Phonon-photon synergistic CNT/BiCuSO phase-change composites for balanced photothermal conversion and thermoelectric generation

Photothermal phase-change materials can directly harvest and store solar heat. However, their multifunctional integration is often limited by competing requirements for heat localization, heat transfer, and latent-heat retention. In this study, a paraffin-wax composite integrating carbon nanotubes (CNTs) and bismuth copper oxysulfide nanosheets was developed as a single material platform for in situ photothermal conversion, latent-heat thermal energy storage, and thermoelectric generation. CNTs formed a conductive network that facilitates heat redistribution, whereas bismuth copper oxysulfide acted as a broadband photothermal component that localizes light-induced heating. The composites were evaluated using structural, thermophysical, photothermal, and device-level thermoelectric analyses. Incorporation of CNTs increased thermal conductivity by 126.6% relative to paraffin wax. The subsequent addition of bismuth copper oxysulfide maintained thermal conductivity above that of paraffin wax while progressively increasing photothermal conversion efficiency to 91.1%. However, the temperature and voltage responses under thermoelectric generator coupling did not directly follow the standalone photothermal trends of the composites. This indicates that device output is governed by the combined effects of latent-heat consumption, internal heat transfer, and heat extraction across the module. An intermediate filler-dispersion state produced the most balanced performance, coupling strong photothermal conversion with preserved through-thickness heat transport, which resulted in stable voltage generation. These results demonstrate that optimal multifunctional performance is not achieved simply by maximizing filler loading. Instead, it requires the design of a distributed photothermal–conductive architecture that preserves latent-heat capacity while sustaining cross-plane thermal transport for efficient thermoelectric conversion.