Three-Dimensional Electrodes for High-Performance Bioelectrochemical Systems

Advanced electrolysis sulfur-based biofiltration for simultaneous total nitrogen removal and estrogen toxicity reduction from low carbon-to-nitrogen ratio wastewater

A sulfur-based biofilter enhanced by phosphate modified activated carbon as particle electrodes was constructed to simultaneously remove total nitrogen (TN) and estrogen from low carbon-to-nitrogen ratio (C/N) wastewater containing 1 mg/L 17-alpha-ethinylestradiol (EE2). Results showed that the enhanced biofilter achieved outstanding performance in EE2 removal (93.2 %) and TN reduction (effluent < 5 mg/L), demonstrating robustness against C/N fluctuations. It was noteworthy that it successfully reduced both acute toxicity (59.5 %) and estrogenic activity (88.6 %). Comprehensive characterization investigations and microbial community structure analysis revealed that enhanced electron transfer and increased microbial abundance likely contributed to improved biofilter performance. Core microorganisms, such as Pseudomonas and Chryseobacterium were identified as key contributors to synergistic estrogen degradation and denitrification. This study presented a feasible and promising strategy of combined process with three-dimensional electrodes and sulfur-based biofilter, highlighting substantial potential for advanced purification and safe reuse of wastewater.

Electrofermentation increases concentration of poly γ-glutamic acid in Bacillus subtilis biofilms

Fluctuations in redox conditions in bioprocesses can alter the end-products, reduce their concentration, and lengthen the process time. Electrofermentation enables rapid metabolic modulation of biosynthesis and allows control of redox imbalances in biofilm-based fermentation processes. In this study, electrofermentation is used to boost the production of the bacterial biopolymer poly-γ-glutamic acid (γ-PGA) from Bacillus subtilis ATCC 6051. When compared to control experiments (3.3 ± 0.99 g L-1 ), the application of an electrode potential E = 0.4 V versus Ag/AgCl results in a more than two-fold increase in the production of γ-PGA (9.13 ± 1.4 g L-1 ). Using an engineered B. subtilis strain, in which γ-PGA production is driven by isopropyl β-d-1-thiogalactopyranoside, electrofermentation improves polymer concentrations from 15.4 ± 1.5 to 23.1 ± 1.6 versus g L-1 . These results confirm that electrofermentation conditions can be adopted to increase the concentration of γ-PGA and perhaps other extracellular biopolymers in industrial strains.

Scaling up of dual-chamber microbial electrochemical systems - An appraisal using systems design approach

Impetus to minimise the energy and carbon footprints of evolving wastewater resource recovery facilities has promoted the development of microbial electrochemical systems (MES) as an emerging energy-neutral and sustainable platform technology. Using separators in dual-chamber MES to isolate anodic and cathodic environments creates endless opportunities for its myriad applications. Nevertheless, the high internal resistance and the complex interdependencies among various system factors have challenged its scale-up. This critical review employed a systems approach to examine the complex interdependencies and practical issues surrounding the implementation and scalability of dual-chamber MES, where the anodic and cathodic reactions are mutually appraised to improve the overall system efficiency. The robustness and stability of anodic biofilms in large-volume MES is dependent on its inoculum source, antecedent history and enrichment strategies. The composition and anode-respiring activity of these biofilms are modulated by the anolyte composition, while their performance demands a delicate balance between the electrode size, macrostructure and the availability of substrates, buffers and nutrients when using real wastewater as anolyte. Additionally, the catholyte governed the reduction environment and associated energy consumption of MES with scalable electrocatalysts needed to enhance the sluggish reaction kinetics for energy-efficient resource recovery. A comprehensive assessment of the dual-chamber reactor configuration revealed that the tubular, spiral-wound, or plug-in modular MES configurations are suitable for pilot-scale, where it could be designed more effectively using efficient electrode macrostructure, suitable membranes and bespoke strategies for continuous operation to maximise their performance. It is anticipated that the critical and analytical understanding gained through this review will support the continuous development and scaling-up of dual-chamber MES for prospective energy-neutral treatment of wastewater and simultaneous circular management of highly relevant environmental resources.

