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Electrochimica Acta 322 (2019) 134724

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Electrochimica Acta

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Chloride-ion concentration flow cells for efficient salinity gradientenergy recovery with bismuth oxychloride electrodes

Guangcai Tan a, Sidan Lu a, Jizhou Fan b, Guoqiang Li b, Xiuping Zhu a, *

a Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA, 70803, USAb Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA, 70803, USA

a r t i c l e i n f o

Article history:Received 7 July 2019Accepted 18 August 2019Available online 20 August 2019

Keywords:Salinity gradient energyBismuth oxychlorideConcentration flow cellDonnan potentialCation-exchange membrane

* Corresponding author.E-mail address: xzhu@lsu.edu (X. Zhu).

https://doi.org/10.1016/j.electacta.2019.1347240013-4686/© 2019 Published by Elsevier Ltd.

a b s t r a c t

Salinity gradient (SG) energy is a large, renewable, and clean energy source, which naturally exists be-tween river water and seawater and can also be created by engineering processes. Here, bismuth oxy-chloride (BiOCl) electrodes was used for efficient SG energy recovery in a chloride-ion concentration flowcell (Cl-CFC) based on a new mechanism, that is, chloride ion (Cl�) intercalation/deintercalation.Compared to previous Cl-CFC with BiCl3 electrodes based on Cl

� extraction/insertion (3.17Wm�2), thepeak power density of the cell with BiOCl electrodes (4.36Wm�2) was higher under the same testingconditions when using synthetic river water (1 g L�1 NaCl) and seawater (30 g L�1 NaCl). Further in-vestigations demonstrated that the relatively higher power density was attribute to the faster kinetics ofCl� intercalation/deintercalation at BiOCl electrodes than that of Cl� extraction/insertion at BiCl3 elec-trodes. In addition, three BiOCl electrodes with different carbon black ratios (CB, 10%, 20% and 30%) werealso prepared and examined in this study. Compared to BiOCl/10%CB and BiOCl/30%CB electrodes, thelower charge transfer and ion diffusion resistances, larger ion storage capacities and higher cell opencircuit voltages of the BiOCl/20%CB electrodes were all beneficial to its higher power output.

© 2019 Published by Elsevier Ltd.

1. Introduction

As a kind of “blue energy”, salinity gradient (SG) energy can begained when two different salt solutions are mixed, making the useof the chemical potential difference [1]. Naturally, there isapproximately 2 kJ of free energy released when 1 L river waterpours into the sea, which is comparable to the energy released bythe same amount of water falling over dam with a height of 260m[2e4]. The global potential for energy production from SG energycould be up to 2.6 TW, which is even higher than the global elec-tricity consumption (2.0 TW) [5]. In addition, compared to theintermittent renewable energy like sun and wind, SG energy ismuch more available and predictable.

So far, several technologies have been developed for SG energyrecovery, including pressure-retarded osmosis (PRO), reverseelectrodialysis (RED), capacitive mixing (CapMix), and hydrogelexpansion (HEx). In PRO, electricity is produced by taking advan-tage of the pressure differences between two different salt solu-tions using salt-rejecting membranes [6,7]. In RED, electricity is

produced by Donnan potentials developed from the flow of ionsacross ion-exchange membranes [8]. The highest reported powerdensity to date are 7.5Wm�2 for PRO and 3.5Wm�2 for RED[6e10]. However, both PRO and RED are faced with the problem ofmembrane fouling. Although membranes are not required forCapMix that produces electricity by making the use of saltconcentration-dependent electrode potentials [11e13], and HExthat recovers SG energy by extracting work done in the processionof the expansion and contraction of hydrogel particles, their powerdensities are relatively low (highest 0.4Wm�2) [14].

