Article pubs.acs.org/est
In Situ Investigation of Cathode and Local Biofilm Microenvironments Reveals Important Roles of OH− and Oxygen Transport in Microbial Fuel Cells Yong Yuan, Shungui Zhou,* and Jiahuan Tang Guangdong Institute of Eco-environmental and Soil Sciences, Guangzhou 510650, China S Supporting Information *
ABSTRACT: Mass transport within a cathode, including OH− transport and oxygen diffusion, is important for the performance of air-cathode microbial fuel cells (MFCs). However, little is known regarding how mass transport profiles are associated with MFC performance and how they are affected by biofilm that inevitably forms on the cathode surface. In this study, the OH− and oxygen profiles of a cathode biofilm were probed in situ in an MFC using microelectrodes. The pH of the catalyst layer interface increased from 7.0 ± 0.1 to 9.4 ± 0.3 in a buffered MFC with a bare cathode, which demonstrates significant accumulation of OH− in the cathode region. Furthermore, the pH of the interface increased to 10.0 ± 0.3 in the presence of the local biofilm, which indicates that OH− transport was severely blocked. As a result of the significant OH− accumulation, the maximum power density of the MFC decreased from 1.8 ± 0.1 W/m2 to 1.5 ± 0.08 W/m2. In contrast, oxygen crossover, which was significant under low current flow conditions, was limited by the cathode biofilm. As a result of the blocked oxygen crossover, higher MFC coulombic efficiency (CE) was achieved in the presence of the cathode biofilm. These results indicate that enhanced OH− transport and decreased oxygen crossover would be beneficial for high-performance MFC development. concentration of OH− at the catalyst layer can significantly affect the performance of the MFC.10 It is worth noting that the theoretical potential of the ORR in reaction 2 is much lower than that in reaction 1, thus representing a large potential loss because of the pH change in the microenvironment of the cathode. To the best of our knowledge, there is still a lack of in situ investigations on how pH microenvironments change in MFC cathodes. Additionally, several other problems that can significantly decrease MFC performance arising from the air-driven cathode MFC because of the absence of a membrane. For example, a cathode biofilm will inevitably form over the site of cathodic catalysis because of direct contact with the anode electrolyte in the reactor, thus resulting in an extra biofilm layer in the cathode zone. This cathode biofilm is believed to increase the internal MFC resistance, which subsequently decreases the generation of electricity.11,12 It has been assumed that the decrease in power output partially results from the physical blockage of mass transfer (i.e., H+ and charged ionic species) in the cathode region.13,14 This hypothesis has not been experimentally confirmed. Additionally, the power output can also be decreased by the adverse effect of oxygen leaking into
■. INTRODUCTION A microbial fuel cell (MFC) with an air-driven cathode has the advantage of high power generation and low cost, which is the most popular and practically feasible configuration. In such an MFC, membranes are unnecessary, and oxygen is passively transferred to a cathode using air without energy-intensive air sparging.1−3 As a result of the membraneless configuration, the cathode reactions occur directly in a medium with neutral pH, which is favorable for bacterial growth at the anode. However, the kinetics of the oxygen reduction reaction (ORR) are slow at a pH of 7.0, which has been considered to be an important limitation in MFC power output.4,5 Platinum (Pt) has been widely used to accelerate the slow cathodic ORR in MFCs. Generally, oxygen is reduced at the Pt cathode surface through one of the following reactions: O2 + 4e− + 4H+ → 2H 2O E 0 = +1.230V
(1)
O2 + 4e− + 2H 2O → 4OH− E 0 = +0.40V
(2)
On the basis of reaction 1, the ORR is accompanied by the consumption of protons in solution. Thus, the limiting factor for the cathode has been identified as low proton transport and availability to catalyst sites.6,7 In contrast, cathode alkalization has been observed in the MFC,8,9 which suggests that OH− is a main product of the ORR. The ORR has been proposed to occur through reaction 2, in which the microenvironmental © 2013 American Chemical Society
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Figure 1. Single-chamber air-cathode MFC equipped with microelectrodes for monitoring OH− and oxygen profiles (A), air-driven cathode structures and cathode reactions (B), SEM images of cathode biofilm (C), and (D, high magnification), digital photographs of a bare cathode and a biofilm-covered cathode (E), confocal laser microscope image of a cathode biofilm cross section showing its thickness (F).
