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Enhancing hydrogen production efficiency in microbial electrolysis cell with membrane electrode assembly cathode
Journal of Industrial and Engineering Chemistry 18 (2012) 715–719
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Enhancing hydrogen production efficiency in microbial electrolysis cell with membrane electrode assembly cathode Yu Hong Jia a, Jae Hun Ryu a, Cho Hui Kim a, Woo Kyung Lee a, Thi Van Trinh Tran a, Hyo Lee Lee a, Rui Hong Zhang b, Dae Hee Ahn a,* a b
Department of Environmental Engineering and Biotechnology, Myongji University, San 38-2 Namdong, Yongin, Kyonggido 449-728, Republic of Korea Department of Biological and Agricultural Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
A R T I C L E I N F O
Article history: Received 28 April 2011 Accepted 11 July 2011 Available online 12 November 2011 Keywords: Microbial electrolysis cell Hydrogen production Membrane electrode assembly Organic removal Stainless steel mesh
A B S T R A C T
Microbial electrolysis cell is a device which can produce hydrogen gas from biomass through microbial catalyzed process and thus reduce the organic matter. For the real application in wastewater treatment, the scale-up of microbial electrolysis cell is an important issue but few tests were conducted with relatively large size. In this study, a 3.7 L microbial electrolysis cell (liquid volume 3.2 L) equipped with a membrane electrode assembly cathode was designed and tested. The internal resistance was examined, hydrogen generation and organic removal performance was investigated under different conditions. A maximum overall hydrogen efficiency of 41% was achieved at an applied voltage of 1.2 V with acetate as substrate, corresponding to a volumetric hydrogen production rate of approximately 0.12 m3 H2/m3 reactor liquid volume/day. The results obtained in this study could help to further develop pilot-MEC for practical applications. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Renewable energy production technologies are currently gaining great attraction in the society, due to the depletion of fossil fuels and the threat of global warming. Microbial fuel cells (MFCs) have been discovered as an innovative method for renewable energy production simultaneously with organic compounds removal from wastewater [1,2]. In the MFC system, the produced electrons by the oxidation of substrate are transferred to the anode, which then flows to the cathode through the external wire, where they normally combine with oxygen and protons to form water [3–6]. While in MFCs, the maximum voltage that could be obtained is less than 1.14 V [7,8], the voltage limitation restricted the energy recovery. The need of oxygen in the cathode chamber is another disadvantage of MFC [9], for oxygen may leak into the anode chamber through the membrane, either lower the energy recovery or inhibit the growth of obligate anaerobes [10,11]. Based on the MFCs technologies, by removing the oxygen from the cathode chamber and applying a small voltage to the circuit, hydrogen can be produced directly from protons and electrons transferred from the anode [10]. This hydrogen evolution process is
* Corresponding author. Tel.: +82 31 330 6692; fax: +82 31 336 6336. E-mail address: [email protected] (D.H. Ahn).
termed microbial electrolysis cells (MECs). As the system is operated as MECs for hydrogen production, the voltage limitation of MFCs could be bypassed. Meanwhile, the use of oxygen could be omitted, because MECs are completely anaerobic systems. The MEC enables the possibility of direct fuel production from a diverse range of waste streams [12]. In additional, hydrogen is an entirely carbon-free fuel with a high combustion enthalpy of 185 kJ/L [13,14], which considered to be a feasible alternative to fossil fuels [15,16]. Most MECs contain a membrane, although membrane could bring up the pH gradient and thus the cell internal resistance. In the MEC system, the presence of a membrane is essential to increase the coulombic efficiency, and also for the purity of the hydrogen that is produced at the cathode. Without the membrane, the produced hydrogen could be polluted with gaseous metabolic products from the anode chamber, such as CO2, CH4 or H2S. Furthermore, a significant amount of the produced hydrogen will be lost by hydrogen consumption by methanogens growing on the electrodes or in the solution [17]. Membrane electrode assembly (MEA) has been investigated in MFCs and MECs. MEA plays a key role as an electrolyte medium for ion transport and a barrier to avoid the direct contact between anolyte and catholyte, and also significantly reduce the electrode spacing and lower the internal resistance, thus improve the reactor performance [18]. Previous study proved that the use of MEA (Nafion cation exchange membrane hot-press to carbon cloth
