Electrochemical Desalination of Seawater and Hypersaline Brines

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Letter

Electrochemical Desalination of Seawater and Hypersaline Brines with Coupled Electricity Storage Divyaraj Desai, Eugene S Beh, Saroj K Sahu, Vedasri Vedharathinam, Quentin van Overmeere, Charles-François de Lannoy, Arun P Jose, Armin R Volkel, and Jessy Rivest ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01220 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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ACS Energy Letters

Electrochemical Desalination of Seawater and Hypersaline Brines with Coupled Electricity Storage Divyaraj Desai,†* Eugene S. Beh,† Saroj Sahu,† Vedasri Vedharathinam,†‡ Quentin van Overmeere,† Charles F. de Lannoy,†# Arun P. Jose,† Armin R. Vӧlkel† and Jessy B. Rivest†* †

‡

PARC, A Xerox Company, Palo Alto, California, USA

Lawrence Livermore National Laboratory, Livermore, California, USA #

McMaster University, Ontario, Canada

* Corresponding authors: [email protected] (Divyaraj Desai); [email protected] (Jessy B. Rivest)

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Abstract

We present a zinc|ferricyanide hybrid flow battery that achieves extensive first-pass desalination while simultaneously supplying electrical energy (10 Wh/L). We demonstrate 85% salt removal from simulated seawater (35 g/L NaCl) and 86% from hypersaline brine (100 g/L NaCl), together with reversible battery operation over 100 hours with high round-trip efficiency (84.8%). The system has a high operating voltage (E0 = +1.25 V), low specific energy consumption (2.11 Wh/L for 85% salt removal), and a desalination flux (4.7 mol/m2·h) on par with reverse osmosis membranes. Salt removal was similarly effective at higher feed salinities, for which reverse osmosis becomes physically impossible due to the pressure required. The results have positive implications for regions that rely on desalination for their fresh water needs, especially where sea salinity is high. Alternatively, the battery may also be useful in minimal liquid discharge (MLD) wastewater treatment if operated as a brine concentrator.


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Water scarcity is expected to affect 48% of the global population by 2025, and result in depletion of 90% of available freshwater sources.1 This projected demand requires the rapid development and adoption of energy-efficient and affordable desalination technologies to replace current energy-intensive processes. The current state of the art, seawater reverse osmosis (SWRO), requires large capital investments and incurs high operating costs,2 resulting in water that is expensive (>$0.53/m3) to produce.3 Furthermore, the specific energy consumption and desalination cost escalates with increasing feed salinity due to increased osmotic pressure and reduced water flux (see Section S1). For this reason, very energy-intensive thermal processes such as multi-stage flash distillation or multiple-effect distillation ($0.52–1.75/m3) 4 are used for high salinity water (Middle East seawater or saline aquifers).5 Electrochemical processes hold promise for energy-efficient and economical desalination. Capacitive electrochemical desalination approaches6-9 require extremely high surface area electrodes to maximize their electrosorption capacity, and are therefore only economical for desalinating brackish water (0.5–10 ppt TDS).10 On the other hand, Faradaic electrochemical desalination performs better at higher salinities because of increased ionic conductivity of the feed. The electroactive materials can be identical11-12 or dissimilar13-25 and have been reported as desalination batteries. However, the desalination batteries reported to date all have a low nominal cell potential (0.25–0.65 V), a low cathodic desalination capacity (27–35 mAh/g), and have demonstrated only low depths of seawater desalination (25-40% salt removal; see Section S2 for a detailed comparison of desalination systems that utilize Faradaic reactions). In principle, incorporating electrical energy storage into a desalination battery could reduce cost by facilitating load shifting on the electrical grid over a period of several hours while simultaneously reducing its demand charge, or by enabling deferral of investments into


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ACS Energy Letters

transmission and distribution infrastructure.26 Aqueous flow batteries would integrate well into an electrochemical desalination system because they share many common desalination capital requirements such as pumps and plumbing,2,

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which could be exploited to perform both

desalination and energy storage. By tapping the revenue streams that are available to an energy storage technology,26 the system can defray desalination costs while simultaneously enabling increased adoption of renewables.29 A technology that successfully brings together the fields of desalination or water purification and energy storage would be transformative.30 Here, we present a design and experimental results of a zinc|ferricyanide hybrid desalination flow battery operating at a high nominal cell potential (E0 = +1.25 V), which enables high roundtrip efficiency and electrical storage capacity.23-25 The cell architecture is analogous to previously reported flow battery designs,31-33 but utilizes different redox-active materials at moderate pH (49). (Figure 1) During operation, the discharge half-cycle reactions result in the removal of NaCl from the central electrolyte (desalination). The zinc anode is oxidized to Zn2+ ions, drawing Cl− ions into the anolyte tank through the anion-exchange membrane, while ferricyanide is reduced to ferrocyanide in the catholyte, drawing Na+ ions into the catholyte tank through the cationexchange membrane. Before the subsequent charge half-cycle, the desalinated water is removed and replaced with a new volume of seawater. Na+ and Cl− ions are driven into the central reservoir by the reverse reactions (cell charging), thereby producing concentrated brine. The net result over a full electrochemical discharge-charge cycle is therefore the production of two streams of water, one depleted and the other enriched in NaCl. This is distinct from methods that utilize one or two intercalation electrodes, which simultaneously produce both streams of water


