Construction of innovative 3D-weaved carbon mesh anode network to boost electron transfer and microbial activity in bioelectrochemical system

High-performance anode electrocatalyst of MnCo2S4-Co4S3/bamboo charcoal for stimulating power generation in microbial fuel cell

Microbial fuel cell (MFC) is a promising technology for recovering energy in wastewater through bacterial metabolism. However, it always suffers from low power density and electron transfer efficiency, restricting the application. This study fabricated the MnCo2S4-Co4S3/bamboo charcoal (MCS-CS/BC) through an easy one-step hydrothermal method, and the material was applied to carbon felt (CF) to form high-performance MFC anode. MCS-CS/BC-CF anode exhibited lower Rct (10.1 Ω) than BC-CF (17.24 Ω) and CF anode (116.1 Ω), exhibiting higher electrochemical activity. MCS-CS/BC-CF anode promoted the electron transfer rate and resulted in enhanced power density, which was 9.27 times higher (980 mW m-2) than the bare CF (105.7 mW m-2). MCS-CS/BC-CF anode showed the best biocompatibility which attracted distinctly larger biomass (146.27 mg/μL) than CF (20 mg/μL) and BC-CF anode (20.1 mg/μL). The typical exoelectrogens (Geobacter and etc.) took dramatically higher proportion on MCS-CS/BC-CF anode (59.78%) than CF (2.99%) and BC-CF anode (26.67%). In addition, MCS-CS/BC stimulated the synergistic effect between exoelectrogens and fermentative bacteria, greatly favouring the extracellular electron transfer rate between bacteria and the anode and the power output. This study presented an efficient way of high-performance anode electrocatalyst fabrication for stimulating MFC power generation, giving suggestions for high-efficient energy recovery from wastewater.

Newly graphene/polypyrrole (rGO/PPy) modified carbon felt as bio-cathode in bio-electrochemical systems (BESs) achieving complete denitrification

Nitrate reduction in bio-electrochemical systems (BESs) has attracted wide attention due to its low sludge yields and cost-efficiency advantages. However, the high resistance of traditional electrodes is considered to limit the denitrification performance of BESs. Herein, a new graphene/polypyrrole (rGO/PPy) modified electrode is fabricated via one-step electrodeposition and used as cathode in BES for improving nitrate removal from wastewater. The formation and morphological results support the successful formation of rGO/PPy nanohybrids and confirm the part covalent bonding of Py into GO honeycomb lattices to form a three-dimensional cross-linked spatial structure. The electrochemical tests indicate that the rGO/PPy electrode outperforms the unmodified electrode due to the 3.9-fold increase in electrochemical active surface area and 6.9-fold decrease in the charge transfer resistance (Rct). Batch denitrification activity tests demonstrate that the BES equipped with modified rGO/PPy biocathode could not only achieve the full denitrification efficiency of 100% with energy recovery (15.9 × 10-2 ± 0.14 A/m2), but also favor microbial attach and growth with improved biocompatible surface. This work provides a feasible electrochemical route to fabricate and design a high-performance bioelectrode to enhance denitrification in BESs.

Spatially-assembled binary carbon anode synergizing directional electron transfer and enriched microbe accommodation for wastewater treatment and energy conversion: From simulation to experiments

Bioelectrochemical systems (BESs) hold prospects in wastewater energy and resource recovery. Anode optimization is important for simultaneous enhancement of wastewater energy conversion and effluent quality in BESs. In this study, a multi-physics model coupling fluid flow, organic degradation and electrochemical process was constructed to guide the design and optimization of BES anodes. Based on the multi-physics simulation, spatially-assembled binary carbon anodes composed of three-dimensional carbon mesh skeleton and granular activated carbon were proposed and established. The granular activated carbon conducive to microbe accommodation played a vital role in improving effluent water quality, while the carbon mesh skeleton favoring electron collection and transfer could enhance the bioelectricity output. With an average chemical oxygen demand (COD) removal rate of 0.442 kg m-3 d-1, a maximum power density of 20.6 W m-3 was achieved in the optimized composite anode BES, which was 25% and 154% higher than carbon mesh skeleton BES and granular activated carbon BES. Electroactive bacteria were enriched in composite anodes and performed important functions related to microbial metabolism and energy production. The spatially-assembled binary carbon anode with low carbon mesh packing density was more cost-effective with a daily energy output per anode cost of 221 J d-1 RMB-1. This study not only provides a cost-efficient alternative anode to simultaneously improve organic degradation and power generation performance, but also demonstrates the potential of multi-physics simulation in offering theoretical support and prediction for BES configuration design as well as optimization.

