Cathodic plasma–induced syntheses of graphene nanosheet/MnO₂/WO₃ architectures and their use in supercapacitors

In this study, we synthesized new 0.01–2 μm graphene nanosheet/MnO₂/WO₃ (G/MnO₂/WO₃) architectures through an electrochemically induced cathodic plasma process in a single batch at a lower temperature (70 °C) and for a shorter time (2 h) than those required for the syntheses of similar structures when using a hydrothermal method. We first obtained 0.01–1 μm leaf-like graphene (G) nanosheets, then 0.1–0.3 μm long and approximately 10 nm diameter petiole-like MnO₂ nanowires on the G nanosheets, and finally 0.20–2.0 μm petal-like WO₃ on MnO₂/G — thereby forming the G/MnO₂/WO₃ architectures — as evidenced using scanning electron microscopy and transmission electron microscopy. We deciphered the step-wise reaction mechanism behind the formation of the G/MnO₂/WO₃ architectures during the plasma process. The high surface area of 291 m² g⁻¹ in the G/MnO₂/WO₃ architecture was contributed mainly by the G nanosheets, providing a suitable surface area for diffusion of the charge carriers during the charging and discharging process. As a result, an electrode incorporating the G/MnO₂/WO₃ architectures exhibited an excellent specific capacitance of 620 F g⁻¹ — 45 and 200% higher than those of G/MnO₂ (421 F g⁻¹) and G (189 F g⁻¹) electrodes, respectively — at a current density of 0.5 A g⁻¹. Moreover, the G/MnO₂/WO₃–incorporated electrode exhibited good electrochemical cycling stability, with 90% capacitance retention over 5000 cycles at 1 A g⁻¹. Such new G/MnO₂/WO₃ heterojunction structures, not only provide high-performance electrode applications, but also suggest a potential approach toward fabricating other heterojunction structures having high surface areas for energy storage applications.