- Email: [email protected]
Hydrogen production in microbial electrolysis cells: Choice of catholyte
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 x x x ( 2 0 1 3 ) 1 e6
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Short Communication
Hydrogen production in microbial electrolysis cells: Choice of catholyte Siriporn Yossan a, Li Xiao b, Poonsuk Prasertsan c,d, Zhen He b,* a
Program of Science, Faculty of Liberal Arts and Science, Sisaket Rajabhat University, Sisaket 33000, Thailand Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA c Department of Industrial Biotechnology, Faculty of Agro-Industry, Prince of Songkla University, Songkhla 90112, Thailand d Palm Oil Products and Technology Research Center (POPTEC), Faculty of Agro-Industry, Prince of Songkla University, Songkhla 90112, Thailand b
article info
abstract
Article history:
A catholyte is a key factor to hydrogen production in microbial electrolysis cells (MECs).
Received 18 January 2013
Among the four groups of catholytes investigated in this study, a 100 mM phosphate buffer
Received in revised form
solution (PBS) resulted in the highest hydrogen production rate of 0.237 0.031 m3H2/m3/d,
16 May 2013
followed by 0.171 0.012 m3H2/m3/d with a 134 mM NaCl solution and 0.171 0.004 m3H2/
Accepted 19 May 2013
m3/d with the acidified water adjusted with sulfuric acid. The MEC with all catholytes
Available online xxx
achieved good organic removal efficiency, but the removal rate varied following the trend of the hydrogen production rate. The reuse of the catholyte for an extended period led to a
Keywords:
decreasing hydrogen production rate, affected by the elevated pH. The cost of both the
Hydrogen
acidified water and the NaCl solution was much lower than the PBS, and therefore, they
Microbial electrolysis cells
could be a better choice as an MEC catholyte with further consideration of cost reduction
Catholyte
and chemical reuse/disposal.
Buffer
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is a promising alternative to fossil fuel and is considered as a clean fuel because its combustion generates no pollutants but water [1]. Hydrogen gas can be produced by both thermochemical (pyrolysis/gasification) and biological (biophotolysis, dark and photo fermentation) processes [2]. In recent years, a new method of hydrogen production has been developed based on bioelectrochemical principles in microbial electrolysis cells (MECs). The advantage of MEC technology is to produce hydrogen gas from a non-fermenting
substrate (e.g., in wastewater) with a very low input of external energy. Therefore, MECs can function as an energyefficient process for simultaneous waste treatment and energy (hydrogen) production. Our understanding of MECs has been greatly advanced by intensive research on microbiology, substrates, and reactor configuration and operation [3,4]. MECs usually contain two chambers, an anode and a cathode separated by an ion exchange membrane (IEM). A single-chamber MEC was developed by omitting IEM to considerably simplify the reactor structure and potentially improve hydrogen production by
* Corresponding author. Tel.: þ1 414 229 5846; fax: þ1 414 229 6958. E-mail address: [email protected] (Z. He). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
2
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 x x x ( 2 0 1 3 ) 1 e6
reducing the MEC’s internal resistance [5]. However, a great concern with the single-chamber MECs is methane production, which is caused by methanogens inhabiting the adjacent cathode and has been observed in several MECs including a pilot test [6]. Therefore, to produce high-purity hydrogen gas in an MEC, IEM may still be a preferable separator to prevent microbial contamination in the cathode. With an IEM and a separated cathode chamber, a simple but important question arises: what should be the catholyte? A commonly used catholyte is a phosphate buffered solution (PBS), which can minimize the pH elevation and provide a certain electrolyte conductivity; however, the use of PBS may not be economical and environmentally friendly. The anode effluent (treated wastewater) could be a low-cost catholyte, which has been studied in microbial fuel cells (MFCs) [7], but it can introduce microorganisms including methanogens into the cathode chamber, resulting in methane production. Thus, we should avoid any catholytes containing microbial sources. In addition to PBS, several other catholytes have been studied, including salt solutions and bicarbonate buffers. In general, there is very limited information available about the comparison between these and other potential catholytes in terms
of performance and cost. In this study, we aim to examine four different catholytes and provide suggestions on optimal choices of a catholyte in an abiotic cathode for sustainable hydrogen production in an MEC.
2.
Materials and methods
2.1.