Recently, concentration flow cells were introduced as a newtechnology for efficient SG energy recovery based on both theDonnan potential and the electrode potentials [15]. A concentrationflow cell (CFC) consisted of two identical electrodes (e.g., copperhexacyanoferrate, CuHCF) with an anion-exchange membrane(AEM) between the electrodes to form two channels which wereseparately fed with synthetic seawater and freshwater. Therefore,the cell can take advantage of the Donnan potential across the AEMas well as the electrode potential resulted from sodium-ion inter-calation and deintercalation, called sodium-ion concentration flowcells (Na-CFC) [12,15,16]. Later, our studies demonstrated that theelectrode potential of concentration flow cells can be also derived

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G. Tan et al. / Electrochimica Acta 322 (2019) 1347242

from capacitive double layer expansion (CDLE) using carbonizedpeat moss (CPM) electrodes, called capacitive concentration flowcells (Cap-CFC) [17]. The concentration flow cell can also workbased on chloride-ion (Cl�) extraction/insertion with metal chlo-ride (e.g., BiCl3) electrodes and the Donnan potential across acation-exchange membrane (CEM), called chloride-ion concentra-tion flow cells (Cl-CFC) [18]. All the three kinds of concentrationflow cells can produce electricity continuously through rechargingthe cell by switching the solution paths periodically, and thehighest power density was reported to be 12.6Wm�2 [15].

Here, a new type of CleCFCs was developed with BiOCl elec-trodes, which worked based on Cl� intercalation/deintercalation(Fig. 1). Compared to previous CleCFCs with BiCl3 electrodes basedon Cl� extraction/insertion (3.17Wm�2), higher peak power output(4.36Wm�2) was obtained under the same testing conditions us-ing synthetic river water (1 g L�1 NaCl) and seawater (30 g L�1

NaCl). Although the peak power density was relatively lower thanthose of NaeCFCs and Cap-CFCs, the low cost and high stability ofBiOCl electrodes make it still practically attractive.

2. Materials and methods

2.1. Preparation of BiOCl electrodes

Bismuth oxychloride (BiOCl, 98%) was bought from Alfa Aesar.The different weight ratios of BiOCl to carbon black (conductivecarbon, 99.5%, Alfa Aesar) were prepared as 9:1 (10% CB), 8:2 (20%CB) and 7:3 (30% CB). The BiOCl powder and carbon black weremixed using ball-milling (PQ-N04, USA) for 24 h at a rotation speedof 400 rpm. To prepare electrodes, 90mg as-prepared active pow-der, and 10mg polyvinylidene fluoride (PVDF, Alfa Aesar) as binderwere mixed with 0.5mL dimethylformamide (Alfa Aesar, 99%) andground by hand. A slurry containing this mixture was painted ontoboth sides of the carbon paper (3 cm� 3 cm) with aworking area ofca. 3 cm2 (3 cm� 1 cm), and followed by drying under vacuum at70 �C overnight. Themass loading of the as-prepared active powderwas about 4.8mg cm�2. The electrodes with different weight ratiosof BiOCl to carbon blackwere designated as BiOCl/10%CB, BiOCl/20%CB and BiOCl/30%CB.

Fig. 1. The working principle of concentration flow cells with BiOCl/2

2.2. Characterizations of BiOCl electrodes

Cyclic voltammetry (CV), electrochemical impedance spectros-copy (EIS), galvanostatic charge/discharge, and electrode potentialsof BiOCl electrodes were measured by a potentiostat (VMP3, Bio-Logic, French) in a single-chamber three-electrodes cell. A plat-inum wire was used as the counter electrode with an Ag/AgClreference electrode. CVs were performed in the potentialrange �0.8 to 0.9 V under a scan rate of 5mV s�1. EIS was carriedout at the open-circuit potential (OCP) in the frequency range of10�2-105 Hz with a 10mV peak-to-peak sinusoidal potentialperturbation. The results were reported as Nyquist plots. Galva-nostatic charge/discharge was measure in the potential range of0e1.0 V at a current density of 1.6 A g�1 (based on the active mass ofthe electrode). The discharge capacity was calculated according toC ¼ IDtDV m, where I and Dt are the discharge current and time, DV isthe potential change, and m is the active mass of the electrode.Electrode potentials in different NaCl solutions (1e50 g L�1) weredetermined when the values were stabilized.

X-ray diffractometer (Empyrean, Malvern Panalytical, UK) withCu Ka radiationwas used for X-ray diffraction (XRD) analysis in a 2qrange from 5� to 90�. X-ray photoelectron spectroscopy (XPS) wasconducted on an AXIS165 spectrometer (ESCA 2SR, Scienta Omi-cron, US) using a twin-anode Al Ka radiation as the X-ray source.

