External CO2 and Water Supplies for Enhancing Electrical Power

Sep 2, 2014 - *Satoshi Okabe [email protected], Telephone/Fax: ..... cathode electrode of the cation exchange membrane (CEM)-based MFC used ...
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External CO2 and Water Supplies for Enhancing Electrical Power Generation of Air-Cathode Microbial Fuel Cells So Ishizaki,† Itto Fujiki,† Daisuke Sano,† and Satoshi Okabe*,† †

Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan S Supporting Information *

ABSTRACT: Alkalization on the cathode electrode limits the electrical power generation of air-cathode microbial fuel cells (MFCs), and thus external proton supply to the cathode electrode is essential to enhance the electrical power generation. In this study, the effects of external CO2 and water supplies to the cathode electrode on the electrical power generation were investigated, and then the relative contributions of CO2 and water supplies to the total proton consumption were experimentally evaluated. The CO2 supply decreased the cathode pH and consequently increased the power generation. Carbonate dissolution was the main proton source under ambient air conditions, which provides about 67% of total protons consumed for the cathode reaction. It is also critical to adequately control the water content on the cathode electrode of air-cathode MFCs because the carbonate dissolution was highly dependent on water content. On the basis of these experimental results, the power density was increased by 400% (143.0 ± 3.5 mW/m2 to 575.0 ± 36.0 mW/m2) by supplying a humid gas containing 50% CO2 to the cathode chamber. This study demonstrates that the simultaneous CO2 and water supplies to the cathode electrode were effective to increase the electrical power generation of air-cathode MFCs for the first time.

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INTRODUCTION Microbial fuel cells (MFCs) are able to directly convert chemical energy in organic compounds to electrical energy by microbial metabolisms.1,2 One of the promising application fields of MFCs is wastewater treatment, in which the removal of organic pollutants from wastewater and the generation of electricity are simultaneously expected.2,3 Advantages of MFCs in wastewater treatment also include the lack of the necessity of aeration, lower operational temperature than methane fermentation, and sludge reduction.4−7 However, insufficient electrical power generation is currently a critical bottleneck for the practical use of MFCs for wastewater treatment, and thus significant increase in electrical power generation has to be achieved for any styles of application of MFCs.8,9 Air-cathode MFCs are a practical configuration of MFCs, which use oxygen in air as an electron acceptor, because the utilization efficiency of oxygen is superior to other MFC configurations.8,10,11 The cathode reaction (the electrons and protons transferred from the anode chamber are used for reduction of oxygen) is generally expressed as follows: O2 + 4H+ + 4e− → 2H 2O

cathode pH is elevated and consequently the cathodic potential is reduced.2 In addition, H+/OH− mitigation caused by progression of the cathode reaction also causes the alkalization.13 Therefore, reaction 2 is considered to occur mainly under such high cathode pH conditions.14 O2 + 2H 2O + 4e− → 4OH−

Alkalization on the cathode electrode is considered to limit the electrical power generation of air-cathode MFCs. The mitigation of alkalization, i.e., proton supply to the cathode electrode, is essential to enhance the cathode reaction. It has been proven that buffer systems slow down the alkalization on a cathode electrode.9 The effects of the buffer systems have been investigated.15−18 Torres et al. reported that the electrical power generation increased when a gas containing CO2 was supplied to a cathode electrode, and concluded that carbonic acid and bicarbonate ions generated by carbonate dissolution maintain the charge balance in anode and cathode chambers as substitute for hydroxide ion.17 Since water is an important solution for carbonate dissolution, the water content of cathode electrodes must be adequately maintained. Thus, external supplies of not only CO2, but also water are considered to be essential to enhance the cathode reaction. However, there

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However, the proton flux from the anode chamber is much lower than the amount of protons consumed by the cathode reaction since the proton concentration in an anode chamber is far lower than those of cationic species, such as sodium and potassium ions.12 Thus, as the cathode reaction proceeds, the © 2014 American Chemical Society

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Received: December 24, 2013 Accepted: September 2, 2014 Published: September 2, 2014 11204