Mechanism and application of modified bioelectrochemical system anodes made of carbon nanomaterial for the removal of heavy metals from soil

Bioelectrochemical techniques are quick, efficient, and sustainable alternatives for treating heavy metal soils. The use of carbon nanomaterials in combination with electroactive microorganisms can create a conductive network that mediates long-distance electron transfer in an electrode system, thereby resolving the issue of low electron transfer efficiency in soil remediation. As a multifunctional soil heavy metal remediation technology, its application in organic remediation has matured, and numerous studies have demonstrated its potential for soil heavy metal remediation. This is a ground-breaking method for remediating soils polluted with high concentrations of heavy metals using soil microbial electrochemistry. This review summarizes the use of bioelectrochemical systems with modified anode materials for the remediation of soils with high heavy metal concentrations by discussing the mass-transfer mechanism of electrochemically active microorganisms in bioelectrochemical systems, focusing on the suitability of carbon nanomaterials and acidophilic bacteria. Finally, we discuss the emerging limitations of bioelectrochemical systems, and future research efforts to improve their performance and facilitate practical applications. The mass-transfer mechanism of electrochemically active microorganisms in bioelectrochemical systems emphasizes the suitability of carbon nanomaterials and acidophilic bacteria for remediating soils polluted with high concentrations of heavy metals. We conclude by discussing present and future research initiatives for bioelectrochemical systems to enhance their performance and facilitate practical applications. As a result, this study can close any gaps in the development of bioelectrochemical systems and guide their practical application in remediating heavy-metal-contaminated soils.

Performance and microbial community analysis on nitrate removal in a bioelectrochemical reactor

In this experiment, we took reflux sludge, sludge from an aeration tank, and soil from roots as microbial inoculating sources for an electrochemical device for denitrification with high-throughput sequencing on cathodic biofilms. The efficiency of nitrate nitrogen removal using different microbial inoculates varied among voltages. The optimal voltages for denitrification of reflux sludge, aeration tank sludge, and root soil were 0.7V, 0.5V, and 0.5V, respectively. Further analysis revealed that the respective voltages had a significant effect upon microbial growth from the respective inoculates. Proteobacteria and Firmicutes were the main denitrifying microbes. With the addition of low current (produced by the applied voltage), the Chao1, Shannon and Simpson indexes of the diversity of microorganisms in soil inoculation sources increased, indicating that low current can increase the diversity and richness of the microorganisms, while the reflux sludge and aeration tank sludge showed different changes. Low-current stimulation decreased microbial diversity to a certain extent. Pseudomonas showed a trend of decline with increasing applied voltage, in which the MEC (microbial electrolysis cell) of rhizosphere soil as inoculates decreased most significantly from 77.05% to 12.58%, while the MEC of Fusibacter showed a significant increase, and the sludge of reflux sludge, aeration tank and rhizosphere soil increased by 31.12%, 18.7% and 34.6%, respectively. The applied voltage also significantly increased the abundance of Azoarcus in communities from the respective inoculates.

3D Printed lattice-structured metal electrodes for enhanced current production in bioelectrochemical systems

Bioelectrochemical systems (BESs) are emerging techniques that use biological production of current for versatile activities, including energy recovery and bioremediation. The development of high-performance three-dimensional (3D) electrodes has attracted attention for facilitating current production in BESs. Carbon-based electrodes have been commonly used in BESs, but metal electrodes are not generally employed because of their low biocompatibility with microbes. In this study, 3D stainless-steel electrodes, composed of octahedral lattice, were fabricated using the 3D printing technique. Heat treatment was conducted to form an iron-oxide layer on the electrode surface for increasing biocompatibility. Another crucial parameter that determines current production is the pitch length of a lattice electrode as it affects the surface area and substrate diffusion. The pitch length was optimized by testing the lattice electrodes with pitches ranging from 1.5 mm to 6.0 mm. The highest current, obtained with the 3.0 mm-pitch electrode, was 50% higher than that obtained with common 3D carbon-felt electrodes. These results demonstrate the usefulness of 3D lattice-structured metal electrodes in BESs.

Self-assembly of cell-embedding reduced graphene oxide/ polypyrrole hydrogel as efficient anode for high-performance microbial fuel cell

A three-dimensional (3D) macroporous reduced graphene oxide/polypyrrole (rGO/Ppy) hydrogel assembled by bacterial cells was fabricated and applied for microbial fuel cells. By taking the advantage of electroactive cell-induced bioreduction of graphene oxide and in-situ polymerization of Ppy, a facile self-assembly by Shewanella oneidensis MR-1and in-situ polymerization approach for 3D rGO/Ppy hydrogel preparation was developed. This facile one-step self-assembly process enabled the embedding of living electroactive cells inside the hydrogel electrode, which showed an interconnected 3D macroporous structures with high conductivity and biocompatibility. Electrochemical analysis indicated that the self-assembly of cell-embedding rGO/Ppy hydrogel enhanced the electrochemical activity of the bioelectrode and reduced the electron charge transfer resistance between the cells and the electrode. Impressively, extremely high power output of 3366 ± 42 mW m^-2 was achieved from the MFC with cell-embedding rGO/Ppy hydrogel rGO/Ppy, which was 8.6 times of that delivered from the MFC with bare electrode. Further analysis indicated that the increased cell loading by the hydrogel and improved electrochemical activity by the rGO/Ppy composite would be the underlying mechanism for this performance improvement. This study provided a facile approach to fabricate the biocompatible and electrochemical active 3D nanocomposites for MFC, which would also be promising for performance optimization of various bioelectrochemical systems.