MEC set up and operation
An MEC was made from two glass bottles, which were joined by a cation exchange membrane (Membranes International, Inc., GlenRock, NJ, USA). Each bottle had a liquid volume of 120 mL. The anode electrode was a carbon brush (Gordon Brush Mfg. Co., Inc., Commerce, CA, USA), and the cathode electrode was a piece of carbon cloth (Zoltek Corporation, St. Louis, MO, USA) with a length of 4 cm and a width of 3 cm. The cathode electrode was coated with Pt as a catalyst (0.5 mg Pt/ cm2) [8]. Both the anolyte and the catholyte were continuously mixed by magnetic stirrers. An external voltage (0.8 V) was applied to the circuit by connecting the negative pole of a power supply (3644 A, Circuit Specialists, Inc., Mesa, AZ, USA)
Fig. 1 e Current generation in the MEC with different catholytes: (A) 100, 50, 25 and 10 mM PBS; (B) 134, 100 and 200 mM NaCl; (C) DI and tap water; and (D) acidified water at pH 4 and pH 2 adjusted by H2SO4 or HCl.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
3
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 x x x ( 2 0 1 3 ) 1 e6
Table 1 e Coulombic efficiencies and hydrogen production in the MEC with different catholytes. Catholytes
Time period (day)
Coulombic recovery: CR (%)
0.10 0.06 0.07 0.07
34.6 18.7 13.2 8.0
100 mM PBS 50 mM PBS 25 mM PBS 10 mM PBS
1.44 1.06 1.11 1.29
134 mM NaCl 100 mM NaCl 200 mM NaCl
2.01 0.12 1.55 0.06 1.30 0.07
26.5 1.5 19.8 1.4 19.7 3.5
DI water Tap water
5.10 1.47 3.14 0.61
42.8 6.8 10.9 2.6
Acidified water (pH 4)a Acidified water (pH 2)a Acidified water (pH 2)b
2.81 0.18 1.19 0.05 1.37 0.07
11.0 2.1 21.5 0.8 18.5 2.0
Cathodic H2 recovery: Rcat (%)
1.0 2.0 0.4 0.5
89.2 89.2 80.1 69.9
Overall H2 recovery: RH2 (%)
9.2 6.6 3.7 8.1
30.8 16.7 10.6 5.6
120.4 4.4 91.2 3.4 93.7 5.0
H2 production rate: QH2 (m3H2/m3/d)
2.3 1.4 0.8 0.3
0.237 0.165 0.100 0.048
0.031 0.005 0.007 0.007
31.8 1.2 18.0 0.8 18.4 2.8
0.171 0.012 0.116 0.004 0.143 0.009
67.9 12.6 93.8 22.1
28.5 1.4 9.8 0.1
0.065 0.021 0.034 0.007
156.3 17.3 84.7 1.8 65.5 8.4
17.0 1.3 18.2 0.5 12.0 0.8
0.065 0.002 0.171 0.004 0.089 0.002
a Adjusted by sulfuric acid. b Adjusted by hydrochloric acid.
to an external resistor (10 U), then to the cathode electrode, and the positive pole to the anode electrode. The MEC was operated in a fed-batch mode at 20 C. The anode chamber was inoculated using anaerobic sludge from South Shore Water Reclamation Facility (Milwaukee, WI, USA). The anode feeding solution was prepared in tap water consisting of 1.0 g/L sodium acetate, 0.3 g/L NH4Cl, 1.0 g/L NaCl, 0.03 g/L MgSO4, 0.04 g/L CaCl2, 0.2 g/L NaHCO3, 5.3 g/L KH2PO4, 10.7 g/L K2HPO4 and 1 mL/L trace elements [8]. Four groups of catholytes were examined in the MEC: PBS, NaCl solution, water, and acidified water. The PBS was tested in four concentrations, 10, 25, 50, and 100 mM. The NaCl solution was prepared by dissolving NaCl in tap water to 100, 134, and 200 mM. Two waters, tap water and deionized (DI) water, were studied. The acidified water was prepared in two pHs, 2 and 4, using hydrochloric acid (HCl) or sulfuric acid (H2SO4). The catholyte was completely replaced when the voltage dropped below 2 mV. The new catholyte was flushed with nitrogen gas
for 15 min to remove any oxygen before starting MEC operation. The anolyte was replaced by 80% with a fresh anode feeding solution at the same time when the catholyte was replaced. During the extended-catholyte study, the anolyte was replaced when the voltage decreased below 2 mV, but the catholyte was not replenished.
2.2.
