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

Abstract

Room-temperature stabilization of metastable β-NiMoO 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, α-NiMoO, has also been borne out to support the enhanced electrochemical activities of the as-obtained β-NiMoO. A specific capacitance of ∼4188 F g (at a current density of 5 A g) 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 β-NiMoO better energy storage and conversion material compared to α-NiMoO. Likewise, from the oxygen evolution reaction experiment, it is found that the state of art current density of 10 mA cm is achieved at overpotential of 300 mV, which is much lower than that of IrO/C. First-principles calculations also confirm a lower overpotential of 350 mV for β-NiMoO

Recent Progress on Nickel‐Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

“…(c) Schematic illustration of the exposed crystal planes and (d) LSV curves of the NiCo 2 O 4 nanosheet, NiCo 2 O 4 octahedron, NiCo 2 O 4 truncated octahedron . (e) Polarization curves for α‐NiMoO 4 and β‐NiMoO 4 , and IrO 2 /C in 1 m KOH solution . (f) Schematic illustration of atomically thin NiFe 2 O 4 quantum dots (AT NiFe 2 O 4 QDs) for OER electrocatalysis …”

“…They discovered that 0.61NiMnO 3 /NiMn 2 O 4 delivered optimal overall bifunctional activity since the surface Ni 3+ favors OER activity while the surface Mn 3+ with a small amount of Mn 4+ facilitates ORR processes. Rout's group demonstrated that metastable β‐NiMoO 4 was more active for OER than α‐NiMoO 4 (Figure e) . Ma's group fabricated atomically thin NiFe 2 O 4 quantum dots with different lateral sizes as an efficient OER catalyst through a mild and controllable method (Figure f) …”

“…Rout's group demonstrated that metastable b-NiMoO 4 was more active for OER than a-NiMoO 4 (Figure 6e). [56] Ma'sg roup fabricated atomically thin NiFe 2 O 4 quantum dots with differentl aterals izes as an efficient OER catalystt hrough am ild and controllable method ( Figure 6f). [57] Figure 6.…”

“…[54] (e) Polarizationc urves for a-NiMoO 4 and b-NiMoO 4 ,a nd IrO 2 /C in 1 m KOH solution. [56] (f) Schematic illustration of atomically thin NiFe 2 O 4 quantumd ots (ATNiFe 2 O 4 QDs) for OER electrocatalysis. [57] .…”

Hierarchical Porous NiO/β‐NiMoO4 Heterostructure as Superior Anode Material for Lithium Storage

“…At atmospheric pressure, there are two phases of NiMoO 4 both with a monoclinic structure: α‐NiMoO 4 (low‐temperature stable phase) and β‐NiMoO 4 (high‐temperature stable phase). The most striking difference between those two polymorphs is the coordination of the Mo 6+ ions, with pseudo‐octahedral coordination in the α‐NiMoO 4 phase and distorted tetrahedral coordination in the β‐NiMoO 4 phase . Generally, β‐NiMoO 4 has a higher intrinsic conductivity and reactivity than α‐NiMoO 4 , but it is very difficult to prepare and maintain.…”