Effects of metal ratios and post treatments on energy storage ability of cobalt manganese metal organic frameworks

Metal organic framework (MOF) with large surface area and pore tunability has been intensively applied on energy storage. Tubular structure is one of promising configurations due to abundant contacts to electrolyte at inner/outer surface and straight charge transfer paths. Bimetallic compounds with multiple redox states are favorable for generating redox reactions. Possible conversions to oxides or sulfides can also induce additional active sites and enhance conductivity. In this study, cobalt and manganese bimetallic MOF (CoMn-MOF) tubular structures are firstly synthesized using polypyrrole nanotubes as the template. Effects of metal ratio on energy storage is investigated to understand contributions from Co and Mn. The CoMn-MOF derived oxide and sulfide are further synthesized to enhance energy storage ability. A larger specific capacitance (C_F) of 670.1 F/g is attained for CoMn-MOF derived sulfide (S-CoMn-MOF) electrode, respectively compared to those of 247.0 and 426.7 F/g for the CoMn-MOF and oxidized CoMn-MOF electrodes, because of surface connected tubular structure and balanced components of MOF and sulfides for S-CoMn-MOF. An energy storage device with S-CoMn-MOF and graphene electrodes shows a maximum energy density of 17.9 Wh/kg at 785.7 W/kg. The C_F retention of 78% and Coulombic efficiency of 97% after 10,000 charge/discharge cycles are also obtained.