Different methods used to form oxygen reducing biocathodes lead to different biomass quantities, bacterial communities, and electrochemical kinetics

Six biocathodes catalyzing oxygen reduction were designed from the same environmental inoculum but using three different methods. Two were formed freely at open circuit potential, two using conventional aerobic polarization at -0.2V/SCE and two by reversion of already established acetate-fed bioanodes. Observation of the biofilms by SEM and epifluorescence microscopy revealed that reversible bioelectrodes had the densest biofilms. Electrochemical characterization revealed two different redox systems for oxygen reduction, at -0.30 and +0.23V/SCE. The biocathodes formed under aerobic polarization gave higher electrocalatytic performance for O2 reduction, due to production of the redox systems at +0.23V/SCE. Analyses of the bacterial communities on the biocathodes by 16S-rRNA pyrosequencing showed different selection (or enrichment) of microorganisms depending on the method used. This study highlights how the method chosen for designing oxygen biocathodes can affect the cathode coverage, the selection of bacterial populations and the electrochemical performanc

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. Out of several thousands of studies, only a minority have used 3-electrode set-ups to ensure well-controlled electroanalysis conditions. The present article reviews these electroanalytical studies with the admitted objective of promoting this type of investigation. A first recall of basics emphasises the advantages of the 3-electrode set-up compared to microbial fuel cell devices if analytical objectives are pursued. Experimental precautions specifically relating to microbial anodes are then noted and the existing experimental set-ups and procedures are reviewed. The state-of-the-art is described through three aspects: the effect of the polarisation potential on the characteristics of microbial anodes, the electroanalytical techniques, and the electrode. We hope that the final outlook will encourage researchers working with microbial anodes to strengthen their engagement along the multiple exciting paths of electroanalysis

The sloped limiting current region during ion transfer at arrays of nanointerfaces between immiscible electrolyte solutions

The formation of arrays of nano-interfaces between immiscible electrolyte solutions using nanoporous membranes opens up new opportunities in electrochemical analysis. However, an unusual feature in the voltammetry, in the form of a consistently sloped current in the limiting current region, has been observed. This sloped limiting current was observed using different alkylammonium cation transfers at the nanoITIES arrays, showing the generality of the feature. Experiments with variable concentrations of organic or aqueous phase electrolytes revealed that the sloped limiting current was impacted by the concentration of the aqueous phase electrolyte. A plausible explanation for the effect is discussed based on ternary electrodiffusion which occurs due to facilitated ion transfer of the aqueous phase background electrolyte cation. As a result, the use of nanoITIES arrays as the basis for chemical sensors and detection needs to be carefully considered

Multi-system Nernst–Michaelis–Menten model applied to bioanodes formed from sewage sludge

Bioanodes were formed under constant polarization at 0.2 V/SCE from fermented sewage sludge. Current densities reached were 9.3 ± 1.2 A m2 with the whole fermented sludge and 6.2 ± 0.9 A m2 with the fermented sludge supernatant. The bioanode kinetics was analysed by differentiating among the contributions of the three redox systems identified by voltammetry. Each system ensured reversible Nernstian electron transfer but around a different central potential. The global overpotential required to reach the maximum current plateau was not imposed by slow electron transfer rates but was due to the potential range covered by the different redox systems. The microbial communities of the three bioanodes were analysed by 16S rRNA gene pyrosequencing. They showed a significant microbial diversity around a core of Desulfuromonadales, the proportion of which was correlated with the electrochemical performance of the bioanodes

Multiple electron transfer systems in oxygen reducing biocathodes revealed by different conditions of aeration/agitation

Oxygen reducing biocathodes were formed at − 0.2 V/SCE (+ 0.04 V/SHE) from compost leachate. Depending on whether aeration was implemented or not, two different redox systems responsible for the electrocatalysis of oxygen reduction were evidenced. System I was observed at low potential (− 0.03 V/SHE) on cyclic voltammetries (CVs). It appeared during the early formation of the biocathode (few hours) and resisted the hydrodynamic conditions induced by the aeration. System II was observed at higher potential on CV (+ 0.46 V/SHE); it required a longer lag time (up to 10 days) and quiescent conditions to produce an electrochemical signal. The hydrodynamic effects produced by the forced aeration led to its extinction. From their different behaviors and examples in the literature, system I was identified as being a membrane-bound cytochrome-related molecule, while system II was identified as a soluble redox mediator excreted by the biofilm. This study highlighted the importance of controlling the local hydrodynamics to design efficient oxygen reducing biocathodes able to operate at high potential

Garden compost inoculum leads to microbial bioanodes with potential-independent characteristics

International audienceGarden compost leachate was used to form microbial bioanodes under polarization at 0.4, 0.2 and +0.1 V/SCE. Current densities were 6.3 and 8.9 A m2 on average at 0.4 and +0.1 V/SCE respectively, with acetate 10 mM. The catalytic cyclic voltammetry (CV) showed similar electrochemical characteristics for all bioanodes and indicated that the lower currents recorded at 0.4 V/SCE were due to the slower interfacial electron transfer rate at this potential, consistently with conventional electrochemical kinetics.RNA- and DNA-based DGGE evidenced that the three dominant bacterial groups Geobacter, Anaerophaga and Pelobacter were identical for all bioanodes and did not depend on the polarization potential. Only non-turnover CVs showed differences in the redox equipment of the biofilms, the highest potential promoting multiple electron transfer pathways. This first description of a potential-independent electroactive microbial community opens up promising prospects for the design of stable bioanodes for microbial fuel cells

