Sustainable energy harvesting via a scalable Janus photonic metamaterial for thermoelectric generation

Sustainable power generation in outdoor environments is crucial for realizing self-powered Internet of Things (IoT) and wearable applications. Among various approaches, thermoelectric generation is particularly attractive. However, its widespread deployment is inevitably hindered by unstable energy harvesting during diurnal environmental shifts. Here, we report a monolithic thermoelectric device based on Janus photonic metamaterials (JPM), which utilizes radiative cooling to drive continuous thermoelectric generation. Featuring an asymmetric concentration distribution of hexagonal boron nitride nanosheets (h-BNNs) within a polydimethylsiloxane (PDMS) matrix, coupled with pyramidal metastructures on the top surface, the resulting Janus photonic metamaterials simultaneously maximize solar reflectance and mid-infrared (MIR) emissivity to achieve radiative cooling. With single-walled carbon nanotube (CNT) arrays sandwiched between the JPM and a low-filler h-BNN composite substrate, the monolithic device achieves a considerable temperature difference of 27.5 K and an output voltage of 23.4 mV under a solar irradiance of 620 W m^-2. Even at zero solar irradiance, it maintains a 1 mV voltage output driven solely by the cooling effect. Furthermore, the rear surface of the device successfully achieves subambient cooling once the solar irradiance falls below 500 W m^-2. Ultimately, this work presents a viable strategy to realize sustainable self-powered operation and highly effective thermal management for diverse outdoor electronic systems.