Progress in proton conducting sol-gel glasses

Sol-gel processed phosphosilicates have been investigated for over 30 years for applications requiring a proton conducting electrolyte. The mechanism for proton conductivity most often claimed in these materials is the hopping mechanism. In this mechanism, the proton hops from hydroxyl group to hydroxyl group. The measured conductivity at room temperature ranges from about 10⁻³ to 10⁻² (ohm.cm)⁻¹, where the value depends strongly on relative humidity (%RH) and on chemical composition. Generally, substitutions of organic functions on the siloxane precursor increase the conductivity of the phosphosilicate gel. From this historical perspective, it is seen that phosphosilicate gels, especially those with organic modifications or in composites, are suitable for applications that incorporate a proton conductor. [Figure not available: see fulltext.].