Carbonate Species as OH− Carriers for Decreasing the pH Gradient

Environ. Sci. Technol. 2008, 42, 8773–8777

Carbonate Species as OH- Carriers for Decreasing the pH Gradient between Cathode and Anode in Biological Fuel Cells ´ SAR I. TORRES,* HYUNG-SOOL LEE, CE AND BRUCE E. RITTMANN Center for Environmental Biotechnology, Biodesign Institute, Arizona State University, Tempe, Arizona 85287

Received July 13, 2008. Revised manuscript received September 19, 2008. Accepted September 29, 2008.

Anodes of biological fuel cells (BFCs) normally must operate at a near-neutral pH in the presence of various ionic species required for the function of the biological catalyst (e.g., substrate, nutrients, and buffers). These ionic species are in higher concentration than protons (H+) and hydroxides (OH-); slow transport of H+ and OH- equivalents between anode and cathode compartments can lead to a large pH gradient that can inhibit the function of biological components, decrease voltage efficiency in BFCs, or both. We evaluate the use of carbonate species as OH- carriers from the cathode to the anode compartment. This is achieved by adding CO2 to the influent air in the cathode. CO2 is an acid that combines with OH- in the cathode to produce bicarbonate and carbonate. These species can migrate to the anode compartment as OH- carriers at a rate much greater than can OH- itself when the pH is not extremely high in the cathode compartment. We demonstrate this concept by feeding different air/CO2 mixtures to the cathode of a dual-chamber microbial fuel cell (MFC) fed with acetate as substrate. Our results show a 45% increase in power density (from 1.9 to 2.8 W/m2) by feeding air augmented with 2-10% CO2. The cell voltage increased by as much as 120 mV, indicating that the pH gradient decreased by as much as 2 pH units. Analysis of the anode effluent showed an average increase of 4.9 mM in total carbonate, indicating that mostly carbonate was transferred from the cathode compartment. This process provides a simple way to minimize potential losses in BFCs due to pH gradients between anode and cathode compartments.

Introduction Biological fuel cells (BFCs) are widely researched today as a means to produce combustionless electrical energy from a wide variety of organic compounds present in water. BFCs use microorganisms and/or enzymes as catalysts and are capable of oxidizing many complex organic compounds as well as simple organic molecules present in water (1-4). This capability of BFCs opens up the possibility of producing electrical energy directly from biomass-based feed stocks that are renewable and carbon-neutral fuels (5-7). All fuel cells have certain common features: (a) an electron donor (the fuel) is oxidized at the anode with a catalyst; (b) * Corresponding author phone: 480-965-7495; fax: 480-727-0889; e-mail: [email protected]. 10.1021/es8019353 CCC: $40.75

Published on Web 10/29/2008

 2008 American Chemical Society

electrons from the electron donor move through an electrical circuit from the anode to the cathode; (c) at the cathode, the electrons are transferred to an electron acceptor, usually oxygen (O2); and (d) either protons (H+) move separately from the anode compartment to the cathode compartment or hydroxide ions (OH-) move from the cathode compartment to the anode compartment to maintain electroneutrality in the anode compartment. Failure to transfer the H+ ions from the anode compartment or OH- ions into the anode compartment can result in a pH gradient between the compartments. In proton exchange membrane fuel cells (PEMFCs) an acidic pH condition (i.e., high concentration of H+ ions and a low pH) can facilitate the transport of protons that is required between anode and cathode. However, most of the applications that use microbes or other biological catalysts in the anodic compartment of a BFC require a near-neutral pH (1, 8). The low proton concentration in the anodic compartment (0.1 µM at pH 7) contrasts with the relatively high concentration of other ionic components of biological media (buffers, salts in millimolar range), which are often needed to maintain operation of BFC biological components. These high concentrations result in a limitation in proton transport between anode and cathode compartments; to maintain electroneutrality, other ions are transported between the compartments (9, 10). The result is a pH gradient, especially at high current densities, in which the anodecompartment pH decreases and the cathode-compartment pH increases (11). The pH gradient causes a drop in voltage efficiency, which consequently decreases power generation. In microbial systems this pH difference was calculated by Rozendal et al. (12) to be more than 4.4 pH units, which resulted in a potential loss of more than 0.26 V or approximately 20% loss in available energy. Thus, a pH gradient between anode and cathode compartments is one of the main sinks of voltage efficiency in microbial systems; it should also be a significant sink in enzymatic systems, although it has not been well documented. We propose an approach to alleviate a large pH gradient between anode and cathode compartments in BFCs by providing acid to the cathode compartment in the form of CO2. To help understand the principle we write the cathodic oxygen-reduction reaction with hydroxide ion (OH-) production 0.25O2 + 0.5H2O + e- f OH-

