The performance of a microbial fuel cell (MFC) depends on a complex system of parameters. Apart from technical variables like the anode or fuel cell design, it is mainly the paths and mechanisms of the bioelectrochemical energy conversion that decisively determine the MFC power and energy output. Here, the electron transfer from the microbial cell to the fuel cell anode, as a process that links microbiology and electrochemistry, represents a key factor that defines the theoretical limits of the energy conversion. The determination of the energy efficiency of the electron transfer reactions, based on the biological standard potentials of the involved redox species in combination with the known paths (and stoichiometry) of the underlying microbial metabolism, is an important instrument for this discussion. Against the sometimes confusing classifications of MFCs in literature it is demonstrated that the anodic electron transfer is always based on one and the same background: the exploitation of the necessity of every living cell to dispose the electrons liberated during oxidative substrate degradation.

In this communication we discuss, by means of the metal reducing bacterium Geobacter sulfurreducens, a strategy to use cyclic voltammetry for the study of anodic bioelectrocatalytic electron transfer in microbial fuel cells.

The electrochemical properties of the room temperature ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM⁺BF₄ ⁻), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM⁺PF₆ ⁻) and 1-methyl-3-[2,6-(S)-dimethylocten-2-yl]imidazolium tetrafluoroborate (MDIM⁺BF₄ ⁻) as solvents have been studied using micro-samples, with a volume of 10 μL, of the ionic liquids under vacuum conditions and under conditions with controlled gas and moisture supplies. The impact of water—absorbed into the ionic liquid in a controlled manner from the gas phase—on the voltammetry of dissolved redox systems and on the accessible potential window of the ionic liquids was investigated. The diffusion coefficients for three representative redox systems, the oxidation of neutral N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), the reduction of cationic methyl viologen (MV²⁺) and reduction of anionic hexacyanoferrate(III), Fe(CN)₆ ³⁻, have been determined as a function of the water content of the ionic liquids. Water is shown to have a much more dramatic acceleration effect on the diffusion of the ionic compounds compared to its effect on neutral species in ionic liquids. A model based on nanoscale structural features of wet ionic liquid materials is proposed. The novel methodology, which employs redox-active compounds dissolved or partitioned in microdroplets of ionic liquid, uses conditions suitable for the study of ionic liquids for applications in electrochemical gas phase reactors and gas sensor systems.

Electron transfer between microorganisms and an electrode — even across long distances — enables the former to live by coupling to an electronic circuit. Such a system integrates biological metabolism with artificial electronics; studying these systems adds to our knowledge of charge transport in the chemical species involved, as well as, perhaps most importantly, to our knowledge of charge transport and chemistry at the cell–electrode interfaces. This understanding may lead to microbial electrochemical systems finding widespread application, particularly in the energy sector. Bioelectrochemical systems have already shown promise for electricity generation, as well as for the production of biochemical and chemical feedstocks, and with improvement are likely to give rise to viable applications.

Microbial electrochemistry is the study and application of interactions between living microbial cells and electrodes (i.e. electron conductors, capacitive materials). For a long time this subfield of bioelectrochemistry has been the interest of mainly fundamental researchers. This has considerably changed during the last decade and microbial electrochemistry gained interest from applied researchers and engineers. These researchers took the microbial fuel cell (MFC), which is a system that converts the chemical energy of organic material in wastewater into electric power, from a concept to a technology. In addition, a plethora of derivative technologies, such as microbial electrolysis cells (MECs), microbial desalination cells (MDCs), photomicrobial fuel cells (photoMFCs), microbial electrosynthesis (MES), and biocomputing have been developed. The growing number of systems is often referred to in literature under the termini bioelectrochemical system (BES), microbial electrochemical technology (MET), or electrobiotechnology. Within this article we introduce a classification of technologies based on interfacing microbiology and electrochemistry. We argue that BESs comprise all systems based on bioelectrochemistry, with a further layer of termini through the use of METs. Primary METs are based on extracellular electron transfer (direct or mediated), whereas secondary METs include systems in which electrochemistry is connected – at least through ionic contact – with a microbial process via the electrochemical control or adaptation of environmental parameters, such as pH or metabolite concentration level.