Nanostructured thin film pseudocapacitive electrodes for enhanced electrochemical energy storage

Abstract

This thesis presents work relating to the fabrication of novel thin film electrodes for energy storage applications, with a focus on low cost, nanostructured transition metal oxides, and electrode manufacture by atomised spray deposition. Iron oxide (FeO_x) nanowires were synthesised hydrothermally and combined with multi-walled carbon nanotubes (MWNT) in sprayed electrodes, which provided the necessary conductivity enhancement for effective energy storage. The spray processing technique allowed for facile control over the relative fraction of MWNTs in the sprayed electrodes. Optimised electrodes were investigated in a range of aqueous electrolytes, and the best energy storage behaviour occurred in Na₂SO₃ with a maximum capacitance from cyclic voltammetry of 312 Fg^-1 at a scan rate of 2 mVs^-1. The FeO_x/MWNT electrodes were investigated for their suitability as lithium-ion battery anodes and showed reasonable energy storage behaviour. Nickel oxide (NiO) electrodes were manufactured by hydrothermal synthesis and annealing followed atomised spray deposition. The performance of the NiO electrodes was enhanced though combination with aqueous graphene suspensions, produced in-house by ultrasonic exfoliation of graphite. The processing route used to combine the nanomaterials was considered and a co-synthesis route resulted in the best performing electrodes. Different substrates were investigated, as the most commonly used Ni-foam substrate reacted with the basic electrolytes necessary for electrochemical activity of NiO. NiO/graphene electrodes showed charge/discharge capacitances of up to 571 Fg^-1 at a current density of 10 Ag^-1, which was maintained at over 300 F/g at a very high current density of 100 Ag^-1. Asymmetric supercapacitor devices were constructed using various combinations of FeO_x, NiO, and commercial carbon black electrodes to extend the operating potential window beyond the ~1.23 V limit of symmetric aqueous-electrolyte devices. Power densities of over 20 kWkg^-1 were achieved for an FeO_x/MWNT-carbon device, which was comparable with current commercial carbon-only supercapacitors.