Optimization of catholyte concentration and anolyte pHs in two ...
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Optimization of catholyte concentration and anolyte pHs in two chamber microbial electrolysis cells Joo-Youn Nam, Bruce E. Logan* Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA
article info
abstract
Article history:
The hydrogen production rate in a microbial electrolysis cell (MEC) using a non-buffered
Received 29 July 2012
saline catholyte (NaCl) can be optimized through proper control of the initial anolyte pH
Received in revised form
and catholyte NaCl concentration. The highest hydrogen yield of 3.3 0.4 mol H2/mole
17 September 2012
acetate and gas production rate of 2.2 0.2 m3 H2/m3/d were achieved here with an initial
Accepted 23 September 2012
anolyte pH ¼ 9 and catholyte NaCl concentration of 98 mM. Further increases in the salt
Available online 18 October 2012
concentration substantially reduced the anolyte pH to as low as 4.6, resulting in reduced MEC performance due to pH inhibition of exoelectrogens. Cathodic hydrogen recovery was
Keywords:
high (rcat > 90%) as hydrogen consumption by hydrogenotrophic methanogens was pre-
Anolyte pH
vented by separating the anode and cathode chambers using a membrane. These results
Conductivity
show that the MEC can be optimized for hydrogen production through proper choices in
Hydrogen
the concentration of a non-buffered saline catholyte and initial anolyte pH in two chamber
Sodium chloride
MECs.
Microbial electrolysis cell
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is an important energy carrier, but environmentally benign and cost-effective production processes are needed. A microbial electrolysis cell (MEC) is a nascent technology for producing hydrogen gas from organic matter [1]. In a typical two chamber MEC, the anode and cathode chambers are operated under anaerobic conditions with the two chambers divided by either a cation (CEM) or anion exchange membrane (AEM). Electrochemically active bacteria growing on the anode surface oxidize organic matter and produce electrons and protons, which acidify the anolyte solution. Protons are consumed at the cathode in the presence of an applied voltage to produce hydrogen gas, which then results in the alkalization of the catholyte [2,3]. These changes in pH adversely
affected performance, and therefore methods were needed to improve current densities. MECs can be modified to improve hydrogen production yields and rates through changes that reduce internal resistance. One approach was the development of a single chamber MEC that lacked a membrane [4]. Small MECs had improved performance in terms of hydrogen production rates and current densities due to reductions in internal resistance [4e7]. For example, single chamber MECs produced 3.1e17.8 m3 H2/ m3/d [4,8] compared to 0.2e0.3 m3 H2/m3/d [9e11] due to the absence of the membrane. However, in many tests using mixed culture inocula in small- and larger-scale MECs with single chamber designs, there was very little net hydrogen production due to hydrogenotrophic methanogenesis [12e15]. This indicated that in order to produce and recover a high purity
* Corresponding author. Tel./fax: þ1 814 863 7908. E-mail address: [email protected] (B.E. Logan). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.140
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hydrogen gas in larger MECs, two chamber configurations had to be used. Internal resistance in a two chamber MEC can be reduced through better control of the catholyte solution chemistry. Phosphate buffers are usually used in both the cathode and anode chambers of MECs, at concentrations ranging from 50 to 100 mM [10,16e18]. Using a buffer helps to control the pH, but depending on the cathode volume and current generation, even these typical buffer concentrations fail to prevent catholyte alkalization [19]. If a buffer is not used in the cathode chamber, the pH will substantially increase above neutral pH. Based on the Nernst equation and typical solution conditions, each unit of pH increase could increase overpotential by 59 mV [20]. Thus, it has been assumed that a high cathode pH will be detrimental to MEC performance. However, pH is not the only factor that affects overpotential. For example, the use of a catholyte with a high conductivity (7.3 mS/cm, with 66 mM NaCl) can greatly reduce the catholyte solution resistance [19]. Hydrogen production in a two chamber system without pH control was superior to that of an MEC with a phosphate buffer, despite the increase of the pH in the non-buffered catholyte to a pH ¼ 11 [19]. When a 0.1 M KCl solution (no buffer) was used in a two chamber MEC with a Ni foam cathode, the catholyte increased to a pH ¼ 12 but the gas production rates were higher than those obtained in a neutral pH buffered system [21]. These results suggest that catholyte pH may not be as critical to MEC performance as other operational conditions. In order to better identify the effect of catholyte conditions on hydrogen production in MECs, and improve gas production rates, tests were conducted here with different salt concentration in the cathode chamber. A low anode pH is well known to inhibit current generation [22,23]. Therefore, the anode pH was examined in conjunction with different cathode salt concentrations to better optimize the performance of the two chamber MEC for hydrogen gas production.
2.
Material and methods
2.1.
