Acid solutions exhibit
proton-conductivity, while pure proton conductors are usually dry solids. Typical materials are polymers or ceramic. Typically, the pores in practical materials are small such that
protons dominate
direct current and transport of cations or bulk solvent is prevented.
Water ice is a common example of a pure proton conductor, albeit a relatively poor one.[2] A special form of water ice,
superionic water, has been shown to conduct much more efficiently than normal water ice.[3]
Proton conduction has also been observed in the new type of proton conductors for fuel cells – protic organic ionic
plastic crystals (POIPCs), such as 1,2,4-triazolium perfluorobutanesulfonate[5] and
imidazolium methanesulfonate.[6] In particular, a high ionic conductivity of 10 mS/cm is reached at 185 °C in the plastic phase of imidazolium methanesulfonate.
When in the
form of thin membranes, proton conductors are an essential part of small, inexpensive
fuel cells. The polymer
nafion is a typical proton conductor in fuel cells. A jelly-like substance similar to Nafion residing in the
ampullae of Lorenzini of sharks has proton conductivity only slightly lower than nafion.[7][8]
High proton conductivity has been reported among alkaline-earth cerates and
zirconate based perovskite materials such as acceptor doped SrCeO3, BaCeO3 and BaZrO3.[9] Relatively high proton conductivity has also been found in rare-earth ortho-niobates and ortho-tantalates as well as rare-earth tungstates.[10]
References
^Traditionally, but not precisely, H+ ions are referred as "
protons".