Recent advances in electron transfer between biofilms and metals

Microbial biofilms produce electrochemical interactions with metal surfaces by following a wide variety of different electron exchange pathways. Reviewing the mechanisms identified in the biocorrosion of steels leads us to distinguish direct and indirect mechanisms for biofilm-catalysed cathodic reactions. Indirect mechanisms are due to the production of metal oxides or hydrogen peroxide (aerobic corrosion) or metal sulphides (anaerobic corrosion), which further react with the metal surface. Direct mechanisms involve adsorbed biocompounds, generally enzymes or their active sites, which catalyse the cathodic reduction of oxygen for aerobic biocorrosion or the proton/water reduction in anaerobic processes. Recent studies dealing with the role of hydrogenases in anaerobic corrosion have shed light on the important role of phosphate species via so-called cathodic deprotonation. Advances in the development of microbial fuel cells have also resulted in new concepts, mainly for oxidation processes. Some microbial cells have been shown to be able to produce their own electron mediators. Others can transfer electrons directly to electrodes through membrane-bound electron shuttles or achieve long-range transfer through conductive pili

First air-tolerant effective stainless steel microbial anode obtained from a natural marine biofilm

Microbial anodes were constructed with stainless steel electrodes under constant polarisation. The seawater medium was inoculated with a natural biofilm scraped from harbour equipment. This procedure led to efficient microbial anodes providing up to 4 A/m2 for 10 mM acetate oxidation at −0.1 V/SCE. The whole current was due to the presence of biofilm on the electrode surface, without any significant involvement of the abiotic oxidation of sulphide or soluble metabolites. Using a natural biofilm as inoculum ensured almost optimal performance of the biofilm anode as soon as it was set up; the procedure also proved able to form biofilms in fully aerated media, which provided up to 0.7 A/m2. The current density was finally raised to 8.2 A per square meter projected surface area using a stainless steel grid. The inoculating procedure used here combined with the control of the potential revealed, for the first time, stainless steel as a very competitive material for forming bioanodes with natural microbial consortia

Catalysis of oxygen reduction in PEM fuel cell by seawater biofilm

The catalysis of oxygen reduction on metallic materials has been widely studied in the domain of aerobic corrosion. In this framework, it has been stated that seawater biofilms are able to catalyse efficiently oxygen reduction on stainless steels. This capacity was transferred here to the catalysis of the cathodic reaction of a proton exchange membrane fuel cell. A laboratory-scale fuel cell was designed with a stainless steel cathode, a platinum anode, and two separated liquid loops. The cathodic loop was air-saturated, while the anodic loop was hydrogen saturated. Seawater biofilm was previously grown on the stainless steel cathode, which was then set up into the fuel cell. The presence of the seawater biofilm on the stainless steel surface led to efficient catalysis of oxygen reduction. The biofilm-covered cathode was able to support current density up to 1.89 A/m2. Power density of 0.32 W/m2 was supplied with 1.34 A/m2 current density

Microbial catalysis of the oxygen reduction reaction for microbial fuel cells: a review.

The slow kinetics of the electrochemical oxygen reduction reaction (ORR) is a crucial bottleneck in the development of microbial fuel cells (MFCs). This article firstly gives an overview of the particular constraints imposed on ORR by MFC operating conditions: neutral pH, slow oxygen mass transfer, sensitivity to reactive oxygen species, fouling and biofouling. A review of the literature is then proposed to assess how microbial catalysis could afford suitable solutions. Actually, microbial catalysis of ORR occurs spontaneously on the surface of metallic materials and is an effective motor of microbial corrosion. In this framework, several mechanisms have been proposed, which are reviewed in the second part of the article. The last part describes the efforts made in the domain of MFCs to determine the microbial ecology of electroactive biofilms and define efficient protocols for the formation of microbial oxygen-reducing cathodes. Although no clear mechanism has been established yet, several promising solutions have been recently proposed

Increased power from a two-chamber microbial fuel cell with a low-pH air-cathode compartment

Pt-supported air-cathodes still need improvement if their application in MFC technology is to be sustainable. In this context, the efficiency of an air-cathode was studied with respect to the pH of the solution it was exposed to. Voltammetry showed that oxygen reduction was no longer limited by H+ availability for pH lower than 3.0. A new MFC was designed with a catholyte compartment setup between the anode compartment and the air-cathode. With a catholyte compartment at pH 1.0, the MFC provided up to 5 W/m2, i.e., 2.5-fold the power density obtained with the same anode and cathode in a single-chamber MFC working at pH 7.5. Current density exceeded 20 A/m2. The benefit of low-pH in the catholyte chamber largely counterbalanced the mass transfer hindrance due the membrane that separated the two compartments. The MFC kept 66% its performance during nine days of continuous operation

From microbial fuel cell (MFC) to microbial electrochemical snorkel (MES): maximizing chemical oxygen demand (COD) removal from wastewater

