Opportunities for visual techniques to determine characteristics and limitations of electro-active biofilms

Optimization of bio-electrochemical systems (BESs) relies on a better understanding of electro-active biofilms (EABfs). These microbial communities are studied with a range of techniques, including electrochemical, visual and chemical techniques. Even though each of these techniques provides very valuable and wide-ranging information about EABfs, such as performance, morphology and biofilm composition, they are often destructive. Therefore, the information obtained from EABfs development and characterization studies are limited to a single characterization of EABfs and often limited to one time point that determines the end of the experiment. Despite being scarcer and not as commonly reported as destructive techniques, non-destructive visual techniques can be used to supplement EABfs characterization by adding in-situ information of EABfs functioning and its development throughout time. This opens the door to EABfs monitoring studies that can complement the information obtained with destructive techniques. In this review, we provide an overview of visual techniques and discuss the opportunities for combination with the established electrochemical techniques to study EABfs. By providing an overview of suitable visual techniques and discussing practical examples of combination of visual with electrochemical methods, this review aims at serving as a source of inspiration for future studies in the field of BESs

Considerations for application of granular activated carbon as capacitive bioanode in bioelectrochemical systems

In the last decades, the research in Microbial Fuel Cells (MFCs) has expanded from electricity production and wastewater treatment to remediation technologies, chemicals production and low power applications. More recently, capacitors have been implemented to boost the power output of these systems when applied as wastewater treatment technology. Specifically, the use of granular capacitive materials (e.g. activated carbon granules) as bioanodes has opened up new opportunities for reactor designs and upscaling of the technology. One of the main features of these systems is that charge and discharge processes can be separated, which offers multiple advantages over more conventional reactor types. In this manuscript, we discuss several aspects to consider for the application of capacitive granules as bioanodes in MFCs and other bioelectrochemical systems, as well as the recent advances that have been made in applying these granules in various reactor systems. Similarly, we discuss the granule properties that are key to determine system operation and performance, and show that biofilm growth is highly dependent on the efficiency of discharge.</p

Quantification of charge carriers and acetate diffusion lengths in intermittent electro-active biofilms using Electrochemical Impedance Spectroscopy

Intermittent anode potential regimes have been used to increase the concentration of charge carriers in electro-active biofilms (EABfs). Even though this increased number of carriers is typically correlated to higher current densities, estimating the concentration of charge carriers in EABfs and linking it to measured current density has never been done. In this study, Electrochemical Impedance Spectroscopy (EIS) and Optical Coherence Tomography (OCT) were used to estimate charge carriers and to study mass transfer limitations in intermittently polarized anodic EABfs. Intermittent potential steps of 20, 60, and 300 s were applied and EABf equilibration times were measured. These times were in the order of 10 s and correlated to the diffusion times obtained from EIS. Acetate consumption rates 100 times faster than the diffusion time of acetate into the EABfs were also estimated with EIS, indicating that current was diffusion limited. Using the capacitance and considering the measured volume of EABf, concentrations of charge carriers ranging from 0.05 molcharge carriers m−3EABf at current densities of 1 A m−2 up to 0.2 molcharge carriers m−3EABf at current densities higher than 2 A m−2 were calculated. This study shows that EIS can be used to study developing EABfs in a non-destructive way and in real-time.</p

Quantification of charge carriers and acetate diffusion lengths in intermittent electro-active biofilms using Electrochemical Impedance Spectroscopy

