Whole-cell biophotovoltaic systems for renewable energy generation: A systematic analysis of existing knowledge

The development of carbon-neutral fuel sources is an essential step in addressing the global fossil energy crisis. Whole-cell biophotovoltaic systems (BPVs) are a renewable, non-polluting energy-generating device that utilizes oxygenic photosynthetic microbes (OPMs) to split water molecules and generate bioelectricity under the driving of light energy. Since 2006, BPVs have been widely studied, with the order magnitudes of power density increasing from 10-4 mW/m2 to 103 mW/m2. This review examines the extracellular electron transfer (EET) mechanisms and regulation techniques of BPVs from biofilm to external environment. It is found that the EET of OPMs is mainly mediated by membrane proteins, with terminal oxidase limiting the power output. Synechocystis sp. PCC6803 and Chlorella vulgaris are two species that produce high power density in BPVs. The use of metal nanoparticles mixing, 3D pillar array electrodes, microfluidic technology, and transient-state operation models can significantly enhance power density. Challenges and potential research directions are discussed, including a deeper analysis of EET mechanisms and dynamics, the development of modular devices, integration of multiple regulatory components, and the exploration of novel BPV technologies.

Influence of support materials on the electroactive behavior, structure and gene expression of wild type and GSU1771-deficient mutant of Geobacter sulfurreducens biofilms

Geobacter sulfurreducens DL1 is a metal-reducing dissimilatory bacterium frequently used to produce electricity in bioelectrochemical systems (BES). The biofilm formed on electrodes is one of the most important factors for efficient electron transfer; this is possible due to the production of type IV pili and c-type cytochromes that allow it to carry out extracellular electron transfer (EET) to final acceptors. In this study, we analyzed the biofilm formed on different support materials (glass, hematite (Fe2O3) on glass, fluorine-doped tin oxide (FTO) semiconductor glass, Fe2O3 on FTO, graphite, and stainless steel) by G. sulfurreducens DL1 (WT) and GSU1771-deficient strain mutant (Δgsu1771). GSU1771 is a transcriptional regulator that controls the expression of several genes involved in electron transfer. Different approaches and experimental tests were carried out with the biofilms grown on the different support materials including structure analysis by confocal laser scanning microscopy (CLSM), characterization of electrochemical activity, and quantification of relative gene expression by RT-qPCR. The gene expression of selected genes involved in EET was analyzed, observing an overexpression of pgcA, omcS, omcM, and omcF from Δgsu1771 biofilms compared to those from WT, also the overexpression of the epsH gene, which is involved in exopolysaccharide synthesis. Although we observed that for the Δgsu1771 mutant strain, the associated redox processes are similar to the WT strain, and more current is produced, we think that this could be associated with a higher relative expression of certain genes involved in EET and in the production of exopolysaccharides despite the chemical environment where the biofilm develops. This study supports that G. sulfurreducens is capable of adapting to the electrochemical environment where it grows.

Green synthesis of reduced graphene oxide by using tropical microalgae and its application in biophotovoltaic devices

The successful commercialization of algal biophotovoltaics (BPV) technology hinges upon a multifaceted approach, encompassing factors such as the development of a cost-efficient and highly conductive anode material. To address this issue, we developed an environmentally benign method of producing reduced graphene oxide (rGO), using concentrated Chlorella sp. UMACC 313 suspensions as the reducing agent. The produced rGO was subsequently coated on the carbon paper (rGO-CP) and used as the BPV device's anode. As a result, maximum power density was increased by 950% for Chlorella sp. UMACC 258 (0.210 mW m-2) and 781% for Synechococcus sp. UMACC 371 (0.555 mW m-2) compared to bare CP. The improved microalgae adhesion to the anode and improved electrical conductivity of rGO brought on by the effective removal of oxygen functional groups may be the causes of this. This study has demonstrated how microalgal-reduced GO may improve the efficiency of algal BPV for producing bioelectricity.