The restricted mass transfer inside the anode pore channel affects the electroactive biofilms formation, community composition and the power production in microbial electrochemical systems

Porous anodes improve system performance in microbial electrochemical systems by increasing the specific surface area for electroactive bacteria. In this study, multilayer anodes with different pore diameters were constructed to assess the impact of pore size and depth on anode performance. This layered structure makes detecting electroactive biofilms more accessible layer by layer, which is the first study to examine electroactive biofilms' molecular biology and electrochemical properties at different depths in pores with varied pore sizes. The millimeter-scale pores inside the bioanode have a limited effect in increasing power. The larger the pore diameter, the higher the maximum power density (Pmax) obtained. The Pmax of anodes with 4 mm pore (1.91 ± 0.15 W m-2) was 1.4 times higher than that of the non-perforated (1.37 ± 0.07 W m-2) and 0.5 mm pore anodes (1.39 ± 0.04 W m-2). Electricigens can colonize into pore channels for at least 10 mm with a pore diameter ≥3 mm and current densities >0.05 A m-2. However, in the pores channel with 0.5 mm diameter, electricigens can only colonize to a depth of 2 mm. The biofilm thickness, electricity output, metabolic activity, and biocommunity changed with pore depth and were restricted by the limited mass transfer. The Geobacter sp. was the dominant species in inter-pore biofilms, with 43.8 %-78.6 % in abundance and decreased in quantity as pore depth increased. The inter-pore biofilms on the outer layer contributed a current density of 0.17 ± 0.003 A m-2, while that of the inner layer was only 0.02 ± 0.01 A m-2. Further studies found that the pore edge mass transfer effect can contribute up to 75 % of the current. The mass transfer process at the pore edge region could be a multidirectional mass transfer rather than a pore channel mass transfer.

Deciphering three-dimensional bioanode configuration for augmenting power generation and nitrogen removal in air-cathode microbial fuel cells

In this study, the engineering-oriented three-dimensional (3D) bioanode concept was applied, demonstrating that spiral-stairs-like/rolled carbon felt (SCF/RCF) configurations achieved good performances in air-cathode microbial fuel cells (ACMFCs). With the 3D anodes, ACMFCs generated significantly higher power densities of 1535 mW/m3 (SCF) and 1800 mW/m3 (RCF), compared with that of a traditional flat carbon felt anode (FCF, 315 mW/m3). The coulombic efficiency of 15.39 % at SCF anode and 14.34 % at RCF anode also is higher than the 7.93 % at FCF anode. The 3D anode ACMFCs exhibited favorable removal of chemical oxygen demand (96 % of SCF and RCF) and total nitrogen (97 % of SCF, 99 % of RCF). Further results show that three-dimensional anode structures could enrich more electrode surface biomass and diversify the biofilm microbial communities for promoting bioelectroactivity, denitrification, and nitrification. These results demonstrate that three-dimensional anodes with active biofilm is a promising strategy for creating scalable MFCs-based wastewater treatment system.

Lignocellulose reconstituted shape-controllable self-supporting carbonaceous capacitance-anodes with high electron transfer rates for high-performance microbial electrochemical system

Natural biomass is a promising candidate for manufacturing an efficient anode in the microbial electrochemical system (MES) for its abundance and low cost. However, the structure and performance of the electrode highly depend on the biomass species. A simple and sustainable method for creating a self-supporting electrode is proposed by freeze-drying and carbonizing a blend of cellulose, lignin, and hemicellulose. This strategy leads to a cork-like structure and improved mechanical strength of the lignocellulose carbon. A power density of 4780 ± 260 mW m^-2 (CLX-800) was achieved, which was the highest record for unmodified lignocellulose-based anodes in the microbial fuel cells. The morphological as lamellar multilayer and rich in hydrophilic functional groups could facilitate the formation of thick electroactive biofilms and enrich Geobacter with the highest abundance of 92.3%. The CLX material is expected to be the ideal electrode for high performance and functionally controllability.

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.

Advanced bioelectrochemical system for nitrogen removal in wastewater

Nitrogen (N) pollution in water has become a serious issue that cannot be ignored due to the harm posed by excessive nitrogen to environmental safety and human health; as such, N concentrations in water are strictly limited. The bioelectrochemical system (BES) is a new method to remove excessive N from water, and has attracted considerable attention. Compared with other methods, it is highly efficient and has low energy consumption. However, the BES has not been applied for N removal in practice due to lack of in-depth research on the mechanism and construction of high-performance electrodes, separators, and reactor configurations; this highlights a need to review and examine the efforts in this field. This paper provides a comprehensive review on the current BES research for N removal focusing on the reaction principles, reactor configurations, electrodes and separators, and treatment of actual wastewater; the corresponding performances in these realms are also discussed. Finally, the prospects for N removal in water using the BES are presented.

