High field MAS NMR and conductivity study of the superionic conductor LiH₂PO₄: Critical role of physisorbed water in its protonic conductivity

LiH₂PO₄ (LDP) is a favored candidate for hydrogen fuel cells, but the mechanism of its high protonic conductivity remains unclear. A complicating factor has been the lack of resolution in the reported proton NMR spectra. We now report multinuclear magic angle spinning NMR in LDP at magnetic fields up to 21.2 T. Well-resolved ¹H NMR spectra are observed that are assignable to protons in the short and long O-H O hydrogen bonds and a peak to physisorbed H₂O. The position and intensity for the H₂O peak depend on the H₂O content, implying fast exchange between the adsorbed H₂O and the O-H O protons. ³¹P and ⁷Li NMR spectra and spin-lattice relaxation measurements showed that the proton hopping/exchange processes involve concerted hindered rotational fluctuations of the phosphate groups. Conductivity data from adsorbed H₂O-controlled samples clearly suggest that the mechanism of LDP's protonic conductivity is dominantly the exchange (and hopping) of the adsorbed H₂O protons with the short O-H O hydrogen bonds, in contrast to an earlier model that ascribed it to intermolecular hopping of O-H O protons. The new findings enable us to modulate LDP's protonic conductivity by several orders of magnitude via controlling physisorbed water.