Salt-tolerant and low-cost flame-treated aerogel for continuously efficient solar steam generation

Highlights

• A solar interface evaporator derived from wood was designed for steam generation.

• The preparation of evaporator only requires simple carbonization on the surface.

• The device gains evaporation rate of 1.4 kg m⁻² h⁻¹ and efficiency of 83.4%.

• The device has excellent salt resistance and self-salt discharge capacity.

Abstract

Interface solar-driven evaporation is a very promising technology used in solar energy harvesting, seawater desalination and water purification. However, salt accumulation is still a serious problem that hinders the long-term stable operation of the evaporator. Herein, we use natural wood with inherent porosity to develop a flexible cellulose aerogel, which can be used as efficient and stable solar evaporator for desalination after simple surface carbonization. This aerogel evaporator achieves a solar steam efficiency of 83.4% and an evaporation rate of 1.40 kg m⁻² h⁻¹ under the sunlight intensity of 1 kW m⁻². In the simulated evaporation experiment for 7 days, the evaporator shows a stable evaporation rate (about 1.39 kg m⁻² h⁻¹), indicating the potential for long-term operation. In addition, the evaporator has the ability to resist salt accumulation and self-cleaning. The low cost, simple preparation and multiple functions make the evaporator promising to be a continuous and stable installation for solar desalination.

Introduction

Freshwater resources are very limited (Zhao et al., 2018, Alvarez et al., 2018, Gao et al., 2019, Thakur et al., 2021). In the global water resources, terrestrial fresh water only accounts for 3.5%, and the remaining 96.5% is the seawater. The freshwater resources available for human use are even scarcer. According to statistics, 77.2% of terrestrial freshwater resources are located in the Arctic and Antarctic, and 22.4% are located in the deep underground that is difficult to exploit, and only 0.4% can be used for human life (Shi et al., 2018, Chen et al., 2020). Therefore, many researchers are committed to solving the problem of how to efficiently convert abundant seawater into freshwater resources (Morciano et al., 2020, Morciano et al., 2017). In recent years, interfacial solar evaporation process based on photothermal materials has attracted great attention (Patel et al., 2020, Wang et al., 2019). Different from the traditional methods of using solar energy to produce fresh water, which directly heat the bulk water by employing volumetric absorbent and cause large heat losses (Xiao et al., 2013, Muftah et al., 2014, Winfred Rufuss et al., 2016), this new process can have a strong light absorption capacity with the help of photothermal material and fix the heat at the interface of air and water, thereby reducing the heat loss and significantly improving the evaporation efficiency (EEI). Therefore, such process is considered an economical and sustainable one for efficient production of clean water (Zhu et al., 2018).