The material-microorganism interface in microbial hybrid electrocatalysis systems

This review presents a comprehensive summary of the material-microorganism interface in microbial hybrid electrocatalysis systems. Microbial hybrid electrocatalysis has been developed to combine the advantages of inorganic electrocatalysis and microbial catalysis. However, electron transfer at the interfaces between microorganisms and materials is a very critical issue that affects the efficiency of the system. Therefore, this review focuses on the electron transfer at the material-microorganism interface and the strategies for building efficient microorganism and material interfaces. We begin with a brief introduction of the electron transfer mechanism in both the bioanode and biocathode of bioelectrochemical systems to understand the material-microorganism interface. Next, we summarise the strategies for constructing efficient material-microorganism interfaces including material design and modification and bacterial engineering. We also discuss emerging studies on the bio-inorganic hybrid electrocatalysis system. Understanding the interface between electrode/active materials and the microorganisms, especially the electron transfer processes, could help to drive the evolution of material-microorganism hybrid electrocatalysis systems towards maturity.

Methodology for In Situ Microsensor Profiling of Hydrogen, pH, Oxidation-Reduction Potential, and Electric Potential throughout Three-Dimensional Porous Cathodes of (Bio)Electrochemical Systems

We developed a technique based on the use of microsensors to measure pH and H2 gradients during microbial electrosynthesis. The use of 3D electrodes in (bio)electrochemical systems likely results in the occurrence of gradients from the bulk conditions into the electrode. Since these gradients, e.g., with respect to pH and reactant/product concentrations determine the performance of the electrode, it is essential to be able to accurately measure them. Apart from these parameters, also local oxidation-reduction potential and electric field potential were determined in the electrolyte and throughout the 3D porous electrodes. Key was the realization that the presence of an electric field disturbed the measurements obtained by the potentiometric type of microsensor. To overcome the interference on the pH measure, a method was validated where the signal was corrected for the local electric field measured with the electric potential microsensor. The developed method provides a useful tool for studies about electrode design, reactor engineering, measuring gradients in electroactive biofilms, and flow dynamics in and around 3D porous electrodes of (bio)electrochemical systems.

Enabling Electron Injection for Microbial Electrosynthesis with n-Type Conjugated Polyelectrolytes

Microbial electrosynthesis-using renewable electricity to stimulate microbial metabolism-holds the promise of sustainable chemical production. A key limitation hindering performance is slow electron-transfer rates at biotic-abiotic interfaces. Here a new n-type conjugated polyelectrolyte is rationally designed and synthesized and its use is demonstrated as a soft conductive material to encapsulate electroactive bacteria Shewanella oneidensis MR-1. The self-assembled 3D living biocomposite amplifies current uptake from the electrode ≈674-fold over controls with the same initial number of cells, thereby enabling continuous synthesis of succinate from fumarate. Such functionality is a result of the increased number of bacterial cells having intimate electronic communication with the electrode and a higher current uptake per cell. This is underpinned by the molecular design of the polymer to have an n-dopable conjugated backbone for facile reduction by the electrode and zwitterionic side chains for compatibility with aqueous media. Moreover, direct arylation polycondensation is employed instead of the traditional Stille polymerization to avoid non-biocompatible tin by-products. By demonstrating synergy between living cells with n-type organic semiconductor materials, these results provide new strategies for improving the performance of bioelectrosynthesis technologies.