Reactor set up
Two chambered MECs were constructed from cubes of polycarbonate drilled to contain a cylindrical chamber 3 cm in diameter and 4 cm long. The anode and cathode chambers were separated by an AEM (AMI-7001, Membranes International Inc.) with working volumes of 28 mL for the anode and 32 mL for the cathode. A cylindrical glass tube for gas collection was attached to the top of the cathode chamber. The tube was sealed using a butyl rubber stopper and an aluminum crimp cap, and the gas produced was collected in a gas bag (0.1 L capacity; Cali-5-Bond, Calibrated Instruments Inc.). The anode was heat-treated graphite brush (25 mm diameter 25 mm length; 0.22 m2 surface area; fiber type; PANEX 33 160 K, ZOLTEK), and the cathode was stainless steel (SS) mesh (type 304 SS, #60 mesh, McMaster-Carr) coated with Pt catalyst layer (5 mg/cm2 10% Pt on Vulcan XC-71 with 33.3 mL/cm2 of 5 wt% Nafion solution as binder, projected cross sectional area of 7 cm2). Each chamber contained an Ag/AgCl reference electrode (RE-5B; BASi) in order to measure individual electrode potentials.
2.2.
18623
Experiments and measurements
Anodes were pre-acclimated in MFCs, which were originally inoculated with wastewater and an acetate-based medium [24], and transferred to MECs. Voltage (Eap ¼ 0.9 V) was added to the circuit using a power source (Model 3645A; Circuit Specialists, Inc.) by connecting the negative lead of the power source in series to a 10 U resistor and the cathode, with the positive lead on the anode. Voltages across the resistor and electrode potential were measured using a multimeter (Model 2700; Keithley Instruments, Inc.). The anode chambers were supplied with a solution containing 1.5 g/L sodium acetate and a 50 mM phosphate buffer solution (PBS) to provide some buffering capacity in the anolyte solution as most wastewaters contain some level of buffering capacity. PBS contained 4.58 g/L Na2HPO4, and 2.45 g/L NaH2PO4$H2O, pH ¼ 7.04, 0.31 g/ L NH4Cl, 0.13 g/L KCl, and trace vitamins and minerals [12]. The initial anolyte pH (pHin) was adjusted to 9 or 10 by using 1 N KOH to provide the desired starting pH condition. The catholyte was a salt solution (no buffer) with NaCl concentrations (CNaCl) ranging from 66 mM to 342 mM (conductivities of 7.3e33.7 mS/cm) as indicated. All MECs were operated in fed batch mode at 30 C. Gas production (V, mL) was measured using a respirometer (AER-200; Challenge Environmental). Gas collected in the gas bags and the reactor headspace was analyzed using gas chromatographs (GCs; SRI Instruments) for H2, N2, CO2 and CH4 as previously described [24]. Total chemical oxygen demand (COD) was measured at the beginning and end of each batch (TNT plus COD Reagent; HACH Company). The initial and final pHs (pHout) and sample conductivities were monitored using pH and conductivity meters (SevenMulti, Mettler-Toledo International Inc.). Anions and cations were analyzed using an ion chromatograph (ICS-3000, Dionex, USA) equipped with an IonPac AS18 (Dionex, USA) anion exchange column or a CS18 (Dionex, USA) cation exchange column, respectively.
2.3.
Calculations
The performance of the MECs was evaluated in terms of: coulombic efficiency (CE, %) based on total coulombs recovered compared to the initial mass of substrate; cathode hydrogen recovery (rcat, %); and volumetric hydrogen production rate (Q, m3 H2/m3/d) normalized to the anode working volume. The volumetric current density (Ivol, A/m3) was an average of the maximum current production over a 2 h period divided by the anolyte volume. Energy efficiencies were calculated relative to electrical input (sE, %) as the ratio of energy content of hydrogen produced to the electrical energy added; and as an overall recovery based on both electric and substrate input (sEþS, %) taking into account both electric and substrate energy input [25]. In order to identify optimum regions for system operation, volumetric current density (Ivol), hydrogen volume (V), and hydrogen production rates (Q) results were fitted with a polynomial equation: F ¼ y0 þ ax þ by þ cx2 þ dy2
(1)
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Fig. 1 e Current generation with varied anolyte pHs and NaCl concentration in the catholyte.
where F is V, Q or Ivol, x is the initial pH, y is the NaCl concentration (mM) in the cathode chamber, and a, b, c, and d, are regression coefficients.
3.