(1)

A buffer present in the cathode compartment, such as phosphate, can bind to the OH- ions and prevent the build up of a large pH gradient over short periods of time, as reported by Kim et al. (13). However, to prevent a pH gradient during long-term operation we must supply an acidic buffer component that can be fed continuously at a low cost. Carbon dioxide gas (CO2) is such buffer, which reacts with OH- to form bicarbonate (HCO3-) or carbonate (CO32-) in the cathode compartment CO2+OH- f HCO3

(2)

CO2+2OH- f CO23 + H2O

(3)

Furthermore, CO2 is continuously produced in the anode chamber as a result of organic matter oxidation, e.g. CH2O f CO2+2H++2e-

(4)

Thus, we can rely on the bicarbonate system in a BFC to transport OH- ions between anode and cathode compartVOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic of the OH- transport process to reduce the pH gradient between the anode and cathode using bicarbonate. Dashed line indicates the possibility to recycle CO2 from the anode compartment into the cathode compartment. ments, as shown in Figure 1. The CO2 fed into the cathode chamber combines with the OH- produced by the cathodic reduction (eq 1) and forms HCO3- and/or CO32-, which flows across an anion exchange membrane (AEM) into the anode chamber to maintain electroneutrality. In the anode chamber HCO3- or CO32- combines with H+ produced as a result of substrate oxidation to produce CO2 again. In practical applications with concentrated influent substrates the overall process would not require any buffer addition as CO2 can be recycled between anode and cathode compartments. Analysis of the conditions required for CO2 recycle is presented in the Supporting Information. Note, from eqs 2 and 3 that a HCO3molecule transports 1OH-, whereas a CO32- molecule transports 2OH-. Thus, CO32- is a more efficient transporter of OH- equivalents than is HCO3-. The pKas of CO2/HCO3- (pKa,1 ) 6.3) and HCO3-/CO32(pKa,2 ) 10.3) are advantageous for OH- transport since a small concentration of CO2 is associated with a high HCO3concentration at near-neutral pH values. For example, 5% CO2 in air results in an equilibrium HCO3- concentration of 168 mM at pH ) 8.3. This high HCO3- concentration should allow high anion current densities across the membrane while still maintaining a near-neutral pH and, thus, minimizing voltage loss. At higher pH values carbonate (CO32-) is significant and can become dominant in the transport of OH- equivalents across the compartments. Our experiments provide proof of concept for CO2 addition in the cathode compartment of a microbial fuel cell. We use an anion exchange membrane (AEM), which is beneficial in this application, since it allows transport of HCO3- and CO32from cathode to anode. However, other membranes can be used as long as they allow the transport of bicarbonate and carbonate into the anode compartment. Previous studies have shown that a simple filter, J-cloth, or no membrane at all are effective at producing electrical power in microbial fuel cells (MFCs) (14, 15) and could be effective in the bicarbonate transport process. Our expected results are higher power densities as a result of a decrease in pH gradient between anode and cathode compartments due to CO2 addition in the cathode compartment.

