Thesis
Hydrothermal synthesis of binder-free tungsten-tungsten oxide electrodes for electrochemical energy storage
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2025
- Thesis identifier
- T17194
- Person Identifier (Local)
- 201959875
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Binder-free metal-metal oxide architectures are a widely reported, highly promising electrode design for catalytic, semi-conducting, and sensing applications. Despite the wealth of existing literature on the synthesis of metal-metal oxide electrodes and, separately, the well-documented pseudocapacitive intercalation of cations in transition metal oxides, no attempt has been made to integrate such a design for applications in batteries or supercapacitors. This design streamlines fabrication and post-life recycling by limiting materials, reduces demand on transition metals, and may be expected to deliver excellent performance as the active charge storage layer is directly bonded to the current collecting material. Tungsten oxide (WO3) and conjugate metal tungsten (W) were considered an excellent initial test case for binder-free energy storage electrodes, with extensive literature to draw from in both these areas. Tungsten oxide is an emergent pseudocapacitive energy storage material, capable of reversibly intercalating protons within its structure upon simultaneous transfer of charge. In particular, the metastable hexagonal crystal phase has been shown to possess excellent kinetics owed to a tunnel structure, achieving attractive specific capacitances of over 300 Fg-1. Similarly, the synthesis and characterisation of tungsten-tungsten oxide electrodes are widely reported in fields such as electrochromism, photocatalysis and gas-sensing. The purpose of this project is to merge these two areas, taking a first step into appraising binder-free electrodes for energy storage using a base case of tungsten-tungsten oxide (W-WO3) electrodes; delivering full physical and electrochemical characterisation of W-WO3 electrodes whilst using the learnings to inform future work. In this study, eight sets of W-WO3 electrodes were synthesised using a hydrothermal reaction pathway adapted from existing literature using four different capping agents with sodium tungstate, successfully growing hexagonal-WO3 upon the surface of tungsten foil. It was demonstrated that a ratio of tungstate to cation is critical in delivering the hexagonal crystal structure and the interplay between anion and cation in solution was scrutinised. The energy storage characteristics of the hexagonal-WO3 electrodes were studied through electrochemical potentiodynamic and galvanostatic investigation as well as potentiation impedance spectroscopy (PEIS). The electrochemical data was thoroughly analysed the extent of pseudocapacitive charge storage quantified, and capacities reported of up to 1.4 mAh/cm2 at 1 mVs-1 and gravimetric capacitances of 178 Fg-1. X-ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM) were used to analyse the crystal phase and microstructure of each of the synthesised electrodes. The nature of the physical surface was linked to the electrochemical performance of each electrode; rubidium sulphate was found to be a particularly effective capping agent delivering the best-performing electrodes of the sample, attributed to a unique nano-urchin morphology with a high surface area porous structure. The hydrothermal synthesis scheme was then empirically analysed by systematic removal of reactants and changing reaction conditions to better understand the factors controlling the growth mechanism of WO3. The output result of each change was quantified using electrochemical potentiodynamic characterisation as a measure of electrochemical activity and SEM as a method of observing the physical change. It was found that the absence of sodium tungstate resulted in thinner coatings and the formation of the monoclinic phase. An acid-only hydrothermal reaction in the absence of both sodium tungstate and capping agent produced an electrode with highly promising performance, pointing to a key design trade-off between thin oxide layers possessing better kinetics but less capacity than thicker ones. To give perspective to the merits and inadequacies of the binder-free W-WO3 design, a straightforward comparison between the synthesised electrodes and conventional powder-based electrodes was completed, using the tungsten oxide powders produced as a by-product from each reaction of the initial synthesis scheme. The physical nature of the powders underwent thorough examination through XRD and SEM analyses, revealing each has a hexagonal crystal structure. The microstructure of each powder exhibited significant similarities to the binder-free electrodes synthesized under identical conditions. Using potentiodynamic and galvanostatic characterisation, the powders were found to have superior electrochemical performance than the binder-free foils, higher gravimetric capacities, and better capacity retention at faster rates of charge/discharge. The improved performance is ascribed to the thin layer of active material better facilitating ion movement and charge transfer. A final investigation applied the synthetic procedure used on tungsten foil to molybdenum instead, of the element above tungsten in the periodic table. The resultant foil exceeded the performance of the WO3 powder and W-WO3 electrodes documented pointing to an exciting avenue of future binder-free research.
- Advisor / supervisor
- Roy, Sudipta
- Brightman, Edward
- Resource Type
- DOI
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