Scalable carbon black deposited fabric/hydrogel composites for affordable solar-driven water purification

Highlights

• Bilayer fabric/hydrogel composites exhibit tough interfacial bonding.

• The hydrogel substrate combines high mechanical strength and water absorption.

• The composites function well as effective interfacial evaporator for water purification.

Abstract

Interfacial solar-driven evaporators have presented great potential for water purification owing to their low energy consumption and high steam generation efficiency. However, their further applications are hindered by the high costs and complicated fabrication processes. Here, a scalable bilayer interfacial evaporator was constructed via an affordable technique, in which carbon black deposited nonwoven fabric (CB@NF) was employed as the upper photothermal layer, as well as PVA/starch hybrid hydrogel for self-floating and water transport. Under simulated one sun irradiation, CB@NF layer displayed excellent photothermal conversion performance, whose temperature could increase 30.4 °C within 15 min. Moreover, the introduction of starch into PVA endowed the hybrid hydrogels with considerable water-absorption capability on the premise of ensuring mechanical properties. The resultant CB@NF/PVA/starch composites achieved superior interfacial adhesion performance with interfacial toughness at about 200 J m⁻². Combining with good evaporation performance, salt-rejection property and high purification efficiency on pollutants, this evaporation system would become a promising candidate to alleviate water shortage.

Introduction

Freshwater is indispensable for human survival on earth. However, the drive towards rapid industrial development by mankind has put forward growing concerns over fresh water scarcity due to severe water contamination [1,2]. To seek sustainable and affordable solutions, various purification technologies have been investigated to obtain safe freshwater from the abundant seawater or sewage. Among them, membrane-based technologies such as reverse osmosis and forward osmosis that block harmful dissolved solids through physical barriers are relatively economical in terms of energy consumption [3]. Nonetheless, their practical application is hindered by the stringent requirements on water quality. For instance, acid, alkali, sodium or microorganism in sewage would compromise membrane performances and need to be removed by tedious pretreatment procedures [4]. Other alternatives based on thermal technologies, which comprise an evaporator and a condenser, are capable of generating high-quality freshwater directly even from the wastewater with complex composition. However, they consume large amounts of electricity and require complex infrastructure [5]. Therefore, it is imperative to explore low-cost heat resources that can provide an affordable water supply.

Recently, efficient utilization of solar energy has become the emphasis in the fields of renewable energy [6], [7], [8]. In such a case, solar-driven water purification technologies are emerging and attractive to alleviate water scarcity [9,10]. The key of solar-driven technologies focuses on the design of evaporators, which combine superior photothermal properties, rapid water replenishment and efficient thermal management. Typically, solar evaporators are classified into three types including bottom heating, bulk heating and interfacial heating [11]. Among these, interfacial heating only selectively heats the water near the surface of the evaporator and reduces the needless energy transfer, thus showing considerable performances [12]. Such systems often involve a photothermal layer for light-heat conversion and a hydrophilic thermal barrier for water transport [13]. Various photothermal conversion materials like metal nanoparticles (Au, Pd, Pt) [14], [15], [16], conductive polymers (PPy, PANi) [17], [18], [19], semiconductors (MoS₂, Ti₂O₃) [20], [21], [22] and carbon materials [23], [24], [25], [26], [27] have been introduced into a photothermal layer for enhancing solar absorption. Particularly, carbon-based materials such as carbon black (CB), reduced graphene oxides (rGO) are less costly and more efficient, exhibiting the ability to absorb broadband solar radiation. Li et al. [28] reported a jellyfish-like evaporator, whose thermal conversion efficiency reached 87.5% owing to 99.0% absorption of the entire solar spectrum with carbon-based materials. However, these materials are often restricted by the difficulty to obtain uniform dispersions, which is not conducive for long-term operation.

As for the hydrophilic substrate, hydrogels consisting of three-dimensional porous networks have aroused the wide attention of scholars. They can absorb/release water reversibly via capillarity, ensuring rapid replenishment as water vaporizes [29,30]. Furthermore, their inherent low thermal conductivity prevents heat conduction from the evaporator towards the underlying water. Above all, hydrogels exhibit a distinctive water state owing to water-polymer interactions, which are classified into bound water, intermediate water and free water [31]. Among these, intermediate water weakly interacts with polymer chains and adjacent water molecules, thus requiring less energy for vaporization [32]. It has been demonstrated that higher evaporation rates can be realized through tailoring the water state in hydrogels, providing a new perspective for the design of hydrogel-based evaporators [33], [34], [35]. Whereas, to function as the substrate of evaporators, hydrogels must possess high mechanical strength, shape stability as well as excellent water-absorbing ability. Compared with fragile hydrogels from chemical design, physically cross-linked hydrogels reveal enhanced mechanical strength [36], [37], [38]. Typical examples of such include poly(vinyl alcohol) (PVA) hydrogels obtained through freezing-thawing method. With microcrystalline regions as cross-linker, these PVA hydrogels present superiorities like low-cost, simple preparation and dimensional stability after swelling. Nevertheless, the dense hydrophilic PVA chains in crystalline regions might inhibit the absorption of intermediate water in such hydrogels [39]. Previous studies show that starch with large amounts of hydrophilic hydroxyl groups is a suitable candidate for superabsorbent hydrogels [40,41]. The structure of starch also enables its modifications of synthetic polymers through blending [42,43]. Therefore, the combination of PVA and starch presents an opportunity to exploit the synergy between good water absorption and high mechanical strength.

Herein, a double-layer solar interfacial evaporator containing carbon black deposited nonwoven fabric (CB@NF) and PVA/starch hybrid hydrogel layer was developed. CB was modified via cellulose nanofibers to obtain well-dispersed solutions, which were deposited on fabrics through vacuum filtration. The obtained CB@NF displayed good photothermal conversion performance, whose temperature increased 30.4 °C after 1 sun irradiation for 15 min. While the introduction of starch into PVA endowed resultant hybrid hydrogels with considerable water absorption without sacrificing their mechanical properties. Furthermore, by overlaying CB@NF on PVA/starch hydrogels via freezing-thawing process, double-layer CB@NF/PVA/starch achieved strong interfacial adhesion performance with the interfacial toughness at around 200 J m⁻². Along with good salt-rejection performance and high purification efficiency on seawater and sewage, the CB@NF/PVA/starch could serve as an interfacial evaporator, opening a new pathway to overcome the dilemma of water shortage in the near future.