Influence of the electrode size on microbial anode performance

The performance of microbial fuel cells and other related microbial electrochemical processes is seen to deteriorate severely when they are scaled up. This crucial problem is addressed here by comparing the kinetics of microbial anodes with projected surface areas of 9 and 50 cm2 under well-controlled electrochemical conditions. The microbial anode kinetics were characterized by low scan rate voltammetry. The 9-cm2 anodes showed Nernstian behaviour, while the 50-cm2 anodes showed significantly lower performance. The distribution of the electrostatic potential in the experimental set-up was modelled numerically. The model predicted the general trend of the voltammetry curves recorded with the 50-cm2 anodes well, showing that part of the performance deterioration was due to ohmic drop and to non-uniformity of the local potential over the anode surface. Furthermore, the biofilm presented slightly different electrochemical characteristics when grown on the 9-cm2 or 50-cm2 anodes, and the difference in local potential over the 50-cm2 anodes induced spatial heterogeneity in biofilm development. The effect of local potential on biofilm characteristics was an additional cause of the lower performance obtained with the 50-cm2 anodes. In the current state of the art, the soundest way to design large-sized microbial anodes is to adopt the dual main aim of minimizing the ohmic drop while keeping the most uniform possible potential over the electrode surface. Modelling potential distribution inside the reactor should make an essential contribution to this

Bio-ingénierie pour les piles à combustible microbiennes

Une Pile à Combustible Microbienne (PCM) convertit l’énergie chimique issue de l’oxydation de la matière organique directement en énergie électrique. L’oxydation du combustible est assurée par un biofilm dit « électroactif » se développant à la surface de l’anode et jouant le rôle de catalyseur microbien. L’anode microbienne formée à partir d’un consortium bactérien, issu dans cette étude de terreau de jardin, est associée à une cathode à air abiotique à la surface de laquelle se produit la réduction de l’oxygène. L’assemblage d’une anode microbienne et d’une cathode à air abiotique pour construire une PCM est un réel challenge tant les conditions optimales de chacune sont différentes. Ces travaux de thèse ont donc pour objectif d'anticiper le fonctionnement global de la PCM pour concevoir une anode microbienne et une cathode abiotique capables de fonctionner ensemble de façon optimale. Une partie expérimentale conséquente vise à concevoir une PCM optimale en menant des essais sur différents designs de réacteur. Un modèle numérique, basé sur l’expérimentation et calculant les distributions secondaires de courant et de potentiel au sein de la PCM, vient compléter l’étude expérimentale afin d’optimiser l’architecture de la PCM et maximiser les performances délivrées. La configuration « Assemblage Séparateur-Electrodes » consiste à intercaler le séparateur entre la bioanode et la cathode à air dans le but de diminuer la résistance interne du système. Ce design a permis de concevoir des PCMs délivrant d’excellentes performances jusqu’à 6.42 W.m-2. In fine, le prototype « Bioelec », utilisé comme modèle de démonstration, est réalisé à l’échelle du laboratoire avec un assemblage en série et en parallèle de plusieurs PCMs élaborées avec cette configuration « ASE »

Chemical mass transfer in shear zones and metacarbonate xenoliths: a comparison of four mass balance approaches

International audienceMass balance calculations have been performed through a comparison of published graphical and statistical approaches applied to two contrasted geological settings: (i) the development of a greenschist-facies ductile shear-zone that recorded a weak volume change but significant mass transfers, and (ii) the formation of exoskarns in metacarbonate xenoliths that recorded a large volume decrease related to huge mass transfers. The comparison of the four mass-balance approaches shows that, if uncertainties are ignored, (1) they yield similar results concerning the mobile vs immobile behaviour of many components; (2) they yield similar mass-change values on bulk rock and on individual chemical elements (bulk-rock mass-change values differ by a maximum of ca. 15 % between graphical and statistical treatments of the metacarbonate xenolith evolution). The main difference concerns the uncertainties on mass changes (for bulk rocks and individual elements), which are much larger with the graphical than with the statistical approaches when uncertainties on chemical elements are taken into account, as they should be.The main advantage of the graphical methods is their rapid implementation and the clarity of the diagrams. Their main disadvantages are that uncertainties on each chemical element and bulk compositions are not taken into account and the difficulty in choosing an accurate immobility field to precisely define errors. Graphical methods need to be completed by a statistical treatment that gives absolute mass transfer results. The statistical approaches have the advantage of taking into account the chemical heterogeneities of the compared populations, in conjunction to a precise data treatment. The statistical treatment is an important and necessary step to decipher and to be pertinent in interpreting mobility/immobility of chemical elements, and, thus, in the absolute quantification of mass and volume changes

