Ionic Thermoelectric Effect within the Framework of Thermoelectricity

The fundamental nature of the ionic thermoelectric (i-TE) effect-whether it represents genuine thermoelectricity or merely an analogy-remains unresolved. Existing devices exploit ion migration within the electrolyte but lack interfacial contributions, and no experimental evidence of the Peltier effect in i-TE materials has been reported. A central question thus arises: if such a Peltier effect exists, do the Seebeck, Peltier, and Thomson coefficients of i-TE systems satisfy the Kelvin relations? Here, we systematically formulate the three thermoelectric effects in the i-TE context and provide the experimental observation of an ionic Peltier effect, whose cooling power exceeds that of conventional electrolytes by more than 2 orders of magnitude. Furthermore, we introduce an electromotive-force-based method to determine thermoelectric coefficients without direct heat-flux measurements, thereby enabling experimental verification of the Kelvin relations. These results resolve fundamental questions of the i-TE effect and establish its position within thermoelectricity.