the reactor.2,15,16 MFC separators have been suggested to overcome this oxygen crossover.17,18 However, little is known regarding how microenvironments at the cathode are associated with MFC performance and how they are affected by the biofilm that inevitably forms on the cathode. The goal of this study was to determine the properties of OH− and oxygen in the cathode biofilm of a real MFC. Generation of bioelectricity in the MFC is related to OH− and oxygen variation at the cathode, which offers constructive indications for improving MFC performance. Microelectrodes have been shown to be useful for monitoring microenvironmental changes at interfaces or in biofilms. However, they have primarily been used to monitor the microenvironments of anode biofilms in MFCs.19,20 In this study, we applied microelectrodes to observe in situ changes in the concentrations of OH− and oxygen at the cathode interface in the presence and absence of a local biofilm on the cathode. In addition, considering the practical wastewater treatment applications of MFCs, we also conducted measurements in the MFC using unbuffered wastewater as fuel.
MFC reactors were inoculated with 2.0 mL of activated anaerobic sludge (Liede Sewage Treatment Plant, Guangzhou, China) and 10 mL of sodium acetate (1000 mg L−1, pH 7.0) culture medium solution. After three repeated feeding cycles, only medium (no activated anaerobic sludge) was added. Reactors were considered to have started if the maximum voltage production was repeatable for at least three consecutive cycles of voltage output. The sodium acetate medium solution contained NaH2PO4·2H2O (2.77 g L−1), Na2HPO4·12H2O (11.40 g L−1), NH4Cl (0.31 g L−1), KCl (0.13 g L−1), a vitamin stock solution (12.5 mL L−1), and a mineral stock solution (12.5 mL L−1). After initiation, some MFCs were fed with domestic wastewater (abbreviated as WW, COD = 510 ± 60 mg L−1) collected from the same wastewater treatment plant (rather than acetate). The domestic wastewater had an average pH of 7.2 ± 0.3, and conductivity of 1.4 ± 0.2 mS cm−1. No additional buffers, vitamins, or minerals were added to the wastewater prior to its use in the MFC. Wastewater samples were stored at 4 °C, and new samples were collected approximately every two weeks. After stable voltage outputs were achieved, power density curves were obtained by changing the circuit resistor from 5000 Ω to 50 Ω, and a single resistor was used for each full batch cycle. All tests were conducted in batch mode in a 30 °C incubator. The power was normalized by the projected surface area of the anode. All tests were conducted in triplicate, and the mean values are presented here. Microelectrode Measurements. Prior to taking measurements, the MFCs were operated for more than three months, which ensured stable generation of electricity and the formation of a mature biofilm inside the systems.22 pH measurements were obtained using a pH microsensor (50 μm in diameter, response time of ca. 30 s) connected to a multimeter (Unisense, Aarhus, Denmark). Prior to its use, the sensor was calibrated using standard pH buffers. Oxygen measurements were obtained using an oxygen microsensor (50 μm in diameter, response time of ca. 5 s) connected to the same equipment. Before the measurements, the oxygen microsensor was polarized at +800 mV to achieve a stable signal output. The
■. MATERIALS AND METHODS MFC Construction and Operation. Single-chamber air cathode MFCs with a liquid volume of 12 mL were constructed as previously described.21 The cylindrical MFC chamber was made of Plexiglas with a length of 1.7 cm and diameters of 3.0 cm on the cathode side and 1.8 cm on the anode side. The cathode and anode surface areas were 7 cm2 and 2.5 cm2, respectively. A non-wet proofed carbon cloth (type A) was purchased from Fuel Cell Earth LLC Inc. (USA) and used for the anode. The cathode was prepared from 30% wet-proofed carbon cloth (type B) with four layers of polytetrafluoroethylene (PTFE) coating. The other side of the cathode was coated with Pt/C (0.5 mg cm−2 Pt loading) as a catalysis layer for oxygen reduction. The anode and cathode were placed on opposite sides of the cell. The catalyst-coated layer faced the anode, and the PTFE-coated gas diffusion layer faced the air. 4912
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where Dbox is the diffusion coefficient for the dissolved oxygen in the biofilm, Cbox is the concentration of dissolved oxygen, and Vmax represents the maximum rate achieved by the system at the maximum substrate concentration. Ks is the substrate concentration at which the reaction rate is half of Vmax. The steady-state concentration (dC/dt = 0)b within a biofilm is achieved when consumption equals the rate of transport because of diffusion. Therefore, eq 6 can be derived as follows:
sensor was calibrated in both oxygen-saturated and oxygenscavenged solutions. The microelectrodes were placed in the MFC as shown in part A of Figure 1. Nitrogen gas was pumped into the headspace of the MFC reactor to prevent oxygen diffusion from the gap between the microelectrode and the reactor. The movement of the microelectrodes was controlled by a micromanipulator (Unisense, Aarhus, Denmark). Variations in pH and oxygen with distance were measured when the MFCs produced a stable power output under controlled conditions. Curves for pH or oxygen concentration versus time were obtained by placing the microelectrodes at a fixed position during a complete MFC batch cycle operation. Scanning Electron Microscopy (SEM) of Cathode Biofilms. SEM was applied to confirm the presence of cathode biofilms. Prior to SEM, the cathode samples were fixed in 2.5% glutaraldehyde solution for 1 h and then in an ethanol dehydration series (i.e., 25%, 50%, 75%, and 100% v/v EtOH, 0.5 h for each treatment) and then dried at the CO2 critical point for 3 h. The resulting specimens were coated with gold using a coating device (Emitech K550X, UK) and observed using an SEM (JEOL, JSM-6330F; Japan) at 20 kV. Confocal Laser Microscopy. The cathode was removed from the MFC and cut into small, narrow pieces (width of approximately 1 mm). The samples were then stained for 20 min with a LIVE/DEAD BacLight viability kit. After rinsed to eliminate excess dye, the samples were fixed to a glass slide by placing the carbon cloth in a standing position. The confocal images were captured with a laser scanning microscope (Leica SPE, Germany) using an argon laser at 488 nm as an excitation source. Calculations. The OH− transport behaviors of different cathode layers were predicted by Fick’s law. The flux of OH− through the biofilm−electrolyte interface and diffusion layer was as follows: ⎛ dC b − ⎞ OH b b ⎟⎟ JOH − = DOH−⎜ ⎜ ⎝ dx b ⎠
(3)
w ⎞ ⎛ dCOH − w w JOH ⎟ − = DOH−⎜ ⎝ dx w ⎠
(4)
⎛ d2C b ⎞−1 Db × K s 1 Doxb ⎜⎜ 2ox ⎟⎟ = ox + Vmax Coxb Vmax ⎝ dx ⎠
or 1/2 ⎡ V ⎛ ⎛ dC b ⎞ K s + Coxb ⎞⎤ ox max b ⎟⎟⎥ ⎜⎜ ⎟⎟ = ⎢2 b ⎜⎜Cox − K s × ln K s ⎠⎥⎦ ⎝ dx ⎠ b ⎢⎣ Dox ⎝
C Jox =
C Dox 0 *) (Cox − Cox L
(9)
where is the oxygen flux at the cathode interface, D ox is the mass transfer coefficient of oxygen, L is the cathode thickness, C0ox is the concentration at the air side of the cathode, and C*ox is the oxygen concentration at the cathode interface. Dcox/L was 2.3 ± 0.2 × 10−3 cm/s with the same cathode configuration according to the literature.24 Similarly, the oxygen flux continuity must be preserved at the catalyst− biofilm interface. Thus, the cathode biofilm oxygen diffusion coefficient Dbox can be calculated. Jcox
c
■. RESULTS AND DISCUSSION Electrochemical Performance of the Cathode under Different Conditions. Without a membrane or separator, it is difficult to prevent the formation of biofilms on the cathode interface because of direct anolyte exposure in a single-chamber air cathode MFC. As shown in parts B and C of Figure 1, SEM revealed a dense biofilm covering the cathode. This cathode biofilm was observable with the naked eye as shown in part D of Figure 1. A representative confocal scanning laser microscopy image shows that the biofilm had a thickness of up to 1000 to 1500 μm (part E of Figure 1). The thickness of cathode biofilms can reach 2 to 3 mm after a long duration of operation, as reported in previous study.26 It is worth mentioning that this thickness is 10 to 100 times larger than the thickness of the anode biofilm of an MFC.27 Biofilms have been considered to be diffusion barriers for mass ion transport to the cathode interface and to therefore affect the overall reaction rate at the electrode surface.28 As depicted in part F of Figure 1, the presence of a cathode biofilm is expected to affect the transport of H+ or OH−. Additionally, the cathodic bacteria may consume a portion of the available oxygen at the catalytic sites by aerobic respiration and thus reduce the cell voltage. The influence of the cathode biofilm on cathode performance was investigated by artificially controlling electrolyte pH using a half-cell from a previous study.10 As shown in Figure S1 of the Supporting Information, the current−potential curves reveal that the current density increased when the cathode biofilm was removed, and the current density decreased when the pH was
(5)
The derivative dC/dx can be calculated from the slope of the OH− concentration−distance profiles in two layers. DwOH− is 5.3 × 10−9 m2/s according to a previous study.10 DbOH− can be calculated from eq 5. As suggested in a previous study,23 the rate of oxygen consumption in a cathode biofilm can be described by the following equation, which is derived from a differential mass balance: ⎛ ∂C b ⎞ ⎛ ∂ 2C b ⎞ V × Coxb ⎜⎜ ox ⎟⎟ = Doxb ⎜⎜ 2ox ⎟⎟ − max K s + Coxb ⎝ ∂t ⎠ b ⎝ ∂x ⎠ b
