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Influence of flowrates to a reverse electro-dialysis (RED) stack on performance and electrochemistry of a microbial reverse electrodialysis cell (MRC)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 7 6 8 5 e2 7 6 9 2
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Influence of flowrates to a reverse electro-dialysis (RED) stack on performance and electrochemistry of a microbial reverse electrodialysis cell (MRC) Heunggu Kang a,b, Eojin Kim a, Sokhee P. Jung a,* a
Department of Environment and Energy Engineering, Chonnam National University, Gwangju 61186, Republic of Korea b Green Energy Institute, Naju 58217, Republic of Korea
article info
abstract
Article history:
An MRC is a bioelectrochemical system combining a microbial fuel cell (MFC) with a RED
Received 31 March 2017
stack to generate electricity from salinity gradient and organic wastewater with simulta-
Received in revised form
neous treatment. Operating an MRC at an optimum flowrate to RED is important because it
21 June 2017
is closely related with energy production rate and economic feasibility. However, influence
Accepted 22 June 2017
of RED flowrates on MRC electrochemistry and power production have not been investi-
Available online 23 July 2017
gated. For this purpose, four different flowrates of high concentration and low concentration solutions were tested. Maximum power density was highest in 10 mL/min (3.71 W/
Keywords:
m2) and optimum current density was highest in 7.5 mL/min (5.36 A/m2). By mere
Microbial reverse electrodialysis cell
increasing the flowrate to MRC, maximum power and optimum current densities increased
Reverse electrodialysis stack
by 17.7% and 16.2%. EIS showed that impedances of anode, cathode and full-cell were
RED flowrate
decreased by 51%, 31% and 19%, respectively. Anode CV showed that peak current density
Electrochemistry
was increased by 25.7%. COD removal and CE were not affected by RED flowrate. Power
Electrochemical impedance
generation at 7.5 mL/min and 10 mL/min were not so different, but current production was
spectroscopy
better at 7.5 mL/min. Therefore, considering energy production, the RED flowrate of 7.5 mL/ min is a reasonable choice for MRC operation. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Salinity gradient energy between sea and fresh water is a renewable energy, and the amount of energy that can be captured from salinity gradients in river water and sea water around the world is estimated at 1.9e2.6 TW [1,2]. There are various technologies such as reverse electrodialysis (RED) and pressure retarded osmosis (PRO) that produce electricity from salinity gradient. Reverse electrodialysis (RED) is a membranebased technology that can convert chemical potentials
directly into electricity through ion transport [3]. The RED stack consists of alternately stacked anion exchange membrane (AEM) and cation exchange membrane (CEM) separated by high concentration (HC) and low concentration (LC) cell. When HC and LC solutions are supplied into the RED stack, due to the concentration gradient, the cations in the HC solution migrate through the cation exchange membrane and the anions migrate through the anion exchange membrane to the LC solution. As a result, the potential difference across the membrane is converted to electrical current through the redox reactions of the electrodes (e.g., ~0.1e0.2 V per cell pair [4,5]).
* Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (S.P. Jung). http://dx.doi.org/10.1016/j.ijhydene.2017.06.187 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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A microbial reverse electrodialysis cell (MRC) is a microbial electrochemical system that combines an MFC with a reverse electrodialysis (RED) stack to generate electricity from the salinity gradient of seawater and fresh water, and treat wastewater simultaneously with electricity generation. In addition, power production can be greatly enhanced in an MRC [6]. There have been several previous studies that tested and characterized MRC systems by applying different RED flowrates, salinity ratios of HC and LC solutions, ion-exchange modification and thermolytic solutions [6e8]. Specifically, Roland et al. (2013) analyzed impedance distribution of an MRC with two different cell pairs (1 and 2) in a RED stack, in which raw domestic wastewater, buffered wastewater and buffered laboratory medium were tested as substrates [9]. Kim and Logan (2011) analyzed performance of an MRC to test effect of salinity ratio of RED inflow and effect of RED flowrate (0.85 and 1.55 mL/min) without EIS analysis [6]. Previous studies tested relatively lower ranges of RED flowrates and only two different flowrates were tested. However, in this study, we tested higher RED flowrates (2.5, 5.0, 7.5 and 10.0 mL/min) to produce more power from MRC, to find the optimum RED flowrate among them and to understand MRC electrochemistry in high RED flowrates.
Materials and methods MRC construction and operation The MRC reactor consisted of an anode chamber, cathode chamber and reverse electrodialysis (RED) stack (Fig. 1). Two chamber MRCs were made of polycarbonate cubic blocks. Anode chamber (45 mL) was 4 cm long, 3 cm diameter and cathode chamber (18 mL) was 2 cm long, 3 cm diameter [8]. A brush anode was made from carbon fibers (25 mm diameter 25 mm length; fiber type: T700, TORAYCA) wound into two twisted titanium wires (length: 7 cm; 17 gauge; #2 grade; Seoul Titanium). The anode was heat treated at 450 C for 30 min in a furnace (FX/FHX, Daihan Scientific, Wonju,
South Korea) before using. The inoculated brush anode was operated for over six months in a temperature room at 30 C at 1000-U external resistance [10,11]. The cathode was made of stainless steel mesh (#60 mesh, type 304, projected area of 7 cm2) spread with the catalytic mixture (300 mg of activated carbon powder, 30 mg of carbon black and 1 mL of 10% PVDF solution) [12e15]. To measure the potential of the anode and cathode electrode and the voltage of the red stack, Ag/AgCl reference electrodes (RE-1S, Qrins, Seoul, South Korea; 0.209 V versus a standard hydrogen electrode, SHE) were inserted into the anode chamber and the cathode chamber [16]. The anolyte consisted of 1.0 g/L sodium acetate and a buffered nutrient medium consisting of 8.4 g/L NaHCO3, 0.31 g/L NH4Cl, 0.13 g/L KCl, 0.05 g/L Na2HPO4, 0.03 g/L NaH2PO4$H2O, 10 mL/L trace vitamins, and 10 mL/L minerals. The cathode solution is almost same as the anode solution, except for sodium acetate. The anolyte and catholyte were replaced every fed-batch cycle. RED stack with five cell pairs was located between the anode chamber and cathode chamber. Once cell pair contained two membranes (anion exchange membrane, cation exchange membrane, AMX, CEX, Astom corporation, Japan), high concentration (HC) and low concentration (LC) cell. Intermembrane distance was separated by silicon gasket (1.3 mm), having rectangular cross section (4 2 cm2) that contained polyester mesh spacer to prevent contact with membranes. The HC solution was 35 g/L NaCl solution, and the LC solution was adjusted to salinity ratio of 50 with 0.7 g/L NaCl solution. The HC solution flows from the HC cell next to the cathode chamber and flows out to the HC cell next to the anode chamber. The LC solution flows from the LC cell next to the anode chamber and flows out to the LC cell next to the cathode chamber. The HC and LC solutions were continuously supplied into the RED stack using a peristaltic pump (BT1001L, Longer Pump, China) [6,17]. To investigate the influence of the flowrates of HC and LC solutions on MRC electrochemistry and optimal flowrate for maximum power production in MRCs, flowrates of HC and LC solutions were equally changed from 2.5 to 10 mL/min. In order to exclude some elements due to differences in anode performance, a same anode brush was used. The Reference electrode (Qrins, RE-1S Ag/AgCl; 209 V vs. a standard hydrogen electrode, SHE) was inserted through the top of the tube, in order to prevent the reference electrode is in contact with the brush anode.
Electrochemical tests
Fig. 1 e Photograph of the MRC system.
