(Invited) Greener Supercapacitors: Aqueous Binders and Moisture Tolerant Electrolytes

Abstract

Supercapacitors possess higher power densities than batteries and can therefore fill important gaps in applications extending from electric cars to grid stabilization. Accordingly, the market for the industrially most relevant type, electrical double layer capacitors (EDLCs), is estimated to grow to 3 billion US$ by 2025. With the widespreading of such technology, concerns about the environmental impact of EDLCs are unavoidably rising. Besides the use of greener materials, sustainable and low cost production processes, as well as easy end-of-life disposal need to be implemented. In this contribution, research on two highly relevant aspects will be presented: aqueous binders and electrolytes.[1]Binders hold together the active materials in EDLC electrodes, allowing high mass loadings while staying flexible. Suitable candidates are preferentially processable in water, cheap, and electrochemically stable, and show good rheological properties for the optimal coating of the current collectors. Within the ULTIMATE project, a national collaboration coordinated by a leading European manufacturer of EDLCs, it could be demonstrated that mixtures of natural polysaccharides such as starch and guar gum enable increased mass loadings with respect to conventional binder formulations.[2,3] Key to this improvement is the reduced shrinkage of the electrode coating upon drying, which causes severe cracking of thick coatings. Higher loading results in higher areal capacitance because the weight fraction of the active materials is larger compared to the inactive ones, e.g., the current collectors. Finally, the new binders were demonstrated inert towards several conventional and novel electrolytes and not affecting the device’s cycle life in long-term, voltage hold tests.The electrolyte choice has also a determining role with regards of environmental sustainability of the device, both with respect to processing and disposal. As traces of moisture can severely limit the electrochemical stability, electrolytes and assembled cells have to be thoroughly dried with energy intensive procedures. This is especially demanding in the case of ionic liquids. On the other hand, ionic liquids (IL) are often soluble in water and can therefore be recovered from end-of-life EDLCs with environmentally friendly and facile aqueous treatments, while volatile organic solvents need to be removed by more energy intensive heating. Regarding requirements for applications, electrolytes must also provide a wide electrochemical stability window in order to maximize energy density. They should ideally be non-flammable, non-toxic, and cheap. These approaches were then used to verify recent studies on the electrochemical stability of hydrophilic ionic liquids, in this case OTf- and BF4 - based ones. Interestingly, they suggested that hydrophilic ionic liquids with significant water content do not show increased decomposition reactions at elevated voltages, based on simulations and experiments on model carbon surfaces.[4] In more realistic setups closer to application, these results could be tentatively confirmed, suggesting cost and energy savings from a much-reduced need for drying of ionic liquids, while retaining the benefits of fire safety and large voltage windows.