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Cation effect of inorganic salts on ionic Seebeck coefficient

Ionic thermoelectric materials attract widespread attention due to their large Seebeck coefficient compared with electronic thermoelectric counterparts whose main charge carriers are electrons or holes. The present work shows that the ion-polymer matrix interaction can alter the Seebeck coefficient to a great extent. To further shed light on the role of ions, seven water-soluble inorganic chlorides are utilized in this work, to highlight the influence of the cationic softness parameter on the Seebeck coefficient in a PEDOT:PSS ionic conductor. We notice that cations with a positive or a small negative softness parameter value exhibit a negative Seebeck coefficient, resulting from limited thermal diffusion of the cations that are strongly bound to PSS- anions. Conversely, cations with a sufficiently negative softness parameter exhibit fast thermal diffusion due to weak binding to PSS- and, thus, lead to a positive Seebeck coefficient. Based on the established relationship, robust thermoelectric materials with a tunable Seebeck coefficient ranging from –9.63 mV/K to +3.07 mV/K are demonstrated. Our work highlights the important role of ionic properties and provides a pathway for the rational selection of ions in ionic thermoelectrics.