Sustainable bioelectricity generation using Cladophora sp. as a biocathode in membrane-less microbial fuel cell

In this study, the Cladophora sp. is used to provide oxygen to the cathode of the photosynthetic biocathode membrane-less microbial fuel cell (PB-MLMFC). Non-aerated (NA-MLMFC) and mechanically-aerated (MA-MLMFC) MLMFCs are operated under similar operating conditions to evaluate the performance of PB-MLMFC with the presence of Cladophora sp. The PB-MLMFC exhibits the highest dissolved oxygen (DO) concentration, which results in a more efficient oxygen reduction reaction and a significant improvement in the electricity generation performance. The maximum power density of PB-MLMFC is 619.1 mW m^-2, which is the highest power density known to be reported for algal cathode MFCs in the literature. The electrochemical analysis shows that theCladophora sp.reduces the charge (R_ct) and mass transfer (R_mt) resistances of the PB-MLMFC, and improves the bioelectrochemical activity of the anode microorganisms. The study reveals that Cladophora sp. provides a cost-effective and renewable approach for practical applications of MLMFCs.

Self-sustained recovery of silver with stainless-steel based Cobalt/Molybdenum/Manganese polycrystalline catalytic electrode in bio-electroreduction microbial fuel cell (BEMFC)

In this study, a novel bio-electroreduction microbial fuel cell (BEMFC) assisted by stainless-steel based Cobalt/Molybdenum/Manganese (Co/Mo/Mn-SS) polycrystalline catalytic electrode was used to achieve high recovery to silver. The exoelectrogens (Shewanella sp. etc.) using organic wastewater (the inflow was controlled at 1.2 L d^-1) as nutrient matrix in the anode chamber generated electrons, while silver ions were simultaneously electroreduced and electrodeposited on the surface of the catalytic electrode as electron acceptors. Silver nanoplates could be observed directly. The products of electroreduction on the cathode were analyzed by Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffractometer (XRD), and the results of electrochemical characterization confirmed the existence of silver in the products. In the operation, the silver ions were in-situ recovered and enriched from the initial concentration of 20-300 mg L^-1 to almost complete recovery (8-18 h), with the maximum power density of 1008.2 mW m^-2 and 5.5 A m^-2 current density. The recovery efficiency of silver in the BEMFC using the Co/Mo/Mn-SS electrode was up to 9.60 kg m^-2h^-1, and the energy efficiency was 27.8 kg kWh^-1. Under the continuous flow operation mode, the BEMFC still achieved 90.2% recovery efficiency of the silver.

A review of recent advances in electrode materials for emerging bioelectrochemical systems: From biofilm-bearing anodes to specialized cathodes

Bioelectrochemical systems (BES), mainly microbial fuel cells (MEC) and microbial electrolysis cells (MFC), are unique biosystems that use electroactive bacteria (EAB) to produce electrons in the form of electric energy for different applications. BES have attracted increasing attention as a sustainable, low-cost, and neutral-carbon option for energy production, wastewater treatment, and biosynthesis. Complex interactions between EAB and the electrode materials play a crucial role in system performance and scalability. The electron transfer processes from the EAB to the anode surface or from the cathode surface to the EAB have been the object of numerous investigations in BES, and the development of new materials to maximize energy production and overall performance has been a hot topic in the last years. The present review paper discusses the advances on innovative electrode materials for emerging BES, which include MEC coupled to anaerobic digestion (MEC-AD), Microbial Desalination Cells (MDC), plant-MFC (P-MFC), constructed wetlands-MFC (CW-MFC), and microbial electro-Fenton (BEF). Detailed insights on innovative electrode modification strategies to improve the electrode transfer kinetics on each emerging BES are provided. The effect of materials on microbial population is also discussed in this review. Furthermore, the challenges and opportunities for materials scientists and engineers working in BES are presented at the end of this work aiming at scaling up and industrialization of such versatile systems.

Comparative evaluation of fibrous artificial carbons and bamboo charcoal in terms of recovery of current from sewage wastewater