2.3. Configuration and performance tests of concentration flow cells

BiOCl electrodes are tested in a home-made concentration flowcell (Fig. S1), which had two identical channels(3 cm� 1 cm� 127 mm) separated by a CEM (5� 5 cm, SelemionASV, Japan). The two flow channels were constructed by two127 mm-thick gaskets with a window of 3 cm� 1 cm, which can beseparately fed with synthetic river water and seawater. The BiOClelectrodes were encircled by gaskets with a 3 cm� 3 cm windowand placed on each channel. Platinum wires were used as currentleads. Then, two end plates (5 cm� 5 cm� 3mm) with each silicongaskets inside (5 cm� 5 cm� 508 mm) were added on the ends ofthe cell and sealed firmly using rods and nuts. It should be notedthat one BiOCl electrode was pretreated for Cl� deintercalation byapplying a potential of �0.5 V for 20min in a three-electrode

0%CB electrodes. HC: high concentration; LC: low concentration.

G. Tan et al. / Electrochimica Acta 322 (2019) 134724 3

electrochemical cell (a platinum wire was used as the counterelectrode with an Ag/AgCl reference electrode in 1M NaCl), and theother BiOCl electrode was used as-prepared.

Low concentration (LC; 1 g L�1, representative of river water)and high concentration (HC; 30 g L�1, representative of seawater)NaCl solutions were employed for most of the tests. A peristalticpump (Cole-Parmer, USA) was used to simultaneously feed the twosolutions (15mLmin�1) to each channel of the cell. Open circuitvoltages (OCVs) of the cells were measured by a potentiostatwithout loading an external resistor. When one flow channel wasfed with LC and the other flow channel was fed with HC, the cellvoltage was generated due to the different potentials of electrodesin HC and LC solutions and the Donnan potential across the CEM.After 2min, the LC and HC solutions were switched. As a result, thecell voltage was reversed.

When an external resistor was connected between the elec-trodes, current was generated and power productionwas obtained.The power output of the cell was tested by loading varied externalresistors (Rext, 3e50U) between the two electrodes with cell volt-ages (U, in V) recorded by a potentiostat. When the cell voltagesdecreased below the cutoff voltage (±5mV), the paths of HC and LCsolutions were switched and a new cycle started. The HC and LCsolutions were switched at least four times at each external resistorand the mean value of the four cycles was reported. The instanta-neous power density (Pins.¼U2/RexAt, inWm�2) produced from thecell was calculated from the recorded cell voltage and the Rext, andA was the working area (~3 cm2) of the electrode. The average po-wer density (Pave: ¼

R tcycle0 Pins:dt=tcycle; tcycle is the cycle time, in W

m�2) was calculated by averaging the instantaneous power densityover one cycle. The energy density (Ed ¼

R tcycle0 Pins:dt, in J m

�2) wasobtained by integrating the instantaneous power density over onecycle. In addition to investigate the performance of the cell torecover SG energy from brine and freshwater, different HC(30e300 g L�1) NaCl solutions were employed with the LC solutionfixed at 1.0 g L�1.

Optimum external resistors for cells were determined based onthe peak instantaneous power density. To test the stability of thissystem, the cell with BiOCl/20%CB electrodes was continuously run100 cycles at optimum external resistor (4U) with the pH of theoutflow monitored in the whole 100 cycles.

3. Results and discussion

3.1. Electrochemical characterizations of BiOCl electrodes

CV tests of BiOCl/10%CB, BiOCl/20%CB and BiOCl/30%CB

Fig. 2. Cyclic voltammograms of (a) BiOCl/10%CB, (b) BiOCl/20%CB, and (c) BiOCl/30%C

electrodes were conducted in 30 g L�1 and 1 g L�1 NaCl solutions(Fig. 2). Compared to the CVs in the 1 g L�1 NaCl solution, a pair ofobvious oxidation and reduction peaks was observed on the CVs ofall BiOCl electrodes in the 30 g L�1 NaCl solution, suggesting theexistence of a redox reaction on them. However, Zhao et al.demonstrated that there were two pairs of redox peaks repre-senting two kinds of reversible reactions where BiOCl used as anefficient cathode in chloride-ion batteries: i) a major conversionreaction that BiOCl transformed to Bi metal and Bi2O3, and ii) aminor intercalation process that BiOCl changed to BiOCl1-x (x> 0)due to Cl� intercalation [19]. The only one redox peak observed inpresent study suggested that only intercalation/deintercalationprocesses occurred on BiOCl electrodes in concentration flow cellswhich possibly due to the lower operation potentials and the use ofaqueous solutions, which was further confirmed by XRD and XPSanalysis below.

