The effect of zinc (Zn) doping and defect formation on the surface of nickel molybdate (NiMoO 4 ) structures with varying Zn content has been studied to produce one-dimensional electrodes and catalysts for electrochemical energy storage and ethanol oxidation, respectively. Zn-doped nickel molybdate (Ni 1-x Zn x MoO 4 , where x = 0.1, 0.2, 0.4, and 0.6) nanorods were synthesized by a simple wet chemical route. The optimal amount of Zn is found to be around 0.25 above which the NiMoO 4 becomes unstable, resulting in poor electrochemical activity. This result agrees with our density functional theory calculations in which the thermodynamic stability reveals that Ni 1-x Zn x MoO 4 crystallized in the β-NiMoO 4 phase and is found to be stable for x≤0.25. Analytical techniques show direct evidence of the presence of Zn in the NiMoO 4 nanorods, which subtly alter the electrocatalytic activity. Compared with pristine NiMoO 4 , Zn-doped NiMoO 4 with the optimized Zn content was tested as an electrode for an asymmetric supercapacitor and demonstrated an enhanced specific capacitance of 122 F g −1 with a high specific energy density of 43 W h kg −1 at a high power density of 384 W kg −1 . Our calculations suggest that the good conductivity from Zn doping is attributed to the formation of excess oxygen vacancies and dopants play an important role in enhancing the charge transfer between the surface and OH − ions from the electrolyte. We report electrochemical testing, material characterization, and computational insights and demonstrate that the appropriate amount of Zn in NiMoO 4 can improve the storage capacity (∼15%) due to oxygen vacancy interactions.

fElectrochemical energy production and storage at large scale and low cost, is a critical bottleneck in renewable energy systems. Oxides and lithium transition metal phosphates have been researched for over two decades and many technologies based on them exist. Much less work has been done investigating the use of sodium phosphates for energy storage. In this work, the synthesis of sodium nickel phosphate at different temperatures is performed and its performance evaluated for supercapacitor applications. The electronic properties of polycrystalline NaNiPO 4 polymorphs, triphylite and maricite, t-and m-NaNiPO 4 are calculated by means of first-principle calculations based on spin-polarized Density Functional Theory (DFT). The structure and morphology of the polymorphs were characterized and validated experimentally and it is shown that the sodium nickel phosphate (NaNiPO 4 ) exists in two different forms (triphylite and maricite), depending on the synthetic temperature (300-550°C). The as-prepared and triphylite forms of NaNiPO 4 vs. activated carbon in 2 M NaOH exhibit the maximum specific capacitance of 125 F g −1 and 85 F g −1 respectively, at 1 A g −1 ; both having excellent cycling stability with retention of 99% capacity up to 2000 cycles. The maricite form showed 70 F g −1 with a significant drop in capacity after just 50 cycles.These results reveal that the synthesized triphylite showed a high performance energy density of 44 Wh kg −1 which is attributed to the hierarchical structure of the porous NaNiPO 4 nanosheets. At a higher temperature (>400°C) the maricite form of NaNiPO 4 possesses a nanoplate-like (coarse and blocky) structure with a large skewing at the intermediate frequency that is not tolerant of cycling. Computed results for the sodium nickel phosphate polymorphs and the electrochemical experimental results are in good agreement.

The race for better electrochemical energy storage systems has prompted examination of the stability in the molybdate framework (MMoO; M = Mn, Co, or Ni) based on a range of transition metal cations from both computational and experimental approaches. Molybdate materials synthesized with controlled nanoscale morphologies (such as nanorods, agglomerated nanostructures, and nanoneedles for Mn, Co, and Ni elements, respectively) have been used as a cathode in hybrid energy storage systems. The computational and experimental data confirms that the MnMoO crystallized in β-form with α-MnMoO type whereas Co and Ni cations crystallized in α-form with α-CoMoO type structure. Among the various transition metal cations studied, hybrid device comprising NiMoO vs activated carbon exhibited excellent electrochemical performance having the specific capacitance 82 F g at a current density of 0.1 A g but the cycling stability needed to be significantly improved. The specific capacitance of the NiMoO electrode material is shown to be directly related to the surface area of the electrode/electrolyte interface, but the CoMoO and MnMoO favored a bulk formation that could be suitable for structural stability. The useful insights from the electronic structure analysis and effective mass have been provided to demonstrate the role of cations in the molybdate structure and its influence in electrochemical energy storage. With improved cycling stability, NiMoO can be suitable for renewable energy storage. Overall, this study will enable the development of next generation molybdate materials with multiple cation substitution resulting in better cycling stability and higher specific capacitance.

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Illustration of Na ion and small hole polaron migration in Kröhnkite based cathode material.