Measurement and analysis
The voltage was monitored and recorded every 5 min by a digital multimeter (Keithley Instruments Co., Ltd., USA). The concentration of chemical oxygen demand (COD) was analyzed by a DR/890 colorimeter (HACH Co., Ltd., USA). The pH was measured using a bench-top pH meter (Oakton Instruments Co., Ltd., USA) and the electrical conductivity was measured by a bench-top conductivity meter (Mettler-Toledo Co., Ltd., USA). The production of hydrogen gas was measured by water replacement and analyzed by using a gas
Table 2 e COD removal in anolyte and characteristics of effluent catholyte with different catholytes. Catholytes
COD removal (%)
COD removal rate (g COD/m3/d)
Conductivity (ms/cm) Initial
94.6 97.0 97.5 97.2
134 mM NaCl 100 mM NaCl 200 mM NaCl
96.7 0.7 95.9 1.8 96.7 0.2
0.37 0.03 0.44 0.02 0.54 0.05
12.2 0.1 12.0 0.1 11.9 0.1
13.88 0.24 7.68 0.66 20.00 0.20
21.07 0.65 15.28 0.92 25.05 0.30
DI water Tap water
94.6 2.3 86.6 0.8
0.15 0.04 0.24 0.05
12.6 0.1 11.6 0.4
0.01 0.00 0.30 0.02
1.56 0.08 2.53 0.50
Acidified water (pH 4)a Acidified water (pH 2)a Acidified water (pH 2)b
97.4 0.1 98.2 0.3 96.8 0.2
0.27 0.01 0.66 0.02 0.51 0.03
11.9 0.4 9.0 0.7 11.0 0.4
0.05 0.01 7.37 0.05 5.46 0.02
2.94 0.38 3.61 0.25 3.27 0.49
0.04 0.07 0.06 0.05
9.2 8.9 10.8 11.3
0.2 0.2 0.2 0.2
13.97 8.52 4.06 1.44
Effluent
100 mM PBS 50 mM PBS 25 mM PBS 10 mM PBS
0.3 0.4 0.4 0.4
0.54 0.69 0.66 0.59
Effluent pH
0.01 0.80 0.04 0.48
12.15 9.67 5.40 2.63
0.23 0.35 0.30 0.57
a Adjusted by sulfuric acid. b Adjusted by hydrochloric acid.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
4
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 x x x ( 2 0 1 3 ) 1 e6
chromatograph (Focus GC, Thermo Scientific, USA). The key parameters were calculated as previously described [8]: the coulombic recovery (CR) is defined as the total output coulombs over the total input coulombs in acetate; the cathodic hydrogen recovery (Rcat) is the ratio between the electrons contained in the produced hydrogen gas and the electrons produced as current; the overall hydrogen recovery ðRH2 Þ is the substrate used for the hydrogen production; and the hydrogen production rate ðQH2 Þ is the hydrogen production per anode working volume per time.
3.
Results and discussion
The MEC performance with different catholytes was described by using three main parameters, electricity generation, hydrogen production, and COD removal.
Fig. 1 shows the profile of batch current generation with four groups of catholytes at different concentrations. In general, the PBS catholytes with higher concentrations had a higher peak current, followed by the acidified water (pH 2), and the NaCl solution. Both tap water and DI water resulted in very low peak currents. Decreasing the PBS concentration from 100 to 10 mM also decreased the peak current from 31.1 1.5 to 8.2 0.4 A/m3 (Fig. 1(A)), while increasing the NaCl concentration from 100 to 200 mM slightly improved the peak current from 15.0 0.5 to 18.7 1.1 A/m3 (Fig. 1(B)). The exceptional performance of the 100-mM PBS is due to its buffer capacity and conductivity (Table 2). The high current with the NaCl catholyte is because of its high conductivity (Table 2), while the high current with the acidified water is likely due to the additional proton input. The higher conductivity of the tap water over that of the DI water did not result in more current production: the current generation with the DI
Fig. 2 e Current generation (white dot) and hydrogen production rate (white square) in the MEC in the extended period of catholyte reuse: (A) 100 mM PBS; (B) 134 mM NaCl; and (C) acidified water at pH 2 adjusted by H2SO4.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
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 x x x ( 2 0 1 3 ) 1 e6
water (11.7 0.1 A/m3) was twice that with the tap water (5.5 0.4 A/m3), possibly because of the pH influence: the initial pH of the DI water (4.7 0.0) was much lower than that of the tap water (7.3 0.3), indicating that the proton supply may have a more important role in current generation than the electrolyte conductivity. When the initial pH was similar, for example, with the pH-2 water prepared by H2SO4 or HCl, a higher influent conductivity of the H2SO4 catholyte led to a higher peak current of (31.1 0.8 A/m3) than that of 25.7 0.3 A/m3 with the HCl catholyte. Hydrogen production efficiencies and rates with different catholytes are summarized in Table 1. The efficiency of the substrate-to-hydrogen (overall hydrogen recovery) varied between 5% and 32%; interestingly, the DI water and the tap water had comparable overall hydrogen recovery, as well as coulombic recovery, with the other catholytes. However, it took a much longer time (3e5 days) for the MEC with the DI or tap water to complete a cycle, resulting in a much lower hydrogen production rate. For the catholytes of the PBS, the NaCl solution or the acidified water, the hydrogen production rates matched their current generation, and a higher current led to a higher hydrogen production rate. The primary function of an MEC using wastewater as its anode substrate is considered to be contaminant removal; thus, COD reduction is a key parameter to evaluate the MEC performance. In general, the MEC achieved more than 94% of COD removal with all catholytes except the tap water (Table 2), demonstrating a good performance in organic removal. However, the COD removal rate was clearly affected by current generation and the period of a batch cycle, and the catholytes such as the DI water and the tap water with low hydrogen rates also yielded low COD removal rates. We chose three catholytes, 100-mM PBS, 134-mM NaCl, and acidified water pH 2 (H2SO4), and conducted an extended study without replacing the catholyte. The peak current densities with the PBS and the acidified water clearly decreased over time, but the NaCl maintained a relatively stable current output. The hydrogen production rates, on the other hand, all decreased in the extended operating period (Fig. 2). Interestingly, the hydrogen production rate with the acidified water showed a dramatic decrease in the second day with a significant increase in pH from 2.0 to 11.2, and then slowly recovered in the following six days with a further increase of pH to 12.6 (Fig. 2(C)). The pH of the PBS increased from 7.3 to 12.3 in ten days and the NaCl catholyte had an elevated pH from 7.5 to 12.9 in six days. The final conductivity of the acidified water was 12.57 mS/cm, half of that of the PBS or NaCl catholytes. All three catholytes had a similar hydrogen production rate of 0.10 m3H2/m3/d at the end of their testing period, indicating that pH had more influence than catholyte conductivity on hydrogen production. The cost of a catholyte was estimated from the amount of the chemicals used and their bulk prices obtained from SigmaeAldrich (St. Louis, USA). For an operation with frequent replacement of the catholyte (as shown in Fig. 1), it would cost $3.39/m3 H2 with the 100-mM PBS, much higher than the hydrogen price ($0.42/m3 estimated from $4.75/kg H2 [9]). The use of the 134-mM NaCl or the acidified water (pH ¼ 2 adjusted by sulfuric acid) would cost $0.29/m3 and $0.21/m3, respectively.