Engineered cytochrome fused extracellular matrix enabled efficient extracellular electron transfer and improved performance of microbial fuel cell

Microbial fuel cell (MFC) was a promising technology for energy harvesting from wastewater. However, inefficient bacterial extracellular electron transfer (EET) limited the performance as well as the applications of MFC. Here, a new strategy to reinforce the EET by engineering synthetic extracellular matrix (ECM) with cytochrome fused curli was developed. By genetically fusing a minimal cytochrome domain (MCD) with the curli protein CsgA and heterogeneously expressing in model exoelectrogen of Shewanella oneidensis MR-1, the cytochrome fused electroactive curli network was successfully constructed and assembled. Interestingly, the strain with the MCD fused synthetic ECM delivered about 2.4 times and 2.0 times higher voltage and power density output than these of wild type MR-1 in MFC. More impressively, electrochemical analysis suggested that this synthetic ECM not only introduced cytochrome of MCD, but also attracted more self-secreted electrochemically active substances, which might facilitate the EET and improve the MFC performance. This work demonstrated the possibility to manipulation the EET with ECM engineering, which opened up new path for exoelectrogen design and engineering.

Three-dimensional biofilm electrode reactors (3D-BERs) for wastewater treatment

Three-dimensional biofilm electrode reactors (3D-BERs) are highly efficient in refractory wastewater treatment. In comparison to conventional bio-electrochemical systems, the filled particle electrodes act as both electrodes and microbial carriers in 3D-BERs. This article reviews the conception and basic mechanisms of 3D-BERs, as well as their current development. The advantages of 3D-BERs are illustrated with an emphasis on the synergy of electricity and microorganisms. Electrode materials utilized in 3D-BERs are systematically summarized, especially the critical particle electrodes. The configurations of 3D-BERs and their integration with wastewater treatment reactors are introduced. Operational parameters and the adaptation of 3D-BERs to varieties of wastewater are discussed. The prospects and challenges of 3D-BERs for wastewater treatment are then presented, and the future research directions are proposed. We believe that this timely review will help to attract more attentions on 3D-BERs investigation, thus promoting the potential application of 3D-BERs in wastewater treatment.

Wastewater-powered high-value chemical synthesis in a hybrid bioelectrochemical system

A microbial electrochemical system could potentially be applied as a biosynthesis platform by extracting wastewater energy while converting it to value-added chemicals. However, the unfavorable thermodynamics and sluggish kinetics of in vivo whole-cell cathodic catalysis largely limit product diversity and value. Herein, we convert the in vivo cathodic reaction to in vitro enzymatic catalysis and develop a microbe-enzyme hybrid bioelectrochemical system (BES), where microbes release the electricity from wastewater (anode) to power enzymatic catalysis (cathode). Three representative examples for the synthesis of pharmaceutically relevant compounds, including halofunctionalized oleic acid based on a cascade reaction, (4-chlorophenyl)-(pyridin-2-yl)-methanol based on electrochemical cofactor regeneration, and l-3,4-dihydroxyphenylalanine based on electrochemical reduction, were demonstrated. According to the techno-economic analysis, this system could deliver high system profit, opening an avenue to a potentially viable wastewater-to-profit process while shedding scientific light on hybrid BES mechanisms toward a sustainable reuse of wastewater.

A novel fragmented anode biofilm microbial fuel cell (FAB-MFC) integrated system for domestic wastewater treatment and bioelectricity generation

BACKGROUND

The critical MFC design challenge is to increase anode surface area. A novel FAB-MFC integrated system was developed and evaluated for domestic wastewater treatment. It was operated in fed-batch flow mode at 1-3 days of HRT with 755 mg/L CODIN and 0.76 kg-COD/m3/day. The study includes anaerobic-MFC and aerobic-MFC integrated systems. Microbial electrode jacket dish (MEJ-dish) with hybrid dimension (HD) was invented, first time to authors' knowledge, to boost anode biofilm growth. The treatment system with MEJ+ (FAB) and MEJ- (MFC) anode are called FAB-MFC and MFC, respectively.

RESULTS

Fragmented variable anode biofilm thickness was observed in FAB than MFC. The FAB-MFC (FAB+) simple technique increases the anode biofilm thickness by ~ 5 times MFC. Due to HD the anode biofilm was fragmented in FAB+ system than MFC. At the end of each treatment cycle, voltage drops. All FAB+ integrated systems reduced voltage drop relative to MFC. FAB reduces voltage drops better than MFC in anaerobic-MFC from 6 to 20 mV and aerobic-MFC from 35-47 mV at 1 kΩ external load. The highest power density was achieved by FAB in anaerobic-MFC (FAB = 104 mW/m2, MFC = 98 mW/m2) and aerobic-MFC integrated system (FAB = 59 mW/m2, MFC = 42 mW/m2).

CONCLUSIONS

The ∆COD and CE between FAB and MFC could not be concluded because both setups were inserted in the same reactor. The integrated system COD removal (78-97%) was higher than the solitary MFC treatment (68-78%). This study findings support the FAB+ integrated system could be applied for real applications and improve performance. However, it might depend on influent COD, the microbial nature, and ∆COD in FAB+ and MFC, which requires further study.

