High-Energy MnO₂ Nanowire/Graphene and Graphene Asymmetric Electrochemical Capacitors

TLDR

The study aims to develop a high‑voltage asymmetric electrochemical capacitor using graphene as the negative electrode and a MnO₂ nanowire/graphene composite as the positive electrode in neutral Na₂SO₄ electrolyte to achieve high energy and power densities. The capacitor was fabricated by assembling graphene sheets with α‑MnO₂ nanowires into a composite positive electrode, paired with a graphene negative electrode, and tested in a neutral Na₂SO₄ aqueous electrolyte. The asymmetric EC cycled reversibly from 0 to 2.0 V, delivering an energy density of 30.4 Wh kg⁻¹ (much higher than symmetric counterparts), a power density of 5000 W kg⁻¹ at 7.0 Wh kg⁻¹, and retained ~79 % capacity after 1000 cycles, demonstrating the viability of graphene‑based composites for high‑energy, high‑power aqueous asymmetric capacitors.

Abstract

In order to achieve high energy and power densities, we developed a high-voltage asymmetric electrochemical capacitor (EC) based on graphene as negative electrode and a MnO2 nanowire/graphene composite (MGC) as positive electrode in a neutral aqueous Na2SO4 solution as electrolyte. MGC was prepared by solution-phase assembly of graphene sheets and α-MnO2 nanowires. Such aqueous electrolyte-based asymmetric ECs can be cycled reversibly in the high-voltage region of 0−2.0 V and exhibit a superior energy density of 30.4 Wh kg−1, which is much higher than those of symmetric ECs based on graphene//graphene (2.8 Wh kg−1) and MGC//MGC (5.2 Wh kg−1). Moreover, they present a high power density (5000 W kg−1 at 7.0 Wh kg−1) and acceptable cycling performance of ∼79% retention after 1000 cycles. These findings open up the possibility of graphene-based composites for applications in safe aqueous electrolyte-based high-voltage asymmetric ECs with high energy and power densities.