The cell voltage was recorded at 1 min intervals using a data acquisition system (3706A, Keithley Instrument, USA). Polarization curve and Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were performed using a potentiostat/galvanostat/impedance analyzer (ZIVE SP1, Wonatech, Seoul, South Korea). To determine the maximum power density, polarization curve was performed. For polarization curve, MRC reactor was held at open circuit potential for 90 min and connected cathode as the working electrode (WE) and anode as the counter electrode (CE). Then, rest time 30 min, scan rate 1 mV/s was applied to the reactor over the potential range OCP to 0 V versus Ag/AgCl. RED stack
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resistance (Rred) at various flowrates was obtained from the slope of RED stack polarization curve [7]. For galvanostatic anode EIS, the MRC reactor connected anode as the working electrode (WE) and cathode as the counter electrode (CE) and Ag/AgCl as the reference electrode (RE). For galvanostatic full cell EIS, MRC reactor connected anode as the CE and cathode as the WE. Bioanode and full cell were poised at each optimal current for 30 min so that it produced stable current. The EIS was performed at each set current with the following conditions: amplitude 0.1 mA, initial frequency: 105 Hz, final frequency 10 mHz, 10 points/ decade of data acquisition [9,18e20]. Impedance data were fitted to the equivalent circuit as previously described [9]. For anode CV tests, MRC reactor connected anode as the WE and cathode as the CE and Ag/AgCl as the RE. Anode potential range was 0.6 to þ0.1 V (vs. Ag/AgCl) at a scan rate of 1 mV/s. The CV measurement was measured in 3 cycles and the data was analyzed using the final cycle [21e23]. COD is consumed in a higher current production, and substrate concentration highly affect COD consumption rate in microbial electrochemical systems. For a fair evaluation, medium samples for COD measurement were taken after about one-day operation of the MRC. Based on measured COD data, COD removal rate, COD removal ratio, CE, EE and ER were calculated. COD was determined according to standard methods (X100, C-MAC, Daejeon, South Korea) [6]. Conductivity of HC solution, LC solution, anolyte and catholyte were measured using conductivity meter (HC3010, Trans Instrument, Singapore). pH of HC solution, LC solution, anolyte, and catholyte were measured using pH/Temp Meter (P25, Istek, Seoul, South Korea). Coulombic efficiency (CE) was defined as Z
t
Idt CE ¼
0
FV
DCOD 8
100
where F is Faraday's constant (96,485.3329 C/e-mol), V is the bed volume of the MRC (0.045 L), DCOD is the change in COD during operation time (g-COD) and 8 is a conversion factor (8 g-COD/e-mol). COD was measured by using a portable spectrophotometer (X-100, C-MAC, Daejeon, Korea). Final CODs were measured by using medium samples when MRC operated about 24 h. The Reynolds number (Re) was defined as [24] Re ¼
rvL m
where r is the density of the fluid, v is a characteristic velocity of the fluid with respect to the object, L is a characteristic linear dimension, m is the dynamic viscosity of the fluid. Energy recovery (ER) is the ratio of power production to the total invested energy in the MRC and it was defined as ER ¼
P 100 in DHc nin s t þX B
where P is the power produced (W), DHc is the heat of combustion of the substrate (870 kJ/mol), nin s is the amount of substrate in the anode chamber at the initial of a batch (mol) and tB is the time range of each batch cycle (s).
X ¼ RTQ in
X i
Cin i;HC
ln
ain i;HC ain i:mixed
þ
Cin i;LC
ln
ain i;LC ain i;mixed
! 100
where Xin is the theoretical energy (W) estimated by a change in free energy, including complete mixing of HC and LC solutions. R is the gas constant, T is the absolute temperature, Q is the flowrate and c is the molar concentration of ionic species i in HC, LC and mixed solutions. Energy efficiency (EE) is the ratio of power production to theoretically extractable energy in the MRC and it was defined as [6,25] hE ¼
P 100 out DHc nin tB þ Xin Xout s ns
is the amount of residual substrate in the anode where nout s chamber at the end of a batch and Xout is the salinity driven energy remaining in the HC and LC effluents. The chemical activity (ai) is calculated by multiplying the molarity concentration by activity coefficient ( fi). Activity coefficient (fi) was calculated by the DebyeeHu¨ckel equation. pffiffiffiffi AjZi j2 Is pffiffiffiffi logð1 þ 0:018mi Þ þ Ki Is log fi ¼ 1 þ Ba0 Is where A ¼ 0.0585 kg1/2/mol1/2, B ¼ 0.3282 Åkg1/2/mol1/2, ion size parameter (a0) ¼ 0.78 Å for both sodium and chloride, KNa ¼ 0.105 kg2/mol2 and KCl ¼ 0.009 kg2/mol2, and m is the molar concentration.
Results Polarization and power curves. According to the polarization and power curve analysis, maximum power density (Pmax) and maximum current density (Imax) increased as the flowrate of the saline and fresh water flowing to the RED stack increased (Fig. 2). Pmax in average was highest in 10 mL/min (3.71 W/m2), followed by 7.5 mL/min (3.70 W/m2), 5 mL/min (3.39 W/m2) and 2.5 mL/min (3.16 W/m2) (Fig. 2A and Table 1). Imax in average was highest in 10 ml/min (10.93 A/m2), followed by 7.5 mL/min (10.51 A/m2), 5 mL/min (9.72 A/m2) and 2.5 mL/min (9.35 A/m2). Maximum power density and maximum current density increased by 17.7% and 16.9%, respectively, as the flowrate increased from 2.5 ml/L to 10 mL/min. Optimum current density (Iopt) also increased as a RED flowrate increased up to 7.5 mL/min. However, it was 2.2% lower in 10 ml/min than 7.5 ml/min. Iopt in average was highest in 7.5 mL/min (5.36 A/m2), followed by 10 mL/min (5.24 A/m2), 5 mL/min (4.96 A/m2) and 2.5 mL/min (4.61 A/m2). Iopt increased by 16.2% as the flowrate increased from 2.5 mL/ min to 7.5 mL/min. Polarization curve analysis revealed that RED stack resistance (Rred), internal resistance (Rint), optimal resistance (Ropt) and cathode resistance (Rcat) generally decreased as a RED flowrate increased up to 7.5 mL/min (Table 1). Rred in average was lowest in 7.5 mL/min (124 U), followed by 5 mL/min (128 U), 10 mL/min (131 U) and 2.5 mL/min (141 U), decreasing by 12.1% as the flowrate increased from 2.5 mL/min to 7.5 mL/ min. Rint in average was lowest in 7.5 mL/min (188 U), followed by 10 mL/min (192 U), 5 mL/min (208 U) and 2.5 mL/min (219 U),
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Fig. 2 e Power density curve (A), full-cell polarization curves (B), RED-stack polarization curves (C) and anode and cathode electrode polarization curves (D) with respect to flowrates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decreasing by 14.2% as the flowrate increased from 2.5 mL/ min to 7.5 mL/min. Ropt in average was lowest in 7.5 mL/min (184 U), followed by 10 mL/min (193 U), 5 mL/min (197 U) and 2.5 mL/min (205 U), decreasing by 10.2% as the flowrate increased from 2.5 mL/min to 7.5 mL/min. Rcat in average was lowest in 7.5 mL/min (32 U), followed by 10 mL/min (36 U), 5 mL/min (41 U) and 2.5 mL/min (45 U), decreasing by 28.9% as the flowrate increased from 2.5 mL/min to 7.5 mL/min. Ran in average ranged 30e39 U, not likely to be correlated with RED flowrate. For more precise measurement of internal resistance distribution, EIS method was applied to the tested MRC. Impedances of an anode, a cathode and a full cell. Galvanostatic EIS was performed for more accurate analysis on the influence of RED flowrates on ohmic resistance (Rohm) and charge transfer resistance (Rct) in an anode, a cathode and a full cell (Table 2 and Fig. 3). EIS was measured at the optimum current condition where an MRC produced maximum power density.
