Polydopamine-integrated melamine-formaldehyde microcapsules with efficient photothermal conversion for solar thermal energy storage

The widespread application of solid-liquid phase change materials (PCMs) is hindered by leakage and their passive thermal response, particularly limiting the direct utilization of intermittent solar energy. While microencapsulation addresses leakage, conventional microcapsules often struggle with the trade-off between high energy density and mechanical/thermal robustness, lacking a synergistic understanding of structural and functional layers. To bridge this gap, this work presents a rational design of double-shell microcapsules (PDA/MF/ODE) via a tailored interfacial strategy, in which n-octadecane (ODE) is encapsulated by a melamine-formaldehyde (MF) shell followed by self-polymerization of a polydopamine (PDA) layer. PDA serves triple synergistic roles as an efficient photothermal layer, a mechanical reinforcement barrier, and an interface adhesive through hydrogen bonding with MF, significantly enhancing shell compatibility and robustness. The resulting PDA/MF/ODE microcapsules exhibit high latent heat (190.11–201.31 J/g), excellent shape stability (92.98% mass retention after 10 thermal cycles), and substantially reduced supercooling (from 12.50 °C for pure ODE to 5.41 °C). Mechanical properties are markedly enhanced—rupture force increases from 0.921 mN to 5.915 mN (6.4-fold) and toughness from 0.154 MPa to 0.300 MPa (1.95-fold). Under simulated sunlight, PDA/MF/ODE achieves 82.5% photothermal conversion efficiency, enabling direct solar-driven phase change energy storage. Furthermore, the microcapsules maintain exceptional mechanical and thermal cycling stability over 50 cycles with negligible performance degradation. This work provides a novel strategy for fabricating high-performance, solar-responsive microencapsulated PCMs by exploiting interfacial chemistry and multifunctional shell design.