Exploration of Dopant Species for Lanthanum Polyphosphate

Yoshinobu Adachi,Akiko Kuramitsu,Tetsuya Uda,Naoyuki Hatada

doi:10.1149/2.0821506jes

Abstract

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 (≡M/(La + M) × 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 H/D 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.

Highlights

LaP3 O9 indicates that proton concentration and/or proton mobility can be different by orders of magnitude, depending on dopant species
Exploration of new dopant species.— In this experiment, the reagents were mixed at the ratio of La:M:P = 0.9:0.1:15 or 0.8:0.2:15 (Cinit. = 10 or 20) and LaP3 O9 was precipitated at 230◦ C in air
This trend can be attributed mainly to increase in the proton concentration, rather than to increase in the orientation strength of grains

Summary

Kyoto University

LaP3 O9 indicates that proton concentration and/or proton mobility can be different by orders of magnitude, depending on dopant species. LaP3 O9 has been doped with only Ca, Sr and Ba.[7] In this study, new appropriate dopant species which can increase concentration and/or mobility of proton has been explored for further conductivity enhancement. For achieving higher doping levels, the ionic radius of dopant is generally required to be similar to that of host cation In light of this empirical rule, Na, K, Mg and Pb were selected as the candidates of the new dopant species in this work (La3+ :1.16 Å, Na+ :1.18 Å, K+ :1.51 Å, Mg2+ :0.89 Å, Pb2+ :1.29 Å in eight-fold coordination[17 ]).

Charge Carrier Identification

Nacalai Tesque

Results and Discussion

Summary