Trace Minerals Recipe for Geobacter
VIDEO: How to Set Up a Microbial Fuel Cell. Research Scientist Michelle Young takes us step-by-step through the set-up of a microbial fuel cell.
Procedure for Making 50mM Acetate Media and Connecting the Media to the Reactor
Design of microbial electrochemical flow cells for microscopic analyses of electrode biofilms
Flow cells are common tools for studying and imaging biofilms on different surfaces. They consist of a flow chamber that allows for continuous medium feed and a glass coverslip viewing window for imaging the biofilm under a microscope. There are a few commercially available flow cells, but a microbial electrochemistry flow cell is not yet available and much more difficult to construct given the requirement for anaerobic conditions and the many electrical connections needed. We present below our own design, which we have used for about a year now (since 2013). This design has the option of using a flat metal anode or an ITO coverslip as your anode. You can either replicate this one, using the Solidworks file below, or get inspiration to build your own version!
The first thing you notice is many screws! The screws give rigidity to the flow cell and provide an equal pressure on the rubber gasket that prevents leakage. Notice the center screws are a bit skewed since we needed a side hole for the reference electrode right at the center of the flow cell (black arrow). The flow cell itself is made of 1.2 cm thick plexiglass and the front and back covers are aluminum plates of 0.2 cm thickness. The front cover has the viewing window that is 0.8 cm X 2.1 cm. It has a sloped surface to accommodate the size of the objective on the microscope. The bottom cover has threads for the scews. It also has a hole around the flow cell viewing window to avoid reflectance of the microscope beam. Also, it has a little hole for a sprig-loaded screw connection (more on this later). Let’s open it!
On the far side of the picture above, you can see the main rubber gasket used. It is ~1/16” thick silicone rubber gasket (McMaster Carr 5827T32). It is cut by hand to fit the flow cell and the screws/viewing window. In this picture, we have introduced the reference electrode, a Leakless Miniature Ag/AgCl Reference Electrode (Model ET072). This is introduced using a PEEK connector and the plexiglass has a thread for this connector. The same thread is used for the influent and effluent lines at each side of the flow cell and stainless steel connectors are used.
The first few times we used this flow cell, we realized there was a small leak at the viewing window, close to the glass coverslip. We added an extra thin gasket (~0.5 mm) around the coverslip to create a better seal.
The plexiglass has a small dip to accommodate a 0.2mm coverslip. The flow cell was designed for a 22 x 40 mm coverslip, as this is a standard size sold coated with ITO in the US. Notice the edges are rounded to allow for the coverslip to be pulled out. The flow cell chamber itself is 0.6 cm X 1.9 cm with a triangular shape along the inlet and outlet. The chamber is 0.8 mm deep, along with a 0.2 mm coverslip gives ~ 1 mm of imaging working distance if using the metal electrode. The electrode is placed close to the center of the chamber and is a stainless steel machined screw with a flat surface that has been sputter coated with gold. Notice the hole next to the electrode, which is the reference electrode connection. Also, on the side and outside of the chamber, there is a stainless steel spring-loaded plunger that can make a connection to an ITO coverslip. In future designs, we will move this plunger closer to the chamber. So…where is the cathode?!? The cathode is actually the effluent screw connection made of stainless steel. This way, the hydrogen produced at the cathode is not being produced inside the flow cell, but on the way out.
Both the gold-coated stainless steel anode and the spring-loaded plunger screw into the plexiglass and come out from the bottom of the flow cell, allowing an electrical connection to either one. This is advantageous for an inverted confocal microscope, like the one we have at SCEB.
Finally, we use a Long-distance 40x objective to image the biofilms on the gold-coated electrode. The one shown here has a working distance of up to 3.3 mm, which is larger than our 1 mm requirement. On the left is a picture of a Geobacter sulfurreducens biofilm grown on the flow cell.
Design of high surface area anode microbial electrolysis cells
As a part of our SERDP project, we are developing high surface area anode microbial electrochemical cells for the production of hydrogen peroxide coupled with oxidation of black water organics. We are first testing the design for hydrogen production in a microbial electrolysis cell. This post shows some of the key aspects of our design. One of the most important features of our design is to minimize the distance between the anode and the cathode, while still using high surface area electrodes. We have characterized that the Ohmic overpotential (apart from that coming from the membrane) in this design at current densities of 10 A/m2(geometric) is <50 mV. This MEC has an anode volume of ~500 mL. There are two anodes and two cathodes. The anode chamber is shared by two anodes. This replicates the modular approach that we believe is essential for scaling up MXCs. Please find below select photos showing the reactor design, along with a Solidworks file.
The cathode chamber: the volume is made up only by the gasket used.
The cathode: we have used SS and Ni meshes as cathodes.
The membrane: we suggest to use an anion exchange membrane between the anode and the cathode. This results in the transport of OH- ions from the cathode chamber to the anode chamber, to help ensure pH neutrality in the anode.
The anode: we used high surface area carbon fibers as the anode. To ensure a high surface area without increasing the third dimension that would result in a large effective distance between the anode and the cathode, we use a mesh-like configuration. The fibers are woven into a titanium current collector plate. The geometric surface area is 100 cm2.
The anode chamber: we design the anode chamber to be 500 mL here. However this was to ensure proper mixing of the anode with a magnetic stirrer. When this design is scaled up, the anode chamber can be designed to be significantly smaller in volume. This will ensure high volumetric current densities.
The anode chamber: we design the anode chamber to be 500 mL here. However this was to ensure proper mixing of the anode with a magnetic stirrer. When this design is scaled up, the anode chamber can be designed to be significantly smaller in volume. This will ensure high volumetric current densities.
The cathode: we use another cathode chamber to close off the MEC. The total cathode volume, including both chambers is 100 mL, while the geometric cathode area is 200 cm2 (100 cm2 each).
The completely assembled MEC. This MEC is capable of producing up to 600 mA of current.