The first electrode 110 and the second electrode 120 each comprise a multi-layer structure, according to the exemplary configuration shown in FIG. 1. As shown in FIG. 1, the first electrode 110 comprises an ionically conductive hydrogel 112 and a metal conductor 114 coupled to the ionically conductive hydrogel 112. The hydrogel 112 and the metal conductor 114 form separate layers that define the first electrode. The second electrode 120 is substantially identical in construction to the first electrode 110.

Referring again to FIG. 1, the second electrode 120 comprises an ionically conductive hydrogel 122 and a metal conductor 124 disposed on the hydrogel 120. The hydrogel 122 and metal conductor 124 form a multi-layer structure defining the second electrode 120. In the first electrode 110 and the second electrode the metal conductor is coupled to the hydrogel such that the hydrogel and the metal conductor are in electrical communication with each other. The metal conductors 114 and 124 may be constructed from aluminium.

The metal conductor being attached to the hydrogel results in an electrical double layer. The area of the metal conductor is only a small portion of the entire area of the hydrogel (112, 122). The small area of the metal conductor on the hydrogel (112, 122) ensures stability of the hydrogel by avoiding interfacial electrochemical reactions.