Supercapacitor component

Cu(OH)₂ nanotube (CuOHNT) arrays are grown directly on a copper-tape (copper ribbon) substrate via a simple one-step reaction reported elsewhere^24,25,26. The copper tape with adhesive side masked is introduced into a chemical bath consisting of 3 g of NaOH, 0.684 g of (NH₄)₂S₂O₈ and 30 ml of de-ionized (DI) water. After ∼20 min submersion, the colour of the copper samples began to turn into a light shade of blue, indicating the growth of Cu(OH)₂ nanotubes. To improve charge transport between the tubes, Ag nanowires (Blue Nano) dispersed in isopropyl alcohol at a concentration of 1 mg ml⁻¹ are drop cast (6 μl) onto the Cu(OH)₂ electrodes and left to dry for 6 h at 45 °C in an oven⁶⁴. Addition of AgNW between CuOHNT provides an interconnected conductive network among the nanotube architecture (Supplementary Fig. 3c). To make the charge transport more efficient, we deposited a thin layer of copper or gold–palladium (AuPd) by sputter coating (Supplementary Fig. 3d) at a constant current of 40 mA for 6 min on top of the CuOHNT and AgNW. AuPd coating provides a better conductivity compared with Cu (refs ^{39, 65}). This highly conductive matrix constitutes a favourable surface for MnO₂ electrodeposition³⁹ on/into the tubular nanoarchitecture (Supplementary Fig. 3e). MnO₂ is deposited for 12 min onto the nanostructures via an anodic electrodeposition method at a constant current density of 0.5 mA cm⁻². The electrolyte for electrodeposition was prepared by dissolving 0.01 M manganese acetate and 0.02 M ammonium acetate into a solvent containing 10 v/v % of dimethyl sulfoxide and 90 v/v % of DI water. Supplementary Fig. 3f,g shows the TEM images of MnO₂-deposited CuOHNT as well as the layer thickness of the deposited material on the final electrodes. The loading of the MnO₂ in a single electrode is ∼0.2 mg. The electron loss spectroscopy (ELS) in TEM is explored to monitor the MnO₂ morphology on the nanotubes surface as shown in Supplementary Fig. 3h. The X-ray photoelectron spectroscopy survey spectrum in Supplementary Fig. 3i reveals the presence of Au (84 eV), Pd (336 eV), Ag (378 eV), Cu (934 eV) and Mn (643 eV) in the final composition of the MnO₂-deposited nanotubes^65,66. This indicates that the deposition of target materials such as MnO₂ provides a smooth nanostructure on the CuOHNT and two peaks of MnO₂ represent the Mn⁴⁺ oxidation state to experience excellent electrochemical response. Moreover, the tubular nature of the high aspect ratio nanostructures on the electrode surface provides highly enhanced surface area and high ion mobility at the electrode–electrolyte interface in contact with electrolyte ions^28,67,68.

The finalized supercapacitor device is then fabricated by wetting the electrode surface with 1 M KOH/PVA gel electrolyte and allowing it to partially dry. Two symmetric electrodes are then pressed together with a pressure of 1 Mpa for 1 h. All electrochemical performances were recorded after cycling the electrodes 20 times.