There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Microbial fuel cell (MFC) research is a rapidly evolving field that lacks established terminology and methods for the analysis of system performance. This makes it difficult for researchers to compare devices on an equivalent basis. The construction and analysis of MFCs requires knowledge of different scientific and engineering fields, ranging from microbiology and electrochemistry to materials and environmental engineering. Describing MFC systems therefore involves an understanding of these different scientific and engineering principles. In this paper, we provide a review of the different materials and methods used to construct MFCs, techniques used to analyze system performance, and recommendations on what information to include in MFC studies and the most useful ways to present results.

Microbial Fuel Cells: Methodology and Technology†

The use of microbial fuel cells to generate electrical current is increasingly being seen as a viable source of renewable energy production In this Progress article, Bruce Logan highlights recent advances in our understanding of the mechanisms used by exoelectrogenic bacteria to generate electrical current and the important factors to consider in microbial fuel cell design There has been an increase in recent years in the number of reports of microorganisms that can generate electrical current in microbial fuel cells Although many new strains have been identified, few strains individually produce power densities as high as strains from mixed communities Enriched anodic biofilms have generated power densities as high as 69 W per m2 (projected anode area), and therefore are approaching theoretical limits To understand bacterial versatility in mechanisms used for current generation, this Progress article explores the underlying reasons for exocellular electron transfer, including cellular respiration and possible cell–cell communication

Exoelectrogenic bacteria that power microbial fuel cells

https://www.cell.com/trends/biotechnology/pdf/S0167-7799(05)00092-2.pdf

Microbial fuel cells: novel biotechnology for energy generation

With the approaching commercialization of PEM fuel cell technology, developing active, inexpensive non-precious metal ORR catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost. This review paper highlights the progress made over the past 40 years with a detailed discussion of recent works in the area of non-precious metal electrocatalysts for oxygen reduction reaction, a necessary reaction at the PEM fuel cell cathode. Several important kinds of unsupported or carbon supported non-precious metal electrocatalysts for ORR are reviewed, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, conductive polymer-based catalysts, transition metal chalcogenides, metal oxides/carbides/nitrides/oxynitrides/carbonitrides, and enzymatic compounds. Among these candidates, pyrolyzed transition metal nitrogen-containing complexes supported on carbon materials (M–Nx/C) are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalysts. Although great progress has been achieved in this area of research and development, there are still some challenges in both their ORR activity and stability. Regarding the ORR activity, the actual volumetric activity of the most active non-precious metal catalyst is still well below the DOE 2015 target. Regarding the ORR stability, stability tests are generally run at low current densities or low power levels, and the lifetime is far shorter than targets set by DOE. Therefore, improving both the ORR activity and stability are the major short and long term focuses of non-precious metal catalyst research and development. Based on the results achieved in this area, several future research directions are also proposed and discussed in this paper.

A review on non-precious metal electrocatalysts for PEM fuel cells

The performance of oxygen reduction catalysts (platinum, pyrolyzed iron(ll) phthalocyanine (pyr-FePc) and cobalt tetramethoxyphenylporphyrin (pyr-CoTMPP)) is discussed in light of their application in microbial fuel cells. It is demonstrated that the physical and chemical environment in microbial fuel cells severely affects the thermodynamics and the kinetics of the electrocatalytic oxygen reduction. The neutral pH in combination with low buffer capacities and low ionic concentrations strongly affect the cathode performance and limit the fuel cell power output. Thus, the limiting current density in galvanodyanamic polarization experiments decreases from 1.5 mA cm(-2) to 0.6 mA cm(-2) (pH 3.3, E(cathode) = 0 V) when the buffer concentration is decreased from 500 to 50 mM. The cathode limitations are superposed by the increasing internal resistance of the MFC that substantially contributes to the decrease of power output. For example, the maximum power output of a model MFC decreased by 35%, from 2.3 to 1.5 mW, whereas the difference between the electrode potentials (deltaE = E(anode) - E(cathode)) decreased only by 10%. The increase of the catalyst load of pyr-FePc from 0.25 to 2 mg cm(-2) increased the cathodic current density from 0.4 to 0.97 mA cm(-2) (pH 7, 50 mM phosphate buffer). The increase of the load of such inexpensive catalyst thus represents a suitable means to improve the cathode performance in microbial fuel cells. Due to the low concentration of protons in MFCs in comparison to relatively high alkali cation levels (ratio C(Na+,K+)/C(H+) = 5 x E5 in pH 7, 50 mM phosphate buffer) the transfer of alkali ions through the proton exchange membrane plays a major role in the charge-balancing ion flux from the anodic into the cathodic compartment. This leads to the formation of pH gradients between the anode and the cathode compartment.

