Nanofiber-reinforced clay-based 2D nanofluidics for highly efficient osmotic energy harvesting

Highlights

• The nanofiber reinforcement strategy is proposed to address key issues.

• The osmotic power output up to ~5.16 W m^‐2 by simulating sea/river junction environment.

• Remarkably superior to almost all reported clay-based 2D nanofluidics.

Abstract

Clay-based 2D nanofluidics are promising candidates for promoting practical application of osmotic energy harvesting owing to their low cost and simple large-scale preparation, but they usually suffer from poor mechanical strength and unsatisfactory ion selectivity. Herein, the nanofiber reinforcement strategy is proposed to address these two key issues of clay-based 2D nanofluidics for achieving highly-efficient osmotic energy harvesting. The aramid nanofibers (ANFs) are intercalated into lameller montmorillonite (MMT) membrane to construct robust 2D nanofluidics. In this configuration, the introduction of negatively-charged ANFs greatly enhances the mechanical strength of MMT nanofluidic membrane, and further improves the cation selectivity towards high-efficient osmotic energy conversion. The ANF-reinforced MMT nanofluidics could delivery a maximum power output up to ~5.16 W m⁻² under 50-fold salinity gradient (KCl) simulating sea/river junction environment, which is remarkably superior to almost all reported clay-based 2D nanofluidics. The osmotic power can be further increased to 6.45 W m⁻² at a higher temperature of 50 ºC. Furthermore, the 2D nanofluidic membrane can withstand extreme water environments such as strong acidity/alkalinity and high salinity for over 20 days. This work is envisaged to provide a new strategy in the construction of robust clay-based 2D nanofluidics towards pushing osmotic energy harvesting into real-world applications.

Introduction

Pursuing clean and sustainable energy is one of the most significant topics over the years in a global context of environmental pollution and fossil fuel crisis [1], [2]. In the nature, oceans and rivers contain tremendous energy, which is known as blue energy. Osmotic energy harvested from the salinity gradient between salt and fresh water is a promising blue energy due to its highly-efficient and clean electric power supply [3]. The collection of osmotic energy is usually based on the reverse electrodialysis (RED) technology, which highly depends on the ion selective membrane (ISM) [4]. As the core component for osmotic energy harvesting, the ISM could selectively conduct single ions (cations or anions) and thus produce net ion current under the transmembrane salinity gradient [5]. The ion selectivity and ion flux of the ISM determines the osmotic energy output. In recent years, the emerging nanofluidic technology provides new opportunities for developing ideal ISMs because the configure of nanofluidic membrane based on dense nanochannels and excess surface charges contributes to both strong ion selectivity and high ion flux [6], [7]. Among various nanofluidic membranes, the two-dimensional (2D) ones have attracted growing interests in the application of osmotic energy harvesting as the stack of 2D nanosheets provides a relatively easy way to achieve the large-scale preparation and functional modification of membranes with nano-confined channels [8]. The currently widely used 2D materials for 2D nanofluidics construction are mostly focused on graphene oxide and MXene [9], [10]. In spite of excellent ion selectivity, the complex and high-price material stripping or synthesis process limit their real-world applications. Thus, it is desirable to expand low-cost 2D raw material selection, as well as develop simple and feasible preparing or stripping methods towards the mass production of 2D nanofluidics for highly-efficient osmotic energy harvesting.

Clay-based nanosheet is one of the good candidates for constructing 2D nanofluidic membranes because of abundant raw materials, low cost and simple preparation process. More importantly, the natural electrification of clay substrate makes it easy to achieve ion selectivity when integrated into 2D nanofluidics. For instance, vermiculite was selected to construct 2D nanofluidic channels and prove extraordinary thermal stability [11], [12]. Besides, the 2D montmorillonite (MMT) nanofluidics developed by our group exhibited cation selectivity and could delivery a 0.15 W m⁻² output power in a concentration cell. Although applicable to osmotic energy harvesting system, the clay-based 2D nanofluidic membranes always suffer from poor mechanical performance and unsatisfactory ion selectivity. From this point, an effective functional moiety modification is necessary to improve the key performance of 2D clay-based nanofluidics towards high-efficient osmotic power harvesting. Recently, many researchers developed a series of high-performance 2D nanofluidics (MXene, GO, BN) enhanced by nanofibers, such as cellulose and Kevlar [13], [14], [15], [16], [17], [18], [19]. The intercalated modification of nanofibers significantly enhanced the mechanical strength of 2D nanofluidic membranes, benefiting for robust osmotic energy harvesting applications [20]. Moreover, the charged groups on the nanofibers would increase the charge density in the nanoconfied space of 2D nanofluidics, which improved the ion selectivity and thus the osmotic energy output [21], [22], [23], [24], [25], [26]. Thus, nanofibrous reinforcement is expected to improve the key performance of clay-based 2D nanofluidics towards large-scale and highly-efficient osmotic energy harvesting.

Herein, the aramid nanofiber (ANF) is applied as intercalating and interlocking agent to address key issues of montmorillonite (MMT)-based 2D nanofluidics towards osmotic energy harvesting applications. The intercalation of ANFs into MMT nanosheets significantly improves the mechanical strength of 2D nanofluidic membranes, benefiting for good structure stability during long-term operation of osmotic energy harvesting. Moreover, the negatively-charged ANFs enhance the space charge density of MMT nanofluidic channels, which contributes to a higher cation-selectivity and thus improves the osmotic energy output. In a solution with same salinity gradient between sea and river, the nanofiber-reinforced 2D nanofluidics could delivery an osmotic power density of ~5.16 W m⁻² at room temperature, almost 3-fold higher than that of pristine MMT nanofluidics, and the value reaches up to 6.45 W m⁻² at 50 °C. Besides, the osmotic energy harvesting from the 2D nanofluidics-based system also exhibits excellent stability in various environments such as high salinity and extreme pH conditions. This work is anticipated to provide new insights into the development of low-cost and robust 2D nanofluidics towards real-world applications in highly-efficient blue energy harvesting and utilization.