Highly optimized nickel cobalt mixed oxide has been derived from zeolite imidazole frameworks. While the pure cobalt oxide gives only 178.7 F g as the specific capacitance at a current density of 1 A g , the optimized Ni:Co 1:1 has given an extremely high and unprecedented specific capacitance of 1931 F g at a current density of 1 A g , with a capacitance retention of 69.5% after 5000 cycles in a three electrode test. This optimized Ni:Co 1:1 mixed oxide is further used to make a composite of nickel cobalt mixed oxide/graphene 3D hydrogel for enhancing the electrochemical performance by virtue of a continuous and porous graphene conductive network. The electrode made from GNi:Co 1:1 successfully achieves an even higher specific capacitance of 2870.8 F g at 1 A g and also shows a significant improvement in the cyclic stability with 81% capacitance retention after 5000 cycles. An asymmetric supercapacitor is also assembled using a pure graphene 3D hydrogel as the negative electrode and the GNi:Co 1:1 as the positive electrode. With a potential window of 1.5 V and binder free electrodes, the capacitor gives a high specific energy density of 50.2 Wh kg at a high power density of 750 W kg .

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have been explored to this end [ 7,8 ] and in particular cobalt-based oxides and their binary oxides with nickel are reported to be potential candidates. [ 9 ] For a favorable OER, the M  OH bond strength should be moderate enough as per the Sabatier's principle of balanced intermediate adsorption in catalysis. [ 4,10,11 ] Cobalt in Co 3 O 4 spinel occupies two different sites, tetrahedral for Co 2+ and both tetrahedral and octahedral for Co 3+ . [ 12 ] Incorporation of nickel in the Co 3 O 4 spinel structure results in the octahedral substitution, which is reported to improve the electronic conductivity and effective surface area and add benefi cial effect on oxygen evolution kinetics. [ 13,14 ] Additionally, nickel-based oxides and hydroxides that are prepared by wet chemical routes are found to produce hierarchical nanostructures, and as a consequence provide high surface area for catalytic reactions. [ 15,16 ] Microstructuring of materials into thin fi lms, [ 17 ] 3D cages, [ 18 ] nanorod, [ 19 ] wires, [ 20 ] and porous structures [ 21 ] are known to enhance the active surface area. The oxygen vacancies and nonstoichiometry introduced during fabrication and annealing processes can further improve the conductivity and lower the hydroxyl adsorption energy. [ 22,23 ] Rational design of morphology is a promising approach to promote material's performance. Building hierarchical hollow structures thus holds promise for more effi cient electrocatalyst which can render large surface area, better electronic conductivity and porosity for electrochemical processes. [ 14,24 ] Great efforts have been devoted to develop such porous and hollow nanostructure, which can not only enhance the desired activity, but also impart new functionalities. [ 24,25 ] Recently, metal-organic framework (MOF) has been demonstrated as excellent precursors and templates for fabrication of 3D structures of metal oxides for highly effi cient supercapacitors, Li ion batteries, and oxygen reduction reactions. [ 26 ] Following similar fabrication methodology, MOF derived coreshell structured NiCo 2 O 4 -Co 3 O 4 has been prepared and used to study supercapacitor and catalytic oxygen evolution behavior. [ 18 ] There is, however, a need for systematic investigations to arrive at an optimum nickel-cobalt oxide for OER based on this fabrication route. Such investigations assume importance in realizing a competitive OER catalyst of this class for use in solar to hydrogen conversion devices.In this paper, we report a simple template based fabrication route for non-stoichiometric Ni  Co metal oxide nanocages with mesoporous structure, and rich in Ni and Co redox centers, and Nonstoichiometric Ni x Co 3− x O 4− y 3D nanocages are fabricated through metalorganic framework template route and their electrocatalytic oxygen evolution reaction (OER) characteristics have been investigated. Substitution of Ni in Co 3 O 4 spinel structure improves the intrinsic catalytic activity. Enhanced OER activity stems from the presence of nonstoichiometry and low co...

The two important factors that affect sunlight assisted water splitting ability of TiO2 are its charge recombination and large band gap. We report the first demonstration of nitrogen doped triphase (anatase-rutile-brookite) TiO2 nanotubes as sun light active photocatalyst for water splitting with high quantum efficiency. Nitrogen doped triphase TiO2 nanotubes, corresponding to different nitrogen concentrations, are synthesized electrochemically. Increase in nitrogen concentration in triphase TiO2 nanotubes is found to induce brookite to anatase phase transformation. The variation in density of intra-band states (Ti3+ and N 2p states) with increase in nitrogen doping are found to be critical in tuning the photocatalytic activity of TiO2 nanotubes. The presence of bulk heterojunctions in single nanotube of different nitrogen doped TiO2 samples is confirmed from HRTEM analysis. The most active nitrogen doped triphase TiO2 nanotubes are found to be 12 times efficient compared to pristine triphase TiO2, for solar hydrogen generation. The band alignment and charge transfer pathways in nitrogen doped TiO2 with triphase heterojunctions are delineated. Bulk heterojunctions among the three phases present in the nanotubes with intra-band defect states is shown to enhance the photocatalytic activity tremendously. Our study also confirms the theory that three phase system is efficient in photocatalysis compared to two phase system.