Great enhancement in the ionic thermopower of ionogels by cationic doping

Ionic thermoelectric (ITE) materials have been attracting more and more attention for heat harvesting mainly because of their high thermopower, which is higher than that of the electronic conductors by 2–3 orders in magnitude. Controlling the ion-ion interaction of ITE materials can increase the ionic thermopower. It is important to deeply understand the mechanism for the ionic thermopower enhancement by ionic doping. Here, acetate salts with various cations including lithium (Li+), sodium (Na+), potassium (K+), cesium (Cs+), ammonia (NH4+), and nickel (Ni2+) are studied as the dopant of the ionogels consisted of gelatin and 1-ethyl-3-methylimidazolium acetate (EMIM:Ac) that is an ionic liquid. All the dopants can increase the ionic thermopower at low doping level, and the increment is related to the lattice energy and melting point of the acetate salts. Among these acetate salts, Na+ doping can give rise to the highest enhancement in the thermoelectric properties of the ionogels. The ionogel doped with Na+ can exhibit a high ionic Seebeck coefficient of 37.3 mV K−1 and ionic conductivity of 12.3 mS cm−1 at room temperature under the relative humidity of 90 %. The corresponding ZTi value is 2.1. The ionic thermopower and the ZTi value are much higher than that of the pristine ionogel.