Preparation and properties of paraffin/alkali modified sludge biochar shape-stabilized phase change material

In response to the issue of paraffin phase change materials being prone to leakage in the molten state, as well as the dual demands of municipal sludge treatment and resource utilization, an alkali-modified biochar derived from municipal sludge was developed as a supporting matrix. Sophorolipid was introduced as a surfactant to improve the interfacial compatibility between paraffin and the biochar substrate. Paraffin/alkali modified biochar shape-stabilized phase change materials were successfully fabricated via vacuum impregnation. Experimental results indicated that NaOH modification significantly enhanced the porous structure of the biochar, with the specific surface area increasing from 46.38 m²/g to 90.61 m²/g, and the total pore volume increasing by 42.3%, thereby substantially improving the paraffin adsorption capacity. Upon the addition of 10 wt% sophorolipid, the leakage rate of the shape-stabilized phase change material was reduced to 3.35%, while the latent heat increased from 25.8 J/g to 36.7 J/g, representing an enhancement of 42.2%, which demonstrates excellent encapsulation stability and thermal energy storage performance. Thermogravimetric analysis revealed that the material exhibited negligible mass loss below 100 ℃. Moreover, after 200 thermal cycling tests, the shape integrity and latent heat remained stable, confirming the material's excellent thermal cycling stability and operational reliability. The thermal energy storage and release performance tests further indicated that the material possesses effective temperature fluctuation control and thermal buffering capabilities. The development of shape-stabilized phase change materials based on sludge-derived biochar not only provides a novel technical route for the resource recovery of municipal sludge but also offers a promising strategy for the design and fabrication of high-performance, low-cost solid-liquid composite phase change materials for thermal energy storage applications.