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.127
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cathode) could increase the coulombic efficiency from 9–12% (MFC lacking a membrane) to 40–50% in MFC [19]. However, the expense of Nafion ion exchange membrane used in normal MEA is a barrier to scale-up and develop, and the relatively cost-effective membranes such as the supported heterogeneous membranes with polyvinylidenefluoride (PVDF) binder have been reported as being unsuitable for hot pressing as they are not heat weldable [20]. A cost-effective MEA fabrication for MEC was developed in this study. Stainless steel mesh was used to press the cathode on to the membrane, and the use of steel mesh avoided the need to hot-press membrane to the cathode. The goal of this research is to determine the effectiveness of the MEC employing a MEA cathode which mechanically combined a cost-effective polymer based ion exchange membrane and a cathode electrode. The performance of the MEC was investigated by examining the internal resistance, and by monitoring the current and hydrogen output. 2. Materials and methods 2.1. Experimental set-up The configuration of MEC used in this study was shown in Fig. 1(a). The two-chamber reactor was separated by a cation exchange membrane (projected area 192.5 cm2, CMI-7000, Membrane International Inc., USA), membrane was used as purchased. The empty bed volume of anodic compartment was 3.7 L. While eight anode modules made of hexahedral stainless steel mesh filled with graphite granules (2–7 mm diameter, a void fraction of 50%, 100 cm3 in volume, Qingdao Grand Graphite Products Co., Ltd., China) were established into the anode chamber, given a net liquid volume of 3.2 L. The cathode consisted of a B1A plain carbon cloth (projected area 160 cm2, no wet proofing, E-TEK Division, Somerset,
New Jersey), with 0.5 mg/cm2 platinum coating as catalyst. The catalyst was applied to the cathode as follows: a commercial Pt catalyst (20 wt% Pt/C) was mixed with 5% Nafion solution in a ratio of 6.67 mL Nafion per mg Pt/C powder; coat the mixture to the membrane faced side of the cathode; dry the cathode at room temperature for 24 h. Cathode was placed above the membrane; stainless steel mesh was placed above the cathode and firmly pressed the cathode onto the membrane. The cathodic chamber was used as a gas chamber for hydrogen collection with a volume of about 1 L. Reactor was covered with aluminum foil to exclude light. The photograph of experimental apparatus was given in Fig. 1(b). 2.2. Inoculation The anode modules were inoculated in a conventional MFC for more than 7 months with acetate as substrate, and then transferred to the MEC for further experiment. The anode chamber was fed with synthetic wastewater modified from Lee et al. [21] with phosphate concentration of 50 mM (pH 7.0) and sodium acetate as sole substrate. The reactor was operated at room temperature during the whole experiment. When operated at MEC mode, the cathodic chamber was flushed with N2 gas prior to the experiment to get a totally anaerobic atmosphere. 2.3. Analytical methods The system voltage was measured (30 min intervals) with a multimeter and a data acquisition system (Keithley 2700, USA) connected to a personal computer. The anode potential was measured by a reference electrode (Ag/AgCl, +200 mV vs. a standard hydrogen electrode, SHE) equipped to anode chamber which was also recorded by the multimeter. A voltage range from 0.1 V to 1.4 V was applied to the circuit using a multichannel potentiostat (WMPG1000, WonATech, Korea) by connecting the counter and reference poles to the anode, and the working pole to the cathode. Polarization curves were examined to evaluate the electrode potential in function of the current production, which were obtained with potentiostat by increasing the applied voltage by 0.03 V increments every 10 min from the voltage obtained in open circuit to minus value. The ohmic cell resistance was determined with the current interrupt method based on cell voltage change in 0.01 s recorded by the potentiostat after cutting off the cell current. The pH of the electrolyte was measured with a pH meter (ion LAB WTW). The chemical oxygen demand (COD) was quantified using Hach COD measurement system and kit, all procedure for pre-treatment of samples was done in accordance to the standard method. The hydrogen concentration was analyzed using a gas chromatograph (GC-17A, Shimadzu, Japan) equipped with a thermal conductivity detector (TCD). A Molecular Sieve 5A column (Alltech Corporate, USA) with argon as carrier gas was used to separate the gas. 2.4. Calculations
Fig. 1. Schematic diagram and photograph of the microbial electrolysis cell.
