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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.

This paper provides a scaffold for the development of a clear and consistent terminology and classification of microbial electrochemistry and microbial electrochemical technologies.

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Subcritical water, that is, water above the boiling and below critical point, is a unique and sustainable reaction medium. Based on its solvent properties, in combination with the often considerable intrinsic water content of natural biomass, it is often considered as a potential solvent for biomass processing. Current knowledge on biomass transformation in subcritical water is, however, still rather scattered without providing a consistent picture. Concentrating on fundamental physical and chemical aspects, this review summarizes the current state of knowledge of hydrothermal biomass conversion in subcritical water. After briefly introducing subcritical water as a reaction medium, its advantages for biomass processing compared to other thermal processes are highlighted. Subsequently, the physical-chemical properties of subcritical water are discussed in the light of their impact on the occurring chemical reactions. The influence of major operational parameters, including temperature, pressure, and reactant concentration on hydrothermal biomass transformation processes are illustrated for selected carbohydrates. Major emphasis is put on the nature of the carbohydrate monomers, since the conversion of the respective polymers is analogous with the additional prior step of hydrolytic depolymerization.

The core of primary microbial electrochemical technologies (METs) is the ability of the electroactive microorganisms to interact with electrodes via extracellular electron transfer (EET), allowing wiring of current flow and microbial metabolism. Geobacter sulfurreducens and Shewanella oneidensis are the model organisms for understanding and engineering EET. Many other microorganisms are reported being electroactive but are often sparsely characterized. Based on a literature survey 94 species are ascribed as electroactive. Their apparent diversity raises questions on the natural importance and distribution of the EET capacity, that is, of the ecological niche of microbial electroactivity. To identify this potential niche the environmental preferences and natural habitat characteristics of all electroactive species were combined with their metabolic, growth and EET characteristics and an extensive meta‐analysis performed. The results indicate that there is not a single ecological niche for electroactive microorganisms. Significantly more electroactive species presumably exist in nature as well as already existing strain collections but due to current cultivation techniques their EET potential is not leveraged. Thus, in the light of specific traits required for industrial application, microbial resource mining based on ecological knowledge bears a great potential for broadening the foundation of microbial electrochemistry as well as for future developments of primary METs.

Microbial fuel cells (MFC) are the archetype microbial bioelectrochemical system (BES), producing electricity from microbially catalyzed anodic oxidation processes. The greatest potential of MFC lies in the use of wastewater as a substrate (fuel), which allows combining waste treatment and energy recovery. Recently, a development has been initiated that expands the scope of these bioelectrochemical systems from power generation to an increasing number of further applications. This development has become possible by the introduction of new cathode catalyst concepts. The corresponding devices, here summarized as MXCs--the X standing for the different types and applications--share one common element: the microbial anode. The cathode, however, has to fulfil rather different tasks and thus differs quite remarkably across these systems. In this critical review we analyze the different cathode tasks and the resulting requirements for the respective cathode and discuss the available catalyst options in the light of their major advantages and weaknesses. These catalyst options comprise inorganic, biomolecular as well microbial catalyst systems. Hereby, special emphasis is put on a comparative analysis of chemical and biological cathodes and their individual potentials and limitations. For this purpose, criteria are defined based on relevant properties (performance, price, longevity, etc.) and are evaluated by means of a multi-factor analysis, based on the individual target reaction and catalyst. This analysis is exemplarily elaborated for the oxygen reduction reaction (typical for MFCs) and for the hydrogen production (in MECs) (91 references).