For BiOCl/10%CB electrodes, a small pair of redox peaks wereobserved at �0.01 V (peak current density¼ 0.21 A g�1)and �0.39 V (peak current density¼�0.51 A g�1), indicating slowCl� intercalation and deintercalation. For BiOCl/20%CB electrodes, abig pair of redox peaks were observed at �0.03 V (peak currentdensity¼ 0.49 A g�1) and �0.29 V (peak current den-sity¼�1.02 A g�1), suggesting fast Cl� intercalation and dein-tercalation. For BiOCl/30%CB electrodes, a big oxidation peak wasobserved at 0.01 V (peak current density¼ 0.81 A g�1) probably dueto fast Cl� intercalation. However, only a small reduction peak wasobserved at �0.4 V (peak current density¼�0.79 A g�1), indicatingslow Cl� deintercalation. The smallest potential difference betweenthe oxidation and reduction peaks for BiOCl/20%CB (0.24 V)compared to 0.38 V for BiOCl/10%CB and 0.41 V for BiOCl/30%CB,also indicated that the kinetics of Cl� intercalation/deintercalationon BiOCl/20%CB electrodes was the fastest.

The EIS of all BiOCl electrodes were tested in 30 g L�1 and 1 g L�1

NaCl solutions to examine the resistances (Fig. 3). The chargetransfer resistance (Rct) was indicated by the diameter of thesemicircle in the range of high frequencies [20]. Obviously, BiOCl/10%CB electrodes had a large Rct value (~35U in HC, ~75U in LC),while those of BiOCl/20%CB and BiOCl/30%CB electrodes weresimilar and much smaller (~10U in HC, ~25U in LC) (Fig. 3a and c).The ionic diffusion resistance (s) was represented by the tail of theNyquist plots. The value of s can be obtained from the slope of thelinear fitting line of the real part of impedance versus the reciprocalof the square root of frequency (u�0.5) in the low to intermediatefrequency range [21]. At the NaCl concentration of 30 g L�1, theBiOCl/10%CB (3.4U s0.5), BiOCl/20%CB (3.7U s0.5) and BiOCl/30%CB(3.8U s0.5) electrodes showed the similar small ionic diffusion

B electrodes in 30 g L�1 NaCl (solid line) and 1 g L�1 NaCl (dotted line) solutions.

Fig. 3. Nyquist plot for the impedance response of BiOCl/10%CB, BiOCl/20%CB and BiOCl/30%CB electrodes at the NaCl concentration of (a) 30 g L�1 and (c) 1 g L�1 (Inset shows aclose-up of near origin section of the figure). Z0 vs. the reciprocal of the square root of frequency (u�0.5) in the intermediate to low frequency range (0.01e0.2 Hz) at the NaClconcentration of (b) 30 g L�1 and (d) 1 g L�1. (e) representative galvanostatic charge/discharge profiles of BiOCl/10%CB, BiOCl/20%CB and BiOCl/30%CB electrodes in 30 g L�1 NaClsolutions, and BiOCl/20%CB electrode in 300 g L�1 NaCl solutions. (f) Electrode potentials of BiOCl/10%CB, BiOCl/20%CB and BiOCl/30%CB electrodes in solutions with different NaClconcentrations. (The solid line is the result of linearization).

G. Tan et al. / Electrochimica Acta 322 (2019) 1347244

resistance (Fig. 3b). While, at the NaCl concentration of 1 g L�1, thes value for BiOCl/10%CB electrodes increased to 14.1U s0.5 whichwas more than twice higher than those of BiOCl/20%CB (5.5U s0.5)and BiOCl/30%CB (5.2U s0.5) electrodes (Fig. 3d). In all, the EIS re-sults demonstrated that BiOCl/10%CB electrodes had much largercharge transfer and Cl� diffusion resistances than those of BiOCl/20%CB and BiOCl/30%CB electrodes.

