Pyrolysis temperature controls the accessible pore volume and capric acid encapsulation efficiency of coconut shell biochar

Waste-derived biochar is a low-cost scaffold for shape-stabilized phase change materials (PCMs), yet how pyrolysis temperature governs the pore volume a molten PCM can reach remains unresolved. Here we decouple surface area from pore accessibility by preparing ZnCl 2 -activated coconut shell biochars at 550, 700, 850 and 1000 °C under an identical protocol and impregnating them with capric acid. Contrary to the assumption that higher BET surface area guarantees greater PCM loading, encapsulation efficiency tracks the pore volume accessible on the adsorption branch. Between the 850 and 1000 °C biochars the BET area differs by only 3.3%, whereas the BJH adsorption cumulative pore volume differs by 17.6% (0.1102 versus 0.1296 cm 3 g –1 ) and the encapsulation efficiency by 18.5% (57.59 versus 68.25%). High-resolution TEM resolves the origin: short, curved graphene fringes constrict pore entrances at 850 °C, whereas extended parallel stacks bound open channels at 1000 °C. Efficiency rises from 42.25% at 550 °C to 68.25% at 1000 °C; the optimized composite delivers 112.88 J g –1 , does not leak at 55 °C and retains 99% of its latent heat after 500 cycles. Accessible pore volume, not surface area, is the key design criterion for biochar PCMs in passive building thermal regulation.