A Reduced Graphene Oxide-Polyoxometalate Composite as Stable and Efficient Pseudocapacitive Material in Aqueous Solution

Abstract

Due to its high electrical conductivity and large specific surface area, graphene is a highly promising material for electrochemical energy storage applications. However, its practical use remains limited due to stability issues, primarily due to π–π stacking interactions between the graphene sheets. Herein, a graphene-based composite is reported that overcomes this limitation. This composite consists of reduced graphene oxide (rGO) decorated with polyoxometalate (POM) nanoclusters, [SiW₁₂O₄₀]⁴⁻. To obtain this composite, first, [SiW₁₂O₄₀]⁴⁻ ions are electrochemically reduced, then the solution is mixed with a suspension of graphene oxide (GO). The reduced POMs reduce GO and deposit on the graphene sheets, leading to a rGO@POM composite. The composite suspension could be drop casted onto an electrode without requiring binders. The interest of [SiW₁₂O₄₀]⁴⁻ is its reversible redox properties with the potentials in cathodic domain allowing to explore an unusual potential domain (1.6 V) in an aqueous electrolyte (Na₂SO₄/H₂SO₄, pH 4). This approach afforded a pseudocapacitive material with excellent stability, showing no capacitance loss over 20,000 cycles at 1 V•s⁻¹. Furthermore, the synergistic effect between the faradaic contributions due to [SiW₁₂O₄₀]⁴⁻ and the rGO capacitive behavior results in a high volumetric capacitance exceeding 300 F cm⁻³ and an outstanding energy density of 26 mWh cm⁻³.