Industrial Electrospinning Machine| Nanofbrous Hydrogel with Highly Salt-ResistantRadial/Vertical-Combined Structure for Efcient SolarInterfacial Evaporation

Prof. Cheng Si & Assoc. Prof. Liu Jinxin (Soochow University): Highly Salt-Resistant Nanofibrous Hydrogel Solar Evaporator with Radial/Vertical-Combined Structure

With the rapid population growth and industrial expansion, the continuous decline in freshwater supply is gradually becoming one of the most severe challenges facing human society. Among all desalination methods, solar interfacial evaporation (SIE) has garnered widespread attention due to its renewable energy source, cost-effectiveness, and environmental friendliness.

Recently, the research team led by Prof. Cheng Si and Assoc. Prof. Liu Jinxin from Soochow University published their latest findings in Small titled "Nanofibrous Hydrogel with Highly Salt-Resistant Radial/Vertical-Combined Structure for Efficient Solar Interfacial Evaporation." Soochow University is the first affiliation, with master student Wen Yong as the first author and Prof. Cheng Si and Assoc. Prof. Liu Jinxin as corresponding authors.The study fabricated an alkaline-hydrolyzed polyacrylonitrile (aPAN) nanofiber/MXene/sodium alginate (SA) hydrogel with an internal vertical structure and external radial structure via electrospinning and freeze-drying. This radial/vertical combined structure synergistically utilizes the rapid water transport capability of vertical channels and the superior thermal confinement of radial structures. Compared to single-array hydrogel evaporators, the resulting aPAN/MXene/SA (PMS) composite radial hydrogel demonstrated enhanced evaporation performance.

Fig. 1: Fabrication of PMS hydrogel evaporator.

The team first prepared PAN nanofiber membranes via electrospinning, followed by alkaline treatment to introduce hydrophilic groups on the fiber surfaces. This not only provided a fibrous skeleton for the subsequent freeze-dried hydrogel but also improved its hydrophilicity. A one-step freeze-casting technique was then employed to construct the nanofibrous hydrogel with a combined radial structure, featuring vertical channels for water transport and radial structures for thermal confinement. After crosslinking, the hydrogel retained its original morphology while gaining strength and elasticity from the ionically crosslinked "egg-box" structure.

Fig. 2: Morphology of PMS hydrogel.

Figure 2 shows the microstructure of PMS-4 aerogel. As seen in Figs. 2a–f, PMS exhibits an internally aligned vertical structure and externally oriented radial channels. Top-view SEM images reveal a radially symmetric pattern (Fig. 2b), with microporous radial channels facilitating salt/water transport (Fig. 2c). The central region contains random pores, while cross-sections confirm distinct vertical channels (Fig. 2d). SEM and photographs (Figs. 2e, h) further verify the hybrid structure: vertical channels at the core and horizontal radial channels on the sides. Ionically crosslinking PMS in CaCl₂ solution preserved the structure (Fig. 2i).

Fig. 3: Evaporation performance, mechanism, and stability.

MXene integration endowed PMS-4 with 92% light absorption (250–2500 nm; Fig. 3a). Under 1 kW m⁻² irradiation, its surface temperature reached 36°C (vs. 27.2°C for pure water), while infrared imaging showed a cool evaporation surface (30.6°C), minimizing heat loss (Figs. 3b, c). Evaporation rates increased with exposure height, peaking at 4.62 kg m⁻² h⁻¹ (146.57% efficiency) at 2 cm due to enhanced lateral heat absorption (Figs. 3d, e). DSC revealed PMS-4’s reduced water evaporation enthalpy (1800 J g⁻¹, 75% of pure water), attributed to hydrophilic group-mediated intermediate water (Figs. 3f, g). Long-term testing in 3.5 wt% NaCl showed stable evaporation (4.62 kg m⁻² h⁻¹), maintaining high efficiency (4.20±0.42 kg m⁻² h⁻¹) over 7 cycles (Figs. 3h, i).

Fig. 4: Salt resistance and mechanism.

PMS-4 demonstrated exceptional salt resistance: no crystallization in 3.5 wt% NaCl for 12 h, minimal crystallization in 20 wt% at 12 h, and onset at 10 h in 25 wt% (Fig. 4a). Its performance surpassed most evaporators (Fig. 4b), owing to synergistic composite structure and superhydrophilicity. Salt diffusion experiments confirmed rapid ion migration via vertical channels (Fig. 4c). The radial/vertical array enabled efficient water transport (Fig. 4d), with vertical channels promoting salt reflux. Stable evaporation was maintained across salinities (seawater to 20 wt% NaCl; 3.99±0.05 kg m⁻² h⁻¹ at 20 wt%). Post-desalination water met drinking standards (Fig. 4f). PMS-4 outperformed peers in rate/efficiency (Fig. 4g), benefiting from vertical water transport and radial thermal confinement.

Fig. 5: Practical application of PMS evaporator.

Outdoor tests at Soochow University validated practicality: PMS-4 achieved 6.79 kg m⁻² h⁻¹ (47.91 kg m⁻² over 10 h) at >30°C (Figs. 5a, b). Under cloudy conditions, a 2.01 cm² evaporator produced 480 g freshwater (Fig. 5c). Seven-day tests showed stable output correlating with irradiance/temperature (Fig. 5d), though condensate collection requires optimization.

This study verifies the efficacy of nanostructured hydrogels for efficient, stable, and salt-resistant solar evaporation. The innovative PMS-4 marks a milestone in sustainable desalination, offering a promising global solution.

Paper link: https://doi.org/10.1002/smll.202411780