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chamber of each MFC reactor at a hydraulic retention time (HRT) of 5 h. All reactors were operated at room temperature (25 ± 2 °C). Cell voltage and electrical current was measured by using a multimeter with a data acquisition system (Agilent HP 34970) and converted to power density, P (mW/m2), according to P = IV/s, where I (A) is the current, V (V) is the voltage, and s (m2) is the surface area of the cathode electrode.20 Ohmic resistance was measured using an Ohm tester (MODEL 3566, Tsuruga Electric co., Osaka, Japan). Effect of Buffer Systems on Electrical Power Generation. The power density was measured during feeding buffer solutions with pH values of 3, 5, 7, or 10 to the cathode electrodes of a DCMFC and compared to confirm proton shortage on the cathode electrodes. Citric acid buffer solutions (pH: 3, 5, or 7) and carbonate buffer solution (pH: 10) were fed to the surface of the cathode electrodes at a flow rate of 2.0 L/h for 30 min (SI Table S2). The average power density was monitored after the power density was stabilized. The experiment using each buffer solution was performed in triplicate. Effect of External CO2 Supply on Electrical Power Generation. In order to evaluate the effect of external CO2 supply to a cathode electrode on electrical power generation, three types of gases prepared by mixing a pure (100%) CO2 gas and air were fed to each cathode chamber of a DCMFC at a flow rate of 150 mL/h. The compositions of each gas were as follow: the ambient air, 30% CO2 gas (pure CO2 gas/ambient air = 3:7) and 60% CO2 gas (pure CO2 gas/ambient air = 3:2). Polarization curves of the DCMFC were generated by changing the external resistance during each gas feeding, which were monitored after the power density was stabilized. The pH value of water on the cathode electrode was continuously monitored by using LIX-type microelectrodes during feeding 50% CO2 gas (pure CO2 gas/ambient air = 1:1) to the cathode chamber of a SCMFC at a flow rate of 80 mL/h.21 This gas flow rate was set because the electrical stability of microsensor measurement is affected at high gas flow rate. The pH measurement was performed in duplicate and the representative data were shown. Effect of External Water Supply on Electrical Power Generation. In order to evaluate the effect of external water supply on electrical power generation, the power density was monitored during feeding humid gases to the cathode chamber of a SCMFC. Three types of gases were prepared by mixing the saturated air and ambient air, and fed to the cathode chamber at a flow rate of 2.5 L/h. The compositions of the gases fed to the cathode chamber as follow: humid gas 1 (saturated air/ambient air = 1:0), humid gas 2 (saturated air/ambient air = 3:1) and humid gas 3 (saturated air/ambient air = 1:1). The cathode electrode was dried up before feeding each gas to the cathode chamber. Cell voltage, electrical current, and Ohmic resistance was monitored before and after the water supply. In the separate operation, Milli-Q water was directly fed to the cathode electrode of the SCMFC at 2.5 L/h for 1 min. Cell voltage, electrical current and Ohmic resistance were monitored before and after 20 min of water supply. The theoretical cell voltage was calculated by using the Nernst equation to evaluate the contribution of the external water supply.2 This experiment was performed in duplicate and the representative data were shown. Relative Contribution of Potential Proton Sources. In this study, we hypothesized that the potential proton sources are (1) proton transfer from an anode chamber and (2) CO2 dissolved into water on a cathode electrode (carbonate

is no information on the effect of the simultaneous supply of CO 2 and water on the electrical power generation. Furthermore, a better understanding of the proton supply mechanisms of air-cathode MFCs is essential for improvement of the process performance and reactor design. The contribution of external CO2 supply (i.e., carbonate dissolution) to total protons consumed for the cathode reaction has never been quantitatively determined under different water contents of cathode electrodes. In this study, the effects of external CO2 and water supplies to cathode electrodes on the electrical power generation of aircathode MFCs were investigated. Furthermore, the relative contribution of carbonate dissolution and proton transfer from an anode chamber to total proton consumption for the cathode reaction was experimentally determined for better understanding on the proton supply mechanisms of air-cathode MFCs. These results demonstrate that the external CO2 and water supplies were effective to increase the electrical power generation of air-cathode MFCs, and carbonate dissolution was a main proton source under the normal air conditions.

