A microbial fuel cell (MFC) represents a novel type of system for converting chemically bound energy in organic waste into electricity [1
]. In MFCs bacteria serve as biocatalysts to oxidize organic matter in the anode chamber and subsequently transfer electrons to the anode electrode. These electrons are then transferred through an external circuit to the cathode, where the reduction reaction takes place. Typically the reduction at the cathode surface involves reduction of O2
]. The latest research has mainly focused on the bacterial metabolism on the anode electrode surface [3
], improving anode electrode materials [4
], optimizing the anode inoculum [5
], speeding up the initiation through selection of substrate [6
], and on improving the reactor design and configuration for maximizing conversion efficiency and output [1
In addition to maximizing the microbial oxidation reaction in the anode chamber it is important to optimize the cathode reaction. One aspect of the cathode performance is the cathode reaction design. Potassium ferricyanide cathode, K3[Fe(CN)6], abbreviated here as FeC, is a well-studied and efficient type of cathode reaction employed in MFCs, relying on Fe3+ → Fe2+ reduction for electron consumption. However, the use of FeC presents some toxicity issues due to the cyanide. Comparison of FeC with other types of cathodes, e.g., oxygen based cathodes, can therefore provide an improved decision base for selecting better and more sustainable electrode reactions for MFC technology.
Another key issue in MFC research is to understand the biotic and abiotic factors that limit electric current output [7
]. The limiting factors are generally represented by internal resistance, which is caused by activation resistance, ohmic resistance and concentration resistance [8
]. The ohmic resistance can be estimated by electrochemical impedance spectroscopy (EIS), which provides information for estimation of the electrochemical reaction rate [7
]. The detailed technique and data analysis have been introduced in several recent reports [7
The current generation is influenced by operational parameters including external resistance and substrate loading rate. The substrate loading rate is usually expressed as chemical oxygen demand (COD) in the unit g/(L∙day). Lowering of external resistance can result in an increase in current generation. However, an increase in substrate loading rate only results in an increased current generation when the external resistance is low enough to enable current generation [12
Substrates that have been tested in MFCs include acetate, which can be directly used by the electrogenic bacterium Geobacter sulfurreducens
] and fermentable sugars such as xylose, which are converted by fermentative bacteria resulting in acetate formation [14
]. Bioethanol effluent is a solution rich in xylose resulting from second generation bioethanol fermentation and distillation [15
]. Bioethanol effluent is suitable as substrate for current generation in MFCs when the MFC is initiated using acetate [6
]. However, further performance improvement can be achieved by the use of inocula from different environmental sources such as lake sediment [5
]. In addition, an efficient cathode process is required before the use of MFC for current generation is feasible.
The aim of this study was to assess the performance of three different cathode configurations (dissolved oxygen cathode (DOC), FeC, and air cathode (AiC)). Moreover, the effects of substrate loading rate and external resistance were examined to optimize the electrical current generation in MFCs running on bioethanol effluent as substrate in the anode chamber. It was thereby expected to maximize power generation and to obtain an improved understanding of the significance of the cathode process for current generation.
The FeC and DOC cathodes were evaluated in dual chamber MFCs with the cathodes designed to constitute the wall in the cathode chamber opposite to the membrane (Figure 1
a), whereas the AiC cathode was examined in a single-chamber MFC reactor configuration (Figure 1
b). The air electrode was constructed as a sandwich composite cathode (Figure 1
c). The reactor was designed so that the anode constituted the wall on the opposite side of the membrane in all three MFC configurations (Figure 1
a,b). The comparisons of cathode performance were done over seven periods; initially, Period 1 (0–10 days), the anode chambers contained minimal medium (M9) and acetate, then in subsequent cycles bioethanol effluent was added at increasing substrate loading rate. The external resistance was set to decrease versus
time period. The cathode compartments tested contained a solution of 10 g/L NaCl bubbled with air for the DOC-MFC, K3
+ phosphate buffer for the FeC-MFC and air for the AiC-MFC, respectively.
2.1. Maximum Current Generation Found from the Polarisation Curves
To assess the effect of cathode reaction on maximum current density and maximum power density, cell voltage and power were plotted versus
current density obtained during the initiation phase with acetate (specifically data were taken at day 4 in Period 1; Figure 2
). The open-circuit voltage was found to be 407, 692 and 674 mV for the DOC-, FeC- and AiC-MFCs, respectively, i.e.
, the FeC-MFC produced highest voltage at zero current (Figure 2
a). The FeC-MFC also showed highest maximum current density (1480 mA/m2
) and maximum power density (580 mW/m2
), followed by the AiC-MFC with 1130 mA/m2
and 340 mW/m2
, respectively. The DOC-MFC thus generated lowest maximum current density (630 mA/m2
) and maximum power density (57 mW/m2
) (Figure 2
b). These results are in agreement with the finding that the FeC-MFC had the lowest internal resistance of 0.18 Ωm2
). The curves in Figure 2
a were linear in the range 200–600 mA/m2
for DOC and AiC and in the range 200–1100 mA/m2
for FeC. The process is limited by electrolyte resistance in these ranges. The normalized internal resistance, obtained from the slopes of the cell voltage curves were 0.46, 0.32 and 0.18 Ωm2
for the MFCs with DOC, AiC and FeC cathodes, respectively.
shows the anode and cathode electrode potentials versus
an Ag/AgCl reference electrode plotted versus
current density. The gap in mV between the curves equals the cell voltage plotted in Figure 2
current density. This plot makes it possible to determine the performance of each electrode reaction individually while Figure 2
a shows the performance of the entire MFC. The electrode which limits the performance will result in a curve with a steep slope versus
current. The regression line of the curves is linear in the ohmic losses region [16
], but the slope can be increased if there is a large over-potential and/or high concentration resistance.