The galvanostatic charge/discharge of BiOCl electrodes wereperformed at a current density of 1.6 A g�1 in 30 g L�1 NaCl to obtain

the ion storage capacity (Fig. 3e). The charge time of BiOCl/20%CBand BiOCl/30%CB electrodes were much longer than that of BiOCl/10%CB electrode, indicating larger ion storage capacities. Accord-ingly, the BiOCl/20%CB and BiOCl/30%CB electrodes possessedmuch larger discharge capacity (~23 mAh g�1) than that of BiOCl/10%CB electrode (7 mAh g�1). When the solution concentrationincreased to 300 g/L, the discharging capacity of BiOCl/20%CBelectrode was further improved to 36 mAh g�1. In addition, thesudden voltage drop (iR) at the beginning of discharging can

G. Tan et al. / Electrochimica Acta 322 (2019) 134724 5

represent the ohmic resistance of ions movement in the electrode[22,23]. The large iR drop for BiOCl/10%CB electrode indicated ahigh inner resistance, which could be due to the low conductivity ofBiOCl/10%CB electrode with a small ratio of carbon black.

In solutions with different salt concentrations, the Cl� interca-lation/deintercalation processes were varied. The electrode poten-tials of all BiOCl electrodes decreased linearly with the natural logof NaCl solution concentrations (lnCNaCl) ranged from 1 to 50 g L�1

(Fig. 3f), which was in accord with the Nernst equation [15,18]. Thedecreasing slope was the highest for the BiOCl/10%CB electrode,followed by BiOCl/20%CB and BiOCl/30%CB electrodes, suggestingthat higher amount of active compound (BiOCl) resulted in higherelectrode potential differences probably due to more chargedspecies transferred across the interface between the electrode andelectrolyte. As a result, the differences of electrode potentials in30 g L�1 and 1 g L�1 NaCl solutions were ~0.06 V for BiOCl/10%CBelectrodes, ~0.05 V for BiOCl/20%CB electrodes, and ~0.04 V forBiOCl/30%CB electrodes.

3.2. Performance of concentration flow cells with BiOCl electrodes

The BiOCl electrodes were then tested in concentration flowcells. The OCVs of the cells were obtained by switching LC and HCflow paths every 2min (Fig. 4). The cell with BiOCl/10%CB elec-trodes had the highest OCV of ca. ± 0.14 V, followed by those withBiOCl/20%CB (ca.± 0.13 V) and BiOCl/30%CB (ca.± 0.12 V) elec-trodes. The OCVs decreased concurrently with the ratio of BiOCl,which was consistent with the electrode potential difference invaried NaCl solutions that the largest difference was observed forBiOCl/10%CB electrodes (Fig. 3f). It is indicated that the amount ofactive compound (BiOCl) affected the electrode potentials indifferent salt solutions and consequently the OCVs.

Similar to the previous studies [17,18], the instantaneous powerdensity and the cell voltage firstly increased and then graduallydecreased in one cycle. In the following cycle, the power was pro-duced again with cell voltages reversed after switching the flowpaths of LC and HC solutions (Fig. 5). Although the cell with BiOCl/10%CB electrodes had the highest OCV, the power output of the cellwith BiOCl/20%CB electrodes was the highest (Figs. 4 and 6).Different external resistances (3e50U) were connected betweenthe two electrodes to obtain the maximum power output pointwhere the external resistance was equal to the internal resistanceof the cell (Fig. 6). The cell with BiOCl/20%CB electrodes had the

Fig. 4. Open circuit voltages (OCVs) of concentration flow cells with BiOCl/10%CB (solidline), BiOCl/20%CB (dashed line) and BiOCl/30%CB (dotted line) electrodes. HC (30 g L�1

NaCl) and LC (1 g L�1 NaCl) solutions were switched every 2min.

highest peak power density of 4.36± 0.14Wm�2 under a 4Uexternal resistance, which was three times of that with BiOCl/10%CB electrodes (1.38± 0.11W m�2 under a 10U external resistance)and 43% higher than that with BiOCl/30%CB electrodes(3.05± 0.09Wm�2 under a 4U external resistance) (Fig. 6a).Similarly, the cell with BiOCl/20%CB electrodes had the highestaverage power density of 1.23± 0.12Wm�2, followed by BiOCl/30%CB electrodes (0.81± 0.05Wm�2) and BiOCl/10%CB electrodes(0.27± 0.04Wm�2) (Fig. 6b). In addition, the cell with BiOCl/20%CBelectrodes possessed the highest energy density(15.98± 0.34 Jm�2), which was more than three times of that withBiOCl/10%CB electrodes (4.26± 0.11 Jm�2) and 55% higher than thatwith BiOCl/30%CB electrodes (10.31± 0.26 Jm�2).

