Tailored glycosylated anode surfaces: Addressing the exoelectrogen bacterial community via functional layers for microbial fuel cell applications

PDA-Fe3O4 decorated carbon felt anode enhancing electrochemical performance of microbial fuel cells: Effect of electrode materials on electroactive biofilm

Anode modification is an effective strategy for enhancing the electrochemical performance of microbial fuel cell (MFC). However, the impacts of the modified materials on anode biofilm development during MFC operation have been less studied. We prepared a novel PDA-Fe3O4-CF composite anode by coating original carbon felt anode (CF) with polydopamine (PDA) and Fe3O4 nanoparticles. The composite anode material was characterized by excellent hydrophilicity and electrical conductivity, and the anodic biofilm exhibited fast start-up, higher biomass, and more uniform biofilm layer after MFC operation. The MFC reactor assembled with the composite anode achieved a maximum power density of 608 mW m-2 and an output voltage of 586 mV, which were 316.4% and 72.4% higher than the MFC with the original CF anode, respectively. Microbial community analysis indicated that the modified anode biofilm had a higher relative abundance of exoelectrogen species in comparison to the unmodified anode. The PICRUSt data revealed that the anodic materials may affect the bioelectrochemical performance of the biofilm by influencing the expression levels of key enzyme genes involved in biofilm extracellular polymer (EPS) secretion and extracellular electron transfer (EET). The growth of the anodic biofilm would exert positive or negative influences on the efficiency of electricity production and electron transfer of the MFCs at different operating stages. This work expands the knowledge of the role that anodic materials play in the development and electrochemical performance of anodic biofilm in MFCs.

Development of bioanodes rich in exoelectrogenic bacteria using iron-rich palaeomarine sediment inoculum

Microbial Fuel Cells (MFC) convert energy stored in chemicals into electrical energy thanks to exoelectrogenic microorganisms who also play a crucial role in geochemical cycles in their natural environment, including that of iron. In this study, we investigated paleomarine sediments as inoculum for bioanode development in MFCs. These sediments were formed under anoxic conditions ca. 113 million years ago and are rich in clay minerals, organic matter, and iron. The marlstone inoculum was incubated in the anolyte of an MFC using acetate as the added electron donor and ferricyanide as the electron acceptor in the catholyte. After seven weeks of incubation, the current density increased to 0.15 mA.cm-2 and a stable + 700 mV open circuit potential was reached. Community analysis revealed the presence of two exoelectrogenic bacterial genera, Geovibrio and Geobacter. Development of electroactive biofilms was correlated to bulk chemical transformations of the sediment inoculum with an increase in the Fe(II) to Fetotal ratio. Comparisons to sediments sterilized prior to inoculation confirmed that bioanode development derives from the native microbiota of these paleomarine sediments. This study illustrates the feasibility of developing exoelectrogenic biofilms from iron-rich marlstone and has implications for the role of such bacteria in broader paleoenvironmental phenomena.

Fe(III) oxide microparticles modulate extracellular electron transfer in anodic biofilms dominated by bacteria of the Pelobacter genus

Exo-electrogenic microorganisms have been extensively studied for their ability to transfer electrons with solid surfaces using a large variety of metabolic pathways. Most of the studies on these microorganisms consist in the replacement of solid electron acceptors such as Fe(III) oxides found in nature by electrodes with the objective of generating harvestable current in devices such as microbial fuel cells. In this study we show how the presence of solid ferric oxide (Fe₂O₃) particles in the inoculum during bio-anode development influences extracellular electron transfer to the electrode. Amplification and sequencing of the 16S rRNA (V4-V5 region) show bacteria and archaea communities with a large predominance of the Pelobacter genus, which is known to be phylogenetically close to the Geobacter genus, regardless of the presence or absence of ferric oxide in the inoculum. Data indicate that the bacteria at the bio-anode surface can preferentially utilize solid ferric oxide as terminal electron acceptors instead of the anode, though extracellular electron transfer to the anode can be restored by removing the particles. Mixed inoculum commonly used to develop bioanodes may produce similar bacterial communities with divergent electrochemical responses due to the presence of alternate electron acceptors, with direct implications for microbial fuel cell performance.