Intermittent anode potential regimes have been used to increase the concentration of charge carriers in electro-active biofilms (EABfs). Even though this increased number of carriers is typically correlated to higher current densities, estimating the concentration of charge carriers in EABfs and linking it to measured current density has never been done. In this study, Electrochemical Impedance Spectroscopy (EIS) and Optical Coherence Tomography (OCT) were used to estimate charge carriers and to study mass transfer limitations in intermittently polarized anodic EABfs. Intermittent potential steps of 20, 60, and 300 s were applied and EABf equilibration times were measured. These times were in the order of 10 s and correlated to the diffusion times obtained from EIS. Acetate consumption rates 100 times faster than the diffusion time of acetate into the EABfs were also estimated with EIS, indicating that current was diffusion limited. Using the capacitance and considering the measured volume of EABf, concentrations of charge carriers ranging from 0.05 molcharge carriers m−3EABf at current densities of 1 A m−2 up to 0.2 molcharge carriers m−3EABf at current densities higher than 2 A m−2 were calculated. This study shows that EIS can be used to study developing EABfs in a non-destructive way and in real-time.</p

Quantification of charge carriers and acetate diffusion lengths in intermittent electro-active biofilms using Electrochemical Impedance Spectroscopy

Intermittent anode potential regimes have been used to increase the concentration of charge carriers in electro-active biofilms (EABfs). Even though this increased number of carriers is typically correlated to higher current densities, estimating the concentration of charge carriers in EABfs and linking it to measured current density has never been done. In this study, Electrochemical Impedance Spectroscopy (EIS) and Optical Coherence Tomography (OCT) were used to estimate charge carriers and to study mass transfer limitations in intermittently polarized anodic EABfs. Intermittent potential steps of 20, 60, and 300 s were applied and EABf equilibration times were measured. These times were in the order of 10 s and correlated to the diffusion times obtained from EIS. Acetate consumption rates 100 times faster than the diffusion time of acetate into the EABfs were also estimated with EIS, indicating that current was diffusion limited. Using the capacitance and considering the measured volume of EABf, concentrations of charge carriers ranging from 0.05 molcharge carriers m−3EABf at current densities of 1 A m−2 up to 0.2 molcharge carriers m−3EABf at current densities higher than 2 A m−2 were calculated. This study shows that EIS can be used to study developing EABfs in a non-destructive way and in real-time.</p

Quantification of charge carriers and acetate diffusion lengths in intermittent electro-active biofilms using Electrochemical Impedance Spectroscopy

Intermittent anode potential regimes have been used to increase the concentration of charge carriers in electro-active biofilms (EABfs). Even though this increased number of carriers is typically correlated to higher current densities, estimating the concentration of charge carriers in EABfs and linking it to measured current density has never been done. In this study, Electrochemical Impedance Spectroscopy (EIS) and Optical Coherence Tomography (OCT) were used to estimate charge carriers and to study mass transfer limitations in intermittently polarized anodic EABfs. Intermittent potential steps of 20, 60, and 300 s were applied and EABf equilibration times were measured. These times were in the order of 10 s and correlated to the diffusion times obtained from EIS. Acetate consumption rates 100 times faster than the diffusion time of acetate into the EABfs were also estimated with EIS, indicating that current was diffusion limited. Using the capacitance and considering the measured volume of EABf, concentrations of charge carriers ranging from 0.05 molcharge carriers m−3EABf at current densities of 1 A m−2 up to 0.2 molcharge carriers m−3EABf at current densities higher than 2 A m−2 were calculated. This study shows that EIS can be used to study developing EABfs in a non-destructive way and in real-time.</p

Carbon nanomaterials for electrode modification in CH4-producing bioelectrochemical systems