5
In an extended use of the catholyte (as shown in Fig. 2), the cost of those three catholytes (calculated from the first four cycles) decreased because of the reuse: $1.50/m3 with the 100-mM PBS, $0.19/m3 with the 134-mM NaCl, and $0.11/m3 with the acidified water (pH ¼ 2). The cost can be even less with additional operating cycles using the same catholyte (Fig. 2(A) and (C)) and lower prices of raw chemicals in a larger quantity. Both the NaCl solution and the acidified water may be a better choice as an MEC catholyte than the PBS, because of the significantly lower cost. The use of PBS can also potentially cause environmental problems by releasing phosphorus into natural water bodies. Of course, challenges also exist with the use of the NaCl or the acidified water and should be further addressed; for example, the anode needs to have a sufficient alkalinity when using a high-concentration NaCl catholyte [10], or the waste sulfate from the acidified water should be properly disposed.
4.
Conclusions
Hydrogen production in an MEC is affected by the pH buffering capacity and the electrolyte conductivity of a catholyte. The 100-mM PBS catholyte exhibited the best buffering capacity and also led to the highest hydrogen production rate, followed by the NaCl solution with a high electrolyte conductivity and the acidified water that supplied additional protons to the cathode reaction. The tap/DI water, the low-concentration PBS and the acidified water at pH 4 had the lowest hydrogen production rates. The reuse of the cathode for an extended period led to decreased hydrogen production rates, but could be justified by cost saving. PBS may not be a good choice as an MEC catholyte because of its high cost and potential environmental effects. Both NaCl and acidified water could be considered as better choices. Future applications of those catholytes in MECs will need to consider issues such as the catholyte cost (with further reductions), the catholyte reuse, and the disposal.
Acknowledgments This work was financially supported by the Research Group for the Development of Microbial Hydrogen Production Processes from Biomass, Office of the Higher Education Commission, Thailand, and a research grant from the National Science Foundation (Award 1033505). We also thank Dr. Marjorie Piechowski (UW-Milwaukee) for proofreading the manuscript.
references
[1] Suwansaard M, Choorit W, Zeilstra-Ryalls JH, Prasertsan P. Isolation of anoxygenic photosynthetic bacteria from Songkhla Lake for use in a two-staged biohydrogen production process from palm oil mill effluent. Int J Hydrogen Energy 2009;34:7523e9. [2] Manish S, Banerjee R. Comparison of biohydrogen production processes. Int J Hydrogen Energy 2008;33:279e86.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
6
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 x x x ( 2 0 1 3 ) 1 e6
[3] Liu H, Hu H, Chignell J, Fan Y. Microbial electrolysis: novel technology for hydrogen production from biomass. Biofuels 2010;1:129e42. [4] Kundu A, Sahu JN, Redzwan G, Hashim MA. An overview of cathode material and catalysts suitable for generating hydrogen in microbial electrolysis cell. Int J Hydrogen Energy 2013;38:1745e57. [5] Call D, Logan BE. Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane. Environ Sci Technol 2008;42:3401e6. [6] Cusick RD, Bryan B, Parker DS, Merrill MD, Mehanna M, Kiely PD, et al. Performance of a pilot-scale continuous flow microbial electrolysis cell fed winery wastewater. Appl Microbiol Biotechnol 2011;89:2053e63.
[7] Zhang F, Jacobson KS, Torres P, He Z. Effects of anolyte recirculation rates and catholytes on electricity generation in a liter-scale upflow microbial fuel cell. Energy Environ Sci 2010;3:1347e52. [8] Xiao L, Wen Z, Ci S, Chen J, He Z. Carbon/iron based nanorod catalyst for hydrogen production in microbial electrolysis cells. Nano Energy 2012;1:751e6. [9] Escapa A, Go´mez X, Tartakovsky B, Mora´n A. Estimating microbial electrolysis cell (MEC) investment costs in wastewater treatment plants: case study. Int J Hydrogen Energy 2012;37:18641e53. [10] Nam J-Y, Logan BE. Optimization of catholyte concentration and anolyte pHs in two chamber microbial electrolysis cells. Int J Hydrogen Energy 2012;37:18622e8.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Short Communication
Hydrogen production in microbial electrolysis cells: Choice of catholyte Siriporn Yossan a, Li Xiao b, Poonsuk Prasertsan c,d, Zhen He b,* a
Program of Science, Faculty of Liberal Arts and Science, Sisaket Rajabhat University, Sisaket 33000, Thailand Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA c Department of Industrial Biotechnology, Faculty of Agro-Industry, Prince of Songkla University, Songkhla 90112, Thailand d Palm Oil Products and Technology Research Center (POPTEC), Faculty of Agro-Industry, Prince of Songkla University, Songkhla 90112, Thailand b
article info
abstract
Article history:
A catholyte is a key factor to hydrogen production in microbial electrolysis cells (MECs).
