Microscale gradients of oxygen, hydrogen peroxide, and pH in freshwater cathodic biofilms

Abstract

Cathodic reactions in biofilms employed in sediment microbial fuel cells is generally studied in the bulk phase. However, the cathodic biofilms affected by these reactions exist in microscale conditions in the biofilm and near the electrode surface that differ from the bulk phase. Understanding these microscale conditions and relating them to cathodic biofilm performance is critical for better-performing cathodes. The goal of this research was to quantify the variation in oxygen, hydrogen peroxide, and the pH value near polarized surfaces in river water to simulate cathodic biofilms. We used laboratory river-water biofilms and pure culture biofilms of Leptothrix discophora SP-6 as two types of cathodic biofilms. Microelectrodes were used to quantify oxygen concentration, hydrogen peroxide concentration, and the pH value near the cathodes. We observed the correlation between cathodic current generation, oxygen consumption, and hydrogen peroxide accumulation. We found that the 2 e(-) pathway for oxygen reduction is the dominant pathway as opposed to the previously accepted 4 e(-) pathway quantified from bulk-phase data. Biofouling of initially non-polarized cathodes by oxygen scavengers reduced cathode performance. Continuously polarized cathodes could sustain a higher cathodic current longer despite contamination. The surface pH reached a value of 8.8 when a current of only -30 μA was passed through a polarized cathode, demonstrating that the pH value could also contribute to preventing biofouling. Over time, oxygen-producing cathodic biofilms (Leptothrix discophora SP-6) colonized on polarized cathodes, which decreased the overpotential for oxygen reduction and resulted in a large cathodic current attributed to manganese reduction. However, the cathodic current was not sustainable.

Keywords: cyclic voltammetry; electrochemistry; electron transfer; fuel cells; oxygen.

Figure 1

Stationary profiles at t = 0 d of A) oxygen and B) hydrogen peroxide at an initially unpolarized cathode (fresh electrode); plots of C) oxygen concentration versus current; and D) hydrogen peroxide concentration versus current derived from CV spectra and stationary profiles. The dashed lines of both (A) and (B) are centered at −180 mV_Ag/AgCl. Red trace in C) is a linear regression fit.

Figure 2

Stationary profile of oxygen at a glassy carbon electrode with L. discophora SP-6 grown on the surface. The dashed line is centered at −40 mV_Ag/AgCl.

Figure 3

Selected images of cathodes during different stages of contamination. The inset red squares in the SEM image (right bottom; scale bar corresponds to 20 μm) are diatoms found at different locations. The white circles indicate bacteria colonizing the diatom surface.

Figure 4

A) Comparison of oxygen depth profiles at an initially unpolarized cathode at t=0 and 23 d; B) stationary profile of oxygen at an initially unpolarized cathode at t=23 d. The dashed line is centered at −135 mV_Ag/AgCl.

Figure 5

Hydrogen peroxide depth profiles at t=0 d above an initially unpolarized cathode.

Figure 6

A) Oxygen depth profile of a cathode after 19 d of continuous polarization at −700 mV_Ag/AgCl; B) pH depth profile of the same polarized cathode after 25 d. For comparison, a pH depth profile of an initially unpolarized cathode is shown. Dashed lines represent the approximate biofilm/river-water interface.

Figure 7

A) Current over time for a continuously polarized cathode at −700 mV_Ag/AgCl; B) oxygen depth profile at t=54 d inside the biofilm showing an increase in oxygen concentration near the biofilm/river-water interface.

Figure 8

Side view of flat-plate reactors: A) single-chamber reactor used for unpolarized cathodes; B) dual-chamber reactor used for long-term polarized cathodes. One polarized cathode and one unpolarized cathode were placed adjacent to each other. The auxiliary chamber was isolated with a cation exchange membrane (CEM). The counter electrode (CE) and reference electrode (RE) were placed in the auxiliary chamber.

Figure 9

Oxygen or hydrogen peroxide stationary profiles. The potential is swept over a range, in which oxygen is reduced and the change in concentration is measured by the microelectrode.