The coulombic hydrogen recovery is calculated as: rCE = nCE/nth. nCE is the moles of hydrogen that could be recovered from the measured current, given by: nCE = CP/2F, where CP is total coulombs calculated by integrating the current over time, F is Faraday’s constant (96,485 C/mol electrons). nth is the total theoretical number of moles of hydrogen that could be recovered based on substrate. The cathodic hydrogen recovery is deduced as follows: r Cat ¼ nH2 =nCE , where nH2 is the number of moles of hydrogen recovered during a batch cycle. The overall hydrogen recovery is calculated according to: RH2 ¼ r CE r Cat [22]. The energy added to the system by power source (WE) can be P calculated as: W E ¼ n1 ðIEa p DtÞ, where Eap is the voltage applied
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by the potentiostat, I is current examined in the circuit, and Dt is the time increment of n data points measured during a batch cycle. Energy balances approach based on heats of combustion, which is commonly used for electrolyzers and for estimating the energy content of organic matter, were used in this study. The amount of energy added by the substrate is given by: WS = DHSnS, where DHS = 870.28 kJ/mol is the heat combustion of the substrate, and ns is the number of moles of substrate consumed during a batch cycle based on COD removal. The amount of energy output as hydrogen is calculated as: W H2 ¼ DHH2 nH2 , where DHH2 ¼ 285:83 KJ=mol is the energy content of the hydrogen based on the heat of combustion (upper heating value), and nH2 is the number of moles of hydrogen produced in a batch cycle. 3. Results and discussion 3.1. Inoculation in MFC mode The anode modules were transferred from a conventional liquid-cathode MFC operated for more than 7 months with oxygen as electron acceptor and acetate as substrate. After transforming, the MEC reactor was first operated in MFC mode for further inoculation with the gas collection cathodic chamber open to air. The performance was tested with polarization behavior (Fig. 2) after stabilization of the voltage production. The maximum power density recorded was 0.25 W/m2 at a current of 25.8 mA in this test. Based on the current–voltage curve, internal resistance was calculated to be 6.1 V. The power density was maintained over 2 months without any deterioration of performance. The results proved that the reactor was successfully inoculated and could proceed to the hydrogen production test. The ohmic cell resistance was further determined with the current interrupt method. At a relatively steady state, when the current is suddenly switching off, the electric double layer will take some time to disperse, which causes the associated overvoltage change gradually. Among them, voltage difference caused by ohmic losses (Ur) will be immediately observed, followed by a slower voltage increase caused by the activation losses (Ua) [23]. As the interruption process was completed within 0.01 s, it could be considered as instantaneous. The ohmic resistance was calculated from R = Ur/I, where I represents the steady state current before interruption. Fig. 3 shows the voltage change over time after interruption at a current of 20 mA, and the internal resistance calculated based on the Ur value was 7 V. Interruption processes were done at current of 5 mA, 10 mA, 29 mA and 38 mA, respectively, similar internal resistance of 7.03 V in average was obtained.
Fig. 2. Polarization curve obtained in MFC mode.
Fig. 3. Voltage changes over time after a current interruption.
High internal resistance is a key problem limiting the hydrogen output of the microbial electrolysis cell [24]. With the innovative design of MEA in this study, the internal resistance was decreased to about 7 V with a relatively large reactor size of 3.7 L, which is lower than the 19 V obtained in a membrane free MEC [25], as it was reported that the absence of membrane could decrease the internal resistance. Tartakovsky et al. [25] achieved a internal resistance of 27 V with MEC (liquid volume 50 mL) employing a MEA (Nafion 117 proton exchange membrane hot-pressed onto carbon cloth cathode with 0.5 mg/cm2 Pt coating), which is higher than the internal resistance obtained in this study. Zhang et al. [26] showed in a MFC equipped with MEA fabrication (similar MEA design as in this study), the internal resistance obtained without a mesh was about 49.4 V, and the value decreased to 16.4 V when stainless steel mesh was used to press the membrane onto the cathode to control the membrane deformation. The results demonstrated that the MEA fabricated in this study could decrease the internal resistance, thus help to improve the MEC performance. 3.2. Hydrogen production in MEC mode The reactor was then transferred to MEC mode by apply an external voltage of 1.0 V to the circuit. Once stable hydrogen production was observed, voltage scans were carried out by stepwise increase the voltage from 0.1 V to 1.4 V (Fig. 4). When acetate was present in the anode chamber, the current significantly increased from 0 to 66.6 mA with the increasing of applied voltage, while only background value of current was obtained during the voltage scan process with none substrate present. The result indicated that the examined current arose from the acetate oxidation in the anodic chamber. Fig. 5 shows the performance of MEC in anode potential, current and H2 production with organic removal at an applied voltage of 1.1 V. The MEC was operated in batch mode and 50 mM acetate was fed to the anodic chamber as sole carbon source. A current of 39.4 mA in average was developed in MEC up to the substrate addition, stabilized over 6 days, and decreased to background value as a result of the complete consumption of acetate. Almost all acetate as COD compounds was consumed (91.1%) in the batch test. The measured value of hydrogen production was used to calculate the process efficiencies. Measurements of current and hydrogen production were done with batch mode operation of MEC under different applied voltages (Fig. 6). Based on the 50 mM acetate injected as substrate, the current output in the batch tests were in the same range as predicated by the applied voltage scans. Overall, at applied voltage
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Fig. 6. Current and hydrogen production as a function of applied voltage. Fig. 4. Current obtained in voltage scans with 10 min intervals between voltage changes. Background values were obtained by excluding acetate from the stock solution.
