Metal-Organic-Framework-Derived N-, P-, and O-Codoped Nickel/Carbon Composites Homogeneously Decorated on Reduced Graphene Oxide for Energy Storage

In this work, a type of high-performance electrode material of Ni/C/rGO-n [n = wt % of reduced graphene oxide (rGO)] derived from nickel-based metal-organic framework (Ni-MOF) for supercapacitors is successfully prepared. First, a hydrogen-bonding-assisted approach to continuous growth of Ni-MOFs on graphene oxide (GO) sheets is developed through the application of a hexadentate ligand, hexakis(4-carboxylphenoxy)cyclotriphosphazene (CTP-COOH), as the anchor with multiple biting points. The thicknesses of the obtained layer cakelike Ni-MOF/GO-n (n: wt % GO) nanosheet composites with GO as the filling content can be controlled by tuning the ratio between GO and Ni-MOF. During the subsequent process of thermolysis treatment of Ni-MOF/GO-n composites, GO acts as a "hard"template and thus the decomposition of loaded metal-organic frameworks (MOFs) affords highly porous metal/C composites homogeneously embedded on the rGO matrix. This highly porous structure enhances the stability of Ni/C composites profoundly. Besides, both the rGO matrix and porous metal/C composites can provide an expressway for electron transport and shorten ion diffusion paths for greatly enhanced diffusion kinetics within the electrode materials, therefore leading to a superior rate capability. Moreover, application of the hexadentate ligand CTP-COOH with a six-membered ring backbone, which consisted of N and P atoms linked together alternatively, enables self-doping of N, O, and P to the derived materials. It is proven that the electrochemical performances, especially the stability, are remarkably enhanced for the derived Ni/C/rGO-n composites compared with the bare Ni-MOF-derived Ni/C composites. More importantly, this work pioneeringly demonstrates the potential application of MOF derivatives in energy storage devices.