An ideal evaporator with high efficiency should have the following key features: (1) air/water interface to generate steam; (2) high light absorption for photothermal conversion; (3) low thermal conductivity to reduce the heat loss; (4) good hydrophilicity and highly porous frame for speeding up the escape of water vapor (Ni et al., 2016, Jiang et al., 2017, Kaur et al., 2017). In order to achieve high-efficiency energy conversion, many kinds of materials have been developed for light absorption, such as carbon materials (Liu et al., 2017, Thakur et al., 2021, Loeb et al., 2018, Thakur et al., 2021, Zhou et al., 2018), polymers (Zhao et al., 2018, Chen et al., 2018, Yin et al., 2018, Jiang et al., 2017, Wu et al., 2018), MXenes (Xie et al., 2020, Lin et al., 2018, Li et al., 2018), semiconductors (Chang et al., 2018, Wu et al., 2019, Zhang et al., 2019, Wang et al., 2017, Yao and Yang, 2018) and plasma metal nanoparticles (Pang et al., 2020, Hong et al., 2016, Hong et al., 2016, Xue et al., 2019, Zhou et al., 2016). To improve the capacity of water transport and heat insulation, low-density and high-hydrophilic porous materials are required as supporting materials for evaporator, like carbon foam (Qiu et al., 2019), polystyrene foam (Wu et al., 2019, Liu et al., 2019), and polymeric compounds (Chen et al., 2019). However, these materials generally have some limitations that hinder their further development. For example, the inherent high cost of MXenes and gold nanomaterials limits their commercial practicability (e.g., retail price of gold nanoshells is $395 per mg) (Chen et al., 2020). In addition, the synthetic polymers that are difficult to degrade and the silver nanoparticles that are easily leached from material may cause additional environmental pollution (Farid et al., 2021). Recently some inexpensive materials and designs for solar evaporation have been developed such as corn straw (Zhang et al., 2020), loofah sponge (Liu et al., 2020), ink enabled wood evaporator (Zhang et al., 2020) and flame-treated wood (Xue et al., 2017), most of which are based on wood-derived materials. Due to its good mechanical property, low density, inherent porosity and low cost, natural wood is an excellent choice as the supporting material for evaporators (Wang et al., 2019). Xue et al. (Xue et al., 2017) invented flame-treated wood with hydrophilicity, high solar absorption rate (∼99%), and low thermal conductivity (0.33 W·m⁻¹ K⁻¹) for solar-driven interface evaporation, which can achieve a solar thermal efficiency of 72% at a solar intensity of 1 KW·m⁻². Liu et al. (Liu et al., 2017) produced a solar steam generator with a wood-graphene oxide double-layer structure, and the EEI reached 83% under 12-sun illumination. Chen et al. (Chen et al., 2017) used carbon nanotubes to modify the flexible natural wood film to achieve a rapid evaporation rate (ER, 11.22 kg m⁻² h⁻¹) and high EEI (81%) under 10 sunlight. Although wood evaporators have made great achievements in improving the EEI, the fabrication processes of light absorption coatings (such as graphene oxide and CuFeSe₂ nanoparticles) are generally complicated (Wang et al., 2019), and the salt deposition is still one of the problems that reduce the EEI of the brine and hinder the long-term stable operation of evaporators. To solve the problem of salt deposition, special salt-resistant devices have been designed. For example, Song et al. (Song et al., 2018) designed a hydrophobic membrane composed of hierarchical nanosheets simulating lotus leaf, which did not accumulate salt during the long-term operation for 310 days. This method strongly relies on the top surface of device, which has high requirements for the hydrophobicity of the evaporation interface. Kuang et al. (2019) designed a self-regenerating evaporator by drilling channels in the wood, which exhibited anti-fouling behavior during continuous evaporation with a multidirectional mass transfer mechanism. Thus the development by simple manufacturing processes to create a sustainable, low-cost and salt tolerant solar-driven evaporator based on wood is still in need.

Here we have developed an easy-to-manufacture and low-cost wood-derived aerogel (WA) as the supporting material of the evaporator to achieve efficient, stable and continuous interface solar desalination. Compared with carbon nanotubes, graphene oxide, silica and other aerogel materials, WA has the advantages of ultra-low thermal conductivity (0.0494 W m⁻¹·K⁻¹), low density (30.42 mg cm⁻³), rich source, low cost and high biocompatibility. At the same time, this WA can serve as an ideal light absorber after a simple carbonization (Fig. 1). The wood aerogel after the surface carbonization (C-WA) has good optical absorption in the spectral range of 250–2500 nm. Owing to its low density and inherent porous structure, it can float on water and transport water to the evaporation surface through fiber and capillary action. Furthermore, C-WA has unique cellular architecture, which can realize the diffusion and convection between the concentrated brine and seawater through the vessel channels and holes in the walls, that is, the concentrated brine near the evaporation interface is transported to the lower bulk water and seawater is supplemented to the evaporation interface at the same time. This performance can reduce the salt concentration at the evaporation interface and prevent salt deposition while maintaining the high ER (Fig. 2). Based on these critical features, C-WA can achieve up to a high EEI of 83.40% and ER of 1.40 kg m⁻² h⁻¹ under the sunlight intensity of 1 kW m⁻² (one sun). In the evaporation experiment for 7 days, C-WA exhibited a stable evaporation rate and there wasn't salt accumulation occurring on the upper surface of it. Simultaneously, C-WA can clean salt crystals of 1 g on the evaporation surface with a diameter of 2.9 cm in 30 min. All these results indicate that C-WA is a high-performance solar steam generation device material which is easy-to-manufacture and has the ability to resist salt accumulation and self-cleaning.