Exploration of Dopant Species for Lanthanum Polyphosphate

Doped lanthanum polyphosphate (LaP3O9) exhibits relatively high proton conductivity. For the practical applications such as the electrolyte of fuel cells, however, its conductivity must be improved by 2 orders of magnitude. Protons are introduced into matrix by lower-valent cation doping, and proton conductivity depends on dopant species. To date, LaP3O9 has been doped with only Ca, Sr and Ba. In this work, we tried to dope LaP3O9 with Na+, K+, Mg2+ and Pb2+, as the new dopant species, due to their close ionic radii to La3+. Among them, only Pb could substitute for La at a comparable concentration to those of alkaline earth metals and its highest doping level was 6.4 mol% (Doping level is defined as the concentration ratio of dopant (M) to host cation (La) site in matrix (equivalent to M/(La + M) x 100 (mol%))). Though Pb can exist as either divalent or tetravalent state, Pb in LaP3O9 was identified to be divalent state by XPS analysis. Proton conduction was demonstrated by FM isotope effect. The electrical conductivity of Pb-doped LaP3O9 increased with Pb-doping level, owing to the increase in proton concentration. The conductivity of 4.5 mol% Pb-doped LaP3O9 was about one order of magnitude lower than that of 7.9 mol% Sr-doped LaP3O9. (C) The Author(s) 2015. Published by ECS. All rights reserved.