The RED stack flowrate significantly affected charge transfer resistance in the anode and ohmic resistance in the cathode. Anode ohmic resistance in average was almost constant (1.1e1.2 U) regardless of the RED stack flowrate. However, when the flowrate increased from 2.5 mL/min to 10 mL/min, anode charge transfer resistance in average decreased by 52.8% from 26.9 to 12.7 U. Cathode charge transfer resistance in average was almost constant (8.1e8.9 U) regardless of the RED stack flowrate. However, when the flowrate increased from 2.5 mL/min to 10 mL/min, cathode ohmic resistance in average decreased by 39.6% from 26.8 to 18.3 U. Full-cell ohmic resistance in average was lowest in 10 mL/ min (96.6 U), followed by 7.5 mL/min (116.6 U), 5 mL/min (122.7 U) and 2.5 mL/min (139.7 U). Full cell total resistance (Rtotal) in average was lowest in 10 mL/min (135.8 U), followed by 7.5 mL/min (141.9 U), 5 mL/min (150.0 U) and 2.5 mL/min (166.7 U). When the flowrate increased from 2.5 mL/min to
Table 1 e Analyses of polarization curves with respect to flowrates. Flowrate 2
Pmax (W/m ) Imax (A/m2) Iopt (A/m2) Ropt (U) Rint (U) Ran (U) Rcat (U) Rred (U)
2.5 mL/min
5.0 mL/min
7.5 mL/min
10.0 mL/min
3.16 ± 0.11 9.35 ± 0.18 4.61 ± 0.15 205 ± 2 219 ± 8 33 ± 5 45 ± 2 141 ± 10
3.39 ± 0.09 9.72 ± 0.11 4.96 ± 0.09 197 ± 2 208 ± 3 39 ± 1 41 ± 3 128 ± 6
3.70 ± 0.04 10.51 ± 0.51 5.36 ± 0.13 184 ± 7 188 ± 2 32 ± 1 32 ± 0 124 ± 2
3.71 ± 0.08 10.93 ± 0.33 5.24 ± 0.18 193 ± 10 192 ± 7 30 ± 2 36 ± 5 131 ± 9
Pmax: Maximum power density, Imax: Maximum current density, Iopt: Optimal current density, Ropt: Optimal resistance, Rint: Internal resistance, Ran: Anode resistance, Rcat: Cathode resistance, Rred: Red stack resistance.
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Table 2 e Impedance analyses of an anode, a cathode and a full cell. Bias current (mA) Anode 2.5 mL/min 5.0 mL/min 7.5 mL/min 10.0 mL/min Cathode 2.5 mL/min 5.0 mL/min 7.5 mL/min 10.0 mL/min Full Cell 2.5 mL/min 5.0 mL/min 7.5 mL/min 10.0 mL/min
3.2 3.5 3.7 3.7
Rohm (U) 1.2 1.2 1.1 1.1
± 0.0 ± 0.0 ± 0.0 ± 0.0
3.2 3.5 3.7 3.7
26.8 30.3 28.5 18.3
± 0.2 ± 0.0 ± 0.1 ± 0.0
3.2 3.5 3.7 3.7
139.7 ± 1.0 122.7 ± 3.2 116.6 ± 0.7 96.6 ± 0.5
Rct (U) 26.9 23.4 14.0 12.7 8.1 8.1 8.9 8.2
± 0.0 ± 0.2 ± 0.1 ± 0.1
± 0.2 ± 0.1 ± 0.0 ± 0.1
27.0 27.4 25.3 39.2
± 0.2 ± 0.2 ± 0.0 ± 0.2
Rtotal (U) 28.1 24.6 15.1 13.7
± 0.0 ± 0.2 ± 0.1 ± 0.0
34.9 38.3 37.4 26.6
± 0.3 ± 0.0 ± 0.0 ± 0.0
166.7 150.0 141.9 135.8
± 0.8 ± 3.0 ± 0.7 ± 0.3
Rohm: Ohmic resistance, Rct: Charge transfer resistance, Rtotal: Total resistance, Bias current: optimum current for the MRC for each flowrate, and it is used for galvanostatic impedance measurement.
Fig. 3 e Nyquist plots and equivalent circuit fittings in an anode (A), a cathode (B) and a full cell (C). Symbols represent experimental data and lines represent fitted data. The inset is the equivalent circuit used for fitting EIS data: Rohm is ohmic resistance, Rct is charge transfer resistance, Q is constant phase element (CPE).
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Fig. 4 e Representative anodic cyclic voltammetry with respect to flowrates. A peak current density at a corresponding peak potential for each flowrate are shown in a read box (5.50 A/m2 for 2.5 mL/min, 5.54 A/m2 for 5 mL/ min, 5.56 A/m2 for 7.5 mL/min, 7.40 A/m2 for 10 mL/min).
Table 3 e Batch time, COD removal rate, COD removal ratio, CE, EE and ER of the MRC with respect to the flowrate on the RED stack. Flowrate Batch time (hr) COD removal rate (mg/ L/hr) COD removal ratio (%) CE (%) EE (%) ER (%)
2.5 mL/ min
5 mL/ min
7.5 mL/ min
10 mL/ min
23.7 10.0
22.9 9.4
24.6 12.0
23.3 11.0
35.4 65.9 17.6 3.7
33.9 69.6 10.3 2.0
46.1 57.7 8.5 1.5
40.1 63.5 6.9 1.1
10 mL/min, full cell Rohm decreased by 30.9%, and full cell Rtotal decreased by 18.5%. When the RED stack flowrate increased from 2.5 mL/min to 7.5 mL/min, the full cell Rct was almost invariable (25.3e27.4 U). However, when RED stack flowrate was 10 mL/min, the full cell Rct increased sharply to 39.2 U. Effect of feed solutions flowrates at bioanode CV tests. Anode CV was measured to investigate the electrochemical activity of the anode biofilm. Anode CV results showed that peak current density increased by 25.7% (from 5.50 to 7.40 A/ m2) and the corresponding peak anode potential also increased from 346 mV to 265 mV by simply increasing RED flowrate in the MRC (Fig. 4). Peak current density was highest in 10 mL/min (7.40 A/m2), followed by 7.5 mL/min
(5.56 A/m2), 5 mL/min (5.54 A/m2) and 2.5 mL/min (5.5 A/m2). Peak anode potential was highest in 10 mL/min (265 mV), followed by 7.5 mL/min (286 mV), 5 mL/min (288 mV) and 2.5 mL/min (346 mV). COD removal, coulombic efficiency (CE), energy efficiency (EE), energy recovery (ER). COD removal rate, COD removal ratio, CE and EE were measured when MRC operated about 24 h. Results showed that they seemed to have no correlation with RED flowrate. On average, COD removal rate was 10.6 mg/ L/hr, COD removal ratio was 38.8% and CE was 64.2% (Table 3). However, EE and ER increased as a RED flowrate decreased. EE was highest in 2.5 mL/min (17.6%), followed by 5 mL/min (10.3%), 7.5 mL/min (8.5%) and 10 mL/min (6.9%). ER was highest in 2.5 mL/min (3.7%), followed by 5 mL/min (2.0%), 7.5 mL/min (1.5%) and 10 mL/min (1.1%). pH and conductivity of Influents and Effluents. Anolyte pH decreased from 8.2 initially to 6.6 in the end of the batch due to the substrate oxidation in the anode [6]. Catholyte pH increased from 8.3 to 10.5 due to the proton consumption by the oxygen reduction in the cathode. Anolyte conductivity increased from 9.2 up to 12.6 mS/cm due to the anion transfer from the RED stack to the anode chamber [25]. Catholyte conductivity increased from 8.5 to 11.6 mS/cm due to the anion transfer from the RED stack to the cathode chamber (Table 4). Influent pHs of HC and LC for the RED stack were 6.36 and 6.93, respectively, and their effluent pHs rose by about 0.5.
