Fast and Anisotropic Proton Conduction in a Crystalline Polyphosphate

Abstract

Lanthanum polyphosphate (LaP 3 O 9 ) is an attractive candidate for the electrolytes in fuel cells because of its relatively high proton conductivity. However, the proton conduction mechanism in LaP 3 O 9 (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 LaP 3 O 9 , 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 PO 4 chains, leading to the conductivity anisotropy, which is readily confirmed using Sr-doped LaP 3 O 9 single crystals. The c-axis oriented, coarse-grained polycrystalline membranes of Sr-doped LaP 3 O 9 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 LaP 3 O 9 will motivate further development of LaP 3 O 9 -based electrolytes as well as exploration of new proton conducting crystalline polyphosphates with infinite chains of PO 4 tetrahedra.

Exploration of Dopant Species for Lanthanum Polyphosphate

“…In that temperature range, the activation energy of LP(Pb4.5) was calculated to be 0.79 eV, and larger than that of LP(Sr7.9) prepared in the similar manner (0.67 eV). 10 The conductivity of LP(Pb4.5) was about one order of magnitude lower than that of LP(Sr7.9), even though the doping level was only twice lower. Since these two samples have the similar morphologies, the difference in conductivity is not attributed to that in morphology, but to those in proton concentration and/or proton mobility in LaP 3 O 9 matrix.…”

“…In addition, the doping levels comparable to those of alkaline earth metals should be attained (The highest value ever reported is 7.9 mol% by Sr-doping 10 ). For achieving higher doping levels, the ionic radius of dopant is generally required to be similar to that of host cation.…”

Growth of thin, c-axis oriented Sr-doped LaP3O9 electrolyte membranes in condensed phosphoric acid solutions

“…Thus the thickness of Sr-doped LaP 3 O 9 membranes can be controlled by precipitation temperature and time unless the thickness is smaller than 60 μm. Considering that self-supported Sr-doped LaP 3 O 9 membranes [10,16] require minimum thickness of ∼200 μm for mechanical strength, the use of carbon paper supports is effective for reducing the thickness.…”

“…Several types of lanthanum phosphates (La 7 P 3 O 18 [1], LaPO 4 [2][3][4], LaP 3 O 9 [5][6][7][8][9][10][11], and LaP 5 O 14 [12]) exhibit proton conductivity of about 10 À 4 S cm À 1 at 600°C when doped with divalent cations (Ca 2 þ , Sr 2 þ , Ba 2 þ , and Pb 2 þ ), and have potential applications as solid electrolytes in fuel cells, sensors, etc. Among them, LaPO 4 is the most well studied material which is characterized by relatively high thermal stability [13][14][15] while LaP 3 O 9 is gaining attention because of its higher proton conductivity than those of other lanthanum phosphates.…”

“…We have recently attempted to further enhance the proton conductivity of LaP 3 O 9 . A key strategy has been the optimization of the grain size and crystal orientation, since the fast proton conduction pathways exist along the b-and caxes in the crystalline grains of LaP 3 O 9 [10]. The c-axis oriented polycrystalline membrane of Sr-doped LaP 3 O 9 with the thickness of ∼270 μm was successfully synthesized by precipitation in condensed phosphoric acid solutions, and it showed the higher conductivity (∼2 Â 10 À 4 S cm À 1 at 400°C) than those of randomly oriented sintered samples [10,16].…”