An apparatus including a first electrode including a substantially homogeneous mixture of graphene oxide and a proton conductor; a second electrode including reduced graphene oxide; and spaced-apart charge collectors for the respective first and second electrodes, wherein the first and second electrodes extend from their respective charge collectors towards one another to form a junction at an interface there between, and wherein the substantially homogeneous mixture of the first electrode is configured to be sufficiently hydrophobic to prevent intermixing of the homogeneous mixture with the reduced graphene oxide of the second electrode in proximity to one or both of the respective charge collectors to prevent short circuiting of the spaced-apart charge collectors.