Development of Greener Electrode Coatings for Next-Generation Supercapacitors
Authors: Worsley Eleri Anne , Jones Daniel Raymond , Bertoncello Paolo , Margadonna Serena 
Published: December 2024

The demand for improved energy storage technologies will increase significantly in coming years, as global ambitions move towards a net-zero future and intermittent renewable energy sources make up a larger proportion of the total energy mix. To meet this demand, worldwide storage technologies must be diversified to include modern electrochemical energy storage, such as batteries and supercapacitors, at handheld-, household- and grid-scale capacities.

Supercapacitors represent a promising alternative and complementary electrochemical storage method to traditional batteries. They possess cycle lifetimes and power densities far superior to batteries, allowing much faster charging and discharging, but currently lag behind in energy density. Additionally, they are more environmentally friendly than batteries through having no reliance on toxic metals such as lithium. However, the most common formulations for supercapacitor electrode coatings in the literature continue to use hazardous solvents. One of the most common polymer/solvent combinations is polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). NMP is very hazardous to human health and has recently been classified as a “Substance of Very High Concern” by the European Chemicals Agency, leading to use restrictions within the EU. As such, there is an urgent requirement for greener alternatives.

This work explores a variety of alternative, greener solvents for PVDF dissolution, examining their effects on the properties and performance of electric double-layer capacitor (EDLC) electrodes, in comparison to NMP. The research aims to find a green solvent with comparable electrochemical performance and physical properties, alongside a much lower health and environmental impact. Dimethyl sulfoxide (DMSO), γ-valerolactone (GVL), triethyl phosphate (TEP) and Cyrene (dihydrolevoglucosenone) have been investigated; chosen based on their Hansen Solubility parameters and position within the Hansen solubility sphere for PVDF. The electrodes have been analysed morphologically using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), and nitrogen adsorption-desorption analysis (BET), to identify any differences in coating performance and pore structure. A particular focus has been placed on electrochemical testing of EDLC electrodes fabricated using these coatings. Three-electrode testing in aqueous electrolyte has comprised cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) and electrode cycling over 2000 charge-discharge cycles. In this work we show that electrodes produced using Cyrene outperformed the specific capacitance and capacitance retention of all tested solvents, including control NMP samples, across the whole range of current densities from 0 to 200 Ag-1.

Electrodes have also been produced for each solvent using either activated carbon or graphene nanoplatelets as the active material, to confirm that the results are comparable for different carbon structures. This has allowed comparison of the electrochemical properties between the two carbon materials. In addition, each solvent has been assessed for suitability for carbon-incorporated polymer nanowires production for supercapacitor electrode application. This could open the doors to developing environmentally friendly, freestanding electrodes for fast-charging electrical energy storage devices.

Figure 1