Results and discussion
3.1. Current generation at different initial anolyte pHs and NaCl concentrations in the cathode chamber Increasing the catholyte NaCl concentration from 66 mM (7.3 mS/cm) to 98 mM (10.4 mS/cm) slightly reduced the peak current from 4.1 0.6 mA to 3.7 0.7 mA, despite the increase in catholyte conductivity (Fig. 1). Also, the pH of the catholyte at the end of the cycle was slightly higher (pH ¼ 11.1 0.3) at 98 mM than it was at 66 mM (pH ¼ 10.9 0.4). This suggested that the increased salt concentration reduced OH transport out of the cathode due to the more favorable transport of Cl. The reduced OH transport with the increased salt concentration resulted in the anode solution (pHin ¼ 7) being reduced to a lower final pH (pHout ¼ 6.0) with the 98 mM catholyte than with 66 mM NaCl (pHout ¼ 6.5) (Table 1). It is known that low anolyte pHs (between 5 and 6 or lower) can adversely affect electricity generation [22,26]. This suggested that a higher initial anolyte pH might help improve the MEC
performance. When the anolyte was initially alkaline (pHin of 9 or 10), the peak current with 98 mM NaCl increased to 5.3 1.3 mA for pHin ¼ 9, and to 5.6 0.6 mA for pHin ¼ 10 (Fig. 1). This showed that high initial anolyte pHs were not detrimental to current generation by exoelectrogens, but that lower pHs could reduce MEC performance. Also, an alkaline anolyte has been shown to hinder methane production by methanogens, while maintaining exoelectrogenic activity, resulting in higher current generation with higher pHs [27]. At a higher initial catholyte NaCl concentration of 162 mM, peak currents at pHin ¼ 9 and 10 were slightly higher, and nearly equal, with values of 5.8 0.7 mA (Ivol ¼ 209 23 A/m3, pHin ¼ 9) and 5.7 0.7 mA (205 27 A/m3, pHin ¼ 10) (Fig. 1). The current did not consistently increase with further increases in the NaCl concentrations to 162 mM and 342 mM. At the very highest salt concentration of 342 mM, the final anode pH reached very low values (pHout ¼ 4.6e4.7), which would adversely affect the anode biofilm. A regression analysis on the volumetric current density to identify the optimal range of anolyte pHs and NaCl concentrations produced Ivol A=m3 ¼ 779:7 þ 202:9x þ 0:5y 10:9x2 0:1 102 y2 R2 ¼ 0:95; P ¼ 0:025
(2)
The use of this regression to create a contour plot (Fig. 2) showed that the cathode chamber optimal conditions were a NaCl concentration of 223 mM and a pH ¼ 9.3, which would produce a maximum current density of 213 A/m3. The contour line plot can be used to show that densities higher than 180 A/m3 (86% of the highest value) require an initial pHin > 8.1 and a catholyte with a NaCl concentration >120 mM (Fig. 2). Thus, it is clear that the anolyte pH would have to be raised to prevent significant pH drops during the fed batch cycle with highly saline catholyte solution. One approach to obtain this initial higher anolyte pH would be to add the high pH cathode solution to the anode chamber [18]. Alternatively, a bipolar membrane (BPM) could be used instead of an AEM, although that would increase the voltage that would need to be applied to achieve the same current densities with the BPM as the AEM [16].
3.2.
Variation in electrode potentials and conductivities
Changing the NaCl concentration also affected the anode potentials at these different pH conditions (Table 1). The lower final pH (pHout ¼ 6.0) with 98 mM NaCl solution produced more
Table 1 e Electrode potential and final pHs at different initial anolyte pHs and NaCl concentration in catholyte. NaCl (mM)
Anode pH Initial
66 98
162 342
7 7 9 10 9 10 9 10
Final cathode pH Final 6.5 6.0 6.4 6.6 6.1 6.3 4.6 4.7
0.2 0 0.1 0.1 0 0.3 0.1 0.1
Electrode potentials (mV) Anode
10.9 11.1 11.6 11.8 11.8 11.4 12.1 12.2
0.4 0.3 0.3 0.1 0.1 0.7 0.1 0
234 193 258 253 246 256 211 188
28 50 36 8 22 24 43 37
Cathode 939 16 955 17 994 52 1069 4 1001 49 978 8 980 6 981 8
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Fig. 2 e Volumetric current density as a function of anolyte pH and NaCl concentration in the cathode chamber. The top-right region indicated by the dashed lines shows conditions where >86% (>180 A/m3) of the maximum current density can be achieved.
anolyte (Table S1). Phosphate ions were detected in the cathode chamber due to their transport through the AEM from the anode to the cathode chamber. However, the total mass of phosphate ions recovered from both chambers (0.6e0.8 mmol) was less than that initially added (1.4 mmol), due to microbial uptake and adsorption in the membrane. The change in the 2 concentration of the other anions (NO 2 , NO3 , and SO4 ) was negligible, based on their initial added masses in the catholyte of 0.9 V) [24,31], which shows that the use of the membrane results in additional energy losses. However, compared to previous studies that used a membrane in MECs, our results show that it is possible to enhance hydrogen recovery in two chamber MECs by using a more optimal and alkaline anolyte pH, and a catholyte that is more saline than that which has previously been used [11,32]. These changes result in improved operation of the system in terms of hydrogen gas production, and they eliminate methane generation.
4.
