The Seebeck coefficient is reported for thermoelectric cells with gas electrodes and a molten electrolyte of one salt, lithium carbonate, at an average temperature of 750 degrees C. We show that the coefficient, which is 0.88mVK(-1), can be further increased by adding an inorganic oxide powder to the electrolyte. We interpret the measurements using the theory of irreversible thermodynamics and find that the increase in the Seebeck coefficient is due to a reduction in the transported entropy of the carbonate ion when adding solid particles to the alkali carbonate. Oxides of magnesium, cerium and lithium aluminate lead to a reduction in the transported entropy from 232 +/- 12 to around 200 +/- 4JK(-1) mol(-1). This is of importance for design of thermoelectric converters. (C) 2015 The Authors. Published by Elsevier Ltd.