Introduction: Unprecedented environmental phenomena have led to emerging and challenging plans to tackle global threats for the humanity namely intensive use of fossil resources and global warming. CO2 emission to the atmosphere is one of the major driver of global climate change. In this context, the development of alternative technologies for carbon capture and utilization has attracting more and more attention. Electrochemically assisted CO2 conversion in bioelectrochemical systems (BESs) for CH4 production is a new and emerging technology. This innovative approach allows the storage of electrical renewable energy in the form of CH4 that can, when needed, be reconverted, but also the simultaneous CO2 capture contributing to mitigate the climate change and the global warming. However, this technology has limitations mainly related to the electrons transference between the electrode and the biocatalysts. Previous results, obtained within the research group, demonstrated that it is possible to increase the efficiency of the process by improving the electrode surface area which, in turn, improved the microbial attachment. Methodology: This work aimed to investigate the effect of the presence of carbon nanomaterials (carbon nanotubes (CNTs)) at the cathode, on the CH4 production via CO2 reduction. It was hypothesized that the presence of carbon nanomaterials will improve the electrode surface area, thus increasing the electron transfer between the electrode and the biocatalysts. The production of CH4 was analyzed in two BESs, one working with a modified electrode (BES-CNT) and another one that works as a control with a non-modified electrode (BES-CTRL). The potential of CNTs to improve CH4 production was investigated under different electrochemical control modes, potentiostatic and galvanostatic. In addition, the microbial community developed at the biocathode was also investigated. Results: The results demonstrated that for both electrochemical control modes, the production of CH4 was higher in the presence of CNTs compared to the control assay. The study of the microbial community developed at the biocathode under galvanostatic control demonstrated a clear enrichment of methanogens compared to the initial inoculum, however no significant differences were observed between both BES. Conclusions: In conclusion, this work contributed with preliminary insights on the effect of carbon nanomaterials, namely CNTs, to improve the biocathode performance on BESs for CH4 production from CO2.This study was supported by the Portuguese Foundation for Science and Technology(FCT) under the scope of the strategic funding of UIDB/04469/2020 unit.info:eu-repo/semantics/publishedVersio

Real-time monitoring of biofilm thickness allows for determination of acetate limitations in bio-anodes

Several studies have reported that current produced by electro-active bacteria (EAB) is dependent on anode potential and substrate concentration. However, information about the relation between biofilm growth and current density is scarce. In this study, biofilm thickness was monitored in-situ and this relation explored at three anode potentials and three acetate concentrations. The highest current densities of 3.7 A·m−2 were obtained for biofilms thinner than 40 μm, even though thicknesses up to 88 μm were measured. Fick's law was used to estimate the acetate penetration depth in the biofilm, acetate diffusion rates in the biofilm, and specific acetate utilization rates. A maximum biofilm thickness of a non-acetate limited biofilm of 55 μm and an acetate diffusion rate of 2.68 × 10−10 m2·s−1 were estimated at −0.2 V vs Ag/AgCl. The results provide information on the target biofilm thickness for which no acetate limitations occur and provide data for modeling works with bio-anodes

Analysis of bio-anode performance through electrochemical impedance spectroscopy

In this paper we studied the performance of bioanodes under different experimental conditions using polarization curves and impedance spectroscopy. We have identified that the large capacitances of up to 1 mF·cm− 2 for graphite anodes have their origin in the nature of the carbonaceous electrode, rather than the microbial culture. In some cases, the separate contributions of charge transfer and diffusion resistance were clearly visible, while in other cases their contribution was masked by the high capacitance of 1 mF·cm− 2. The impedance data were analyzed using the basic Randles model to analyze ohmic, charge transfer and diffusion resistances. Increasing buffer concentration from 0 to 50 mM and increasing pH from 6 to 8 resulted in decreased charge transfer and diffusion resistances; lowest values being 144 Ω·cm2 and 34 Ω·cm2, respectively. At acetate concentrations below 1 mM, current generation was limited by acetate. We show a linear relationship between inverse charge transfer resistance at potentials close to open circuit and saturation (maximum) current, associated to the Butler–Volmer relationship that needs further exploration.The authors wish to acknowledge funding from the European Union Seventh Framework Programme (FP7/2012-2016) project ‘Bioelectrochemical systems for metal production, recycling, and remediation’ under grant agreement no. 282970. AtH is supported by a NWO VENI grant no. 13631. OS was supported by the French environmental agency ADEME, by the Region Bretagne and by Rennes Metropole when doing the experiments. This work was performed in the cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is co-funded by the Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the European Union Regional Development Fund, the Province of Fryslân, and the Northern Netherlands Provinces