Carbon-Based Microbial-Fuel-Cell Electrodes: From Conductive Supports to Active Catalysts

Nano- and Microscale Optical and Electrical Biointerfaces and Their Relevance to Energy Research

Different research fields in energy sciences, such as photovoltaics for solar energy conversion, supercapacitors for energy storage, electrocatalysis for clean energy conversion technologies, and materials-bacterial hybrid for CO₂ fixation have been under intense investigations over the past decade. In recent years, new platforms for biointerface designs have emerged from the energy conversion and storage principles. This paper reviews recent advances in nano- and microscale materials/devices for optical and electrical biointerfaces. First, a connection is drawn between biointerfaces and energy science, and how these two distinct research fields can be connected is summarized. Then, a brief overview of current available tools for biointerface studies is presented. Third, three representative biointerfaces are reviewed, including neural, cardiac, and bacterial biointerfaces, to show how to apply these tools and principles to biointerface design and research. Finally, two possible future research directions for nano- and microscale biointerfaces are proposed.

Metal-Organic-Framework-Derived Nanostructures as Multifaceted Electrodes in Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) are considered up-and-coming future-generation energy storage systems because of their prominent theoretical energy density. However, the practical applications of MSBs are still hampered by several critical challenges, i.e., the shuttle effects, sluggish redox kinetics, and low conductivity of sulfur species. Recently, benefiting from the high surface area, regulated networks, molecular/atomic-level reactive sites, the metal-organic frameworks (MOFs)-derived nanostructures have emerged as efficient and durable multifaceted electrodes in MSBs. Herein, a timely review is presented on recent advancements in designing MOF-derived electrodes, including fabricating strategies, composition management, topography control, and electrochemical performance assessment. Particularly, the inherent charge transfer, intrinsic polysulfide immobilization, and catalytic conversion on designing and engineering of MOF nanostructures for efficient MSBs are systematically discussed. In the end, the essence of how MOFs' nanostructures influence their electrochemical properties in MSBs and conclude the future tendencies regarding the construction of MOF-derived electrodes in MSBs is exposed. It is believed that this progress review will provide significant experimental/theoretical guidance in designing and understanding the MOF-derived nanostructures as multifaceted electrodes, thus offering promising orientations for the future development of fast-kinetic and robust MSBs in broad energy fields.

Simultaneous sulfamethoxazole degradation with electricity generation by microbial fuel cells using Ni-MOF-74 as cathode catalysts and quantification of antibiotic resistance genes

Antibiotic wastewater presents serious challenges in water treatment. Metal-organic frameworks (MOFs) have received significant attention as promising precursors and sacrificial templates in the preparation of porous carbon-supported catalysts. Herein, we investigated the sulfamethoxazole (SMX) degradation and electrochemical performance of microbial fuel cells (MFCs) that applied as-prepared Ni-MOF-74 and Ni-N-C (Ni-MOF-74 underwent pyrolysis treatment at different temperatures) as air-cathode catalyst. Firstly, the electrocatalytic activity towards oxygen reduction reaction (ORR) of the catalyst was investigated by rotating disk electrode. The results showed that electron transfer number for Ni-MOF-74 was 2.12, while that of 800Ni-N-C was 3.44, which was close to four-electron reduction. Applying Ni-MOF-74 in MFCs, a maximum power density of 446 mW/m² was obtained, which was close to that of 800Ni-N-C. Besides, using Ni-MOF-74 as cathode catalyst, a chemical oxygen demand removal rate of about 84% was obtained, and the degradation rate of 10 mg/L SMX was 61%. The degradation rate decreased with increasing antibiotic concentration, but the average degradation efficiency increased stepwise. Additionally, the relative abundance of resistant gene sul1 in the reactors of the new catalytic material was about 62% lower than that of sul1 in the control (Pt/C) reactors, and the relative abundance of sul2 was about 73% lower. Moreover, cost assessments related to the catalyst performance are presented. The findings of this study demonstrated that Ni-MOF-74 could be considered as a two-electron transfer ORR catalyst, and offers a promising technique for preparation of Ni-N-C for use as four-electron transfer ORR catalysts. In comparison, Ni-MOF-74 could be a promising ORR catalyst of MFCs for antibiotic degradation.

