High-Performance SiO₂ Nanoparticle Ionogels for Continuous Heat-To-Electricity Conversion

Ionic thermoelectric (TE) materials recently emerged as the next-generation TE materials mainly owing to their high thermopower. A big challenge for them is the continuous heat-to-electricity conversion because ions cannot transport across the electrodes to external circuit. Here, ionogels using SiO₂ nanoparticles as the gelator are reported, which enable continuous heat harvesting into electricity under a steady temperature gradient. The ionogels consist of 1-ethyl-3-methylimidazolium dicyanamide (EMIM:DCA), SiO₂ nanoparticles, polyethylene glycol (PEG), and sodium dicyanamide (Na:DCA). These ionogels exhibit outstanding thermopower and ionic conductivity because both PEG and Na⁺ ions can retard DCA^- anion transport, while SiO₂ nanoparticles provide ion conduction channels. They can achieve a high thermopower of 35 mV K^-1 and a high ionic conductivity of 27 mS cm^-1 at room temperature. Interestingly, the ionogels can generate a stable output voltage on the external load under a steady temperature gradient, strikingly different from the control ionogels without SiO₂ nanoparticles. This TE behavior is similar to that of conventional TE generators (TEGs). The continuous electricity generation can be attributed to the synergistic effects of the charge tunneling across the SiO₂ nanoparticles and the Soret effect of the ions under temperature gradient.