Electroanalysis of microbial anodes for bioelectrochemical systems: basics, progress and perspectives

Over about the last ten years, microbial anodes have been the subject of a huge number of fundamental studies dealing with an increasing variety of possible application domains.

Marine floating microbial fuel cell involving aerobic biofilm on stainless steel cathodes

Here is presented a new design of a floating marine MFC in which the inter-electrode space is constant. This design allows the generation of stable current for applications in environments where the water column is large or subject to fluctuations such as tidal effects. The operation of the first prototype was validated by running a continuous test campaign for 6months. Performance in terms of electricity generation was already equivalent to what is conventionally reported in the literature with basic benthic MFCs despite the identification of a large internal resistance in the proposed design of the floating system. This high internal resistance is mainly explained by poor positioning of the membrane separating the anode compartment from the open seawater. The future objectives are to achieve more consistent performance and a second-generation prototype is now being developed, mainly incorporating a modification of the separator position and a stainless steel biocathode with a large bioavailable surface.

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/m(2) 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.

Electrochemical checking of aerobic isolates from electrochemically active biofilms formed in compost

AIMS

To design a cyclic voltammetry (CV) procedure to check the electrochemical activity of bacterial isolates that may explain the electrochemical properties of biofilms formed in compost.

METHODS AND RESULTS

Bacteria catalysing acetate oxidation in garden compost were able to form electrochemically active biofilms by transferring electrons to an electrode under chronoamperometry. They were recovered from the electrode surface and identification of the isolates using 16S rRNA sequencing showed that most of them were Gammaproteobacteria, mainly related to Enterobacter and Pseudomonas spp. A CV procedure was designed to check the electrochemical activity of both groups of isolates. Preliminary CVs suggested that the bacteria were not responsible for the catalysis of acetate oxidation. In contrast, both groups of isolates were found to catalyse the electrochemical reduction of oxygen under experimental conditions that favoured adsorption of the microbial cells on the electrode surface.

CONCLUSIONS

Members of the genera Enterobacter and Pseudomonas were found to be able to catalyse the electrochemical reduction of oxygen.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study has shown the unexpected efficiency of Enterobacter and Pseudomonas spp. in catalysing the reduction of oxygen, suggesting a possible involvement of these species in biocorrosion, or possible application of these strains in designing bio-cathode for microbial fuel cells.

Membrane electrochemical reactors (MER) for NADH regeneration in HLADH-catalysed synthesis: comparison of effectiveness

Two membrane electrochemical reactors (MER) were designed and applied to HLADH-catalysed reduction of cyclohexanone to cyclohexanol. The regeneration of the cofactor NADH was ensured electrochemically, using either methyl viologen or a rhodium complex as electrochemical mediator. A semi-permeable membrane (dialysis or ultra-filtration) was integrated in the filter-press electrochemical reactor to confine the enzyme(s) as close as possible to the electrode surface. When methyl viologen was used, the transformation ratio of cyclohexanone varied from 0 to 65% depending on the internal arrangement of the reactor. Matching the reactor configuration to the reaction system was essential in this case. With the rhodium complex, the ultra-filtration MER was tested in continuous and recycling configurations. The best conditions led to 100% transformation of 0.1 L volume of 0.1 M cyclohexanone after 70 h with the recycling mode. Finally, the performances of the reactors are discussed with respect to different evaluations of the production yields.

Glucose oxidase catalysed oxidation of glucose in a dialysis membrane electrochemical reactor (D-MER)

The purpose of this work was to evaluate the effectiveness of a new Membrane Electrochemical Reactor (MER) for the production of gluconic acid by glucose oxidase (GOD) catalysed glucose oxidation. The GOD was confined against the electrode surface with a dialysis membrane. The role of the electrochemical step was to eliminate by oxidation the hydrogen peroxide that appeared as a by-product of the reaction and strongly inhibited and/or inactivated GOD. The dialysis MER gave a transformation ratio of 30% with an initial glucose concentration of around 300 mM. This result is significantly better than the maximum of 10% obtained when hydrogen peroxide was eliminated by addition of a large excess of catalase in solution, as is generally done. The D-MER also revealed unexpected properties of the enzyme kinetics, such as an oscillatory behaviour, which were discussed.

The role of hydrogenases in the anaerobic microbiologically influenced corrosion of steels

The direct electron transfer between 316 L stainless steel and the NAD-dependent hydrogenase from Ralstonia eutropha was studied by spectroelectrochemistry. The presence of hydrogenase and NAD+ clearly increased the quantity of electricity, which was consumed during the electrolysis performed at potential lower than -0.70 V/SCE. The involvement of hydrogenase in the cathodic depolarisation theory was discussed in the light of these results.

Designing membrane electrochemical reactors for oxidoreductase-catalysed synthesis

The purpose of this work was to design an electrochemical reactor to enhance the high selectivity of enzyme-catalysed processes. In order to develop economically efficient syntheses, the enzymes must be confined in the strict vicinity of the electrode surface. Here the confinement was achieved with a dialysis membrane in a so-called Dialysis-Membrane Electrochemical Reactor (D-MER). Oxidation of glucose into gluconic acid catalysed by glucose oxidase was a first example. The ADH-catalysed reduction of cyclohexanone into cyclohexanol was also tested in a new type of MER. NADH was electrochemically regenerated thanks to mediator (methyl viologen or rhodium complex). The key point in developing electro-enzymatic process is to ensure the perfect fitting of the reactor design to the reactions that are to be processed.

Direct electrochemistry of catalase on glassy carbon electrodes

Catalase was investigated as a possible catalyst of the electrochemical reduction of oxygen on glassy carbon electrodes. The presence of catalase dissolved in solution only provoked a moderate current increase, which was fully explained by the catalase-catalysed disproportionation of hydrogen peroxide (Scheme I). When catalase was adsorbed from dimethylsulfoxide on the surface of electrodes that did not undergo any electrochemical pre-treatment (EP), catalase efficiently catalysed oxygen reduction via direct electron transfer from the electrode (Scheme II). The results are discussed with respect to the electrode surface properties and the enzyme structure.

Enzymatic amplification for spectrophotometric and electrochemical assays of NAD+ and NADH

Five different procedures are presented for the enzymatic assay of the sum of NAD+ and NADH concentrations. They are based on the principle of amplification by cycling. The reactions involve oxidation of the formate ion, ethanol, glucose, or carnitine catalyzed by the corresponding dehydrogenases. The detection reactions are based on the 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyltetrazolium chloride (INT)/INT-formazan and ferricyanide/ferrocyanide couples and use a diaphorase. Two of the systems presented--with formate ion and ethanol--were coupled with spectrophotometric detection. The absorbance measurement values were multiplied by 3 in the first case and by 20 in the second, with respect to the values that would have been obtained in the same conditions without the amplification system. The accessible concentration ranges were between 0.05 and 100 microM approximately. Three systems--with formate ion, carnitine, and glucose--used an electrochemical detection based on oxidation of the ferrocyanide ion. The response times were of the order of 10 min and the precision of about 5%. The first brought to light some difficulties concerning the design of such devices. For the second, the proportionality constant had a value of the order of 0.25 microA.microM-1 and an accessible concentration range between 0.2 and 40 microM. The third allowed more precise assays for lower concentration values: between 0.02 and 1.5 microM, with a proportionality constant of 0.49 microA.microM-1. Emphasis was placed on the adaptation possibilities of these systems as a function of the assay requirements.