Impact of Carbon Felt Electrode Pretreatment on Anodic Biofilm Composition in Microbial Electrolysis Cells

Sustainable technologies for energy production and storage are currently in great demand. Bioelectrochemical systems (BESs) offer promising solutions for both. Several attempts have been made to improve carbon felt electrode characteristics with various pretreatments in order to enhance performance. This study was motivated by gaps in current knowledge of the impact of pretreatments on the enrichment and microbial composition of bioelectrochemical systems. Therefore, electrodes were treated with poly(neutral red), chitosan, or isopropanol in a first step and then fixed in microbial electrolysis cells (MECs). Four MECs consisting of organic substance-degrading bioanodes and methane-producing biocathodes were set up and operated in batch mode by controlling the bioanode at 400 mV vs. Ag/AgCl (3M NaCl). After 1 month of operation, Enterococcus species were dominant microorganisms attached to all bioanodes and independent of electrode pretreatment. However, electrode pretreatments led to a decrease in microbial diversity and the enrichment of specific electroactive genera, according to the type of modification used. The MEC containing isopropanol-treated electrodes achieved the highest performance due to presence of both Enterococcus and Geobacter. The obtained results might help to select suitable electrode pretreatments and support growth conditions for desired electroactive microorganisms, whereby performance of BESs and related applications, such as BES-based biosensors, could be enhanced.

Characterization and significance of extracellular polymeric substances, reactive oxygen species, and extracellular electron transfer in methanogenic biocathode

The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. This article reports an experimental investigation of extracellular polymeric substances (EPS), reactive oxygen species (ROS), and the expression of genes associated with extracellular electron transfer (EET) in methanogenic biocathodes. The MEC-AD systems were examined using two cathode materials: carbon fibers and stainless-steel mesh. A higher abundance of hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers. Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions (possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus, these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development.

Whole-Cell Microbial Bioreporter for Soil Contaminants Detection

Anthropogenic activities have released various contaminants into soil that pose a serious threat to the ecosystem and human well-being. Compared to conventional analytical methodologies, microbial cell-based bioreporters are offering a flexible, rapid, and cost-effective strategy to assess the environmental risks. This review aims to summarize the recent progress in the application of bioreporters in soil contamination detection and provide insight into the challenges and current strategies. The biosensing principles and genetic circuit engineering are introduced. Developments of bioreporters to detect and quantify heavy metal and organic contaminants in soil are reviewed. Moreover, future opportunities of whole-cell bioreporters for soil contamination monitoring are discussed.

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.

Status Update on Bioelectrochemical Systems: Prospects for Carbon Electrode Design and Scale-Up

Bioelectrochemical systems (BES) employ enzymes, subcellular structures or whole electroactive microorganisms as biocatalysts for energy conversion purposes, such as the electrosynthesis of value-added chemicals and power generation in biofuel cells. From a bioelectrode engineering viewpoint, customizable nanostructured carbonaceous matrices have recently received considerable scientific attention as promising electrode supports due to their unique properties attractive to bioelectronics devices. This review demonstrates the latest advances in the application of nano- and micro-structured carbon electrode assemblies in BES. Specifically, in view of the gradual increase in the commercial applicability of these systems, we aim to address the stability and scalability of different BES designs and to highlight their potential roles in a circular bioeconomy.

Microbial Electrolysis Cells for Decentralised Wastewater Treatment: The Next Steps

Traditional wastewater treatment methods have become aged and inefficient, meaning alternative methods are essential to protect the environment and ensure water and energy security worldwide. The use of microbial electrolysis cells (MEC) for wastewater treatment provides an innovative alternative, working towards circular wastewater treatment for energy production. This study evaluates the factors hindering industrial adoption of this technology and proposes the next steps for further research and development. Existing pilot-scale investigations are studied to critically assess the main limitations, focusing on the electrode material, feedstock, system design and inoculation and what steps need to be taken for industrial adoption of the technology. It was found that high strength influents lead to an increase in energy production, improving economic viability; however, large variations in waste streams indicated that a homogenous solution to wastewater treatment is unlikely with changes to the MEC system specific to different waste streams. The current capital cost of implementing MECs is high and reducing the cost of the electrodes should be a priority. Previous pilot-scale studies have predominantly used carbon-based materials. Significant reductions in relative performance are observed when electrodes increase in size. Inoculation time was found to be a significant barrier to quick operational performance. Economic analysis of the technology indicated that MECs offer an attractive option for wastewater treatment, namely greater energy production and improved treatment efficiency. However, a significant reduction in capital cost is necessary to make this economically viable. MEC based systems should offer improvements in system reliability, reduced downtime, improved treatment rates and improved energy return. Discussion of the merits of H2 or CH4 production indicates that an initial focus on methane production could provide a stepping-stone in the adoption of this technology while the hydrogen market matures.