(8)
From these derivations, the Ks and dC/dx in the cathode biofilm can be calculated. The oxygen flux in the cathode catalyst layer was calculated as follows:24,25
where J is the OH− flux driven by the concentration gradient, dC/dx is the measured concentration gradient of OH−, and D is the diffusion coefficient in the catalyst and biofilm layer. Subscript b indicates biofilm, and subscript w indicates water. Flux continuity must be preserved at the interface of the liquid film and the biofilm, where Jw and Jb were assumed to be equal. w ⎞ ⎛ dC b − ⎞ ⎛ dCOH − OH w b ⎟⎟ −⎜ −⎜ DOH ⎟ = DOH ⎜ ⎝ dx w ⎠ ⎝ dx b ⎠
(7)
(6) 4913
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increased from 7.0 ± 0.1 to 8.0 ± 0.1. These results demonstrate that the cathodic reaction was affected by both the biofilm and electrolyte pH condition. However, this measurement was obtained by artificially controlling the pH of the electrolyte, which does not reflect the real environment in a well-maintained MFC. OH− Cathode Profiles. In situ pH variations at the cathode interface were measured to determine the real OH− cathode profiles during MFC operation. As shown in Figure 2, the
pH of the cathode solution was measured in a two-chamber MFC, which does not reflect the important microenvironment of the cathode ORR at the electrode interface. OH− accumulation was higher when the cathode biofilm was present under high current flow conditions. As depicted by the blue line in Figure 2, the cathode interface had a pH of 10.0 ± 0.3, and the pH changed greatly with depth in the presence of a cathode biofilm at 100 Ω. A distinct pH peak of 11.6 ± 0.3 appeared at a depth of approximately 1 mm, which was identical to the measured thickness of the cathode biofilm. The pH peak was less obvious in the absence of a cathode biofilm under the same resistance load. This result confirms the influence of the cathode biofilm on OH− accumulation, which may be an important reason for the decreased power output of the system. Similar phenomena were observed for unbuffered wastewater-fed MFC. Theoretically, pH variations should be larger in this case than in a buffered MFC.31 Lower pH variation was discovered in the wastewater-fed MFC that may have resulted from low electron flow following low substrate bioavailability for microbes and ion conductivity. Nevertheless, the cathode zone was more alkaline in the presence of a cathode biofilm under both conditions. The increased OH− accumulation resulted from the blockage of OH− transportation by the cathode biofilm. Cathode Oxygen Profiles. According to reactions 1 or 2, the oxygen concentration is another decisive variable in cathode performance. Figure 3 shows the oxygen profiles under various
Figure 2. Variations in pH at the cathode and local biofilm under closed-circuit mode with different loading conditions and open-circuit (OC) mode in the acetate-fed (upward curves) and wastewater-fed (downward curves) MFCs. The error bars represent ± SD (n = 3).
cathode regions where ORR occurred became alkaline under all closed-circuit modes, but no pH change was observed under the open circuit mode suggesting that pH variations were strongly related to the current generation process. The pH of the cathode started to increase when current generation was initiated, and it decreased as the substrate was depleted. Under control conditions with no current generation (open circuit mode), no pH change was observed during the entire process as shown in Figure S2 of the Supporting Information. Additionally, the pH was 7.7 ± 0.2 at the cathode interface when the fuel cell was under a 1000 Ω load, and the pH increased to 9.4 ± 0.3 with loading of 100 Ω into the acetatefed MFC (Figure 2). These results indicate that the variations in cathode pH were coupled with the amount of electrons provided by the anode. The pH profiles exhibited a sharp peak in the cathode zone, which suggests significant OH − accumulation under all conditions. Phosphate buffer is typically used to minimize pH variation in MFCs. The greatly increased pH of the cathode interface during electricity generation demonstrates that phosphate buffer does not actually maintain a neutral cathode pH value. pH variations in the cathode solution have been previously observed, and they are considered to limit the performance of real MFCs. The pH of the cathode solution increased from 7.0 to 8.3 in a twochamber MFC,29 and the pH increased to over 9.5 in an unbuffered MFC after the fuel was added.30 However, only the
Figure 3. Variations in oxygen concentration in the cathode and local biofilm under open-circuit (OC) and closed-circuit modes with different loading in the MFCs. Error bars represent ± SD (n = 3).