The lowest power output of the cell with BiOCl/10%CB elec-trodes could be due to the large internal resistance (~10U), indi-cating that the internal resistance had a significant role on thepower production. Future studies should play more attention todecrease the internal resistance of the concentration flow cell,especially when the system is in a pilot scale. The reason for thehighest power output and energy density of BiOCl/20%CB elec-trodes should be attributed to the fast Cl� intercalation/dein-tercalation processes, the low charge transfer and Cl� diffusionresistances, and the high ion storage capacities as demonstrated byabove CV, EIS, and galvanostatic charge/discharge tests. Althoughthe charge transfer and ion diffusion resistances, and ion storagecapacities of BiOCl/30%CB electrodes were similar to those of BiOCl/20%CB electrodes, the power output of the cell with BiOCl/20%CBelectrodes was higher than that with BiOCl/30%CB electrodes. Thereason could be due to the relatively faster kinetics of Cl� interca-lation/deintercalation on BiOCl/20%CB electrodes (Fig. 2) and thehigher OCV for the cell with BiOCl/20%CB electrodes (Fig. 4).

In order to investigate the possibility of the concentration flowcell with BiOCl electrodes for SG energy recovery from highly salinewaters, the concentration of HC solutions increased from 30 to300 g L�1 NaCl with LC solutions fixed at 1 g L�1 NaCl. The peak andaverage power densities rose concurrently with the increasingconcentrations of HC solutions for all electrodes (Fig. 7), suggestingthat there was a high potential to use the concentration flow cellwith BiOCl electrodes for SG energy recovery from brines, such aswaste brine from production operations (epichlorohydrin synthesisand chloralkali processes) and reject flows from desalination plants[3,15,18]. When the HC concentration solution was 300 g L�1 NaCl,the cell with BiOCl/20%CB electrodes had the highest peak powerdensity (9.79± 0.43Wm�2) and average power output(1.94± 0.21Wm�2), which increased by 124% (peak P) and 58%(average P) compared to those with a HC solution of 30 g L�1 NaCl.The increases could be attributed to the relative higher gainedvoltage under 300 g L�1 NaCl (Fig. S2) [15]. However, compared tothe ten times increase in HC concentrations from 30 g L�1 to300 g L�1 NaCl, the increase in the power output from 4.36Wm�2

to 9.79Wm�2 was less than two times. One reason could be due tothe limitation in the total discharge capacity of BiOCl/20%CB elec-trode which only increased about 1.6 times in 300 g L�1 NaClcompared to that in 30 g L�1 NaCl (Fig. 3e). Another reason could beascribed to the increased “uncontrolled mixing” which resultedfrom imperfect permselectivity of the membrane and the remainedsolution within the pores of electrode when switching the HC andLC between cycles [15].

To further examine the stability of this system, the cell withBiOCl/20%CB electrodes were run at the optimum external resis-tance of 4U for 100 cycles with 1 g L�1 LC and 30 g L�1 HC solutions(Fig. 8). The peak power density of the cell remained around4.36Wm�2, the average power density stayed at ca. 1.23Wm�2,and the energy density was stable at ca. 15.98 Jm�2. Moreover, thepH of the outflow was monitored over 100 cycles, which only

Fig. 5. The cell voltage and instantaneous power density (Pins.) of concentration flow cells with BiOCl/10%CB (a), BiOCl/20%CB (b) and BiOCl/30%CB (c) electrodes at an externalresistance of 10U fed with 1 g L�1 NaCl (LC) and 30 g L�1 NaCl (HC) solutions.

Fig. 6. Peak power output (a) and average power output (b) of concentration flow cells with BiOCl/10%CB, BiOCl/20%CB and BiOCl/30%CB electrodes at different external resistancesfed with 1 g L�1 NaCl (LC) and 30 g L�1 NaCl (HC) solutions.

Fig. 7. The effect of the concentration of HC solutions (30, 100, 200 and 300 g L�1) on the peak power output (a) and average power output (b) of concentration flow cells with BiOCl/10%CB, BiOCl/20%CB and BiOCl/30%CB electrodes under optimum external resistances of 10U (BiOCl/10%CB), 4U (BiOCl/20%CB) and 4U (BiOCl/30%CB). The LC solution was fixed at1 g L�1.