Received 18 January 2013
Among the four groups of catholytes investigated in this study, a 100 mM phosphate buffer
Received in revised form
solution (PBS) resulted in the highest hydrogen production rate of 0.237 0.031 m3H2/m3/d,
16 May 2013
followed by 0.171 0.012 m3H2/m3/d with a 134 mM NaCl solution and 0.171 0.004 m3H2/
Accepted 19 May 2013
m3/d with the acidified water adjusted with sulfuric acid. The MEC with all catholytes
Available online xxx
achieved good organic removal efficiency, but the removal rate varied following the trend of the hydrogen production rate. The reuse of the catholyte for an extended period led to a
Keywords:
decreasing hydrogen production rate, affected by the elevated pH. The cost of both the
Hydrogen
acidified water and the NaCl solution was much lower than the PBS, and therefore, they
Microbial electrolysis cells
could be a better choice as an MEC catholyte with further consideration of cost reduction
Catholyte
and chemical reuse/disposal.
Buffer
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is a promising alternative to fossil fuel and is considered as a clean fuel because its combustion generates no pollutants but water [1]. Hydrogen gas can be produced by both thermochemical (pyrolysis/gasification) and biological (biophotolysis, dark and photo fermentation) processes [2]. In recent years, a new method of hydrogen production has been developed based on bioelectrochemical principles in microbial electrolysis cells (MECs). The advantage of MEC technology is to produce hydrogen gas from a non-fermenting
substrate (e.g., in wastewater) with a very low input of external energy. Therefore, MECs can function as an energyefficient process for simultaneous waste treatment and energy (hydrogen) production. Our understanding of MECs has been greatly advanced by intensive research on microbiology, substrates, and reactor configuration and operation [3,4]. MECs usually contain two chambers, an anode and a cathode separated by an ion exchange membrane (IEM). A single-chamber MEC was developed by omitting IEM to considerably simplify the reactor structure and potentially improve hydrogen production by
* Corresponding author. Tel.: þ1 414 229 5846; fax: þ1 414 229 6958. E-mail address: [email protected] (Z. He). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
2
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 x x x ( 2 0 1 3 ) 1 e6
reducing the MEC’s internal resistance [5]. However, a great concern with the single-chamber MECs is methane production, which is caused by methanogens inhabiting the adjacent cathode and has been observed in several MECs including a pilot test [6]. Therefore, to produce high-purity hydrogen gas in an MEC, IEM may still be a preferable separator to prevent microbial contamination in the cathode. With an IEM and a separated cathode chamber, a simple but important question arises: what should be the catholyte? A commonly used catholyte is a phosphate buffered solution (PBS), which can minimize the pH elevation and provide a certain electrolyte conductivity; however, the use of PBS may not be economical and environmentally friendly. The anode effluent (treated wastewater) could be a low-cost catholyte, which has been studied in microbial fuel cells (MFCs) [7], but it can introduce microorganisms including methanogens into the cathode chamber, resulting in methane production. Thus, we should avoid any catholytes containing microbial sources. In addition to PBS, several other catholytes have been studied, including salt solutions and bicarbonate buffers. In general, there is very limited information available about the comparison between these and other potential catholytes in terms
of performance and cost. In this study, we aim to examine four different catholytes and provide suggestions on optimal choices of a catholyte in an abiotic cathode for sustainable hydrogen production in an MEC.
2.
Materials and methods
2.1.
