Effect of Transition Metal Cations on Stability Enhancement for Molybdate-Based Hybrid Supercapacitor

The race for better electrochemical energy storage systems has prompted examination of the stability in the molybdate framework (MMoO₄; 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 MnMoO₄ crystallized in β-form with α-MnMoO₄ type whereas Co and Ni cations crystallized in α-form with α-CoMoO₄ type structure. Among the various transition metal cations studied, hybrid device comprising NiMoO₄ vs activated carbon exhibited excellent electrochemical performance having the specific capacitance 82 F g^-1 at a current density of 0.1 A g^-1 but the cycling stability needed to be significantly improved. The specific capacitance of the NiMoO₄ electrode material is shown to be directly related to the surface area of the electrode/electrolyte interface, but the CoMoO₄ and MnMoO₄ 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, NiMoO₄ 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.