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MATERIALS AND METHODS MFC Configurations and Operation. Three air-cathode MFCs, a double-cathode MFC (DCMFC), a single-cathode MFC (SCMFC), and an H-type MFC (HMFC), were constructed and used for different subexperiments in this study (Supporting Information, SI, Figure S1). The type of MFC used for each subexperiment and the reasons for the use are summarized (SI Table S1). The double-cathode MFC (DCMFC) consists of a single anode chamber (15 mL, 3 × 5 × 1 cm3) and two cathode chambers (70 mL, 5 × 7 × 2 cm3 each). The porous carbon (3 cm × 5 cm2, Somerset, NJ) was used as an anode electrode, and the carbon cloth loaded with 0.5 mg/cm2 of platinum (3 × 5 cm2, E-TEK, Somerset, NJ) was used as a cathode electrode. Nafion membrane (Nafion 117, Dupont Co., DE, U.S.A.) was sandwiched between the anode chamber and the cathode electrode at both sides. Two cathode electrodes were connected to an anode electrode with external resistance of 50 ohm, as shown in SI Figure S1. This DCMFC generates about double the electrical power as the SCMFC, indicating that the cathode reaction is limiting the power generation. The SCMFC, which consists of a single anode chamber and one cathode chamber (SI Figure S1b), was used for some subexperiments due to several technical reasons (SI Table S1). In the SCMFC, only one cathode electrode was connected to an anode electrode with an external resistance of 50 ohm (SI Figure S1b). For experimental verification of the effect of carbonate dissolution, an HMFC was used (SI Figure S1d). The HMFC consisted of an anode chamber (350 mL) and a cathode chamber (350 mL).19 The porous carbon (3 × 4 cm2) and carbon cloth loaded with 0.5 mg/cm2 of platinum (1 × 1 cm2) was used as an anode electrode and a cathode electrode for HMFC. The Nafion membrane was sandwiched between the anode chamber and the cathode electrode (SI Figure S1c). The external resistance in the HMFC was fixed at 300 ohm. The anode chamber was inoculated with biomass from an anode chamber of the MFC that has been operated in our laboratory. 19 A synthetic medium containing 200 μM (NH4)2SO4, 200 μM NaCl, 500 μM CaCl2, 500 μM MgCl2· 6H2O, 27 mM K2HPO4, 55 mM KH2PO4 and 20 mM CH3COONa (a sole energy source) was fed to the anode 11205

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Synergistic Effect of External CO2 and Water Supplies on Electrical Power Generation. In order to confirm the synergistic effect of external CO2 and water supplies, the power density was measured during feeding a humid gas containing 50% CO2 to the cathode chambers of a DCMFC at a flow late of 2.5 L/h. The power density was monitored for 1 h after the power density was stabilized. Experimental Verification of the Effect of Carbonate Dissolution on Electrical Power Generation. In order to confirm the effect of carbonate dissolution on electrical power generation, the gas consisting of O2/CO2/N2 = 20.5:17.5:62.0 was initially filled in the cathode chamber of HMFC, and time course of the gas composition in the cathode chamber was monitored for 95 h. The HMFC was submerged into a water bath to prevent the air contamination in the cathode chamber. Ten mL of the head space gas in the cathode chamber was collected, and the gas composite was measured by using Gas Chromatography (GC-14B; Shimadzu Co., Kyoto, Japan).

dissolution) (Figure 1). The contribution of the protons transferred from an anode chamber (pa) and donated by

Figure 1. Schematic diagram of proton flows and reactions at a cathode electrode of air-cathode MFCs. Protons transferred from an anode chamber via a separator and donated by carbonate dissolution were considered as major proton sources. The pH is assumed to be over 12.