MEC set up and operation
An MEC was made from two glass bottles, which were joined by a cation exchange membrane (Membranes International, Inc., GlenRock, NJ, USA). Each bottle had a liquid volume of 120 mL. The anode electrode was a carbon brush (Gordon Brush Mfg. Co., Inc., Commerce, CA, USA), and the cathode electrode was a piece of carbon cloth (Zoltek Corporation, St. Louis, MO, USA) with a length of 4 cm and a width of 3 cm. The cathode electrode was coated with Pt as a catalyst (0.5 mg Pt/ cm2) [8]. Both the anolyte and the catholyte were continuously mixed by magnetic stirrers. An external voltage (0.8 V) was applied to the circuit by connecting the negative pole of a power supply (3644 A, Circuit Specialists, Inc., Mesa, AZ, USA)
Fig. 1 e Current generation in the MEC with different catholytes: (A) 100, 50, 25 and 10 mM PBS; (B) 134, 100 and 200 mM NaCl; (C) DI and tap water; and (D) acidified water at pH 4 and pH 2 adjusted by H2SO4 or HCl.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
3
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 x x x ( 2 0 1 3 ) 1 e6
Table 1 e Coulombic efficiencies and hydrogen production in the MEC with different catholytes. Catholytes
Time period (day)
Coulombic recovery: CR (%)
0.10 0.06 0.07 0.07
34.6 18.7 13.2 8.0
100 mM PBS 50 mM PBS 25 mM PBS 10 mM PBS
1.44 1.06 1.11 1.29
134 mM NaCl 100 mM NaCl 200 mM NaCl
2.01 0.12 1.55 0.06 1.30 0.07
26.5 1.5 19.8 1.4 19.7 3.5
DI water Tap water
5.10 1.47 3.14 0.61
42.8 6.8 10.9 2.6
Acidified water (pH 4)a Acidified water (pH 2)a Acidified water (pH 2)b
2.81 0.18 1.19 0.05 1.37 0.07
11.0 2.1 21.5 0.8 18.5 2.0
Cathodic H2 recovery: Rcat (%)
1.0 2.0 0.4 0.5
89.2 89.2 80.1 69.9
Overall H2 recovery: RH2 (%)
9.2 6.6 3.7 8.1
30.8 16.7 10.6 5.6
120.4 4.4 91.2 3.4 93.7 5.0
H2 production rate: QH2 (m3H2/m3/d)
2.3 1.4 0.8 0.3
0.237 0.165 0.100 0.048
0.031 0.005 0.007 0.007
31.8 1.2 18.0 0.8 18.4 2.8
0.171 0.012 0.116 0.004 0.143 0.009
67.9 12.6 93.8 22.1
28.5 1.4 9.8 0.1
0.065 0.021 0.034 0.007
156.3 17.3 84.7 1.8 65.5 8.4
17.0 1.3 18.2 0.5 12.0 0.8
0.065 0.002 0.171 0.004 0.089 0.002
a Adjusted by sulfuric acid. b Adjusted by hydrochloric acid.
to an external resistor (10 U), then to the cathode electrode, and the positive pole to the anode electrode. The MEC was operated in a fed-batch mode at 20 C. The anode chamber was inoculated using anaerobic sludge from South Shore Water Reclamation Facility (Milwaukee, WI, USA). The anode feeding solution was prepared in tap water consisting of 1.0 g/L sodium acetate, 0.3 g/L NH4Cl, 1.0 g/L NaCl, 0.03 g/L MgSO4, 0.04 g/L CaCl2, 0.2 g/L NaHCO3, 5.3 g/L KH2PO4, 10.7 g/L K2HPO4 and 1 mL/L trace elements [8]. Four groups of catholytes were examined in the MEC: PBS, NaCl solution, water, and acidified water. The PBS was tested in four concentrations, 10, 25, 50, and 100 mM. The NaCl solution was prepared by dissolving NaCl in tap water to 100, 134, and 200 mM. Two waters, tap water and deionized (DI) water, were studied. The acidified water was prepared in two pHs, 2 and 4, using hydrochloric acid (HCl) or sulfuric acid (H2SO4). The catholyte was completely replaced when the voltage dropped below 2 mV. The new catholyte was flushed with nitrogen gas
for 15 min to remove any oxygen before starting MEC operation. The anolyte was replaced by 80% with a fresh anode feeding solution at the same time when the catholyte was replaced. During the extended-catholyte study, the anolyte was replaced when the voltage decreased below 2 mV, but the catholyte was not replenished.
2.2.
Measurement and analysis
The voltage was monitored and recorded every 5 min by a digital multimeter (Keithley Instruments Co., Ltd., USA). The concentration of chemical oxygen demand (COD) was analyzed by a DR/890 colorimeter (HACH Co., Ltd., USA). The pH was measured using a bench-top pH meter (Oakton Instruments Co., Ltd., USA) and the electrical conductivity was measured by a bench-top conductivity meter (Mettler-Toledo Co., Ltd., USA). The production of hydrogen gas was measured by water replacement and analyzed by using a gas
Table 2 e COD removal in anolyte and characteristics of effluent catholyte with different catholytes. Catholytes
COD removal (%)
COD removal rate (g COD/m3/d)
Conductivity (ms/cm) Initial
94.6 97.0 97.5 97.2
134 mM NaCl 100 mM NaCl 200 mM NaCl
96.7 0.7 95.9 1.8 96.7 0.2
0.37 0.03 0.44 0.02 0.54 0.05
12.2 0.1 12.0 0.1 11.9 0.1
13.88 0.24 7.68 0.66 20.00 0.20
21.07 0.65 15.28 0.92 25.05 0.30
DI water Tap water
94.6 2.3 86.6 0.8
0.15 0.04 0.24 0.05
12.6 0.1 11.6 0.4
0.01 0.00 0.30 0.02
1.56 0.08 2.53 0.50
Acidified water (pH 4)a Acidified water (pH 2)a Acidified water (pH 2)b
97.4 0.1 98.2 0.3 96.8 0.2
0.27 0.01 0.66 0.02 0.51 0.03
11.9 0.4 9.0 0.7 11.0 0.4
0.05 0.01 7.37 0.05 5.46 0.02
2.94 0.38 3.61 0.25 3.27 0.49
0.04 0.07 0.06 0.05
9.2 8.9 10.8 11.3
0.2 0.2 0.2 0.2
13.97 8.52 4.06 1.44
Effluent
100 mM PBS 50 mM PBS 25 mM PBS 10 mM PBS
0.3 0.4 0.4 0.4
0.54 0.69 0.66 0.59
Effluent pH
0.01 0.80 0.04 0.48
12.15 9.67 5.40 2.63
0.23 0.35 0.30 0.57
a Adjusted by sulfuric acid. b Adjusted by hydrochloric acid.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
4
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 x x x ( 2 0 1 3 ) 1 e6
chromatograph (Focus GC, Thermo Scientific, USA). The key parameters were calculated as previously described [8]: the coulombic recovery (CR) is defined as the total output coulombs over the total input coulombs in acetate; the cathodic hydrogen recovery (Rcat) is the ratio between the electrons contained in the produced hydrogen gas and the electrons produced as current; the overall hydrogen recovery ðRH2 Þ is the substrate used for the hydrogen production; and the hydrogen production rate ðQH2 Þ is the hydrogen production per anode working volume per time.