Mo2 C/Reduced Graphene Oxide Composites with Enhanced Electrocatalytic Activity and Biocompatibility for Microbial Fuel Cells

A simple, cost-effective strategy was developed to effectively improve the electron transfer efficiency as well as the power output of microbial fuel cells (MFCs) by decorating the commercial carbon paper (CP) anode with an advanced Mo₂ C/reduced graphene oxide (Mo₂ C/RGO) composite. Benefiting from the synergistic effects of the superior electrocatalytic activity of Mo₂ C, the high surface area, and prominent conductivity of RGO, the MFC equipped with this Mo₂ C/RGO composite yielded a remarkable output power density of 1747±37.6 mW m^-2 , which was considerably higher than that of CP-MFC (926.8±6.3 mW m^-2 ). Importantly, the composite also facilitated the formation of 3D hybrid biofilm and could effectively improve the bacteria-electrode interaction. These features resulted in an enhanced coulombic efficiency up 13.2 %, nearly one order of magnitude higher than that of the CP (1.2 %).

Nanomaterials Facilitating Microbial Extracellular Electron Transfer at Interfaces

Electrochemically active bacteria can transport their metabolically generated electrons to anodes, or accept electrons from cathodes to synthesize high-value chemicals and fuels, via a process known as extracellular electron transfer (EET). Harnessing of this microbial EET process has led to the development of microbial bio-electrochemical systems (BESs), which can achieve the interconversion of electrical and chemical energy and enable electricity generation, hydrogen production, electrosynthesis, wastewater treatment, desalination, water and soil remediation, and sensing. Here, the focus is on the current understanding of the microbial EET process occurring at both the bacteria-electrode interface and the biotic interface, as well as some attempts to improve the EET by using various nanomaterials. The behavior of nanomaterials in different EET routes and their influence on the performance of BESs are described. The inherent mechanisms will guide rational design of EET-related materials and lead to a better understanding of EET mechanisms.

A Novel Design Portable Plugged-Type Soil Microbial Fuel Cell for Bioelectricity Generation

Soil microbial fuel cells (SMFCs) are a promising cost-effective power source for on-demand electricity generation applications. So far, reported SMFC configurations are usually bulky and hard to setup. In this study, a low-cost portable plugged-type SMFC (PSMFC) was designed and fabricated for on-demand micropower generation. The PSMFC can be activated just by plugging into natural wet soil, which is easy to access in the natural condition. The PSMFC uses carbon-based electrodes for cost-effectiveness. After setting the PSMFC into the soil to activate, it started to produce electricity after 1 h and reached the power density of 7.3 mW/m2 after 48 h. The proposed PSMFC can potentially generate electricity for remote sensors or soil sensing systems.

Graphene: An Antibacterial Agent or a Promoter of Bacterial Proliferation?

Graphene materials (GMs) are being investigated for multiple microbiological applications because of their unique physicochemical characteristics including high electrical conductivity, large specific surface area, and robust mechanical strength. In the last decade, studies on the interaction of GMs with bacterial cells appear conflicting. On one side, GMs have been developed to promote the proliferation of electroactive bacteria on the surface of electrodes in bioelectrochemical systems or to accelerate interspecies electron transfer during anaerobic digestion. On the other side, GMs with antibacterial properties have been synthesized to prevent biofilm formation on membranes for water treatment, on medical equipment, and on tissue engineering scaffolds. In this review, we discuss the mechanisms and factors determining the positive or negative impact of GMs on bacteria. Furthermore, we examine the bacterial growth-promoting and antibacterial applications of GMs and debate their practicability.

Anode Modification as an Alternative Approach to Improve Electricity Generation in Microbial Fuel Cells

Sustainable production of electricity from renewable sources by microorganisms is considered an attractive alternative to energy production from fossil fuels. In recent years, research on microbial fuel cells (MFCs) technology for electricity production has increased. However, there are problems with up-scaling MFCs due to the fairly low power output and high operational costs. One of the approaches to improving energy generation in MFCs is by modifying the existing anode materials to provide more electrochemically active sites and improve the adhesion of microorganisms. The aim of this review is to present the effect of anode modification with carbon compounds, metallic nanomaterials, and polymers and the effect that these modifications have on the structure of the microbiological community inhabiting the anode surface. This review summarizes the advantages and disadvantages of individual materials as well as possibilities for using them for environmentally friendly production of electricity in MFCs.

Bacteria-affinity aminated carbon nanotubes bridging reduced graphene oxide for highly efficient microbial electrocatalysis

Bioelectrochemical systems (BESs) exhibit great potential for simultaneous wastewater treatment and energy recovery. However, the efficiency of microbial electrocatalysis is fundamentally limited by the high resistance and poor biocompatibility of electrode materials. Herein, we construct a novel "binder-free" 3D biocompatible bioelectrode consists of 1D aminated carbon nanotubes (CNTs-NH₂) and 2D conductive reduced graphene oxide (rGO) nanosheets through one-step electrodeposition. As expected, the maximum current density reached to 3.25 ± 0.03 mA cm^-2 with the rGO@CNTs-NH₂ electrode, which is 4.33-fold higher than that of a bare rGO (0.75 ± 0.01 mA cm^-2), and is among the best performance reported for three-dimensional electrodes. The high microbial electrocatalytic activity is mainly attributed to the excellent performance of electron transfer and bacterial colonization, which originates from the 3D interconnecting scaffold, fast 1D CNTs "e-bridge" and positively charged surface.

