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Concentration flow cells based on chloride-ion extraction and insertion with metal chloride electrodes for efficient salinity gradient energy harvest Guangcai Tan, Hongna Li, Haihui Zhu, Sidan Lu, Jizhou Fan, Guoqiang Li, and Xiuping Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03657 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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ACS Sustainable Chemistry & Engineering

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Concentration flow cells based on chloride-ion extraction and insertion with

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metal chloride electrodes for efficient salinity gradient energy harvest

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Guangcai Tan,a Hongna Li,b Haihui Zhu,a Sidan Lu,a Jizhou Fan,c Guoqiang Li,c Xiuping Zhua,*

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a Department

of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803,

USA. b Agricultural

Clean Watershed Research Group, Institute of Environment and Sustainable Development in

Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, PR China

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c Department

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*Corresponding

of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA. author. Tel./Fax: +1-(225) 578-1523. E-mail address: [email protected].

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1 ACS Paragon Plus Environment

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Abstract

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Salinity gradient (SG) is a natural and renewable energy source existed in estuaries, and can

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also be produced during various desalination and industrial processes. Here, a new method is

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proposed to efficiently recover SG energy based on chloride-ion (Cl-) extraction and insertion with

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metal chloride electrodes and the Donnan potential over a cation-exchange membrane in a

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concentration flow cell. Three different metal chloride electrodes (BiCl3, CoCl2 and VCl3) were

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investigated in the cell and their properties after discharging in 30 g L-1 (seawater) and 1 g L-1

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(river water) NaCl solutions were studied by the cyclic voltammetry, the electrochemical

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impedance spectroscopy, and the X-ray photoelectron spectroscopy. The cell with BiCl3 electrodes

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yielded the largest power density (max. = 3.17 W m-2) compared to that of CoCl2 and VCl3

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electrodes, which was higher than those of mostly previous technologies for SG energy recovery.

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Fast Cl- extraction and insertion processes were observed on BiCl3 electrodes due to small charge

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transfer resistance and Cl– diffusion resistance. BiCl3 was reduced to metal Bi as Cl– released from

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the electrode to river water, while metal Bi was oxidized to BiCl3 as Cl– inserted into the electrode

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from seawater.

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Keywords: Salinity gradient energy; Chloride ion battery; Metal chloride; Concentration flow cell

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Introduction

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When two solutions with different salinities are mixed, a release of free energy is obtained due

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to the difference of chemical potentials, which is known as salinity gradient (SG) energy. A

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tremendous amount of SG energy naturally exists in estuaries. There is about 2.5 MJ energy in

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theory when one liter of river water flows into the ocean.1 It is estimated that approximately 625


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TWh per year of SG energy is practically extractable globally in coasts when accounting for

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geography and other practical constraints, which amounts to 3% of worldwide electricity

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consumption.2 A large amount of salty waters also exits in hypersaline lakes, geothermal wells,

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reject flows from desalination plants, hydraulic fracturing flowback water, and various industrial

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wastewaters (i.e., leather, petroleum, and agro-food industries).3,4

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Several approaches have been put forward to harvest SG energy, such as pressure-retarded

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osmosis (PRO), reverse electrodialysis (RED), capacitive mixing (CapMix), and hydrogel

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expansion (HEx). PRO has received the most attention to date, which produce electrical power by

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using mechanical turbines powered by the different pressure between two solutions with different

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salt concentrations separated with a salt-rejecting semi-permeable membrane.5 PRO can produce

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relatively high power densities with synthetic seawater and freshwater (7.5 W m–2),6,7 but it is

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hindered by rapid membrane fouling issue and not suitable for hypersaline waters as current

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membranes cannot withstand high pressures.8 RED is another extensively studied method, which

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produce electricity driven by the potential difference (i.e., Donnan potential) created across ion-

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exchange membranes as a result of selective ion transport.9,10 The reported highest power density

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of RED is 2.9 W m–2.11 CapMix produces electrical power by taking advantage of the potential

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difference between two salt concentration-dependent electrodes.12,13 The power densities of

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CapMix systems are generally low (maximum 0.4 W m–2) and the power output is intermittent.

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HEx is a quite new method, which recover SG energy depended on extracting work done in the

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procession of the expansion and contraction of hydrogel particles under different salt solutions.14

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Although the power density is low (0.1 W/kg-hydrogel), the low cost of hydrogels makes it

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attractive and the power can be further improved by synthesizing more effective hydrogels.