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carbonate dissolution (pc) to the total protons consumed for cathode reaction were quantitatively determined using the following equations: pa = 1 − Q+/Q−

(3)

pc = H +c /Q−

(4)

RESULTS AND DISCUSSION Effect of Buffer Systems on Electrical Power Generation. Four types of buffer solutions with the pH values of 3, 5, 7, and 10 were separately fed to the cathode electrodes of a DCMFC, and the power density was monitored to confirm the effect of the pH of buffer solutions on electrical power generation of air-cathode MFCs. There was a clear negative correlation between the power density and pH value (R2 = 0.96, Figure 2), indicating that alkalization on the cathode electrodes is a limiting factor for electrical power generation.

+

where, Q (mol) was the amount of cationic species accumulated on the cathode electrode except protons migrated from an anode chamber, Q−(mol) was the quotient of integrated current to the Faraday constant (96485 C/mol), H+c (mol) is the amount of protons donated by carbonate dissolution. In this experiment, the amounts of sodium and potassium ions transferred from an anode chamber to a cathode chamber were defined as Q+, because 99% of cationic species in the synthetic medium were composed of sodium and potassium ions (SI Table S3). It should be noted that one mole of CO2 donates two moles of protons in the carbonate dissolution when the pH value of the cathode electrode is over 10.3 (Figure 1). We confirmed that the pH value of water on the cathode electrode was over 12 in all operations by using pH paper (data not shown). It was assumed that the total amount of protons consumed by the cathode reaction was equal to the electrons transferred from an anode chamber via the external circuit.12 The protons donated by water autoionization would be one of the proton sources, however, this contribution is minor when the cathode pH is over 12. SCMFCs were operated under three different operational conditions; (1) dry condition (dry air was continuously fed to the cathode electrode at a flow rate of 1 L/min to keep the cathode electrode dry), (2) wet condition (the saturated air was fed to the cathode electrode at a flow rate of 2.5 L/h, and (3) normal condition (the SCMFC was placed under ambient air). The cathode electrode was washed with Milli-Q water and dried well before each operation to remove carbonate, sodium, and potassium ion accumulated on the cathode electrode. SCMFCs were operated for 12 h, and then carbonate, sodium and potassium ions accumulated on the cathode electrode were collected by washing the cathode electrode with 5 mL of MilliQ water after each operation. The carbonate ion concentration was measured using High-Performance Liquid Chromatography (LC-10AD; Shimadzu Co., Kyoto, Japan), and sodium and potassium ions were measured by using ICP-AES (ICPE9000; Shimadzu Co., Kyoto, Japan). The experiment was performed under each condition in triplicate.

Figure 2. Variation of power density during feeding buffer solutions of different pH values to the cathode electrode. Citric acid buffer solutions (pH: 3, 5, or 7) and carbonate buffer solution (pH: 10) were continuously fed to the surface of the cathode electrodes of a DCMFC at a flow rate of 2.0 L/h. Power generation was measured for 30 min after the buffer solution supply was started. The error bars indicate the standard deviations of three independent experiments.

CO2 and Water As External Proton Sources. The power density was monitored during feeding the ambient air, 30% CO2 gas, and 60% CO2 gas to the cathode chambers of a DCMFC to examine the effect of external CO2 supply on the electrical power generation. The power density increased with increasing the CO2 concentration in the gases (Figure 3a). The power density was increased by 220% when 60% CO2 gas was fed (189.2 ± 1.8 mW/m2 by feeding ambient air to 421.7 ± 3.2 mW/m2 by feeding 60% CO2 gas). This demonstrated that the CO2 supply was effective to increase the electrical power generation. In order to investigate the effect of CO2 supply on 11206

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Figure 3. (a) Effect of CO2 concentration on the power density of a DCMFC. Ambient air, 30% CO2 gas, and 60% CO2 gas are continuously fed to each cathode chamber at a flow rate of 150 mL/h. (b) Time courses of the power density and the pH value of water on the cathode electrode of a SCMFC during 50% CO2 gas supply. The 50% CO2 gas supply was started at 0 min and stopped at 5 min as indicated by an arrow. This experiment was performed in duplicate and the representative data were shown.