A new method for urine electrofiltration and long term power enhancement using surface modified anodes with activated carbon in ceramic microbial fuel cells

This work is presenting for the first time the use of inexpensive and efficient anode material for boosting power production, as well as improving electrofiltration of human urine in tubular microbial fuel cells (MFCs). The MFCs were constructed using unglazed ceramic clay functioning as the membrane and chassis. The study is looking into effective anodic surface modification by applying activated carbon micro-nanostructure onto carbon fibres that allows electrode packing without excessive enlargement of the electrode. The surface treatment of the carbon veil matrix resulted in 3.7 mW (52.9 W m-3 and 1626 mW m-2) of power generated and almost a 10-fold increase in the anodic current due to the doping as well as long-term stability in one year of continuous operation. The higher power output resulted in the synthesis of clear catholyte, thereby i) avoiding cathode fouling and contributing to the active splitting of both pH and ions and ii) transforming urine into a purified catholyte - 30% salt reduction - by electroosmotic drag, whilst generating - rather than consuming - electricity, and in a way demonstrating electrofiltration. For the purpose of future technology implementation , the importance of simultaneous increase in power generation, long-term stability over 1 year and efficient urine cleaning by using low-cost materials, is very promising and helps the technology enter the wider market.

Air-breathing cathode self-powered supercapacitive microbial fuel cell with human urine as electrolyte

In this work, a membraneless microbial fuel cell (MFC) with an empty volume of 1.5 mL, fed continuously with hydrolysed urine, was tested in supercapacitive mode (SC-MFC). In order to enhance the power output, a double strategy was used: i) a double cathode was added leading to a decrease in the equivalent series resistance (ESR); ii) the apparent capacitance was boosted up by adding capacitive features on the anode electrode. Galvanostatic (GLV) discharges were performed at different discharge currents. The results showed that both strategies were successful obtaining a maximum power output of 1.59 ± 0.01 mW (1.06 ± 0.01 mW mL⁻¹) at pulse time of 0.01 s and 0.57 ± 0.01 mW (0.38 ± 0.01 mW mL⁻¹) at pulse time of 2 s. The highest energy delivered at i_pulse equal to 2 mA was 3.3 ± 0.1 mJ. The best performing SC-MFCs were then connected in series and parallel and tested through GLV discharges. As the power output was similar, the connection in parallel allowed to roughly doubling the current produced. Durability tests over ≈5.6 days showed certain stability despite a light overall decrease.

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Air-breathing microbial fuel cell was tested in supercapacitive mode.

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A double cathode addition lead to a decrease in ohmic resistance.

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Apparent capacitance was boosted up by adding capacitive features.

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Maximum power output of 1.59 mW (1.06 mW mL⁻¹) was reached at t_pulse 0.01s.

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Series and parallel connections improved the galvanostatic discharges.

A Carbon-Cloth Anode Electroplated with Iron Nanostructure for Microbial Fuel Cell Operated with Real Wastewater

Microbial fuel cell (MFC) is an emerging method for extracting energy from wastewater. The power generated from such systems is low due to the sluggish electron transfer from the inside of the biocatalyst to the anode surface. One strategy for enhancing the electron transfer rate is anode modification. In this study, iron nanostructure was synthesized on a carbon cloth (CC) via a simple electroplating technique, and later investigated as a bio-anode in an MFC operated with real wastewater. The performance of an MFC with a nano-layer of iron was compared to that using bare CC. The results demonstrated that the open-circuit voltage increased from 600 mV in the case of bare CC to 800 mV in the case of the iron modified CC, showing a 33% increase in OCV. This increase in OCV can be credited to the decrease in the anode potential from 0.16 V vs. Ag/AgCl in the case of bare CC, to −0.01 V vs. Ag/AgCl in the case of the modified CC. The power output in the case of the modified electrode was 80 mW/m2—two times that of the MFC using the bare CC. Furthermore, the steady-state current in the case of the iron modified carbon cloth was two times that of the bare CC electrode. The improved performance was correlated to the enhanced electron transfer between the microorganisms and the iron-plated surface, along with the increase of the anode surface- as confirmed from the electrochemical impedance spectroscopy and the surface morphology, respectively.

