From waste to energy storage: fabrication of shape-stabilized phase change composites using cellulose extracted from waste paper

Organic phase change materials (PCMs) are promising for sustainable energy due to their high storage capacity, broad temperature control, and minimal volume change during phase transitions. However, their application is limited by low thermal conductivity and high leakage caused by volume instability. To address these issues, a shape-stabilizing approach using a nature-based and porous matrix of cellulose from recyclable resources, is proposed. In this study, a cellulose hydrogel-based composite was used as a support for encapsulating polyethylene glycol (PEG 2000) PCM, creating a phase change composite (PCC). Cellulose was extracted from waste newspaper (WP) through alkaline and peroxide treatments, achieving 16.5% efficiency and 78% purity. The cellulose was then used to synthesize different three-dimensional (3D) hydrogel networks with citric acid (CA) as the cross-linking agent. Carbon monofilaments (CFs) were incorporated into the hydrogels to enhance stability, reduce leakage, and improve thermal properties. The thermophysical and morphological characterization of the prepared system revealed that cellulose-based hydrogels were formed through esterification between cellulose hydroxyl groups and CA carboxyl groups. The leakage rate of the (cell-4/CF/PEG) PCC was measured as 4.25 wt% after 5 heating-cooling cycles. The latent heat of melting was similar to pure PEG 2000, with an energy storage capacity increase of 25%. Furthermore, the addition of CFs improved thermal conductivity (<i>k</i>) by 80% and achieved an enthalpy efficiency of 90%. The thermal diffusivity (<i>α</i>), specific heat capacity (<i>C</i> _p), and effective thermal conductivity (<i>k</i> _eff) of the (cell-4/CF/PEG) PCC were recorded as 8.2 × 10^-9 m² s^-1, 5400 J kg^-1 °C^-1, and 0.027 W m^-1 °C^-1, respectively.