Materials and Methods We used a continuous air-cathode MFC to carry out experiments to evaluate our concept. The configuration of the MFC is shown schematically in Figure 2. The anodecompartment volume of 20 mL was filled with graphite granules with ∼50% void volume made from pieces of graphite rods (graphitestore.com). The anode inoculum was a mixed culture of acclimated anode-respiring bacteria (ARB) from previous experiments (8). We fed the anode a nutrient 8774

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medium with 25 mM acetate as organic substrate for bacterial metabolism and a 100 mM phosphate buffer (Na2HPO4/ KH2PO4) at pH 7.5. The high phosphate buffer allowed us to maintain a neutral pH (pH between 7.3 and 7.5) throughout the experiments and collect all the carbonate species transferred mainly as HCO3-. The medium composition was described previously (4). We fed the medium continuously at 0.4 mL/min, giving a hydraulic retention time (HRT) of 25 min. The cathode was a carbon cloth with 5 g/m2 Pt loading with a surface area of 7.5 cm2 (GDE HT-140 EW, E-TEK, Germany). The carbon cloth was physically placed in direct contact with an AEM (Selemion AMV, Asahi Glass Co., Japan). The cathode compartment was a closed gas chamber to allow a flow of air/CO2 mixtures from two peristaltic pumps, as shown in Figure 2. The total gas flow to the cathode was 80 mL/min during the experiments. We recirculated the anode liquid at 5 mL/min to provide mixing of the medium and maintained the reactor at 32 °C in an incubator. We analyzed effluent samples for acetate concentrations using high-performance liquid chromatography (HPLC, Shimadzu LC-20AT, Japan) with an Aminex HPX-87H column (Biorad Laboratories, Milan, Italy) at 30 °C and with a diode array detector. The eluent was 2.5 mM H2SO4 at 0.6 mL/min. Effluent samples were always >20 mM acetate to ensure substrate saturation in the anode compartment. We also analyzed effluent samples for total carbonate species (H2CO3* + HCO3- + CO32-) concentrations using an ion chromatograph (IC, Dionex ICS-3000, California) with a Dionex IonPac AS18 column (4 mm × 250 mm) and a Dionex suppressor (ASRS 4 mm at 88mA). The eluent was produced with a KOH eluent generator at 1 mL/min using a 22-35 mM gradient (22 mM at 0-8 min, 22-35 mM at 8-9 min, 35 mM at 9-16 min). All HPLC and IC samples were run in duplicate. We generated polarization curves for the MFC by changing the external resistance of the MFC in the range of 5-900 Ω we also evaluated at the open-cell voltage. We collected voltage/current data across the resistor in intervals of 2 min using LABVIEW software and a National Instruments BNC2110 analog interface. We waited at least 45 min for each condition and averaged the last 20 min of data. Given that our experiments focus on optimizing the cathode performance, we normalized the reported current to the geometrical electrode surface area of the cathode (7.5 cm2) for comparison among experiments.

Results and Discussion Figure 3a shows the effects of adding CO2 to the air flow in the cathode over time when using a constant 25 Ω resistor in the MFC circuit. Voltage and power increased when the air flow contained 2% CO2 (versus air, which has only ∼380 ppm CO2). An increase in the CO2 concentration from 2% to 5% yielded a further small increase in voltage/power density, but increasing the CO2 concentration to 10% had a negligible effect. During our CO2 experiments over the course of 12 days we did not observe carbonate precipitation in the cathode surface. The lower pH values due to CO2 addition should minimize precipitation, but precipitation should be evaluated in future studies. In order to quantify the flow of carbonate (as HCO3- and/ or CO32-) we added a high phosphate buffer concentration (100 mM) in the anode medium. The high buffering allowed the carbonate that passed through the AEM to be retained in the liquid phase as HCO3- since the anode pH remained between 7.3 and 7.5 throughout the experiments. Figure 3b shows the total carbonate concentrations in the anode compartment throughout the experiment. Given that our medium did not contain any carbonate, all of the carbonate present was derived from acetate oxidation or CO2 feed in the cathode compartment. At 0% CO2 the total carbonate

FIGURE 2. Schematic of the MFC reactor with air/CO2 feed in the cathode compartment.