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Recently, a new technology, called concentration flow cell, was reported for efficient SG

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energy recovery,4 which combined the mechanisms responsible for power production in RED

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(Donnan potential) and CapMix (electrode potential) to generate an unprecedentedly high power

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density (12.6 W m2-membrane) for an electrochemical system. The concentration flow cell was

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constructed by two identical electrodes composed of copper hexacyanoferrate (CuHCF). Anion-

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exchange membrane (AEM) was used to separate the two electrodes and form two channels which

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can be fed with synthetic freshwater and seawater. The water paths were periodically switched to

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recharge the cell and further produce power. Previous concentration flow cells for SG energy

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harvest were all based on the Na+ intercalation and deintercalation using hexacyanoferrate

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electrodes and the Donnan potential across an AEM.4,13 There is no study on harvesting SG energy

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based on Cl– extraction and insertion, although it has been used for energy storage in chloride-ion

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batteries.15-17

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Here, we demonstrated that a concentration fuel cell can also work well for SG energy

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recovery based on Cl– extraction and insertion with metal chloride electrodes (e.g., BiCl3) and

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Donna potential across a cation-exchange membranes (CEM) (Fig. 1). The maximum power

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density was 3.17 W m-2 in the present study, which was higher than those of previous RED and

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CapMix cells11,13,18 and can be further improved as the development of this technology. In addition,

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this cell can make use of abundant material resources since various chlorine compounds could be

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used as active materials.17 It provides a new way to develop concentration fuel cells for efficient

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SG energy recovery based on Cl– extraction and insertion, which has not been reported so far.

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Experiment Section

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Preparation of metal chloride electrodes


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BiCl3, CoCl2 and VCl3 powders (99%, Alfa Aesar) were dried at 90 ºC overnight in a vacuum

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oven. Then, the dried metal chloride powders were mixed with 20% mass carbon black (conductive

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carbon, Alfa Aesar, 99.5%) by ball milling in a hardened steel container with several 10 mm-

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diameter hardened steel balls in an argon atmosphere. The powder to ball was at a weight ratio of

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1:20. The milling was performed in a planetary mill (PQ-N04, USA). The rotation speed was 400

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rpm for one day. To prepare electrodes, 2.5% polyvinylidene fluoride (PVDF, Alfa Aesar) was

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dissolved in 0.5 mL dimethylformamide (Alfa Aesar, 99%) as binder. Then, 70 mg of as-prepared

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active powder was mixed with the binder solution, and ground by hand. The slurry was painted

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onto both sides of carbon paper current collectors (3 cm × 3 cm) with a mass loading of ~5 mg

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cm-2 and a working area of 1 cm × 3 cm (ca. 3 cm2) in the middle. The electrodes were then dried

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for 12 h at 70 °C under vacuum. The electrodes of pristine carbon black were also prepared

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following the same procedures for comparison.

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Construction and performance tests of the cells

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The cell configuration was similar to our previous study (Fig. S1 in supporting information),19

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which were constructed with two symmetrical metal chloride electrodes (BiCl3, CoCl2 or VCl3).

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The electrodes were separately by a cation exchange membrane (CEM, 5 cm × 5 cm, Selemion

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ASV, Japan). Short platinum wires were connected to each electrode as current leads. Two silicon

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gaskets (5 cm × 5 cm × 127 μm) with a chamber of 3 cm × 1 cm × 127 μm in the middle were

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placed between the membrane and the electrodes to form two channels which could be separately

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and simultaneously fed by high concentration and low concentration NaCl solutions. Then, two

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thicker solid silicon gaskets (5 cm × 5 cm × 508 μm) and two end plates (5 cm × 5 cm × 3 mm)

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with an inlet and an outlet were added on both ends and firmly sealed.



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To test the electrical performance of concentration flow cells, 30 g L-1 NaCl (high

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concentration, HC) and 1 g L-1 NaCl (low concentration, LC) solutions were used to mimic the

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seawater and river water. A peristaltic pump (Cole-Parmer) was used to pump the two solutions

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alternatingly through the two channels of the cells. The flow rate was 7 mL min-1, and the

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theoretical hydraulic retention time was ∼0.9 s. An potentiostat (VMP3, Bio-Logic) was used to

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record the open circuit voltages (OCVs) of the cells when the HC and LC solutions were switched

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every 2 mins.