Figure 4. (a) Time courses of the Ohmic resistance and electrical current after water was supplied to the cathode electrode of a SCMFC. Water was supplied to the cathode electrode for 1 min (from 0 to 1 min) at a flow rate of 2.5 L/h. Minimum Ohmic resistance was marked with an open circle. (b) Time courses of the power density during feeding different types of humid gases to the cathode electrode of the SCMFC. Each humid gas was prepared by mixing the water vapor-saturated air and ambient air, and the humidity was adjusted by changing the ratio of each air flow rate. The compositions of gases fed to the cathode chamber were as follows: humid gas 1 (saturated air/ambient air = 1:0), humid gas 2 (saturated air/ambient air = 3:1), and humid gas 3 (saturated air/ambient air = 1:1).

MFC reported in the literature (pH = 9.0).18 Furthermore, it is reported that the pH value of the permeate water across CEM (pH = 13.2) was higher than that of the permeate water across AEM (pH = 11.1) in microbial electrolysis cells (MECs).22 Thus, the proton shortage, i.e., alkalization, on the cathode electrode of CEM-based MFCs is more critical than AEMbased MFCs. In order to study the effect of external water supply to the cathode electrode on the electrical power generation, the time course of cell voltage and Ohmic resistance before and after temporal water supply to the cathode electrode of a SCMFC were monitored. The cell voltage increased dramatically by 140% after water supply at time 0 while the Ohmic resistance gradually decreased (Figure 4a). The pH values obtained before and after temporal water supply were approximately 12 and 7, respectively based on the pH paper measurement (data not shown). According to the Nernst equation, the cathodic potential increased about 150% when the pH value of the cathode electrode decreased from 12 to 7.2 Time course of the power density of a SCMFC was monitored during continuous feeding of three different humid gases to confirm the effect of water content on the electrical power generation. The power density rapidly increased with increasing humidity of the feed gas (Figure 4b). However, the power density reached a plateau at around 350 mW/m2 within

the cathode pH, the pH value of the water on the cathode electrode was continuously monitored by using LIX-type microelectrodes, and the power density was also monitored (Figure 3b). The power density increased to 204.0 mW/m2, while the pH value decreased to 7.2 as a result of feeding 50% CO2 gas to the cathode chamber. After the gas feed was stopped (as indicated by an arrow in Figure 3b), the power density started declining gradually, and the pH value increased immediately and reached the original value. Furthermore, the cathode pH could be maintained around 7−8 when the 50% CO2 gas was continuously fed to the cathode chamber for 6 h (data not shown). This result suggested that protons were provided by carbonate dissolution into the water on the cathode electrode, which enhanced the electrical power generation of air-cathode MFCs. Torres et al. reported that the amount of CO2 dissolved to the cathode electrode was limited in their study when the CO2 concentration of feed gas increased from 5% to 10%.17 However, our study indicated that the electrical power generation increased with increasing the CO2 concentration of feed gas up to 60%. This difference could be attributed to the difference in the cathode pH. The pH value of the water on the cathode electrode of the cation exchange membrane (CEM)based MFC used in this study was over pH 12, which was higher than that of anion exchange membrane (AEM)-based 11207

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Figure 5. Comparison of the contributions of potential proton sources; proton transfer from an anode chamber and carbonate dissolution for the cathode reaction of a SCMFC operated under dry, wet, and normal conditions. (a) The amounts of protons provided by each proton source. The error bars indicate the standard deviations of independent three experiments. (b) The relative contribution of each proton source. Two moles of protons are assumed to be produced when one mole of CO2 is dissolved in water (see Figure 1). “Others” represents the difference between the integrated current and the sum of the amount of protons transferred from an anode chamber and donated by carbonate dissolution. The number represents the percentage of their relative contribution under each operational condition.

protons transferred from an anode chamber and donated by carbonate dissolution, which is expressed as follows:

20 h for all gas conditions, probably due to the saturation of water on the cathode electrode (completely wet state). This suggested that the water content must be adequately regulated to maximize the electrical power generation. Relative Contribution of Potential Proton Sources. In this study, it is hypothesized that the potential major proton sources are proton transfer from an anode chamber and carbonate dissolution. In order to estimate the relative contribution of these proton sources, the ratios of the amount of protons transferred from the anode chamber (pa) and donated by carbonate dissolution (pc) to the total amount of protons consumed by the cathode reaction were estimated based on the results shown in SI Figure S2 (Figure 4). The total amount of electrons consumed in the SCMFC was measured for 12 h under three operational conditions, which are dry, normal, and wet conditions. Sodium and potassium ions are dominant species in the anodic medium (over 99%), and the amount of calcium and magnesium ion migrated from the anode chamber was less than 0.1% of the total cationic species transferred from the anode chamber (SI Table S3). In normal and wet conditions, carbonate dissolution was the primary proton source for electrical power generation, which accounted for 67% and 80% of total protons consumed for the cathode reaction of air-cathode MFCs, respectively. The contribution of carbonate dissolution increased with increasing water content on the cathode electrode, indicating that the water is an important solution for carbonate dissolution. In dry condition, the total amount of electrons transferred from the anode chamber was 17% and 31% of the normal and wet condition, respectively (SI Figure S2). The contribution of proton transfer from the anode chamber was similar to the normal and wet conditions, while the contribution of carbonate dissolution was very low (only 18%). This indicates that desiccation of the cathode electrode limits carbonate dissolution and consequently lowers the electrical power generation (SI Figure S2). Accordingly, adequate water supply is required to maximize the performance of air-cathode MFCs, even water is permeated from the anode chamber regardless of operational conditions. It should be noted that estimation of the relative contribution of each proton source is prone to accompanying some experimental errors, as indicated by “others” in dry condition (Figure 5a,b). The “others (po)” are defined as the difference between the integrated current and the sum of the amount of

po = 1 − pa − pc

(5)

This term may include the protons donated by water autoionization, but the contribution is considered to be minor at high cathode pH (>12). The high percentage of “others” is probably because the cationic species, such as sodium and potassium ions and carbonate, would not be 100% accumulated on the cathode electrode (they tend to accumulate in between the cathode electrode and the CEM), and thus could not be collected by washing with 5 mL of Milli-Q water especially in dry condition. However, these potential experimental errors do not negate the conclusion of the present study showing that carbonate dissolution is a primary proton source. The relative contribution of the proton supply depends on the type of separators because the type of ions migrating from an anode chamber is dependent on the separator characteristics.22−24 In the case of AEM, the phosphate ion and carbonic ion need to be considered since they could donate protons on the cathode electrode.16,17 The concentrations of cationic species in anodic medium also influence the relative contribution of the proton supply. It was reported that addition of cationic species other than protons in the anodic medium promoted the migration of cationic species and alkalization in the cathode chamber.25,26 The relative contribution would change during the long-term operation because biofilm formation and membrane fouling on the separator prevent the migration of ions.27,28 Recently, alkalization of cathode electrode was mitigated by alternating the anode and cathode reaction periodically by regulating the electrical potential and feeding the anodic effluent to the cathode chamber.29,30 Enhancement of Electrical Power Generation by External CO2 and Water Supplies. The time courses of the power density and concentrations of molecular oxygen and CO2 in the cathode chamber of a HMFC were measured when a gas consisting of O2 (20.5%), CO2 (17.5%) and N2 (62.0%) was initially filled in the cathode chamber (Figure 6). The power density gradually decreased in accordance with the decrease in CO2 concentration and stabilized after 35 h when the CO2 concentration decreased to less than 1%. Meanwhile, the O2 concentration was still more than 15% after 90 h, indicating that O2 was not a limiting factor for the power 11208

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cathode MFCs. Since carbonate dissolution was dependent on the water content on the cathode electrode, the simultaneous supply of CO2 and humid gas was the most effective to increase the electrical power generation. The power density was increased by 400% by supplying a humid air containing 50% CO2 to the cathode chamber. Further studies are required to optimize the CO2 and water supply to the cathode chamber (i.e., the gas composition and feeding rate). For practical application of MFCs to wastewater treatment, a gas containing high CO2 concentrations such as off-gas from a sludge incinerator could be used as a potential CO2 source. However, the feasibility of this gas must be investigated in the future.