conditions. The concentration of oxygen was 150 ± 10 μM (4.8 mg L−1) at the cathode interface, and the oxygen was almost completely consumed at a depth of 1 mm at 100 Ω. Oxygen was almost undetectable in the anode zone, even in the largest part of the electrolyte bulk zone in the presence of substrate. The effect of the cathode biofilm on the oxygen profile was significant under low current density conditions. Oxygen diffused to a depth of almost 3 mm when the cathode biofilm was removed at 1000 Ω, whereas it only diffused to a depth of 1 mm in the presence of the biofilm. The consumption of biofilm oxygen was found to occur through metabolic consumption of aerobic bacteria or catalytic reduction by the biofilm in previous studies.32,33 The oxygen profile in the cathode region was highly related to the electricity generation of the MFC. Under controlled conditions with no current generation (open circuit mode with only PBS solution), the oxygen concentration was almost consistent across the entire measured distance scale (Figure 3). It is worth mentioning that oxygen diffusion was 4914
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Figure 4. MFC voltage output curves at 1000 Ω (A). Power density curves under different conditions (B). Anode (solid symbols) and cathode potentials (open symbols) vs current density (C). CE vs current density (D).
also limited under the open circuit mode in the presence of acetate in the electrolyte in which oxygen could diffuse across a limited distance (ca. 3 mm). As shown in Figure S3 of the Supporting Information, the oxygen concentration at the fixed position of the bulk solution (5 mm away from the cathode interface) gradually decreased to almost zero within 8 h in the presence of acetate under the open circuit mode. In this case, bacterial (plankton and biofilm) respiration must contribute to oxygen consumption.34 Effect of Local Biofilm on the Cathode on Power Generation. The cathode microenvironment was closely associated with the MFC power output. Power generation was greatly affected by cathode biofilms, and the local pH was subsequently affected in both buffered and unbuffered solutions. As shown in part A of Figure 4, the voltage output at 1000 Ω was 0.54 ± 0.02 V in the presence of a biofilm on the cathode with sodium acetate as the substrate. The voltage output increased to 0.59 ± 0.02 V as the cathode biofilm was completely removed. The voltage outputs (0.42 ± 0.01 V in the presence of biofilm and 0.48 ± 0.016 V in the absence of biofilm) from wastewater were significantly lower than those of acetate under the same conditions. Nevertheless, the effect of the cathode biofilm on voltage output was identical for these two substrates. In terms of maximum power density, the acetate-fed MFC produced 1.5 ± 0.08 W/m2 and 1.8 ± 0.1 W/ m2 with and without a cathode biofilm, and the wastewater-fed MFC produced 0.4 ± 0.02 W/m2 and 0.55 ± 0.04 W/m2, respectively. The variations in power output were closely associated with the pH profile of the cathode as mentioned above. Lower power was obtained when using wastewater in the MFC mainly because of the lower ionic strength and substrate availability as well as large pH variation in the
unbuffered electrolyte. Individual electrode potentials verified the influence of a cathode biofilm on power generation in this type of system. As shown in part C of Figure 4, the anode potentials were almost identical for the MFCs fed with the same substrate, but the cathode potentials varied because of the presence of the cathode biofilm, in which lower cathode potentials at all current densities were observed for both acetate-fed and wastewater-fed MFCs. The oxygen profiles were closely associated with the CE characteristics of the MFC. With deeper oxygen diffusion into the electrolyte, the CE decreased (part D of Figure 4). OH − and Oxygen Transport Properties in the Cathode. We depict the possible OH− and oxygen transport in the cathode in the presence of a cathode biofilm in Figure 5 (middle panel). As previously suggested, the cathode can be classified into several layers with differences in OH− or oxygen transport, such as the cathode catalyst layer and diffusion boundary layer.10 However, the cathode biofilm layer has not been considered in this model even though it plays important roles in OH− and oxygen transport in the cathode as revealed by the pH and oxygen profiles measured in the current study. On the basis of the pH profile slopes in Figure 2 (lower panel, pink line), we predicted the OH− transport behavior in these layers as shown in Figure 5 (lower panel). The mechanism in reaction 2 implies that OH− transport rather than proton transport governs cathode performance. The accumulation of OH− at the cathode interface would result in significant OH− concentration overpotential (η [OH−]) and contribute to potential cathodic loss in the MFC. From the Nernst equation (Supporting Information), we can calculate the η[OH−] using the measured pH as suggested by Popat et al.10 For instance, the pH values were 7.0 ± 0.1 in bulk and 10.0 ± 0.3 at the catalytic 4915
