Effect of surface roughness, biofilm coverage and biofilm structure on the electrochemical efficiency of microbial cathodes

A pore-scale reactive transport modeling study for quorum sensing-driven biofilm dispersal in heterogeneous porous media

Microorganisms regulate the expression of energetically expensive phenotypes via a collective decision-making mechanism known as quorum sensing (QS). This study investigates the intricate dynamics of biofilm growth and QS-controlled biofilm dispersal in heterogeneous porous media, employing a pore-scale reactive transport modeling approach. Model simulations carried out under various fluid flow conditions and biofilm growth scenarios reveal that QS processes are influenced not only by the biomass density of biofilm colonies but also by a complex interplay between pore architecture, flow velocity, and the rates of biofilm growth and dispersal. This study demonstrates that pore architecture controls the initiation of QS processes and advection gives rise to oscillatory growth of biofilms. Such oscillation is suppressed if biofilm dynamics are in favor of sustaining a sufficiently high signal concentration, such as fast growth or slow dispersal rates. By establishing a mathematical framework, this study contributes to the fundamental understanding of QS-controlled biofilm dynamics in complex environments.

Evolution and interaction of microbial communities in mangrove microbial fuel cells and first description of Shewanella fodinae as electroactive bacterium

Understanding exoelectrogenic bacteria mechanisms and their interactions in complex biofilm is critical for the development of microbial fuel cells (MFCs). In this article, assumptions concerning the benefits of the complex sediment microbial community for electricity production were explored with both the complex microbial community and isolates identified as Shewanella. Analysis of the microbial community revealed a strong influence of the sediment community on anodes and electrolytes compared to that of only water. Moreover, while Pelobacteraceae-related genera were dominant in our MFCs instead of Desulfuromonas and Geobacter as usually reported, the electroactive Shewanella algae and Shewanella fodinae were isolated and cultivated from the anodic biofilm. S. fodinae, described for the first time as an electroactive bacterium to the best of our knowledge, led to a maximal current density of 3.6 A/m² set as 0.3 V/SCE in a three-electrode set-up fed with lactate. S. algae, in a complex medium containing several available substrates, showed several preferential oxidative behaviors including a diauxic behavior. In pure culture and under our conditions, S. fodinae and S. algae were not able to use acetate as a sole electron donor. However, their presence in our acetate-fed MFCs and the adaptive behavior of S. algae hint a syntrophic interaction between the bacteria to optimize the use of the substrate in a complex environment.

Progress and perspectives on microbial electrosynthesis for valorisation of CO2 into value-added products

Microbial electrosynthesis (MES) is a neoteric technology that facilitates biocatalysed synthesis of organic compounds with the aid of homoacetogenic bacteria, while feeding CO₂ as an inorganic carbon source. Operating MES with surplus renewable electricity further enhances the sustainability of this innovative bioelectrochemical system (BES). However, several lacunae exist in the domain knowledge, stunting the widespread application of MES. Despite significant progress in this area over the past decade, the product yield efficiency is not on par with other contemporary technologies. This bottleneck can be overcome by adopting a holistic approach, i.e., applying innovative and integrated solutions to ensure a robust MES operation. Further, the widespread deployment of MES exclusively relies on its ability to mature a sessile biofilm over a biocompatible electrode, while offering minimal charge transfer resistance. Additionally, operating MES preferably at H₂-generating reduction potential and valorising industrial off-gas as carbon substrate is crucial to accomplish economic sustainability. In light of the aforementioned, this review collates the latest progress in the design and development of MES-centred systems for valorisation of CO₂ into value-added products. Specifically, it highlights the significance of inoculum pre-treatment for promoting biocatalytic activity and biofilm growth on the cathodic surface. In addition, it summarizes the diverse materials that are commonly used as electrodes in MES, with an emphasis on the importance of inexpensive, robust, and biocompatible electrode materials for the practical application of MES technology. Further, the review presents insights into media conditions, operational factors, and reactor configurations that affect the overall performance of MES process. Finally, the product range of MES, downstream processing requirements, and integration of MES with other environmental remediation technologies are also discussed.

