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.