The curves for cathode electrode potential for DOC showed a fast linear drop (slope = −0.43 Ωm2) due to the slow cathodic redox reaction rate caused by slow transport of dissolved oxygen, which limits maximum current density for the cathode. The similar slopes for FeC- and AiC-cathodes were −0.11 Ωm2 and −0.10 Ωm2, respectively, in the linear current range 200–600 mA/m2. The slope on anode potentials of the three different cathode compositions were +0.08, +0.07 and +0.22 Ωm2 for DOC, FeC and AiC, respectively.
2.2. Resistance in the Anode and Cathode Electrodes
The experimentally obtained EIS data are presented in Figure 4
as Nyquist plots showing the imaginary impedance (−Z
the real impedance (Z
) of the measured data. The three cathode setups were plotted for cathode and anode impedance contribution in Figure 4
a,b, respectively. For all three cathodes, ohmic resistance was above zero due to resistance caused by ion migration between the electrodes through the solution. Ohmic resistance for the cathodes were 0.05, 0.04 and 0.02 Ωm2
for the DOC-, FeC- and AiC-MFCs, respectively (Figure 4
a). Especially for DOC- and AiC-MFCs, these resistance values were lower (9 and 5 times, respectively) than the total internal resistance estimated by linear regression of voltage in Figure 2
a and electrode potentials in Figure 3
). The discrepancies can be explained by larger concentration resistance in these two cases at low frequencies (large Z
’), and since oxygen dissolution is limiting with the DOC cathode. For all three anodes, the ohmic resistance was low due to a low electrolyte resistance. Ohmic resistance for the anode were all in the range of 0.05 ± 0.02 Ωm2
, namely at 0.02, 0.07 and 0.02 Ωm2
for DOC-, FeC- and AiC-MFCs, respectively (discernable on the x
-axis in Figure 4
These ohmic resistances were for the DOC and AiC-MFCs lower than the internal resistance estimated by linear regression in Table 1
. Again, the difference may be due to larger concentration resistance found in these two cases at low frequencies (large Z
’). The data corroborated that the K3
solution was much more efficient than the NaCl solution for current generation. The EIS data for the anode and cathode thus explained that the FeC-MFC gave the highest maximum current density, followed by the AiC-MFC and the DOC-MFC.
2.3. Current Generation in Relation to Substrate Loading Rate and Switching to Bioethanol Effluent
It has been found that substrate loading rate and external resistance determines the maximum charge and current, which can be generated in a MFC [5
]. Increasing substrate loading rate at decreasing external resistance was investigated to optimize electricity generation in the MFCs as shown in Figure 5
The thereby resulting current generation, electrode potentials and coulombic efficiency (CE
period number are summarized in Table 2
. During the first 10 days, the MFCs were fed with acetate at an external resistance of 150 Ω (Period 1). The FeC-MFC generated highest average current density (444 mA/m2
), followed by AiC-MFC (397 mA/m2
) and DOC-MFC (142 mA/m2
). This result agreed with the EIS results that internal resistance was lower with the FeC- and AiC-MFC than with the DOC-MFC (Table 1
and Figure 4
). The lowest average current density of the DOC-MFC is partly due to the low open circuit cathode electrode potential of −0.28 V generated by reduction of dissolved oxygen compared to +0.23 V for the FeC and AiC cathodes (Figure 4
). Afterwards, all the MFCs were switched to utilize bioethanol effluent containing 65 g/L COD, 20.5 g/L xylose, 1.8 g/L arabinose and 2.5 g/L propionic acid. At an external resistance of 150 Ω, the substrate loading rate was increased from 0.5 g·COD/(L∙day) to 1.0 g·COD/(L∙day) (Periods 2 and 3). Remarkably, among all the parameters only CE
decreased from Periods 1 to 3. This may be due to the addition of bioethanol effluent, which can enrich the population of fermentable bacteria [6
During the subsequent Periods 3–5 the substrate loading rate was kept at 1.0 g·COD/(L∙day). At the same time, the external resistance was decreased from 150 Ω in Period 3 to 75 Ω in Period 4 and to 47 Ω in Period 5. This decrease in external resistance resulted in a significant increase in current density and CE
). The FeC-MFC showed highest average current density (1114 mA/m2
) and CE
(62%), followed by AiC-MFC with 920 mA/m2
and 49%, respectively and by DOC-MFC with 186 mA/m2
and 19%, respectively (Period 5). The decrease in external resistance also decreased the cathode electrode potential in DOC-MFC from −290 mV to −390 mV and in AiC-MFC from +150 mV to +50 mV due to the increase in current density.
At an external resistance of 47 Ω, the average current density in FeC- and AiC-MFCs was increased by doubling the substrate loading rate from 1 g·COD/(L∙day) to 2 g·COD/(L∙day) (Periods 5 and 6). This indicates that an increase in substrate loading rate results in increased current when the resistance is low. However, CE decreased significantly because of consumption of excess substrate by fermentative bacteria. The decrease in external resistance from 47 Ω to 27 Ω at a substrate loading rate of 2 g·COD/(L∙day) resulted in an increase in average current density from 1279 mA/m2 to 1640 mA/m2 in FeC-MFC, while the average current density in AiC-MFCs decreased from 1043 mA/m2 to 802 mA/m2. The decreased average current density in AiC-MFC was accompanied by a significant rise of anode electrode potential, from −400 mV to −50 mV, while the FeC-MFC only showed slight rise from −390 mV to −340 mV. This indicates that the optimal external resistance for the AiC reaction was 47 Ω. However, the FeC-MFC generated by far the highest average current density (1640 mA/m2) at the lowest external resistance (27 Ω).