Encapsulating phase change materials into melamine formaldehyde sponge assembled with polypyrrole modified halloysite nanotube for effective solar-thermal energy storage and solar-thermal-electric conversion

Exploiting inexpensive composite phase change materials (PCMs) with comprehensive characteristics of high encapsulation efficiency, good leakage resistance, strong flexibility, high heat conductivity, and powerful light absorption is considerably imperative for their solar-thermal and solar-thermal-electric conversion. Herein, polypyrrole (PPy) modified halloysite (HNT/PPy) was assembled into the pores of melamine formaldehyde (MF) sponge to construct hierarchical porous structure (MHP), in which PPy serves as light absorber and heat conducting agent to consolidate the light absorption and thermal conductivity, while the interwoven HNT in MF not only acts as carrier to provide sufficient space for guaranteeing more PCMs' encapsulation, but also dramatically narrows MF's pore size and prevents PCMs' leakage. As expected, the MHP can encapsulate as high as 95 wt% of polyethylene glycol (PEG) with extremely high latent heat of 177.8 J g^-1 (PEG@MHP). The rich hydroxyl groups on PEG, PVA and HNT form strong hydrogen bond interaction, effectively improving the leakage-proof performance of PEG@MHP. Especially, PEG@MHP with 82 wt% encapsulation ratio (PEG@MHP-82) demonstrates unexceptionable leakage resistance (negligible leakage at 100 °C), strong light absorption (97 %), good photothermal conversion efficiency (90.36 %), and high thermal conductivity (0.235 W m^-1 K^-1). Except that, the solar-thermal-electric (STE) conversion system assembled with PEG@MHP-82 enables efficient voltage and current outputs of 587 mV and 83.3 mA respectively under 3 kW m^-2 with heat dissipation in water. Encouragingly, the voltage output still achieves 154.7 mV under actual outdoor illumination (0.984 kW m^-2). This work provides a simple method to efficiently encapsulate PCMs for realizing high-efficiency solar-thermal and solar-thermal-electric conversion.