Reductive processing of single walled carbon nanotubes for high volumetric performance supercapacitors

Abstract

Intrinsically, single walled carbon nanotubes (SWCNTs) are excellent candidates for electrochemical double layer supercapacitor (EDLC) electrodes, owing to their high electrical conductivity, high accessible surface area, and high aspect ratio/connectivity, which provide exceptional intrinsic gravimetric energy and power densities. However, in practice, local bundling due to strong intertube van der Waals interactions reduces the effective surface area; at larger scales, the bundling also creates low density networks that limit the volumetric electrochemical performance of practical electrodes. In this study, reductive charging is used to dissolve individual SWCNTs and assemble them to form relatively dense (0.34 g cm⁻³), thick (38 μm) ‘buckypaper’ electrodes, with high electrical conductivity (>400 S cm⁻¹). Intermediate charging ratios (C : Na = 10 : 1) and carbon concentrations (0.125 M) provide greater SWCNT solubilisation and individualisation, and correlate with maximum volumetric capacitance of 74 F cm_electrode⁻³ at 10 mV s⁻¹ in 1 M H₂SO₄. These optimised half-cell electrodes were implemented in full symmetric cell devices, prepared in both aqueous and ionic liquid electrolytes, using a bespoke bacterial cellulose (BC) ultrathin separator (7 microns) to minimize parasitic mass/volume. The full cell performance in ionic liquid reached maximum energy and power densities of 2.6 Wh kg⁻¹ (2.2 mWh cm⁻³), and 10.2 kW kg⁻¹ (8.3 W cm⁻³), respectively, normalised by the total mass and volume of device (electrodes, electrolyte, and separator; no separate current collector is needed). The relatively effective transfer of half-cell to full-cell performance is encouraging but could be optimized further in future. Appropriate normalisations for supercapacitor electrodes and devices are discussed in detail. Thin BC-based separators have wide applicability to other electrochemical devices.