A Dual-Metal Oxide Strategy: Fabricating 3D Hierarchical SnO₂/MnO₂ Composites for Enhanced Supercapacitors

Metal oxides have been widely studied as an efficient and robust electrode material for supercapacitors for a couple of decades due to their affordability, high capacitance, multiple oxidation states, pseudocapacitive nature, and excellent redox activity. In this paper, a SnO 2 -MnO 2 hierarchical nanostructure (HNS) composite has been developed via a cost-effective and straightforward hydrothermal reaction. FESEM and TEM analysis revealed flower-like and nanowire-like morphologies for SnO 2 and MnO 2 , respectively , with highly pure, crystalline tetragonal structure. The average size of SnO 2 microflowers and MnO 2 nanorods is 1.5 μm and 2.3 μm. The chemical composition and oxidation states of the samples were investigated using Energy-Dispersive X-ray (EDX) and X-ray Photoelectron Spectroscopy (XPS), specifying the adequate formation of the SnO 2 -MnO 2 HNS composite. The surface area and pore analysis of the samples reveal that the SnO 2 -MnO 2 HNS composite yields the highest BET surface area of 62.93 m 2 /g with mesoporous features centered at ~9   nm. Remarkably, at a low current rate of 1   A/g, the SnO 2 -MnO 2 HNS composite achieved a high capacitance of 520   F/g as compared to pure SnO 2 and MnO 2 in a three-electrode mode assembly at identical conditions. The emergence of heterointerfaces enhanced the capacitance of the composite by providing more surface area for charge transfer and storage at the electrode/electrolyte interface. An asymmetric hybrid supercapacitor (HASC) was constructed utilizing two pseudocapacitive electrode materials: a SnO 2 -MnO 2 HNS composite as the anode and FeSe 2 as the cathode, represented as SnO 2 -MnO 2 ||FeSe 2 . The fabricated HASC attained a high specific capacitance of 111   F/g, a high energy density of 44.5   Wh/kg, and a power density of 852   W/kg at 1   A/g. At higher current rates, the power density extends to 4153   W/kg, with an energy density of 15   Wh/kg. Similarly, notable stability was observed, with 93.5% capacitance retention after 10,000 cycles during HASC assembly. Therefore, metal oxides are extensively explored as electrode materials for supercapacitors, and their superiority remains a top priority for researchers and engineers in the design of viable and renewable energy technologies.