Electrode properties of layered tungsten-based oxides for electrochemical capacitors

Abstract

Electrochemical properties of layer-structured H₂W₂O₇ for electrochemical capacitors were investigated in an aqueous electrolyte for the first time. H₂W₂O₇ was synthesized from Bi₂W₂O₉ by proton-exchanges with acid treatment. H₂W₂O₇ exhibited a low redox potential of about −0.3 V in a H₂SO₄ electrolyte solution. The H₂W₂O₇ electrode showed an oxidation capacity of 56 mAh g⁻¹ at a current density of 300 mA g⁻¹, which is equal to the theoretical capacity assuming that 0.5 electrons react with one tungsten ion. H₂W₂O₇ also retained 88% oxidation capacity at a high current density of 2 A g⁻¹ against that at a low current density of 300 mA g⁻¹. It is assumed that redox reaction rate mainly depends on proton diffusion in H₂W₂O₇. Moreover, this material showed excellent cycle stability over 1000 cycles without capacity loss. It is clarified that a H₂W₂O₇ electrode is a promising electrode for electrochemical capacitors featuring a high rate capability and good cycle life.