The paper introduces the concept of the microbial electrochemical snorkel (MES), a simplified design of a “short-circuited” microbial fuel cell (MFC). The MES cannot provide current but it is optimized for wastewater treatment. An electrochemically active biofilm (EAB) was grown on graphite felt under constant polarization in an urban wastewater. Controlling the electrode potential and inoculating the bioreactor with a suspension of an established EAB improved the performance and the reproducibility of the anodes. Anodes, colonized by an EAB were tested for the chemical oxygen demand (COD) removal from urban wastewater using a variety of bio-electrochemical processes (microbial electrolysis, MFC, MES). The MES technology, as well as a short-circuited MFC, led to a COD removal 57% higher than a 1000 Ω-connected MFC, confirming the potential for wastewater treatment

Electrochemical activity of Geobacter sulfurreducens biofilms on stainless steel anodes

Stainless steel was studied as anode for the biocatalysis of acetate oxidation by biofilms of Geobacter sulfurreducens. Electrodeswere individually polarized at different potential in the range−0.20V to +0.20V vs. Ag/AgCl either in the same reactor or in different reactors containing acetate as electron donor and no electron acceptor except the working electrode. At +0.20V vs. Ag/AgCl, the current increased after a 2-day lag period up to maximum current densities around 0.7Am−2 and 2.4Am−2 with 5mMand 10mM acetate, respectively. No current was obtained during chronoamperometry (CA) at potential values lower than 0.00V vs. Ag/AgCl, while the cyclic voltammetries (CV) that were performed periodically always detected a fast electron transfer, with the oxidation starting around −0.25V vs. Ag/AgCl. Epifluorescent microscopy showed that the current recorded by chronoamperometrywas linked to the biofilm growth on the electrode surface, while CVswere more likely linked to the cells initially adsorbed from the inoculum. A model was proposed to explain the electrochemical behaviour of the biofilm, which appeared to be controlled by the pioneering adherent cells playing the role of “electrochemical gate” between the biofilm and the electrode surface

Forming electrochemically active biofilms from garden compost under chronoamperometry

Dimensionally stable anodes (DSA) were polarized at different constant potential values for several days in garden compost. After an initial lag period ranging from 1 to 10.5 days, the current increased fast and then stabilized for days. Current densities higher than 100 mA m2 and up to 385 mA m2 were obtained with the sole organic matter contained in compost as substrate. Control experiments performed with sterilized compost, oscillations of the current with the temperature, kinetics of the exponential phase of current increase and observations of the surface of electrodes by epifluorescence microscopy showed that the current was controlled by the colonization of the electrode surface by a biofilm which originated the indigenous flora of compost. Three individually addressed electrodes polarized at different potentials in the same reactor led to identical current evolutions on each electrode, which underlined the key role of the microbial flora of the compost in the discrepancy observed in the other experiments. Chronoamperometry revealed a promising technique to check natural environments for new electrochemically active microbial species

Electroactive cytochromeP450BM3 cast polyion films on graphite electrodes

Films of electrochemically active cytochrome P450BM3 were constructed on graphite electrodes using alternate assembly with polyethyleneimin(PEI). The original layer-by-layer adsorption method was slightly modified here to form so-called “cast polyion” films. The cast polyion films were elaborated by immobilizing two successive layers of PEI and protein in very large excess with respect to a monolayer, without any intermediate washing step. Following the immobilization steps by SEM showed that uniform films of a few micrometers were deposited on the graphite surface. The electrochemically activity of the immobilized cytP450 was tested with regard to the reduction of oxygen and the one-electron reduction of the heme. Cyclic voltammetry indicated surface concentration of electrochemically active cytP450 around 0.6 nmol/cm2, which corresponded to 5% of the total amount of protein that was consumed by the immobilisation process. Adapting the procedure to a graphite felt electrode with the view of scaling up porous electrodes for large scale synthesis increased the concentration to 0.9 nmol/cm2. Cast polyion films may represent a simple technique to immobilize high amount of electrochemically active protein, keeping the advantage of the electrostatic interactions of the regular layer-by-layer method

Effect of surface roughness, biofilm coverage and biofilm structure on the electrochemical efficiency of microbial cathodes

Biofilms of Geobacter sulfurreducens were formed under chronoamperometry at −0.5 V and −0.6 V vs. Ag/AgCl on stainless steel cathodes and tested for fumarate reduction. Increasing the surface roughness Ra from 2.0 μm to 4.0 μm increased currents by a factor of 1.6. The overall current density increased with biofilm coverage. When the current density was calculated with respect to the biofilm-coated area only, values up to 280 A/m2 were derived. These values decreased with biofilm coverage and indicated that isolated cells or small colonies locally provide higher current density than dense colonies. Steel composition affected the current values because of differences in biofilm structure and electron transfer rates. Biofilms formed under polarisation revealed better electrochemical characteristics than biofilm developed at open circuit. This work opens up new guidelines for the design of microbial cathodes: a uniform carpet of isolated bacteria or small colonies should be targeted, avoiding the formation of large colonies