When an experimental parameter such as the initial electron donor concentration is changed, the current can then be correlated without the effect of a varying elec- trode potential. Reactor configurations with an ion-selective membrane Fig. For membrane-less reactor configurations, the membrane potential loss is eliminated [67, ].
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The disad- vantage is that the supporting electrode reaction products are free to diffuse to the biofilm electrode. This could potentially generate uncontrolled experimental param- eters. Chapter 5 gives several example cases. In membrane-less microbial electrolysis cells, the diffusion of hydrogen from the supporting electrode to the working elec- trode can be utilized by EABs to produce a current higher than that expected with the supplied electron donor [77, 78]. Regardless of the use of ion-selective mem- branes, potentiostatic systems provide more control over electron transfer than the MFC mode of operation Fig.
There is often a misconception that the use of a potentiostat damages the EABs on a biofilm electrode. Potentiostats allow users to set the biofilm electrode potential to a wide range of values, including those that would irreversibly damage the EAB. In reality, it is usually inexperienced users who damage EABs with potentiostats because such an outcome was not possible in an MFC configuration.
The knowledge of which electrode limits the current is critical when BESs are studied.
The ins and outs of microorganism–electrode electron transfer reactions
The simplest way to determine which electrode in an MFC setup is the current-limiting electrode is to monitor the individual elec- trode potentials using an RE Fig. When the resistor load is changed, the current-limiting electrode will undergo a significant change in electrode potential, whereas the non-current-limiting electrode will not .
The current-limiting electrode will always be the highest priority and the electrode in need of immediate improvement. The current-limiting electrode concept also applies to potentiostatic systems Fig. The potentiostatic mode controls the biofilm electrode potential such that perturbations of the biofilm electrode potential cause a measurable change in the EAB under investigation.
Thus, the current is controlled by the EAB.
Microbial Community Characterization on Polarized Electrode Surfaces — Penn State
If, at any time, reactions at the supporting elec- trode affect the EAB under investigation, the controlled electrode cannot be called the limiting electrode, and steps must be taken to ensure that the effect of the supporting electrode can be assumed to be negligible. This is especially important in BESs that place the biofilm electrode of interest and the supporting electrode in the same solu- tion Fig.
For fundamental electron transfer investigations, this concept cannot be ignored. There is no consensus on the exact magnitude of the potential to apply; however, there is a clear understanding that applying a polar- ization potential more positive than the OCP of the biofilm electrode is sufficient to drive electrons from the EAB to the biofilm electrode. The concept of an optimal polarization potential is misleading, as the polarization potential can be limited by external factors such as the energy efficiency of the BES . The preferred polar- ization potential when electron transfer from an EAB is studied must be explored experimentally and chosen from a range of polarization potentials from near OCP to a few hundred millivolts more positive than OCP.
For example, if maximum cur- rent is desired, then a polarization potential that is in the current-limiting region for EABs current independent from polarization potential should be used . The polarization potential could also be used to select for different types of EABs, with different abilities for extracellular electron transfer .
However, for use in MFCs and other BESs, the polarization potential should be comparable to what is observed in the actual selection process or application. Interestingly, this is one of the more important concepts for studying extracellular electron transfer in SMFC applications. There, conditioning of anodes and cathodes would dramatically affect the performance. Therefore, techniques have been developed to adjust the potential of the electrode to a desired value incrementally, often slowly, to mimic the behavior seen during power generation.
The method of acclimatization affects the type of EAB grown on the biofilm electrode and can be focused on control of the current or of the biofilm electrode potential. Four acclimatization methods are common in the literature: 1. Closed circuit: The biofilm electrode and the supporting electrode are short cir- cuited or connected across a resistor. Open circuit: The biofilm electrode is left disconnected. Controlled cell potential: A constant potential is applied between the biofilm electrode and the supporting electrode. Controlled electrode potential: A constant polarization potential is placed between the biofilm electrode and the RE.
