High Field MAS NMR and Conductivity Study of the Superionic Conductor LiH2PO4: Critical Role of Physisorbed Water in Its Protonic Conductivity

Abstract

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 H NMR spectra are observed that are assignable to protons in the short and long O-H center dot center dot center dot 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 center dot center dot center dot O protons. P-31 and Li-7 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 center dot center dot center dot O hydrogen bonds, in contrast to an earlier model that ascribed it to intermolecular hopping of O-H center dot center dot center dot O protons. The new findings enable us to modulate LDP's protonic conductivity by several orders of magnitude via controlling physisorbed water.