We planned to design a reactor system that is suitable to investigate the electrochemical behaviour in bioprocesses. That includes the possibility to characterize mediators and different electrode materials on the one hand and the electron uptake into the cells on the other.
During our research we discovered the H-cell reactor that seemed to meet with our needs. (Park et al., 1999)
We approached two different concepts to realize the reactor construction. One of our H-cell reactors was constructed with the possibilities given to us by the facillities of our university. We instructed the glass workshop to modify two glass bottles by adding a glass-flange. Besides that the technical workshop build the lids from stainless steel. This approach had the advantage that we could influenze the design and had to make precise design drawings especially for the connections in the lids.
The second H-cell reactor was a commercially available system by Adams & Chittenden scientific glass. The commercial system had a smaller volume and the benefit of a larger flange diameter. The necessary lids for that system were also custom design by our workshop. In figure 1 you can see both reactors in comparison.

Figure 1: 1 Custom designed lids that profide connections for: a pO₂-electrode, a pH-electrode, an entrance for reference and working electode, air output, heating coils and acid/ base input for pH control, 2 Heating coils, 3 Clamps for the flange connection 4 Sealing rings.

The H-cell is suitable for experiments concerning the investigation of mediator redox-characteristics and indirect electron transfer into electrotrophes.
In addition to the H-cell design we thought of an alternative reactor design that meets with the requirements of direct electron transfer. To enable direct electron transfer it is necessary that there is a large electrode surface provided to the microorganisms. Furthermore substrate limitation should be avoided. To meet with these requirements it is favourable to have an reactor that can be continiously driven. Our proposed solution is a flow cell reactor (FCR) which could be driven continiously.

Testing the set up

Our first experiments were carried out with a constant power supply and we measured the voltage input and the current. The set up is shown in figure 2.

Figure 2: Set up of our first experiments with the H-cell: 1 Ammeter 2 Power supply and voltmeter 3 Cathode compartment 4 Anode compartment

During our first experiments we filled both, the cathode- and the anode-space with phosphate buffer where neutral red was added to final concentration of 100 µM.
It turned out that we could not use the pH-electrode and the pO₂-electrode during our cultivations, because they affected the measurement. Especially the pO₂-electrode was not suitable in this set up, due to the fact that it is completely made of steel. It turned out that the electrode achieved a grounding of the system which set the lid under electric power. This resulted in a couple of unwanted oxidation processes at the weldseam of the lid. The consequence was to remove both electrodes from the system.

Different electrode materials

We tested different electrode materials for their potential to work in our reactor. We decided to investigate fabric carbon, fabric fleece als platinum electrodes. The different materials are shown in figure 3.

Figure 3: Different electrode materials for characterization experiments: 1 Carbon fabric 2 Carbon fleece 3 Platinum electrode.

Carbonic materials have the advantage that they are relativly cheap and are available in huge amounts. The nature of the processing of the material has a major influence on its electrochemical behavior.
Carbon fabric is made up of individual fibres and has therefore a good stability. Another advantage is that the fibres consist of one piece and therefore has a good electrical conductivity.
The carbon fleece instead is thicker and provides a larger surface for the microorganisms to attach to the electrode material. This advantage goes at expense of stability.

Cultivation - constant voltage

The first experiments in the H-cell reactor were performed under constant direct voltage. These experiments were performed to test the set up. We investigated if E. coli was able to grow in the needed voltage range and if the different mediators influence the cells if a small electric current is applied.

Cyclic voltammetry - mediator characterization

During our experiments we used a Ag/AgCl reference electrode for measuring the working electrode potential. The counter electrode, which completes the cell circuit, was made from platinum wire. Platinum has the advantage that it is an inert conducter.

Figure 4: 1 Ag/AgCl reference electrode 2 Platinum working electrode 3 Platinum wire counter electrode 4 Heating water system.

Figure 5:

Parameter	Value
Scan rate [mV-s]	35
step size [mV]	1
Scan limit E1 [V]	0.3
Scan limit E2 [V]	-0.6
Electrode material	Platinum
Aeriation	Oxygen free by aeriation with nitrogen

Chronoamperometry - current consumption

Figure 2: Overlay of the chronoamperometric measurements from cultivation A B and C.

Flow Cell

Figure 10: Basical set up of the FCR: 1 Hose pump 2 FCR connectet to an energy supply source and to the pump 3 Stock bottles for media and buffer.

Figure 11: Basical set up of the FCR: 1 Connection for the power supply cabel at the anode 2 Connection for the power supply cabel at the cathode 3 Hose connection nipple.

Figure 12: Individual components of the FCR: 1 Stable base for the cell 2 End cover plate with electrical plug for power supply 3 Separation partitions for anode- and cathode-space 4 Sealing rings 5 Screws for the fixation of the single parts 6 Carbonic electrode material.