Thermally Chargeable Ammonium‐Ion Capacitor for Energy Storage and Low‐Grade Heat Harvesting

Abstract Harvesting heat from the low‐grade heat (below 100 °C) into electricity has the signification to enhance the utilization of energy and lower carbon emissions by a simple device. Herein we demonstrate a thermally chargeable ammonium ion capacitor (TAIC) by employing graphene‐polyimide (rGO‐PI) synthesized through polycondensation of 1,4,5,8‐naphthalenetetracarboxylic dianhydride and ethylenediamine as cold electrode, N‐doped hollow carbon nanofibers as hot electrode to directly convert waste heat into electricity. Combining thermodiffusion effect of electrolyte with thermogalvanic effect of a redox couple (−C=O/−C−O−NH₄⁺), as‐assembled TAIC can deliver a high output voltage of 624 mV, power density of 82 μW cm⁻² and average Seebeck coefficient of 9.07 mV K⁻¹ at temperature difference of 45 K. Meanwhile, with the introduction of polyacrylamide‐polyacrylic acid‐based gel electrolyte, the assembled flexible device can well serve in various bending states, and the power density can attain a satisfying value of 1.92 μW cm⁻². This quasi‐solid‐state TAIC shows great potential as one promising candidate for high value‐added conversion from low‐grade heat into electricity as well as wearable applications.