Separator electrode assembly (SEA) with 3-dimensional bioanode and removable air-cathode boosts microbial fuel cell performance

Separator electrode assemblies (SEAs) were designed by associating a microbial anode with an air-cathode on each side of three different kinds of separator: plastic grid, J-cloth and baking paper. The SEA was designed to allow the air-cathode be removed and replaced without disturbing the bioanode. Power densities up to 6.4 W m−2 were produced by the Grid-SEAs (on average 5.9 ± 0.5 W m−2) while JCloth-SEAs and Paper-SEAs produced 4.8 ± 0.3 and 1.8 ± 0.1 W m−2, respectively. Power densities decreased with time mainly because of fast deterioration of the cathode kinetics. They always increased again when the air-cathodes were replaced by new ones; the Grid-SEAs were thus boosted above 4 W m−2 after 7 weeks of operation. The theoretical analysis of SEA functioning suggested that the high performance of the Grid-SEAs was due to the combination of several virtuous phenomena: the efficient pH balance thanks to free diffusion through the large-mesh grid, the likely mitigation of oxygen crossover thanks to the 3-dimensional structure of the bioanode and the possibility of overcoming cathode fouling by replacing it during MFC operation. Finally, the microbial community of all bioanodes showed stringent selection of Proteiniphilum acetatigenes in proportion with the performance

Removable air-cathode to overcome cathode biofouling in microbial fuel cells

An innovative microbial fuel cell (MFC) design is described, which allows the air-cathode to be replaced easily without draining the electrolyte. MFCs equipped with 9-cm2 or 50-cm2 bioanodes provided 0.6 and 0.7 W/m2 (referred to the cathode surface area) and were boosted to 1.25 and 1.96 W/m2, respectively, when the initial air-cathode was replaced by a new one. These results validate the practical interest of removable air-cathodes and evidence the importance of the cathode biofouling that takes place during the MFC starting phase. As this biofouling is compensated by the concomitant improvement of the bioanodes it cannot be detected on the power curves and may be a widespread cause of performance underestimation

Microbial fuel cells connected in series in a common electrolyte underperform: understanding why and in what context such a set-up can be applied

Microbial fuel cells (MFCs) have the outstanding ability to transform the chemical energy contained in organic matter directly to electrical energy. Unfortunately, they give only low cell voltage at maximum power. Connecting several MFCs electrically in series inside the same reactor may be a way to increase the cell voltage, but experimental attempts have shown poor efficiency for such single-electrolyte stacks.The present study uses numerical modelling to understand the behaviour of single-electrolyte MFC stacks and to assess possible ways to improve it. The numerical model was validated by comparison with two experimental MFCs that produced 0.85 ± 0.05 mW each at 0.23 V cell voltage. Connected in series in a common electrolyte, the stack produced only 0.7 mW at 0.21 V, while, in theory, 1.7 mW could be reached at 0.47 V. The model showed that the drastic power loss was due to ionic short-circuiting, which may, however, be an interesting phenomenon to be exploited for designing an electro-microbial snorkel. The model also showed that decreasing the anode-cathode distance, increasing the distance between the MFCs or using baffles between them could optimize the single-electrolyte stack to produce up to 80% of the theoretical maximum power. Nevertheless, such designs are appropriate only for specific applications, e.g. biosensing. The model further suggests that benthic MFCs could be effectively connected in series

Brittle-Ductile Rheological Behavior in Subduction Zones: Effects of Strength Ratio Between Strong and Weak Phases in a Bi-Phase System

The brittle-ductile rheological behavior in subduction zones is commonly proposed to explain deep transient slips. Generally observed at large scales in tectonic “mélanges”, here we show that it is also observed at the grain scale in exhumed blueschist metagabbros. In these rocks, petrologic and microstructural observations show a bi-phase material constituted by strong microfractured magmatic pyroxene clasts located in a weak and ductile lawsonite-rich metamorphic matrix. To constrain the mechanical conditions allowing the brittle deformation of a clast in a ductile matrix, we used two-dimensional simple shear numerical experiments. Results show four behaviors: (a) entirely brittle; (b) brittle-ductile with clast fracturing in a ductile matrix; (c) ductile-dominant with limited plastic deformation at clast edges; and (d) entirely ductile. We propose that the conditions of the brittle-ductile behavior, commonly associated with deep transient slips, are controlled by the strength ratio between the strong brittle phase and the weak ductile phase