less than 1.2 V, the current and hydrogen production increased in response to the increasing voltage. A maximum current of 44.5 mA (2.8 A/m2) and a highest hydrogen production of 1.83 L were both obtained at 1.2 V, and the value decreased with the further increase of voltage to 1.4 V. In this study, the performance of hydrogen production at voltages below 0.8 V was limited, this go along with the results obtained in previous studies [24,25], which might be due to the high ohmic potential loss in large reactor, even though the ohmic resistance was relatively low. For the electrical currents that flow through the cells were larger (2.1–2.8 A/m2), the
Fig. 5. The performance of MEC in anode potential, current and H2 production with COD removal at an applied voltage of 1.1 V.
ohmic potential losses caused by the resistance was higher than the one with same resistance in smaller systems [24]. Fig. 7 presents the coulombic efficiency, cathodic efficiency and overall hydrogen efficiency with different applied voltages. The overall hydrogen efficiency increased with the increasing of applied voltage, and achieved a maximum of 41% at the applied voltage of 1.2 V, corresponding to a volumetric hydrogen production rate of 0.12 m3-H2/m3-day and a hydrogen yield of 1.64 mol H2/mol acetate. The hydrogen yield achieved in this study is comparable to the one obtained by Tartakovsky et al. [25], where a hydrogen yield of 1.3 mol H2/mol acetate was developed at a voltage of 1.15 V from MEC equipped with a MEA (Nafion 117 proton exchange membrane hot-pressed onto carbon cloth cathode with 0.5 mg/cm2 Pt coating). The overall hydrogen yield of 35% achieved at 1.0 V in this system is higher than the 23% obtained by Rozendal et al. [24] in a MEC with similar reactor size (liquid volume 3.3 L) as in this study; while Nafion 117 membrane and Ti-mesh (5 mg Pt/cm2 coated) was used to fabricated their MEA. Applied voltage between 1.0 and 1.2 V was determined as the optimum for the hydrogen production, where the total hydrogen efficiency was over 35%. The coulombic efficiency, defined as the recovery of total electrons in acetate as current, was over 54% at applied voltage ranged 1.0–1.2 V. Coulombic efficiency of 55–71% was obtained in a MFC (equipped with a MEA) [20], which is comparable as this study.
Fig. 7. Coulombic efficiency, cathodic efficiency and overall hydrogen efficiency as a function of applied voltage.
Y.H. Jia et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 715–719
At an applied voltage of 0.9 V, the energy efficiency based on electrical energy input was over 102%. The acetate removal rates obtained in the batch operation were ranged 81.3–92.4%, and the volumetric acetate removal rates were calculated to be 0.18– 0.32 g-acetate/m3-day. The acetate removal rate was proportional to applied voltage (data not shown), confirming that acetate was consumed by anodophilic microorganisms. The results demonstrated that organic compounds from wastewater could be feasibly treated based on this MEC system. 4. Conclusions This study investigated the hydrogen generated from a 3.7 L microbial electrolysis cell employing a cost-effective MEA fabrication. The use of MEA allowed for reduced distance between the electrodes, thus reduced the internal resistance and increased the hydrogen production. An internal resistance of 7 V was examined from the reactor with the relatively large working volume. The MEC achieved a coulombic efficiency and cathodic efficiency of 55% and 74% respectively at an applied voltage of 1.2 V, results an overall hydrogen efficiency of 41%. This corresponding to a volumetric hydrogen production rate of 0.12 m3-H2/m3-day at a hydrogen yield of 1.64 mol H2/mol acetate and a current density of 2.8 A/m2. The hydrogen performance at voltages below 0.8 V was limited, which might be caused by the high potential losses in large system. Acknowledgement This study was financially supported by the Ministry of Environment as the Eco-Technopia 21 project, Republic of Korea.
