Platinum-free, graphene based anodes and air cathodes for single chamber microbial fuel cells

N-Doped Carbon Nanowire-Modified Macroporous Carbon Foam Microbial Fuel Cell Anode: Enrichment of Exoelectrogens and Enhancement of Extracellular Electron Transfer

Microbial fuel cell (MFC) performance is affected by the metabolic activity of bacteria and the extracellular electron transfer (EET) process. The deficiency of nanostructures on macroporous anode obstructs the enrichment of exoelectrogens and the EET. Herein, a N-doped carbon nanowire-modified macroporous carbon foam was prepared and served as an anode in MFCs. The anode has a hierarchical porous structure, which can solve the problem of biofilm blockage, ensure mass transport, favor exoelectrogen enrichment, and enhance the metabolic activity of bacteria. The microscopic morphology, spectroscopy, and electrochemical characterization of the anode confirm that carbon nanowires can penetrate biofilm, decrease charge resistance, and enhance long-distance electron transfer efficiency. In addition, pyrrolic N can effectively reduce the binding energy and electron transfer distance of bacterial outer membrane hemin. With this hierarchical anode, a maximum power density of 5.32 W/m3 was obtained, about 2.5-fold that of bare carbon cloth. The one-dimensional nanomaterial-modified macroporous anodes in this study are a promising strategy to improve the exoelectrogen enrichment and EET for MFCs.

A review on graphene / graphene oxide supported electrodes for microbial fuel cell applications: Challenges and prospects

Microbial Fuel Cell (MFC) has gained great interest as an alternative green technology for bioenergy generation along with reduced sludge production, nutrient recovery, removal of COD and color, etc. during wastewater treatment. However, the MFC has several challenges for real-time applications due to less power output and high ohmic resistance and fabrication (electrode and membrane) cost. Several kinds of research have been carried out to increase energy production by reducing various losses associated with electrodes in the MFC. Though, carbonaceous electrodes (carbon and graphite) are the key materials for the anode and cathode side, since these have a higher surface area, good biocompatibility, low cost, and good mechanical strength. Graphene or graphene oxide-based nanocomposite can be an ideal substitute for electrode modifications and an alternative for an expensive anode and cathode catalyst in MFC. Graphene oxide synthesis from waste material such as waste biomass, agricultural, plastic waste, etc. is added advantages of minimizing the cost of the electrodes. But, the synthesis of graphene is quite expensive and has limitations in economic feasibility for bioelectricity production in MFC. Hence, the present review deals with the anode and cathode electrode modification with graphene-based nanocomposites, synthesis of graphene/graphene oxide from various raw materials, and its application in MFC. The current challenges and future outlook on graphene-based composites on MFC performance are also discussed.

Enhanced production of microalgae-originated photosensitizer by integrating photosynthetic electrons extraction and antibiotic induction towards photocatalytic degradation of antibiotic: A novel complementary treatment process for antibiotic removal from effluent of conventional biological wastewater treatment

Antibiotic residues in effluents from bio-treated wastewaters are mainly responsible for the spread of antibiotic resistance genes in the environment. Conventional physicochemical treatments are thought to be unsustainable due to high energy consumption, large consumption of chemicals and environmental unfriendly processing step. In this study, a novel approach by integrating photosynthetic electrons extraction from microalgae with antibiotic induction was used to enhance the production of microalgae-originated photosensitizer for photolytic removal of antibiotic residues in effluents from conventional bio-treated wastewaters. Results showed that the accumulation of photoactive substances in extracellular polymeric substance (EPS) of chlorella vulgaris was positively related to the amounts of photosynthetic electrons extracted by the electrode which is a potential-dependent process and can be further enhanced by tetracycline (TC) induction. The protein and humic acid which are considered two main photoactive substances in EPS produced at 0.6 V accumulated to a high level of 320 and 24 μg/cm³ and were further increased to 380 and 48 μg/cm³ when TC was added which were 4.7 and 6.4-folds higher than that produced at potential free in the absence of TC. The EPS produced at 0.6 and 0.8 V led to 1.34 and 1.53-fold acceleration in photosensitized degradation of TC compared to that of EPS free in secondary effluent of municipal wastewater treatment plant. The complex heterocyclic ring structure of TC was broken down into simple monocyclic aromatic compounds, indicating a marked reduction in biotoxicity and recalcitrance. The hydroxyl radical played a main role for the photolysis of TC followed by singlet oxygen. This technology provides a new alternative to conventional physicochemical treatment as complementary treatment processes for biological wastewater treatment in terms of antibiotics removal.

