Excess of oxygen in thermodynamically unstable H2MoO5 enables high-performance all solid-state supercapacitors

Pseudocapacitors with oxygen-enriched vacancies have been the state-of-the-art surface chemistry to invoke various intrinsic mechanisms. Nevertheless, the electrochemical behavior of vacancies-induced properties of MoO₃ is still under debate. In this work, we report an oxygen-enriched polymorph of molybdenum trioxide (MoO₃), i.e., H₂MoO₅, which is a thermodynamically unstable phase of MoO₃ with aliovalent oxygen ions (O₂^2- and O₂^-), to achieve a higher amount of pseudocapacitance compared to its thermodynamically stable phase (alpha-MoO₃). Mott-Schottky analysis identified a higher proportion of oxygen vacancies in H₂MoO₅ compared to MoO₃. A symmetric supercapacitor of H₂MoO₅ with PVA/H₂SO₄ displayed a high charge storage of 46.54 F/g at a current density of 0.5 A/g, maintaining a remarkable cycle life of up to 6000 cycles. Furthermore, the oxygen-enriched cell could deliver a high-power density of 470 W/kg at a higher energy density of 22.8472 Wh/kg. The ability to tune oxygen vacancies in metal oxide systems opens a new platform to enhance pseudocapactive character without compromising the energy density.