FIGURE 3. Voltage (a), power (a), and bicarbonate concentration in the anode compartment (b) during different periods of CO2 addition with a fixed resistor (25 Ω). was 5.3 ( 0.3 mM as a result of acetate oxidation alone, indicating that approximately 2.5 mM acetate was converted to HCO3-; this result is consistent with our influent/effluent acetate concentrations. On the basis of the influent rate (0.4 mL/min), acetate oxidation produced 2.1 × 10-3 mmol HCO3-/min. When 2% CO2 was added to the air flow in the cathode, the total carbonate concentration in the anode compartment increased to 10.2 ( 0.9 mM, a clear indication that CO2 was flowing through the AEM as HCO3- or CO32-. However, the increase of CO2 from 2% to 5% and 10% did not increase the HCO3- concentration significantly. This is consistent with our results in Figure 3a, in which an increase

in CO2 concentration over 2% did not increase the voltage and power density significantly. In order to obtain electroneutrality using HCO3-/CO32as the means of transporting OH- equivalents from the cathode to the anode, the rate of OH- transport as carbonate should equal the amount of electron equivalents circuited through the MFC. Our total current in the MFC was ∼8.5 mA when fed with 5-10% CO2. This corresponds to an e- flow of 5.3 × 10-3 e- meq/min, which needs to be balanced by a counter flow of 5.3 × 10-3 mmol HCO3-/min. On the other hand, if the transport occurs through CO32-, only one-half of the flow is needed since CO32- transports an equivalent of 2OH- per mol. Therefore, only a counter flow of 2.65 × 10-3 mmol CO32-/min is needed. On the basis of the influent flow rate of 0.4 mL/min, transfer of only HCO3- should result in an increase in total carbonate in the anode medium of 13.3 mM. On the other hand, the expected increase in total carbonate in the medium is 6.65 mM for CO32-. The measured increase in total carbonate in the anode medium was an average of 4.9 mM, a value that is more consistent with CO32transport rather than HCO3-, as depicted in Figure 4. The difference between 6.65 and 4.9 mM was from transport of OH-. In our analysis we do not include the possible diffusion of H2PO4- (present at 15.2 mM in the anode compartment) from anode to cathode, from which it could also serve as an OH- carrier by diffusing back to the anode. Diffusion of anions across an AEM to the cathode should be minimal given that migration forces are moving anions the opposite way. Figure 4 illustrates the OH- flows without and with addition of CO2. With 0% CO2, all of the OH- flow (4.66 × 10-3 e- meq/min for 7.5 mA) was carried by OH-. When CO2 was added in the cathode compartment at 2% or greater, the OH- equivalents were carried mostly by CO32- (3.91 × 10-3 e- meq/min), which constituted 81% of the total flow of 5.3 × 10-3 e- meq/min (for 8.5 mA), while OH- carried 19%. Even though OH- transport was still occurring, CO32transport allowed a 70% reduction in the flow of OH- through the AEM. The dominance by CO32- transport, along with some OH- transport, is an indication that the pH gradient was minimized but not completely avoided since CO32- would be dominant at high pH values (above pH 10). Figure 4 also shows how the OH- equivalents reacted with H+ equivalents released from the anode. H+ equivalents produced by bacteria as a result of acetate oxidation could be neutralized in three different reactions with OH-, CO32-, or HPO42- in the anode compartment, which had a stable pH in the range of 7.3-7.5; these three reactions are shown in VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Schematic of OH--equivalent fluxes across the AEM and reactions at the anode compartment to neutralize H+ produced as result of acetate oxidation for (a) air only and (b) air/CO2 fed into the cathode compartment. Fluxes of carbonate are estimated based on the total carbonate concentration increase in the anode compartment. Figure 4. When no CO2 was added to the cathode air, only OH- flowed through the membrane, stoichiometrically neutralizing the H+ produced (H+ + OH- f H2O). This is consistent with our observed effluent pH ) 7.5, which is the same as the influent pH. When CO2 was added to the cathode air, the CO32- transported through the membrane carried 2OH- (eq 3). However, only one OH- carried by CO32- was released to neutralize the H+ produced at the anode because pKa,1 for CO2/HCO3- is lower than the pH maintained by the phosphate buffer. Since all carbonates stayed in the liquid as HCO3-, HPO42- had to neutralize some of the H+ (H+ + HPO42- f H2PO4-). Consequently, the anode pH decreased from 7.5 to 7.3, which is consistent with H+ being neutralized roughly equally by protonation of CO32-, H2PO4-, and OH(Figure 4b). Figure 5a shows the decrease in potential as a function of current density. Anode potentials decreased only ∼60 mV from OCV (OCV was -0.453 vs Ag/AgCl; data not shown), indicating that most of the potential drop was due to cathode and transport processes. Addition of 5% CO2 to the air flow in the cathode increased the operational voltage between 80 and 120 mV in the area of maximum power output. An increase of 120 mV is consistent with a decrease of two pH units in the cathode compartment. Since pH values in the cathode compartment has been reported to be as high as pH 13 (12), it is likely that a pH gradient remained between anode and cathode compartments but was significantly reduced by CO2 addition. As the current density increased the gap between experiment operational voltage of the control experiment and CO2 addition decreased to 80 mV. This decrease in the gap is an indication of an increase in the pH of the liquid present in the cathode as the current density increased. The maximum power output increased from 1.9 8776

