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

Local analysis of oxygen reduction catalysis by scanning vibrating electrode technique : a new approach to the study of biocorrosion

The scanning vibrating electrode technique (SVET)was employed to investigate oxygen reduction catalysis by the presence of enzyme in an aerobic medium. Heme protoporphyrin (hemin) was chosen as a model of the enzymes that are able to catalyze oxygen reduction. A strict experimental protocol was defined for preparing the graphite surface by deposition of hemin with a simple configuration mimicking the presence of enzyme on the samples. The same configuration was adapted to a stainless steel electrode. Different geometric arrangementswere investigated by SVET to approach the local conditions. The results demonstrated that hemin deposited on the electrode surface led to an increase in the cathodic current, which indicated a catalytic effect. Based on the SVET analysis, itwas demonstrated that hemin caused the appearance of galvanic cells on the material surface. The SVET proved able to locate active catalytic centres and therefore to foresee the contribution of the enzyme to the creation of galvanic cells, thus leading to localized corrosion. The application of SVET to the study of the interaction between biological molecules and material provides a newapproach for visualizing and understanding microbially influenced corrosion (MIC) in an aerobic medium

Forming microbial anodes under delayed polarisation modifies the electron transfer network and decreases the polarisation time required.

Microbial anodes were formed from compost leachate on carbon cloth electrodes. The biofilms formed at the surface of electrodes kept at open circuit contained microorganisms that switched their metabolism towards electrode respiration in response to a few minutes of polarisation. When polarisation at -0.2 V/SCE (+0.04 V/SHE) was applied to a pre-established biofilm formed at open circuit (delayed polarisation), the bacteria developed an extracellular electron transport network that showed multiple redox systems, reaching 9.4 A/m(2) after only 3-9 days of polarisation. In contrast, when polarisation was applied from the beginning, bacteria developed a well-tuned extracellular electron transfer network concomitantly with their growth, but 36 days of polarisation were required to get current of the same order (6-8 A/m(2)). The difference in performance was attributed to the thinner, more heterogeneous structure of the biofilms obtained by delayed polarisation compared to the thick uniform structure obtained by full polarisation

Checking graphite and stainless anodes with an experimental model of marine microbial fuel cell

A procedure was proposed to mimic marine microbial fuel cell (MFC) in liquid phase. A graphite anode and a stainless steel cathode which have been proven, separately, to be efficient in MFC were investigated. A closed anodic compartment was inoculated with sediments, filled with deoxygenated seawater and fed with milk to recover the sediment’s sulphide concentration. A stainless steel cathode, immersed in aerated seawater, used the marine biofilm formed on its surface to catalyze oxygen reduction. The cell implemented with a 0.02 m2-graphite anode supplied around 0.10 W/m2 for 45 days. A power of 0.02 W/m2 was obtained after the anode replacement by a 0.06 m2-stainless steel electrode. The cell lost its capacity to make a motor turn after one day of operation, but recovered its full efficiency after a few days in open circuit. The evolution of the kinetic properties of stainless steel was identified as responsible for the power limitation

Marine microbial fuel cell : use of stainless steel electrodes as anode and cathode materials

Numerous biocorrosion studies have stated that biofilms formed in aerobic seawater induce an efficient catalysis of the oxygen reduction on stainless steels. This property was implemented here for the first time in a marine microbial fuel cell (MFC). A prototype was designed with a stainless steel anode embedded in marine sediments coupled to a stainless steel cathode in the overlying seawater. Recording current/potential curves during the progress of the experiment confirmed that the cathode progressively acquired effective catalytic properties. The maximal power density produced of 4mWm−2 was lower than those reported previously with marine MFC using graphite electrodes. Decoupling anode and cathode showed that the cathode suffered practical problems related to implementation in the sea, which may found easy technical solutions. A laboratory fuel cell based on the same principle demonstrated that the biofilm-covered stainless steel cathode was able to supply current density up to 140mAm−2 at +0.05V versus Ag/AgCl. The power density of 23mWm−2 was in this case limited by the anode. These first tests presented the biofilm-covered stainless steel cathodes as very promising candidates to be implemented in marine MFC. The suitability of stainless steel as anode has to be further investigated