Role of electrode and proton exchange membrane configurations on microbial fuel cell performance toward bioelectricity generation integrated wastewater treatment

In the present study, the effects of electrode surface area, proton exchange membrane area, and volume of the anodic chamber were investigated on the performance of five different dual chamber microbial fuel cells (MFC) using synthetic wastewater toward wastewater treatment coupled electricity generation. In the batch mode, the five different MFC's were operated with the anodic chamber volumes of 93-890 mL, 17.33-56.77 cm² electrode surface area, obtained volumetric power densities of 137.72-58.13 mW/m³, and unit area power densities ranging from 27.04 to 11.94 mW/m². Fed-batch studies were done with the MFC having 740 mL anodic chamber volume at different wastewater COD concentrations. The power density per unit area increased from 22.93 mW/m² to 36.25 cm² when the distance between electrodes was reduced from 10 to 6 cm. A maximum volumetric power density of 135.21 mW/m³ has been attained with a 6 cm electrode distance with the accomplished COD reduction of 93.21%. The presence of biofilm on the anode has been visualized through the SEM images. The higher COD concentration of wastewater and the fed-batch operation resulted in increased power output and wastewater treatment efficiency.

Bio-electrochemical frameworks governing microbial fuel cell performance: technical bottlenecks and proposed solutions

Microbial fuel cells (MFCs) are recognized as a future technology with a unique ability to exploit metabolic activities of living microorganisms for simultaneous conversion of chemical energy into electrical energy. This technology holds the promise to offer sustained innovations and continuous development towards many different applications and value-added production that extends beyond electricity generation, such as water desalination, wastewater treatment, heavy metal removal, bio-hydrogen production, volatile fatty acid production and biosensors. Despite these advantages, MFCs still face technical challenges in terms of low power and current density, limiting their use to powering only small-scale devices. Description of some of these challenges and their proposed solutions is demanded if MFCs are applied on a large or commercial scale. On the other hand, the slow oxygen reduction process (ORR) in the cathodic compartment is a major roadblock in the commercialization of fuel cells for energy conversion. Thus, the scope of this review article addresses the main technical challenges of MFC operation and provides different practical approaches based on different attempts reported over the years.

Sustainable operation requires addressing key MFC-bottleneck issues. Enhancing extracellular electron transfer is the key to elevated MFC performance.

Energetics, electron uptake mechanisms and limitations of electroautotrophs growing on biocathodes - A review

Electroautotrophs are microorganisms that can take the electrons needed for energy generation, CO₂ fixation and other metabolic reactions from a polarized electrode. They have been the focus of intense research for its application in wastewater treatment, bioelectrosynthetic processes and hydrogen generation. As a general trend, current densities produced by the electron uptake of these microorganisms are low, limiting their applicability at large scale. In this work, the electron uptake mechanisms that may operate in electroautotrophs are reviewed, aiming at finding possible causes for this low performance. Biomass yields, growth rates and electron uptake rates observed when these microorganisms use chemical electron donors are compared with those typically obtained with electrodes, to explore limitations and advantages inherent to the electroautotrophic metabolism. Also, the factors affecting biofilm development are analysed to show how interfacial interactions condition bacterial adhesion, biofilm growth and electrons uptake. Finally, possible strategies to overcome these limitations are described.

Tuning of Electrode Surface for Enhanced Bacterial Adhesion and Reactions: A Review on Recent Approaches

The study of bacterial adhesion and its consequences has great significance in different fields such as marine science, renewable energy sectors, soil and plant ecology, food industry, and the biomedical field. Generally, the adverse effects of microbial surface interactions have attained wide visibility. However, herein, we present distinct approaches to highlight the beneficial aspects of microbial surface interactions for various applications rather than deal with the conventional negative aspects or prevention strategies. The surface microbial reactions can be tuned for useful biochemical or bio-electrochemical applications, which are otherwise unattainable through conventional routes. In this context, the present review is a comprehensive approach to highlight the basic principles and signature parameters that are responsible for the useful microbial-electrode interactions. It also proposes various surface tuning strategies, which are useful for tuning the electrode characteristics particularly suitable for the enhanced bacterial adhesion and reactions. The tuning of surface characteristics of electrodes is discussed with a special reference to the Microbial Fuel Cell as an example.

