Elsevier

Desalination

Volume 506, 15 June 2021, 115018
Desalination

Investigating the influence of catholyte salinity on seawater battery desalination

https://doi.org/10.1016/j.desal.2021.115018Get rights and content

Highlights

  • Salinity of the catholyte influences the volume of the desalination compartment.

  • Ion removal efficiencies of seawater batteries do not depend on catholytes used.

  • High-saline catholyte yields less time for desalination.

  • Low saline catholyte prevents high current charging and discharging.

Abstract

The seawater battery (SWB) is a promising desalination technology that utilizes abundant sodium ions as an energy storage medium. Recently, the alternative desalination system, seawater battery desalination (SWB-D), was developed by placing an SWB next to the desalination compartment. This SWB-D system can desalt water while charging the SWB next to it. However, only a fixed catholyte solution has been investigated, although the catholytes impact the overall SWB-D performance. Therefore, we evaluated the effect of different catholytes on the desalination performance. High-saline reverse osmosis (RO) concentrate or brackish water exhibited excellent salt removal capability (>85.3% of sodium and >76.6% of chloride ions) with relatively short operation times (36.4 h for RO concentrate and 39.5 h for brackish water) upon charging, whereas the relatively low-saline river water showed the longest operation time (81.0 h), implying that river water should be excluded as a potential catholyte. The amount of desalinated water was marginally reduced due to osmosis through the anion exchange membrane; however, the amount of treated salt was >82.9% even after the reduction in water volume. These findings suggest that the catholyte with a resistance of >0.041 kΩ·cm can be ideal for the SWB-D.

Introduction

The increasing demand for potable water has remained a major global issue for decades [1]. Many countries in the Middle East, North Africa, East Asia, North America, and Australia have built seawater desalination plants due to the lack of freshwater resources such as river water, rainwater, and groundwater [2]. Since the early 2000s, RO has been widely adopted to produce clean water owing to its high energy efficiency and productivity compared to previous thermal desalination technologies [3,4]. However, membrane-based desalination technologies are still energy-intensive and environmentally detrimental [[5], [6], [7]]. For instance, the development of RO has gradually reduced the energy consumption from ~10 to 3 kWh/m3; however, this is still higher than the theoretical minimum energy consumption of ~0.8 kWh/m3 [[8], [9], [10], [11], [12], [13]]. Additionally, a high concentration of RO brine, together with toxic chemicals, is inevitably discharged into the sea, which could adversely impact the environment [7]. Hence, there is great interest in the development of more energy-efficient and cleaner desalination technologies [[14], [15], [16]].

Alternative desalination technologies based on electrochemistry, such as capacitive deionization (CDI), membrane capacitive deionization (MCDI), electro-deionization (EDI), and electrode forward osmosis, have been actively investigated [[17], [18], [19]]. In the early stages of development, these electrochemical desalination technologies seemed to have relatively low energy consumption compared to RO under brackish water conditions (<30 mM NaCl) [20]. However, a recent modeling study has reported that RO could outperform CDI even in brackish water desalination [21], while other studies reported remarkably low or competitive energy demands of electrochemical processes [22,23]. Further, one of the drawbacks of CDI is its low salt adsorption capacity (~30 mg/g for brackish water desalination), as it adsorbs dissolved ions through a non-Faradaic reaction with a low charge capacity [24]; thus, it is unlikely feasible for seawater desalination, which requires a significantly large salt adsorption capacity [25].

The recently proposed seawater battery (SWB) system utilizes the sodium ions present in seawater [[26], [27], [28]]. The SWB extracts sodium ions from seawater during the charging of the battery by storing electrical energy in the anode (i.e., sodium metal); therefore, it has a higher theoretical salt adsorption capacity (~2520 mg/g) than that of CDI [29]. During charging, the counter ions (mostly chlorine ions) are extracted from the cathode compartment by chlorine gas evolution [30]. Thus, the SWB could extract over 70% of the dissolved ions from seawater due to the large quantity of sodium (~30%) and chlorine ions (~55%) present in seawater [31]. Additionally, the SWB is rechargeable as only the sodium ions can enter or leave the SWB anode [18,20]. Thus, the desalination aspect of the SWB has received interest from environmental engineers engaged in water desalination [32,33]. However, only a few studies using a single desalination compartment of catholyte have been conducted [33,34].

A novel SWB-D system consisting of the SWB, desalination compartment, and catholyte compartment was introduced by combining the SWB with additional membrane filtration systems, such as nanofiltration (NF) and RO, to remove residual ions [35]. According to a study [35], the SWB-D system successfully reduced the total energy consumption for desalination from 2.83 (using only RO) to 1.35 kWh/m3 (RO combined with SWB-D). However, several aspects remain unexplored regarding the practical application of the SWB-D system for water desalination. Most studies primarily focused on material development, such as electrolyte synthesis or electrode replacement [29,36,37]. Additionally, only a fixed catholyte of a synthetic electrolyte [32] or seawater [35] has been used to evaluate the salt removal and electrochemical profiles. Hence, the SWB-D system still requires further investigation through comprehensive research using various catholytes.

In this regard, we investigated the influence of cathode compartments on the performance of the SWB-D system and discussed its implications and practical applications for water desalination based on the following objectives: 1) evaluating the effect of different catholytes—three natural water resources with different salinities (RO retentate, brackish water, or river water) on the charging/discharging and desalination efficiencies of the SWB-D system; 2) evaluating the quality and quantity of the desalinated/catholyte solution under different catholyte conditions; and 3) providing a guideline for the catholyte selection for the SWB-D system.

Section snippets

Seawater battery coin cell assembly

SWB (SWB2465, 4TOONE, Republic of Korea) was assembled using a previously described procedure [35]. The coin cell compartments consisted of seven components in the following order: 1) the coin bottom equipped with a NASICON (Na3Zr2Si2PO12; CE&Chem Co. Ltd., Republic of Korea) membrane with a diameter of 16 mm and a thickness of 1 mm [38], 2) polyethylene (PE; Celgard, Republic of Korea) separator with a 19 mm diameter, 3) 200 μL of a mixed organic electrolyte (1 M NaCF3SO3) dissolved in

Feed water characteristics

The salinity of C1 was expected to be the most influential factor in water desalination using the SWB-D system as it determines the solution resistance of C1 and ion migration rate from C2 to C1. The ionic conductivity of the three catholytes was in the range of 0.28 ± 0.08–90.55 ± 0.04 mS/cm, which was selected based on that of the C2 solution (22.20 ± 0.52 mS/cm) (Table 1). A fixed feed water of NF permeate was used in C2 as a previous study has shown that an ion conductivity of >48.7 mS/cm

Conclusion

Here, we explored the influence of the feed water of the cathode compartment on the performance of the SWB-D system. Each type of feed water yielded different charging and discharging performances based on the resistivity and salinity of the feed water. This study proposes the necessary consideration for the catholyte solution to improve the desalination efficiency of SWB. The major findings are summarized as follows:

  • High-saline feed water maintained the high step-currents during charging

CRediT authorship contribution statement

  • Sanghun Park: Writing - Original draft preparation, Methodology, Visualization

  • Mayzonee Ligaray: Methodology, Conceptualization

  • Youngsik Kim: Investigation, Validation

  • Kangmin Chon: Investigation, Validation

  • Moon Son: Supervision, Writing - Reviewing and editing

  • Kyung Hwa Cho: Supervision, Conceptualization.

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.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) [grant number 2020R1A4A1019568].

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