Conclusions
Increasing the concentration of salt in a non-buffered catholyte enhanced hydrogen production (39 4 mL) in two chamber MECs as high as that obtained from a single chamber MEC [12]. However, sufficient buffering capacity of the anolyte was essential when using a highly saline catholyte. The highest hydrogen gas yield (3.3 0.4 mol H2/mol acetate) and production rate (Q ¼ 2.2 0.2 m3 H2/m3/d) were obtained with 98 mM NaCl when initial anolyte pH was set to 9. These conditions also produce the highest energy efficiencies based on either electrical energy (sE ¼ 180 38%) or substrate and electrical energy (sEþS ¼ 66 6%). Further increases in salt concentration (>98 mM NaCl, 10.4 mS/cm) resulted in low anolyte pHs of 4.6, which reduced performance. These results show that non-buffered, saline solution can be used for hydrogen evolution reaction in the cathode chamber to increase MEC performance in terms of hydrogen production rates. However, sufficient alkalinity should be provided to the anolyte in order to maintain good exoelectrogenic activity.
Acknowledgments This study was supported by the National Renewable Energy Laboratory (NREL) and the King Abdullah University of Science and Technology (KAUST) (Award KUS-I1-003-13).
Appendix A. Supplementary material Supplementary data related to this article can be found on line at http://dx.doi.org/10.1016/j.ijhydene.2012.09.140.
references Fig. 5 e Efficiencies with varied anolyte pHs and NaCl concentration in the cathode chamber (A) Coulombic efficiency (B) COD removal efficiency (C) Electrical energy recovery (hE) (D) Overall energy recovery (hEþS).
The highest overall energy recovery (hEþS ¼ 66%) was achieved with 98 mM NaCl at pHin ¼ 9, which an electrical energy efficiency of hE ¼ 180% (Fig. 5C and D). This overall energy recovery was slightly lower than that previously achieved
[1] Logan BE, Call D, Cheng S, Hamelers HVM, Sleutels THJA, Jeremiasse AW, et al. Microbial electrolysis cells for high yield hydrogen gas production from organic matter. Environ Sci Technol 2008;42:8630e40. [2] Fornero JJ, Rosenbaum M, Cotta MA, Angenent LT. Carbon dioxide addition to microbial fuel cell cathodes maintains sustainable catholyte pH and improves anolyte pH, alkalinity, and conductivity. Environ Sci Technol 2010;44: 2728e34. [3] Kim Y, Logan BE. Hydrogen production from inexhaustible supplies of fresh and salt water using
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microbial reverse-electrodialysis electrolysis cells. Proc Nat Acad Sci 2011;108:16176e81. Call D, Logan BE. Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane. Environ Sci Technol 2008;42:3401e6. Lee H-S, Rittmann BE. Significance of biological hydrogen oxidation in a continuous single-chamber microbial electrolysis cell. Environ Sci Technol 2009;44:948e54. Liu H, Cheng SA, Logan BE. Power generation in fed-batch microbial fuel cells as a function of ionic strength, temperature, and reactor configuration. Environ Sci Technol 2005;39:5488e93. Wang A, Liu W, Ren N, Cheng H, Lee D-J. Reduced internal resistance of microbial electrolysis cell (MEC) as factors of configuration and stuffing with granular activated carbon. Int J Hydrogen Energy 2010;35:13488e92. Cheng S, Logan BE. High hydrogen production rate of microbial electrolysis cell (MEC) with reduced electrode spacing. Bioresour Technol 2011;102:3571e4. Liu H, Grot S, Logan BE. Electrochemically assisted microbial production of hydrogen from acetate. Environ Sci Technol 2005;39:4317e20. Kyazze G, Popov A, Dinsdale R, Esteves S, Hawkes F, Premier G, et al. Influence of catholyte pH and temperature on hydrogen production from acetate using a two chamber concentric tubular microbial electrolysis cell. Int J Hydrogen Energy 2010;35:7716e22. Rozendal RA, Hamelers HVM, Molenkamp RJ, Buisman CJN. Performance of single chamber biocatalyzed electrolysis with different types of ion exchange membranes. Water Res 2007;41:1984e94. Nam J-Y, Tokash JC, Logan BE. Comparison of microbial electrolysis cells operated with added voltage or by setting the anode potential. Int J Hydrogen Energy 2011;36:10550e6. Parameswaran P, Torres CI, Lee H-S, Rittmann BE, Krajmalnik-Brown R. Hydrogen consumption in microbial electrochemical systems (MXCs): the role of homoacetogenic bacteria. Bioresour Technol 2011;102:263e71. Rader GK, Logan BE. Multi-electrode continuous flow microbial electrolysis cell for biogas production from acetate. Int J Hydrogen Energy 2010;35:8848e54. Cusick R, Bryan B, Parker D, Merrill M, Mehanna M, Kiely P, et al. Performance of a pilot-scale continuous flow microbial electrolysis cell fed winery wastewater. Appl Microbiol Biotechnol 2011;89:2053e63. Chen S, Liu G, Zhang R, Qin B, Luo Y. Development of the microbial electrolysis desalination and chemical-production cell for desalination as well as acid and alkali productions. Environ Sci Technol 2012;46:2467e72. Jeremiasse AW, Hamelers HVM, Buisman CJN. Microbial electrolysis cell with a microbial biocathode. Bioelectrochem 2010;78:39e43.
