Exploration of Dopant Species for Lanthanum Polyphosphate

Abstract

Doped lanthanum polyphosphate (LaP 3 O 9 ) 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, LaP 3 O 9 has been doped with only Ca, Sr and Ba. In this work, we tried to dope LaP 3 O 9 with Na + , K + , Mg 2+ and Pb 2+ , as the new dopant species, due to their close ionic radii to La 3+ . 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 LaP 3 O 9 was identified to be divalent state by XPS analysis. Proton conduction was demonstrated by H/D isotope effect. The electrical conductivity of Pb-doped LaP 3 O 9 increased with Pb-doping level, owing to the increase in proton concentration. The conductivity of 4.5 mol% Pb-doped LaP 3 O 9 was about one order of magnitude lower than that of 7.9 mol% Sr-doped LaP 3 O 9 .

Selecting the best dopant sites in proton-conducting pyrochlore oxides (La2(Nb1−xYx)2O7−δ) by probing hydration-induced local distortion

“…s total of LNY65 in Ar-3% H 2 O at 700 C is 4.07 Â 10 À4 S cm À1 , about 1.59 times higher than that in Ar-3% D 2 O (2.56 Â 10 À4 S cm À1 ). This ratio is typical for the H/D isotope effect of proton-conducting solid oxides, 24,25 indicating that protons are probably the major charge carriers.…”

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

“…On the other hand, membrane A consisted mainly of small crystals with sizes less than 10 μm. It has recently been shown that fast proton conduction pathways exist along the b-and c-axes in the crystal lattice of Sr-doped LaP 3 O 9 and that grain boundaries have relatively high resistivity [10,20]. This suggests that the layer of small-grain polycrystals without bor c-axis orientation exhibit relatively high resistance.…”

“…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.…”

Effect of Pb Substitution on Electrical Conduction and Sinterability of LaPO₄-Based Protonic Conductor

“…In previous works [8][9][10][11] , it was reported that a spark plasma sintering (SPS) method and a modified coprecipitation process are effective ways for densification of the phosphate-based materials, but a development of more simple strategy is expected from the industrial viewpoint.…”

“…From such background, we have focused on other substituent elements than alkaline-earth elements, and reported here an effect of Pb partial substitution 11) on sinterability and electrical conductivity of LaPO 4 . On the basis of the results, we also performed co-substitution of Pb and Sr for La, and then evaluated sinterability and electrical conductivity of (La, Sr, Pb)PO 4 .…”