A facile and fast strategy for cathodic electroactive-biofilm assembly via magnetic nanoparticle bioconjugation

Microbial electrosynthesis is a promising electricity-driven technology for converting carbon dioxide into value-added compounds, but the formation of cathodic electroactive-biofilms (CEBs) is challenging. Herein, we have demonstrated an innovative strategy for CEBs assembly via magnetic nanoparticle bioconjugation, which lies in the synergistic interactions among a bonder (Streptavidin, SA), conductive nanomaterials (Fe₃O₄), and a methanogen (M. barkeri). The results showed that the bioconjugated M. barkeri-SA-Fe₃O₄ biohybrids significantly enhanced both methane yield (33.2-fold) and faradaic efficiency (5.6-fold), compared with that of bare M. barkeri. Such an enhancement was attributed to the improved viability of CEBs with a higher biomass density. Particularly, more live cells were presented in the inner biofilms and promoted the long-distance electron exchange between the live outer-layer biofilm and the cathode electrode. Meanwhile, the higher redox activity of CEBs with the M. barkeri-SA-Fe₃O₄ biohybrids resulted in an improved transient charge storage capability, which was beneficial for the biological CO₂-to-CH₄ conversion via acting as an additional electron donor. This work has provided a new approach to accelerate the formation of CEBs and subsequent electron transfer, which holds a great potential for accomplishing electrosynthesis and CO₂ fixation.

Effect of Electrode Material and Hydrodynamics on the Produced Current in Double Chamber Microbial Fuel Cells

In recent decades, there has been huge interest in exploring cost-effective and sustainable ways for energy production using fuel cells. In this study, different electrode materials, namely, nickel, stainless steel, brass, and graphite were used to investigate the energy production in double chamber microbial fuel cells. Yeast microorganisms (MOs) (Saccharomyces cerevisiae) were used at different concentrations for electricity production under different operating conditions with glucose as a substrate. The produced current and potential of the electrode were measured for ranges of operating conditions such as MO concentration (1-8 g/L), flow velocity (0-600 rpm), and aeration of the catholyte. It was found that there was a different performance exhibited by each electrode material, with nickel and graphite giving the highest efficiency. Increasing the flow velocity and aeration in the cathode compartment led to increasing the produced current while the flow and aeration in the anode compartment had a negative effect on the produced current. Simultaneous aeration and agitation gave high produced current values, while high agitation with aeration reduced the efficacy. The increased concentration of substrate glucose showed different influences on the produced current depending on electrode materials.

Preparation and Formula Analysis of Anti-Biofouling Titania–Polyurea Spray Coating with Nano/Micro-Structure

This paper proposes the preparation and formula analysis of anti-biofouling Titania–polyurea (TiO2–SPUA) spray coating, which uses nano-scale antibacterial and photocatalytic agents, titanium dioxide, to construct regularly hydrophobic surface texture on the polyurea coating system. Through formulating analysis of anti-biofouling performance, it is found the causal factors include antibacterial TiO2, surface wettability and morphology in order of their importance. The most optimized formula group is able to obtain uniform surface textures, high contact angle (91.5°), low surface energy (32.5 mJ/m2), and strong hardness (74 A). Moreover, this newly fabricated coating can effectively prevent Pseudomonas aeruginosa and biofilm from enriching on the surface, and there is no toxins release from the coating itself, which makes it eco-friendly, even after long-time exposure. These studies provide insights to the relative importance of physiochemical properties of Titania–polyurea spray coatings for further use in marine, as well as bio medical engineering.

