Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Final Activity Report Summary - FRRMN (Effect of electrode porosity and nature of electrolyte on performance of double layer electrochemical capacitors. A study on template carbons and ionic liquids)

The increasing demand for highly effective energy storage devices has stimulated intensive research efforts on electrical double-layer capacitors (EDLCs). EDLCs fill the gap between high-energy and high-power devices; they provide a higher energy density than dielectric capacitors and a higher power density than batteries. A growing interest in electric vehicles and hybrid power vehicles (HEV), in which EDLCs are used for providing the peak power required during acceleration, has stimulated on-going research into EDLCs. Energy storage in EDLCs proceeds through the electrosorption of ions on high-surface area porous materials; nanoporous activated carbon have received a widespread use as electrode material for supercapacitors due to available natural precursors and low-cost production technologies.

Current research is focusing essentially on improving the energy performance of EDLCs, which is their main drawback in comparison to most batteries. Energy improvements require increasing two main characteristics of an EDLC, capacitance and voltage, with more impact from voltage. That is why organic electrolytes are predominantly employed in the present-day industrial systems owing to a maximum achivable voltage of 2.7 V as compared to a maximum of 1 V for the aqueous ones. Recent developments have shown that subnanometer pores are the most effective for enhancing specific capacitance, owing to ion desolvation in organic electrolyte. This suggests that carbon porosity be tailored to match the size of desolvated ions. By contrast, our work has revealed the limits of matching the sizes of ions and pores.

By employing a set of cations with a gradually changing cation size in the systems with nanoporous carbons of tuned porosity, we have realised that a close size match of carbon pores and electrolyte ions can lead to the complete filling of the electrode pores before maximum operational voltage is reached. Thus, matching the size of electrode pores to that of electrolyte ions provides gains in capacitance, but can simultaneously give rise to capacitance loss at voltages approaching the maximum. This finding demonstrates the pitfalls and limitations of recently-established strategies for higher energy density, stimulating further research on adapting the electrode materials to electrolytes for prospective EDLC systems.

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