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Different external resistors (Rext = 30, 20, 15, 10, 8, 5, 4, 3 Ω) were connected between the

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electrodes to test the power output of the cells with HC and LC solutions were switched. For each

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external resistor, at least four full cycles were conducted. Each cycle ended when the cell voltage

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decreased below ±5 mV and restarted by changing the flow path of HC and LC solutions. The cell

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voltage (U, in V) was recorded every 0.1 s using a potentiostat. Instantaneous power density (Pins.,

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in W m-2) was calculated from the Rext and the recorded cell voltage (Pins. = U2/ARext, A was the

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working electrode area). The average power density (Pave., in W m-2) and energy density (Ed, in J

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m-2) were obtained from the instantons power densities over one cycle (𝑃𝑎𝑣𝑒. = ∫0𝑐𝑦𝑐𝑙𝑒𝑃𝑖𝑛𝑠.𝑑𝑡/𝑡𝑐𝑦𝑐𝑙𝑒

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and 𝐸𝑑 = ∫0𝑐𝑦𝑐𝑙𝑒𝑃𝑖𝑛𝑠.𝑑𝑡; tcycle is the cycle time). Optimum external resistors for the cells with

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different metal chloride electrodes were determined based on the maximum instantaneous power

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density.

𝑡

𝑡

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To test the stability of these systems, the cells were run at the optimum external resistor (4 Ω

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for BiCl3, 5 Ω for CoCl2, 5 Ω for VCl3) for 50 cycles. Similarly, cell voltage, power density, and

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energy density were obtained for every cycle. Different combinations of LC (1.0 g L-1) and HC

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(30, 100, 200 and 300 g L-1) NaCl solutions were also used to test power output of concentration

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flow cells from brines and freshwaters. 6 ACS Paragon Plus Environment

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Electrochemical characterization of metal chloride electrodes

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Electrochemical characterization of metal chloride electrodes was conducted using a

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potentiostat in a single-chamber cell containing an 80 mL NaCl solution with a metal chloride

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electrode (BiCl3, CoCl2 or VCl3) as the working electrode. The counter electrode was a platinum

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coated titanium electrode and the reference electrode was an Ag/AgCl electrode. All potentials

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reported here were referred to the Ag/AgCl reference electrode (+210 mV vs. a standard hydrogen

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electrode, SHE).

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Electrode potentials of metal chloride electrodes in different NaCl concentration solutions (1,

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5, 10, 20, 30, 40, 50 g L-1) were recorded using a potentiostat until steady values were obtained.

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Cyclic voltammetry (CV) was scanned from –1.5 V (BiCl3) or –0.8 V (CoCl2 and VCl3) to 0.9 V

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(BiCl3) or 0.8 V (CoCl2 or VCl3) in 1 g L-1 and 30 g L-1 NaCl solutions. The scan rate was 1 mV

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s-1. Electrochemical impedance spectra (EIS) were recorded with a perturbation amplitude of 10

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mV around the equilibrium potential (OCV) at the frequency range from 10 mHz to 100 kHz. 1 g

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L-1 and 30 g L-1 NaCl solutions were used.

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X-ray photoelectron spectroscopy analysis

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In order to illustrate the working principle of the concentration flow cell, the original BiCl3

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electrodes and the electrodes after discharging in LC (1 g L-1 NaCl) and HC (30 g L-1 NaCl)

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solutions in the cells were analyzed by X-ray photoelectron spectroscopy (XPS). XPS data were

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gathered on an AXIS165 spectrometer using a twin-anode Al Kα radiation as the X-ray source.

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The analyzer chamber was kept in vacuum at about 1.3 × 10-8 Pa during the measurements. The

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high-resolution spectra used a pass energy of 23.5 eV and steps of 0.2 eV. Linear-type background

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subtraction was used to treat the XPS spectra firstly, and then a standard program for data



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processing was employed to fit the XPS spectra with mixed Gaussian–Lorenzian functions.20 The

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peak positions were calibrated against the C 1s peak at 284.3 eV.

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Results and Discussion

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Electrical performance of the cells with metal chloride electrodes

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The OCVs of the cells with metal chloride electrodes (BiCl3, CoCl2, and VCl3) were obtained

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with LC and HC flow channels switched every 2 min (Fig. 2). The cell with BiCl3 electrodes

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possessed the highest OCV of ca. ± 0.16 V, followed by CoCl2 (ca. ± 0.11 V) and VCl3 (ca. ± 0.09

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V). The power densities of the cells were then examined at varied external resistances in the range

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of 3 ~ 30 Ω. As shown in Fig. 3, the cell voltage and instantaneous power density firstly increased

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and then gradually decrease to almost zero for each cycle because of the limited charging capacity

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of the electrodes. When the flow paths of HC and LC solutions were switched in next cycle, the

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cell voltage was reversed, and power was produced again. Fig. 4 shown the maximum

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instantaneous power density and the average power density at different external resistances. The

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maximum instantaneous power density of the cell with BiCl3 electrodes (3.17 ± 0.19 W m-2 when