Challenges and Constraints of Using Oxygen Cathodes in Microbial Fuel Cells

Application of pyrolysed iron(II) phthalocyanine and CoTMPP based oxygen reduction catalysts as cathode materials in microbial fuel cells

The concept of employing microbial catabolic activity to directly generate electricity from the degradation of organic matter provides access to cheap and environmentally friendly energy sources. The energy conversion can be achieved with the help of microbial fuel cells, in which mostly fermentative microorganisms serve as biocatalysts and from which electrons are diverted and transferred to an electrode to generate electricity. Finding an efficient way to “wire” the microbial activity to an electrode is the key to the success of microbial fuel cells.[1] Hitherto, electricity generation was limited to current densities of about 20 mAcm 2 achieved by the use of dissolved artificial redox mediators[2] and 120 mAcm 2 by using the oxidation of metabolic products at platinum black anodes.[3] Herein we report a microbial fuel cell that continuously generates a current output more than one order of magnitude larger than the known microbial fuel cells (up to 1.5 mAcm 2). The novel fuel cell concept uses polymer-modified catalytically active anodes which shuttle electrons from the bacterial suspension to the anode. Until now three concepts have been proposed to wire microbial catabolic activity to electrodes for in situ electricity generation. The differences in these concepts is found at the stage at which electrons are diverted from the microbial catabolic path, and consequently in the way that the electrons are captured: 1) Dissolved artificial redox mediators serve as electron shuttles that penetrate the bacterial cells, divert electrons from the respiration chain and from internal metabolites, and transfer electrons to the fuel cell anode.[4] Current densities of between 5–20 mAcm 2 were reported.[5] However, the low current output, environmental and cost concerns disfavor this concept. 2) Metal-reducing bacteria, such as Shewanella putrefaciens that have special cytochromes bound to their outer membrane, can pass electrons directly to an electrode, which mimics the presence of metal ions as the terminal electron acceptors making the presence of artificial electron mediators obsolete.[6] However, the current and power output are rather low. Recently, Park and Zeikus could increase the current output to 16 mA cm 2 by using resting cells of Shewanella putrefaciens and enhancing the electron transfer to the anode by using Mn4+ ions or neutral red modified anode materials.[7] 3) Fermentation products like hydrogen, methanol, or ethanol have been used for in situ electricity generation. In this case, platinum black electrodes were used as electrocatalytically active anode materials; one such example, a microbial fuel cell based on the hydrogen evolution by immobilized cells of Clostridium butyricum was reported.[3] Short circuit currents of 120 mAcm 2 were reached by using lactate as the substrate. The fuel cell concept reported herein overcomes the problems of the previous microbial fuel cells. Central parts of the concept are a novel layered multifunctional anode consisting of a metallic electrocatalyst (present in the form of platinum or platinized carbon cloth electrodes) covered by an electrocatalytic conductive polymer (e.g., polyaniline, poly(neutral red), poly(methylene blue)) and a regenerative-potential program applied to maintain the long-term electrode activity. The cyclic voltammograms of a polyaniline modified platinum electrode immersed in a stirred anaerobic culture of Escherichia coli K12 (Figure 1) demonstrate the electrocatalytic behavior of the electrode material at different stages of

A Generation of Microbial Fuel Cells with Current Outputs Boosted by More Than One Order of Magnitude

Basic Electrochemistry. The electrical double layer and its structure. Thermodynamics of electrochemical reactions. Kinetics of electrochemical reactions.- Electroanalytical techniques. Cyclic voltammetry. Pulse voltammetry. Square-wave voltammetry. Chronocoulometry. Electrochemical impedance spectroscopy. UV/Vis/NIR-Spectroelectrochemistry. Stripping voltammetry. Electrochemical studies of solid compounds and materials. Potentiometry.- Electrodes and electrolytes. Working electrodes. Reference electrodes. Electrolytes. Experimental setup.- Publications in Electrochemistry.- Seminal Publications in Electrochemistry and Electroanalysis.- Textbooks, monographs, reference books, series, journals and WWW sources.

Fritz Scholz

Papers

Challenges and Constraints of Using Oxygen Cathodes in Microbial Fuel Cells

Application of pyrolysed iron(II) phthalocyanine and CoTMPP based oxygen reduction catalysts as cathode materials in microbial fuel cells

A Generation of Microbial Fuel Cells with Current Outputs Boosted by More Than One Order of Magnitude

Electroanalytical Methods : Guide to Experiments and Applications

Exploiting complex carbohydrates for microbial electricity generation: a bacterial fuel cell operating on starch