Expression of Alternative Nitrogenases in Rhodopseudomonas palustris Is Enhanced Using an Optimized Genetic Toolset for Rapid, Markerless Modifications

The phototrophic bacterium Rhodopseudomonas palustris is emerging as a promising biotechnological chassis organism, due to its resilience to a range of harsh conditions, a wide metabolic repertoire, and the ability to quickly regenerate ATP using light. However, realization of this promise is impeded by a lack of efficient, rapid methods for genetic modification. Here, we present optimized tools for generating chromosomal insertions and deletions employing electroporation as a means of transformation. Generation of markerless strains can be completed in 12 days, approximately half the time for previous conjugation-based methods. This system was used for overexpression of alternative nitrogenase isozymes with the aim of improving biohydrogen productivity. Insertion of the pucBa promoter upstream of vnf and anf nitrogenase operons drove robust overexpression up to 4000-fold higher than wild-type. Transcript quantification was facilitated by an optimized high-quality RNA extraction protocol employing lysis using detergent and heat. Overexpression resulted in increased nitrogenase protein levels, extending to superior hydrogen productivity in bioreactor studies under nongrowing conditions, where promoter-modified strains better utilized the favorable energy state created by reduced competition from cell division. Robust heterologous expression driven by the pucBa promoter is thus attractive for energy-intensive biosyntheses suited to the capabilities of R. palustris. Development of this genetic modification toolset will accelerate the advancement of R. palustris as a biotechnological chassis organism, and insights into the effects of nitrogenase overexpression will guide future efforts in engineering strains for improved hydrogen production.

Separation of CH4, H2S, N2 and CO2 gases using four types of nanoporous graphene cluster model: a quantum chemical investigation

Nanoporous graphene is being regarded as a promising candidate for reliable gas separation and purification applications. In the present research, the permeation barrier, selectivity and all thermodynamic functions for passing of four different molecules including CH₄, H₂S, N₂ and CO₂ gases on four types of porous graphene which is doped by two, three and six nitrogen atoms using quantum mechanical modelling, based on the density functional theory, B97D, and cc-pVTZ basis set have been evaluated. We find that the permeation barrier of all studied gases especially carbon dioxide decreased by considering the functionalized porous graphene by two, three and six nitrogens-doped, respectively. The results of our study propose using a porous graphene sheet as highly efficient and highly selective membranes for gas separations.

Type IV Pili-Independent Photocurrent Production by the Cyanobacterium Synechocystis sp. PCC 6803

Biophotovoltaic devices utilize photosynthetic organisms such as the model cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis) to generate current for power or hydrogen production from light. These devices have been improved by both architecture engineering and genetic engineering of the phototrophic organism. However, genetic approaches are limited by lack of understanding of cellular mechanisms of electron transfer from internal metabolism to the cell exterior. Type IV pili have been implicated in extracellular electron transfer (EET) in some species of heterotrophic bacteria. Furthermore, conductive cell surface filaments have been reported for cyanobacteria, including Synechocystis. However, it remains unclear whether these filaments are type IV pili and whether they are involved in EET. Herein, a mediatorless electrochemical setup is used to compare the electrogenic output of wild-type Synechocystis to that of a ΔpilD mutant that cannot produce type IV pili. No differences in photocurrent, i.e., current in response to illumination, are detectable. Furthermore, measurements of individual pili using conductive atomic force microscopy indicate these structures are not conductive. These results suggest that pili are not required for EET by Synechocystis, supporting a role for shuttling of electrons via soluble redox mediators or direct interactions between the cell surface and extracellular substrates.

Enhanced removal of veterinary antibiotic from wastewater by photoelectroactive biofilm of purple anoxygenic phototroph through photosynthetic electron uptake

