Quantifying the factors limiting performance and rates in microbial fuel cells using the electrode potential slope analysis combined with electrical impedance spectroscopy

Abstract

Improving power generation of microbial fuel cells (MFCs) requires better methods to quantify the impact of the solution chemistry on performance. While buffer concentration and conductivity have been indicated to impact performance of flat (carbon cloth) anodes, we were able for the first to time to quantitatively determine the impact of the phosphate buffer solution (PBS) concentration separate from conductivity on electrode resistances of commonly used brush anodes and activated carbon cathodes. Using the electrode potential slope (EPS) method, we showed that the anode resistance decreased by 64% (from 14.6 ± 0.1 mΩ m2 to 5.3 ± 0.1 mΩ m2) by increasing the PBS concentration from 50 to 200 mM. There was no appreciable change in the cathode resistance (17.0 ± 0.1 mΩ m2, 50 mM; 18.9 ± 0.2 mΩ m2, 200 mM) although overall performance increased due to a larger cathode experimental working potential (ECat,e0-50= 268.9 ± 0.9 mV, ECat,e0-200= 370 ± 1 mV). Adding phosphate buffer to low conductivity synthetic wastewater containing 0.25 g L−1 sodium acetate decreased the anode resistance by 52% (from 59.1 ± 0.2 mΩ m2 to 28.1 ± 0.1 mΩ m2), but increasing only conductivity by adding NaCl or acetate had little impact on electrode resistances. Using electrochemical impedance spectroscopy (EIS), we determined the reasons for these responses. The cathode was limited by oxygen reduction reaction (ORR) kinetics, while the brush anode was limited by mass transfer (proton diffusion), consistent with the impact of buffer concentration on performance. These results have important implications for operation of MFCs as increasing solution conductivity alone, for example, by adding seawater, will not be sufficient to improve anode performance.