Discussion As RED flowrate increased, maximum power density and maximum current density increased. Because salinity gradient becomes larger at higher flowrate, ion exchange increases at higher flowrates and power production also increases. Maximum power densities and maximum current densities at 7.5 mL/min and 10 mL/min were similar. However, operating current density was 5.36 A/m2 in 7.5 mL/min, which is higher than 5.24 A/m2 in 10 mL/min. So, in terms of electricity production, 7.5 mL/min is the best flowrate. Energy efficiency and energy recovery were also higher in 7.5 mL/min than those of 10 mL/min. Therefore, considering electricity power production, energy efficiency and energy recovery, RED flowrate 7.5 mL/min is the best choice among the tested flowrates in the tested MRC. Because pressure in the RED stack increases in a high RED flowrate (i.e. 10 mL/min in this experiment), RED stack
Table 4 e Influent (HC solution, LC solution, Anolyte, Catholyte) and Effluent (HC solution, LC solution, Anolyte, Catholyte) conductivity with respect to flowrates. Flowrate Influent
Effluent
HC LC Anolyte Catholyte HC LC Anolyte Catholyte
2.5 mL/min
5 mL/min
7.5 mL/min
10 mL/min
51.4 mS 1.42 mS 9.15 mS 8.57 mS 51.4 mS 2.64 mS 12.33 mS 11.66 mS
2.56 mS 12.29 mS 11.32 mS
2.27 mS 12.61 mS 11.83 mS
2.14 mS 12.47 mS 11.92 mS
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resistance can be also increased as RED flowrate reach a certain level. And increased RED stack resistance can add more resistance to internal resistance and optimum resistance of the MRC. In this study, RED flowrates up to 7.5 mL/ min decreased RED stack resistance, internal resistance and optimum resistance of the MRC due to the enhanced ion exchange, but the 10 mL/min increased those resistance possibly due to high internal pressure in the RED stack. In the anode EIS results, as RED flowrate increased, the anode ohmic resistance remained almost constant, but the anode charge transfer resistance decreased, resulting in an overall decrease in total anode resistance. This is possibly due to a higher anolyte conductivity and a higher optimum current for the EIS measurement in a higher RED flowrate. As the optimum current increases, respiration rate and energy production of anode biofilm increases, reducing anode charge transfer resistance. In the cathode EIS, as the flowrate increased, the cathode charge transfer resistance remained constant, but the cathode ohmic resistance decreased, resulting in an overall decrease in cathode total resistance (Rtotal). As the flowrate of the RED stack increased, ion exchange becomes more activated, increasing catholyte conductivity and decreasing cathode ohmic resistance. According to the full cell EIS results, total internal resistance of the full cell decreased with increasing RED stack flowrate. Our results showed that this was mainly due to decreasing fullcell ohmic resistance. As RED flowrate increased from 2.5 to 10.0 mL/min, total anode resistance decreased by 14.4 U and total cathode resistance decreased by 8.3 U, but ohmic resistance of the MRC decreased by 43.1 U. Because ohmic resistance of the MRC is closely associated with the RED stack, decreasing internal resistance of the MRC in a high RED flowrate can be mainly due to decreasing RED stack resistance. As RED flowrate increased, peak current density and peak potential of the anode increased. In an end of a batch operation for MRC, anolyte pH decreased and conductivity increased. It seems that the anolyte ion concentration was increased by substrate oxidation and ion influx through the anion exchange membrane. This high conductivity condition in high RED flowrate may enhance anode current generation, just as revealed by CV.
Conclusions We tested influence of the RED flowrates on performance, electrochemistry and optimal flowrate for maximum power production in MRCs. For this purpose, different flowrates of high- and low-salinity concentrations were tested. Considering electricity power production, energy efficiency and energy recovery, RED flowrate 7.5 mL/min is the best choice among the tested flowrates in the tested MRC.
Acknowledgments This research was supported by Korea Electric Power Corporation through Korea Electrical Engineering and Science
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Research Institute (R15XA03-04), a research grant from Gwangju Green Environment Center in Ministry of Environment (15-05-03-02-02) and Basic Science Research Program of National Research Foundation of Korea (NRF) in Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02037493). The authors certify that there is no authorship dispute among the authors.
references
[1] Post JW, Hamelers HV, Buisman CJ. Energy recovery from controlled mixing salt and fresh water with a reverse electrodialysis system. Environ Sci Technol 2008;42:5785e90. [2] Ramon GZ, Feinberg BJ, Hoek EM. Membrane-based production of salinity-gradient power. Energy Environ Sci 2011;4:4423e34. [3] Luo X, Cao X, Mo Y, Xiao K, Zhang X, Liang P, et al. Power generation by coupling reverse electrodialysis and ammonium bicarbonate: implication for recovery of waste heat. Electrochem Commun 2012;19:25e8. [4] Veerman J, Saakes M, Metz SJ, Harmsen GJ. Electrical power from sea and river water by reverse electrodialysis: a first step from the laboratory to a real power plant. Environ Sci Technol 2010;44:9207e12. [5] Weinstein JN, Leitz FB. Electric power from differences in salinity: the dialytic battery. Science 1976;191:557e9. [6] Kim Y, Logan BE. Microbial reverse electrodialysis cells for synergistically enhanced power production. Environ Sci Technol 2011;45:5834e9. [7] Cusick RD, Kim Y, Logan BE. Energy capture from thermolytic solutions in microbial reverse-electrodialysis cells. Science 2012;335:1474e7. [8] Liu J, Geise GM, Luo X, Hou H, Zhang F, Feng Y, et al. Patterned ion exchange membranes for improved power production in microbial reverse-electrodialysis cells. J Power Sources 2014;271:437e43. [9] Cusick RD, Hatzell M, Zhang F, Logan BE. Minimal RED cell pairs markedly improve electrode kinetics and power production in microbial reverse electrodialysis cells. Environ Sci Technol 2013;47:14518e24. [10] Jung S, Regan JM. Influence of external resistance on electrogenesis, methanogenesis, and anode prokaryotic communities in microbial fuel cells. Appl Environ Microbiol 2011;77:564e71. [11] Jung SP, Yoon M-H, Lee S-M, Oh S-E, Kang H, Yang J-K. Power generation and anode bacterial community compositions of sediment fuel cells differing in anode materials and carbon sources. Int J Electrochem Sci 2014;9:315e26. [12] Yang W, He W, Zhang F, Hickner MA, Logan BE. Single-step fabrication using a phase inversion method of poly (vinylidene fluoride)(PVDF) activated carbon air cathodes for microbial fuel cells. Environ Sci Technol Lett 2014;1:416e20. [13] Zhang Y, Sun J, Hu Y, Li S, Xu Q. Bio-cathode materials evaluation in microbial fuel cells: a comparison of graphite felt, carbon paper and stainless steel mesh materials. Int J Hydrogen Energy 2012;37:16935e42. [14] Othman R, Dicks AL, Zhu Z. Non precious metal catalysts for the PEM fuel cell cathode. Int J Hydrogen Energy 2012;37:357e72. [15] Zhang Y, Merrill MD, Logan BE. The use and optimization of stainless steel mesh cathodes in microbial electrolysis cells. Int J Hydrogen Energy 2010;35:12020e8. [16] Cheng S, Liu H, Logan BE. Increased performance of singlechamber microbial fuel cells using an improved cathode structure. Electrochem Commun 2006;8:489e94.