Biophotovoltaics: Recent advances and perspectives

Biophotovoltaics (BPV) is a clean power generation technology that uses self-renewing photosynthetic microorganisms to capture solar energy and generate electrical current. Although the internal quantum efficiency of charge separation in photosynthetic microorganisms is very high, the inefficient electron transfer from photosystems to the extracellular electrodes hampered the electrical outputs of BPV systems. This review summarizes the approaches that have been taken to increase the electrical outputs of BPV systems in recent years. These mainly include redirecting intracellular electron transfer, broadening available photosynthetic microorganisms, reinforcing interfacial electron transfer and design high-performance devices with different configurations. Furthermore, three strategies developed to extract photosynthetic electrons were discussed. Among them, the strategy of using synthetic microbial consortia could circumvent the weak exoelectrogenic activity of photosynthetic microorganisms and the cytotoxicity of exogenous electron mediators, thus show great potential in enhancing the power output and prolonging the lifetime of BPV systems. Lastly, we prospected how to facilitate electron extraction and further improve the performance of BPV systems.

Microbiomics for enhancing electron transfer in an electrochemical system

In microbial electrochemical systems, microorganisms catalyze chemical reactions converting chemical energy present in organic and inorganic molecules into electrical energy. The concept of microbial electrochemistry has been gaining tremendous attention for the past two decades, mainly due to its numerous applications. This technology offers a wide range of applications in areas such as the environment, industries, and sensors. The biocatalysts governing the reactions could be cell secretion, cell component, or a whole cell. The electroactive bacteria can interact with insoluble materials such as electrodes for exchanging electrons through colonization and biofilm formation. Though biofilm formation is one of the major modes for extracellular electron transfer with the electrode, there are other few mechanisms through which the process can occur. Apart from biofilm formation electron exchange can take place through flavins, cytochromes, cell surface appendages, and other metabolites. The present article targets the various mechanisms of electron exchange for microbiome-induced electron transfer activity, proteins, and secretory molecules involved in the electron transfer. This review also focuses on various proteomics and genetics strategies implemented and developed to enhance the exo-electron transfer process in electroactive bacteria. Recent progress and reports on synthetic biology and genetic engineering in exploring the direct and indirect electron transfer phenomenon have also been emphasized.

Biophotoelectrochemistry for renewable energy and environmental applications

Biophotoelectrochemistry (BPEC) is an interdisciplinary research field and combines bioelectrochemistry and photoelectrochemistry through the utilization of the catalytic abilities of biomachineries and light harvesters to accomplish the production of energy or chemicals driven by solar energy. The BPEC process may act as a new approach for sustainable green chemistry and waste minimization. This review provides the state-of-the-art introduction of BPEC basics and systems, with a focus on light harvesters and biocatalysts, configurations, photoelectron transfer mechanisms, and the potential applications in energy and environment. Several examples of BPEC applications are discussed including H2 production, CO2 reduction, chemical synthesis, pollution control, and biogeochemical cycle of elements. The challenges about BPEC systems are identified and potential solutions are proposed. The review aims to encourage further research of BPEC toward development of practical BPEC systems for energy and environmental applications.

Quantitative analysis of the effects of morphological changes on extracellular electron transfer rates in cyanobacteria

BACKGROUND

Understanding the extracellular electron transport pathways in cyanobacteria is a major factor towards developing biophotovoltaics. Stressing cyanobacteria cells environmentally and then probing changes in physiology or metabolism following a significant change in electron transfer rates is a common approach for investigating the electron path from cell to electrode. However, such studies have not explored how the cells' concurrent morphological adaptations to the applied stresses affect electron transfer rates. In this paper, we establish a ratio to quantify this effect in mediated systems and apply it to Synechococcus elongatus sp. PCC7942 cells grown under different nutritional regimes.

RESULTS

The results provide evidence that wider and longer cells with larger surface areas have faster mediated electron transfer rates. For rod-shaped cells, increase in cell area as a result of cell elongation more than compensates for the associated decline in mass transfer coefficients, resulting in faster electron transfer. In addition, the results demonstrate that the extent to which morphological adaptations account for the changes in electron transfer rates changes over the bacterial growth cycle, such that investigations probing physiological and metabolic changes are meaningful only at certain time periods.

