Abstract
A method for assessing the performance of microbial fuel cells (MFCs) is the polarisation sweep where different external resistances are applied at set intervals (sample rates). The resulting power curves often exhibit an overshoot where both power and current decrease concomitantly. To investigate these phenomena, small-scale (1 mL volume) MFCs operated in continuous flow were subjected to polarisation sweeps under various conditions. At shorter sample rates the overshoot was more exaggerated and power generation was overestimated; sampling at 30 s produced 23% higher maximum power than at 3 min. MFCs with an immature anodic biofilm (5 days) exhibited a double overshoot effect, which disappeared after a sufficient adjustment period (5 weeks). Mature MFCs were subject to overshoot when the anode was fed weak (1 mM acetate) feedstock with low conductivity (< 100 μS) but not when fed with a higher concentration (20 mM acetate) feedstock with high conductivity (>1500 μS). MFCs developed in a pH neutral environment produced overshoot after the anode had been exposed to acidic (pH 3) conditions for 24 h. In contrast, changes to the cathode both in terms of pH and varying catholyte conductivity, although affecting power output did not result in overshoot suggesting that this is an anodic phenomenon.
Highlights
Secondary wastewater treatment processes currently employ the biological activities of complex microbial biofilms to remove organic pollutants
There are various published accounts showing overshoot at longer sample rates such as 20 min (e.g. [9,12]) and there are many more instances where the sample rate is not provided. Another method for verifying the suitability of a sample rate is to perform two polarisation sweeps; the first starting at open circuit and decreasing in a step-wise manner down to zero, the second performed by increasing the resistance values from zero back up to open circuit
At each new resistance value, following the initial drop, the voltage can clearly be seen to stabilise as exemplified by the magnified area showing the step-like decline at the higher current densities. This highlights the suitability of 1 min as a sample rate for this design of Microbial fuel cells (MFCs) under optimal conditions. These results suggest that while faster sample rates can exaggerate the overshoot in underperforming MFCs, it might be that suboptimal operating conditions affecting internal resistance are the reason for the presence of this phenomenon
Summary
Secondary wastewater treatment processes currently employ the biological activities of complex microbial biofilms to remove organic pollutants. Microbial fuel cells (MFCs) are able to capture the electrons produced during microbial processes to generate electricity and offer the promise of a new sustainable source of energy, for the wastewater industry. As the applied external resistance becomes lower (i.e. the load gets ‘heavier’) there is a greater electron demand requiring that the microbial consortium increase metabolic activity improving power and treatment efficiency. As long as this is approximately at the point of maximum power transfer (MPT) it will be sustainable. If the performance is pushed beyond this critical parameter power output becomes non-sustainable
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