Microbial cathode reactions

Babauta JT, Hsu L, Atci E, Kagan J, Chadwick B, Beyenal H. Multiple cathodic reaction mechanisms in seawater cathodic biofilms operating in sediment microbial fuel cells. ChemSusChem 2014 7 2898-2906. 

Electrical power can also be applied to the BES to increase the electrical potential between the anode and cathode electrodes and enable cathodic reactions (reviewed in [1]) not attainable in cathode-limited systems such as MECs. Energy is added to the system with an external power source or by poising the anode potential with a potentiostat. This type of BES is often referred to as a MEC because it enables the electrochemical reduction of protons to hydrogen at the cathode (reviewed in [8]). These BESs are especially suitable for microbial studies and have been used extensively for Geohflcter bacteria. For this reason, we describe the theory and practice of MEC operation using Geobacter cultures in detail in the following sections. 

Biofilms, Electroactive, Fig. 5 Principles of the most abundant microbial bloelectrochemlcal systems (a) microbial fuel cells and (b) microbial electrosynthesis cells on the example of the H2 production. (Note Here the anodic and cathodic reactions are catalyzed by blofilms, yet as described in the text also other catalysts can be exploited)... 

Pham, T.H. Improvement of cathode reaction of a mediatorless microbial fuel-cell. J. Microbiol. Biotechnol. 14 (2004), pp. 324-329. 

It will now be evident, from consideration of possible cathodic reactions, that corrosion cells may arise because of localized compositional differences in the electrolyte with respect to, for example, pH, dissolved O2, metal ions, oxyanions and the metabolism of microbial infections. 

Write a one-page summary of microbial fuel cells. Describe the anodic and cathodic reactions and electrolyte used. 

Z. He, Y. Huang, A.K. Manohar and F. Mansfeld, Effect of electrolyte pH on the rate of the anodic and cathodic reactions in an air-cathode microbial fuel cell, Bioelectrochemistry 74,2008,78-82. 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

A relatively high degree of corrosion arises from microbial reduction of sulfates in anaerobic soils [20]. Here an anodic partial reaction is stimulated and the formation of electrically conductive iron sulfide deposits also favors the cathodic partial reaction. 

Fuel cell applications Manganese dioxide as a new cathode catalyst in microbial fuel cells [118] OMS-2 catalysts in proton exchange membrane fuel cell applications [119] An improved cathode for alkaline fuel cells [120] Nanostructured manganese oxide as a cathodic catalyst for enhanced oxygen reduction in a microbial fuel cell [121] Carbon-supported tetragonal MnOOH catalysts for oxygen reduction reaction in alkaline media [122]... 

In Chapter 12 micro-organisms in suspension were referred to as "living colloids". It is to be expected therefore, that microbial fouling will be influenced by cathodic protection. Maines [1993] however, suggests that the picture is far from clear with respect to electrode material, types of organism and the technique adopted for cathodic protection. It would appear that in some situations biofouling is increased and in others it is reduced. Where a biofllm is present it would be expected to modify the electrochemical conditions at the metal surface and thereby influence the metal dissolution reactions [Dexter et al 1989]. 

Non-oxidizing biocides include a variety of organic compounds such as glutaraldehyde, quarternary ammonium salts (QUATS), isothiazolones etc. These compounds are less critical in terms of corrosion because they do not accelerate the cathodic partial reaction. Many of them are active over a wide range of pH and temperatures. THPS, a water soluble organic compound, has been found particularly effective for preventing microbial corrosion due to SRB in oilfields [33]. Biocide... 

Microbial electrocatalysis relies on microorganisms as catalysts for reactions occurring at electrodes. The microorganisms involved are able to transport electrons in and out of the cell, a process known as extracellular electron transfer (EET), and can catalyze both oxidation and reduction reactions [80, 81]. Their catalytic properties have been confirmed by the fact that they are able to lower the overpotentials (lower energy loss) at both anodes [82] and cathodes [56, 69], giving an increased performance of the system. Nevertheless, they cannot be considered as true catalysts since part of the substrate/electron donor is consumed for growth. 

Rhoads, A., Beyenal, H., and Lewandowski, Z. (2005) Microbial fuel cell using anaerobic respiration as an anodic reaction and biomineralized manganese as a cathodic reactant. Environ. Sci. Technol, 39 (12), 4666-4671. 

Bioelectrochemical Hydrogen Production, Fig.1 Schematic layout of a microbial electrolysis cell (MEC) showing the anode and cathode chamber, the electrodes with attached biocatalysts, the membrane separator and the power supply, as well as the anodic and cathodic half reactions. AEM anion exchange membrane, CEM cation exchange membrane... 

A microbial electrolysis cell is based on the concept of a microbial fuel cell (MFC) and consists of an anode and cathode chamber, a membrane that electrically separates the electrodes, and an external power supply [8] (Fig. 1). The anodic reaction is the same as in a microbial fuel cell ElectrochemicaUy active microorganisms oxidize organic compounds such as acetate, generating carbon dioxide (CO2), protons (H" ), and electrons (e ), using the anode as terminal... 

However, hydrogen production from acetate oxidation, as aimed for in an MEC, is thermodynamically not feasible. Indicated by the equilibrium potentials of the individual half reactions at microbial conditions (pH 7, acetate 1 M, 1 bar), the electromotive force (emf) (= cathode potential - anode potential) of this reaction is -0.14 V, which means that additional electrical energy is required to support electrolytic hydrogen formation (Fig. 2). This is provided by applying a circuit voltage that is... 

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