Electrochemical Capacitance of Ni-Doped Metal Organic Framework and Reduced Graphene Oxide Composites: More than the Sum of Its Parts

Abstract

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.

Recent Progress in Metal–Organic Frameworks and Their Derived Nanostructures for Energy and Environmental Applications

“…For example, Yang et al synthesized Zn-doped Ni-MOF as an active material for SCs. [87] In comparison with ah igh charge-transferr esistance of 184 mW for pure MOF-5,t he Ni-doped MOF-5 exhibits much lower resistanceo fo nly 74 mW.W ith reducedc harge-transferr esistance, the Ni-dopedM OF-5 provides ad ramatically increased capacitance of 240 Fg À1 at ac urrent density of 50 mA g À1 , whereas the capacitance of pure MOF-5 is only 100 Fg À1 . Another example, Co 8 -MOF-5e lectrode with Zn cations partially substituted by Co cations, has been reported for enhanced performance of SCs.…”

“…[86] The mixed-metal-based MOF structure shows much better SC performance than that of most single metal oxide based electrodes, owing to improvede lectric conductivity.R ecently,B anerjee et al demonstrated the enhanced electric conductivity of Ni-doped MOF-5 by using electrochemical impedance spectroscopy (EIS). [87] In comparison with ah igh charge-transferr esistance of 184 mW for pure MOF-5,t he Ni-doped MOF-5 exhibits much lower resistanceo fo nly 74 mW.W ith reducedc harge-transferr esistance, the Ni-dopedM OF-5 provides ad ramatically increased capacitance of 240 Fg À1 at ac urrent density of 50 mA g À1 , whereas the capacitance of pure MOF-5 is only 100 Fg À1 .…”

One‐pot self‐assembly and photoreduction synthesis of silver nanoparticle‐decorated reduced graphene oxide/MIL‐125(Ti) photocatalyst with improved visible light photocatalytic activity

“…It seems reasonable to assign the bands to vibrations in the organic part H 2 BDC of MIL‐125(Ti) . More precisely, these peaks correlate to the asymmetric and symmetric stretching vibrations of the carboxylate as well as the vibrations of CC and CH bonds from the benzene ring . For rGO/MIL‐125(Ti) and Ag/rGO/MIL‐125(Ti), no obvious peak shifts of the MOF bands are observed except for the appearance of bands associated with rGO.…”

Recent advancements in metal organic framework based electrodes for supercapacitors

“…Banerjee et al [114] synthesized MOF-5 NiRGO in bulk (gram scale) quantities. The composites were prepared by mixing Ni-doped MOF-5 with RGO (Fig.…”