Hydrothermally Enhanced Tungsten-Tungsten Bronze (W-MxWO3) Electrodes for Pseudocapacitive Energy Storage

Abstract

Hexagonal tungsten bronzes (h-MxWO3) are an emergent class of pseudocapacitive energy storage material: a metastable phase arranged in a hexagonal crystal structure that enhances the oxide’s rapid intercalation mechanism with impressive capacitances of over 200 F/g and excellent cyclability figures reported. A symmetrical pseudocapacitive device composed of h-MxWO3 electrodes negates the need for expensive and problematic rare minerals such as cobalt and lithium instead using low-cost tungsten whilst offering more efficient high-rate energy storage than commercial lithium-ion batteries.Hexagonal tungsten bronze has shown some promising capacitances in literature in conventional electrode configurations, i.e. composite structures with binder support materials which increase electrode mass impacting specific capacitance. If the bronze was instead grown directly upon a current-collecting substrate, this would negate the need for additional electrode components, with the entire electrode mass participating in storing charge. The present work explores this concept of a novel binder-free electrode design where the nanostructured tungsten-bronze (active material) is grown directly upon a tungsten-based substrate (current collector). This simple design offers potentially higher specific capacitances than conventionally fabricated electrodes.Nanostructured W/h-MxWO3 electrodes were fabricated in a twostep process. Firstly, tungsten foil was oxidised in hot nitric acid, forming a seed layer of monoclinic-WO3. A second step saw the hexagonal bronze deposited upon substrates through a hydrothermal dehydration reaction of sodium tungstate (Na2WO4) where the effect of the directing agent (NaCl, Na2SO4, (NH4)2SO4& Rb2SO4) on the deposited nanostructure was investigated. To assess the benefit of the acid pretreatment step, vis-à-vis a pristine untreated tungsten foil, the hydrothermal reaction was completed upon both a pristine tungsten foil and an acid treated foil. The nanostructured surface deposited from the hydrothermal reaction was characterised by SEM, and the crystal phase identified by XRD. The electrochemical behaviour was analysed using a Biologic SP-300 potentiostat in a 3-electrode cell: as-synthesised foils employed as working electrode; Pt Wire as counter electrode; a saturated calomel electrode (SCE) as reference; and 0.5M H2SO4 as electrolyte. The performance of devices fabricated from two symmetrical as-synthesised electrodes was also characterised using both the potentiostat and a NEWARE battery cycling test system in a two-electrode cell configuration.XRD analysis confirmed that hexagonal-tungsten bronzes were successfully deposited upon substrates irrespective of the substrate pretreatment. The acid pretreatment generally resulted in larger masses of bronze being deposited upon the substrate, but this did not necessarily translate to superior electrochemical performance. The choice of directing agent present in hydrothermal synthesis drastically altered the nature of the deposited layer as may be seen in the provided figure of as-synthesised electrodes (figure b-d). Sodium directed samples formed powder-like deposits (figure b), ammonium sulphate produced larger colloidal bronze layers (figure c) and rubidium sulphate developed glossy smooth sheets (figure d). Additionally, the ratio of tungstate ions (WO4 2-) to directing agent cations (M+) was identified as being critical to obtaining the hexagonal phase. The mechanical stability of the deposited bronze proved problematic, with the layer prone to delamination and disintegration from a variety of mechanisms. A bespoke electrode holder, device testing apparatus and handling procedure was therefore developed to enable repeatable electrode characterization.The cyclic voltammetry response of the hydrothermally deposited layers was typical of an intercalation system, though an initial pseudocapacitive behaviour deteriorated to a battery-like response at sweeprates above 20 mV/s. Galvanostatic charge-discharge testing displayed a significant pseudocapacitive response over limited potential windows; moving beyond these limits produced steep non-linear curves indicating a departure from pseudocapacitive charge transfer processes. The charging at current densities above 1 A/g occurred over rapid time scales showing the electrodes’ suitability for high power applications. Preliminary 3-electrode capacitance results are encouraging with specific capacitances of over 100 Fg-1 by dry mass. The tunnelated M+ ion in the bronze is found to greatly influence the electrochemical performance of the electrode.This work reports the first example of hexagonal tungsten bronze being successfully grown directly on tungsten substrates for application as energy storage electrodes. The binder free electrodes were found to deliver promising performance characteristics in both 3-electrode analysis and in a two-electrode symmetrical device. The performance so far is hampered by the mechanical fragility of the electrode structures, while scale-up of the manufacturing process may prove challenging. The observed mechanism of deposition of hexagonal tungsten bronze from solution to substrate is of interest for future work, as is the significance of tunnelated M+ cations to the hexagonal bronze (h-MxWO3) structure, stability, and electrode performance (figure a). Figure 1