An apparatus comprising an insulating substrate (101); at least two charge collectors (102, 103) spaced apart on the substrate (101) surface; a proton-generating electrode layer (104) configured to generate proton charge carriers; a proton-accepting electrode layer (105) configured to accept the generated proton charge carriers, wherein each of the proton-generating electrode layer (104) and proton-accepting electrode layer (105) are configured to be electrically connected to a different one of the respective charge collectors (102, 103), and to overlap in a junction region such that the charge carriers of the layers can be transferred between the proton-generating (104) and proton-accepting (105) electrode layers to thereby generate a voltage.