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the external resistance was 4 Ω) was much higher than those of the cells with CoCl2 electrodes

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(1.97 ± 0.14 W m-2 when the external resistance was 5 Ω) and VCl3 electrodes (1.19 ± 0.02 W m-

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2

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external resistances indicated that the internal resistances of the cells varied since the maximum

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power output was usually obtained when the external resistance is the same as the internal

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resistance. Similarly, the cell with BiCl3 electrodes had the highest average power density of 0.82

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± 0.02 W m-2, which was about two times of that with CoCl2 electrodes (0.47 ± 0.06 W m-2), and

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nearly three times of that with VCl3 electrodes (0.28 ± 0.04 W m-2) (Fig. 4b and Table 1). The

when the external resistance was 5 Ω) electrodes (Fig. 4a and Table 1). The different optimum


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much higher power production of the cell with BiCl3 electrodes could be ascribed to its highest

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gained voltage (Fig. 2) and a relative lower internal resistance (4 Ω compared to 5 Ω for the cells

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with CoCl2 and VCl3 electrodes).13

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The cells with metal chloride electrodes were then operated under their optimum external

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resistances of BiCl3 (4 Ω), CoCl2 (5 Ω) and VCl3 (5 Ω) for 50 cycles to test the stability of these

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systems. Although there were few fluctuations for maximum instantaneous power density, average

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power density and energy densities as solutions were switched manually, all concentration flow

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cells showed good stability generally (Fig. 5). The maximum instantaneous power density

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remained at ca. 3.17 W m-2 for the cell with BiCl3 electrodes, ca. 1.97 W m-2 for that with CoCl2

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electrodes, and ca. 1.19 W m-2 for that with VCl3 electrodes over the 50 cycles. The average power

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density was ~0.82 W m-2 for the cell with BiCl3 electrodes, ~0.47 W m-2 for that with CoCl2

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electrodes, and ~0.28 W m-2 for that with VCl3 electrodes. The energy densities of the cells were

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also stable. BiCl3 electrodes had the highest energy density of 10.65 ± 0.46 J m-2, followed by

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CoCl2 (4.21 ± 0.16 J m-2) and VCl3 (2.25 ± 0.11 J m-2) electrodes (Fig. 5 and Table 1).

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The power output of the cells with metal chloride electrodes from highly saline waters were

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also investigated under optimum external resistances (Fig. 6). For all electrodes, both the

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maximum instantaneous power density and the average power density rose concurrently with the

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increased HC solution concentration, when the LC was fixed at 1 g L-1. When the HC concentration

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increased to 300 g L-1 NaCl, the cell with BiCl3 electrodes produced the highest maximum

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instantaneous power (7.05 ± 0.28 W m-2) and highest average power density (1.66 ± 0.34 W m-2),

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which were nearly 2 times of those when the HC concentration was of 30 g L-1 NaCl. These results

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indicated that the concentration flow cell with metal chloride electrodes had the potential to capture



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energy from brines, such as reject flows from seawater desalination plants and waste brine

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producing from epichlorohydrin synthesis and chloralkali processes.3

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Electrochemical characterizations of metal chloride electrodes

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In order to comprehend the varied electrical performance of the cells, CV and EIS were

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employed to examine the electrochemical properties of BiCl3, CoCl2, and VCl3 electrodes. CVs of

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metal chloride electrodes in the HC solution (30 g/L NaCl) compared to that in the LC solution (1

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g/L NaCl) indicated that Cl- extraction and insertion could occur on the three electrodes (Fig. 7).

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In general, the relative scale of oxidation and reduction peaks were similar to the report of Zhao et

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al.15 which using these three electrodes in chloride ion batteries. For BiCl3 electrodes, there was a

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large oxidation peak observed at 0.28 V (peak current = 16.91 mA) and a large reduction peak

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observed at –1.11 V (peak current = –17.47 mA), suggesting easy Cl- extraction and insertion into

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BiCl3 electrode. Similar to chloride ion batteries, BiCl3 could be reduced to Bi metal as Cl- released

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from the electrode and Bi metal could be oxidized to BiCl3 as Cl- inserted into the electrodes.15

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For CoCl2 electrodes, an oxidation peak appeared at 0.24 V (peak current = 1.75 mA) and a

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reduction showed at –0.01 V (peak current = –0.87 mA). Another reduction peak appeared at 0.21

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V (peak current = –0.67 mA), indicating there could be an intermediate Co reduction state

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responding to Cl- released from CoCl2 electrodes. For VCl3 electrodes, there was a small oxidation

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peak at -0.08 V (peak current = 0.70 mA) which was probably ascribed to slow Cl- insertion and a

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small reduction peak at -0.3 V (peak current = -1.25 mA) could be due to sluggish Cl- release.