3.
Results and discussion
The MEC performance with different catholytes was described by using three main parameters, electricity generation, hydrogen production, and COD removal.
Fig. 1 shows the profile of batch current generation with four groups of catholytes at different concentrations. In general, the PBS catholytes with higher concentrations had a higher peak current, followed by the acidified water (pH 2), and the NaCl solution. Both tap water and DI water resulted in very low peak currents. Decreasing the PBS concentration from 100 to 10 mM also decreased the peak current from 31.1 1.5 to 8.2 0.4 A/m3 (Fig. 1(A)), while increasing the NaCl concentration from 100 to 200 mM slightly improved the peak current from 15.0 0.5 to 18.7 1.1 A/m3 (Fig. 1(B)). The exceptional performance of the 100-mM PBS is due to its buffer capacity and conductivity (Table 2). The high current with the NaCl catholyte is because of its high conductivity (Table 2), while the high current with the acidified water is likely due to the additional proton input. The higher conductivity of the tap water over that of the DI water did not result in more current production: the current generation with the DI
Fig. 2 e Current generation (white dot) and hydrogen production rate (white square) in the MEC in the extended period of catholyte reuse: (A) 100 mM PBS; (B) 134 mM NaCl; and (C) acidified water at pH 2 adjusted by H2SO4.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
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 x x x ( 2 0 1 3 ) 1 e6
water (11.7 0.1 A/m3) was twice that with the tap water (5.5 0.4 A/m3), possibly because of the pH influence: the initial pH of the DI water (4.7 0.0) was much lower than that of the tap water (7.3 0.3), indicating that the proton supply may have a more important role in current generation than the electrolyte conductivity. When the initial pH was similar, for example, with the pH-2 water prepared by H2SO4 or HCl, a higher influent conductivity of the H2SO4 catholyte led to a higher peak current of (31.1 0.8 A/m3) than that of 25.7 0.3 A/m3 with the HCl catholyte. Hydrogen production efficiencies and rates with different catholytes are summarized in Table 1. The efficiency of the substrate-to-hydrogen (overall hydrogen recovery) varied between 5% and 32%; interestingly, the DI water and the tap water had comparable overall hydrogen recovery, as well as coulombic recovery, with the other catholytes. However, it took a much longer time (3e5 days) for the MEC with the DI or tap water to complete a cycle, resulting in a much lower hydrogen production rate. For the catholytes of the PBS, the NaCl solution or the acidified water, the hydrogen production rates matched their current generation, and a higher current led to a higher hydrogen production rate. The primary function of an MEC using wastewater as its anode substrate is considered to be contaminant removal; thus, COD reduction is a key parameter to evaluate the MEC performance. In general, the MEC achieved more than 94% of COD removal with all catholytes except the tap water (Table 2), demonstrating a good performance in organic removal. However, the COD removal rate was clearly affected by current generation and the period of a batch cycle, and the catholytes such as the DI water and the tap water with low hydrogen rates also yielded low COD removal rates. We chose three catholytes, 100-mM PBS, 134-mM NaCl, and acidified water pH 2 (H2SO4), and conducted an extended study without replacing the catholyte. The peak current densities with the PBS and the acidified water clearly decreased over time, but the NaCl maintained a relatively stable current output. The hydrogen production rates, on the other hand, all decreased in the extended operating period (Fig. 2). Interestingly, the hydrogen production rate with the acidified water showed a dramatic decrease in the second day with a significant increase in pH from 2.0 to 11.2, and then slowly recovered in the following six days with a further increase of pH to 12.6 (Fig. 2(C)). The pH of the PBS increased from 7.3 to 12.3 in ten days and the NaCl catholyte had an elevated pH from 7.5 to 12.9 in six days. The final conductivity of the acidified water was 12.57 mS/cm, half of that of the PBS or NaCl catholytes. All three catholytes had a similar hydrogen production rate of 0.10 m3H2/m3/d at the end of their testing period, indicating that pH had more influence than catholyte conductivity on hydrogen production. The cost of a catholyte was estimated from the amount of the chemicals used and their bulk prices obtained from SigmaeAldrich (St. Louis, USA). For an operation with frequent replacement of the catholyte (as shown in Fig. 1), it would cost $3.39/m3 H2 with the 100-mM PBS, much higher than the hydrogen price ($0.42/m3 estimated from $4.75/kg H2 [9]). The use of the 134-mM NaCl or the acidified water (pH ¼ 2 adjusted by sulfuric acid) would cost $0.29/m3 and $0.21/m3, respectively.
