Electrochemical capacitors, also called supercapacitors, are very
promising energy storage devices, applied both in stationary and
mobile systems for delivering high power pulses [1e3]. Since the
energy density of supercapacitors is proportional to the square of
voltage, most of the devices available in the market are based on
organic electrolytes, e.g., tetraethylammonium tetrafluoroborate in
acetonitrile or propylene carbonate, despite their harmfulness