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These results clearly demonstrated that the rates of Cl- extraction and insertion with the electrodes

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played a key role in the power densities of concentration flow cells. For BiCl3, fast Cl- extraction

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and insertion processes (obvious redox peaks) resulted in high power densities, while CoCl2 and


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VCl3 had relative lower power densities because of sluggish Cl- extraction and insertion (small

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redox peaks). On the contrary, there were no obvious oxidation and reduction peaks over the entire

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range for the carbon black electrode (Fig. S2 in supporting information), suggesting that carbon

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black was inactive electrochemically from -1.5 V to 0.9 V and did not contribute to the capacity

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of Cl- extraction and insertion. This result was consistent with the low power density of

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concentration flow cells with carbon black electrodes which was less than 1 mW m-2 (data not

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shown). Similarly, previous studies also indicated that carbon nanotube as conducting network had

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no contribution to the capacity of electrodes in lithium battery.21

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EIS was used to further understand the electrochemical behaviors of metal chloride electrodes.

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The Nyquist plots of each metal chloride electrode in HC and LC solutions were shown in Fig. S3

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of supporting information. The semicircle in the medium-frequency region was observed for all

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the electrodes, corresponding to the charge transfer resistance (Rct) at the interface between the

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NaCl solution and the electrodes. Obviously, the smallest diameter of the semicircle was found for

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BiCl3 electrodes for both conditions of HC and LC NaCl concentrations, suggesting that a smaller

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Rct value of BiCl3 than those of CoCl2 and VCl3 electrodes.22 These results were also in agreement

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with CV analysis which found that BiCl3 electrode had the biggest reduction/oxidation peaks at 30

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g L-1 NaCl concentration, indicating fast Cl- extraction and insertion. The low Rct could be due to

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the faster conduction of both electrons and chloride ions during the extraction and insertion process

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for BiCl3 electrode. The crystal structure of BiCl3 is a pyramidal structure, while the crystal

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structure is monoclinic for CoCl2 and hexagonal for VCl3.23 The electrons and chloride ions could

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be easier to transfer in the pyramidal structure.

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The results of Fig. S3b in supporting information indicated that ionic diffusion resistance in

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metal chloride electrodes in 1 g L-1 NaCl was significantly larger than that in 30 g L-1 NaCl. The



Page 12 of 29

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reason could be contributed to the lower solution conductivity or ion hydration in low NaCl

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concentration.24 In addition, it was noted that the straight line which associated with Warburg

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impedance more likely vertical to the real axis in lower frequency region for both CoCl2 and VCl3

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electrodes, suggesting higher Cl- diffusion resistance. The EIS results further demonstrated that

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Cl- extraction and insertion was easy with BiCl3 electrodes due to low charge transfer resistance

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and Cl- diffusion resistance.

246 247

Working principle of the concentration flow cells with metal chloride electrodes

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The cell voltage of the cells was resulted from two parts: the Donnan potential developed

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across the CEM (∆E, Eq. (1)) and the electrode potential difference developed at the metal chloride

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electrodes. As shown in Eq. (1), the theoretical Donnan potential across the CEM was ~0.08 V. 𝑅𝑇

251

𝑎𝑁𝑎 + , HC

∆𝐸 = 𝑛𝐹ln ( 𝑎

𝑁𝑎 + , LC

)

(1)

252

where R is the gas constant (8.314 J mol-1 K-1), T is absolute temperature (298 K in our

253

experiments), n is the electron number (n = 1), F is the Faraday constant (96 485 C mol–1), and a

254

is activity.

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As the Donnan potential across the CEM was the same, the differences in OCVs of the cells

256

should be attributed to their respective electrode potential differences. As shown in Fig. 8, the

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electrode potentials of metal chlorides nearly linearly decreased with the natural log of NaCl

258

concentrations (lnCNaCl) in the range of 1 ~ 50 g L-1. This was consistent with Nernst equation.4

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The decreasing slope was highest for BiCl3 electrode, followed by CoCl2 and VCl3 electrodes. The

260

difference of BiCl3 electrode potentials between 1 g L-1 and 30 g L-1 NaCl was ~0.08 V. Although

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the VCl3 electrode had higher potential in 1 g L-1 NaCl compared to that of CoCl2, the potential

262

difference between 1 g L-1 and 30 g L-1 NaCl was only ~0.02 V, while the potential difference in 12 ACS Paragon Plus Environment

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1 g L-1 and 30 g L-1 NaCl solutions for CoCl2 electrode was ~0.03 V. Since the OCVs were 0.16

264

V for BiCl3, 0.11 V for CoCl2, and 0.09 V for VCl3 electrodes (Fig. 2), these results clearly

265

indicated that the cell voltage was result from electrode potential difference (~0.08 V for BiCl3,

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~0.03 V for CoCl2, and ~0.02 V for VCl3) and Donnan potential (~0.08 V).4 In addition, the

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highest electrode potential difference of BiCl3 electrodes in HC and LC solutions indicated that

268

the cell with BiCl3 electrodes had higher electromotive force for ions movement, thereby higher

269

power density.