G. Tan et al. / Electrochimica Acta 322 (2019) 1347246

Fig. 8. The peak power density (Peak P), average power density (Average P) and energydensities recorded for 100 cycles of concentration flow cells with BiOCl/20%CB elec-trode under optimum external resistances of 4U fed with 1 g L�1 NaCl (LC) and 30 g L�1

NaCl (HC) solutions.

G. Tan et al. / Electrochimica Acta 322 (2019) 134724 7

slightly changed from 6.10 to 5.95 in the whole process with noobvious fluctuations (Fig. S3). These results demonstrated that thecell with BiOCl electrodes was very stable for SG energy recovery.

3.3. Working mechanisms of the concentration flow cell with BiOClelectrodes

According to previous research on chlorine-ion batteries [19],there are two possible reversible reactions on BiOCl electrodes. Oneis the conversion reaction (BiOCl transformed to Bi metal andBi2O3) and the other is the intercalation process (BiOCl changed toBiOCl1-x (x> 0)). The CV results indicated that there was only onereaction occurred on BiOCl electrodes in concentration flow cells. Inorder to distinguish this reaction, the crystal structures andchemical compositions of origin BiOCl/20%CB electrode and theelectrodes after discharging in 1 g L�1 and 30 g L�1 NaCl solutionswere analyzed by XRD (Fig. S4) and XPS (Figs. S5 and S6). Comparedto the origin electrode, the crystal structure of BiOCl electrodesafter discharging in LC and HC had no changes (Fig. S4), suggestingthat BiOCl kept its crystal structure without phase transformation[19]. The stable crystal structures of BiOCl after discharging alsodemonstrated the chemical stability and thereby the high cyclingstability of the cell. Moreover, only two bands corresponding to Bi3þ

(159.9 and 165.3 eV) were observed in XPS for the original BiOClelectrode as well as the electrode after discharging in LC and HCsolutions (Fig. S5), further demonstrating that BiOCl was nottransformed to Bi metal or Bi2O3 (no conversion reaction). Instead,the ratio of chloride element decreased to 5.47% after discharging inthe LC solution and increased to 7.25% after discharging in the HCsolution in comparison with the original BiOCl electrode (6.31%)(Fig. S6). All of these results indicated that only Cl� intercalation/deintercalation occurred on BiOCl electrodes in concentration flowcells (BiOCl þ x ee 4 BiOCl1-x þ x Cl�1). In the HC solution, theBiOCl electrode captured Cl� ions (intercalation) and lost electrons,while Cl� ions were released from the BiOCl electrode (dein-tercalation) with electrons received by the electrode in the LC so-lution (Fig. 1).

Except for the electrode potential difference resulted from Cl�

intercalation/deintercalation in HC and LC solutions, the Donnanpotential across the AEM due to Naþ transport was another

important contributor to the cell voltage of the concentration flowcell [15,18]. According to equation (1) [15], the cells with differentBiOCl electrodes should have the same Donnan potential (~0.08 V)across the AEM. Therefore, the voltage of the cells with BiOCl/10%CB(0.14 V), BiOCl/20%CB (0.13 V) and BiOCl/30%CB (0.12 V) electrodes(Fig. 4) should consist of the Donnan potential (~0.08 V) and theelectrode potential difference in HC and LC solutions (~0.06 V forBiOCl/10%CB, ~0.05 V for BiOCl/20%CB, and ~0.04 V for BiOCl/30%CB,Fig. 3f).

DE ¼ RTnF

ln�aNaþ; HCaNaþ; LC

�(1)

where T is absolute temperature (298 K in the present study), R isthe gas constant (8.314 Jmol�1 K�1), n is the electron number(n¼ 1), a is the activity of sodium ion, and F is the Faraday constant(96 485 Cmol�1).

