Vanadium Pentoxide with H₂O, K⁺, and Na⁺ Spacer between Layered Nanostructures for High-Performance Symmetric Electrochemical Capacitors

Vanadium oxide 2D layered nanostructures with the hydrous form of potassium (K⁺) and sodium (Na⁺) are synthesized via hydrothermal reaction between VOSO₄ · xH₂O and different persulfate oxidants ((NH₄)₂S₂O₈, K₂S₂O₈, and Na₂S₂O₈). The physicochemical characterization suggests that the synthesized V₂O₅ · 3H₂O nanostructures possess layered morphology with considerable amount of water molecules accommodated between the interlayer spacing of nanostructures. Moreover, samples obtained using K₂S₂O₈ and Na₂S₂O₈ oxidants have K⁺ (6.41%) and Na⁺ (0.38%) ions intercalated on the 2D nanostructure along with the water molecules. Subsequently, the synthesized samples are heat-treated at 400 °C for 3 h in the air and it is observed that the structural and morphological aspects are highly affected due to the removal of surface- and lattice-bonded water molecules. The electrochemical properties of the samples are studied by assembling symmetric supercapacitor devices utilizing the bare and heat-treated samples in 1 m Na₂SO₄ electrolyte. The fabricated device with bare V₂O₅ · 3H₂O samples shows maximum specific capacitance (>60%) than that of heat-treated V₂O₅ samples, which represents the positive influence of water molecule on electrochemical behavior of V₂O₅. Moreover, the bare sample V₂O₅ · 3H₂O (using (NH₄)₂S₂O₈) symmetric supercapacitor exhibits an excellent cyclic stability with high capacitance retention of ≈97% after 25 000 cycles.