270

In order to elaborate the Cl– extraction and insertion processes on metal chloride electrodes

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(e.g., BiCl3), XPS was used to analyze the Cl compositions (Fig. S4 in supporting information)

272

and Bi chemical states (Fig. S5 in supporting information) of the as-prepared electrode, and the

273

electrodes after discharged in HC (30 g L-1 NaCl) and LC (1 g L-1 NaCl) solutions. Compared to

274

the as-prepared BiCl3 electrode (17.75%), the ratio of Cl element increased to 19.98% after

275

oxidized in the HC solution while decreased to 15.21% after reduced in the LC (Fig. S4 in

276

supporting information). These results evidently indicated that Cl- insertion into metal chloride

277

electrodes would happen in the 30 g L-1 NaCl solution, while Cl- extraction occurred in the 1 g L-

278

1

279

showed two bands at 159.9 eV and 165.3 eV for the as-prepared BiCl3 electrode (Fig. S5a in

280

supporting information), attributed to the Bi 4f7/2 and Bi 4f5/2 peaks of Bi3+.15 After discharged in

281

1 g L-1 NaCl, another two bands at 163.1 and 168.8 eV related to metallic state Bi were observed

282

(Fig. S5b in supporting information),17 indicating that Bi3+ was reduced to Bi metal due to Cl–

283

release. The electrode after discharged in 30 g L-1 NaCl had the same Bi 4f bands as that of the as-

284

prepared BiCl3 (Fig. S5c in supporting information), suggesting that Bi metal was oxidized to

NaCl solution during the discharging processes. As for Bi chemical states, the Bi 4f spectrum



Page 14 of 29

285

BiCl3 again attributed to Cl– insertion. Based on the results and discusses above, the reaction on

286

BiCl3 electrodes could be presented as the following expression (Eq. (2)): BiCl3 + 3e– ↔ Bi + 3Cl-1

287

(2)

288

In sum, the work principle of the cell with metal chloride electrodes was similar to a chloride-

289

ion battery based on Cl- extraction and insertion (Fig. 1). However, the concentration flow cell was

290

charged by solutions with different salt concentrations not the electrical power. The salinity

291

gradient can produce a Donnan potential across the CEM and an electrode potential difference

292

between the two symmetrical electrodes. At the anode immersed in the HC solution, Cl- insertion

293

occurred and the redox couple (e.g., Bi metal) in the electrode was oxidized, thereby providing

294

electrons to the cathode. At the cathode immersed in the LC solution, the redox couple (e.g., BiCl3)

295

was reduced with Cl- released from the electrode. In the solution, Na+ ions moved from the HC to

296

the LC channels across the CEM driven by the Donnan potential. Through switching the HC and

297

LC flow paths, the cell can be recharged again after discharging. Thereby, there is continuous

298

power output in the consecutive cycles.

299 300

Outlook

301

This study demonstrated that concentration flow cells can also work well for SG energy

302

recovery based on Cl– extraction and insertion with metal chloride electrodes and the Donnan

303

potential across a CEM. The maximum power density of the cell with BiCl3 electrodes was 3.17

304

W m-2 when synthetic river water (1 g L-1 NaCl) and seawater (30 g L-1 NaCl) were used, which

305

was higher than those of RED (max. 2.9 W m-2),11 CapMix (max. 0.4 W m-2)13 and HEx (0.1 W

306

kg-hydrogel-1),14 and in the same order with PRO (calculated: 9.2 W m–2-membrane area).6,7

307

Because the battery electrodes have finite charging capacity and the voltage depends on the


Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


308

charged state of the electrodes, the peak power density of the concentration flow cells could be

309

transient. However, using flow electrodes could overcome this drawback and obtain continuous

310

power production while maintaining the high power density.4 The highest power density was

311

further improved (7.05 W m-2) when using brine (300 g L-1 NaCl) in combination with synthetic

312

river water (1 g L-1 NaCl), which was noteworthy due to previously reported challenges in

313

harvesting the high potential energy contained in brines relative to river water.3 For PRO, current

314

semipermeable membranes cannot withstand the high pressures resulted from large salinity

315

gradients. For RED, decreased perm-selectivity of ion-exchange membranes with higher salinity

316

differences caused large voltage losses. Demonstrated by this study as well as previous studies,4

317

battery electrodes can develop cell voltages using highly concentrated solutions and thus overcome

318

the limitations of using brines.

