Poised potentials

Most often, the tabulated values of formal potential are given with respect to the normal hydrogen electrode (NHE), which has the defined potential 0 V. However, in practise, a silver/silver chloride (Ag/AgCl) electrode or a plain metal surface (e.g. Au or Pt) is commonly used as a RE. An Ag/AgCl RE, having an internal electrolyte of saturated KCl, has a characteristic potential of 197 mV with respect to the NHE. A plain metal surface, on the other hand, does not have a characteristic potential that can be expressed in terms of NHE. Instead, its potential depends on the prevailing conditions, affected by the deposited species and the electrolyte. E.g., if a Au surface is used as an RE to adjust the potential of, for instance, another Au surface (WE), both the RE and WE are affected by the same conditions. The equilibrium potential between such electrodes is ideally 0 V and a poised potential is directly an overpotential with respect to the equilibrium potential. 

Srikanth S, Mohan SV, Sarma PN. Positive anodic poised potential regulates microbial fuel cell performance with the function of open and closed circuitry. Bioresour Technol... 

Wang X, Feng YJ, Ren NQ, Wang HM, Lee H, Li N, Zhao QL. Accelerated start-up of two-chambered microbial fuel cells effect of anodic positive poised potential. Elec-trochim Acta 2009 54 1109-1114. 

The sequence of reactions in which the cytochromes participate is a mechanism for transferring electrons to molecular oxygen via iron complexes that are alternately in ferric and ferrous states. The order of the transfer has been deduced from studies with inhibitors, in which the electron-transport chain is broken so that components below the break are reduced, those above are oxidized from studies with poised potentials, in which the relative degrees of oxidation and reduction define the oxidation-reduction potentials of the various components and from rapid kinetic measurements, in which the order of reduction or oxidation can be seen. These methods agree on the following sequence ... 

Propionibacterium freudenreichii in a poised-potential amperometric culture system. Arch Microbiol 153 506-512... 

In this case, the electrochemical reactions are forced to happen at the working electrode under the influence of an externally poised potential controlled by a potentiostat. 

Betaproteobacteria. Rhodoferax ferrireducens is an iron-reducing bacterium that was isolated from subsurface sediments, and shown to produce power in MFCs (Chaudhuri and Lovley 2003). Using glucose as an electron donor, 6.8 to 33 mW/m (graphite rods, carbon felt or foam electrodes) was produced with a two-chamber system. Coulombic efficiency was 83% with a poised potential working electrode and was 81% using ferricyanide at the cathode. Fructose, sucrose, and xylose also produced power. No mixed cultures were examined in the apparatus for comparison of power densities. 

While most Geobacter spp. are not known to produce mediators, there is evidence for mediator production by Geothrix fermentans based on MFC tests using poised potential anodes (0.2 V vs. Ag/AgCl). When the medium was replaced in an MFC that had stable power generation with this isolate, power dropped by 50% and required 10 days to resume the original level. This observation was in contrast to experiments with Geobacter sulfurreducens which immediately produced comparable power levels with fresh medium. Filtrate from the anode chamber suspension with G. fermentans (obtained... 

Poised potentials. In order to conduct more detailed electrochemical analyses of MFCs, a potentiostat is essential. These can be obtained from various vendors e.g., Ecochemie, The Netherlands Princeton Applied Research, USA Gamry Scientific, USA Uniscan, UK) and may have one or more channels for monitoring a system. Each channel consists of a three-electrode setup with a reference electrode, a working electrode (anode or cathode), and a counter electrode which is usually Pt. Using a potentiostat, the current or the potential (but not both) in a circuit can be set at a defined value, allowing study of the system under a well controlled condition. 

Only one study has been done specifically to examine the effect of a poised potential on the response of the bacterial biofilm. Finkelstein et al. (2006) set the potentials of anodes in sediment fuel cells at three potentials meant to mimic different reduction reaction potentials (vs. Ag/AgCl) at the cathode under acetate oxidizing conditions -0.058 V (arsenate, As0/7As03 ), 0.103 V (selenite, SeOs /Se ), and 0.618 V (O2/H2O). They found the microorganisms obtained most of the energy (95%) from oxidation of the acetate. When they switched each system to an open-circuit condition, the potentials dropped only 0.040-0.050 V in each case (i.e., they did not all drop to the same value). However, they allowed only 30 s for the system to obtain an OCV, and thus the final potential that each reactor would have reached given more time is not known. Still, this study does suggest that the mixed culture adapted in each case to the set potential—as the bacteria would have to do in order to use the electrode as a terminal electron acceptor. 

Two-chamber reactors with soluble catholytes or poised potentials... 

Poised potential reactors. In a reactor with oxygen as the electron acceptor at the cathode, the working potential is typically ca. 0.25 V, with the anode at ca. -0.2 V, producing 0.45 V (see pig. 3,1 ) In a poised potential experiment, the anode potential can be set at any value using a potentiostat. Once that is done, the bacteria see a terminal... 

When a poised potential is used, energy is put into the system. For example, if the working potential for the anode was -0.3 V but the potential was poised at 0.2 V, then the input power would be P = 0.5 /, where I is the current achieved in the experiment. Thus, power is input into this system directly in a non-sustainable manner, as opposed to indirectly using the chemical energy of catholytes such as ferricyanide. Another potential disadvantage of this set up is that H2 gas produced in the cathode chamber. If this gas diffuses back into the anode chamber, it can become another substrate for the bacteria which can artificially raise the Coulombic efficiency or power production. 

Emde, R., Swain, A. and Schink, B. (1989) Anaerobic oxidation of glycerol by Escherichia coli in an amperometric poised-potential culture system. Appl. Microbiol. Biotechnol. 32, 170-175. 

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