Marine aerobic biofilm as biocathode catalyst

Stainless steel electrodes were immersed in open seawater and polarized for some days at − 200 mV vs. Ag/AgCl. The current increase indicated the formation of biofilms that catalysed the electrochemical reduction of oxygen. These wild, electrochemically active (EA) biofilms were scraped, resuspended in seawater and used as the inoculum in closed 0.5 L electrochemical reactors. This procedure allowed marine biofilms that are able to catalyse oxygen reduction to be formed in small, closed small vessels for the first time. Potential polarisation during biofilm formation was required to obtain EA biofilms and the roughness of the surface favoured high current values. The low availability of nutrients was shown to be a main limitation. Using an open reactor continuously fed with filtered seawater multiplied the current density by a factor of around 20, up to 60 µA/cm2, which was higher than the current density provided in open seawater by the initial wild biofilm. These high values were attributed to continuous feeding with the nutrients contained in seawater and to suppression of the indigenous microbial species that compete with EA strains in natural open environments. Pure isolates were extracted from the wild biofilms and checked for EA properties. Of more than thirty different species tested, only Winogradskyella poriferorum and Acinetobacter johsonii gave current densities of respectively 7% and 3% of the current obtained with the wild biofilm used as inoculum. Current densities obtained with pure cultures were lower than those obtained with wild biofilms. It is suspected that synergetic effects occur in whole biofilms or/and that wild strains may be more efficient than the cultured isolates

Single medium microbial fuel cell: Stainless steel and graphite electrode materials select bacterial communities resulting in opposite electrocatalytic activities

A graphite electrode and a stainless steel electrode immersed in exactly the same medium and polarised at the same potential were colonised by different microbial biofilms. This difference in electroactive microbial population leads stainless steel and graphite to become a microbial cathode and a microbial anode respectively. The results demonstrated that the electrode material can drive the electrocatalytic property of the biofilm opening perspectives for designing single medium MFC. This new discovery led to of the first demonstration of a “single medium MFC.” Such a single medium MFC designed with a graphite anode connected to a stainless steel cathode, both buried in marine sediments, produced 280 mA m−2 at a voltage of 0.3 V for more than 2 weeks

Stainless steel is a promising electrode material for anodes of microbial fuel cells.

The abilities of carbon cloth, graphite plate and stainless steel to form microbial anodes were compared under identical conditions. Each electrode was polarised at −0.2 V vs. SCE in soil leachate and fed by successive additions of 20 mM acetate. Under these conditions, the maximum current densities provided were on average 33.7 A m−2 for carbon cloth, 20.6 A m−2 for stainless steel, and 9.5 A m−2 for flat graphite. The high current density obtained with carbon cloth was obviously influenced by the three dimensional electrode structure. Nevertheless, a fair comparison between flat electrodes demonstrated the great interest of stainless steel. The comparison was even more in favour of stainless steel at higher potential values. At +0.1 V vs. SCE stainless steel provided up to 35 A m−2, while graphite did not exceed 11 A m−2. This was the first demonstration that stainless steel offers a very promising ability to form microbial anodes. The surface topography of the stainless steel did not significantly affect the current provided. Analysis of the voltammetry curves allowed two groups of electrode materials to be distinguished by their kinetics. The division into two well-defined kinetics groups proved to be appropriate for a wide range of microbial anodes described in the literature

Electroactivity of phototrophic river biofilms and constitutive cultivable bacteria

Electroactivity is a property of microorganisms assembled in biofilms that has been highlighted in a variety of environments. This characteristic was assessed for phototrophic river biofilms at the community scale and at the bacterial population scale. At the community scale, electroactivity was evaluated on stainless steel and copper alloy coupons used both as biofilm colonization supports and as working electrodes. At the population scale, the ability of environmental bacterial strains to catalyze oxygen reduction was assessed by cyclic voltammetry. Our data demonstrate that phototrophic river biofilm development on the electrodes, measured by dry mass and chlorophyll a content, resulted in significant increases of the recorded potentials, with potentials of up to +120 mV/saturated calomel electrode (SCE) on stainless steel electrodes and +60 mV/SCE on copper electrodes. Thirty-two bacterial strains isolated from natural phototrophic river biofilms were tested by cyclic voltammetry. Twenty-five were able to catalyze oxygen reduction, with shifts of potential ranging from 0.06 to 0.23 V, cathodic peak potentials ranging from −0.36 to −0.76 V/SCE, and peak amplitudes ranging from −9.5 to −19.4 μA. These isolates were diversified phylogenetically (Actinobacteria, Firmicutes, Bacteroidetes, and Alpha-, Beta-, and Gammaproteobacteria) and exhibited various phenotypic properties (Gram stain, oxidase, and catalase characteristics). These data suggest that phototrophic river biofilm communities and/or most of their constitutive bacterial populations present the ability to promote electronic exchange with a metallic electrode, supporting the following possibilities: (i) development of electrochemistry-based sensors allowing in situ phototrophic river biofilm detection and (ii) production of microbial fuel cell inocula under oligotrophic conditions