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and can operate with one or even no membranes. The reversible half-cell and overall reactions are represented below. Anode: 2 ⇌ 2 SHE)

(E0 = −0.76 V vs

Cathode: 2 2 2 ⇌ 2 (E0 = +0.49 V vs SHE) Overall: 2 2 ⇌ 2

(E0 = +1.25 V)

Figure 1. Schematic of the desalination battery operation during discharge (desalination) and charge (salination). Briefly, a central reservoir (3 mL) containing aqueous NaCl was bounded on one side by an anolyte reservoir (50 mL) containing an aqueous solution of ZnCl2 (0.3 M) and a zinc anode, and on the other side by a catholyte reservoir (50 mL) containing an aqueous solution of K4Fe(CN)6 (0.3 M) + K3Fe(CN)6 (0.3 M) and a graphite cathode. This combination of reactants gives an energy density of 10 Wh/L based on the anolyte and catholyte volume. An anion-exchange membrane (ASTOM Neosepta AFX, 8 cm2) separated the anolyte reservoir from the central reservoir, which was in turn separated from the catholyte reservoir by a cation-exchange membrane (Fumasep FKE-50, 8 cm2). All three electrolyte solutions were circulated to and from external reservoirs using peristaltic pumps (Control Company, 10 mL/min); Viton gaskets (0.76


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mm thick) were used to seal each reservoir. To improve the surface area of the electrodes and provide structural support for the membranes, copper mesh (60×60 mesh, McMaster-Carr) was pressed onto the zinc anode, and carbon fiber paper (Sigracet 10BI, 2 sheets, pre-baked in air for 4 hours at 400°C) was pressed onto the graphite cathode. A nylon mesh (31×31 mesh, McMaster-Carr) provided structural support within the central reservoir. Where employed, the extent of desalination was measured using a TDS meter (HI-98192, Hanna Instruments), which was calibrated using purchased stock solutions. A detailed electrochemical characterization of the zinc|ferricyanide system was performed using a full cell with a 0.8 mm seawater chamber width. Shallow cycling at various fixed current densities and two different seawater salinities (35 and 100 g/mL) produced a galvanostatic polarization curve showing a short-circuit current density of 29 mA/cm2 (corresponding to a maximum achievable flux of 10.8 mol/m2·h) and a cell area-specific resistance (ASR) of approximately 30 Ω·cm2, which was in good agreement from the ASR extracted from linear sweep voltammetry (29 Ω·cm2 at a rate of 0.1 V/s). (See Sections S3–S7 for further details, as well as results from cyclic voltammetry and electrochemical impedance spectroscopy). Deep cycling experiments were performed to investigate the ability of the desalination battery to achieve a high depth of desalination. Another cell with a 0.8 mm seawater chamber width was freshly assembled with 3.00 mL of 35 g/L NaCl in the central reservoir, and discharged at a constant current of 0.70 mA/cm2, whereupon the salinity of the water in the central reservoir was found to be 4.81 g/L, corresponding to 85% salt removal (Figure 2). The specific energy consumption can be found from the area between the charge/discharge curves, and corresponds to an energy consumption of 4.08 Wh/mol (or 2.11 Wh/L). We also experimented with high salinity brines; when the central reservoir of the cell was filled with 100 g/L NaCl (resulting in a


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nearly unchanged starting ASR of 30 Ω·cm2; vide supra) and discharged at 1.25 mA/cm2, the final salinity was measured to be 13.4 g/L NaCl, corresponding to 86% salt removal at an energy consumption of 12.7 Wh/L. The higher ultimate salinity was most likely a consequence of the higher current density, which was employed to allow for practical experimental timescales. The cell is therefore capable of supporting first-pass desalination of seawater or concentrated brine down to as low as 5 ppt TDS. Below 5 ppt TDS, the conductivity of the seawater chamber (and therefore the entire cell) becomes too low for efficient operation,31 and a second stage of desalination is needed. This could be achieved using brackish water RO or capacitive deionization systems for economical second-pass desalination, thereby producing potable water.