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cathode zone and the circulation of the anolyte and catholyte are additional solutions, but additional energy is required to purge high concentrations of CO2 and force the liquid to flow.36,37 The selection of anion-conducting binders will also be a good option to enhance OH− transport.10 Oxygen crossover to the anode is not significant in the presence of a substrate in the MFC. Both plankton and the cathodic biofilm consumed oxygen, which affected the CE of the MFC. Membranes and separators may help to achieve a high CE.14,17,18 In summary, we have experimentally verified high OH− accumulation in a single-chamber air-driven MFC by measuring in situ pH profiles for the cathode interface using microelectrodes. The accumulation of OH− increased in the presence of a cathode biofilm. We did not observe oxygen diffusing to the anode region. Rather, oxygen crossed over to the bulk electrolyte and was consumed by plankton, which was associated with substrate consumption in the MFC that consequently decreased the CE. The findings of the present study could be instructive for developing high-performance air cathodes in the future.
Figure 5. Schematic representation of OH− and oxygen transport in the cathode and local biofilm layers.
site in acetate-fed MFCs in the presence of the cathode biofilm, and 9.4 ± 0.3 at the catalytic site in the absence of the cathode biofilm. Thus, the η[OH−] values were calculated to be 0.18 ± 0.02 V and 0.14 ± 0.02 V, respectively. The higher value of η[OH−] suggested a detrimental effect of the cathode biofilm on the cathode performance. Note that the difference between these two values was 0.035 V, which is consistent with the results derived from the measured individual cathodic potentials as shown in part C of Figure 4. The difference increased to 0.051 V at a higher current density, which suggests that the loss of cathodic potential mainly resulted from OH− accumulation. The OH− diffusion coefficient (DbOH−) in the biofilm was obtained in the calculation depicted in the Experimental section. The DbOH− was calculated to be 1.7 × 10−9 m2/s in the unbuffered MFC and 2.2 × 10−9 m2/s in the buffered MFC, which is lower than that of water (D = 5.3 × 10−9 m2/s). The difference in these two values is not as large as expected because the wastewater had some buffering capacity from the presence of weak acids and salts. Additionally, the CO2 produced by anodic organic oxidation may be converted into bicarbonate buffer in a closed MFC system.35 Similarly, the Dbox of the biofilm was also calculated to be 0.96 × 10−9 m2/s in the cathode biofilm, which was also much lower than that in the water (D = 2.5 × 10−9 m2/s). Implications for Cathode Development. The cathode is believed to be one of the key factors in achieving higher electricity output in an MFC. Efforts have been made to search for alternatives to expensive Pt to reduce cathode costs. In contrast, the present study indicates that cathode performance is greatly affected by the microenvironment in the cathode zone, which has not been considered in previous studies. Based on the results of this study, we can constructively suggest that several issues should be further addressed in future cathode development. One issue is that the electrochemical performance of new catalysts used in MFCs should be examined under alkaline conditions. Thus far, many new cathode catalysts for oxygen reduction reactions have been developed, but the catalytic reaction has been examined under neutral conditions in most cases. Another suggestion is that the formation of a cathode biofilm should be prevented to achieve higher power output. One solution is to introduce bacterial growth inhibitors such as silver nanoparticles to the cathode, as suggested by An et al.31 Finally, OH− transport should be accelerated to prevent overpotential loss in the ORR. The addition of CO2 in the
■
ASSOCIATED CONTENT
S Supporting Information *
Electrochemical measurement, calculation of the overpotentials, polarization curves of cathodes; pH vs time curves at a fixed position at 100 Ω; and oxygen concentration vs time curves at a fixed position. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: 86-20-3730-0951; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This study was supported jointly by the National Natural Science Foundation of China (Nos. 41101211, 21277035, and 41222006).
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REFERENCES
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Environmental Science & Technology
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dx.doi.org/10.1021/es400045s | Environ. Sci. Technol. 2013, 47, 4911−4917
