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

Highlights: • The p-n conversion for the iodide/triiodide (I^-/I₃^-) redox couple induced by thermo-sensitive nanogels. • The Seebeck coefficient for I^-/I₃^- redox couple was inverted from 0.71 mV K⁻¹ to − 1.91 mV K⁻¹ induced by nanogels. • A wearable device consisting of alternating I^-/I₃^- and I^-/I₃^-/nanogels unites in series can harvest body. -- Abstract: 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.