Graphene-Like Layers from Carbon Black: In Vivo Toxicity Assessment

Graphene-like (GL) layers, a new graphene-related material (GRM), possess peculiar chemical, colloidal, optical and transport properties. Considering the very recent promising application of GL layers in biomedical and bioelectronic fields, it is of utmost importance to investigate the toxicological profile of these nanomaterials. This study represents an important first report of a complete in vivo toxicity assessment of GL layers on embryonic zebrafish (Danio rerio). Our results show that GL layers do not lead to any perturbations in the different biological parameters evaluated, indicating their good biocompatibility on a vertebrate model. The new insight into the biosafety of GL layers will expand their applications in nanomedicine.

Long-term bio-power of ceramic microbial fuel cells in individual and stacked configurations

In order to improve the potential of Microbial Fuel Cells (MFCs) as an applicable technology, the main challenge is to engineer practical systems for bioenergy production at larger scales and to test how the prototypes withstand the challenges occurring during the prolonged operation under constant feeding regime with real waste stream. This work presents the performance assessment of low-cost ceramic MFCs in the individual, stacked (modular) and modular cascade (3 modules) configurations during long term operation up to 19 months, utilising neat human urine as feedstock. During 1 year, the performance of the individual MFC units reached up to 1.56 mW (22.3 W/m3), exhibiting only 20% power loss on day 350 which was significantly smaller in comparison to conventional proton or cation exchange membranes. The stack module comprising 22 MFCs reached up to 21.4 mW (11.9 W/m3) showing power recovery to the initial output levels after 580 days, whereas the 3-module cascade reached up to 75 mW (13.9 W/m3) of power, showing 20% power loss on day 446. In terms of chemical oxygen demand (COD) removal, the 3-module cascade configuration achieved a cumulative reduction of >92%, which is higher than that observed in the single module (56%).

Composite of medium-chain-length polyhydroxyalkanoates-co-methyl acrylate and carbon nanotubes as innovative electrodes modifier in microbial fuel cell

A microbial fuel cell is a sustainable and environmental-friendly device that combines electricity generation and wastewater treatment through metabolic activities of microorganisms. However, low power output from inadequate electron transfer to the anode electrode hampers its practical implementation. Nanocomposites of oxidized carbon nanotubes and medium-chain-length polyhydroxyalkanoates (mcl-PHA) grafted with methyl acrylate monomers enhance the electrochemical function of electrodes in microbial fuel cell. Extensive polymerization of methyl acrylate monomers within mcl-PHA matrix, and homogenous dispersion of carbon nanotubes within the graft matrix are responsible for the enhancement. Modified electrodes exhibit high conductivities, better redox peak and reduction of cell internal resistance up to 76%. A stable voltage output at almost 700 mV running for 225 H generates maximum power and current density of 351 mW/m² and 765 mA/m² , respectively. Superior biofilm growth on modified surface is responsible for improved electron transfer to the anode hence stable and elevated power output generation.

Enhanced Anaerobic Digestion by Stimulating DIET Reaction

Since the observation of direct interspecies electron transfer (DIET) in anaerobic mixed cultures in 2010s, the topic “DIET-stimulation” has been the main route to enhance the performance of anaerobic digestion (AD) under harsh conditions, such as high organic loading rate (OLR) and the toxicants’ presence. In this review article, we tried to answer three main questions: (i) What are the merits and strategies for DIET stimulation? (ii) What are the consequences of stimulation? (iii) What is the mechanism of action behind the impact of this stimulation? Therefore, we introduced DIET history and recent relevant findings with a focus on the theoretical advantages. Then, we reviewed the most recent articles by categorizing how DIET reaction was stimulated by adding conductive material (CM) and/or applying external voltage (EV). The emphasis was made on the enhanced performance (yield and/or production rate), CM type, applied EV, and mechanism of action for each stimulation strategy. In addition, we explained DIET-caused changes in microbial community structure. Finally, future perspectives and practical limitations/chances were explored in detail. We expect this review article will provide a better understanding for DIET pathway in AD and encourage further research development in a right direction.

