Tailoring polypyrrole-based Janus aerogel for efficient and stable solar steam generation

Highlights

• A free-standing Janus aerogel was tailored.

• A solar-to-vapor efficiency of 94.7% with an evaporation rate of 1.68 kg^.m^-2.h^-1 was obtained.

• The Janus aerogel demonstrated good recyclability and long-term durability.

• The Janus aerogel reduced the equivalent vaporization enthalpy of the trapped water.

Abstract

Seeking an efficient and robust evaporator is the central goal of solar-driven interfacial steam generation. Here, a free-standing Janus aerogel with rough hydrophobic upper layer and smooth hydrophilic bottom layer was tailored by freeze drying of polypyrrole-polyvinyl alcohol (PPy/PVA) colloidal solution, followed by acetalization treatment and selective fluorination decoration. PPy/PVA-F Janus aerogel has a structure integrated in the form of different microscopic layers and chemical compositions, endowing it with omnidirectional light absorption, stable floating on water, enhanced evaporation performance, and salt rejection capability. Due to the reduced vaporization enthalpy, evaporation rate of 1.68 kg·m⁻²·h⁻¹ with solar-to-vapor efficiency of 94.7% was obtained under 1-sun irradiation using the Janus aerogel. Furthermore, the Janus aerogel demonstrated good recyclability and long-term durability at different radiation intensities. This Janus aerogel had satisfactory availability for solar desalination and sewage treatment. The strategy to integrate different microscopic layers and chemical compositions in a Janus aerogel holds great promise for the design and application of Janus evaporators in the field of solar steam generation.

Introduction

Nowadays, freshwater shortage has become a critical global challenge caused by climate change and environmental pollution [1], [2]. Tremendous efforts have been made to obtain fresh water, among which solar steam generation is expected to be a green and sustainable technology for sea water desalination [3], [4], [5]. Unlike bulk water heating technologies [6], [7], solar-driven interfacial steam generation (SISG) can maximize the harvest of solar radiation and minimize thermal loss via localization of the solar heating at the air-water interface, and thus greatly improve the efficiency in SISG [8], [9], [10], [11], [12], [13], [14], [15]. Considerable efforts have been made to engineer the materials and systems in SISG, while the practical application of SISG is still hindered.

One of the main challenges in SISG is pursuing an efficient and durable evaporator. Various photothermal nanomaterials were used and engineered as floating evaporators, such as carbon-based nanomaterials [10], [16], [17], [18], [19], metallic nanoparticles [20], [21], [22], [23], [24], semiconductor nanomaterials [25], [26], [27], [28], [29], [30], [31], organic polymers [32], [33], [34], [35], biomasses [36], [37], [38], [39], [40], etc. A crude strategy is to place a thin layer of photothermal nanomaterials on a hydrophobic and low-density support to form a plausible floating evaporator [16], [24], [33], [41], [42], [43]. Actually, the nanomaterial only acted as a light absorber and photothermal converter, while equally important water supply and thermal insulation require assistance from other elaborate designs. These types of non-integrated evaporators are difficult to ensure evaporation stability, even though they are commonly used to test the performance of some newly developed photothermal nanomaterials. Another elegant strategy is to incorporate photothermal nanomaterials in porous monoliths capable of floating on water [20], [21], [35], [44]. Representative examples of metal nanoparticles (e.g., Pd, Al, Au) decorated wood and anodic aluminum array have shown efficient and stable solar steam generation [20], [21]. However, the loading quantity and assembly form of the photothermal nanomaterials are influenced by the pore structure and surface chemical properties of the carriers.

An alternative approach is to construct aerogel with photothermal capability. The combined advantages of low density, enormous layer areas, high porosity, tunable shape, enhanced radiation absorption, and flexible composition make aerogels ideal for free-standing evaporators in SISG [45], [46], [47], [48], [49], [50], [51]. Different dimensional nanomaterials with intrinsic photothermal conversion capability have been used as building blocks for the construction of aerogels. However, the overall structure of aerogel only expresses hydrophilicity or hydrophobicity, making it unable to combine floating with water supply. Therefore, it is still essential to design free-standing photothermal aerogel with improved structural stability and alternative wettability used as an evaporator in SISG.

Salt deposition on the layer of evaporator will decrease radiation absorption efficiency and obstruct water transport, both of which surely reduce the photothermal conversion efficiency and evaporation rate. Thus, salt resistance of evaporator is also important for efficient and stable solar steam generation. Recently, Janus evaporator with a hydrophobic upper layer and a hydrophilic bottom layer has been determined to have an efficient and stable evaporation rate associated with salt rejection capability [35], [52], [53], [54], [55], [56]. This elegantly integrated evaporator prevents salt precipitation on the hydrophobic upper layer and meanwhile resolves the salt into bulk solution driven by convection. Selective decoration of the upper layer of aerogel with hydrophobic organic groups, such as alkyl and fluoroalkyl, is a good strategy to construct a Janus evaporator [56], [57]. The degree of hydrophobic decoration is extremely important because excessive hydrophobicity of upper layer will prevent water supply to the heating region, while insufficient hydrophobicity cannot maintain the buoyancy of the Janus evaporator. Unobstructed water supply and adequate buoyancy require selective and effective hydrophobic decoration on aerogels.

The aim in this work is to develop an evaporator with efficient and stable performance in SISG. Accordingly, a free-standing Janus aerogel was constructed by freeze drying of polypyrrole-polyvinyl alcohol (PPy/PVA) colloidal solution, followed by glutaraldehyde treatment and selective fluorination decoration. Both different microscopic layer structures and chemical composites were integrated into PPy/PVA-F Janus aerogel, which has rough hydrophobic upper layer and smooth hydrophilic bottom layer. The Janus aerogel has omnidirectional light absorption, stable floating on water, enhanced evaporation performance, and salt rejection capability. Due to the reduced vaporization enthalpy of PVA matrix, typical PPy/PVA-F4 showed high solar-to-vapor efficiency and solar steam generation under simulated solar light and natural solar irradiation. Furthermore, PPy-PVA-F4 Janus aerogel expressed good recyclability and long-term durability at different radiation intensities without noticeable performance degradation. This Janus aerogel was available for solar desalination and sewage treatment.