Team:Pretoria UP/Hardware
Hardware To test the attachment of the thylakoids and to prove the functionality of the laccase, a working prototype of the PBEC fuel cell was needed. The design methodology used when developing the PBEC fuel cell structure is shown in Figure 1. The design process followed the general sequence as indicated in the figure. It should, however, be mentioned that design is an inherently iterative process, and therefore the procedure was repeated many times both forwards and backward until a satisfactory design was obtained. Stages 1 and 2 are used to gather as much information about the cell, to assist with any gaps in information about PBEC cells. At stages 3 and 4, results from the literature survey were used to create design requirements that are used to solve the problem presented in stage 1. Lastly, stage 5 is used to finalize the design. The purpose of this page is to illustrate the entire design process used to create the structure of the PBEC cell. Included is how feedback from potential users, as well as the expert opinion, was used to improve the design of the cell. Thus integrating human practices within the design process of the cell.
Literature survey
What is a fuel cell? A fuel cell is a device that converts chemical energy directly into electrical energy. Fuel cells come in many shapes, sizes and forms. One common feature, however special the cell’s design may be, is that they all produce an electrical current through a chemical reaction between two electrodes, specifically the cathode and anode. Fuels cells also have an electrolyte that assists in the transport of ions from the anode to the cathode. These electrolytes come in various guises, from Polymer Electrolyte Membranes (PEM) to liquid electrolytes such as trycine. Each with their own respective advantages and disadvantages. Lastly, most fuel cells generally have a catalyst that either assist the chemical reaction or increases the rate of electron transport.
Design specifications
The three biggest questions that arose during the design specification phase were: whether the cell required a PEM; Should the cell be isolated from the environment and lastly what material should the cell and electrodes be made from? The answers to these questions were obtained from questionnaires and expert opinion, shown later.
Concept generation
An initial concept was developed based on design specifications that were assumed from above-mentioned questions. The concept comprised of a rectangular cell housing with two separate compartments to house the anode and cathode, respectively. The electrodes would sit in each compartment. The cell would be manufactured out of Perspex or acrylic glass, from plates that were cut to size and glued together using epoxy. The two compartments would be separated by a PEM. It was further assumed that the cell was to be left open during testing. Figure 2 shows the initial hand sketch of the concept.
It was decided that the concept would be built into a prototype for testing. Perspex was kindly donated by Mr Van Niekerk at Mr Plastic Group Bedford view. The Perspex was measured and cut using a milling machine (See Figure 3 and Figure 4). The pieces were glued together to form one-half of the cells compartment shown in Figure 5. A piece of Nafion 117 (PEM) was then sandwiched between a 3D printed frame also shown in Figure 5.
Before testing the cell, the PEM must undergo a deoxidizing process to remove any oxides that may have formed on the membrane. Unfortunately, the epoxy used to glue the PEM to the frame was incorrect and the PEM “burnt” during the process of deoxidizing. It was clear that the design needed to be upgraded and therefore this concept was scrapped.
Visit our Human Practices - Gold page where the results of the surveys are summarised. Furthermore, we consulted Dr S. Radhakrishnan* who is an expert in the field. In this consultation, the initial concept was discussed in which Dr Radhakrishnan gave her opinion on the design and gave advice on ways to improve the overall efficiency and functionality of the PBEC cell. It was Dr Radhakrishnan who highlighted the fact that the epoxy used was incorrect and suggested that silicon should be used for all gluing purposes. Dr Radhakrishnan then advised us against the use of Perspex to construct the cell as it is at its very best only a translucent material and would not allow for the optimal light transmittance. Lastly, Dr Radhakrishnan recommended that the overall design of the cell be updated to allow for the electrodes to sandwich the PEM.
Detail design
The new concept was shown to Dr Radhakrishnan for her final opinion. She agreed that the new design would be more efficient than the initial concept. The concept was then transformed into the final design. The 3D printed frame of the new PBEC cell was designed with the following overall dimensions shown in Figure 7:
The final design comprised of two 3D printed plates. A glass window was added on either and stuck into place with silicon. The two frames fit into each other with the top frame that has a raised ridge that fits snugly in a crest in the bottom frame. This ensures the cell is water tight. The cell was further sealed with silicon to ensure complete water tightness. The design allowed for the use of four countersunk screws and bolts to hold the two frames together. Figure 8 show a rendering of the final design of the PBEC. The graphene electrodes fit snugly in their own compartments. PBEC compounds can be added to the fluid channels which double as the electrode wire ports.