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FIGURE 5. Increase in microbial fuel cell voltage (a) and power (b) as a result of CO2 addition to the air fed in the cathode compartment. to 2.8 W/m2 by addition of 5% CO2 in the air stream (Figure 5b). This is a 45% increase in power as a result of an increase in voltage efficiency. However, an increase to 10% CO2 did not show any significant improvement in the voltage output. In our experimental setup we were not able to document the pH gradient between the anode and cathode compartment directly because of the lack of water in the cathode compartment (an air cathode). However, the increase in operational voltage is a clear indication of a decrease in pH gradient as this problem is known to be responsible for as much as 240 mV of the potential loss (12). Our results show an increase of as much as 120 mV in operational voltage using an air/CO2 mixture. Given that our experiments were performed in the same reactor, the increase in voltage can only be attributed to a decrease in pH gradient, as ohmic resistances and electrode overpotentials should be similar throughout the experiments. The increase in 120 mV corresponds to a 2 pH unit decrease in the pH gradient. However, our analysis seems to indicate that the pH in the cathode compartment is still higher than the anode compartment as CO32- and OH- were flowing through the AEM; it is unlikely that the pH in the cathode compartment was as low as in the anode compartment (pH 7.3-7.5). Therefore, a pH gradient in our MFC still caused a voltage loss, but it was less with the CO2 added to the air. We hypothesize that the rate of disolution of CO2 into the cathode water can limit the use of carbonate species as OHcarriers. Our calculations show that we dissolved CO2 in the cathode surface at a rate of 2.3 × 10-3 mmol CO2/min (based on 4.9 mM total carbonate increase in the anode). This is equivalent to dissolving 57 µL of CO2/min in a 7.5 cm2 of cathode surface area, or 7.6 µL/cm2 of cathode. Dissolution of CO2 into a small amount of water present at the cathode surface could be the limiting step in the process of CO2 as OH- carrier. Therefore, this approach could be improved by finding ways to maximize the rate of CO2 dissolution in the cathode compartment. For example, the benefits of CO2 addition could be even more advantageous in systems in

which the cathode compartment is filled with water, such as some microbial electrolytic cells (4, 16), since the rate of CO2 dissolution would be higher and not depend on the cathode surface area. Our experiments demonstrate a means to mitigate the pH gradient between the anode and cathode compartments in BFCs. Decreasing the pH gradient resulted in an increase in voltage efficiency and power output by simply adding CO2 to the cathode air at current densities of ∼1 mA/cm2. The process of CO2 addition can be applied to many types of BFCs other than the MFC application shown in our experiments. It can be applied to enzyme-catalyzed fuel cells that operate at a near-neutral pH as well as electrolytic cells that produce H2 gas from organic materials.

Acknowledgments We thank Asahi Glass Co. for providing the AEM to carry out the experiments. Funding for this work was provided by OpenCEL, LLC.

Supporting Information Available Analysis of the conditions required for CO2 recycle from the anode to cathode compartment in an MFC. This material is available free of charge via the Internet at http://pubs.acs.org.

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