Thermoelectric effects in ion conducting membranes and perspectives for thermoelectric energy conversion

Thermoelectric phenomena in ion conducting membranes have only been studied superficially in the literature and have, to the best of our knowledge, not been considered for direct thermoelectric conversion of heat into electricity. In the present article, it is shown that there exist two types of thermoelectric effects in membranes, isobaric and non-isobaric, that differ by experimental constraints of the thermoosmotic volume flux. In literature only isobaric thermoelectric effects have been investigated. Using the Onsager relations and the phenomenological transport equations, we derive a relation between the isobaric Seebeck coefficient (image) and the non-isobaric Seebeck coefficient (image) in a similar manners as for the Saxén relations. Experimental literature data suggests that the absolute value of image is of the same order as in aqueous electrolyte solutions∼1.0 mV K−1. Based on literature values of measured transport properties the absolute value of the non-isobaric Seebeck coefficient imageis estimated for a few membrane systems. It is found that imagehas a larger variation than image and can attain absolute values above 10 mV K−1. This makes efficient thermoelectric energy conversion in ion conducting membranes more realistic. It is expected that large values of imagemay be found in charged/ion-conducting nano-porous membranes with pore sizes in the range 5–200 nm.