We
develop an environmentally benign supercapacitor, which decomposes
by contact with water, incorporating an ionic liquid, carbon powder,
a cellulose separator, and a molybdenum electrode. The ionic liquid
is dispersed into a water-dissolvable polymer, poly(vinyl alcohol),
to produce a solid electrolyte, so-called ionic gel. A carbon composite
mixed with the ionic liquid maintains a gel form. The ionic gel and
the carbon composite enable an all-solid-state supercapacitor, which
can be charged at a voltage of 1.5 V. The supercapacitor shows areal
and volumetric capacitances of 65 mF/cm2 and 2.2 F/cm3, respectively. A cycle test reveals that capacitance retention
and Coulombic efficiency are 77 and 90%, respectively. As for the
dissolution test, the ionic gel and carbon composite dissolves in
phosphate buffer solution in 18 days, and the Mo electrode is able
to fully dissolve in 500–588 days. Potential applications of
the environmentally benign supercapacitor include smart agriculture
by monitoring of soil and disaster prevention by a wireless sensor
network without the need for retrieval of devices after use.