[18] Qu Y, Feng Y, Wang X, Liu J, Lv J, He W, et al. Simultaneous water desalination and electricity generation in a microbial desalination cell with electrolyte recirculation for pH control. Bioresour Technol 2012;106:89e94. [19] Nam J-Y, Logan BE. Enhanced hydrogen generation using a saline catholyte in a two chamber microbial electrolysis cell. Int J Hydrogen Energy 2011;36:15105e10. [20] Zhuang L, Zhou S, Li Y, Yuan Y. Enhanced performance of air-cathode two-chamber microbial fuel cells with high-pH anode and low-pH cathode. Bioresour Technol 2010;101: 3514e9. [21] Jeremiasse AW, Hamelers HVM, Saakes M, Buisman CJN. Ni foam cathode enables high volumetric H2 production in a microbial electrolysis cell. Int J Hydrogen Energy 2010;35: 12716e23. [22] Yuan Y, Zhao B, Zhou S, Zhong S, Zhuang L. Electrocatalytic activity of anodic biofilm responses to pH changes in microbial fuel cells. Bioresour Technol 2011;102:6887e91. [23] He Z, Huang Y, Manohar AK, Mansfeld F. Effect of electrolyte pH on the rate of the anodic and cathodic reactions in an air-cathode microbial fuel cell. Bioelectrochem 2008;74:78e82. [24] 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. [25] Selembo PA, Merrill MD, Logan BE. The use of stainless steel and nickel alloys as low-cost cathodes in microbial electrolysis cells. J Power Sources 2009;190:271e8. [26] He Z, Kan JJ, Wang YB, Huang YL, Mansfeld F, Nealson KH. Electricity production coupled to ammonium in a microbial fuel cell. Environ Sci Technol 2009;43:3391e7. [27] Liu M, Yuan Y, Zhang L-x, Zhuang L, Zhou S-g, Ni J-r. Bioelectricity generation by a Gram-positive Corynebacterium sp. strain MFC03 under alkaline condition in microbial fuel cells. Bioresour Technol 2010;101:1807e11. [28] Luo H, Jenkins PE, Ren Z. Concurrent desalination and hydrogen generation using microbial electrolysis and desalination cells. Environ Sci Technol 2011;45:340e4. [29] Hu H, Fan Y, Liu H. Hydrogen production using singlechamber membrane-free microbial electrolysis cells. Water Res 2008;42:4172e8. [30] Chae K-J, Choi M-J, Kim K-Y, Ajayi FF, Chang I-S, Kim IS. Selective inhibition of methanogens for the improvement of biohydrogen production in microbial electrolysis cells. Int J Hydrogen Energy 2010;35:13379e86. [31] Guo K, Tang X, Du Z, Li H. Hydrogen production from acetate in a cathode-on-top single-chamber microbial electrolysis cell with a mipor cathode. Biochem Eng J 2010;51:48e52. [32] Ditzig J, Liu H, Logan BE. Production of hydrogen from domestic wastewater using a bioelectrochemically assisted microbial reactor (BEAMR). Int J Hydrogen Energy 2007;32: 2296e304.
Author's personal copy 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 3 7 ( 2 0 1 2 ) 1 8 6 2 2 e1 8 6 2 8
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Optimization of catholyte concentration and anolyte pHs in two chamber microbial electrolysis cells Joo-Youn Nam, Bruce E. Logan* Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA
article info
abstract
Article history:
The hydrogen production rate in a microbial electrolysis cell (MEC) using a non-buffered
Received 29 July 2012
saline catholyte (NaCl) can be optimized through proper control of the initial anolyte pH
Received in revised form
and catholyte NaCl concentration. The highest hydrogen yield of 3.3 0.4 mol H2/mole
17 September 2012
acetate and gas production rate of 2.2 0.2 m3 H2/m3/d were achieved here with an initial
Accepted 23 September 2012
anolyte pH ¼ 9 and catholyte NaCl concentration of 98 mM. Further increases in the salt
Available online 18 October 2012
concentration substantially reduced the anolyte pH to as low as 4.6, resulting in reduced MEC performance due to pH inhibition of exoelectrogens. Cathodic hydrogen recovery was
Keywords:
high (rcat > 90%) as hydrogen consumption by hydrogenotrophic methanogens was pre-
Anolyte pH
vented by separating the anode and cathode chambers using a membrane. These results
Conductivity
show that the MEC can be optimized for hydrogen production through proper choices in
Hydrogen
the concentration of a non-buffered saline catholyte and initial anolyte pH in two chamber
Sodium chloride
MECs.
