High thermal anisotropy in aligned carbon fiber/chitosan aerogel composites for ultra-efficient solar thermal energy harvesting

• Record axial thermal conductivity and anisotropy ratio via aligned carbon fibers. • Near-zero PEG leakage enabled by chitosan aerogel’s hydrogen-bonding network. • High solar-thermal efficiency from broadband absorption and directional heat transfer. • High latent heat and mechanical strength retained at 79.5 wt % PEG loading. To overcome the critical limitations of conventional phase change composites (PCCs)—including low thermal anisotropy, phase change material (PCM) leakage, and inefficient photo-thermal conversion—we develop a hierarchically engineered PCC by integrating chitosan (CS) aerogel, unidirectionally aligned carbon fibers (CFs), and polyethylene glycol (PEG2000). The CS aerogel matrix suppresses PEG leakage via hydrogen-bonding networks while enabling directional CF alignment, achieving record axial thermal conductivity (6.61 W·m⁻¹·K⁻¹) and exceptional anisotropy (ratio: 11.9). Despite reduced PEG loading with CF content, the composites retain high latent heat (136.6 kJ·kg⁻¹ at 15 wt% CFs) and mechanical robustness. The aligned CF network synergistically enhances broadband solar absorption and directional heat transfer, enabling ultrahigh solar-thermal efficiency (98.5 % at 3.0 sun irradiation). Remarkably, the system exhibits excellent cyclic stability and outdoor operational reliability. This work establishes a scalable paradigm for sustainable thermal energy storage by concurrently resolving the trade-offs among PCM encapsulation, anisotropic thermal management, and photo-thermal conversion.