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[17] Kim Y, Logan BE. Hydrogen production from inexhaustible supplies of fresh and salt water using microbial reverseelectrodialysis electrolysis cells. Proc Natl Acad Sci 2011;108:16176e81. [18] Jung S, Mench MM, Regan JM. Impedance characteristics and polarization behavior of a microbial fuel cell in response to short-term changes in medium pH. Environ Sci Technol 2011;45:9069e74. [19] Jung S. Impedance analysis of Geobacter sulfurreducens PCA, Shewanella oneidensis MR-1, and their coculture in bioeletrochemical systems. Int J Electrochem Sci 2012;7:11091e100. [20] Jung S, Ahn Y-H, Oh S-E, Lee J-H, Cho K-T, Kim Y-J, et al. Impedance and thermodynamic analysis of bioanode, abiotic anode, and riboflavin-amended anode in microbial fuel cells. Bull Korean Chem Soc 2012;33: 3349e54. [21] Yoho RA, Popat SC, Rago L, Guisasola A, Torres CI. Anode biofilms of Geoalkalibacter ferrihydriticus exhibit
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electrochemical signatures of multiple electron transport pathways. Langmuir 2015;31:12552e9. Zhang F, Liu J, Ivanov I, Hatzell MC, Yang W, Ahn Y, et al. Reference and counter electrode positions affect electrochemical characterization of bioanodes in different bioelectrochemical systems. Biotechnol Bioeng 2014;111:1931e9. voteau A, Patil SA, Guo K, Soeriyadi AH, Feng H, Pre Gooding JJ, et al. Heat-treated stainless steel felt as scalable anode material for bioelectrochemical systems. Bioresour Technol 2015;195:46e50. Sommerfeld A. Ein beitrag zur hydrodynamischen erklaerung der turbulenten fluessigkeitsbewegungen. In: Proceedings of the 4th international congress of mathematicians III, Rome, Italy; 1908. p. 116e24. Nam J-Y, Cusick RD, Kim Y, Logan BE. Hydrogen generation in microbial reverse-electrodialysis electrolysis cells using a heat-regenerated salt solution. Environ Sci Technol 2012;46:5240e6.
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Influence of flowrates to a reverse electro-dialysis (RED) stack on performance and electrochemistry of a microbial reverse electrodialysis cell (MRC) Heunggu Kang a,b, Eojin Kim a, Sokhee P. Jung a,* a
Department of Environment and Energy Engineering, Chonnam National University, Gwangju 61186, Republic of Korea b Green Energy Institute, Naju 58217, Republic of Korea
article info
abstract
Article history:
An MRC is a bioelectrochemical system combining a microbial fuel cell (MFC) with a RED
Received 31 March 2017
stack to generate electricity from salinity gradient and organic wastewater with simulta-
Received in revised form
neous treatment. Operating an MRC at an optimum flowrate to RED is important because it
21 June 2017
is closely related with energy production rate and economic feasibility. However, influence
Accepted 22 June 2017
of RED flowrates on MRC electrochemistry and power production have not been investi-
Available online 23 July 2017
gated. For this purpose, four different flowrates of high concentration and low concentration solutions were tested. Maximum power density was highest in 10 mL/min (3.71 W/
Keywords:
m2) and optimum current density was highest in 7.5 mL/min (5.36 A/m2). By mere
Microbial reverse electrodialysis cell
increasing the flowrate to MRC, maximum power and optimum current densities increased
Reverse electrodialysis stack
by 17.7% and 16.2%. EIS showed that impedances of anode, cathode and full-cell were
RED flowrate
decreased by 51%, 31% and 19%, respectively. Anode CV showed that peak current density
Electrochemistry
was increased by 25.7%. COD removal and CE were not affected by RED flowrate. Power
Electrochemical impedance
generation at 7.5 mL/min and 10 mL/min were not so different, but current production was
spectroscopy
better at 7.5 mL/min. Therefore, considering energy production, the RED flowrate of 7.5 mL/ min is a reasonable choice for MRC operation. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Salinity gradient energy between sea and fresh water is a renewable energy, and the amount of energy that can be captured from salinity gradients in river water and sea water around the world is estimated at 1.9e2.6 TW [1,2]. There are various technologies such as reverse electrodialysis (RED) and pressure retarded osmosis (PRO) that produce electricity from salinity gradient. Reverse electrodialysis (RED) is a membranebased technology that can convert chemical potentials
directly into electricity through ion transport [3]. The RED stack consists of alternately stacked anion exchange membrane (AEM) and cation exchange membrane (CEM) separated by high concentration (HC) and low concentration (LC) cell. When HC and LC solutions are supplied into the RED stack, due to the concentration gradient, the cations in the HC solution migrate through the cation exchange membrane and the anions migrate through the anion exchange membrane to the LC solution. As a result, the potential difference across the membrane is converted to electrical current through the redox reactions of the electrodes (e.g., ~0.1e0.2 V per cell pair [4,5]).
* Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (S.P. Jung). http://dx.doi.org/10.1016/j.ijhydene.2017.06.187 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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A microbial reverse electrodialysis cell (MRC) is a microbial electrochemical system that combines an MFC with a reverse electrodialysis (RED) stack to generate electricity from the salinity gradient of seawater and fresh water, and treat wastewater simultaneously with electricity generation. In addition, power production can be greatly enhanced in an MRC [6]. There have been several previous studies that tested and characterized MRC systems by applying different RED flowrates, salinity ratios of HC and LC solutions, ion-exchange modification and thermolytic solutions [6e8]. Specifically, Roland et al. (2013) analyzed impedance distribution of an MRC with two different cell pairs (1 and 2) in a RED stack, in which raw domestic wastewater, buffered wastewater and buffered laboratory medium were tested as substrates [9]. Kim and Logan (2011) analyzed performance of an MRC to test effect of salinity ratio of RED inflow and effect of RED flowrate (0.85 and 1.55 mL/min) without EIS analysis [6]. Previous studies tested relatively lower ranges of RED flowrates and only two different flowrates were tested. However, in this study, we tested higher RED flowrates (2.5, 5.0, 7.5 and 10.0 mL/min) to produce more power from MRC, to find the optimum RED flowrate among them and to understand MRC electrochemistry in high RED flowrates.
