Unique applications and improvements of reverse electrodialysis: A review and outlook
Introduction
It is estimated that the salinity gradient power (SGP) in the world's estuaries may reach 30 TW, and 2.6 TW can be extracted [1]. To explore this vast renewable energy source, many technologies, such as reverse electrodialysis (RED) [2], pressure retarded osmosis [3], mixing entropy batteries [4] and capacitive mixing [5], have been developed. Among these technologies, RED is promising due to its high power density and operability. With the growing fossil fuel energy crisis, RED has drawn increasing attention, as shown in Fig. 1. A standard RED configuration contains a membrane stack, anode chamber and cathode chamber (Fig. 2). The membrane stack is formed by alternatively stacked anion exchange membranes (AEMs) and cation exchange membranes (CEMs). Spacers between the ion exchange membranes (IEMs) form inlet compartments. When a high-salinity solution (HSS) and low-salinity solution (LSS) flow into the compartments, anions and cations on the HSS side move to the LSS side through the AEMs and CEMs in opposite directions. Then, the potential can be converted to electricity via a redox reaction at the electrodes.
RED was first presented by Pattle as early as 1954 [6]. However, its development stagnated for a long time due to poor membrane performance. Development of IEMs, spacers, electrodes, redox couples and operational parameters of RED is increasing. In recent years, substantial progress in RED performance capabilities has been reported on IEMs with high ionic conductivity, low resistance, and monovalent ion selectivity [7], [8], [9]. The spacers have both positive and negative effects on RED performance. The net-like spacers can promote fluid mixing to reduce polarization phenomena. However, they can also increase pressure drop and electrical resistance and reduce active membrane area due to shadow effects [10], [11], which lead to the development of spacerless RED stacks [12], [13]. The study of electrodes and redox species in standard RED is minimal. Currently, the most widely used electrodes and redox couples are Ru/Ir covered titanium electrodes and Fe2+/Fe3+ or [Fe(CN)6]2−/[Fe(CN)6]3−, respectively, in standard RED [14]. In this review, we introduce several electrodes and redox couples used in modified RED. Operational parameters were assessed to determine the optimal parameters to generate maximum power density. A higher flow velocity [15] and elevated temperature [16] of the feed solutions has a positive impact on RED performance.
More encouragingly, a few pilot-scale RED tests have been reported. In 2014, the first RED demonstration plant was built in Italy. In this pilot installation, a RED stack with 125 cell pairs with a 44 × 44 cm2 membrane area (50 m2) was tested with real samples of brackish water and saturated brine from saltworks. A maximum power density of 1.3 W/m2 was achieved using this RED [17]. Later, the RED demonstration plant was scaled up to three RED stacks with a total membrane area of 400 m2. Although the total output power increased, there was no improvement in the power density [18]. In a recent report, Nam et al. performed RED equipped with 1000 cell pairs with a total membrane area of 250 m2 using municipal wastewater effluent (1.3–5.7 mS/cm) and seawater (52.9–53.8 mS/cm) as feed solutions [19]. The pilot plant produced a power density of 0.38 W/m2. Although many efforts have been made toward the commercial application of RED, substantial challenges remain. First, the minimum power density of RED for commercial application is 2.2 W/m2 [20], which is still not achieved by RED fed with natural waters. Second, although the technical requirements are already met by currently available membranes, the cost prices are out of range to make RED affordable. According to [21], the price of low-resistance IEMs must be reduced a hundred times, which seems to be the desired cost level. In addition, there are also some challenges for RED fed with natural waters, such as membrane fouling [22], electrode solution leakage [14] and geographic constraints for location [23].
Recently, how to use or store electricity produced by RED in situ has been given more attention due to the high cost of membranes for RED applications. Many researchers have developed improvements in RED and RED-based applications. RED developed for hydrogen generation and energy storage in batteries was presented for the storage of RED-derived electricity. RED was also developed for wastewater remediation with in situ use of the electricity. Forming a synergistic effect by combining RED with other technologies is another promising development trend. The integration of RED with desalination technologies can achieve a high power density and good water recovery rate with minimized brine discharge. In fact, there are already reviews of extraordinary unique RED improvements and RED applications [24], [25]. However, the reviews have mainly focused on membrane development. These reviews provide a basic introduction of unique RED improvements and RED applications.
In this review, unique RED applications for energy conversion, desalination, and wastewater remediation in the last decade are reviewed and analysed in detail. Moreover, novel electrodes (capacitive flow electrode, membrane electrode), feed solutions (H2CO3/H2O, waste acid/base, BaCl2/AgSO4, feed solution filled with ion exchange resin beads), membranes (inorganic nanopore membrane) and membrane cell operation that are different from those used in standard RED, are summarized. The relevant literature on unique RED applications and RED improvements is shown in Table 1.
Section snippets
Theoretical background for RED
Fig. 2 illustrates a standard RED unit whose main component is a membrane stack and electrodes. The membrane stack is composed of alternately stacked CEMs and AEMs. A series of adjacent high-salinity compartments and low-salinity compartments are then formed and fed with a HSS and a LSS, respectively. AEMs are selective for anions, while CEMs allow the transport of cations. The difference in concentration between HSS and LSS generates a transport of ions from the HSS towards LSS, which is
Energy conversion
As described in the introduction, RED is still uneconomical due to low current power densities and costs of commercial membranes. Therefore, the power generated from RED must be used in situ or stored. In this part, two ways of storing energy for RED are introduced. One way is to convert the salinity gradient power (SGP) to hydrogen production by RED, and the other way is to convert the SGP to an energy storage battery by RED.
RED integration with desalination technologies
The brine from desalination technologies can be used as a high-salinity solution (HSS) for improved power production from RED [86], [87]. At the same time, this approach avoids the environmental hazards of brine discharge [88], [89]. The concept of combining RED with desalination technologies has long been proposed [90]. The concept is illustrated in Fig. 10. Different desalination technologies are employed in seawater desalination units (SWDUs) for potable water. After desalination, the brine
Wastewater remediation
As shown in Fig. 16, RED can be used for wastewater remediation by direct or indirect reactions in the electrode chambers. Some studies have reported that the reduction of pollutants such as Cr(VI) in wastewater occurred at the cathode [116] and oxidation of organic wastewater at the anode by the direct reaction [117]. The indirect oxidation includes the electro-Fenton reaction in the cathode chamber and active chlorine (Cl2, HClO) oxidation in the anode chamber for the abatement of organic
Unique improvements of standard RED devices
In addition to the RED applications mentioned above, there are also some improvements in RED related to electrodes, feed solutions, membranes and membrane cell operation.
There are two improvements in the RED electrode based on the capacitive flow electrode and membrane electrode. One is a continuously recirculated capacitive flow electrode RED (FE-RED), and the other is a combination of a membrane electrode assembly (MEA) with RED (MEA-RED). The schematic of FE-RED is shown in Fig. 17. The
Conclusion
This review summarized most of the unique RED applications in the last decade. Innovations in energy conversion, desalination, and wastewater remediation based on RED are summarized. RED-HE can convert waste heat to electricity, which explands RED application. The electricity produced by RED or combined RED can be converted to hydrogen or stored as other forms of energy, which indirectly solves the problem that RED is still uneconomical due to low current power densities and costs of commercial
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
We greatly acknowledged the financial support from the National Natural Science Foundation of China (No. 51978055, 51578070), Special Project of International Science and Technology Cooperation (No. 18393611D), Department of Science and Technology, HeBei Province.
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