Photothermal membrane distillation (PMD) is an emerging desalination technique for purifying saline water by using abundant and sustainable sunlight. Unlike other desalination techniques, such as reverse osmosis and thermal distillation, PMD can be efficiently operated in a small modular configuration, facilitating its use in decentralized communities. Among various PMD configurations, airgap membrane distillation (AMD) is considered the best, owing to its high solar-to-water efficiency. However, most PMD studies operate PMD with an airgap thickness that differs from the theoretical optimum value, due to water contact between the condensed water and the photothermal membrane. To operate at the optimum airgap thickness in a PMD module, we employed super-hydrophilic aluminum (Al) plates, fabricated by acid etching. For efficient photothermal conversion, polypyrrole (PPy) was utilized as the photothermal material, because of its effective photothermal conversion and scalable coating process. The resulting PPy-based membrane exhibited efficient photothermal conversion, while the PPy coating on the membrane did not compromise its micropore structure for vapor diffusion. The super-hydrophilic Al plate enabled operating the PMD at an airgap of 1.5 mm without water contact, whereas the smallest airgap thickness with an unmodified Al plate was 3 mm. As a result, the PMD module with the super-hydrophilic Al plate reached a water flux of 2.29 kg/m<sup>2</sup>/hr under 2.40 kW/m<sup>2</sup>, an ~12% higher solar efficiency than that of an unmodified Al plate with a 3 mm airgap. Moreover, owing to film-wise vapor condensation, the PMD module with the super-hydrophilic Al plate substantially shortened the induction time needed to begin collecting purified water from the module. These findings demonstrate the importance of the vapor condensation pathway via chemical modification of engineered surfaces to thermally engineer PMD for practical and sustainable desalination.

Engineered photothermal airgap membrane distillation using a super-hydrophilic aluminum plate

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