CONCLUSION

A simple ratio for quantitatively evaluating the effects of cell morphology adaptations on electron transfer rates has been defined. Furthermore, the study points to engineering cell shape, either via environmental conditioning or genetic engineering, as a potential strategy for improving the performance of biophotovoltaic devices.

Silk Fibroin: An Emerging Biocompatible Material for Application of Enzymes and Whole Cells in Bioelectronics and Bioanalytical Sciences

Enzymes and whole cells serve as the active biological entities in a myriad of applications including bioprocesses, bioanalytics, and bioelectronics. Conserving the natural activity of these functional biological entities during their prolonged use is one of the major goals for validating their practical applications. Silk fibroin (SF) has emerged as a biocompatible material to interface with enzymes as well as whole cells. These biomaterials can be tailored both physically and chemically to create excellent scaffolds of different forms such as fibers, films, and powder for immobilization and stabilization of enzymes. The secondary structures of the SF-protein can be attuned to generate hydrophobic/hydrophilic pockets suitable to create the biocompatible microenvironments. The fibrous nature of the SF protein with a dominant hydrophobic property may also serve as an excellent support for promoting cellular adhesion and growth. This review compiles and discusses the recent literature on the application of SF as a biocompatible material at the interface of enzymes and cells in various fields, including the emerging area of bioelectronics and bioanalytical sciences.

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.

3D Flower-Like FeWO4/CeO2 Hierarchical Architectures on rGO for Durable and High-Performance Microalgae Biophotovoltaic Fuel Cells

A facile chemical reduction approach is adopted for the synthesis of iron tungstate (FeWO₄)/ceria (CeO₂)-decorated reduced graphene oxide (rGO) nanocomposite. Surface morphological studies of rGO/FeWO₄/CeO₂ composite reveal the formation of hierarchical FeWO₄ flower-like microstructures on rGO sheets, in which the CeO₂ nanoparticles are decorated over the FeWO₄ microstructures. The distinct anodic peaks observed for the cyclic voltammograms of studied electrodes under light/dark regimes validate the electroactive proteins present in the microalgae. With the cumulative endeavors of three-dimensional FeWO₄ microstructures, phase effect between rGO sheet and FeWO₄/CeO₂, highly exposed surface area, and light harvesting property of CeO₂ nanoparticles, the relevant rGO/FeWO₄/CeO₂ nanocomposite demonstrates high power and stable biophotovoltaic energy generation compared with those of previous reports. Thus, these findings construct a distinct horizon to tailor a ternary nanocomposite with high electrochemical activity for the construction of cost-efficient and environmentally benign fuel cells.

Improved performance of carbon nanotubes embedded photomicrobial solar cell

Enhancing the energy efficiency of power out is a key issue of microorganisms based energy harvesting. Here, we introduced carbon nanotubes (CNTs) into a photomicrobial solar cell (PMSC) system in order to increase the harvesting energy power. Microcystis aeruginosa was used as a solar energy converter, microorganism. It revealed that when a small amount of CNTs (e.g. 0.001 wt%) were added in the cyanobacterium suspension, the photocurrents were enhanced dramatically. The optical and electrical properties of the CNT suspension were analyzed. The biochemical features of the PMSC were evaluated under dark and light conditions. This study is expected to offer a strategic way for harvesting living cell-based solar energy in a more efficient manner.

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.

Facile fabrication of carbon brush with reduced graphene oxide (rGO) for decreasing resistance and accelerating pollutants removal in bio-electrochemical systems

Low electrode resistance is crucial for achieving efficient reactions in bio-electrochemical system (BES), especially considering the factors of BES scaling-up and microbial effects. Graphene has revealed a cornucopia of potential applications due to its high conductivity and extraordinary electrochemical properties. Here, significant reduction of electrode resistance and increment of electrochemical activity were achieved by fabricating the three-dimensional carbon brush using reduced graphene oxide (rGO/carbon brush) through one-step electro-deposition without any binder. The rGO/carbon brush was employed as cathode in BES equipped with bio-anode for azo compound (AO7) removal. The charge transfer resistances of cathode part and whole cell were decreased by 89% and 65%, respectively. The reactor showed quickly start-up within 48 h with peak cycle current six fold increase relative to the control. AO7 decolorization efficiency reached 91.1 ± 0.1% at 4 h and 97.6 ± 0.4% at 6 h. Effective decolorization of AO7 was at rate up to 650.7 g AO7/m³·h. The results indicated that the advantages of graphene and three-dimensional carbon brush successfully improved the overall performance of BES and enhanced refractory pollutants removal when applied to specific wastewater.