Impact of electrode micro- and nano-scale topography on the formation and performance of microbial electrodes

From a fundamental standpoint, microbial electrochemistry is unravelling a thrilling link between life and materials. Technically, it may be the source of a large number of new processes such as microbial fuel cells for powering remote sensors, autonomous sensors, microbial electrolysers and equipment for effluent treatment. Microbial electron transfers are also involved in many natural processes such as biocorrosion. In these contexts, a huge number of studies have dealt with the impact of electrode materials, coatings and surface functionalizations but very few have focused on the effect of the surface topography, although it has often been pointed out as a key parameter impacting the performance of electroactive biofilms. The first part of the review gives an overview of the influence of electrode topography on abiotic electrochemical reactions. The second part recalls some basics of the effect of surface topography on bacterial adhesion and biofilm formation, in a broad domain reaching beyond the context of electroactivity. On these well-established bases, the effect of surface topography is reviewed and analysed in the field of electroactive biofilms. General trends are extracted and fundamental questions are pointed out, which should be addressed to boost future research endeavours. The objective is to provide basic guidelines useful to the widest possible range of research communities so that they can exploit surface topography as a powerful lever to improve, or to mitigate in the case of biocorrosion for instance, the performance of electrode/biofilm interfaces.

Biofilm Formation by Clostridium ljungdahlii Is Induced by Sodium Chloride Stress: Experimental Evaluation and Transcriptome Analysis

The acetogen Clostridium ljungdahlii is capable of syngas fermentation and microbial electrosynthesis. Biofilm formation could benefit both these applications, but was not yet reported for C. ljungdahlii. Biofilm formation does not occur under standard growth conditions, but attachment or aggregation could be induced by different stresses. The strongest biofilm formation was observed with the addition of sodium chloride. After 3 days of incubation, the biomass volume attached to a plastic surface was 20 times higher with than without the addition of 200 mM NaCl to the medium. The addition of NaCl also resulted in biofilm formation on glass, graphite and glassy carbon, the latter two being often used electrode materials for microbial electrosynthesis. Biofilms were composed of extracellular proteins, polysaccharides, as well as DNA, while pilus-like appendages were observed with, but not without, the addition of NaCl. A transcriptome analysis comparing planktonic (no NaCl) and biofilm (NaCl addition) cells showed that C. ljungdahlii coped with the salt stress by the upregulation of the general stress response, Na⁺ export and osmoprotectant accumulation. A potential role for poly-N-acetylglucosamines and D-alanine in biofilm formation was found. Flagellar motility was downregulated, while putative type IV pili biosynthesis genes were not expressed. Moreover, the gene expression analysis suggested the involvement of the transcriptional regulators LexA, Spo0A and CcpA in stress response and biofilm formation. This study showed that NaCl addition might be a valuable strategy to induce biofilm formation by C. ljungdahlii, which can improve the efficacy of syngas fermentation and microbial electrosynthesis applications.

Versatile plasmonic-effects at the interface of inverted perovskite solar cells

Plasmonics is a highly promising approach to enhancing the light-harvesting properties of hybrid organic/inorganic perovskite solar cells. In the present work, our cells have a p-i-n inverted planar structure. An ultrathin NiO film with two different thicknesses of 5 and 10 nm prepared by a pulsed laser deposition process on an ITO substrate with a faceted and furrowed surface enabled the formation of a continuous and compact layer of well-crystallized CH3NH3PbI3via an anti-solvent chlorobenzene process. The coverage mechanism of the NiO film on the ITO was clearly demonstrated through the J-V and external quantum efficiency (EQE) curves. Moreover, the results demonstrated that the gold nanoislands (Au NIs) increased the power conversion efficiency to 5.1%, almost double that of the samples without Au NIs. This result is due to the excitation of surface plasmons, which is characterized by strong scattering and enhancement of the electric field in the vicinity of the Au NIs loaded at the interface between the NiO and perovskite films. Additionally, we observed an enhancement of the EQE at wavelengths shorter than the plasmon resonance peak. In the current state, we speculate that the plasmoelectric potential effect is considered to be a good explanation of the photocurrent enhancement at the off-resonance region. Our work provides good guidance for the design and fabrication of solar-energy-related devices employing NiO electrodes and plasmonic Au NIs.

