Full Length ArticleSimultaneous engineering on absorption window and transportation geometry of graphene-based foams toward high-performance solar steam generator
Graphical abstract
Introduction
The supply of fresh water is of essential importance to economic growth and sustainable development of human civilization; [1], [2] however, the situation of water shortage is becoming more and more extensive globally [3], [4]. By 2050, two-thirds of the world population will suffer the environment of water-stress. Besides the applications of a series of methods for water saving, innovative technologies to extend the supply of water are in hot pursuit to meet this major challenge, such as water harvesting from air [5], [6], seawater desalination [7], [8], etc. Specifically, desalination technologies, mainly including multi-stage flash [9] (MSF, introduce heated brine into chambers with gradually descending pressure lower than saturated vapor pressure to obtain drinkable water), reverse osmosis [10] (RO, use external pressure to drive sea water across a semipermeable membrane to gain fresh water), electrodialysis [11] (ED, drive salt ions through anion/cation membranes by electric field to produce clean water), as well as solar steam generation [12], [13], [14] (SSG, solar-to-vapor conversion through photothermal materials followed by condensation of steam to harvest fresh water), may serve as the most viable approaches to produce water and alleviate the water scarcity.
Compared with other technologies of seawater desalination, the SSG method has high energy conversion efficiency and is fully driven by clean energy, which therefore has attracted intensive attentions in the past decade [15], [16]. The core component for a SSG device is the solar absorber, which should typically satisfy the following criteria: (1) it is capable of absorbing light across the full solar spectrum (300–2500 nm; the wider, the better); (2) it has a high light-to-heat conversion efficiency; (3) it has a low thermal conductivity to achieve heat localization by diminishing heat conduction, radiation and convection; (4) it possesses a good wettability and porous structure to ensure water transport and evaporation; (5) it has excellent mechanical and chemical stabilities to achieve high performance over a long term, even in harsh environments [17], [18], [19], [20]. Consequently, three categories of photothermal materials, plasmonic nanoparticles [21], [22], [23], semiconductors [18], [19] and carbon-based materials [24], [25], [26], are the recent focus of study. It should be added here that new computational methodologies, such as artificial intelligence (AI), may play a more and more significant role in searching of new material candidates [27], [28], [29].
Plasmonic nanoparticles (Au, Ag, Cu, Al et al.) possess rather high solar absorption ability owing to surface plasmon resonance (SPR) effect, but their capability of absorption is usually confined in a narrow range of solar spectrum [20]. Besides, the fabrication of these nanoparticles normally involves complex procedures and is expensive, which makes them not suitable for large-scale desalination industry [30]. Upon illumination, plenty of electron-hole pairs in semiconductors with their energy similar to bandgap can be generated, and electrons eventually return to low-level states and release the corresponding heat energy [31]. The key limitation for semiconductors rises from their bandgap energy which generally responds to ultraviolet (UV) region in the solar spectrum. Therefore, additional efforts should be made to narrow the bandgap of semiconductors for practical applications. In this context, carbon-based materials, with their intrinsic advantages such as wide solar absorption window, chemical and structural stability, facile production process and cheap price, have been regarded as one of the most promising photothermal materials in SSG devices [32], [33]. A significant number of studies have designed and fabricated SSG devices based on graphite [24], carbon nanotubes (CNTs) [34], [35], reduced graphene oxide (RGO) [36], [37] and so on. Li et al. developed a covalent organic framework (COF)/graphene dual-region hydrogel via controlled deposition of a sulfonic acid-functionalized COF (COF-SO3H) onto RGO, which achieves a high solar vapor conversion efficiency up to 92% under one sun illumination [36]. Zhang et al. fabricated a lotus-inspired interfacial evaporator with 3D biomimetic architectures that can effectively suppress heat loss and gain energy from the environment; the evaporator not only shows an extremely high evaporation rate of 3.23 kg m−2 h−1, but also exhibits an energy conversion efficiency of 153.20% [38].
To further improve the solar thermal conversion efficiency of SSG devices based on carbon materials, a wide range of efforts have been contributed recently. On one hand, it is quite straightforward that increasing the solar absorption (via either increasing the amount of light or extending the adsorption window) can lead to a higher efficiency. Ren and co-workers designed a hierarchical graphene (h-G) foam consisting of a secondary structure of graphene nanoplates, which anchored on a porous Ni foam by plasma-enhanced CVD growth method. In contrast with ordinary graphene foam, the h-G foam minimized the reflection and transmittance of incident light so the external solar-thermal energy conversion efficiency can reach up to ≈93.4% [30]. Ming et al. reported a 3D porous aerogel monolith assembled from 2D Ti3C2Tx (MXene) by a graphene oxide (GO)-assisted process; the GO is weakly reduced to RGO by MXene during synthesis, leading to an enhanced photothermal conversion performance. As a results, the floating GO/MXene evaporator achieves a high solar evaporation efficiency up to 90.7% under one sun illumination [39]. On the other hand, it is now realized that accelerating the water transportation from bulk water to surface of photothermal materials can mitigate the energy dissipation and maintain ample water supply in the process of water evaporation, which is also a key factor for improving the liquid–vapor conversion in SSG devices. For example, Wu et al. designed a bayberry tannin (BT) directed assembly of bifunctional graphene aerogel, in which BT acts as an hydrophilic segment for improving water infiltration, and the device has shown a high solar vapor conversion efficiency of 95.5% under one sun irradiation [40].
