Anodophilic Biofilm Catalyzes Cathodic Oxygen Reduction

Abstract

Poor cathodic oxygen reduction and the detrimental buildup of a pH gradient between anode and cathode are the major hurdles in the development of sustainable microbial fuel cells (MFCs). This article describes and tests a concept that can help overcoming both of these limitations, by inverting the polarity of the MFC repeatedly, allowing anodic and cathodic reactions to occur alternately in the same half-cell and hence neutralizing its respective pH effects. For simplicity, we studied polarity inversion exclusively in one half-cell, maintaining its potential at -300 mV (vs Ag/AgCl) by a potentiostat. An alternating supply of acetate and dissolved oxygen to the biofilm resulted in the tested half-cell repeatedly changing from an anode to a cathode and vice versa. This repeated inversion of current direction avoided the detrimental drifting of the electrolyte pH. Control runs without current inversion ceased to produce current, as a result of anode acidification. The presence of the anodophilic biofilm survived the intermittent oxygen exposure and could measurably facilitate the cathodic reaction by reducing the apparent oxygen overpotential. It enabled cathodic oxygen reduction at about -150 mV (vs Ag/AgCl) compared to -300 mV (vs Ag/AgCl) for the same electrode material (granular graphite) without biofilm. Provided that a suitable cathodic potential was chosen, the presence of "anodophilic bacteria" at the cathode could enable a 5-fold increase in power output. Overall, the ability of an electrochemically active biofilm to catalyze both substrate oxidation and cathodic oxygen reduction in a single bioelectrochemical system has been documented. This property could be useful to alleviate both the cathodic oxygen reduction and the detrimental drifting of electrolyte pH in an MFC system. Further research is warranted to explore the application of such bidirectional microbial catalytic properties for sustainable MFC processes.

Ammonium as a sustainable proton shuttle in bioelectrochemical systems

“…Within 48 hours, the pH split had reduced the BES performance to 20 % of its original, a finding that is in line with the literature (Cheng et al, 2010;Harnisch et al, 2008).…”

“…Na + ) rather than protons will become the dominant charge balancing species, resulting in a severe pH gradient between anolyte and catholyte . The decrease in pH of the anolyte is more detrimental than the pH increase in the catholyte (Cheng et al, 2010;Harnisch et al, 2008). This is because lowering pH from 7 towards 5 typically stifles microbial activity and hence the current flow.…”

Enrichment of Microbial Electrolysis Cell Biocathodes from Sediment Microbial Fuel Cell Bioanodes

“…However, platinum is expensive, nonrenewable, and ineffective for catalyzing CO 2 reduction, and it is susceptible to poisoning by sulfur and carbon monoxide (6). Alternatives to platinum as a catalyst include several inorganic materials (stainless steel, nickel, and MoS 2 ) or microorganisms (4,7,15,21).…”

“…Biofilms that develop on electrodes in MFCs under oxic cathode conditions have been shown to sustain current generation when the functions of the electrodes are switched (the cathode, for example, becomes the anode) (7,8,35). This indicates that both exoelectrogenic and electrotrophic microorganisms can be maintained in the electrode biofilms when the cathode is switched from oxic to anoxic conditions.…”

Novel Applications of Microbial Fuel Cells in Sensors and Biosensors

“…As in every biological system, the pH is an essential factor in MFCs, affecting both anodic microbial activities and cathodic reactions [76,77]. Protons, produced by the oxidation of organic matter in the anode compartment of an MFC, pass into the cathode compartment through the PEM and usually react with oxygen generating water molecules [58].…”