Materials and methods MRC construction and operation The MRC reactor consisted of an anode chamber, cathode chamber and reverse electrodialysis (RED) stack (Fig. 1). Two chamber MRCs were made of polycarbonate cubic blocks. Anode chamber (45 mL) was 4 cm long, 3 cm diameter and cathode chamber (18 mL) was 2 cm long, 3 cm diameter [8]. A brush anode was made from carbon fibers (25 mm diameter 25 mm length; fiber type: T700, TORAYCA) wound into two twisted titanium wires (length: 7 cm; 17 gauge; #2 grade; Seoul Titanium). The anode was heat treated at 450 C for 30 min in a furnace (FX/FHX, Daihan Scientific, Wonju,
South Korea) before using. The inoculated brush anode was operated for over six months in a temperature room at 30 C at 1000-U external resistance [10,11]. The cathode was made of stainless steel mesh (#60 mesh, type 304, projected area of 7 cm2) spread with the catalytic mixture (300 mg of activated carbon powder, 30 mg of carbon black and 1 mL of 10% PVDF solution) [12e15]. To measure the potential of the anode and cathode electrode and the voltage of the red stack, Ag/AgCl reference electrodes (RE-1S, Qrins, Seoul, South Korea; 0.209 V versus a standard hydrogen electrode, SHE) were inserted into the anode chamber and the cathode chamber [16]. The anolyte consisted of 1.0 g/L sodium acetate and a buffered nutrient medium consisting of 8.4 g/L NaHCO3, 0.31 g/L NH4Cl, 0.13 g/L KCl, 0.05 g/L Na2HPO4, 0.03 g/L NaH2PO4$H2O, 10 mL/L trace vitamins, and 10 mL/L minerals. The cathode solution is almost same as the anode solution, except for sodium acetate. The anolyte and catholyte were replaced every fed-batch cycle. RED stack with five cell pairs was located between the anode chamber and cathode chamber. Once cell pair contained two membranes (anion exchange membrane, cation exchange membrane, AMX, CEX, Astom corporation, Japan), high concentration (HC) and low concentration (LC) cell. Intermembrane distance was separated by silicon gasket (1.3 mm), having rectangular cross section (4 2 cm2) that contained polyester mesh spacer to prevent contact with membranes. The HC solution was 35 g/L NaCl solution, and the LC solution was adjusted to salinity ratio of 50 with 0.7 g/L NaCl solution. The HC solution flows from the HC cell next to the cathode chamber and flows out to the HC cell next to the anode chamber. The LC solution flows from the LC cell next to the anode chamber and flows out to the LC cell next to the cathode chamber. The HC and LC solutions were continuously supplied into the RED stack using a peristaltic pump (BT1001L, Longer Pump, China) [6,17]. To investigate the influence of the flowrates of HC and LC solutions on MRC electrochemistry and optimal flowrate for maximum power production in MRCs, flowrates of HC and LC solutions were equally changed from 2.5 to 10 mL/min. In order to exclude some elements due to differences in anode performance, a same anode brush was used. The Reference electrode (Qrins, RE-1S Ag/AgCl; 209 V vs. a standard hydrogen electrode, SHE) was inserted through the top of the tube, in order to prevent the reference electrode is in contact with the brush anode.
Electrochemical tests
Fig. 1 e Photograph of the MRC system.
The cell voltage was recorded at 1 min intervals using a data acquisition system (3706A, Keithley Instrument, USA). Polarization curve and Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were performed using a potentiostat/galvanostat/impedance analyzer (ZIVE SP1, Wonatech, Seoul, South Korea). To determine the maximum power density, polarization curve was performed. For polarization curve, MRC reactor was held at open circuit potential for 90 min and connected cathode as the working electrode (WE) and anode as the counter electrode (CE). Then, rest time 30 min, scan rate 1 mV/s was applied to the reactor over the potential range OCP to 0 V versus Ag/AgCl. RED stack
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resistance (Rred) at various flowrates was obtained from the slope of RED stack polarization curve [7]. For galvanostatic anode EIS, the MRC reactor connected anode as the working electrode (WE) and cathode as the counter electrode (CE) and Ag/AgCl as the reference electrode (RE). For galvanostatic full cell EIS, MRC reactor connected anode as the CE and cathode as the WE. Bioanode and full cell were poised at each optimal current for 30 min so that it produced stable current. The EIS was performed at each set current with the following conditions: amplitude 0.1 mA, initial frequency: 105 Hz, final frequency 10 mHz, 10 points/ decade of data acquisition [9,18e20]. Impedance data were fitted to the equivalent circuit as previously described [9]. For anode CV tests, MRC reactor connected anode as the WE and cathode as the CE and Ag/AgCl as the RE. Anode potential range was 0.6 to þ0.1 V (vs. Ag/AgCl) at a scan rate of 1 mV/s. The CV measurement was measured in 3 cycles and the data was analyzed using the final cycle [21e23]. COD is consumed in a higher current production, and substrate concentration highly affect COD consumption rate in microbial electrochemical systems. For a fair evaluation, medium samples for COD measurement were taken after about one-day operation of the MRC. Based on measured COD data, COD removal rate, COD removal ratio, CE, EE and ER were calculated. COD was determined according to standard methods (X100, C-MAC, Daejeon, South Korea) [6]. Conductivity of HC solution, LC solution, anolyte and catholyte were measured using conductivity meter (HC3010, Trans Instrument, Singapore). pH of HC solution, LC solution, anolyte, and catholyte were measured using pH/Temp Meter (P25, Istek, Seoul, South Korea). Coulombic efficiency (CE) was defined as Z
t
Idt CE ¼
0
FV
DCOD 8
100
where F is Faraday's constant (96,485.3329 C/e-mol), V is the bed volume of the MRC (0.045 L), DCOD is the change in COD during operation time (g-COD) and 8 is a conversion factor (8 g-COD/e-mol). COD was measured by using a portable spectrophotometer (X-100, C-MAC, Daejeon, Korea). Final CODs were measured by using medium samples when MRC operated about 24 h. The Reynolds number (Re) was defined as [24] Re ¼
rvL m
where r is the density of the fluid, v is a characteristic velocity of the fluid with respect to the object, L is a characteristic linear dimension, m is the dynamic viscosity of the fluid. Energy recovery (ER) is the ratio of power production to the total invested energy in the MRC and it was defined as ER ¼
P 100 in DHc nin s t þX B
where P is the power produced (W), DHc is the heat of combustion of the substrate (870 kJ/mol), nin s is the amount of substrate in the anode chamber at the initial of a batch (mol) and tB is the time range of each batch cycle (s).
X ¼ RTQ in
X i
Cin i;HC
ln
ain i;HC ain i:mixed
þ
Cin i;LC
ln
ain i;LC ain i;mixed
! 100
where Xin is the theoretical energy (W) estimated by a change in free energy, including complete mixing of HC and LC solutions. R is the gas constant, T is the absolute temperature, Q is the flowrate and c is the molar concentration of ionic species i in HC, LC and mixed solutions. Energy efficiency (EE) is the ratio of power production to theoretically extractable energy in the MRC and it was defined as [6,25] hE ¼
P 100 out DHc nin tB þ Xin Xout s ns
is the amount of residual substrate in the anode where nout s chamber at the end of a batch and Xout is the salinity driven energy remaining in the HC and LC effluents. The chemical activity (ai) is calculated by multiplying the molarity concentration by activity coefficient ( fi). Activity coefficient (fi) was calculated by the DebyeeHu¨ckel equation. pffiffiffiffi AjZi j2 Is pffiffiffiffi logð1 þ 0:018mi Þ þ Ki Is log fi ¼ 1 þ Ba0 Is where A ¼ 0.0585 kg1/2/mol1/2, B ¼ 0.3282 Åkg1/2/mol1/2, ion size parameter (a0) ¼ 0.78 Å for both sodium and chloride, KNa ¼ 0.105 kg2/mol2 and KCl ¼ 0.009 kg2/mol2, and m is the molar concentration.
