In Situ Investigation of Cathode and Local Biofilm Microenvironments Reveals Important Roles of OH^– and Oxygen Transport in Microbial Fuel Cells

Abstract

Mass transport within a cathode, including OH(-) transport and oxygen diffusion, is important for the performance of air-cathode microbial fuel cells (MFCs). However, little is known regarding how mass transport profiles are associated with MFC performance and how they are affected by biofilm that inevitably forms on the cathode surface. In this study, the OH(-) and oxygen profiles of a cathode biofilm were probed in situ in an MFC using microelectrodes. The pH of the catalyst layer interface increased from 7.0 ± 0.1 to 9.4 ± 0.3 in a buffered MFC with a bare cathode, which demonstrates significant accumulation of OH(-) in the cathode region. Furthermore, the pH of the interface increased to 10.0 ± 0.3 in the presence of the local biofilm, which indicates that OH(-) transport was severely blocked. As a result of the significant OH(-) accumulation, the maximum power density of the MFC decreased from 1.8 ± 0.1 W/m(2) to 1.5 ± 0.08 W/m(2). In contrast, oxygen crossover, which was significant under low current flow conditions, was limited by the cathode biofilm. As a result of the blocked oxygen crossover, higher MFC coulombic efficiency (CE) was achieved in the presence of the cathode biofilm. These results indicate that enhanced OH(-) transport and decreased oxygen crossover would be beneficial for high-performance MFC development.

Microbial fuel cells: From fundamentals to applications. A review

“…It is not yet fully understood if the reduction reaction follows the acidic or alkaline pathway with final generation of H 2 O or OH − respectively. Some reports have elucidated the presence of high concentration of OH − in the cathode proximity [323], [324], [325].

Fig. 5(a) H 2 O 2 percentage (top); half-way potential for ORR (middle) and kinetic current ik (bottom) by Fe–4 phenantroline-based catalyst at a pH range of 1–13.7.

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Sand amendment enhances bioelectrochemical remediation of petroleum hydrocarbon contaminated soil

“…At 2-3.7 cm of soil layer, the profile of pH-distance was almost straight line, showing the pure diffusion of H + . It was noted that the concave profile of pH-distance faced left at 5.5-6 cm, suggesting the net consumption of H + (Xiao et al, 2013;Yuan et al, 2013).…”

Microbial fuel cells for energy production from wastewaters: the way toward practical application

“…(ii) Growth of biofilm on the solution-facing side of air-cathodes may reduce the long-term cathode performance, especially for single chamber MFCs (Liu et al, 2005a;Yang et al, 2009;Santoro et al, 2012;Yuan et al, 2013). Oxygen is reduced at the cathode through the reaction O 2 +4e…”

“….40V) depending on the catalyst selected (Yuan et al, 2013). A thick aerobic biofilm on the cathodes may function as a diffusion barrier to H + transfer to the catalyst site (Zhang X. et al, 2009;Ahmed, 2011), and it can severely block OH − transport outside the electrode, resulting in an significant OH − accumulation in the cathode microenvironment and thus a lower cathode potential (Yuan et al, 2013); the aerobic bacteria may consume a portion of the available oxygen at the catalytic sites and thus reduce the oxygen reduction kinetics Yuan et al, 2013); the EPS of attached microorganisms may also cause catalyst poisoning.…”

“…A thick aerobic biofilm on the cathodes may function as a diffusion barrier to H + transfer to the catalyst site (Zhang X. et al, 2009;Ahmed, 2011), and it can severely block OH − transport outside the electrode, resulting in an significant OH − accumulation in the cathode microenvironment and thus a lower cathode potential (Yuan et al, 2013); the aerobic bacteria may consume a portion of the available oxygen at the catalytic sites and thus reduce the oxygen reduction kinetics Yuan et al, 2013); the EPS of attached microorganisms may also cause catalyst poisoning. Up to now, only the mechanisms of blockage of OH − and oxygen transfer by the cathodic biofilms have been demonstrated Yuan et al, 2013), the mechanisms how the aerobic biofilm affects cathode performance still need to be investigated. As a result of biofilm growth, an increased internal resistance coupled with a decreased electricity (power) generation of MFCs was obtained in many studies (Cheng et al, 2006;Yang et al, 2009;Zhang X. et al, 2009;Zhang F. et al, 2011a).…”