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A summary of the type of reactors used for each subexperiment and its reasons (Table S1), the compositions of the buffer solutions fed to the cathode electrode of a DCMFC (Table S2), relative percentage of species contained in the anodic medium (Table S3), schematics and a photograph of reactor configurations used in this study (Figure S1), sums of amounts of electrons transferred from the anode chamber to the cathode electrode via the external circuit of a SCMFC, carbonate ion, and cationic species accumulated on the cathode electrode of a SCMFC (Figure S2), and time course of water content of carbon cloth (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org/.

generation. These results clearly indicate that the electrical power generation of air-cathode MFCs was dependent on the CO2 concentration, namely, proton supply via carbonate dissolution. In order to confirm the synergistic effects of the simultaneous supply of CO2 and humid gas on the electrical power generation, the power densities were compared during feeding four types of gases to the cathode chamber of a DCMFC (Table 1). As a result, the simultaneous supply of CO2 and

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ambient air 60% CO2 gas humid gas 3 Humid gas containing 50% CO2

*Satoshi Okabe [email protected], Telephone/Fax: +81-11-706-6266. Notes

The authors declare no competing financial interest.

power density (mW/m2) 143.0 421.7 250.0 575.0

± ± ± ±

AUTHOR INFORMATION

Corresponding Author

Table 1. Power Density Obtained during Feeding Ambient Air, 60% CO2 Gas, Humid Gas 3 and Humid Gas Containing 50% CO2 to the Cathode Chambers of a DCMFC operational condition

ASSOCIATED CONTENT

S Supporting Information *

Figure 6. Time courses of the power density and concentration of molecular oxygen and CO2 in the cathode chamber of HMFC. A gas consisting of O2 (20.5%), CO2 (17.5%), and N2 (62.0%) was initially filled in the cathode chamber at time 0 h, and no gas was fed thereafter.

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3.5 3.2 21.5 36.0

ACKNOWLEDGMENTS The authors would like to thank to Dr. Hisashi Sato for the technical advice in using the microelectrode, Dr. Yutaka Tabe for valuable technical advice in measuring the Ohmic resistance, and Ms. Nozomi Takeda for the technical advice in using ICPAES. This work has been supported by Core Research of Evolutional Science and Technology (CREST) project of Japan Sciences and Technology Agency (JST) and Grant-in-Aid for Challenging Exploratory Research (23656324) of the Japan Society for the Promotion of Science (JSPS).

humid gas was the most effective to increase the electrical power of a DCMFC. The power density increased by 400% when a humid air containing 50% CO2 gas was fed to the cathode electrodes (the power density increased from 143.0 ± 3.5 mW/m2 to 575.0 ± 36.0 mW/m2). Carbonate dissolution into water also decreases the vapor pressure, which retards the evaporation of water from the cathode electrode. Water molecules in the carbon cloth soaked in Milli-Q water completely evaporated within 3 h, while 36% of the water still remained in the carbon cloth soaked in 5 M sodium bicarbonate (SI Figure S3). This result clearly indicates that the depression of vapor pressure by carbonate dissolution results in holding water molecules longer in a cathode electrode, according to Raoult’s law, in which the vapor pressure decreases in accordance with the increase in solute concentration. However, since formation of chemical scale on the cathode electrode decreases the electrical power generation, carbonate concentration must be carefully controlled.31 In conclusion, the effects of external CO2 and water supplies to the cathode electrode on the electrical power generation were investigated. Carbonate dissolution was the main proton source under ambient air conditions, which accounted for 67% of the total protons consumed for the cathode reaction of air-

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dx.doi.org/10.1021/es5021197 | Environ. Sci. Technol. 2014, 48, 11204−11210