De Novo Approach to Encapsulating Biocatalysts into Synthetic Matrixes: From Enzymes to Microbial Electrocatalysts

Biocatalysts hold great promise in chemical and electrochemical reactions. However, biocatalysts are prone to inhospitable physiochemical conditions. Encapsulating biocatalysts into a synthetic host matrix can improve their stability and activity, and broaden their operational conditions. In this Review, we summarize the emerging de novo approaches to encapsulating biocatalysts into synthetic matrixes. Here, de novo means that embedding of biocatalysts and construction of matrixes take place simultaneously. We discuss the advantages and limitations of the de novo approach. On the basis of the nature of the biocatalysts and the synthetic frameworks, we specifically focus on two aspects: (1) encapsulation of enzymes (in vitro) in metal-organic frameworks and (2) encapsulation of microbial electrocatalysts (in vivo) on the electrode. For both cases, we discuss how the encapsulation improves biocatalysts' performance (stability, viability, activity, and etc.). We also highlight the benefit of encapsulation in facilitating the transport of charge carriers in microbial electrocatalysis.

Bacterial signalling mechanism: An innovative microbial intervention with multifaceted applications in microbial electrochemical technologies: A review

Microbial electrochemical technologies (METs) are a set of inventive tools that generate value-added by-products with concomitant wastewater remediation. However, due to the bottlenecks, like higher fabrication cost and inferior yield of resources, these inventive METs are still devoid of successful field-scale implementation. In this regard, application of quorum sensing (QS) mechanism to improve the power generation of the METs has gained adequate attention. The QS is an intercellular signalling mechanism that controls the bacterial social network in its vicinity via the synthesis of diffusible signal molecules labelled as auto inducers, thus ameliorating yield of valuables produced through METs. This state-of-the-art review elucidates different types of QS molecules and their working mechanism with the special focus on the widespread application of QS in the field of METs for their performance enhancement. Thus, this review intends to guide the researchers in rendering scalability to METs by integrating innovative QS mechanisms into them.

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.

Bioelectrochemical treatment of actual carwash wastewater associated with sustainable energy generation in three-dimensional microbial fuel cell

Carwash garages are worldwide cleaning facilities. Discharge of their untreated or improperly treated wastewaters highly contributes to the pollution of water resources. Sodium dodecyl sulfate (SDS), a widely used anionic surfactant in the carwash shampoos represents the major constituent of the carwash wastewater. In this study, a new configuration of three-dimensional MFC packed with irregular shaped graphite granules to support and join the plain anodes in the anodic section. The performance of the 3D-MFC was evaluated in two operational modes; continuous and batch. The evaluation was carried out mainly in terms of the removal efficiency of organic content, in particular SDS as well as oil and grease associated with bioenergy generation from actual carwash wastewater used to fuel the MFC. The results demonstrated that maximum removal efficiencies of COD, SDS, and oil and grease were 87%, 88%, and 90%, respectively. Also, the results demonstrated that during the continuous mode operation maximum current density and power output were 1786 mA/m³ and 482 mW/m³, respectively. At batch operation mode, the maximum current density and power output were 1793 mA/m³ and 478 mW/m³, respectively indicating that the performance of the 3D-MFC was comparable in both operation modes.

Anodic electro-fermentation: Empowering anaerobic production processes via anodic respiration

In nature as well as in industrial microbiology, all microorganisms need to achieve redox balance. Their redox state and energy conservation highly depend on the availability of a terminal electron acceptor, for example oxygen in aerobic production processes. Under anaerobic conditions in the absence of an electron acceptor, redox balance is achieved via the production of reduced carbon-compounds (fermentation). An alternative strategy to artificially stabilize microbial redox and energy state is the use of anodic electro-fermentation (AEF). This emerging biotechnology empowers respiration under anaerobic conditions using the anode of a bioelectrochemical system as an undepletable terminal electron acceptor. Electrochemical control of redox metabolism and energy conservation via AEF can steer the carbon metabolism towards a product of interest and avoid the need for continuous and cost-inefficient supply of oxygen as well as the production of mixed reduced by-products, as is the case in aerobic production and fermentation processes, respectively. The great challenge for AEF is to establish efficient extracellular electron transfer (EET) from the microbe to the anode and link it to central carbon metabolism to enhance the synthesis of a target product. This article reviews the advantages and challenges of AEF, EET mechanisms, microbial energy gain, and discusses the rational choice of substrate-product couple as well as the choice of microbial catalyst. Besides, it discusses the potential of the industrial model-organism Bacillus subtilis as a promising candidate for AEF, which has not been yet considered for such an application. This prospective review contributes to a better understanding of how industrial microbiology can benefit from AEF and analyses key-factors required to successfully implement AEF processes. Overall, this work aims to advance the young research field especially by critically revisiting the fundamental aspects of AEF.