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Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Enhancing hydrogen production efficiency in microbial electrolysis cell with membrane electrode assembly cathode Yu Hong Jia a, Jae Hun Ryu a, Cho Hui Kim a, Woo Kyung Lee a, Thi Van Trinh Tran a, Hyo Lee Lee a, Rui Hong Zhang b, Dae Hee Ahn a,* a b
Department of Environmental Engineering and Biotechnology, Myongji University, San 38-2 Namdong, Yongin, Kyonggido 449-728, Republic of Korea Department of Biological and Agricultural Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
A R T I C L E I N F O
Article history: Received 28 April 2011 Accepted 11 July 2011 Available online 12 November 2011 Keywords: Microbial electrolysis cell Hydrogen production Membrane electrode assembly Organic removal Stainless steel mesh
A B S T R A C T
Microbial electrolysis cell is a device which can produce hydrogen gas from biomass through microbial catalyzed process and thus reduce the organic matter. For the real application in wastewater treatment, the scale-up of microbial electrolysis cell is an important issue but few tests were conducted with relatively large size. In this study, a 3.7 L microbial electrolysis cell (liquid volume 3.2 L) equipped with a membrane electrode assembly cathode was designed and tested. The internal resistance was examined, hydrogen generation and organic removal performance was investigated under different conditions. A maximum overall hydrogen efficiency of 41% was achieved at an applied voltage of 1.2 V with acetate as substrate, corresponding to a volumetric hydrogen production rate of approximately 0.12 m3 H2/m3 reactor liquid volume/day. The results obtained in this study could help to further develop pilot-MEC for practical applications. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Renewable energy production technologies are currently gaining great attraction in the society, due to the depletion of fossil fuels and the threat of global warming. Microbial fuel cells (MFCs) have been discovered as an innovative method for renewable energy production simultaneously with organic compounds removal from wastewater [1,2]. In the MFC system, the produced electrons by the oxidation of substrate are transferred to the anode, which then flows to the cathode through the external wire, where they normally combine with oxygen and protons to form water [3–6]. While in MFCs, the maximum voltage that could be obtained is less than 1.14 V [7,8], the voltage limitation restricted the energy recovery. The need of oxygen in the cathode chamber is another disadvantage of MFC [9], for oxygen may leak into the anode chamber through the membrane, either lower the energy recovery or inhibit the growth of obligate anaerobes [10,11]. Based on the MFCs technologies, by removing the oxygen from the cathode chamber and applying a small voltage to the circuit, hydrogen can be produced directly from protons and electrons transferred from the anode [10]. This hydrogen evolution process is
* Corresponding author. Tel.: +82 31 330 6692; fax: +82 31 336 6336. E-mail address: [email protected] (D.H. Ahn).
termed microbial electrolysis cells (MECs). As the system is operated as MECs for hydrogen production, the voltage limitation of MFCs could be bypassed. Meanwhile, the use of oxygen could be omitted, because MECs are completely anaerobic systems. The MEC enables the possibility of direct fuel production from a diverse range of waste streams [12]. In additional, hydrogen is an entirely carbon-free fuel with a high combustion enthalpy of 185 kJ/L [13,14], which considered to be a feasible alternative to fossil fuels [15,16]. Most MECs contain a membrane, although membrane could bring up the pH gradient and thus the cell internal resistance. In the MEC system, the presence of a membrane is essential to increase the coulombic efficiency, and also for the purity of the hydrogen that is produced at the cathode. Without the membrane, the produced hydrogen could be polluted with gaseous metabolic products from the anode chamber, such as CO2, CH4 or H2S. Furthermore, a significant amount of the produced hydrogen will be lost by hydrogen consumption by methanogens growing on the electrodes or in the solution [17]. Membrane electrode assembly (MEA) has been investigated in MFCs and MECs. MEA plays a key role as an electrolyte medium for ion transport and a barrier to avoid the direct contact between anolyte and catholyte, and also significantly reduce the electrode spacing and lower the internal resistance, thus improve the reactor performance [18]. Previous study proved that the use of MEA (Nafion cation exchange membrane hot-press to carbon cloth
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.127