Results Polarization and power curves. According to the polarization and power curve analysis, maximum power density (Pmax) and maximum current density (Imax) increased as the flowrate of the saline and fresh water flowing to the RED stack increased (Fig. 2). Pmax in average was highest in 10 mL/min (3.71 W/m2), followed by 7.5 mL/min (3.70 W/m2), 5 mL/min (3.39 W/m2) and 2.5 mL/min (3.16 W/m2) (Fig. 2A and Table 1). Imax in average was highest in 10 ml/min (10.93 A/m2), followed by 7.5 mL/min (10.51 A/m2), 5 mL/min (9.72 A/m2) and 2.5 mL/min (9.35 A/m2). Maximum power density and maximum current density increased by 17.7% and 16.9%, respectively, as the flowrate increased from 2.5 ml/L to 10 mL/min. Optimum current density (Iopt) also increased as a RED flowrate increased up to 7.5 mL/min. However, it was 2.2% lower in 10 ml/min than 7.5 ml/min. Iopt in average was highest in 7.5 mL/min (5.36 A/m2), followed by 10 mL/min (5.24 A/m2), 5 mL/min (4.96 A/m2) and 2.5 mL/min (4.61 A/m2). Iopt increased by 16.2% as the flowrate increased from 2.5 mL/ min to 7.5 mL/min. Polarization curve analysis revealed that RED stack resistance (Rred), internal resistance (Rint), optimal resistance (Ropt) and cathode resistance (Rcat) generally decreased as a RED flowrate increased up to 7.5 mL/min (Table 1). Rred in average was lowest in 7.5 mL/min (124 U), followed by 5 mL/min (128 U), 10 mL/min (131 U) and 2.5 mL/min (141 U), decreasing by 12.1% as the flowrate increased from 2.5 mL/min to 7.5 mL/ min. Rint in average was lowest in 7.5 mL/min (188 U), followed by 10 mL/min (192 U), 5 mL/min (208 U) and 2.5 mL/min (219 U),
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Fig. 2 e Power density curve (A), full-cell polarization curves (B), RED-stack polarization curves (C) and anode and cathode electrode polarization curves (D) with respect to flowrates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decreasing by 14.2% as the flowrate increased from 2.5 mL/ min to 7.5 mL/min. Ropt in average was lowest in 7.5 mL/min (184 U), followed by 10 mL/min (193 U), 5 mL/min (197 U) and 2.5 mL/min (205 U), decreasing by 10.2% as the flowrate increased from 2.5 mL/min to 7.5 mL/min. Rcat in average was lowest in 7.5 mL/min (32 U), followed by 10 mL/min (36 U), 5 mL/min (41 U) and 2.5 mL/min (45 U), decreasing by 28.9% as the flowrate increased from 2.5 mL/min to 7.5 mL/min. Ran in average ranged 30e39 U, not likely to be correlated with RED flowrate. For more precise measurement of internal resistance distribution, EIS method was applied to the tested MRC. Impedances of an anode, a cathode and a full cell. Galvanostatic EIS was performed for more accurate analysis on the influence of RED flowrates on ohmic resistance (Rohm) and charge transfer resistance (Rct) in an anode, a cathode and a full cell (Table 2 and Fig. 3). EIS was measured at the optimum current condition where an MRC produced maximum power density.
The RED stack flowrate significantly affected charge transfer resistance in the anode and ohmic resistance in the cathode. Anode ohmic resistance in average was almost constant (1.1e1.2 U) regardless of the RED stack flowrate. However, when the flowrate increased from 2.5 mL/min to 10 mL/min, anode charge transfer resistance in average decreased by 52.8% from 26.9 to 12.7 U. Cathode charge transfer resistance in average was almost constant (8.1e8.9 U) regardless of the RED stack flowrate. However, when the flowrate increased from 2.5 mL/min to 10 mL/min, cathode ohmic resistance in average decreased by 39.6% from 26.8 to 18.3 U. Full-cell ohmic resistance in average was lowest in 10 mL/ min (96.6 U), followed by 7.5 mL/min (116.6 U), 5 mL/min (122.7 U) and 2.5 mL/min (139.7 U). Full cell total resistance (Rtotal) in average was lowest in 10 mL/min (135.8 U), followed by 7.5 mL/min (141.9 U), 5 mL/min (150.0 U) and 2.5 mL/min (166.7 U). When the flowrate increased from 2.5 mL/min to
Table 1 e Analyses of polarization curves with respect to flowrates. Flowrate 2
Pmax (W/m ) Imax (A/m2) Iopt (A/m2) Ropt (U) Rint (U) Ran (U) Rcat (U) Rred (U)
2.5 mL/min
5.0 mL/min
7.5 mL/min
10.0 mL/min
3.16 ± 0.11 9.35 ± 0.18 4.61 ± 0.15 205 ± 2 219 ± 8 33 ± 5 45 ± 2 141 ± 10
3.39 ± 0.09 9.72 ± 0.11 4.96 ± 0.09 197 ± 2 208 ± 3 39 ± 1 41 ± 3 128 ± 6
3.70 ± 0.04 10.51 ± 0.51 5.36 ± 0.13 184 ± 7 188 ± 2 32 ± 1 32 ± 0 124 ± 2
3.71 ± 0.08 10.93 ± 0.33 5.24 ± 0.18 193 ± 10 192 ± 7 30 ± 2 36 ± 5 131 ± 9
Pmax: Maximum power density, Imax: Maximum current density, Iopt: Optimal current density, Ropt: Optimal resistance, Rint: Internal resistance, Ran: Anode resistance, Rcat: Cathode resistance, Rred: Red stack resistance.
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Table 2 e Impedance analyses of an anode, a cathode and a full cell. Bias current (mA) Anode 2.5 mL/min 5.0 mL/min 7.5 mL/min 10.0 mL/min Cathode 2.5 mL/min 5.0 mL/min 7.5 mL/min 10.0 mL/min Full Cell 2.5 mL/min 5.0 mL/min 7.5 mL/min 10.0 mL/min
3.2 3.5 3.7 3.7
Rohm (U) 1.2 1.2 1.1 1.1
± 0.0 ± 0.0 ± 0.0 ± 0.0
3.2 3.5 3.7 3.7
26.8 30.3 28.5 18.3
± 0.2 ± 0.0 ± 0.1 ± 0.0
3.2 3.5 3.7 3.7
139.7 ± 1.0 122.7 ± 3.2 116.6 ± 0.7 96.6 ± 0.5
Rct (U) 26.9 23.4 14.0 12.7 8.1 8.1 8.9 8.2
± 0.0 ± 0.2 ± 0.1 ± 0.1
± 0.2 ± 0.1 ± 0.0 ± 0.1
27.0 27.4 25.3 39.2
± 0.2 ± 0.2 ± 0.0 ± 0.2
Rtotal (U) 28.1 24.6 15.1 13.7
± 0.0 ± 0.2 ± 0.1 ± 0.0
34.9 38.3 37.4 26.6
± 0.3 ± 0.0 ± 0.0 ± 0.0
166.7 150.0 141.9 135.8
± 0.8 ± 3.0 ± 0.7 ± 0.3
Rohm: Ohmic resistance, Rct: Charge transfer resistance, Rtotal: Total resistance, Bias current: optimum current for the MRC for each flowrate, and it is used for galvanostatic impedance measurement.
Fig. 3 e Nyquist plots and equivalent circuit fittings in an anode (A), a cathode (B) and a full cell (C). Symbols represent experimental data and lines represent fitted data. The inset is the equivalent circuit used for fitting EIS data: Rohm is ohmic resistance, Rct is charge transfer resistance, Q is constant phase element (CPE).
