Electrochemical capacitance of Ni-doped metal organic framework and reduced graphene oxide composites: more than the sum of its parts.

Composites of a Ni-doped metal organic framework (MOF) with reduced graphene oxide (rGO) are synthesized in bulk (gram scale) quantities. The composites are composed of rGO sheets, which avoid restacking from the physical presence of MOF crystals. At larger concentration of rGO, the MOF crystals are distributed on the overlapping and continuous rGO sheets. Ni in Ni-doped MOF is found to engage in a two-electron, reversible, efficient, redox reaction shuttling between Ni and Ni(OH)2 in aqueous potassium hydroxide (KOH) electrolyte. The reaction is rather unique as Ni-based supercapacitors use a one-electron transfer Faradaic redox reaction between Ni(OH)2 and NiO(OH). Employing electrochemical impedance spectroscopy, we determined the charge transfer resistance to be 184 mΩ for MOF, 74 mΩ for a Ni-doped MOF and 6 mΩ for a rGO-Ni-doped MOF composite, but these modifications do not affect the mass transfer resistance. This novel redox reaction in conjunction with the lowered charge transfer resistance from the introduction of rGO underpins the synergy that dramatically increases the capacitance to 758 F/g in the rGO-Ni-doped MOF composite, when the parent MOF could store only 100 F/g and a physical composite of rGO and Ni-doped MOF could algebraically achieve about 240 F/g. A generic approach of doping MOFs with a redox active metal and forming a composite with rGO transforms an electro-inactive MOF to high capacity energy storage material with energy density of 37.8 Wh/kg at a power density of 227 W/kg. These results can promote the development of high-performance energy storage materials from the wide family of MOFs available.