In light of these pioneering contributions, we consider that the simultaneous widening of absorption window for photothermal materials toward full solar spectrum and facilitating water transportation kinetics throughout water–solid interfaces can be beneficial for a more ideal SSG device. In this perspective, graphene-based materials are typical for investigating because of their intrinsic broad light absorption, good processability and outstanding chemical stability [41]. However, the “2D” structure of graphene cannot effectively confine and transport water on its surface, and the smooth surface is also not operative for water evaporation [42], [43]. We further noted that SiC nanoparticles have high absorption in the near-infrared region, which is capable of improving the solar absorption window of graphene [44], [45]. Herein, we try to overcome the aforementioned disadvantages of graphene-based materials by distributing zero-dimensional (0D) SiC nanoparticles on the surfaces of reduced graphene oxide (RGO) and then constructing a monolithic foam. The 3D interconnected porous architecture consisting of RGO flakes anchored with SiC nanoparticles was synthesized via a bubble/ice dual-template method followed by a thermal treatment. As a consequence, the interface of RGO and SiC is chemically linked by a thermal-insulated SiO2 layer (which is resulted from the interfacial reaction between graphene oxide (GO) and SiC during synthesis) that can be helpful in localizing the heat (i.e., to create hotspots) for evaporation of water on SiC sites [46] and improving the chemical stability of RGO-SiC foam. Our systematical characterizations confirm that the hybrid foam achieved synergistic improvements on absorption window, photothermal conversion and heat localization when the structure was used in an SSG system, which finally led to a solar-vapor conversion efficiency of up to 95% under one sun illumination. Additionally, in our previous work, we have found that in situ environmental scanning electron microscopy (ESEM) is a feasible and effective tool for the observation of water–solid interactions [47], [48], [49]. Therefore, we have also performed in situ ESEM observation toward the water condensation and transportation behaviors on the surface of RGO-SiC foam, which microscopically substantiated that SiC nanoparticles are indeed necessary to significantly boost the kinetics of surface transportation for water flow.
Section snippets
Experimental section
Materials. Graphene oxide (1–3 wt%, the size and thickness are 0.5–3 μm and 0.55–1.2 nm, respectively) and SiC (99.9%, average diameter ≤ 150 nm) were purchased from Aladdin Reagent Co., Ltd. (China). Pluronic F127 was purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd. Chemicals were used without further purification. Deionized water was utilized for all experiments and tests unless otherwise mentioned.
Synthesis of RGO-SiC foam. Typically, GO aqueous solution (1.5 mL) and SiC powder
Materials synthesis and structural characterizations
The RGO-SiC foam was synthesized via a facile ice/bubble dual-template method followed by a thermal reduction (see Fig. 1a and Experimental section for details). Briefly, GO aqueous solution, SiC nanoparticles and Pluronic F127 bubble clusters were mixed together and placed in liquid nitrogen followed by freeze-drying to produce GO-SiC foam. In this process, SiC nanoparticles were distributed on the surfaces of GO flakes, and this GO-SiC hybrid flakes were interconnected into porous monolith on
Conclusions
An interconnected porous structure of RGO-SiC foam constructed by RGO flakes modified with SiC nanoparticles was designed and synthesized via a facile ice/bubble dual-template method followed by a thermal treatment. Owing to the introduction of SiC nanoparticles, the solar absorption window of RGO-SiC foam is greatly extended toward full solar spectrum. Notably, the growth and transportation kinetics of water droplets on surfaces of RGO-SiC foam is observed by in situ ESEM, which
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the financial support from the National Natural Science Foundation of China (51972159 and U21A20500), Jiangxi Provincial Natural Science Foundation (20212ACB204016, 20212BAB204056, 20192ACB21018, and 20171ACB20006), and Interdisciplinary Innovation Fund of Nanchang University (IIFNCU) (9166-27060003-ZD05). Yi Liu acknowledges the support from the Fundamental Research Funds for Central Universities.
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Y. Li and R. Zan contributed equally to this work.