Electrochemical characterization of microbial bioanodes formed on a collector/electrode system in a highly saline electrolyte

Bioanodes were formed with electrodes made of carbon felt and equipped with a titanium electrical collector, as commonly used in microbial fuel cells. Electrochemical impedance spectroscopy (EIS) performed on the abiotic electrode system evidenced two time constants, one corresponding to the “collector/carbon felt” contact, the other to the “carbon felt/solution” interface. Such a two time constant system was characteristics of the two-material electrode, independent of biofilm presence. EIS was then performed during the bioanode formation around the constant applied potential of 0.1 V/SCE. The equivalent electrical model was similar to that of the abiotic system. Due to the high salinity of the electrolyte (45 g·L− 1 NaCl) the electrolyte resistance was always very low. The bioanode development induced kinetic heterogeneities that were taken into account by replacing the pure capacitance of the abiotic system by a constant phase element for the “carbon felt/solution” interface. The current increase from 0 to 20.6 A·m− 2 was correlated to the considerable decrease of the charge transfer resistance of the “carbon felt/solution” interface from 2.4 104 to 92 Ω·cm2. Finally, EIS implemented at 0.4 V/SCE showed that the limitation observed at high potential values was not related to mass transfer but to a biofilm-linked kinetics

Biocathodes reducing oxygen at high potential select biofilms dominated by Ectothiorhodospiraceae populations harboring a specific association of genes

Biocathodes polarized at high potential are promising for enhancing Microbial Fuel Cell performances but the microbes and genes involved remain poorly documented. Here, two sets of five oxygen-reducing biocathodes were formed at two potentials (−0.4 V and +0.1 V vs. saturated calomel electrode) and analyzed combining electrochemical and metagenomic approaches. Slower start-up but higher current densities were observed at high potential and a distinctive peak increasing over time was recorded on cyclic voltamogramms, suggesting the growth of oxygen reducing microbes. 16S pyrotag sequencing showed the enrichment of two operational taxonomic units (OTUs) affiliated to Ectothiorodospiraceae on high potential electrodes with the best performances. Shotgun metagenome sequencing and a newly developed method for the identification of Taxon Specific Gene Annotations (TSGA) revealed Ectothiorhodospiraceae specific genes possibly involved in electron transfer and in autotrophic growth. These results give interesting insights into the genetic features underlying the selection of efficient oxygen reducing microbes on biocathodes

Electrochemical Characterisation of Nanoscale Liquid | Liquid Interfaces Located at Focused Ion Beam-Milled Silicon Nitride Membranes

The electrochemical behaviour of single and arrayed nanoscale interfaces between two immiscible electrolyte solutions (single and array nanoITIES) is presented. The interfaces were formed at nanopores fabricated through the focused ion beam (FIB) milling of silicon nitride (SiN) membranes by using nanopores with approximately 30–80 nm radii and with pore-to-pore separations to pore radius ratios in the range of 16–32. Electrochemistry was performed through the interfacial transfer of tetrapropylammonium (TPrA+) across single and array nanoITIES between water and 1,6-dichlorohexane. The ion-transfer limiting current at the single nanoITIES was in excellent agreement with the current predicted by using an inlaid disc interface model. At nanoITIES arrays, experimental currents were lower than predicted for an array of inlaid interfaces, which is attributed to overlapped diffusion zones. As a result, FIB milling offers an attractive strategy to form nanoITIES for diverse investigations

Hydrogen Evolution at Liquid|Liquid Interfaces Catalysed by 2D Materials

The hydrogen evolution reaction (HER) plays a crucial role in clean energy production in hydrogen fuel cells. In order to utilise this process effectively, new catalysts are required that are cheap, non-toxic and efficient. In this context, 2D materials such as transition metal dichalcogenides (e.g. MoS2) should offer the desired properties but have so far proven difficult to manufacture into useful devices. In this work, liquid|liquid interfaces are used for the assembly and testing of the catalytic efficiency of a number of 2D materials and their composites, exploiting the ability of the materials to self-assemble at these interfaces and be tested electrochemically in situ. MoS2, WS2, and graphene were developed for hydrogen evolution at the water|1,2-dichlorobenzene (DCB) interface. The exfoliation process was carried out in DCB and resulted in multi-layer MoS2, few layer WS2 and graphene: when assembled at the water|DCB interface, these materials acted as efficient HER catalysts. HER was investigated using voltammetry, with bulk reaction kinetics monitored by in-situ UV-visible spectroscopy at a constant potential. MoS2 exhibited the highest performance of the catalysts examined, with an average rate constant of 0.0132 ± 0.063 min-1 at an applied Galvani potential of +0.5 V. This is ascribed to the sulphur edge sites of MoS2, which are known to be active for hydrogen evolution predominantly