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.

Porous translucent electrodes enhance current generation from photosynthetic biofilms

Some photosynthetically active bacteria transfer electrons across their membranes, generating electrical photocurrents in biofilms. Devices harvesting solar energy by this mechanism are currently limited by the charge transfer to the electrode. Here, we report the enhancement of bioelectrochemical photocurrent harvesting using electrodes with porosities on the nanometre and micrometre length scale. For the cyanobacteria Nostoc punctiforme and Synechocystis sp. PCC6803 on structured indium-tin-oxide electrodes, an increase in current generation by two orders of magnitude is observed compared to a non-porous electrode. In addition, the photo response is substantially faster compared to non-porous anodes. Electrodes with large enough mesopores for the cells to inhabit show only a small advantage over purely nanoporous electrode morphologies, suggesting the prevalence of a redox shuttle mechanism in the electron transfer from the bacteria to the electrode over a direct conduction mechanism. Our results highlight the importance of electrode nanoporosity in the design of electrochemical bio-interfaces.

Some microorganisms are able to generate electrons that can be externally harvested. Here the authors show an increase by two orders of magnitude in the photocurrent when two cyanobacterial strains are grown on nanopourous transparent conducting substrates, compared to traditional solid substrates.

Interaction of bacteria and archaea in a microbial fuel cell with ITO anode

A microbial fuel cell with an indium tin oxide (ITO) coated glass anode was used to study the mechanism of electricity generation and electron transfer of electrochemically active microbes (EAMs). A simple method of ITO anode pretreatment (pickling) was developed to improve the performance of the microbial fuel cell. After proper treatment, ITO-glass anodes maintained their conductivity with a slight increase in resistance. Using this pickling pretreatment, the ITO-glass microbial fuel cell with an anode area of only 8.3 cm², was successfully initiated and obtained a stable voltage and power output of 418.8 mW m⁻². The electrode material with pretreatment showed optimal performance for the in situ study of EAMs. DNA was extracted from various parts of the reactor and the microbial communities were analyzed. The results indicated that the large proportion of methane-related microbes on the cathode of the MFC was one of the reasons for its high COD removal and low columbic efficiency. ITO glass is suitable as an anode material for the in situ study of EAMs, and shows potential for practical application.

A microbial fuel cell with an indium tin oxide coated glass anode was used to study the mechanism of electricity generation and electron transfer of electrochemically active microbes.

A Synthetic Biology Approach to Engineering Living Photovoltaics

The ability to electronically interface living cells with electron accepting scaffolds is crucial for the development of next-generation biophotovoltaic technologies. Although recent studies have focused on engineering synthetic interfaces that can maximize electronic communication between the cell and scaffold, the efficiency of such devices is limited by the low conductivity of the cell membrane. This review provides a materials science perspective on applying a complementary, synthetic biology approach to engineering membrane-electrode interfaces. It focuses on the technical challenges behind the introduction of foreign extracellular electron transfer pathways in bacterial host cells and the past and future efforts to engineer photosynthetic organisms with artificial electron-export capabilities for biophotovoltaic applications. The article highlights advances in engineering protein-based, electron-exporting conduits in a model host organism, E. coli, before reviewing state-of-the-art biophotovoltaic technologies that use both unmodified and bioengineered photosynthetic bacteria with improved electron transport capabilities. A thermodynamic analysis is used to propose an energetically feasible pathway for extracellular electron transport in engineered cyanobacteria and identify metabolic bottlenecks amenable to protein engineering techniques. Based on this analysis, an engineered photosynthetic organism expressing a foreign, protein-based electron conduit yields a maximum theoretical solar conversion efficiency of 6-10% without accounting for additional bioengineering optimizations for light-harvesting.

