3D Hierarchical Electrodes Boosting Ultrahigh Power Output for Gelatin‐KCl‐FeCN4−/3− Ionic Thermoelectric Cells

Abstract Ionic thermoelectric (i‐TE) cells, using ions as energy carriers, have the advantage of achieving a high voltage of 1−5 V at approximately ambient temperature, showing a promise as a technology for powering Internet‐of‐Things (IoT) sensors. However, the low output power of i‐TE cells restricts their applications. Here, a 3D hierarchical structure electrode is designed to enlarge the electroactive surface area, significantly increasing the thermogalvanic reaction sites and decreasing the interface charge transfer resistance. The quasi‐solid‐state gelatin‐KCl‐FeCN 4–/3– i‐TE cells achieve a record instantaneous output power density (8.9 mW m –2 K –2 ) and an ultrahigh 2 h output energy density ( E 2h ) (80 J m –2 ) under an optimal temperature range. An average E 2h value of 59.4 J m –2 is obtained over the course of a week of operation. A wearable device consisting of 24 i‐TE cells can generate a high voltage of 2.8 V and an instantaneous output power of 68 µW by harvesting body heat. A simple and easy‐to‐operate electrode optimization strategy is provided here to increase the long‐term output power performance of i‐TE cells. This work represents a promising approach to develop reliable and green power sources for IoT sensors near room temperature.