The pore size effect on water state and transport probed by 1H LF-NMR relaxation: a case study of MF/PVA-Co2C photothermal conversion materials

Abstract

In-depth investigation of the existence states of water in porous photothermal conversion materials and effective reduction of the evaporation enthalpy are of great significance for constructing high-performance interfacial solar evaporation devices. In this study, photothermal conversion materials with different pore sizes were constructed using melamine foam (MF) as the substrate, low-cost quasi-metallic Co₂C nanoparticles as the light absorber, and polyvinyl alcohol (PVA) as the binder. The hydrophilic PVA-Co₂C formed an interconnected and open porous network within the MF framework. Low-field nuclear magnetic resonance, dark evaporation, and differential scanning calorimetry analyses indicated that the incorporation of PVA-Co₂C enhanced the interaction between water molecules and the porous framework, facilitating the transformation of free bulk water into surface bound water with weaker hydrogen bonding, thereby effectively reducing the evaporation enthalpy. Furthermore, the pore size and thickness of MF/PVA-Co₂C significantly influenced water transport and distribution. At appropriate pore sizes and thicknesses, water could not completely fill the interconnected pores but instead formed a thin water layer, which enabled continuous water supply and efficient evaporation through confined capillary transport. This not only exposes a larger evaporation area and increases vapor escape channels but also ensures a balance between water-supply and evaporation. These synergistic effects significantly enhance water evaporation performance. Under 1.0 kW m⁻² light irradiation, the water evaporation rate reaches 3.1 kg m⁻² h⁻¹. During continuous operation for 6 hours under partly cloudy weather with an average solar intensity of 577.2 W m⁻², the cumulative water production remains as high as 8.4 kg m⁻². This work presents valuable guidance for the preparation of photothermal conversion materials using porous sponges as substrates, offering an effective solution to address freshwater scarcity.