Increasing the temperature is a relevant strategy to form microbial anodes intended to work at room temperature

Reducing the time required for the formation of microbial anodes from environmental inocula is a great challenge. The possibility of reaching this objective by increasing the temperature during the bioanode preparation was investigated here. Microbial anodes were formed at 25 °C and 40 °C under controlled potential with successive acetate additions. At 25 °C, around 40 days were required to perform three acetate batches, which led to current density of 9.4 ± 2 A.m−2, while at 40 °C, 20 days were sufficient to complete three similar batches, leading to 22.9 ± 4.2 A.m−2. The bioanodes formed at 40 °C revealed three redox systems and those formed at 25 °C only one. The temperature also impacted the biofilm structure, which was less compact at 40 °C. When the bioanodes formed at 40 °C were switched to 25 °C, they produced current densities similar to those of bioanodes formed at 25 °C; they recovered the single redox system that was developed by the bioanodes formed at 25 °C and the difference in biofilm structures was mitigated. It is consequently fully appropriate to accelerate the formation of microbial anodes by increasing the temperatures to 40 °C even if they are finally intended to operate at room temperature

Ion transport in microbial fuel cells: Key roles, theory and critical review

Microbial fuel cells (MFCs) offer the possibility to convert the chemical energy contained in low-cost organic matter directly into electrical energy. Nevertheless, in the current state of the art, microbial electrocatalysis imposes the use of electrolytes of low ionic conductivity, at around neutral pH and with complex chemical compositions, which are far from being ideal electrolytes for an electrochemical process. In this context, ion transport through the electrolyte is a key step, which strongly conditions the electrode kinetics and the global cell performance. The fundamentals of ion transport in electrolytes are recalled and discussed in the light of MFC constraints. The concept of transport number is emphasized in order to provide an easy-to-use theoretical framework for analysing the pivotal roles of ion transport through the electrolyte. Numerical illustrations show how the concept of transport number can be used to predict MFC behaviour on the basis of the electrolyte composition. The tricky problem of pH balance is discussed, the interest of separator-less MFCs is emphasized, and the concept of “microbial separator” is proposed as a worthwhile future research direction. The role of separators in driving ion transport is then reviewed following a comprehensive classification, from the most compact, non-porous membranes to the most porous separators. For each group, basic are first recall to guide the critical analysis of the experimental data. This analysis brings to light a few research directions that may be reoriented and some critical issues that need in-depth investigation in the future. The suitability of cation-/anion-exchange membranes is discussed in comparison to that of porous separators. Finally, the large discrepancy observed on data obtained for the same separator type suggests that, in the future, specific analytical set-ups should be developed

Pervasive carbonation of peridotite to listvenite (Semail Ophiolite, Sultanate of Oman): clues from iron partitioning and chemical zoning

Earth's long-term cycling of carbon is regulated from mid-ocean ridges to convergent plate boundaries by mass transfers involving mantle rocks. Here we examine the conversion of peridotite to listvenite (magnesite + quartz rock) during CO2 metasomatism along the basal thrust of the Semail Ophiolite (Fanja, Sultanate of Oman). At the outcrop scale, this transformation defines reaction zones, from serpentinized peridotites to carbonated serpentinites and listvenites. Based on a detailed petrological and chemical study, we show that carbonation progressed through three main stages involving the development of replacive textures ascribed to early stages, whilst carbonate (± quartz) veining becomes predominant in the last stage. The pervasive replacement of serpentine by magnesite is characterized by the formation of spheroids, among which two types are identified based on the composition of their core regions: Fe-core and Mg-core spheroids. Fe zoning is a type feature of matrix and vein magnesite formed during the onset carbonation (Stage 1). While Fe-rich magnesite is predicted to form at low fluid XCO2 from a poorly to moderately oxidized protolith, our study evidences that the local non-redox destabilization of Fe oxides into Fe-rich magnesite is essential to the development of Fe-core spheroids. The formation of Fe-core spheroids is followed by the pervasive (over-)growth of Mg-rich spheroids and aggregates (Stage 2) at near-equilibrium conditions in response to increasing fluid XCO2. Furthermore, the compositions of carbonates indicate that most siderophile transition elements released by the dissolution of primary minerals are locally trapped in carbonate and oxides during matrix carbonation, while elements with a chalcophile affinity are the most likely to be leached out of reaction zones.</p