Purple anoxygenic phototrophs have been recently attracted substantial attention for their growing potential in wastewater treatment and their diverse metabolic patterns can be regulated for process control and optimization. In this study, the photoheterotrophic metabolism of Rhodopseudomonas palustris (R. palustris) was modified by photosynthetic electron uptake using a poised electrode which was explored to enhance removal of veterinary antibiotic from aqueous medium. The results showed that R. palustris grown as biofilm on electrode surface had excellent photoelectroactive activity and the photosynthetic electron uptake from the photoelectroactive biofilm significantly enhanced antibiotic florfenicol (FLO) degradation. The specific degradation rate of FLO at the set electrode potential of 0 V was 2.59-fold higher than that without applied potential. Enhanced co-metabolic reductive dehalogenation by use of the photosynthetic electrons extracted from co-substrate was mainly responsible for FLO degradation which eliminated the antibacterial activity of FLO. The electrode potential controlled the processes of photosynthetic electron uptake and its resultant FLO degradation. The fastest degradation of FLO was achieved at 0 V because the electrode poised at this potential stroke a proper balance between the enhancing photosynthetic electron uptake by serving as electron acceptor and minimizing competition with FLO for the photosynthetic electron from co-substrate. The activity of photoelectroactive biofilm was not negatively affected by FLO at environmental relevant concentration, suggesting its great potential for removal of antibiotic contaminants in wastewater. R. palustris could serve as a reservoir for floR resistance gene but its abundance can be diminished by choosing appropriate electrode potential.

Bioelectrochemical systems for a circular bioeconomy

In the past two decades, bioelectrochemical systems (BESs) have received a considerable attention as a way of transforming wastewater directly to electricity and chemicals. Since BESs are capable of energy and chemical production by removing wastewater, these systems are considered promising sustainable waste-to-energy/chemical platforms and parts of circular bioeconomy. For the estimation of practical applicability of BESs in the circular bioeconomy, economic assessment of these systems is reviewed in this work. This estimation is necessary to decide whether BESs can be further developed for commercialization or there is any limiting factor for making the systems commercially viable in circular bioeconomy. This review also presents current developments of BESs, providing a critical review of the current status and challenges of techno-economic analysis for BESs. The results highlight the key factors to suggest the future research directions to make BESs economically available as a part of promising circular bioeconomy.

The Development of Biophotovoltaic Systems for Power Generation and Biological Analysis

Biophotovoltaic systems (BPVs) resemble microbial fuel cells, but utilise oxygenic photosynthetic microorganisms associated with an anode to generate an extracellular electrical current, which is stimulated by illumination. Study and exploitation of BPVs have come a long way over the last few decades, having benefited from several generations of electrode development and improvements in wiring schemes. Power densities of up to 0.5 W m-2 and the powering of small electrical devices such as a digital clock have been reported. Improvements in standardisation have meant that this biophotoelectrochemical phenomenon can be further exploited to address biological questions relating to the organisms. Here, we aim to provide both biologists and electrochemists with a review of the progress of BPV development with a focus on biological materials, electrode design and interfacial wiring considerations, and propose steps for driving the field forward.

Carbon-Based Metal-Free ORR Electrocatalysts for Fuel Cells: Past, Present, and Future

Replacing precious platinum with earth-abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last 10 years, the fastest-growing branch in this area has been carbon-based metal-free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals. Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most up-to-date results are presented, along with remarks and perspectives for the future development of carbon-based metal-free ORR electrocatalysts.

Biomimetic Carbon Fiber Systems Engineering: A Modular Design Strategy To Generate Biofunctional Composites from Graphene and Carbon Nanofibers

Carbon-based fibrous scaffolds are highly attractive for all biomaterial applications that require electrical conductivity. It is additionally advantageous if such materials resembled the structural and biochemical features of the natural extracellular environment. Here, we show a novel modular design strategy to engineer biomimetic carbon fiber-based scaffolds. Highly porous ceramic zinc oxide (ZnO) microstructures serve as three-dimensional (3D) sacrificial templates and are infiltrated with carbon nanotubes (CNTs) or graphene dispersions. Once the CNTs and graphene coat the ZnO template, the ZnO is either removed by hydrolysis or converted into carbon by chemical vapor deposition. The resulting 3D carbon scaffolds are both hierarchically ordered and free-standing. The properties of the microfibrous scaffolds were tailored with a high porosity (up to 93%), a high Young's modulus (ca. 0.027-22 MPa), and an electrical conductivity of ca. 0.1-330 S/m, as well as different surface compositions. Cell viability, fibroblast proliferation rate and protein adsorption rate assays have shown that the generated scaffolds are biocompatible and have a high protein adsorption capacity (up to 77.32 ± 6.95 mg/cm3) so that they are able to resemble the extracellular matrix not only structurally but also biochemically. The scaffolds also allow for the successful growth and adhesion of fibroblast cells, showing that we provide a novel, highly scalable modular design strategy to generate biocompatible carbon fiber systems that mimic the extracellular matrix with the additional feature of conductivity.