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Fig. 4 e Representative anodic cyclic voltammetry with respect to flowrates. A peak current density at a corresponding peak potential for each flowrate are shown in a read box (5.50 A/m2 for 2.5 mL/min, 5.54 A/m2 for 5 mL/ min, 5.56 A/m2 for 7.5 mL/min, 7.40 A/m2 for 10 mL/min).
Table 3 e Batch time, COD removal rate, COD removal ratio, CE, EE and ER of the MRC with respect to the flowrate on the RED stack. Flowrate Batch time (hr) COD removal rate (mg/ L/hr) COD removal ratio (%) CE (%) EE (%) ER (%)
2.5 mL/ min
5 mL/ min
7.5 mL/ min
10 mL/ min
23.7 10.0
22.9 9.4
24.6 12.0
23.3 11.0
35.4 65.9 17.6 3.7
33.9 69.6 10.3 2.0
46.1 57.7 8.5 1.5
40.1 63.5 6.9 1.1
10 mL/min, full cell Rohm decreased by 30.9%, and full cell Rtotal decreased by 18.5%. When the RED stack flowrate increased from 2.5 mL/min to 7.5 mL/min, the full cell Rct was almost invariable (25.3e27.4 U). However, when RED stack flowrate was 10 mL/min, the full cell Rct increased sharply to 39.2 U. Effect of feed solutions flowrates at bioanode CV tests. Anode CV was measured to investigate the electrochemical activity of the anode biofilm. Anode CV results showed that peak current density increased by 25.7% (from 5.50 to 7.40 A/ m2) and the corresponding peak anode potential also increased from 346 mV to 265 mV by simply increasing RED flowrate in the MRC (Fig. 4). Peak current density was highest in 10 mL/min (7.40 A/m2), followed by 7.5 mL/min
(5.56 A/m2), 5 mL/min (5.54 A/m2) and 2.5 mL/min (5.5 A/m2). Peak anode potential was highest in 10 mL/min (265 mV), followed by 7.5 mL/min (286 mV), 5 mL/min (288 mV) and 2.5 mL/min (346 mV). COD removal, coulombic efficiency (CE), energy efficiency (EE), energy recovery (ER). COD removal rate, COD removal ratio, CE and EE were measured when MRC operated about 24 h. Results showed that they seemed to have no correlation with RED flowrate. On average, COD removal rate was 10.6 mg/ L/hr, COD removal ratio was 38.8% and CE was 64.2% (Table 3). However, EE and ER increased as a RED flowrate decreased. EE was highest in 2.5 mL/min (17.6%), followed by 5 mL/min (10.3%), 7.5 mL/min (8.5%) and 10 mL/min (6.9%). ER was highest in 2.5 mL/min (3.7%), followed by 5 mL/min (2.0%), 7.5 mL/min (1.5%) and 10 mL/min (1.1%). pH and conductivity of Influents and Effluents. Anolyte pH decreased from 8.2 initially to 6.6 in the end of the batch due to the substrate oxidation in the anode [6]. Catholyte pH increased from 8.3 to 10.5 due to the proton consumption by the oxygen reduction in the cathode. Anolyte conductivity increased from 9.2 up to 12.6 mS/cm due to the anion transfer from the RED stack to the anode chamber [25]. Catholyte conductivity increased from 8.5 to 11.6 mS/cm due to the anion transfer from the RED stack to the cathode chamber (Table 4). Influent pHs of HC and LC for the RED stack were 6.36 and 6.93, respectively, and their effluent pHs rose by about 0.5.
Discussion As RED flowrate increased, maximum power density and maximum current density increased. Because salinity gradient becomes larger at higher flowrate, ion exchange increases at higher flowrates and power production also increases. Maximum power densities and maximum current densities at 7.5 mL/min and 10 mL/min were similar. However, operating current density was 5.36 A/m2 in 7.5 mL/min, which is higher than 5.24 A/m2 in 10 mL/min. So, in terms of electricity production, 7.5 mL/min is the best flowrate. Energy efficiency and energy recovery were also higher in 7.5 mL/min than those of 10 mL/min. Therefore, considering electricity power production, energy efficiency and energy recovery, RED flowrate 7.5 mL/min is the best choice among the tested flowrates in the tested MRC. Because pressure in the RED stack increases in a high RED flowrate (i.e. 10 mL/min in this experiment), RED stack
Table 4 e Influent (HC solution, LC solution, Anolyte, Catholyte) and Effluent (HC solution, LC solution, Anolyte, Catholyte) conductivity with respect to flowrates. Flowrate Influent
Effluent
HC LC Anolyte Catholyte HC LC Anolyte Catholyte
2.5 mL/min
5 mL/min
7.5 mL/min
10 mL/min
51.4 mS 1.42 mS 9.15 mS 8.57 mS 51.4 mS 2.64 mS 12.33 mS 11.66 mS
2.56 mS 12.29 mS 11.32 mS
2.27 mS 12.61 mS 11.83 mS
2.14 mS 12.47 mS 11.92 mS
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resistance can be also increased as RED flowrate reach a certain level. And increased RED stack resistance can add more resistance to internal resistance and optimum resistance of the MRC. In this study, RED flowrates up to 7.5 mL/ min decreased RED stack resistance, internal resistance and optimum resistance of the MRC due to the enhanced ion exchange, but the 10 mL/min increased those resistance possibly due to high internal pressure in the RED stack. In the anode EIS results, as RED flowrate increased, the anode ohmic resistance remained almost constant, but the anode charge transfer resistance decreased, resulting in an overall decrease in total anode resistance. This is possibly due to a higher anolyte conductivity and a higher optimum current for the EIS measurement in a higher RED flowrate. As the optimum current increases, respiration rate and energy production of anode biofilm increases, reducing anode charge transfer resistance. In the cathode EIS, as the flowrate increased, the cathode charge transfer resistance remained constant, but the cathode ohmic resistance decreased, resulting in an overall decrease in cathode total resistance (Rtotal). As the flowrate of the RED stack increased, ion exchange becomes more activated, increasing catholyte conductivity and decreasing cathode ohmic resistance. According to the full cell EIS results, total internal resistance of the full cell decreased with increasing RED stack flowrate. Our results showed that this was mainly due to decreasing fullcell ohmic resistance. As RED flowrate increased from 2.5 to 10.0 mL/min, total anode resistance decreased by 14.4 U and total cathode resistance decreased by 8.3 U, but ohmic resistance of the MRC decreased by 43.1 U. Because ohmic resistance of the MRC is closely associated with the RED stack, decreasing internal resistance of the MRC in a high RED flowrate can be mainly due to decreasing RED stack resistance. As RED flowrate increased, peak current density and peak potential of the anode increased. In an end of a batch operation for MRC, anolyte pH decreased and conductivity increased. It seems that the anolyte ion concentration was increased by substrate oxidation and ion influx through the anion exchange membrane. This high conductivity condition in high RED flowrate may enhance anode current generation, just as revealed by CV.
Conclusions We tested influence of the RED flowrates on performance, electrochemistry and optimal flowrate for maximum power production in MRCs. For this purpose, different flowrates of high- and low-salinity concentrations were tested. Considering electricity power production, energy efficiency and energy recovery, RED flowrate 7.5 mL/min is the best choice among the tested flowrates in the tested MRC.
Acknowledgments This research was supported by Korea Electric Power Corporation through Korea Electrical Engineering and Science
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Research Institute (R15XA03-04), a research grant from Gwangju Green Environment Center in Ministry of Environment (15-05-03-02-02) and Basic Science Research Program of National Research Foundation of Korea (NRF) in Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02037493). The authors certify that there is no authorship dispute among the authors.
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