Microbial electrolysis cell
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is an important energy carrier, but environmentally benign and cost-effective production processes are needed. A microbial electrolysis cell (MEC) is a nascent technology for producing hydrogen gas from organic matter [1]. In a typical two chamber MEC, the anode and cathode chambers are operated under anaerobic conditions with the two chambers divided by either a cation (CEM) or anion exchange membrane (AEM). Electrochemically active bacteria growing on the anode surface oxidize organic matter and produce electrons and protons, which acidify the anolyte solution. Protons are consumed at the cathode in the presence of an applied voltage to produce hydrogen gas, which then results in the alkalization of the catholyte [2,3]. These changes in pH adversely
affected performance, and therefore methods were needed to improve current densities. MECs can be modified to improve hydrogen production yields and rates through changes that reduce internal resistance. One approach was the development of a single chamber MEC that lacked a membrane [4]. Small MECs had improved performance in terms of hydrogen production rates and current densities due to reductions in internal resistance [4e7]. For example, single chamber MECs produced 3.1e17.8 m3 H2/ m3/d [4,8] compared to 0.2e0.3 m3 H2/m3/d [9e11] due to the absence of the membrane. However, in many tests using mixed culture inocula in small- and larger-scale MECs with single chamber designs, there was very little net hydrogen production due to hydrogenotrophic methanogenesis [12e15]. This indicated that in order to produce and recover a high purity
* Corresponding author. Tel./fax: þ1 814 863 7908. E-mail address: [email protected] (B.E. Logan). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.140
Author's personal copy 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 3 7 ( 2 0 1 2 ) 1 8 6 2 2 e1 8 6 2 8
hydrogen gas in larger MECs, two chamber configurations had to be used. Internal resistance in a two chamber MEC can be reduced through better control of the catholyte solution chemistry. Phosphate buffers are usually used in both the cathode and anode chambers of MECs, at concentrations ranging from 50 to 100 mM [10,16e18]. Using a buffer helps to control the pH, but depending on the cathode volume and current generation, even these typical buffer concentrations fail to prevent catholyte alkalization [19]. If a buffer is not used in the cathode chamber, the pH will substantially increase above neutral pH. Based on the Nernst equation and typical solution conditions, each unit of pH increase could increase overpotential by 59 mV [20]. Thus, it has been assumed that a high cathode pH will be detrimental to MEC performance. However, pH is not the only factor that affects overpotential. For example, the use of a catholyte with a high conductivity (7.3 mS/cm, with 66 mM NaCl) can greatly reduce the catholyte solution resistance [19]. Hydrogen production in a two chamber system without pH control was superior to that of an MEC with a phosphate buffer, despite the increase of the pH in the non-buffered catholyte to a pH ¼ 11 [19]. When a 0.1 M KCl solution (no buffer) was used in a two chamber MEC with a Ni foam cathode, the catholyte increased to a pH ¼ 12 but the gas production rates were higher than those obtained in a neutral pH buffered system [21]. These results suggest that catholyte pH may not be as critical to MEC performance as other operational conditions. In order to better identify the effect of catholyte conditions on hydrogen production in MECs, and improve gas production rates, tests were conducted here with different salt concentration in the cathode chamber. A low anode pH is well known to inhibit current generation [22,23]. Therefore, the anode pH was examined in conjunction with different cathode salt concentrations to better optimize the performance of the two chamber MEC for hydrogen gas production.
2.
Material and methods
2.1.
Reactor set up
Two chambered MECs were constructed from cubes of polycarbonate drilled to contain a cylindrical chamber 3 cm in diameter and 4 cm long. The anode and cathode chambers were separated by an AEM (AMI-7001, Membranes International Inc.) with working volumes of 28 mL for the anode and 32 mL for the cathode. A cylindrical glass tube for gas collection was attached to the top of the cathode chamber. The tube was sealed using a butyl rubber stopper and an aluminum crimp cap, and the gas produced was collected in a gas bag (0.1 L capacity; Cali-5-Bond, Calibrated Instruments Inc.). The anode was heat-treated graphite brush (25 mm diameter 25 mm length; 0.22 m2 surface area; fiber type; PANEX 33 160 K, ZOLTEK), and the cathode was stainless steel (SS) mesh (type 304 SS, #60 mesh, McMaster-Carr) coated with Pt catalyst layer (5 mg/cm2 10% Pt on Vulcan XC-71 with 33.3 mL/cm2 of 5 wt% Nafion solution as binder, projected cross sectional area of 7 cm2). Each chamber contained an Ag/AgCl reference electrode (RE-5B; BASi) in order to measure individual electrode potentials.
2.2.
