Complementary Hydrogel-Based Ionic Thermoelectric Systems for Photovoltaic Waste Heat Harvesting and Energy Storage

Photovoltaic (PV) operation generates substantial low-grade waste heat, motivating energy-recovery strategies that can accommodate dynamically varying thermal conditions. Here, thermogalvanic and thermodiffusive hydrogel-based ionic thermoelectric (iTE) devices were separately integrated with the same PV-phase change composite (PCC) platform to compare their energy-conversion and energy-delivery characteristics. The PAM-F thermogalvanic cell (TC) exhibited an ionic Seebeck coefficient of 1.37 mV K-1 and a maximum power density of 4.16 mW m-2 at ΔT = 10 K, while maintaining electrical output under an external load for over 1 h. In contrast, the thermodiffusive PVA-K iTE supercapacitor exhibited a larger negative ionic Seebeck coefficient of -4.28 mV K-1 and enabled thermal charging followed by time-shifted electrical discharge. The PCC suppressed the cold-side temperature rise during irradiation and maintained a larger temperature gradient across the iTE layer, while releasing stored latent heat after irradiation to reverse the thermal gradient. Under simulated PV operation, the PVA-K-based system generated -36.8 mV at a temperature difference of approximately 11 K and subsequently developed a reversed open-circuit voltage of 21.4 mV, delivering 12.65 mJ m-2 of electrical energy during post-irradiation discharge. These results reveal two complementary modes for PV waste-heat utilization: sustained thermogalvanic energy generation and thermodiffusive charge storage with time-shifted release, highlighting the importance of coordinating ionic transport and thermal management. This complementary strategy provides a promising route toward distributed low-power energy harvesting, self-powered sensing, and storage-assisted monitoring in PV systems.