Figure 2. Charge-discharge curves for deep-cycling experiments with seawater (left, 35 4.81 g/L NaCl) and concentrated brine (right, 100 13.4 g/L NaCl) in the central reservoir. The energy consumption (22.8 J and 137.2 J, grey shaded areas) is the area bounded by the two curves. Data taken on a cell with 0.8 mm seawater chamber width. Reversible battery cycling was demonstrated with a 10 mm seawater chamber width, flowing anolyte and catholyte (25 mL each) and a stagnant water reservoir (8 mL) to avoid potential water losses through the silicone pump tubing. This design limited the maximum current and


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resulted in increased resistive losses, but allowed adequate water volume for repeated salinity measurements and enabled continuous operation over 100 hours. The central reservoir was filled with simulated seawater (35 g/L NaCl) and the cell was discharged at a current density of 0.7 mA/cm2 for 12 hours. Subsequently, the central reservoir was drained and replaced with new seawater, then the cell was charged at 1.3 mA/cm2 for 6 hours (salination, brine formation), thereby completing one full cycle. Four identical cycles were performed, then one last cycle was done at the same current densities but for a longer duration (18 hour discharge, 9 hour charge) in order to effect a higher degree of desalination (60% vs. 40%). To evaluate the actual amount of salt transferred, the salinity of the NaCl solution in the central reservoir was measured at the beginning and end of each half-cycle. The cycling results are summarized in Figure 3. As expected, a drop or rise in salinity was observed at the end of each discharge or charge half-cycle, respectively (Figure 3a). The observed degree of desalination or salination was always slightly lower (by ~10%) than expected when considering the amount of charge passed, and therefore the amount of Na+ and Cl− ions transported. The discrepancy is most likely caused by a small amount of electro-osmotic water crossover between the central reservoir and the anolyte or catholyte reservoirs, though we note that no significant change in volume (within 5%) could be observed after each half-cycle. The median round-trip efficiency at 40% salt removal was 84.8% for the 5 cycles, although discharge was continued beyond 40% salt removal for the fifth cycle. The net energy expenditure per cycle is effectively the energy required to separate the feed seawater into two streams, namely desalinated water and brine. From the cycling results, the specific energy consumption of this cell (1.3 mA/cm2 charge, 0.7 mA/cm2 discharge) was 1.63 Wh/L (40% salt removal) and 2.19 Wh/L (60% salt removal).


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(a)

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(b)

Figure 3. (a) Charge-discharge profile (top) and water salinity (bottom) during successive charge-discharge half-cycles with 35 g/L NaCl in the central reservoir of the 10 mm cell. (b) Deep cycling demonstrating brine concentration (35 100 g/L NaCl) and desalination (35 7.6 g/L NaCl). Circles in the TDS plot represent measured data points; the dashed line is a guide for the eye. In order to explore the performance limit of the redox desalination battery and assess its feasibility for large-scale seawater desalination, it is important to know the maximum extent of desalination that the cell can achieve. To this end, a cell with a 10 mm seawater chamber width was charged an average salt flux of 0.63 mol/m2·h (Figure 3b) for 22 h until the salinity in the center reservoir was found to reach 100 ppt. The brine formed was replaced with more seawater and discharged in stages at progressively lower current densities for a further 22 hours, after which the salinity in the center reservoir was found to have decreased to 7.60 g/L (78% salt removal) at an average salt flux of 0.25 mol/m2·h. Periodic measurements of the salinity and the high-frequency cell impedance during discharge revealed increasing impedance as the salinity of the center reservoir decreases. In comparison, the cell with a 0.8 mm seawater chamber width was capable of a higher depth of desalination (35 4.80 g/L NaCl, 86%) at a specific energy consumption of 2.11 Wh/L and a flux of 0.34 mol/m2·h.


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Because the cell ASR rises sharply once the center reservoir has been desalinated below 10 g/L NaCl, secondary desalination to produce drinkable fresh water (TDS 80 ppt) on which SWRO can be used. For hypersaline brines above 50 ppt, the redox desalination battery becomes the most economically viable non-thermal option. Potable water production from concentrated brine (100 g/L NaCl) using the desalination battery in conjunction with downstream BWRO would consume 14.06 Wh/L, and is an improvement over electrodialysis (30–40 Wh/L)35 or thermal distillation processes (20–60 Wh/L).34 At the same time, we have also demonstrated that the technology is capable of directly concentrating brine to at least 100 ppt, which has clear applicability towards minimal liquid discharge (or MLD) processes that have collocated energy storage needs.


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Figure 4. Projected energy consumption comparison for the zinc|ferricyanide desalination battery and SWRO system to achieve first pass desalination (TDS

Electrochemical Desalination of Seawater and Hypersaline Brines

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