P-N conversion in thermogalvanic cells induced by thermo-sensitive nanogels for body heat harvesting

Thermogalvanic cells (TGC) are promising devices for directly converting heat into a stable electric output. The practical applications of TGCs are presently significantly hindered by the low voltage (millivolt level) generated from a limited temperature difference. One general strategy for improving the voltage is to alternately connect n-type and p-type redox units in series. However, the number of the possible redox species is limited, hindering the optimization of the series stacking of devices. In this work, we report a novel concept that enables p-n conversion for the iodide/triiodide (I-/I3-) redox couple induced by poly (N-isopropylacrylamide) (PNIPAM) thermo-sensitive nanogels, with the Seebeck coefficient changing from 0.71 mV K−1 to − 1.91 mV K−1. The results prove that the nanogels enable selective capture of I3- at the hot side followed by the release of I3- at the cold side, yielding a concentration gradient of the free I3-, resulting in the p-n inversion. Furthermore, we designed a wearable device consisting of alternating I-/I3- and I-/I3-/nanogels unites in series that generated the open-circuit voltage of approximately 1 V and output power of approximately 9 μW by utilizing body heat. This work developed a new method for inverting the Seebeck effect of redox couples and is highly important for extending the library of possible redox species in TGCs.