Electrochemical and energy storage properties of layer-by-layer assembled vanadium oxide electrode-based solid-state supercapacitor in n+-SnO2:F/n-V2O5 heterostructure device form using ionic liquid gel electrolyte
Farshad Azadian, A. C. Rastogi
Abstract
Vanadium oxide film electrodes synthesized by layer-by-layer assembly using sol–gel spin casting and variable 3 & 1 h annealing process over SnO 2 :F film coated glass substrates are investigated for supercapacitor energy storage using ionic liquid gel-electrolyte. The X-ray photoelectron spectroscopy analysis of V2p 3/2 core-level and O1s peak show short-term (1 h) annealing forms vanadium as V 2 O 5 alongside multivalent and oxygen deficient phases, whereas synthesis by 3-h annealing forms stochiometric V 2 O 5 film. In stochiometric V 2 O 5 , capacitive contributions are dominantly from redox processes with peaks identified from V 5+ /V 4+ , and V 4+ /V 3+ valance states change and partially from electrical double layer (EDL) yielding high specific capacitance of 346.9 Fg −1 . Supercapacitor with V 2 O 5 in mixed valence and oxygen defects phases shows enhanced EDL contribution alongside Faradaic yielding 316.2 Fg −1 specific capacitance. These mechanistic differences are analyzed for ionic diffusion limitations. The linear charge/discharge curves at 0.04–0.15 A/g current density show 85–90% Coulomb efficiency and steady energy density 6.8–5.5 Whkg −1 as specific power increases from 1.9 to 5.7 kWkg −1 . Oxygen-deficient V 2 O 5 -based supercapacitor shows higher 19.2 Whkg −1 energy density declining to 8.9 Whkg −1 with specific power change from 1.1 to 4.6 kWkg −1 . Raman spectra show during charge/discharge, the V 5+ - V 3+ redox is mediated by V 4.67+ , V 4.57+ , and V 3.33+ intermediate valence states. The pseudocapacitive energy storage depends on charge transfer across n + -SnO 2 :F/n-V 2 O 5 heterostructure with different band alignments for stochiometric and oxygen deficient V 2 O 5 . The conduction band shift to the flat band position determines the potential range and extent of V 5+ -V 3+ redox reaction.