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Energy and the Environment
The Impact of Ohmic Resistance on Measured Electrode Potentials and Maximum Power Production in Microbial Fuel Cells Bruce E. Logan, Emily Zikmund, Wulin Yang, Ruggero Rossi, Kyoung-Yeol Kim, Pascal E. Saikaly, and Fang Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02055 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018
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Environmental Science & Technology
Date: June 5, 2018 (originally submitted April 16, 2018) Submitted to: Environmental Science & Technology
The Impact of Ohmic Resistance on Measured Electrode Potentials and Maximum Power Production in Microbial Fuel Cells
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ABSTRACT
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Low solution conductivity is known to adversely impact power generation in microbial fuel cells (MFCs),
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but its impact on measured electrode potentials has often been neglected in the reporting of electrode
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potentials. While errors in the working electrode (typically the anode) are usually small, larger errors can
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result in reported counter electrode potentials (typically the cathode) due to large distances between
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the reference and working electrodes, or the use of whole cell voltages to calculate counter electrode
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potentials. As shown here, inaccurate electrode potentials impact conclusions concerning factors
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limiting power production in MFCs at higher current densities. To demonstrate how the electrochemical
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measurements should be adjusted using the solution conductivity, electrode potentials were estimated
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in MFCs with brush anodes placed close to the cathode (1 cm), or with flat felt anodes placed further
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from the cathode (3 cm) to avoid oxygen crossover to the anodes. The errors in the cathode potential
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for MFCs with brush anodes reached 94 mV using acetate in a 50 mM phosphate buffer solution. With a
Bruce E. Logan1*, Emily Zikmund1, Wulin Yang1, Ruggero Rossi1, Kyoung-Yeol Kim1, Pascal Saikaly2, Fang Zhang3 1
Department of Civil and Environmental Engineering, The Pennsylvania State University, 231Q Sackett Building, University Park, PA 16802, USA 2 Biological and Environmental Sciences and Engineering Division, Water Desalination and Reuse Research Center, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia 3 School of Environment and State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing, 100084, China *Corresponding author: e-mail:
[email protected]; phone: +1-814-863-7908; fax: +1-814-863-7304
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Environmental Science & Technology
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felt anode and acetate, cathode potential errors increased to 394 mV. While brush anode MFCs
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produced much higher power densities than flat anode MFCs under these conditions, this better
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performance was shown primarily to result from electrode spacing following correction of electrode
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potentials. Brush anode potentials corrected for solution conductivity were the same for brushes set 1
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or 3 cm from the cathode, although the range of current produced was different due to ohmic losses
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with the larger distance. These results demonstrate the critical importance of using corrected electrode
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potentials for to understand factors limiting power production in MFCs.
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Introduction
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Many different types of electrodes have been used in microbial fuel cells (MFCs) in order to try to
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increase power production.1-9 Anodes such as graphite fiber brushes10 or highly porous carbon felt11, 12
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produce higher power densities in MFCs due to their high surface areas and porosities. A comparison of
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anode types based on reports in the literature showed that cylindrical brush anodes generally produced
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higher current densities than flat carbon felt, carbon cloth and carbon paper anodes, when coupled to a
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cathode with a platinum catalyst.3 Placing the electrodes close to each other should improve power
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production as it reduces ohmic resistance. However, flat anodes cannot be placed close to the cathode
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unless a separator or membrane is used to block oxygen transfer from the cathode to the anode, as
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oxygen crossover reduces power production.13-18 In contrast, a brush anode can be placed close to the
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cathode in the absence of a separator without an appreciable loss in power, as long as the brush is more
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than ~1 cm thick.10, 19-22 The brush is quite thick relative to many felt or cloth anodes, which should
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facilitate maintenance of anaerobic conditions within the brush. Removing up to 65% of a 2.5 cm long
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brush on the side most distant from the cathode, so that the brush was only 0.9 cm thick, did not impact
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power production suggesting that the portion of the anode closest to the cathode was important for
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power production.19 Small diameter brushes (0.8 cm) have produced higher power densities than larger
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Environmental Science & Technology
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brushes (2.5 cm diameter) when acetate concentrations were kept high (~ 1 g/L), but they have failed to
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produce stable power with lower strength wastewater (
