the different potentials. The biocathode controlled at +0.15 V vs Ag/AgCl had the highest current, while the biocathode with the highest overpotential, the one controlled at +0.05 V vs Ag/AgCl had considerably lower current. We hypothesize that this lower current may be caused by possible production of H2O2 at the electrode. The thermodynamic potential at which H2O2 is produced is +0.269 V vs NHE at pH=7 and pO2=0.2 bar (Hamelers et al., 2010), which corresponds to a cathode potential of +0.064 V vs Ag/AgCl. Only at potentials lower than this thermodynamic potential, H2O2 can be formed. So, the biocathode controlled at +0.05 V vs A/gAgCl may have been negatively affected by H2O2 and therefore produced a lower current. No H2O2 measurements have been done in this study, however, these would be useful to study if H2O2 negatively affects biocathode performance.
5.3.2 Biocathode performance increased with time
Performance of the biocathodes during and after start-up was analyzed with time by polarization curves. The result for each cell is shown in Figure 2A-C, where the first two biocathodes were studied from day 0 to day 23, and the third biocathode was studied from day 0 to day 76. Performance of all three biocathodes improved with time: the maximum current density that is reached in the polarization curves increases with time, and also cathode potential increases with time at the same current density. The increase in performance with time was most pronounced for the cell with the biocathode controlled at +0.15 V vs Ag/AgCl, and less pronounced for the cell with the biocathode controlled at +0.25 V vs Ag/AgCl, when considering the same time frame from 0 until 23 days. This supports the observed differences in start-up time. The biocathode controlled at +0.25 V vs Ag/AgCl was tested and
characterized during a longer time period, which resulted in a similar polarization curve as the biocathode at +0.05 V vs Ag/AgCl, only after 76 days instead of 23 days. Especially the polarization curve for the biocathodes controlled at +0.15 V vs Ag/AgCl shows a limiting current: below a cathode potential of +0.2 V vs Ag/AgCl, a further decrease in potential does not result in an increase in current density. This may indicate that maximum activity of the biofilm is reached, or that mass transfer of oxygen is limiting biocathode performance.
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Figure 2. Development of the biocathodes can be seen in the polarization curves of the three cathodes. The three cells show an increase in performance with time, showing activity of the biofilm. The cathodes were controlled at (A) +0.05 V vs Ag/AgCl, (B) +0.15 V vs Ag/AgCl, and (C) +0.25 V vs Ag/AgCl. Note that Figure A and B show the development from day 0 to day 23, while Figure C shows the development from day 0 to day 76.
5.3.3 Cyclic voltammetry showed catalytic behavior for oxygen reduction
At the end of the experiment, the catalytic behavior of the biocathodes was tested using cyclic voltammetry. The biocathodes were tested under aerobic and anaerobic conditions. The result for the biocathode controlled at +0.15 V vs Ag/AgCl is shown in Figure 3. It can be seen that the maximum current density of 295 mA/m2 was reached at a cathode potential of +0.2 V vs Ag/AgCl. At lower cathode potential, this current density was constant. Under constant nitrogen flushing, the current density was considerably decreased to 45 mA/m2, indicating that
A
B
C
time
time
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Figure 3. Biocathode controlled at +0.15 V vs Ag/AgCl shows catalytic behavior for oxygen reduction,at an oxygen concentration of 6.5 mg/L (O2) and <0.1 mg/L (N2). The abiotic control produced a
current only below a cathode potential of +0.1 V vs Ag/AgCl at an oxygen concentration of 7.5 mg/L (O2) and 0.2 mg/L (N2).
indeed oxygen reduction is the reaction that is catalyzed. When comparing these results with the cyclic voltammograms obtained for chemical oxygen reduction, it can be seen that indeed the oxygen reduction reaction is catalyzed by the microorganisms: the abiotic control does not produce a current before the cathode potential decreased below +0.1 V vs Ag/AgCl, while the biocathode produced 295 mA/m2 at this same potential. Under constant nitrogen flushing, current production decreased as a result of low oxygen concentration.
The other two biocathodes showed similar catalytic behaviour to the biocathode controlled at +0.15 V vs Ag/AgCl, the main difference being a lower maximum current density for both cells compared to the biocathode controlled at +0.15 V vs Ag/AgCl. The solution alone was also tested for catalytic behaviour by means of cyclic voltammetry. No calalytic behavior was found, and there was no distinct difference between anaerobic and aerobic cyclic voltammetry scans of the solution, meaning that the catalytic activity was caused by the biofilm on the electrode.
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5.3.4 Biocathodes were both mass transfer and charge transfer limited
To investigate the limiting factors in biocathode performance, the current density of the biocathode was studied at different oxygen concentrations, different recirculation rates, and different potentials. This experiment was done in the cell where cathode potential was originally controlled at +0.15 V vs Ag/AgCl. Current was limited by mass transfer of oxygen, as a decrease in oxygen concentration from 100% to 65% to 0.8% resulted in a decrease in current density of 241 to 194 to 15 mA/m2. This mass transfer effect was further studied by changing the recirculation rate, hereby changing oxygen transfer.
Figure 4 shows the current density at different recirculation rates when cathode potential was controlled at +0.2 V, +0.28 V, and +0.35 V vs Ag/AgCl. Two effects can be seen here: current density increases with increasing recirculation rate, and with decreasing cathode potential. This shows that the biocathode was both limited by mass transfer of oxygen and by potential (charge transfer of the electrons from the electrode to oxygen). When cathode potential is lower, current density increases because the driving force increases, and charge transfer increases. The limitations in charge and mass transfer can also been seen in the cyclic voltammogram (Figure 3). In Figure 3, we can see that for the biocathode controlled at +0.15 V vs Ag/AgCl, the performance at 0.28 V and 0.35 V vs Ag/AgCl is limited. This limited performance may be caused by the fact that these potentials are too positive for the active catalytic enzymatic component that interacts with the electrode. At potentials lower than +0.2 V vs Ag/AgCl, oxygen mass transfer becomes limiting, while at the other two potentials charge transfer is more dominant (Figure 4).
Increase in current density or decrease in flow velocity may lead to local pH increase at the electrode, resulting in higher cathode overpotential (Jeremiasse et al., 2009). It was shown by Jeremiasse et al. (2009) that cathode overpotential did not change at current densities up to 2 A/m2 when using a buffer of 20 mM, which indicates that proton supply from bulk to the electrode was sufficient to keep constant pH at the electrode surface. It is therefore likely that under the circumstances in our biocathode with the same buffer concentration and a lower current density, proton transport was sufficiently high, and thus mass transfer can be attributed to oxygen only and not to protons.