Proton and Protonic Species: The Hidden Face of Solid State Chemistry. How to Measure H-Content in Materials?

The first objectives of Chemistry are the description of the reactions and the determination of the composition of each compound involved in a process. Solid State Chemistry tries to establish the composition and the structure, the (micro/nano)structure as well as the relationship between composition, structure, and material properties. In the case of Hydrogen element, most of the literature data do not follow this approach: the contents of protons and associated protonic species are rarely measured. Based on the examples of proton conducting materials with different water content: (i) high (solgel materials), (ii) medium (polyaniline (PANI) polymer and salts), and (iii) anhydrous (protonated perovskites), we will discuss the main methods (TGA, thermal expansion, infrared and Raman spectroscopy, incoherent (elastic, quasi- and in-elastic) and coherent neutron scattering, neutronography, NRA, RBS/ERDA, ERCS, etc.) able to detect/measure the proton content and/or to identify the nature of the protonic species and their interaction with the host framework: hydroxyl ion, free proton, physi/chemi-sorbed water, strong H-bonded water, proton hydrates, aquo-hydroxycarbonates, etc. Emphasis is given to the difficulties encountered for the identification and the quantification of protonic species and the way to overcome them, in particular the discrimination between the surface and bulk moieties.