The CAD model used to print the top and bottom frame can be obtained upon request from the team engineers. Figure 9 shows the final PBEC that was tested. This prototype was shown to produce a maximum current of 100 µA.
Results
The PBEC cell that was designed was tested following the same procedure that was layed out in a paper by Calkins et al,2013. The cell consisted out of 2 graphene foam electrodes, forming the anode and cathode. Thylakoids were attached to the anode electrode using PBSE. Whereas laccase was attached to the cathode also using PBSE. Glycine, instead of tricine, was used as a pH buffer to stabilize the cell. The electrodes were then put into the cell together with potassium ferrocyanide. Unfortunately, this was all done in a room with lights, thus the process of photosynthesis was already initiated.
The cell was put on ice while being transported to the lab where the current measurements were taken. Two triaxial probes were inserted into the PBEC cell fluid channels after which lab view was used to initiate the measuring process and log the data. The cell was tested in a dark faraday case with all light sources off for 400 seconds, refer to point 1 in Figure 10. Then, a growing LED light strip was switched on at point 2. At this point it ws observed that the current increased by 2 µA. Since the thylakoids were already activated, a significant increase was not seen, the light source was placed 60cm directly above the cell. After 16 minutes the LED strip was lowered to 5cm above the cell. An 8 µA increase was observed at point 3. Since the LED strip is also radiating heat the thylakoid cells began to die, which can be seen by the decrease in current up to point 4. The cell was then placed into a dark ice container to forcefully stop the thylakoids function which resulted in the sudden current drop of 0 A. The cell was removed from the ice at point 5 and put under the light, 5cm away, during which the thylakoids started to function again. The current then gradually decreased, a possible reason can be that the LED light’s heat resulted to the thylakoid dying and no current was observed anymore.
Electrodes are one of the most important components of the fuel cell. They serve as the link between the chemical reaction and the circuit that requires power. Similar to the fuel cell, electrodes can come in an abundant range of shapes, sizes, and materials. The most common material used is copper, due to its properties as well as its high electrical conductivity. However, depending on the application, titanium, silver, platinum, as well as carbon-based electrodes, have also been successfully used.
Traditional fuel cells generate electricity from a chemical reaction between hydrogen and oxygen. Typically, hydrogen is sourced from a hydrocarbon rich fuel source such as coal or natural gas. During the reaction, hydrogen is oxidized and the ions produced are transported through the electrolyte to the cathode, where oxygen is reduced to produce water and heat. The electrolyte used mostly in this instance is the PEM. Earlier designs of the fuels cells required platinum electrodes, causing an increase in the cost to manufacture. This high cost in manufacturing and the reliance on fossil fuels for hydrogen sparked research in improving the fuel cells design as well as developing new fuel cells altogether. One such development was the microbial fuel cell.
Microbial fuel cells generate electricity by converting chemical energy stored in biological matter into electrical energy. The anodes in this type of cell are kept in an anaerobic environment, whilst the cathodes are kept in an aerobic environment. In these environments, microorganisms are used to oxidise an electron donating chemical, most typically water. These cells gained popularity in waste-water treatment when it was realized that effluent could be used to provide the necessary biomaterial to generate electricity. A further development of the cell included converting entire eutrophic lakes into fuel cells that were powered by the naturally occurring microorganisms found in the lake. Eutrophic lakes’ anaerobic and aerobic layers, formed due to sedimentation, makes conditions ideal for the lake water’s treatment.
A further development of the microbial fuel cell gave rise to the photo-bioelectrochemical fuel cells. These devices use sunlight to power the oxidizing microorganisms. There are two distinct versions of the cell namely PBEC fuel cell and a PBEC solar cell. In both instances, photonic biomaterials are used as either a catalyst or electron producer. The biomaterials used in the cell can come in the form of whole plant cells, subcellular plant cells or various light absorbing plant protein complexes such as PSII. Cyanobacteria have also been used extensively to generate an electric current in the presence of light. The electrodes used mostly in PBEC fuel cells are carbon based electrodes.
