Urea-Assisted Room Temperature Stabilized Metastable beta-NiMoO4: Experimental and Theoretical Insights into its Unique Bifunctional Activity toward Oxygen Evolution and Supercapacitor

Abstract

Room-temperature stabilization of metastable beta-NiMoO4 is achieved through urea-assisted hydrothermal synthesis technique. Structural and morphological studies provided significant insights for the metastable phase. Furthermore, detailed electrochemical investigations showcased its activity toward energy storage and conversion, yielding intriguing results. Comparison with the stable polymorph, alpha-NiIVoO(4), has also been borne out to support the enhanced electrochemical activities of the as-obtained beta-NiMoO4. A specific capacitance of similar to 4188 F g(-1) (at a current density of 5 A g(-1)) has been observed showing its exceptional faradic capacitance. We qualitatively and extensively demonstrate through the analysis of density of states (DOS) obtained from first-principles calculations that, enhanced DOS near top of the valence band and empty 4d orbital of Mo near Fermi level make beta-NiMoO4 better energy storage and conversion material compared to alpha-NiMoO4. Likewise, from the oxygen evolution reaction experiment, it is found that the state of art current density of 10 mA cm(-2) is achieved at overpotential of 300 mV, which is much lower than that of IrO2/C. First-principles calculations also confirm a lower overpotential of 350 mV for beta-NiMoO4.