Multifunctional carbon nanotube networks for energy storage applications

Abstract

Efficient electrochemical energy storage is a central challenge for the 21st century. Single-walled carbon nanotubes (SWCNTs) are attractive for energy storage applications owing to their excellent electrical conductivity, stability, and surface area.1, 2 This Thesis focuses on the fabrication of multifunctional supercapacitors from SWCNTs, specifically maximising gravimetric and volumetric performance in full devices by systematically optimising the nanostructures of electrodes and separator. Strong inter-tube van der Waals interactions typically drive the formation of ‘bundles’ that reduce electrochemical performance.3, 4 This work creates the first examples of SWCNT buckypapers using reductive methods to minimise rebundling (“nanotubide buckypapers”). The nanotubide anions are generated using sodium naphthalide as a charging agent. The carbon and sodium concentrations control the dissolution process, as well as the microstructure and electrochemical performance of the resulting electrodes. The nanotubide buckypapers were optimised in three-electrode system, then assembled to form full symmetric supercapacitors. With an ionic liquid electrolyte, the energy and power densities reached 2.2 mWh/cm3and 8.3 W/cm3, respectively, normalised by total volume of the device. The use of an ultrathin bacterial cellulose (BC) film (7 microns) as a separator lowered resistive loses and maximised volumetric performance. The reductive chemistry provides a further opportunity to limit rebundling, by covalently cross-linking the individualised SWCNT anions.5, 6 The cross-linking reaction with p diiodobenzene (DIB) was explored as a function of reagent concentration and stoichiometry. Cross-linking generally improves the gravimetric performance and enhances rate capability at high scan rate. The performance of the most successful cross-linked nanotubide electrodes was then extended by varying the electrode thickness, to find the optimum balance between maximising the active material and its rate performance; ionic transport limitations were found to dominate at SWCNT loadings above 2.8 mg/cm2 (55 microns). In order to increase energy density, a redox active polymer, polypyrrole (PPy), was hybridised with the cross-linked nanotubide buckypapers (CNBs) via electrodeposition. By adjusting the electrodeposition potential and pulse sequence, it was possible to generate idealised coatings of PPy throughout the CNB. The introduction of PPy increased the specific energy to 63 Wh/kg, compared to 28 Wh/kg for the pure CNB, in three-electrode measurements using 1 M sulphuric acid as the electrolyte. The optimised hybrid electrode was combined with a pure electrochemical double layer electrode to form an asymmetric device. To balance the overall capacitance, a new design was developed by sandwiching one positive hybrid electrode (PPy-CNB) between two negative electrodes (CNBs). Whilst acting as an active electrode, CNBs have significant mechanical strength (~20 MPa) and Young’s modulus (~0.3 GPa), indicating potential importance in the field of structural energy storage. Although mechanical and electrochemical performance typically conflict, the addition of PPy was found to enhance both aspects.