Anchoring phase change interface enhances solar thermal energy storage

Solar thermal energy storage based on phase change materials (PCMs) provides a compact, near-isothermal pathway toward decarbonizing thermal energy supplies. However, the intrinsic recession of the solid-liquid interface away from the irradiated surface leads to gradually increasing thermal resistance and decaying charging rate with time. To address this challenge, we demonstrate that anchoring the phase change interface achieves rapid, efficient, and continuous solar thermal charging. This is enabled by dynamically circulating photothermal core-shell composite phase change particles (CPCPs) to continuously renew the irradiated surface. Scalable CPCPs, fabricated via extrusion-spheronization, consist of a MnFe₂O₄ photothermal shell and an MgO/h-BN/NaCl-KCl core integrating broadband photon absorption with high thermal conductivity within a single particle. The MnFe₂O₄ shell delivers a solar-weighted absorptance of 91.1%, facilitated by near-surface photon confinement and internal multiple scattering. Within the core, a percolated MgO skeleton combined with in-plane h-BN phonon transport pathways forms a hierarchical ceramic network. The interfacial phonon-spectrum overlap in this structure reduces Kapitza resistance, elevating the effective thermal conductivity to 6.84 W m^-1 K^-1, while maintaining a high energy storage density of 844.7 kJ kg^-1. By matching the incident solar flux with particle stream mass flow rates, the system attains a charging power of 0.54 kW under 1.08 kW solar input. This yields a solar thermal storage efficiency of 49.7%, a 26-fold improvement over conventional diffusion-limited approaches (1.9%). This work introduces a paradigm shift from the conventional diffusion-limited "material-static, interface-retreating" mode to "material-moving, interface-anchoring" mode, leading to rapid, efficient, and continuous solar thermal energy storage.