319

Although the power output of the concentration flow cell based on Cl– extraction and insertion

320

and the Donnan potential across a CEM was lower compared to that of the concentration flow cell

321

based on Na+ intercalation/deintercalation and the Donnan potential across an AEM4 possibly due

322

to the slower kinetics, it is still attractive. Chloride materials are widely available, high safety and

323

environmentally friendly.17 Here, we made the use of cheap metal chlorides (Bi, Co and V) as

324

electrodes, instead of precious silver metal electrodes which used as battery electrodes in

325

CapMix.25 In particular, BiCl3 has been demonstrated as a kind of relatively non-toxic cathode

326

material in chloride ion battery,15,17 and also has been used in synthetic green chemistry as eco-

327

friendly mild Lewis acid catalyst.26 Compared to Na+ intercalation/deintercalation electrodes (e.g.,

328

CuHCF), metal chlorides were more stable in acidic and basic solutions. There were many other

329

Cl– extraction and insertion electrode materials used in chloride-ion batteries, such as metal

330

oxychlorides,16,27,28 chloride ion-doped nanocomposite29 and chloride-doped polymer,30 which



331

could also work in concentration flow cells for efficient SG energy recovery. High ion conductive

332

and resilient CEM were also extensively available, which could be used in concentration flow

333

cells.31,32 Although the ion exchange membranes are expensive (around $100 per m2), their cost

334

could be reduced as new membrane materials and preparation methods are developed. Selective

335

electrodes with a coating of charged polyelectrolytes or polymers could also be used to obtain the

336

Donnan potential without ion exchange membranes.33,34 Moreover, the power output of the

337

concentration flow cell based on Cl– extraction and insertion and the Donnan potential across a

338

CEM can be further improved by modifying the configuration (e.g., reduce the electrode distance

339

with profiled membranes35 and adjust the geomtery of flow channels with better mass transfer9),

340

optimzing the operation conditions (e.g., SG ratios, flow rates, discharging rates), using more

341

active electrode materials and more conductive membranes.4

342 343

Conclusions

344

This study provides a new way for efficient SG energy recovery. The concentration flow cells

345

based on Cl– extraction and insertion work likely to chloride-ion batteries, with Cl- inserted into

346

the anode and Cl- released from the cathode during discharging processes, but SG energy was used

347

to charge the cell instead of electricity. The electrochemical characteristics of the electrodes were

348

essential for the power output of the system. The cell with BiCl3 electrodes produced the maximum

349

power densities of 3.17 W m-2, followed by CoCl2 (1.97 W m-2) and VCl3 (1.19 W m-2) electrodes,

350

attributed to easy and fast Cl- extraction and insertion into the BiCl3 electrodes resulting from

351

smaller charge transfer resistance and ionic diffusion resistance.

352 353

Acknowledgments


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


354

This study was financially supported by Louisiana State University (PG008101), Louisiana

355

Board of Regents (GR-00001674), and Louisiana Water Resources Research Institute (GR-

356

00001844). The XPS was conducted at the Shared Instrumentation Facility at Louisiana State

357

University.

358 359

Conflicts of interest

360

The authors declare no competing interest.

361 362

References

363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393

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455 456



457 458 459 460

Page 20 of 29

Table 1. The maximum instantaneous power density (Maximum Pins), average power density (Average P) and energy density of concentration flow cells with BiCl3, CoCl2 and VCl3 electrodes under optimum external resistances of 4 Ω (BiCl3), 5 Ω (CoCl2) and 5 Ω (VCl3). Maximum Pins (W m–2) 3.17 ± 0.19

Average P (W m–2) 0.82 ± 0.02

Energy density (J m-2) 10.65 ± 0.46

CoCl2 5

1.97 ± 0.14

0.47 ± 0.06

4.21 ± 0.16

VCl3

1.19 ± 0.02

0.28 ± 0.04

2.25 ± 0.11

BiCl3

External resistance (Ω) 4 5

461 462 463


Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

464 465 466


Fig. 1. The working principle of concentration flow cells with metal chloride (e.g., BiCl3) electrodes. HC: high concentration; LC: low concentration.

