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

Abstract

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 P_max/(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 FeCN^4−/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, P_max/(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 P_max/(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.