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Y.H. Jia et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 715–719
cathode) could increase the coulombic efficiency from 9–12% (MFC lacking a membrane) to 40–50% in MFC [19]. However, the expense of Nafion ion exchange membrane used in normal MEA is a barrier to scale-up and develop, and the relatively cost-effective membranes such as the supported heterogeneous membranes with polyvinylidenefluoride (PVDF) binder have been reported as being unsuitable for hot pressing as they are not heat weldable [20]. A cost-effective MEA fabrication for MEC was developed in this study. Stainless steel mesh was used to press the cathode on to the membrane, and the use of steel mesh avoided the need to hot-press membrane to the cathode. The goal of this research is to determine the effectiveness of the MEC employing a MEA cathode which mechanically combined a cost-effective polymer based ion exchange membrane and a cathode electrode. The performance of the MEC was investigated by examining the internal resistance, and by monitoring the current and hydrogen output. 2. Materials and methods 2.1. Experimental set-up The configuration of MEC used in this study was shown in Fig. 1(a). The two-chamber reactor was separated by a cation exchange membrane (projected area 192.5 cm2, CMI-7000, Membrane International Inc., USA), membrane was used as purchased. The empty bed volume of anodic compartment was 3.7 L. While eight anode modules made of hexahedral stainless steel mesh filled with graphite granules (2–7 mm diameter, a void fraction of 50%, 100 cm3 in volume, Qingdao Grand Graphite Products Co., Ltd., China) were established into the anode chamber, given a net liquid volume of 3.2 L. The cathode consisted of a B1A plain carbon cloth (projected area 160 cm2, no wet proofing, E-TEK Division, Somerset,
New Jersey), with 0.5 mg/cm2 platinum coating as catalyst. The catalyst was applied to the cathode as follows: a commercial Pt catalyst (20 wt% Pt/C) was mixed with 5% Nafion solution in a ratio of 6.67 mL Nafion per mg Pt/C powder; coat the mixture to the membrane faced side of the cathode; dry the cathode at room temperature for 24 h. Cathode was placed above the membrane; stainless steel mesh was placed above the cathode and firmly pressed the cathode onto the membrane. The cathodic chamber was used as a gas chamber for hydrogen collection with a volume of about 1 L. Reactor was covered with aluminum foil to exclude light. The photograph of experimental apparatus was given in Fig. 1(b). 2.2. Inoculation The anode modules were inoculated in a conventional MFC for more than 7 months with acetate as substrate, and then transferred to the MEC for further experiment. The anode chamber was fed with synthetic wastewater modified from Lee et al. [21] with phosphate concentration of 50 mM (pH 7.0) and sodium acetate as sole substrate. The reactor was operated at room temperature during the whole experiment. When operated at MEC mode, the cathodic chamber was flushed with N2 gas prior to the experiment to get a totally anaerobic atmosphere. 2.3. Analytical methods The system voltage was measured (30 min intervals) with a multimeter and a data acquisition system (Keithley 2700, USA) connected to a personal computer. The anode potential was measured by a reference electrode (Ag/AgCl, +200 mV vs. a standard hydrogen electrode, SHE) equipped to anode chamber which was also recorded by the multimeter. A voltage range from 0.1 V to 1.4 V was applied to the circuit using a multichannel potentiostat (WMPG1000, WonATech, Korea) by connecting the counter and reference poles to the anode, and the working pole to the cathode. Polarization curves were examined to evaluate the electrode potential in function of the current production, which were obtained with potentiostat by increasing the applied voltage by 0.03 V increments every 10 min from the voltage obtained in open circuit to minus value. The ohmic cell resistance was determined with the current interrupt method based on cell voltage change in 0.01 s recorded by the potentiostat after cutting off the cell current. The pH of the electrolyte was measured with a pH meter (ion LAB WTW). The chemical oxygen demand (COD) was quantified using Hach COD measurement system and kit, all procedure for pre-treatment of samples was done in accordance to the standard method. The hydrogen concentration was analyzed using a gas chromatograph (GC-17A, Shimadzu, Japan) equipped with a thermal conductivity detector (TCD). A Molecular Sieve 5A column (Alltech Corporate, USA) with argon as carrier gas was used to separate the gas. 2.4. Calculations
Fig. 1. Schematic diagram and photograph of the microbial electrolysis cell.
