A Switchable Dual-Mode Janus Composite for All-Weather Passive Radiative Cooling and Solar-Joule Heating in Energy-Efficient Buildings.

Adaptive thermal regulation across diurnal and seasonal cycles remains a critical challenge for energy-efficient buildings. A switchable dual-mode Janus-structured composite is developed by integrating a polyoxymethylene fiber/acrylate resin/BaSO4 aerogel cooling layer with a graphene aerogel/paraffin wax (PW) phase-change composite heating layer to achieve synergistic passive radiative cooling, solar-Joule heating, and latent-heat storage. The heating layer exhibits high thermal conductivity, high solar-to-latent heat conversion efficiency, high latent heat capacity, and high electrical conductivity. This permits a maximum sub-ambient temperature rise of 17.4°C across the day. The cooling layer delivers a solar reflectivity of 96.2%, a mid-infrared emissivity of 94.0%, and a low thermal conductivity of 0.053 W m-1 K-1, achieving a maximum sub-ambient temperature drop of 21.3°C and a genuine radiative net cooling power of 110.4 W m-2, with an additional transient thermal buffering provided by the latent heat of PW (239.7 J g-1). Outdoor tests verify diurnal regulation and paraffin-induced temperature hysteresis. Building energy simulations across 15 Chinese cities demonstrate annual energy savings exceeding 10.8% in most regions, with reductions of 9.5 and 15.1 MJ m-2 in heating and cooling demands, respectively. This work provides an effective strategy for all-weather thermal management and sustainable energy savings under diverse climatic conditions.