Extracellular electron transfer and the conductivity in microbial aggregates during biochemical wastewater treatment: A bottom-up analysis of existing knowledge

Microbial extracellular electron transfer (EET) plays a crucial role in bioenergy production and resource recovery from wastewater. Interdisciplinary efforts have been made to unveil EET processes at various spatial scales, from nanowires to microbial aggregates. Electrical conductivity has been frequently measured as an indicator of EET efficiency. In this review, the conductivity of nanowires, biofilms, and granular sludge was summarized, and factors including subjects, measurement methods, and conducting conditions that affect the conductivity difference were discussed in detail. The high conductivity of nanowires does not necessarily result in efficient EET in microbial aggregates due to the existence of non-conductive substances and contact resistance. Improving the conductivity measurement of microbial aggregates is important because it enables the calculation of an EET flux from conductivity and a comparison of the flux with mass transfer coefficients. This review provides new insight into the significance, characterization, and optimization of EET in microbial aggregates during a wastewater treatment process.

Sprayable biofilm – Agarose hydrogels as 3D matrix for enhanced productivity in bioelectrochemical systems

Bio-based energy production utilizing renewable resources can be realized by exoelectrogenic organisms and their application in bioelectrochemical systems (BES). These organisms catalyze the direct conversion of chemical into electrical energy and are already widely used in bioelectronics and biosensing. However, the biofilm-electrode interaction is a factor that limits sufficient space-time-yields for industrial applications. In this study, a hydrogel matrix consisting of agarose fibers was utilized as a scaffold for S. oneidensis cells to improve anodic processes in BES. This synthetic, scalable biofilm reached a higher current density in BES in comparison to naturally formed biofilms. Complemented with carbon nanofibers and riboflavin, the application of this functionalized hydrogel containing S. oneidensis cells led to an overall 9.1-fold increase in current density to 1324 mA m⁻² in comparison to 145 mA m⁻² for the planktonic control. In addition, the synthetic biofilm can be applied by spraying onto surfaces using a novel spray applicator. The latter allows to apply the biofilm effortless on large surfaces which will facilitate scalability and thus industrial application.

Microbial Interactions in Electroactive Biofilms for Environmental Engineering Applications: A Role for Nonexoelectrogens

Microbial electrochemical systems have gained much attention over the past decade due to their potential for various environmental engineering applications ranging from energy production to wastewater treatment to bioproduction. At the heart of these systems lie exoelectrogens-microorganisms capable of exporting electrons generated during metabolism to external electron acceptors such as electrodes. The bacterial biofilm communities on these electrodes are dominated by exoelectrogens but are nonetheless extremely diverse. So far, within the field, the main focus has been on the electroactive bacteria. However, to broaden our understanding of these communities, it is crucial to clarify how the remaining inhabitants of electrode-respiring biofilms contribute to the overall function of the biofilm. Ultimately, such insights may enable improvement of microbial electrochemical systems by reshaping the community structure with naturally occurring beneficial strains.

Prospective review on bioelectrochemical systems for wastewater treatment: Achievements, hindrances and role in sustainable environment

Bioelectrochemical systems (BESs) are a relatively new arena for producing bioelectricity, desalinating sea water, and treating industrial effluents by removing organic matter. Microbial electrochemical technologies (METs) are promising for obtaining value-added products during simultaneous remediation of pollutants from wastewater. The search for more affordable desalination technology has led to the development of microbial desalination cells (MDCs). MDC combines the operation of microbial fuel cells (MFC) with electrodialysis for water desalination and energy generation. It has received notable interest of researchers in desalination and wastewater treatment because of low energy requirement and eco-friendly nature. Firstly, this article provides a brief overview of MDC technology. Secondly, factors affecting functioning of MDC and its applications have been accentuated. Additionally, challenges and future outlook on the development of this technology have been delineated. State-of-the-art information provided in this review would expand the scope of interdisciplinary and translational research.