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Experiments and measurements
Anodes were pre-acclimated in MFCs, which were originally inoculated with wastewater and an acetate-based medium [24], and transferred to MECs. Voltage (Eap ¼ 0.9 V) was added to the circuit using a power source (Model 3645A; Circuit Specialists, Inc.) by connecting the negative lead of the power source in series to a 10 U resistor and the cathode, with the positive lead on the anode. Voltages across the resistor and electrode potential were measured using a multimeter (Model 2700; Keithley Instruments, Inc.). The anode chambers were supplied with a solution containing 1.5 g/L sodium acetate and a 50 mM phosphate buffer solution (PBS) to provide some buffering capacity in the anolyte solution as most wastewaters contain some level of buffering capacity. PBS contained 4.58 g/L Na2HPO4, and 2.45 g/L NaH2PO4$H2O, pH ¼ 7.04, 0.31 g/ L NH4Cl, 0.13 g/L KCl, and trace vitamins and minerals [12]. The initial anolyte pH (pHin) was adjusted to 9 or 10 by using 1 N KOH to provide the desired starting pH condition. The catholyte was a salt solution (no buffer) with NaCl concentrations (CNaCl) ranging from 66 mM to 342 mM (conductivities of 7.3e33.7 mS/cm) as indicated. All MECs were operated in fed batch mode at 30 C. Gas production (V, mL) was measured using a respirometer (AER-200; Challenge Environmental). Gas collected in the gas bags and the reactor headspace was analyzed using gas chromatographs (GCs; SRI Instruments) for H2, N2, CO2 and CH4 as previously described [24]. Total chemical oxygen demand (COD) was measured at the beginning and end of each batch (TNT plus COD Reagent; HACH Company). The initial and final pHs (pHout) and sample conductivities were monitored using pH and conductivity meters (SevenMulti, Mettler-Toledo International Inc.). Anions and cations were analyzed using an ion chromatograph (ICS-3000, Dionex, USA) equipped with an IonPac AS18 (Dionex, USA) anion exchange column or a CS18 (Dionex, USA) cation exchange column, respectively.
2.3.
Calculations
The performance of the MECs was evaluated in terms of: coulombic efficiency (CE, %) based on total coulombs recovered compared to the initial mass of substrate; cathode hydrogen recovery (rcat, %); and volumetric hydrogen production rate (Q, m3 H2/m3/d) normalized to the anode working volume. The volumetric current density (Ivol, A/m3) was an average of the maximum current production over a 2 h period divided by the anolyte volume. Energy efficiencies were calculated relative to electrical input (sE, %) as the ratio of energy content of hydrogen produced to the electrical energy added; and as an overall recovery based on both electric and substrate input (sEþS, %) taking into account both electric and substrate energy input [25]. In order to identify optimum regions for system operation, volumetric current density (Ivol), hydrogen volume (V), and hydrogen production rates (Q) results were fitted with a polynomial equation: F ¼ y0 þ ax þ by þ cx2 þ dy2
(1)
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Fig. 1 e Current generation with varied anolyte pHs and NaCl concentration in the catholyte.
where F is V, Q or Ivol, x is the initial pH, y is the NaCl concentration (mM) in the cathode chamber, and a, b, c, and d, are regression coefficients.
3.
Results and discussion
3.1. Current generation at different initial anolyte pHs and NaCl concentrations in the cathode chamber Increasing the catholyte NaCl concentration from 66 mM (7.3 mS/cm) to 98 mM (10.4 mS/cm) slightly reduced the peak current from 4.1 0.6 mA to 3.7 0.7 mA, despite the increase in catholyte conductivity (Fig. 1). Also, the pH of the catholyte at the end of the cycle was slightly higher (pH ¼ 11.1 0.3) at 98 mM than it was at 66 mM (pH ¼ 10.9 0.4). This suggested that the increased salt concentration reduced OH transport out of the cathode due to the more favorable transport of Cl. The reduced OH transport with the increased salt concentration resulted in the anode solution (pHin ¼ 7) being reduced to a lower final pH (pHout ¼ 6.0) with the 98 mM catholyte than with 66 mM NaCl (pHout ¼ 6.5) (Table 1). It is known that low anolyte pHs (between 5 and 6 or lower) can adversely affect electricity generation [22,26]. This suggested that a higher initial anolyte pH might help improve the MEC
performance. When the anolyte was initially alkaline (pHin of 9 or 10), the peak current with 98 mM NaCl increased to 5.3 1.3 mA for pHin ¼ 9, and to 5.6 0.6 mA for pHin ¼ 10 (Fig. 1). This showed that high initial anolyte pHs were not detrimental to current generation by exoelectrogens, but that lower pHs could reduce MEC performance. Also, an alkaline anolyte has been shown to hinder methane production by methanogens, while maintaining exoelectrogenic activity, resulting in higher current generation with higher pHs [27]. At a higher initial catholyte NaCl concentration of 162 mM, peak currents at pHin ¼ 9 and 10 were slightly higher, and nearly equal, with values of 5.8 0.7 mA (Ivol ¼ 209 23 A/m3, pHin ¼ 9) and 5.7 0.7 mA (205 27 A/m3, pHin ¼ 10) (Fig. 1). The current did not consistently increase with further increases in the NaCl concentrations to 162 mM and 342 mM. At the very highest salt concentration of 342 mM, the final anode pH reached very low values (pHout ¼ 4.6e4.7), which would adversely affect the anode biofilm. A regression analysis on the volumetric current density to identify the optimal range of anolyte pHs and NaCl concentrations produced Ivol A=m3 ¼ 779:7 þ 202:9x þ 0:5y 10:9x2 0:1 102 y2 R2 ¼ 0:95; P ¼ 0:025
(2)
The use of this regression to create a contour plot (Fig. 2) showed that the cathode chamber optimal conditions were a NaCl concentration of 223 mM and a pH ¼ 9.3, which would produce a maximum current density of 213 A/m3. The contour line plot can be used to show that densities higher than 180 A/m3 (86% of the highest value) require an initial pHin > 8.1 and a catholyte with a NaCl concentration >120 mM (Fig. 2). Thus, it is clear that the anolyte pH would have to be raised to prevent significant pH drops during the fed batch cycle with highly saline catholyte solution. One approach to obtain this initial higher anolyte pH would be to add the high pH cathode solution to the anode chamber [18]. Alternatively, a bipolar membrane (BPM) could be used instead of an AEM, although that would increase the voltage that would need to be applied to achieve the same current densities with the BPM as the AEM [16].
