Hydrogen in oxides

Abstract

The paper reviews the history and present understanding of protons in oxides; their defect chemistry, thermodynamics, and transport. Focus is put on correlations between hydration thermodynamics and other materials properties which may help to predict proton uptake and proton conduction in oxides. Also effects of defect association and the particular problem of high grain boundary resistance in high temperature proton conductors are addressed. In the second part, a number of experimental observations attributed to the presence of hydride ions under mildly reducing conditions are discussed in relation to the unlikelihood that general thermodynamic considerations predict of finding these species under such conditions.

Dense Ceramic Membranes for Hydrogen Separation

“…Moreover, the apparent indications of hydride ions in the literature have now been rationalised by other, more credible phenomena, actually arising from transport of neutral hydrogen [2]. Therefore, it seems, at present, that hydride ions play no role in hydrogen permeation in oxides.…”

“…Characterization of hydrogen permeation itself is more difficult as there are more sources of error and misinterpretation. It has been suggested that diffusion of neutral hydrogen species can be a major contributor to hydrogen permeationnot only in metals, but also in ceramics, to a larger extent than formerly anticipated [2]. Finally, framework materials with sub-nano-sized cages appear to provide interesting borderline cases between microporous hydrogen permeation and mixed ionic-electronic conduction [3].…”

“…Others have reported mixed proton transport in the same material [99], providing an illustration of the uncertainty of the relative roles of various oxidation states of hydrogen. If we move on to even more covalent oxides (still with large band gaps) such as SiO 2 , it appears that neutral species take over even more in permeation; both H and H 2 O are believed to diffuse in SiO 2 much as neutral entities [100,101].…”

“…Indications in the literature of negative charge on hydrogen ions under reducing atmospheres -appearing to reflect hydride ion (H -) transport -are now attributed to diffusion of neutral hydrogen in the materials, polarizing the electrodes and giving rise to false gradients [2].…”

The atomic structure of protons and hydrides in Sm1.92Ca0.08Sn2O7−δ pyrochlore from DFT calculations and FTIR spectroscopy

“…This is in accordance with previous assumptions and recent ab initio calculations. 33,34 Formally, H O is a hydride and thus, similar to an oxide, has two valence electrons and an ionic radius of ca. 1.4 Å .…”