Composition-Dependent Pseudocapacitive Behavior of MoO₃-V₂O₅ Composite Thin Films in an Aprotic Electrolyte

Transition-metal oxide electrodes offer high theoretical capacitance for electrochemical energy storage. Yet, their practical application is fundamentally constrained by chemical instability in aqueous electrolytes, where high-valence metal centers undergo rapid hydrolytic dissolution. Here, we prepared MoO₃-V₂O₅ composite thin films across different compositions. Structural characterization reveals a composition-dependent evolution from orthorhombic phases in the pure oxides to the coexistence of both the metal oxides in intermediate compositions. We employ an aprotic organic electrolyte (LiBF₄ in propylene carbonate), which suppresses hydrolytic degradation pathways and enables stable operation over an expanded electrochemical window. Under these conditions, V₂O₅-rich thin-film electrodes exhibit pronounced pseudocapacitive behavior, delivering a specific capacitance of 25 F g^-1 and capacitance retention of 89.7% at a current density of 1 A g^-1 after 10,000 cycles along with a columbic efficiency of 98.4% in a symmetric device configuration. This work demonstrates a synergistic interaction between MoO₃ and V₂O₅ phases, leading to enhanced charge storage behavior compared to the individual oxides.