The coulombic hydrogen recovery is calculated as: rCE = nCE/nth. nCE is the moles of hydrogen that could be recovered from the measured current, given by: nCE = CP/2F, where CP is total coulombs calculated by integrating the current over time, F is Faraday’s constant (96,485 C/mol electrons). nth is the total theoretical number of moles of hydrogen that could be recovered based on substrate. The cathodic hydrogen recovery is deduced as follows: r Cat ¼ nH2 =nCE , where nH2 is the number of moles of hydrogen recovered during a batch cycle. The overall hydrogen recovery is calculated according to: RH2 ¼ r CE r Cat [22]. The energy added to the system by power source (WE) can be P calculated as: W E ¼ n1 ðIEa p DtÞ, where Eap is the voltage applied
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by the potentiostat, I is current examined in the circuit, and Dt is the time increment of n data points measured during a batch cycle. Energy balances approach based on heats of combustion, which is commonly used for electrolyzers and for estimating the energy content of organic matter, were used in this study. The amount of energy added by the substrate is given by: WS = DHSnS, where DHS = 870.28 kJ/mol is the heat combustion of the substrate, and ns is the number of moles of substrate consumed during a batch cycle based on COD removal. The amount of energy output as hydrogen is calculated as: W H2 ¼ DHH2 nH2 , where DHH2 ¼ 285:83 KJ=mol is the energy content of the hydrogen based on the heat of combustion (upper heating value), and nH2 is the number of moles of hydrogen produced in a batch cycle. 3. Results and discussion 3.1. Inoculation in MFC mode The anode modules were transferred from a conventional liquid-cathode MFC operated for more than 7 months with oxygen as electron acceptor and acetate as substrate. After transforming, the MEC reactor was first operated in MFC mode for further inoculation with the gas collection cathodic chamber open to air. The performance was tested with polarization behavior (Fig. 2) after stabilization of the voltage production. The maximum power density recorded was 0.25 W/m2 at a current of 25.8 mA in this test. Based on the current–voltage curve, internal resistance was calculated to be 6.1 V. The power density was maintained over 2 months without any deterioration of performance. The results proved that the reactor was successfully inoculated and could proceed to the hydrogen production test. The ohmic cell resistance was further determined with the current interrupt method. At a relatively steady state, when the current is suddenly switching off, the electric double layer will take some time to disperse, which causes the associated overvoltage change gradually. Among them, voltage difference caused by ohmic losses (Ur) will be immediately observed, followed by a slower voltage increase caused by the activation losses (Ua) [23]. As the interruption process was completed within 0.01 s, it could be considered as instantaneous. The ohmic resistance was calculated from R = Ur/I, where I represents the steady state current before interruption. Fig. 3 shows the voltage change over time after interruption at a current of 20 mA, and the internal resistance calculated based on the Ur value was 7 V. Interruption processes were done at current of 5 mA, 10 mA, 29 mA and 38 mA, respectively, similar internal resistance of 7.03 V in average was obtained.
Fig. 2. Polarization curve obtained in MFC mode.
Fig. 3. Voltage changes over time after a current interruption.
High internal resistance is a key problem limiting the hydrogen output of the microbial electrolysis cell [24]. With the innovative design of MEA in this study, the internal resistance was decreased to about 7 V with a relatively large reactor size of 3.7 L, which is lower than the 19 V obtained in a membrane free MEC [25], as it was reported that the absence of membrane could decrease the internal resistance. Tartakovsky et al. [25] achieved a internal resistance of 27 V with MEC (liquid volume 50 mL) employing a MEA (Nafion 117 proton exchange membrane hot-pressed onto carbon cloth cathode with 0.5 mg/cm2 Pt coating), which is higher than the internal resistance obtained in this study. Zhang et al. [26] showed in a MFC equipped with MEA fabrication (similar MEA design as in this study), the internal resistance obtained without a mesh was about 49.4 V, and the value decreased to 16.4 V when stainless steel mesh was used to press the membrane onto the cathode to control the membrane deformation. The results demonstrated that the MEA fabricated in this study could decrease the internal resistance, thus help to improve the MEC performance. 3.2. Hydrogen production in MEC mode The reactor was then transferred to MEC mode by apply an external voltage of 1.0 V to the circuit. Once stable hydrogen production was observed, voltage scans were carried out by stepwise increase the voltage from 0.1 V to 1.4 V (Fig. 4). When acetate was present in the anode chamber, the current significantly increased from 0 to 66.6 mA with the increasing of applied voltage, while only background value of current was obtained during the voltage scan process with none substrate present. The result indicated that the examined current arose from the acetate oxidation in the anodic chamber. Fig. 5 shows the performance of MEC in anode potential, current and H2 production with organic removal at an applied voltage of 1.1 V. The MEC was operated in batch mode and 50 mM acetate was fed to the anodic chamber as sole carbon source. A current of 39.4 mA in average was developed in MEC up to the substrate addition, stabilized over 6 days, and decreased to background value as a result of the complete consumption of acetate. Almost all acetate as COD compounds was consumed (91.1%) in the batch test. The measured value of hydrogen production was used to calculate the process efficiencies. Measurements of current and hydrogen production were done with batch mode operation of MEC under different applied voltages (Fig. 6). Based on the 50 mM acetate injected as substrate, the current output in the batch tests were in the same range as predicated by the applied voltage scans. Overall, at applied voltage
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Fig. 6. Current and hydrogen production as a function of applied voltage. Fig. 4. Current obtained in voltage scans with 10 min intervals between voltage changes. Background values were obtained by excluding acetate from the stock solution.
