO-66 and carbonized UIO-66 electrodes respectively show smaller CF values of 335.7 mF/g and 32.78 F/g at 20 mV/s. The better energy storage ability for CMOFAT4 is owing to enough surface area, sound pore structure, high conductivity, and suitable defect/graphene ratio. The symmetric solid-state supercapacitor composed of CMOFAT4 electrodes presents the maximum energy density of 2.13 Wh/kg at 200 W/kg. The CF retention of 84.5% and Coulombic efficiency of 99.9% are achieved after 7000 times charge/discharge process.