3.2.
Variation in electrode potentials and conductivities
Changing the NaCl concentration also affected the anode potentials at these different pH conditions (Table 1). The lower final pH (pHout ¼ 6.0) with 98 mM NaCl solution produced more
Table 1 e Electrode potential and final pHs at different initial anolyte pHs and NaCl concentration in catholyte. NaCl (mM)
Anode pH Initial
66 98
162 342
7 7 9 10 9 10 9 10
Final cathode pH Final 6.5 6.0 6.4 6.6 6.1 6.3 4.6 4.7
0.2 0 0.1 0.1 0 0.3 0.1 0.1
Electrode potentials (mV) Anode
10.9 11.1 11.6 11.8 11.8 11.4 12.1 12.2
0.4 0.3 0.3 0.1 0.1 0.7 0.1 0
234 193 258 253 246 256 211 188
28 50 36 8 22 24 43 37
Cathode 939 16 955 17 994 52 1069 4 1001 49 978 8 980 6 981 8
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Fig. 2 e Volumetric current density as a function of anolyte pH and NaCl concentration in the cathode chamber. The top-right region indicated by the dashed lines shows conditions where >86% (>180 A/m3) of the maximum current density can be achieved.
anolyte (Table S1). Phosphate ions were detected in the cathode chamber due to their transport through the AEM from the anode to the cathode chamber. However, the total mass of phosphate ions recovered from both chambers (0.6e0.8 mmol) was less than that initially added (1.4 mmol), due to microbial uptake and adsorption in the membrane. The change in the 2 concentration of the other anions (NO 2 , NO3 , and SO4 ) was negligible, based on their initial added masses in the catholyte of 0.9 V) [24,31], which shows that the use of the membrane results in additional energy losses. However, compared to previous studies that used a membrane in MECs, our results show that it is possible to enhance hydrogen recovery in two chamber MECs by using a more optimal and alkaline anolyte pH, and a catholyte that is more saline than that which has previously been used [11,32]. These changes result in improved operation of the system in terms of hydrogen gas production, and they eliminate methane generation.
4.
Conclusions
Increasing the concentration of salt in a non-buffered catholyte enhanced hydrogen production (39 4 mL) in two chamber MECs as high as that obtained from a single chamber MEC [12]. However, sufficient buffering capacity of the anolyte was essential when using a highly saline catholyte. The highest hydrogen gas yield (3.3 0.4 mol H2/mol acetate) and production rate (Q ¼ 2.2 0.2 m3 H2/m3/d) were obtained with 98 mM NaCl when initial anolyte pH was set to 9. These conditions also produce the highest energy efficiencies based on either electrical energy (sE ¼ 180 38%) or substrate and electrical energy (sEþS ¼ 66 6%). Further increases in salt concentration (>98 mM NaCl, 10.4 mS/cm) resulted in low anolyte pHs of 4.6, which reduced performance. These results show that non-buffered, saline solution can be used for hydrogen evolution reaction in the cathode chamber to increase MEC performance in terms of hydrogen production rates. However, sufficient alkalinity should be provided to the anolyte in order to maintain good exoelectrogenic activity.
Acknowledgments This study was supported by the National Renewable Energy Laboratory (NREL) and the King Abdullah University of Science and Technology (KAUST) (Award KUS-I1-003-13).
Appendix A. Supplementary material Supplementary data related to this article can be found on line at http://dx.doi.org/10.1016/j.ijhydene.2012.09.140.
references Fig. 5 e Efficiencies with varied anolyte pHs and NaCl concentration in the cathode chamber (A) Coulombic efficiency (B) COD removal efficiency (C) Electrical energy recovery (hE) (D) Overall energy recovery (hEþS).
The highest overall energy recovery (hEþS ¼ 66%) was achieved with 98 mM NaCl at pHin ¼ 9, which an electrical energy efficiency of hE ¼ 180% (Fig. 5C and D). This overall energy recovery was slightly lower than that previously achieved
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