Engineering of Microbial Electrodes

This chapter provides an overview of the current state-of-the-art in the engineering of microbial electrodes for application in microbial electrosynthesis. First, important functional aspects and requirements of basic materials for microbial electrodes are introduced, including the meaningful benchmarking of electrode performance, a comparison of electrode materials, and methods to improve microbe-electrode interaction. Suitable current collectors and composite materials that combine different functionalities are also discussed. Subsequently, the chapter focuses on the design of macroscopic electrode structures. Aspects such as mass transfer and electrode topology are touched upon, and a comparison of the performance of microbial electrodes relevant for practical application is provided. The chapter closes with an overall conclusion and outlook, highlighting the future prospects and challenges for the engineering of microbial electrodes toward practical application in the field of microbial electrosynthesis. Graphical Abstract.

Cobalt Oxide (CoOx) as an Efficient Hole-Extracting Layer for High-Performance Inverted Planar Perovskite Solar Cells

CoO_x is a promising hole-extracting layer (HEL) for inverted planar perovskite solar cells with device configuration ITO/CoO_x/CH₃NH₃PbI₃/PCBM/Ag. The devices fabricated according to a simple solution procedure showed the best photovoltaic performance attaining power conversion efficiency (PCE) of 14.5% under AM 1.5 G 1 sun irradiation, which is significantly superior to those of materials fabricated with a traditional HEL such as PEDOT:PSS (12.2%), NiO_x (10.2%), and CuO_x (9.4%) under the same experimental conditions. We characterized the chemical compositions with XPS, crystal structures with XRD, and film morphology with SEM/AFM techniques. Photoluminescence (PL) spectra and the corresponding PL decays for perovskite deposited on varied HEL films were recorded to obtain the hole-extracting characteristics, for which the hole-extracting times show the order CoO_x (2.8 ns) < PEDOT:PSS (17.5 ns) < NiO_x (22.8 ns) < CuO_x (208.5 ns), consistent with the trend of their photovoltaic performances. The reproducibility and enduring stability of those devices were examined to show the outstanding long-term stability of the devices made of metal oxide HEL, for which the CoO_x device retained PCE ≈ 12% for over 1000 h.

Adaptively Evolving Bacterial Communities for Complete and Selective Reduction of Cr(VI), Cu(II), and Cd(II) in Biocathode Bioelectrochemical Systems

Bioelectrochemical systems (BESs) have been shown to be useful in removing individual metals from solutions, but effective treatment of electroplating and mining wastewaters requires simultaneous removal of several metals in a single system. To develop multiple-reactor BESs for metals removal, biocathodes were first individually acclimated to three different metals using microbial fuel cells with Cr(VI) or Cu(II) as these metals have relatively high redox potentials, and microbial electrolysis cells for reducing Cd(II) as this metal has a more negative redox potential. The BESs were then acclimated to low concentrations of a mixture of metals, followed by more elevated concentrations. This procedure resulted in complete and selective metal reduction at rates of 1.24 ± 0.01 mg/L-h for Cr(VI), 1.07 ± 0.01 mg/L-h for Cu(II), and 0.98 ± 0.01 mg/L-h for Cd(II). These reduction rates were larger than the no adaptive controls by factors of 2.5 for Cr(VI), 2.9 for Cu(II), and 3.6 for Cd(II). This adaptive procedure produced less diverse microbial communities and changes in the microbial communities at the phylum and genus levels. These results demonstrated that bacterial communities can adaptively evolve to utilize solutions containing mixtures of metals, providing a strategy for remediating wastewaters containing Cr(VI), Cu(II), and Cd(II).

Coupling interaction of cathodic reduction and microbial metabolism in aerobic biocathode of microbial fuel cell

Certain mixed consortia colonized on aerobic biocathodes can improve the 4-electron oxygen reduction of cathodes; however, the coupling interaction of the cathodic reaction and microbial metabolism remains unclear.