Biohybrid Cathode in Single Chamber Microbial Fuel Cell

The aim of this work is to investigate the properties of biofilms, spontaneously grown on cathode electrodes of single-chamber microbial fuel cells, when used as catalysts for oxygen reduction reaction (ORR). To this purpose, a comparison between two sets of different carbon-based cathode electrodes is carried out. The first one (Pt-based biocathode) is based on the proliferation of the biofilm onto a Pt/C layer, leading thus to the creation of a biohybrid catalyst. The second set of electrodes (Pt-free biocathode) is based on a bare carbon-based material, on which biofilm grows and acts as the sole catalyst for ORR. Linear sweep voltammetry (LSV) characterization confirmed better performance when the biofilm is formed on both Pt-based and Pt-free cathodes, with respect to that obtained by biofilm-free cathodes. To analyze the properties of spontaneously grown cathodic biofilms on carbon-based electrodes, electrochemical impedance spectroscopy is employed. This study demonstrates that the highest power production is reached when aerobic biofilm acts as a catalyst for ORR in synergy with Pt in the biohybrid cathode.

Metal-Organic Framework-Derived Co3O4/Au Heterostructure as a Catalyst for Efficient Oxygen Reduction

Porous nanostructures with a yolk-shell complex interior will provide lots of virtues to construct advanced catalysts. In our work, the preparation of novel yolk-shell Au nanocrystal-loaded Co₃O₄ nanocages (Co₃O₄/Au heterostructure) from a metal-organic framework-derived composite was reported. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, and so forth were used to analyze the morphology, structure, and composition of the heterostructures. Most importantly, Co₃O₄/Au heterostructures are a kind of low-cost, good performance catalysts for the oxygen reduction reaction to replace the noble-Pt catalysts. The high surface area of the porous structure and the excellent electron transfer properties of well-dispersed Au nanocrystals and also the electronic coupling effect between Co₃O₄ and Au in the composites are attributed to the good performance.

Spray-Coating Thin Films on Three-Dimensional Surfaces for a Semitransparent Capacitive-Touch Device

Here, we formulate low surface tension (∼30 mN/m) and low boiling point (∼79 °C) inks of graphene, single-wall carbon nanotubes and conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and demonstrate their viability for spray-coating of morphologically uniform ( S_q ≈ 48 ± 3 nm), transparent conducting films (TCFs) at room temperature (∼20 °C), which conform to three dimensional curved surfaces. Large area (∼750 cm²) hybrid PEDOT:PSS/graphene films achieved an optical transmission of 67% in the UV and 64% in the near-infrared wavelengths with a conductivity of ∼10⁴ S/m. Finally, we demonstrate the spray-coating of TCFs as an electrode on the inside of a poly(methyl methacrylate) sphere, enabling a semitransparent (around 360°) and spherical touch sensor for interactive devices.

Electrochemical Characterisation of Bio-Bottle-Voltaic (BBV) Systems Operated with Algae and Built with Recycled Materials

Photobioelectrochemical systems are an emerging possibility for renewable energy. By exploiting photosynthesis, they transform the energy of light into electricity. This study evaluates a simple, scalable bioelectrochemical system built from recycled plastic bottles, equipped with an anode made from recycled aluminum, and operated with the green alga Chlorella sorokiniana. We tested whether such a system, referred to as a bio-bottle-voltaic (BBV) device, could operate outdoors for a prolonged time period of 35 days. Electrochemical characterisation was conducted by measuring the drop in potential between the anode and the cathode, and this value was used to calculate the rate of charge accumulation. The BBV systems were initially able to deliver ~500 mC·bottle⁻¹·day⁻¹, which increased throughout the experimental run to a maximum of ~2000 mC·bottle⁻¹·day⁻¹. The electrical output was consistently and significantly higher than that of the abiotic BBV system operated without algal cells (~100 mC·bottle⁻¹·day⁻¹). The analysis of the rate of algal biomass accumulation supported the hypothesis that harvesting a proportion of electrons from the algal cells does not significantly perturb the rate of algal growth. Our finding demonstrates that bioelectrochemical systems can be built using recycled components. Prototypes of these systems have been displayed in public events; they could serve as educational toolkits in schools and could also offer a solution for powering low-energy devices off-grid.