Redox Potential Heterogeneity in Fixed-Bed Electrodes Leads to Microbial Stratification and Inhomogeneous Performance

Bed electrodes provide high electrode area-to-volume ratios represent a promising configuration for transferring bioelectrochemical systems close to industrial applications. Nevertheless, the intrinsic electrical resistance leads to poor polarization behavior. Therefore, the distribution of Geobacter spp. and their electrochemical performance within exemplary fixed-bed electrodes are investigated. A minimally invasive sampling system allows characterization of granules from different spatial locations of bed electrodes. Cyclic voltammetry of single granules (n=63) demonstrates that the major share of electroactivity (134.3 mA L-1 ) is achieved by approximately 10 % of the bed volume, specifically that being close to the current collector. Nevertheless, analysis of the microbial community reveals that Geobacter spp. dominated all sampled granules. These findings clearly demonstrate the need for engineered bed electrodes to improve electron exchange between microorganisms and granules.

Microbial extracellular electron transfer and strategies for engineering electroactive microorganisms

Electroactive microorganisms (EAMs) are ubiquitous in nature and have attracted considerable attention as they can be used for energy recovery and environmental remediation via their extracellular electron transfer (EET) capabilities. Although the EET mechanisms of Shewanella and Geobacter have been rigorously investigated and are well characterized, much less is known about the EET mechanisms of other microorganisms. For EAMs, efficient EET is crucial for the sustainable economic development of bioelectrochemical systems (BESs). Currently, the low efficiency of EET remains a key factor in limiting the development of BESs. In this review, we focus on the EET mechanisms of different microorganisms, (i.e., bacteria, fungi, and archaea). In addition, we describe in detail three engineering strategies for improving the EET ability of EAMs: (1) enhancing transmembrane electron transport via cytochrome protein channels; (2) accelerating electron transport via electron shuttle synthesis and transmission; and (3) promoting the microbe-electrode interface reaction via regulating biofilm formation. At the end of this review, we look to the future, with an emphasis on the cross-disciplinary integration of systems biology and synthetic biology to build high-performance EAM systems.

Three-Dimensional Cathodes for Electrochemical Reduction of CO2: From Macro- to Nano-Engineering

Rising anthropogenic CO2 emissions and their climate warming effects have triggered a global response in research and development to reduce the emissions of this harmful greenhouse gas. The use of CO2 as a feedstock for the production of value-added fuels and chemicals is a promising pathway for development of renewable energy storage and reduction of carbon emissions. Electrochemical CO2 conversion offers a promising route for value-added products. Considerable challenges still remain, limiting this technology for industrial deployment. This work reviews the latest developments in experimental and modeling studies of three-dimensional cathodes towards high-performance electrochemical reduction of CO2. The fabrication-microstructure-performance relationships of electrodes are examined from the macro- to nanoscale. Furthermore, future challenges, perspectives and recommendations for high-performance cathodes are also presented.

Electrogenic Biofilm Development Determines Charge Accumulation and Resistance to pH Perturbation

The electrogenic biofilm and the bio-electrode interface are the key biocatalytic components in bioelectrochemical systems (BES) and can have a large impact on cell performance. This study used four different anodic carbons to investigate electrogenic biofilm development to determine the influence of charge accumulation and biofilm growth on system performance and how biofilm structure may mitigate against pH perturbations. Power production was highest (1.40 W/m3) using carbon felt, but significant power was also produced when felt carbon was open-circuit acclimated in a control reactor (0.95 W/m3). The influence of carbon material on electrogenic biofilm development was determined by measuring the level of biofilm growth, using sequencing to identify the microbial populations and confocal microscopy to understand the spatial locations of key microbial groups. Geobacter spp. were found to be enriched in closed-circuit operation and these were in close association with the carbon anode, but these were not observed in the open-circuit controls. Electrochemical analysis also demonstrated that the highest mid-point anode potentials were close to values reported for cytochromes from Geobacter sulfurreductans. Biofilm development was greatest in felt anodes (closed-circuit acclimated 1209 ng/μL DNA), and this facilitated the highest pseudo-capacitive values due to the presence of redox-active species, and this was associated with higher levels of power production and also served to mitigate against the effects of low-pH operation. Supporting carbon anode structures are key to electrogenic biofilm development and associated system performance and are also capable of protecting electrochemically active bacteria from the effects of environmental perturbations.

