Ultra-high performance of ionic thermoelectric-electrochemical gel cells for harvesting low grade heat

Gel-based ionic thermoelectric (i-TE) conversion systems have attracted more attention to harvest energy from ambient waste heat to power sensors in IoT systems, due to their high ionic thermopower, cost-effectiveness, and environmental friendliness. However, their thermoelectric performance, particularly output power density, remains low, hindering their further application. To date, there is still a lack of an effective strategy to reverse this serious situation. Herein, we originally designed an ionic thermoelectric-electrochemical (i-TE-EC) cell by combining two asymmetric gels into a double sandwich structure, resulting in high thermoelectric performance. By tailoring gel compositions, including the concentration and ratio of redox couples as well as additive contents, a maximum output power density Pmax/(ΔT)2 of 10.0 mW m−2 K−2, a cell ionic thermopower of 5.2 mV K−1, and an energy density of 3.4 J m−2 K−2 in two hours were obtained at 313 K for the i-TE-EC cell Gp | G-m/n FeCN4−/3− | Gp | G-x/y I−/I3−-z CF3SO3K | Gp (m/n = 0.175/0.025 M, x/y = 0.10/0.05 M, z = 0.4 M). Moreover, Pmax/(ΔT)2 increased to a higher value of 20.5 mW m−2 K−2 for the i-TE-EC cell using a CEM as a built-in electrode. A gel-device assembled by nine single i-TE-EC cells connected in series generated a voltage of 1.55 V and a high Pmax/(ΔT)2 of 5.2 mW m−2 K−2 at 313 K and ΔT = 5 K. This work provides a promising route for improving the gel-based thermoelectric performance, available for other energy conversion systems.