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

Bioelectrochemical system (BES) is promising technology to simultaneously treat wastewater and recover energy, and electrode material is important for the system performance. Microbial fuel cell (MFC) is one of typical BES to be applied in wastewater treatment. How to improve the electrode material is significant to improve wastewater treatment, energy recovery and cost effectiveness. In this study, 3D-weaved carbon electrode entity, assembled by multiple pieces of carbon mesh (CM), was proposed to combine all electrode components as entity to facilitate electron conduction and ionic migration, compared with carbon brush (CB) and granular activated carbon (GAC). The result showed that current density and internal resistance of MFC using 3D-weaved CM as horizontally extended inside anode (CM(T)) were 30.9 A m-3 and 4.5 Ω, respectively, with higher output than traditional GAC (22.6 A m-3 and 6.2 Ω). Though GAC had greater electrode filling and surface area for biomass growth, the electron transfer efficiency per unit electrode biomass was only at 0.0019 ± 0.0002 mol g-1 d-1, much lower than CM(T) at 0.0077 ± 0.0009 mol g-1 day-1. Higher ionic migration rate of CM(T) suggested the assisting effect of composite electrode to enhance ionic transportation towards the cathode. Microbial analysis further indicated that 3D-CM electrode network could simultaneously enhance Geobacter abundance and methanogen activity, suggesting the importance of electrode network on electricigens. Furthermore, CM(T) could obtain 10 times higher energy output efficiency than traditional GAC when applied inside anode chamber. This study proved that network construction of anode electrode could promote the electrode performance and cost effectiveness, suggesting the future development of reactor design of bioelectrochemical system.

High-Power Microbial Fuel Cells Based on a Carbon-Carbon Composite Air Cathode

Microbial fuel cells (MFCs) can convert organics in wastewater directly to electricity, and improving oxygen reduction reaction (ORR) performance is critical to their development and future applications. Electrocatalytic ORR performance is determined by the intrinsic activity and accessible amounts of active sites. A surface nitrogen-enriched carbon coaxial nanocable (NCCN) is applied as an ORR electrocatalyst and combined with activated carbon (AC) with 80 wt% addition as a carbon-carbon composite air cathode in MFCs. The fully exposed nitrogen active sites of NCCN contribute to the enhanced ORR activity, while the graphitized core affords a rapid pathway for electron transportation. AC serves as a spacer to construct a porous framework with interconnected ion diffusion channels. This cathode thus exhibits a maximum power density of 2090 mW m^-2 , 120% higher than commercial Pt/C electrocatalysts, and also 6% higher than the pure NCCN, indicating a synergistic effect between NCCN and AC. A high-performance NCCN-AC air cathode with a great promise for future MFC applications is reported and an effective strategy to bridge the electrocatalytic performance from nanomaterials to practical devices is presented.

Microbial Fuel Cell stack performance enhancement through carbon veil anode modification with activated carbon powder

The chemical energy contained in urine can be efficiently extracted into direct electricity by Microbial Fuel Cell stacks to reach usable power levels for practical implementation and a decentralised power source in remote locations. Herein, a novel type of the anode electrode was developed using powdered activated carbon (PAC) applied onto the carbon fibre scaffold in the ceramic MFC stack to achieve superior electrochemical performance during 500 days of operation. The stack equipped with modified anodes (MF-CV) produced up to 37.9 mW (21.1 W m-3) in comparison to the control (CV) that reached 21.4 mW (11.9 W m-3) showing 77% increase in power production. The novel combination of highly porous activated carbon particles applied onto the conductive network of carbon fibres promoted simultaneously electrocatalytic activity and increased surface area, resulting in excellent power output from the MFC stack as well as higher treatment rate. Considering the low cost and simplicity of the material preparation, as well as the outstanding electrochemical activity during long term operation, the resulting modification provides a promising anode electrocatalyst for high-performance MFC stacks to enhance urine and waste treatment for the purpose of future scale-up and technology implementation as an applied off-grid energy source.

Carbon dots-fed Shewanella oneidensis MR-1 for bioelectricity enhancement

Bioelectricity generation, by Shewanella oneidensis (S. oneidensis) MR-1, has become particularly alluring, thanks to its extraordinary prospects for energy production, pollution treatment, and biosynthesis. Attempts to improve its technological output by modification of S. oneidensis MR-1 remains complicated, expensive and inefficient. Herein, we report on the augmentation of S. oneidensis MR-1 with carbon dots (CDs). The CDs-fed cells show accelerated extracellular electron transfer and metabolic rate, with increased intracellular charge, higher adenosine triphosphate level, quicker substrate consumption and more abundant extracellular secretion. Meanwhile, the CDs promote cellular adhesion, electronegativity, and biofilm formation. In bioelectrical systems the CDs-fed cells increase the maximum current value, 7.34 fold, and power output, 6.46 fold. The enhancement efficacy is found to be strongly dependent on the surface charge of the CDs. This work demonstrates a simple, cost-effective and efficient route to improve bioelectricity generation of S. oneidensis MR-1, holding promise in all relevant technologies.

Bacterial fuel cells have generated attention with the prospect of green energy production; current research is focused on optimising the system to improve efficiency. Here, the authors report on the feeding of carbon dots to S. oneidensis MR-1 to enhance metabolic activity and bioelectric generation.

