Conductivity Enhancement in Lanthanum Phosphates

Abstract

Current proton exchange membranes (PEM) are based on polymeric materials such as perfluorosulfonic acid (Nafion™), which have an upper limit to their temperature of operation (<100°C). To overcome this limitation, ceramic PEM materials are being investigated for transportation applications, where operating temperatures in the range of 200°–600°C are desired. In this study, the conductivity behavior of lanthanum orthophosphate (LaPO4) has been compared and contrasted with lanthanum ultraphosphate (LaP5O14) in order to better understand crystal structure—proton conduction relationships in ceramic materials. The conductivity of the lanthanum phosphates (doped and undoped) was measured using impedance spectroscopy in the temperature range 300°–600°C. The conductivity of 5 mol% Sr2+‐doped LaP5O14 (1.01 × 10−4 S/cm, 600°C) was found to be an order of magnitude higher than similarly doped LaPO4 (7.00 × 10−6 S/cm, 600°C), which is a well‐investigated proton conducting material. In addition, it was observed that the activation energy for protonic conduction was much lower for doped LaP5O14 (0.80 ± 0.01 eV) as compared with LaPO4 (1.09 ± 0.01 eV). A hypothesis relating the oxygen‐to‐oxygen ion distance in a material to the activation energy for proton conduction is presented and the experimental results obtained have been critically examined on the basis of the hypothesis and other relevant literature. From this analysis, it is shown that the condensed nature of the phosphate anion in LaP5O14 can provide low‐energy avenues for proton transport within the material leading to enhanced conductivity in the material. Limitations of the currently proposed model for proton conduction along with some other plausible explanations for the conductivity enhancement have also been discussed.

Electrical conductivities of strontium-doped rare earth ultraphosphates and oxyphosphates

“…The ratio of the electrical conductivities, σ H 2 O / σ D 2 O , are within the range expected in the case of the proton migration, explained as classical theory [45]. This suggests that the major carrier in (La,Sr)P 5 O 14 is protons similar to the other phosphates [11][12][13][14][15][16][17][18][19][20][21][22][23][24]26]. Therefore, (La,Sr)P 5 O 14 as well as (La,Sr)P 3 O 9 can be utilized in unhumidified environment as purely proton conducting electrolytes regardless of oxygen partial pressure.…”

“…Figure 11 compares the conductivities of the various phosphates such as (La,Sr)PO 4 [13], (Nd,Sr)PO 4 [14], (La,Sr) P 3 O 9 [19], (La,Sr) 7 P 3 O 18 [16], polycrystalline (La,Sr)P 5 O 14 and (Nd,Sr) 3 PO 7 in humidified environments. The electrical conductivity of a single crystal of (La,Sr)P 5 O 14 [26] is also provided in the figure.…”

“…To date, phase relationships of La 2 O 3 -P 2 O 5 and Nd 2 O 3 -P 2 O 5 were revealed by Niinisto and Keskela [36], and Wong and Kreidler [37] [38,39], crystal structural analysis [40,41], luminescent properties of the rare-earth oxyphosphates [42][43][44], and recently electrical conductivity of non-dope LaP 5 O 14 [25,26] and a single crystal of Sr-doped LaP 5 O 14 [26] have been investigated. However, there still remain questions on the correlation between the defect structure and the electrical conductivities for both phosphates.…”

“…Some acceptor-doped ceramics such as perovskite-type oxides [1][2][3][4][5][6][7][8][9][10], phosphates [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], borates [27], niobates and tantalates [28] etc. are known to significantly conduct protons at high temperatures.…”

Exploration of Dopant Species for Lanthanum Polyphosphate

“…11 After that, Pt was deposited on the both sides of the plates as electrodes by sputtering using Eiko ion coater IB3. During the electrical conductivity measurements, humidified gas mixture of H 2 and Ar was flowed over the samples.…”