Characterization of a microbial fuel cell with reticulated carbon foam electrodes

A microbial fuel cell with open-pore reticulated vitreous carbon electrodes is studied to assess the suitability of this material in a batch mode, in the perspective of flow-through reactors for wastewater treatment with electricity generation. The cell shows good stability and fair robustness in regards to substrate cycles. A power density of 40 W/m(3) is reached. The cell efficiency is mainly limited by cathodic transfers, representing 85% of the global overpotential in open circuit. Through impedance spectrocopy, equivalent circuit modeling reveals the complex nature of the bioelectrochemical phenomena. The global electrical behavior of the cell seems to result in the addition of three anodic and two cathodic distinct phenomena. On the cathode side, the Warburg element in the model is related to the diffusion of oxygen. Warburg resistance and time are respectively 2.99 kΩ cm(2) and 16.4s, similar to those published elsewhere.

Forming microbial anodes under delayed polarisation modifies the electron transfer network and decreases the polarisation time required

Microbial anodes were formed from compost leachate on carbon cloth electrodes. The biofilms formed at the surface of electrodes kept at open circuit contained microorganisms that switched their metabolism towards electrode respiration in response to a few minutes of polarisation. When polarisation at -0.2 V/SCE (+0.04 V/SHE) was applied to a pre-established biofilm formed at open circuit (delayed polarisation), the bacteria developed an extracellular electron transport network that showed multiple redox systems, reaching 9.4 A/m(2) after only 3-9 days of polarisation. In contrast, when polarisation was applied from the beginning, bacteria developed a well-tuned extracellular electron transfer network concomitantly with their growth, but 36 days of polarisation were required to get current of the same order (6-8 A/m(2)). The difference in performance was attributed to the thinner, more heterogeneous structure of the biofilms obtained by delayed polarisation compared to the thick uniform structure obtained by full polarisation.

Impact of an extracellular polymeric substance (EPS) precoating on the initial adhesion of Burkholderia cepacia and Pseudomonas aeruginosa

Extracellular polymeric substances (EPS) significantly influence bacterial adhesion to solid surfaces, but it is difficult to elucidate the role of EPS on bacterial adhesion due to their complexity and variability. In the present study, the effect of EPS on the initial adhesion of B. cepaciaepacia PC184 and P. aeruginosa PAO1 on glass slides with and without an EPS precoating was investigated under three ionic strength conditions. The surface roughness of EPS coated slides was evaluated by atomic force microscopy (AFM), and its effect on initial bacterial adhesion was found to be trivial. X-ray photoelectron spectroscopy (XPS) studies were performed to determine the elemental surface compositions of bacterial cells and substrata. The results showed that an EPS precoating hindered bacterial adhesion on solid surfaces, which was largely attributed to the presence of proteins in the EPS. This observation can be attributed to the increased steric repulsion at high ionic strength conditions. A steric model for polymer brushes that considers the combined influence of steric effects and DLVO interaction forces is shown to adequately describe bacterial adhesion behaviors.

Impact of conditioning films on the initial adhesion of Burkholderia cepacia

Bacterial initial adhesion to inert surfaces in aquatic environments is mainly governed by the surface properties of the substratum, which can be altered significantly by the formation of conditioning films. Bacteria were tested for ability to adhere to bare glass slides and to slides coated with alginate, bovine serum albumin (BSA), or Suwannee River natural organic matter (SR-NOM). Three Burkholderia cepacia strains with different extracellular polymeric substance (EPS) secretion capacities were tested. The surface roughness of the slides was measured by atomic force microscopy (AFM), but its effect on bacterial initial adhesion was not significant. Our results showed the degree (number of cells per cm(2)) of initial adhesion among the three strains of B. cepacia was not significantly different, indicating that B. cepacia surface EPS did not impact adhesive capacity in the conditions tested. Depending on the conditioning film types and ionic strength conditions, conditioning film coatings can either enhance or reduce bacterial initial adhesion. Bacterial adhesion to bare slides and to alginate or SR-NOM coated slides increased with increasing ionic strength; however, a similar trend was not observed on BSA coated slides. Although BSA coated slides were the most hydrophobic and had the lowest negative surface charge among the surfaces tested, bacterial adhesion was not enhanced by the BSA coating. The extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was applied to explain bacterial adhesion to solid surfaces.