A three-dimensional hybrid electrode with electroactive microbes for efficient electrogenesis and chemical synthesis

Addressing the global challenge of sustainability calls for cost-effective and eco-friendly pathways to go beyond the existing energy-intense synthetic routes. Biohybrid electrochemical systems integrate electroactive bacteria with synthetic electrodes to leverage the power of biocatalysis for energy conversion and chemical synthesis. This work presents a three-dimensional electrode scaffold to couple the intracellular metabolism with extracellular redox transformations by means of electrochemistry. The large population of bacteria actively metabolizing within the electrode scaffold produces a benchmark current density. The biohybrid electrode can also carry out synthetic reactions within or beyond biochemical pathways driven by solar light. This hierarchical electrode provides a robust and versatile platform to wire bacteria’s intrinsic physiological functionalities with artificial electronics for sustainable energy conversion and chemical production.

Integration of electroactive bacteria into electrodes combines strengths of intracellular biochemistry with electrochemistry for energy conversion and chemical synthesis. However, such biohybrid systems are often plagued with suboptimal electrodes, which limits the incorporation and productivity of the bacterial colony. Here, we show that an inverse opal-indium tin oxide electrode hosts a large population of current-producing Geobacter and attains a current density of 3 mA cm⁻² stemming from bacterial respiration. Differential gene expression analysis revealed Geobacter’s transcriptional regulations to express more electron-relaying proteins when interfaced with electrodes. The electrode also allows coculturing with Shewanella for syntrophic electrogenesis, which grants the system additional flexibility in converting electron donors. The biohybrid electrode containing Geobacter can also catalyze the reduction of soluble fumarate and heterogenous graphene oxide, with electrons from an external power source or an irradiated photoanode. This biohybrid electrode represents a platform to employ live cells for sustainable power generation and biosynthesis.

Co8FeS8 wrapped in Auricularia-derived N-doped carbon with a micron-size spherical structure as an efficient cathode catalyst for strengthening charge transfer and bioelectricity generation

The main issues regarding the practical application of microbial fuel cells (MFCs) are the poor activity and tolerance of oxygen reduction reaction (ORR) catalysts in wastewater. In this study, Auricularia chelated with Co, Fe and S ions is used as a nitrogen (N)-enriched carbon source to prepare N-doped bimetallic sulfide (Co₈FeS₈)-embedded carbon spheres (Co₈FeS₈/NSC) using a hydrothermal method. The effects of various temperatures (800-950 °C) on the structure and catalytic activity of Co₈FeS₈/NSC catalysts are investigated. The MFC with a Co₈FeS₈/NSC (900 °C) cathode obtained the maximum power density of 1.002 W m^-2, which is higher than that of Pt/C (0.88 W m^-2). After 1440 h of operation, the power density of the Co₈FeS₈/NSC (900 °C) cathode only declined by 5.49%, indicating that the Co₈FeS₈ activity, charge transfer and O₂ transport were slightly influenced by the attached microbes and poisonous substances in the wastewater. The electrochemical results indicate that Co₈FeS₈/NSC (900 °C) mainly proceeds by a 4e^- ORR pathway, indicating that Co₈FeS₈ (Co²⁺ and Fe²⁺) wrapped in NSCs (carbon spheres) can trigger synergistic effects to provide more active sites and high electrical conductivity to achieve the rapid kinetics required for the ORR. Moreover, the porous structures of the NSCs (220.97 m² g^-1) with incorporated pyridinic N, pyrrolic N and graphitic N can provide abundant available channels for O₂ and OH^- transport to ensure the preferential accessibility of the reactant molecules to active sites. This indicates that Auricularia-derived Co₈FeS₈/NSC catalysts have great potential as alternatives for precious metal-based catalysts in neutral electrolyte MFCs.

Semi-biological approaches to solar-to-chemical conversion

This review provides an overview of the cross-disciplinary field of semi-artificial photosynthesis, which combines strengths of biocatalysis and artificial photosynthesis to develop new concepts and approaches for solar-to-chemical conversion.

Current recovery from sewage wastewater using electrochemically oxidized graphite felt

The oxidation of a carbon anode has been reported to enhance electricity recovery in a microbial fuel cell (MFC). This study investigates the applicability of electrochemically oxidized graphite felt (EOGF) as the anode for the recovery of electricity from sewage wastewater when polarized at 0.2 V during MFC operation. EOGFs were prepared by polarizing graphite felt (GF) at 2 V in 1% sulfuric acid or nitric acid. The nitric acid-treated EOGF inoculated with an sewage sludge produced a maximum current of 110 μA cm-3, which exceeds that produced by the original GF (91 μA cm-3) under electrochemical cultivation at 0.2 V vs. Ag/AgCl. This outcome is attributed to a decrease in charge-transfer resistance and an increase in the capacitance of the anode. In contrast, electrochemical oxidation did not affect the chemical oxygen demand (COD) removal rate or the microbial community structure of the anode. The MFC equipped with the EOGF delivered 340-560 mW m-3-MFC of electricity during operation in the drainage water channel of a primary sedimentation tank, which corresponds to 11-15 μA cm-3 of current density. The lower current produced in the MFC compared to that observed during electrochemical cultivation indicates that factors other than the anode material restrict current production in the MFC. Even with the small amount of generated electricity, when operated for more than three days, the MFC provides a positive net energy balance when integrated with post-aeration treatment.