Closed circuit and open circuit acclimatization are the simplest methods to config- ure. Closed circuit acclimatization allows the biofilm electrode to reach a steady state cell potential and is focused on enhanced steady state electron transfer. The choice of resistor controls the amount of current allowed to pass [79, 99, ]. Chapter 1 1 gives an example of this procedure being used to acclimatize the cathode and anodes of an SMFC. Open circuit acclimatization allows the biofilm electrode potential to develop a steady state OCP utilizing natural redox processes in the environment.
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Controlled cell potential acclimatization and controlled electrode potential acclima- tization require powered external equipment irrespective of the natural redox pro- cesses in the environment. Controlled cell potential acclimatization ensures steady state electron transfer at a researcher-specified level. Controlled electrode potential acclimatization ensures steady state electron transfer irrespective of the supporting electrode.
Both methods allow the user to expose the system to potentials not normally sustainable or possible. However, only controlled electrode potential acclimatization gives the researcher direct and consistent control of the biofilm electrode potential. To choose one method over the other, critical decisions must be made.
Firstly, does the researcher prefer to select for biofilm processes or natural redox processes that can take advantage of an applied potential or polarization potential? Does the researcher prefer to produce electrodes that reflect only the natural redox processes? When a current is passed through an electrode to or from its surroundings, this will affect the state of redox processes around it.
The choice of how electrodes are acclimatized affects the end result and should be reported clearly. Growing EABs refers normally to pure cultures in the laboratory and can be achieved using two distinct acclimation methods. We should note that acclimatization refers to field experiments, whereas acclimation refers to laboratory experiments. This method is in line with open circuit acclimatization because no polarization potential is required for EAB growth.
Once the EAB has reached a desired state thickness, surface coverage, metabolic activity, and OCP , the soluble electron acceptor can be removed and the EAB can be switched to respiring on the biofilm electrode where a current is measured. The second method is to grow the EAB on an electrode that acts as the sole electron acceptor.
This method is in line with closed circuit, controlled cell potential, and controlled electrode potential acclimatization in which the current produced reflects biofilm growth. Once an EAB reaches a desired state thickness, surface coverage, metabolic activity, and current , it can be used for further investigation. Both methods are able to produce laboratory-scale EABs; however, the EABs resulting from these two methods have different biofilm properties and electron transfer capabilities.
Most likely, this is due to the acclimation of the EAB to each electron acceptor. Nevin et al. We have also observed structural differences between EABs grown using the two methods in our laboratory. Once the correct reactor configuration has been chosen and the EAB has been successfully grown or acclimatized on an electrode, the next step is to study the electron transfer properties of the biofilm electrode using electrochemi- cal techniques.
There are many electrochemical techniques available; however, we introduce only those frequently used to study electron transfer processes in EAB literature. In this way, the total charge transferred in a batch system or the steady state current produced by the EABs in a continuous system can be measured. A long-term electrode polar- ization experiment identifies sustainable current generation that can be systematically related to controlled parameters such as polarization potential.
While the technique appears similar to controlled electrode potential acclimatization, in which EABs are grown on polarized electrodes, the intents of the two are distinct and should be dis- tinguished. For example, G. Subsequently, the sustainable current productions of the two strains were measured using long-term electrode polarization experiments. The key value of performing electrode polarization experiments is to observe the electrochemical response of an EAB to a change in a controlled experimental parameter or the effects of acclimatization methods.
When a significant current above background and noise levels is generated by an EAB, such as that shown in Figure 1. To confirm that the current generated is related to the metabolism of the EAB or its metabolic by-products, certain long-term electrode polarization experiments can be performed. In batch experiments, the total consump- tion of the electron donor can be correlated to the total charge transferred. If the current trends toward zero when the electron donor concentration goes to zero, then the oxidation of the electron donor is the source of electrons and the coulombic efficiency can be calculated , For example, the current production by G.
In continuous experiments, the electron donor feed concentration can be altered and subsequent steady state current values measured. Replacing the bulk solution with fresh growth medium during electrode polarization experiments has been done to probe soluble extracellular electron transfer mechanisms.
Related Biofilms in bioelectrochemical systems : from laboratory practice to data interpretation
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