Effects of A and B site acceptor doping on hydration and proton mobility of LaNbO4

Acceptor doping of the high temperature proton conductor LaNbO4 has been studied by impedance spectroscopy in various atmospheres at 300-1100 degrees C and by X-ray powder diffraction and scanning electron microscopy. Doping LaNbO4 on both A and B site (with Ca and Ti, respectively) resulted in a two-phase composition of LaNbO4 and LaNb3O9. This composite is interesting as the two phases make it a mixed proton and electron conductor. The electrical characterisation of Ti-doped LaNbO4 revealed mixed electronic (n- and p-type) and ionic conductivity at temperatures above approx. 750 degrees C, while proton conductivity was dominating below this temperature under wet conditions. Ti-doping resulted in higher activation enthalpy and lower mobility of protons as compared to Ca-doping, attributed to stronger proton-acceptor association in the former case. Thermodynamic constants for hydration of associated protons and proton-acceptor association as well as mobility parameters were fitted to the experimental data and came out as Delta S-hydr(0) = 173 +/- 8 J/mol K, Delta H-hydr(0) = -145 +/- 5 kJ/mol, Delta H-assoc(0) = -65 +/-2 kJ/mol, mu(O,H+) = 8 +/- 2 cm(2)K/Vs and Delta H-mob,H-H+ = 55 +/-3 kJ/mol, mu((sic))(O,vo) = 2 +/- 0.5 cm(2)K/Vs and Delta H-mob,v0(similar to) = 60 +/- 5 kJ/mol. Neither B site doping nor A and B site co-doping showed indications of increased solubility relative to sole A site doping in LaNbO4. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.