467 468



469 470

Fig. 2. Open circuit voltages (OCVs) of concentration flow cells with BiCl3, CoCl2 and VCl3 electrodes. HC (30 g L-1 NaCl) and LC (1 g L-1 NaCl) solutions were switched every 2 min.

471 0.2

Open circuit voltages (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

0.1 BiCl3 VCl3

-0.1

-0.2

472 473

CoCl2

0.0

0

2

4

6

8

10

12

Time (min)


14

16

Page 23 of 29

474 475

Fig. 3. The cell voltage and instantaneous power density (Pins.) of concentration flow cells with BiCl3 (a), CoCl2 (b) and VCl3 (c) electrodes at external resistance of 4 Ω.

476

1.5

0.00

0.0

-0.03

-1.5

-0.06

-3.0 0

477

Pins (W m-2)

0.03

1

2

Time (min)

b

0.06

0.6

0.00

0.0

-1

-0.03

-0.6

-2

-0.06

1

0.00

0

-0.03

0

1

1.2

0.03

0.03

-0.06

c

0.06

2

2

Time (min)

478 479 480 481 482


Pins (W m-2)

3.0

Pins (W m-2) Cell voltages (V)

a

0.06

Cell voltages (V)

Cell voltages (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.2 0

1

2

Time (min)

3


Fig. 4. Maximum instantaneous power density (a) and average power density (b) of concentration flow cells with BiCl3, CoCl2 and VCl3 electrodes at different external resistances.

BiCl3

3

CoCl2 VCl3

2 1

0

486 487 488 489

0.9

a Average P (W m-2)

483 484 485

Maximum Pins (W m-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

10

20

b

0.6

0.3

0

30

External resistance (Ω)

10

20

30

External resistance (Ω)


Page 25 of 29

2

15 10

1

5 10

20

30

40

Cycle number

50

6

1.6 1.2

4

0.8

2

0.4 0

10 20 30 40 50

Cycle number


c

1.2

4

Energy density (J m-2)

Energy density Average P

b

2.0

Power density (W m-2)

Maximum Pins

0

496 497

20


a 3


Fig. 5. The maximum instantaneous power density (Maximum Pins), average power density (Average P) and energy densities recorded for 50 cycles of concentration flow cells with BiCl3 (a), CoCl2 (b) and VCl3 (c) electrodes under optimum external resistances of 4 Ω (BiCl3), 5 Ω (CoCl2) and 5 Ω (VCl3).


490 491 492 493 494 495


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9

3

0.6

2

0.3

1 0

10 20 30 40 50

Cycle number


Fig. 6. The effect of the concentration of HC solutions (30, 100, 200 and 300 g L-1) on the maximum instantaneous power density (a) and average power density (b) of concentration flow cells with BiCl3, CoCl2 and VCl3 electrodes under optimum external resistances of 4 Ω (BiCl3), 5 Ω (CoCl2) and 5 Ω (VCl3).

8

VCl3

4 2

30

100

200

300

b

2.0

CoCl2

6

0 504 505 506 507

a

BiCl3

Average P (W m-2)

498 499 500 501 502 503

Maximum Pins (W m-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

1.5 1.0 0.5 0.0

-1

30

100

200

300

Concentration of HC (g L-1)

Concentration of HC (g L )


Page 27 of 29

508 509 510

Fig. 7. Cyclic voltammograms of (a) BiCl3, (b) CoCl2, and (c) VCl3 electrodes in 1 g L-1 NaCl and 30 g L-1 NaCl solutions. 20

Current (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

a

2

b

c

10 0 0

0 -3

1 g L-1 30 g L-1

-10

-2

-20 -1

0

-6

1

Potential (V vs. Ag/AgCl)

-0.8

0.0

0.8


511 512


-0.8

0.0

0.8



513 514 515

Fig. 8. Electrode potentials of BiCl3, CoCl2 and VCl3 electrodes in solutions with different NaCl concentrations. (The solid line is the result of linearization) 0.5

Electrode potential (V vs. AgCl)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

516 517 518

y = -0.0258x + 0.4677 (BiCl3)

0.4

0.3 y = -0.0065x + 0.3129 (VCl3)

0.2

0.1

y = -0.0118x + 0.1806 (CoCl2)

0

1

2

3

lnCNaCl(ln(g L-1))

519


4

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

520 521

522 523 524 525 526 527


Table of Contents

Synopsis: Concentration flow cells based on chloride-ion extraction and insertion with metal chloride electrodes for green, continuous and sustainable salinity gradient energy harvest.

528


Concentration Flow Cells Based on Chloride-Ion ... - ACS Publications

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