Photosynthetic Microbial Fuel Cells

This chapter presents the current state of research on bioelectrochemical systems that include phototrophic organisms. First, we describe what is known of how phototrophs transfer electrons from internal metabolism to external substrates. This includes efforts to understand both the source of electrons and transfer pathways within cells. Second, we consider technological progress toward producing bio-photovoltaic devices with phototrophs. Efforts to improve these devices by changing the species included, the electrode surfaces, and chemical mediators are described. Finally, we consider future directions for this research field.

A review into the use of ceramics in microbial fuel cells

Microbial fuel cells (MFCs) offer great promise as a technology that can produce electricity whilst at the same time treat wastewater. Although significant progress has been made in recent years, the requirement for cheaper materials has prevented the technology from wider, out-of-the-lab, implementation. Recently, researchers have started using ceramics with encouraging results, suggesting that this inexpensive material might be the solution for propelling MFC technology towards real world applications. Studies have demonstrated that ceramics can provide stability, improve power and treatment efficiencies, create a better environment for the electro-active bacteria and contribute towards resource recovery. This review discusses progress to date using ceramics as (i) the structural material, (ii) the medium for ion exchange and (iii) the electrode for MFCs.

An investigation of anode and cathode materials in photomicrobial fuel cells

Photomicrobial fuel cells (p-MFCs) are devices that use photosynthetic organisms (such as cyanobacteria or algae) to turn light energy into electrical energy. In a p-MFC, the anode accepts electrons from microorganisms that are either growing directly on the anode surface (biofilm) or are free floating in solution (planktonic). The nature of both the anode and cathode material is critical for device efficiency. An ideal anode is biocompatible and facilitates direct electron transfer from the microorganisms, with no need for an electron mediator. For a p-MFC, there is the additional requirement that the anode should not prevent light from perfusing through the photosynthetic cells. The cathode should facilitate the rapid reaction of protons and oxygen to form water so as not to rate limit the device. In this paper, we first review the range of anode and cathode materials currently used in p-MFCs. We then present our own data comparing cathode materials in a p-MFC and our first results using porous ceramic anodes in a mediator-free p-MFC.

Iron reduction by the cyanobacterium Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803 uptakes iron using a reductive mechanism, similar to that exhibited by many other microalgae. Various bio-electrochemical technologies have made use of this reductive cellular capacity, but there is still a lack of fundamental understanding of cellular reduction rates under different conditions. This study used electrochemical techniques to further investigate the reductive interactions of Synechocystis cells with Fe(III) from the iron species potassium ferricyanide, with varying cell and ferricyanide concentrations present. At the lowest cell concentrations tested, cell reduction machinery appeared to kinetically limit the reduction reaction, but ferricyanide reduction rates were mass transport controlled at the higher cell and ferricyanide concentrations studied. Improving the understanding of the reduction of Fe(III) by whole cyanobacterial cells is important for improving the efficiencies of technologies that rely on this interaction.

A micro-sized bio-solar cell for self-sustaining power generation

Self-sustainable energy sources are essential for a wide array of wireless applications deployed in remote field locations. Due to their self-assembling and self-repairing properties, "biological solar (bio-solar) cells" are recently gaining attention for those applications. The bio-solar cell can continuously generate electricity from microbial photosynthetic and respiratory activities under day-night cycles. Despite the vast potential and promise of bio-solar cells, they, however, have not yet successfully been translated into commercial applications, as they possess persistent performance limitations and scale-up bottlenecks. Here, we report an entirely self-sustainable and scalable microliter-sized bio-solar cell with significant power enhancement by maximizing solar energy capture, bacterial attachment, and air bubble volume in well-controlled microchambers. The bio-solar cell has a ~300 μL single chamber defined by laser-machined poly(methyl methacrylate) (PMMA) substrates and it uses an air cathode to allow freely available oxygen to act as an electron acceptor. We generated a maximum power density of 0.9 mW m(-2) through photosynthetic reactions of cyanobacteria, Synechocystis sp. PCC 6803, which is the highest power density among all micro-sized bio-solar cells.