5
In an extended use of the catholyte (as shown in Fig. 2), the cost of those three catholytes (calculated from the first four cycles) decreased because of the reuse: $1.50/m3 with the 100-mM PBS, $0.19/m3 with the 134-mM NaCl, and $0.11/m3 with the acidified water (pH ¼ 2). The cost can be even less with additional operating cycles using the same catholyte (Fig. 2(A) and (C)) and lower prices of raw chemicals in a larger quantity. Both the NaCl solution and the acidified water may be a better choice as an MEC catholyte than the PBS, because of the significantly lower cost. The use of PBS can also potentially cause environmental problems by releasing phosphorus into natural water bodies. Of course, challenges also exist with the use of the NaCl or the acidified water and should be further addressed; for example, the anode needs to have a sufficient alkalinity when using a high-concentration NaCl catholyte [10], or the waste sulfate from the acidified water should be properly disposed.
4.
Conclusions
Hydrogen production in an MEC is affected by the pH buffering capacity and the electrolyte conductivity of a catholyte. The 100-mM PBS catholyte exhibited the best buffering capacity and also led to the highest hydrogen production rate, followed by the NaCl solution with a high electrolyte conductivity and the acidified water that supplied additional protons to the cathode reaction. The tap/DI water, the low-concentration PBS and the acidified water at pH 4 had the lowest hydrogen production rates. The reuse of the cathode for an extended period led to decreased hydrogen production rates, but could be justified by cost saving. PBS may not be a good choice as an MEC catholyte because of its high cost and potential environmental effects. Both NaCl and acidified water could be considered as better choices. Future applications of those catholytes in MECs will need to consider issues such as the catholyte cost (with further reductions), the catholyte reuse, and the disposal.
Acknowledgments This work was financially supported by the Research Group for the Development of Microbial Hydrogen Production Processes from Biomass, Office of the Higher Education Commission, Thailand, and a research grant from the National Science Foundation (Award 1033505). We also thank Dr. Marjorie Piechowski (UW-Milwaukee) for proofreading the manuscript.
references
[1] Suwansaard M, Choorit W, Zeilstra-Ryalls JH, Prasertsan P. Isolation of anoxygenic photosynthetic bacteria from Songkhla Lake for use in a two-staged biohydrogen production process from palm oil mill effluent. Int J Hydrogen Energy 2009;34:7523e9. [2] Manish S, Banerjee R. Comparison of biohydrogen production processes. Int J Hydrogen Energy 2008;33:279e86.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094
6
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 x x x ( 2 0 1 3 ) 1 e6
[3] Liu H, Hu H, Chignell J, Fan Y. Microbial electrolysis: novel technology for hydrogen production from biomass. Biofuels 2010;1:129e42. [4] Kundu A, Sahu JN, Redzwan G, Hashim MA. An overview of cathode material and catalysts suitable for generating hydrogen in microbial electrolysis cell. Int J Hydrogen Energy 2013;38:1745e57. [5] Call D, Logan BE. Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane. Environ Sci Technol 2008;42:3401e6. [6] Cusick RD, Bryan B, Parker DS, Merrill MD, Mehanna M, Kiely PD, et al. Performance of a pilot-scale continuous flow microbial electrolysis cell fed winery wastewater. Appl Microbiol Biotechnol 2011;89:2053e63.
[7] Zhang F, Jacobson KS, Torres P, He Z. Effects of anolyte recirculation rates and catholytes on electricity generation in a liter-scale upflow microbial fuel cell. Energy Environ Sci 2010;3:1347e52. [8] Xiao L, Wen Z, Ci S, Chen J, He Z. Carbon/iron based nanorod catalyst for hydrogen production in microbial electrolysis cells. Nano Energy 2012;1:751e6. [9] Escapa A, Go´mez X, Tartakovsky B, Mora´n A. Estimating microbial electrolysis cell (MEC) investment costs in wastewater treatment plants: case study. Int J Hydrogen Energy 2012;37:18641e53. [10] Nam J-Y, Logan BE. Optimization of catholyte concentration and anolyte pHs in two chamber microbial electrolysis cells. Int J Hydrogen Energy 2012;37:18622e8.
Please cite this article in press as: Yossan S, et al., Hydrogen production in microbial electrolysis cells: Choice of catholyte, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.094