3.4. Outlook

The peak power density of the Cl-CFC with BiOCl/20%CB elec-trodes reached 4.36Wm�2, which was higher than our previousstudy on CleCFCs with BiCl3/20%CB (3.17Wm�2) electrodes underthe same experimental conditions when using synthetic river wa-ter (1 g L�1 NaCl) and seawater (30 g L�1 NaCl) [18]. The higherpower output should be attributed to the faster kinetics of Cl�

intercalation/deintercalation on BiOCl electrodes with a layeredstructure [24] compared to that of Cl� extraction/insertion on BiCl3electrodes with a pyramidal structure [18,25]. This was supportedby the CV test results: the potential difference between theoxidation and reduction peaks of the BiOCl/20% CB electrode(0.24 V) was much smaller than that of the BiCl3/20% CB electrode(1.35 V) [18]. Moreover, the conversion reaction (BiCl3 þ3e 4Bi þ 3Cl-) on BiCl3 electrodes due to Cl� extraction/insertion couldcause more than two times of volume changes, which may reducethe electrical contact between the carbon black and the BiCl3 par-ticles and also the stability [26]. On the contrary, the volume changedue to the Cl� intercalation/deintercalation into the layer structureof BiOCl was minor [19], indicated by the similar XRD after dis-charging in the cell. As a result, the cell with BiOCl electrodes hadhigher power output and was more stable than that with BiCl3electrodes.

Compared to NaeCFCs and Cap-CFCs, the peak power density ofthe CleCFCs was relatively lower. A maximum power density of12.6Wm�2 was obtained by the Na-CFC with CuHCF electrodes[15], probably due to the fast kinetics of Naþ intercalation/dein-tercalation as the Shannon radius of Naþ (1.02 Å) is much smallerthan that of Cl� (1.81 Å) [15,16,27]. However, the BiOCl used in thisstudywas an industrial product with a commercial price of $ 25e50per kilogram, while the CuHCF was self-synthesized in the lab witha possible price of $ 45e70 per kilogram calculated based onconsumed chemicals to make it [15,16]. Moreover, the BiOCl wasinsoluble while the CuHCF was soluble in water [28e30], suggest-ing the usage of CuHCF electrodes may cause environmentalpollution. Therefore, BiOCl electrodes were considered to be moreattractive on practical application in terms of the cost, stability, andenvironmental impacts. Although the peak power density of the Cl-CFC with BiOCl/20%CB electrodes (4.36Wm�2) was a little lowerthan that of the Cap-CFC with carbonized peat moss electrodes(5.33Wm�2), the average power density (1.23Wm�2) was nearly30% higher than that of the Cap-CFC (0.95Wm�2) [17]. The higheraverage power density could be attributed to the relatively fasterkinetics of Cl� intercalation/deintercalation than that of CDLE in theCap-CFC. As less time is needed for Cl� intercalation/dein-tercalation, the higher average power density was obtained. In

G. Tan et al. / Electrochimica Acta 322 (2019) 1347248

addition, the electrode potential of Cap-CFCs was derived fromCDLE, which may suffer from charge leakage in long term running[31].

4. Conclusions

The Cl-CFC with BiOCl/20%CB electrodes worked well for SGenergy harvest based on Cl� intercalation/deintercalation.Although the cell with BiOCl/10%CB electrodes had the highestOCV, the largest power density was obtained for the cell with BiOCl/20%CB electrodes (4.36Wm�2), followed by those with BiOCl/30%CB (3.05Wm�2) and BiOCl/10%CB (1.38Wm�2) electrodes. Thehigh power output of the cell with BiOCl/20%CB electrodes wasmainly attributed to the fast Cl� intercalation/deintercalation pro-cesses, the low charge transfer and ion diffusion resistances, andthe high ion storage capacities.

Conflicts of interest

No potential conflict of interest was reported by the authors.

Acknowledgments

The study was supported by Louisiana Water ResourcesResearch Institute (GR-00001844) and Louisiana Board of Regents(GR-00001674). The XPS and XRD were conducted at the SharedInstrumentation Facility (SIF) at Louisiana State University.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.electacta.2019.134724.

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Chloride-ion concentration flow cells for efficient salinity gradient energy recovery with bismuth oxychloride electrodes1. Introduction2. Materials and methods2.1. Preparation of BiOCl electrodes2.2. Characterizations of BiOCl electrodes2.3. Configuration and performance tests of concentration flow cells

3. Results and discussion3.1. Electrochemical characterizations of BiOCl electrodes3.2. Performance of concentration flow cells with BiOCl electrodes3.3. Working mechanisms of the concentration flow cell with BiOCl electrodes3.4. Outlook

4. ConclusionsConflicts of interestAcknowledgmentsAppendix A. Supplementary dataReferences