Living Bioelectrochemical Composites

Composites, in which two or more material elements are combined to provide properties unattainable by single components, have a historical record dating to ancient times. Few include a living microbial community as a key design element. A logical basis for enabling bioelectronic composites stems from the phenomenon that certain microorganisms transfer electrons to external surfaces, such as an electrode. A bioelectronic composite that allows cells to be addressed beyond the confines of an electrode surface can impact bioelectrochemical technologies, including microbial fuel cells for power production and bioelectrosynthesis platforms where microbes produce desired chemicals. It is shown that the conjugated polyelectrolyte CPE-K functions as a conductive matrix to electronically connect a three-dimensional network of Shewanella oneidensis MR-1 to a gold electrode, thereby increasing biocurrent ≈150-fold over control biofilms. These biocomposites spontaneously assemble from solution into an intricate arrangement of cells within a conductive polymer matrix. While increased biocurrent is due to more cells in communication with the electrode, the current extracted per cell is also enhanced, indicating efficient long-range electron transport. Further, the biocomposites show almost an order-of-magnitude lower charge transfer resistance than CPE-K alone, supporting the idea that the electroactive bacteria and the conjugated polyelectrolyte work synergistically toward an effective bioelectronic composite.

Purposely Designed Hierarchical Porous Electrodes for High Rate Microbial Electrosynthesis of Acetate from Carbon Dioxide

Carbon-based products are crucial to our society, but their production from fossil-based carbon is unsustainable. Production pathways based on the reuse of CO₂ will achieve ultimate sustainability. Furthermore, the costs of renewable electricity production are decreasing at such a high rate, that electricity is expected to be the main energy carrier from 2040 onward. Electricity-driven novel processes that convert CO₂ into chemicals need to be further developed. Microbial electrosynthesis is a biocathode-driven process in which electroactive microorganisms derive electrons from solid-state electrodes to catalyze the reduction of CO₂ or organics and generate valuable extracellular multicarbon reduced products. Microorganisms can be tuned to high-rate and selective product formation. Optimization and upscaling of microbial electrosynthesis to practical, real life applications is dependent upon performance improvement while maintaining low cost. Extensive biofilm development, enhanced electron transfer rate from solid-state electrodes to microorganisms and increased chemical production rate require optimized microbial consortia, efficient reactor designs, and improved cathode materials. This Account is about the development of different electrode materials purposely designed for improved microbial electrosynthesis: NanoWeb-RVC and EPD-3D. Both types of electrodes are biocompatible, highly conductive three-dimensional hierarchical porous structures. Both chemical vapor deposition (CVD) and electrophoretic deposition were used to grow homogeneous and uniform carbon nanotube layers on the honeycomb structure of reticulated vitreous carbon. The high surface area to volume ratio of these electrodes maximizes the available surface area for biofilm development, i.e., enabling an increased catalyst loading. Simultaneously, the nanostructure makes it possible for a continuous electroactive biofilm to be formed, with increased electron transfer rate and high Coulombic efficiencies. Fully autotrophic biofilms from mixed cultures developed on both types of electrodes rely on CO₂ as the sole carbon source and the solid-state electrode as the unique energy supply. We present first the synthesis and characteristics of the bare electrodes. We then report the outstanding performance indicators of these novel biocathodes: current densities up to -200 A m^-2 and acetate production rates up to 1330 g m^-2 day^-1, with electron and CO₂ recoveries into acetate being very close to 100% for mature biofilms. The performance indicators are still among the highest reported by either purposely designed or commercially available biocathodes. Finally, we made use of the titration and off-gas analysis sensor (TOGA) to elucidate the electron transfer mechanism in these efficient biocathodes. Planktonic cells in the catholyte were found irrelevant for acetate production. We identified the electron transfer to be mediated by biologically induced H₂. H₂ is not detected in the headspace of the reactors, unless CO₂ feeding is interrupted or the cathodes sterilized. Thus, the biofilm is extremely efficient in consuming the generated H₂. Finally, we successfully demonstrated the use of a synthetic biogas mixture as a CO₂ source. We thus proved the potential of microbial electrosynthesis for the simultaneous upgrading of biogas, while fixating CO₂ via the production of acetate.