Nanoscaffold effects on the performance of air-cathodes for microbial fuel cells: Sustainable Fe/N-carbon electrocatalysts for the oxygen reduction reaction under neutral pH conditions

Nanostructured electrocatalysts for microbial fuel cell air-cathodes were obtained via use of conductive carbon blacks for the synthesis of high performing 3D conductive networks. We used two commercially available nanocarbons, Black Pearls 2000 and multiwalled carbon nanotubes, as conductive scaffolds for the synthesis of nanocomposite electrodes by combining: a hydrothermally carbonized resin, a sacrificial polymeric template, a nitrogenated organic precursor and iron centers. The resulting materials are micro-mesoporous, possess high specific surface area and display N-sites (N/C of 3-5 at%) and Fe-centers (Fe/C < 1.5at.%) at the carbon surface as evidenced from characterization methods. Voltammetry studies of oxygen reduction reaction activity were carried out at neutral pH, which is relevant to microbial fuel cell applications, and activity trends are discussed in light of catalyst morphology and composition. Tests of the electrocatalyst using microbial fuel cell devices indicate that optimization of the nanocarbon scaffold for the Pt-free carbon-based electrocatalysts results in maximum power densities that are 25% better than those of Pt/C cathodes, at a fraction of the materials costs. Therefore, the proposed Fe/N-carbon catalysts are promising and sustainable high-performance cathodic materials for microbial fuel cells.

Controlling the Carbon-Bio Interface via Glycan Functional Adlayers for Applications in Microbial Fuel Cell Bioanodes

Surface modification of electrodes with glycans was investigated as a strategy for modulating the development of electrocatalytic biofilms for microbial fuel cell applications. Covalent attachment of phenyl-mannoside and phenyl-lactoside adlayers on graphite rod electrodes was achieved via electrochemically assisted grafting of aryldiazonium cations from solution. To test the effects of the specific bio-functionalities, modified and unmodified graphite rods were used as anodes in two-chamber microbial fuel cell devices. Devices were set up with wastewater as inoculum and acetate as nutrient and their performance, in terms of output potential (open circuit and 1 kΩ load) and peak power output, was monitored over two months. The presence of glycans was found to lead to significant differences in startup times and peak power outputs. Lactosides were found to inhibit the development of biofilms when compared to bare graphite. Mannosides were found, instead, to promote exoelectrogenic biofilm adhesion and anode colonization, a finding that is supported by quartz crystal microbalance experiments in inoculum media. These differences were observed despite both adlayers possessing thickness in the nm range and similar hydrophilic character. This suggests that specific glycan-mediated bioaffinity interactions can be leveraged to direct the development of biotic electrocatalysts in bioelectrochemical systems and microbial fuel cell devices.

Biocatalyst physiology and interplay: a protagonist of MFC operation

Microbial fuel cells (MFC) have been foreseen as a sustainable renewable energy resource to meet future energy demand. In the past, several studies have been executed in both benchtop and pilot scale to produce electrical energy from wastewater. The key role players in this technology that leads to the operation are microbes, mainly bacteria. The dominant among them is termed as "exoelectrogens" that have the capability to produce and transport electron by utilizing waste source. The current review focuses on such electrogenic bacteria's involvement for enhanced power generation of MFC. The pathway of electron transfer in their cell along and its conduction to the extracellular environment of the MFC system are critically discussed. The interaction of the microbes in various MFC operational conditions, including the role of substrate and solid electron acceptors, i.e., anode, external resistance, temperature, and pH, was also discussed in depth along with biotechnological advancement and future research perspective.

Sustainable Syntheses and Sources of Nanomaterials for Microbial Fuel/Electrolysis Cell Applications: An Overview of Recent Progress

The use of microbial fuel cells (MFCs) is quickly spreading in the fields of bioenergy generation and wastewater treatment, as well as in the biosynthesis of valuable compounds for microbial electrolysis cells (MECs). MFCs and MECs have not been able to penetrate the market as economic feasibility is lost when their performances are boosted by nanomaterials. The nanoparticles used to realize or decorate the components (electrodes or the membrane) have expensive processing, purification, and raw resource costs. In recent decades, many studies have approached the problem of finding green synthesis routes and cheap sources for the most common nanoparticles employed in MFCs and MECs. These nanoparticles are essentially made of carbon, noble metals, and non-noble metals, together with a few other few doping elements. In this review, the most recent findings regarding the sustainable preparation of nanoparticles, in terms of syntheses and sources, are collected, commented, and proposed for applications in MFC and MEC devices. The use of naturally occurring, recycled, and alternative raw materials for nanoparticle synthesis is showcased in detail here. Several examples of how these naturally derived or sustainable nanoparticles have been employed in microbial devices are also examined. The results demonstrate that this approach is valuable and could represent a solid alternative to the expensive use of commercial nanoparticles.