Augmenting Intrinsic Fenton-Like Activities of MOF-Derived Catalysts via N-Molecule-Assisted Self-catalyzed Carbonization

To overcome the ever-growing organic pollutions in the water system, abundant efforts have been dedicated to fabricating efficient Fenton-like carbon catalysts. However, the rational design of carbon catalysts with high intrinsic activity remains a long-term goal. Herein, we report a new N-molecule-assisted self-catalytic carbonization process in augmenting the intrinsic Fenton-like activity of metal-organic-framework-derived carbon hybrids. During carbonization, the N-molecules provide alkane/ammonia gases and the formed iron nanocrystals act as the in situ catalysts, which result in the elaborated formation of carbon nanotubes (in situ chemical vapor deposition from alkane/iron catalysts) and micro-/meso-porous structures (ammonia gas etching). The obtained catalysts exhibited with abundant Fe/Fe-Nx/pyridinic-N active species, micro-/meso-porous structures, and conductive carbon nanotubes. Consequently, the catalysts exhibit high efficiency toward the degradation of different organic pollutions, such as bisphenol A, methylene blue, and tetracycline. This study not only creates a new pathway for achieving highly active Fenton-like carbon catalysts but also takes a step toward the customized production of advanced carbon hybrids for diverse energy and environmental applications.

Salt-induced silk gel-derived N and trace Fe co-doped 3D porous carbon as an oxygen reduction catalyst in microbial fuel cells

Inexpensive and high-efficiency oxygen reduction reaction (ORR) catalysts play a significant role in achieving practical applications of microbial fuel cells (MFCs). Hence, herein, novel nitrogen (N) and trace iron (Fe) co-doped three-dimensional (3D) porous carbon (NFe_x-C) was synthesized as an excellent ORR catalyst from an interesting salt-induced silk gel, which was beneficial to the spontaneously formation of porosity and boosted the ORR activity. Among the series of NFe_x-C, NFe_0.5-C (1.20% N-ORR/C, 0.07 at% Fe) possessed a higher specific surface area (538.94 m² g^-1) and pore volume (2.158 cm³ g^-1). Note that NFe_0.5-C exhibited a significantly higher positive initial potential (0.274 V vs. Ag/AgCl) and half-wave potential (0.095 V vs. Ag/AgCl) than other catalysts and commercial Pt/C (20 wt%); this implied that it possessed prominent ORR catalytic activity. In the MFC tests, the output-voltage and maximum power density of NFe_0.5-C were enhanced to 517.37 ± 7.87 mV and 605.35 ± 15.39 mW m^-2, respectively. Moreover, NFe_0.5-C (0.15 $ g^-1) exhibits excellent anti-poisoning ability and is thousands of times cheaper than commercial Pt/C (20 wt%, 220.04 $ g^-1); therefore, NFe_0.5-C should be a prospective catalyst to substitute precious commercial Pt/C in MFCs and even for application in other types of fuel cells.

Self-stratified and self-powered micro-supercapacitor integrated into a microbial fuel cell operating in human urine

A self-stratified microbial fuel cell fed with human urine with a total internal volume of 0.55 ml was investigated as an internal supercapacitor, for the first time. The internal self-stratification allowed the development of two zones within the cell volume. The oxidation reaction occurred on the bottom electrode (anode) and the reduction reaction on the top electrode (cathode). The electrodes were discharged galvanostatically at different currents and the two electrodes were able to recover their initial voltage value due to their red-ox reactions. Anode and cathode apparent capacitance was increased after introducing high surface area activated carbon embedded within the electrodes. Peak power produced was 1.20 ± 0.04 mW (2.19 ± 0.06 mW ml-1) for a pulse time of 0.01 s that decreased to 0.65 ± 0.02 mW (1.18 ± 0.04 mW ml-1) for longer pulse periods (5 s). Durability tests were conducted over 44 h with ≈2600 discharge/recharge cycles. In this relatively long-term test, the equivalent series resistance increased only by 10% and the apparent capacitance decreased by 18%.