less than 1.2 V, the current and hydrogen production increased in response to the increasing voltage. A maximum current of 44.5 mA (2.8 A/m2) and a highest hydrogen production of 1.83 L were both obtained at 1.2 V, and the value decreased with the further increase of voltage to 1.4 V. In this study, the performance of hydrogen production at voltages below 0.8 V was limited, this go along with the results obtained in previous studies [24,25], which might be due to the high ohmic potential loss in large reactor, even though the ohmic resistance was relatively low. For the electrical currents that flow through the cells were larger (2.1–2.8 A/m2), the
Fig. 5. The performance of MEC in anode potential, current and H2 production with COD removal at an applied voltage of 1.1 V.
ohmic potential losses caused by the resistance was higher than the one with same resistance in smaller systems [24]. Fig. 7 presents the coulombic efficiency, cathodic efficiency and overall hydrogen efficiency with different applied voltages. The overall hydrogen efficiency increased with the increasing of applied voltage, and achieved a maximum of 41% at the applied voltage of 1.2 V, corresponding to a volumetric hydrogen production rate of 0.12 m3-H2/m3-day and a hydrogen yield of 1.64 mol H2/mol acetate. The hydrogen yield achieved in this study is comparable to the one obtained by Tartakovsky et al. [25], where a hydrogen yield of 1.3 mol H2/mol acetate was developed at a voltage of 1.15 V from MEC equipped with a MEA (Nafion 117 proton exchange membrane hot-pressed onto carbon cloth cathode with 0.5 mg/cm2 Pt coating). The overall hydrogen yield of 35% achieved at 1.0 V in this system is higher than the 23% obtained by Rozendal et al. [24] in a MEC with similar reactor size (liquid volume 3.3 L) as in this study; while Nafion 117 membrane and Ti-mesh (5 mg Pt/cm2 coated) was used to fabricated their MEA. Applied voltage between 1.0 and 1.2 V was determined as the optimum for the hydrogen production, where the total hydrogen efficiency was over 35%. The coulombic efficiency, defined as the recovery of total electrons in acetate as current, was over 54% at applied voltage ranged 1.0–1.2 V. Coulombic efficiency of 55–71% was obtained in a MFC (equipped with a MEA) [20], which is comparable as this study.
Fig. 7. Coulombic efficiency, cathodic efficiency and overall hydrogen efficiency as a function of applied voltage.
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At an applied voltage of 0.9 V, the energy efficiency based on electrical energy input was over 102%. The acetate removal rates obtained in the batch operation were ranged 81.3–92.4%, and the volumetric acetate removal rates were calculated to be 0.18– 0.32 g-acetate/m3-day. The acetate removal rate was proportional to applied voltage (data not shown), confirming that acetate was consumed by anodophilic microorganisms. The results demonstrated that organic compounds from wastewater could be feasibly treated based on this MEC system. 4. Conclusions This study investigated the hydrogen generated from a 3.7 L microbial electrolysis cell employing a cost-effective MEA fabrication. The use of MEA allowed for reduced distance between the electrodes, thus reduced the internal resistance and increased the hydrogen production. An internal resistance of 7 V was examined from the reactor with the relatively large working volume. The MEC achieved a coulombic efficiency and cathodic efficiency of 55% and 74% respectively at an applied voltage of 1.2 V, results an overall hydrogen efficiency of 41%. This corresponding to a volumetric hydrogen production rate of 0.12 m3-H2/m3-day at a hydrogen yield of 1.64 mol H2/mol acetate and a current density of 2.8 A/m2. The hydrogen performance at voltages below 0.8 V was limited, which might be caused by the high potential losses in large system. Acknowledgement This study was financially supported by the Ministry of Environment as the Eco-Technopia 21 project, Republic of Korea.
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