Capacitive biocathodes driving electrotrophy towards enhanced CO2 reduction for microbial electrosynthesis of fatty acids

Electron transfer towards biocathode is a rate limiting step for CO₂ reduction during microbial electrosynthesis (MES). Current study is designed to offer an understanding on electrotrophy using four different electrode materials viz., carbon cloth (CC), stainless-steel mesh (SS), combination of both (CC-SS) and a hybrid material (CC-SS-AC with activated carbon (AC)) as capacitive biocathodes for MES. Non turn-over and turn-over electrochemical investigations revealed electrode properties in terms of electron transfer, capacitance and redox catalytic currents relatively higher with CC-SS-AC and CC-SS. Acetic acid production was higher in CC-SS-AC (4.31 g/l) than CC-SS (4.21 g/l), CC (3.5 g/l) and SS (2.83 g/l) along with noticeable ethanol production with all the biocathodes except SS. Interestingly, long-term operation of all biocathodes witnessed reduction in resistance visualized through Nyquist impedance spectra relatively efficient with CC-SS-AC. Biocompatible property of CC-SS-AC with increased surface area was presumed to be a critical factor for enhancing electrotrophy linked with capacitive nature of biocathode towards enhanced bioelectrochemical CO₂ reduction.

Effect of pore size on the current produced by 3-dimensional porous microbial anodes: A critical review

Microbial anodes are the cornerstone of most electro-microbial processes. Designing 3-dimensional porous electrodes to increase the surface area of the electroactive biofilm they support is a key challenge in order to boost their performance. In this context, the critical review presented here aims to assess whether an optimal range of pore size may exist for the design of microbial anodes. Pore sizes of a few micrometres can enable microbial cells to penetrate but in conditions that do not favour efficient development of electroactive biofilms. Pores of a few tens of micrometres are subject to clogging. Sizes of a few hundreds of micrometres allow penetration of the biofilm inside the structure, but its development is limited by internal acidification. Consequently, pore sizes of a millimetre or so appear to be the most suitable. In addition, a simple theoretical approach is described to establish basis for porous microbial anode design.

Development and characterisation of self-assembled graphene hydrogel-based anodes for bioelectrochemical systems

In this work, we report a simple and scalable method to produce high efficiency 3D graphene-based electrodes (GH) for bioelectrochemical systems. GH were obtained by self-assembly of graphene oxide, through slow reduction with ascorbic acid over conductive mesh-works (carbon cloth and stainless-steel). The GH structure and composition were characterised by electron microscopy (SEM) and spectroscopy (FTIR and Raman), whereas the electrodes' performance was tested by chronoamperometry and cyclic voltammetry in a microbial electrolysis cell (MEC) inoculated with a pure culture of G. sulfurreducens. The hydrogel had a broad pore size distribution (>1 μm), which allowed bacterial colonisation within the framework. The macro-porous structure and chemical properties of the hydrogel rendered a higher bacterial loading capacity and substrate oxidation rate than other carbonaceous materials, including different reported graphene electrodes, which significantly increased MEC performance.

A cheap, robust and versatile hydrogel-electrode is easily obtained by reduction of graphene-oxide; its colonisation by Geobacter resulted in high current densities.

Corrugated stainless-steel mesh as a simple engineerable electrode module in bio-electrochemical system: Hydrodynamics and the effects on decolorization performance

The application of bio-electrochemical system (BESs) is strongly depended on the development of the engineering applicable electrode. Here we described an economical and readily processable electrode module with three-dimensional structure, the corrugated stainless-steel mesh electrode module (c-SMEM). This novel developed electrode module was demonstrated to provide a good hydrodynamic characteristic and significantly enhanced the decolorization performance of the BES when serving for treating azo dye (acid orange 7, AO7) containing wastewater. Compared to the conventional planar electrodes module (p-SMEM), c-SMEM was found to prolong the mean residence time (MRT_θ) of AO7 and change the flow pattern closer to the plug flow. As a result, the maximum enhancement of the volumetric decolorization rate (vDR) can reach to 255%, even when the c-SMEM and p-SMEM have the same electrode surface area. In addition, a techno-economic analysis model was established to elucidated the effects of the decolorization performance and the material cost on the initial capital cost, which revealed the BES with c-SMEM could be economically comparable to or even better than the traditional bio-decolorization technologies. These results suggest c-SMEM holds great potential for engineering application, which may help paving the way of applying BES at large-scale.

Improving the extracellular electron transfer of Shewanella oneidensis MR-1 for enhanced bioelectricity production from biomass hydrolysate

Direct electricity production from biomass hydrolysate by microbial fuel cells (MFC) holds great promise for the development of the sustainable biomass industry.