In this study, two fibrous carbon anodes (namely, pleated non-woven graphite (PNWG) and carbon brush (CB) made from artificial carbon) and bamboo charcoal (BC) were evaluated for current recovery from sewage wastewater. When these anodes were polarized at 0.2 V vs. Ag/AgCl in sewage wastewater, CB produced a maximum current of 2.9 A/m². This exceeded that produced by PNWG (1.5 A/m²) and BC (1.4 A/m²). The accumulative charge recovery achieved with CB was superior to those achieved with the other two (1.6- and 2.2-fold higher than that with PNWG and BC, respectively). During the cyclic voltammetry analysis, CB demonstrated the highest catalytic current with maximum potential in the range of -0.6 to 0.4 V vs. Ag/AgCl and the smallest anode resistance (0.20 Ωm²). Direct cell counting revealed that the fibrous anodes (CB and PNWG) attached most of the cells in the anodes (80%), whereas BC did not. In contrast, the proportion of Geobacter species, a representative electrogenic microorganism in the total bacteria, was observed to be similar among the three anodes (4.4-5.8%). The tubular microbial fuel cell (ø 5.0 cm) equipped with an air-chamber core wrapped with an anion exchange membrane (AEM) and the CB delivered a current of 1.8 A/m². This is higher than those reported in the existing literature for the same microbial fuel cell (MFC) configuration. This indicates that the alteration of the anode from planar to brush can contribute toward improving the current recovery through the air-cathode-AEM-MFC. The BC needs improvement to have more specific surface area, whereas it showed superiority in cost efficiency considering material and processing.

Customizable design strategies for high-performance bioanodes in bioelectrochemical systems

Bioelectrochemical systems (BESs) can fulfill the demand for renewable energy and wastewater treatment but still face significant challenges to improve their overall performance. Core efforts have been made to enhance the bioelectrode performance, yet, previous approaches are fragmented and have limited applicability, unable to flexibly adjust physicochemical and structural properties of electrodes for specific requirements in various applications. Here, we propose a facile electrode design strategy that integrates three-dimensional printing technology and functionalized modular electrode materials. A customized graphene-based electrode with hierarchical pores and functionalized components (i.e., ferric ions and magnetite nanoparticles) was fabricated. Owing to efficient mass and electron transfer, a high volumetric current density of 10,608 ± 1,036 A/m³ was achieved, the highest volumetric current density with pure Geobacter sulfurreducens to date. This strategy can be readily applied to existing BESs (e.g., microbial fuel cells and microbial electrosynthesis) and provide a feasibility for practical application.

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A 3D-printed graphene aerogel electrode was proposed for BESs

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The optimized electrode mass transfer resistance was less than 5% of carbon felt

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A high volumetric current density of 10,608 ± 1,036 A/m³ was achieved

Effect of waterproof breathable membrane based cathodes on performance and biofilm microbiomes in bioelectrochemical systems

A novel method for fabricating air-cathodes was developed by assembling an activated carbon (AC) catalyst together with a waterproof breathable membrane (WBM) and stainless steel mesh (SSM) to reduce manufacturing costs of bioelectrochemical systems (BESs). WBMs made of different materials were tested in the assembly, including a hybrid of polypropylene and polyolefin (PPPO), polyethylene (PE), and polyurethane (PU), and compared against poly tetrafluoroethylene (PTFE)-based cathodes. Results showed that the maximum power density of the activated carbon-stainless steel mesh-polyurethane (AC@SSM/PU) assembly was 2.03 W/m² while that of conventional carbon cloth cathode assembly (Pt@CC/PTFE) was 1.51 W/m². Compared to conventional cathode fabrication, AC@SSM/PU had a much lower cost and simpler manufacturing process. Illumina Miseq sequencing of 16S rRNA gene amplicons indicated that microbiomes were substantially different between anode and cathode biofilms. There was also a difference in the community composition between different cathode biofilms. The predominant population in the anode biofilms was Geobacter (38-75% relative abundance), while Thauera and Pseudomonas dominated the cathode biofilms. The results demonstrated that different types of air-cathodes influenced the microbial community assembly on the electrodes.

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.

Biofilm's morphology design for high sensitivity of bioelectrochemical sensor: An experimental and modeling study

High sensitivity is essential for the application of bioelectrochemical system-based sensor (BES sensor) in water quality early-warning, where the electroactive biofilm is of vital importance as it delivers a responsive electric signal to toxic substances. This study artificially designed the morphology of a naturally formed biofilm by employing a serrated knife to scrape the biofilm and thus obtained a reduced thickness and roughness. Then it was further cut by half to halve the biomass. BES sensors equipped with control and processed biofilms were operated under constant anode potential (CAP) and tested at different Cu(II) concentrations to study their sensitivities. Results revealed that the scraped biofilms delivered much increased sensitivity towards Cu(II) shock, which was attributed to a reduced thickness as illustrated by macroscopic and microscopic morphology analysis. Another finding was that biomass per unit interfacial area, rather than the biomass, also affected the sensitivity. To further describe how the inner biofilm responded the toxicity after morphology design, a one-dimension mass transfer model was developed to simulate the mass transfer of Cu(II) in the biofilms with different thicknesses. The relative threshold value of inlet Cu(II) concentration was employed to fit the modeling and experimental results, indicating that decreased biofilm thickness was beneficial for improving the sensitivity.