Novel thin film composite hollow fiber membranes incorporated with carbon quantum dots for osmotic power generation

Highlights

• TFC membranes with 1 wt% Na-CQD-9 show a peak power density of 34.20 W/m² at 23 bar.

• CQDs modify both physicochemical properties and morphology of polyamide layers.

• CQDs could enhance MPD migration during the interfacial polymerization process.

• The pH of MPD solution significantly influences interfacial polymerization process.

Abstract

By means of carbon quantum dots incorporation, we have developed novel thin film composite (TFC) membranes for osmotic power generation. The newly developed TFC membrane exhibits a peak power density as high as 34.20 W/m² at 23 bar using 1.0 M NaCl and deionized water as the feed pair. To our best knowledge, this is the highest ever power density reported in the literature. The carbon quantum dots (CQDs) are a new class of carbon nanomaterials with advantages of excellent hydrophilicity, low toxicity, environmental friendliness, easy synthesis and low cost. The CQDs are incorporated into the polyamide selective layers via the conventional interfacial polymerization reaction. The effects of incorporating different CQDs and their loadings on membrane morphology, properties and PRO performance have been examined. It is found that the addition of Na⁺–functionalized CQDs not only increases the existence of hydrophilic oxygen-containing groups and surface area of the polyamide layer, but also changes the morphology with a looser and thinner polyamide network. The TFC membrane comprising 1 wt% Na–CQD-9 has the optimal performance. Compared with the control, the water flux and power density at 23 bar increase from 44.52 to 53.54 LMH and 28.44 to 34.20 W/m² respectively, while the reverse salt flux remains unchanged.

Introduction

Global energy demand has expanded dramatically over the last decades due to the rapid growth of world population and economy [1]. The total primary energy consumption worldwide was about 160,310 million MWh in 2014, and would reach around 240,318 million MWh in 2040 [2]. The search for renewable energy has received substantial attention in recent years because it may not only provide sustainable energy for the future but also mitigate the greenhouse gas emissions from fossil fuels [2], [3].

Osmotic energy, also known as salinity gradient energy, is increasingly acknowledged as one of the promising renewable and sustainable energy sources [4], [5], [6], [7], [8]. Currently, reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are the two main techniques that aim to harvest the osmotic energy from two solutions with different salinities [6], [7], [8], [9], [10], [11], [12], [13]. Different from RED which uses ionic exchange membranes, PRO employs semi-permeable membranes between these two solutions. Since water spontaneously diffuses across the semi-permeable membranes from the low concentration side (i.e., feed solution) to the pressurized high concentration side (i.e., draw solution) due to the chemical potential difference, it results in a higher pressure or higher volume in the draw solution compartment. One can therefore convert the hydrostatic potential via hydro-turbines or pressure exchangers for power generation [11], [12], [13], [14], [15]. Integrations between PRO and seawater reverse osmosis (SWRO) desalination as well as membrane distillation (MD) have been proposed and demonstrated recently from both academia and industries [11], [14], [15], [16], [17], [18]. The performance of hybrid systems and the amount of energy saving for SWRO are strongly dependent on (1) the performance of PRO membranes, (2) the energy to water price ratio and (3) feed quality and compositions [11], [14], [19], [20], [21], [22], [23], [24], [25], [26].

The heart of PRO process is the semi-permeable membrane, which determines the overall power generation, plant size, capital costs and profitability. According to the experience of Statkraft, who built the first commercial PRO prototype in the world, the employment of high performance PRO membranes is crucial for the commercialization of the PRO technology [24], [27]. The energy generated from PRO must be sufficiently higher than the energy consumption for pretreatments of the feed pair and feed pumps in order to have positive economical values. Therefore, intensive efforts have been focused on the development of high performance PRO membranes [8], [22], [28], [29], [30], [31], [32]. Among them, thin film composite (TFC) membranes made from interfacial polymerization have received most attention because they have (1) superior permeation properties compared with traditional phase inversion ones and (2) flexibility to optimize the substrate and the polyamide selective layer separately. Several strategies have been implemented to improve the mechanical strength and to mitigate the internal concentration polarization (ICP) of the TFC membranes, such as (1) modifying the physicochemical properties of the substrate by pre-compression and polydopamine (PDA) cross-linking [33], [34], and (2) molecularly engineering the substrate with different morphology and structures by controlling the phase inversion process [33], [34], [35], [36]. Meanwhile, many other approaches have been employed to improve the water permeability of the TFC membranes by appropriate modifications of the polyamide selective layers, such as (1) using additives or surfactants in the monomer solution, and (2) post-treating the nascent polyamide selective layer with chloride, alkaline, alcohol etc. [37], [38], [39].

Various nano-materials, such as zeolites, inorganic salts, graphene oxide (GO), carbon nanotubes (CNTs), zeolitic imidazolate frameworks (ZIFs) and metal–organic frameworks (MOFs), have also been employed to tailor the polyamide selective layer with improved separation performance [40], [41], [42], [43], [44], [45], [46], [47]. Significant performance enhancements have been reported with a reasonable loading of these nanoscale materials. Carbon quantum dots (CQDs) are a new class of carbon nanomaterials discovered in the last decade. In addition to optical properties, they have unique features such as excellent hydrophilicity, low toxicity, environmental friendliness and low cost [48], [49]. They are currently used in chemical sensing, nano-medicine and photo-catalysis [48]. To our best knowledge, there is no exploration to include them in the polyamide layer during interfacial polymerization. Therefore, the first objectives of this study are to (1) explore if CQDs can be incorporated into the polyamide layer during interfacial polymerization and (2) produce novel TFC membranes for PRO applications with a much enhanced power density. Various CQDs would be synthesized and embedded into the polyamide layer using an aqueous mixture of m-phenylenediamine (MPD) and CQDs during interfacial polymerization. The second objectives of this work are to investigate (1) the fundamentals of performance enhancement and (2) the effects of CQDs chemistry and loading on membrane morphology and PRO performance. This study may provide new insights and open up novel strategies to design better PRO membranes for osmotic power generation.