Effect of the Anionic Counterpart: Molybdate vs. Tungstate in Energy Storage for Pseudo-Capacitor Applications

Nickel-based bimetallic oxides (BMOs) have shown significant potential in battery-type electrodes for pseudo-capacitors given their ability to facilitate redox reactions. In this work, two bimetallic oxides, NiMoO₄ and NiWO₄, were synthesized using a wet chemical route. The structure and electrochemical properties of the pseudo-capacitor cathode materials were characterized. NiMoO₄ showed superior charge storage performance in comparison to NiWO₄, exhibiting a discharge capacitance of 124 and 77 F^.g^-1, respectively. NiMoO₄, moreover, demonstrates better capacity retention after 1000 cycles with 87.14% compared to 82.22% for NiWO₄. The lower electrochemical performance of the latter was identified to result from the redox behavior during cycling. NiWO₄ reacts in the alkaline solution and forms a passivation layer composed of WO₃ on the electrode, while in contrast, the redox behavior of NiMoO₄ is fully reversible.