To PEM or not to PEM? Proton exchange membranes, also known as Polymer Electrolyte Membranes or PEM are used as an electrolyte in cells, assisting the transport of hydrogen ions from the anode to the cathode of the cell. Like most electrolytes, the membrane helps regulate the pH in either side of the cell chambers. PEM is far more efficient than the salt bridge used in earlier designs of the fuel cell. Depending on the application, PEM is not necessarily required if a liquid electrolyte is used. The internal resistance to electron transfer of the fuel cell can be increased slightly by opting for the liquid electrolyte instead of the PEM. It was decided that the first concept would use a PEM.
During research of current fuel cells, it was found that in most instances, fuel cells were kept isolated from the environment. However, it was still not clear whether this was a necessary design requirement. Leaving the cells open to the environment can leave the cell at risk of becoming contaminated. The biggest concern was whether products produced during the chemical reaction would stay inside the cell or be exposed to the environment. To test this, it was decided that the initial concept would be tested open to the lab environment.
The last of the three questions was what materials should the cell structure be assembled from. Materials used in literature ranged from using micromachined silicon to glass lab beakers with custom designed fuel cell housings to contain the reaction. In most cases, fuel cells require pumps to replenish spent fuel and remove unwanted products from the reaction chambers. These designs, therefore, require special channels to allow the transport of these fluids and gases. In some instances, fuel does not need to be replenished and the products formed during the chemical reaction can be reused in a later reaction. It is for this reason that most fuel cells are manufactured out of plastic, as it can be easily formed into the various structures that are needed.
The materials that were available for electrode manufacturing was carbon nanowire, carbon nanotubes, graphene electrodes and graphene composite electrodes. These potential carbon-based electrodes could be sourced directly from the university. It was decided that graphene composite electrodes would be used to test the cells. A composite of graphene coated on nickel foam was used. The nickel foam provided support for the graphene. The electrodes were also chosen as the manufacturing method was cost-effective and allowed the production of multiple electrodes. An electrode of 4 cm x 4 cm was used for all testing purposes.
It was at this point that it was decided that the design of the structure required the opinion of those that would be potentially using the cell as well as to obtain advice on the design from an expert in the field.
The improved concept: integrating human practice with hardware design.
We gave a presentation SANEDI, during which a questionnaire was filled out by the attendees. We asked their opinion on the drawbacks of a photo-bioelectrochemical cell. Analysis of the questionnaires highlighted two main concerns, above all, that the attendees had with PBEC cells. The first of which was the sustainability of the cell regarding the materials used as well as the amount of water needed to power the cell. The second concern was considering the scalability of the cell. This translated to the cells efficiency and its ability to compete with current solar and fuel cells available today.
With the information collected, a new concept was developed. In this concept, Perspex was substituted for glass. Glass was chosen as it is transparent as well as being 100% recyclable, making it more sustainable than the Perspex option. Due to the long manufacturing processing of the previous concept, it was decided that the new structure would be 3D printable. This meant a larger number of cells could be produced in a much shorter time frame, thus improving the scalability of the project. Additionally, to improve the sustainability of the cell it was decided that the PEM would be substituted for a liquid electrolyte. This meant that the cells could be completely sealed from the environment, with water inside the cell regenerating itself as oxygen produced on the anode side of the cell would be reduced back to water on the cathode side. This meant that far less water is needed to run the cell. Figure 6 shows a schematic of the improved concept.
*Dr Shankara Radhakrishnan is a Research Fellow in the Department of Chemistry in the Faculty of Natural and Agricultural Sciences. She is currently doing research in physical chemistry. Her Research interests include Physical Chemistry, Materials Chemistry, Fuel Cell, Solar Cells, Surface and Interface Chemistry. She has authored and co-authored several publications in journals such as Journal of the American Chemical Society. Pretoria_UP iGEM team of 2016 would like to give a gigantic THANK YOU to Dr Radhakrishnan for her willingness to share her wealth of knowledge and for her patience with helping design the fuel cell. Also for the very kind donation of Nafion.
