Fast and Anisotropic Proton Conduction in a Crystalline Polyphosphate

Abstract

Lanthanum polyphosphate (LaP3O9) is an attractive candidate for the electrolytes in fuel cells because of its relatively high proton conductivity. However, the proton conduction mechanism in LaP3O9 (i.e., proton transport pathways and its relationship with the crystal structure) still remains poorly understood; thus, there has been no clear strategy for enhancing the conductivity. In this paper, we show that the fast and anisotropic proton conduction pathways exist along the b- and c-axes in the crystal lattice of LaP3O9, and the proton conductivity can be remarkably improved by controlling the microstructure of the electrolyte membranes. The first-principles calculations reveal that protons migrate only along the neighbors of specific oxide ions in the PO4 chains, leading to the conductivity anisotropy, which is readily confirmed using Sr-doped LaP3O9 single crystals. The c-axis oriented, coarse-grained polycrystalline membranes of Sr-doped LaP3O9 prepared by solution synthesis techniques exhibit markedly enhanced conductivity compared to randomly oriented polycrystals prepared by solid state reaction and have direct applicability to fuel cell electrolytes. The discovery of fast proton conduction pathways in LaP3O9 will motivate further development of LaP3O9-based electrolytes as well as exploration of new proton conducting crystalline polyphosphates with infinite chains of PO4 tetrahedra.