Post-synthesis Physical and Hydrothermal Mixing of MnO₂, SnO₂, and rGO Influence on Ultracapacitor Electrode Performance

The application of nanoscale materials in electrochemical capacitors (ECs) is promising; however, further advanced development and understanding are required to enable their practical use. The aim here was to investigate how simple physical mixing and the hydrothermal method influence EC electrode performance when MnO₂ nanorods and SnO₂ nanoparticles were combined as binary and ternary nanocomposites with ascorbic acid-reduced graphene oxide (rGO). The key advantage of this approach is the combination of metal oxides after synthesis, unlike the common in situ synthesis during compositing. The MnO₂ nanorods exhibited higher specific capacitance (C_s, 288.9 F g⁻¹) compared to SnO₂ nanoparticles (126.1 F g⁻¹). The physical-mixed composite, namely, MnO₂+ SnO_2, produced electrodes with ≈ 400% and ≈ 900% C_s enhancement relative to MnO₂ and SnO₂, respectively. Meanwhile, electrodes from the MnO₂+ SnO₂-rGO improved the C_s of MnO₂ nanorods and SnO₂ nanoparticles by ≈ 544% and ≈ 1247%, respectively. Additionally, physical mixing achieved power densities of 22321 W kg⁻¹ (MnO₂+ SnO₂) and 22500 W kg⁻¹ (MnO₂-SnO₂). The electrodes from hydrothermally prepared MnO₂–SnO₂ and MnO₂–SnO₂–rGO attained the highest C_s retention between 93% and 80%, respectively, after 5000 cycles. The study highlights the tremendous potential of the cost-effective, simple, quick, and energy-saving physical mixing method in materials engineering for ECs.