Thermogravimetric relaxation study of the proton conductor lanthanum tungstate, La28−xW4+xO54+δv2−δ, x = 0.85

Abstract

Diffusion- and surface exchange coefficients for incorporation and transport of protons and water in lanthanum tungstate, La27.15W4.85O55.28v0.73 have been determined from thermogravimetric relaxation. Tracer diffusion of protons has proved to be considerably faster than chemical diffusion of water, whereas the tracer surface exchange process is somewhat slower than its chemical counterpart. Consequently, tracer relaxations display larger critical thicknesses than the chemical ones and are moreover found to be predominantly surface controlled. The activation energy of the chemical diffusion coefficient changes above 700 °C and this transition and a corresponding change in its water vapor dependency are discussed in light of chemical diffusion of water. The activation energy of the chemical surface exchange coefficient under reducing conditions is the half of that under oxidizing. Platinum nano particles have proved to increase the rate of water incorporation under reducing conditions.