A High Power-Density, Mediator-Free, Microfluidic Biophotovoltaic Device for Cyanobacterial Cells

Biophotovoltaics has emerged as a promising technology for generating renewable energy because it relies on living organisms as inexpensive, self-repairing, and readily available catalysts to produce electricity from an abundant resource: sunlight. The efficiency of biophotovoltaic cells, however, has remained significantly lower than that achievable through synthetic materials. Here, a platform is devised to harness the large power densities afforded by miniaturized geometries. To this effect, a soft-lithography approach is developed for the fabrication of microfluidic biophotovoltaic devices that do not require membranes or mediators. Synechocystis sp. PCC 6803 cells are injected and allowed to settle on the anode, permitting the physical proximity between cells and electrode required for mediator-free operation. Power densities of above 100 mW m^-2 are demonstrated for a chlorophyll concentration of 100 μM under white light, which is a high value for biophotovoltaic devices without extrinsic supply of additional energy.

Synergetic effect of conductive polymer poly(3,4-ethylenedioxythiophene) with different structural configuration of anode for microbial fuel cell application

PEDOT was synthesized by chemical polymerisation and characterised for its electrochemical insights. Three different anode configuration, namely graphite plate (GP), carbon cloth (CC) and graphite felt (GF) were then loaded with a fixed amount of PEDOT (2.5 mg/m(2)) denoted as GP-P, CC-P and GF-P respectively. The PEDOT coating improved the electrochemical characteristics and electron transfer capabilities of the anodes. They also contributed for enhanced MFC performances with maximum energy generation along with coulombic efficiency than the unmodified anodes. The morphological characteristics like higher surface area and open structure of felt material promoted both microbial formation and electrochemical active area. A maximum current density of 3.5A/m(2) was achieved for GF-P with CE and COD of 51% and 86% respectively. Thus, the GF-P anode excelled among the studied anodes with synergetic effect of PEDOT coating and structural configuration, making it as a potential optimum anode for MFC application.

Trapping of redox-mediators at the surface of Chlorella vulgaris leads to error in measurements of cell reducing power

The reduction of the redox mediator ferricyanide, [Fe(CN)6](3-), by a range of algal and bacterial species, is frequently measured to probe plasma membrane ferrireductase activity or to quantify the reducing power of algal/bacterial biofilms and suspensions. In this study we have used rotating disk electrochemistry (RDE) to investigate the reduction of ferricyanide by the model organism Chlorella vulgaris. Importantly, we have seen that the diffusion limited current due to the oxidation of ferrocyanide, [Fe(CN)6](4-), at the electrode decreased linearly as C. vulgaris was added to the solution, even though in a pure ferrocyanide solution the algae are not able to reduce the mediator further and are simply spectator 'particles'. We attribute this effect to trapping of ferrocyanide at the cell surface, with up to 14% of the ferrocyanide missing from the solution at the highest cell concentration. The result has important implications for all techniques that use electrochemistry and other concentration dependent assays (e.g. fluorescence and colourimetry) to monitor ferrocyanide concentrations in the presence of both biofilms and cell suspensions. Analyte trapping could lead to a substantial underestimation of the concentration of reduced product.

A comprehensive review of microbial electrochemical systems as a platform technology

Microbial electrochemical systems (MESs) use microorganisms to covert the chemical energy stored in biodegradable materials to direct electric current and chemicals. Compared to traditional treatment-focused, energy-intensive environmental technologies, this emerging technology offers a new and transformative solution for integrated waste treatment and energy and resource recovery, because it offers a flexible platform for both oxidation and reduction reaction oriented processes. All MESs share one common principle in the anode chamber, in which biodegradable substrates, such as waste materials, are oxidized and generate electrical current. In contrast, a great variety of applications have been developed by utilizing this in situ current, such as direct power generation (microbial fuel cells, MFCs), chemical production (microbial electrolysis cells, MECs; microbial electrosynthesis, MES), or water desalination (microbial desalination cells, MDCs). Different from previous reviews that either focus on one function or a specific application aspect, this article provides a comprehensive and quantitative review of all the different functions or system constructions with different acronyms developed so far from the MES platform and summarizes nearly 50 corresponding systems to date. It also provides discussions on the future development of this promising yet early-stage technology.