3D-Printable Cathode Electrode for Monolithically Printed Microbial Fuel Cells (MFCs)
Authors: Theodosiou Pavlina , Greenman John , Ieropoulos Ioannis 
Published: February 2020

Introduction
Biological fuel cells (BFCs) are an increasingly growing area of research as it beholds long-term sustainable advantages over conventional fuel cells. Microbial Fuel Cells (MFCs) are just one type of BFCs, which as the name implies, employ microbial electroactive species to facilitate the conversion of chemical energy stored in organic matter, into electricity. The properties of MFCs have successfully made the technology a primary source of energy for low-power autonomous robots 1 and off-grid urinal units 2. However, a hindrance to the mass production of MFC units is the time-consuming assembly process, which could perhaps be overcome using additive manufacturing (AM) processes. AM or 3D-printing has played an increasing role in advancing the MFC technology, by substituting essential structural components i.e. chassis and separators, with 3D-printed parts 3,4. This is precisely the line of work in the EVOBLISS project, which is investigating materials that can be extruded from the EVOBOT platform 5 for a potentially monolithically printed MFC. The development of such inexpensive, conductive, printable electrode material is described below as well as the advances of this material as a cathode electrode on air-breathing cathodes.
Material and Methods
Three triplicates of analytical size MFCs were assembled for this experiment using laser-cut acrylic sheets. The MFCs had a 25mL anode chamber, a CMI-700 cation exchange membrane (Membrane International, USA) as separator and three different electrodes forming the air-breathing cathodes. A gas diffusion electrode with polytetrafluoroethylene (PTFE) (60% wt. Sigma Aldrich, UK) painted carbon veil sheet that acted as the hydrophilic supporting material for a microporous layer (MPL) was used as the control. This was prepared with a mixture of activated carbon (80 g/120 mL solution. G Baldwin & Co, UK), PTFE and distilled water. The materials tested were a) a solid commercially available sintered Carbon Block CTO (Water Filter Man LTD, UK) and b) a custom made activated carbon-alginate paste which was made using ground activated carbon (80g) and alginate (Minerals Water Ltd, 20 g) that was then mixed with distilled water until a thick paste was made. The paste was then extruded from a syringe directly onto the membrane (10 ml) and dried/solidified on the bench for 24 hrs. The final weight of all the dried electrodes was 3.8 ± 0.2 g. The cells were inoculated with activated sludge (Wessex Water, UK) supplemented with full strength Tryptone Yeast Extract (1.5% w/v) and fed with human urine.
Results
The results showed that the MFCs using alginate electrode as cathode electron and oxygen receiver performed better compared to the MPL or sintered carbon having a maximum power transfer point at 286 μW, 98 μW and 85 μW respectively. An important factor to consider in the effort to improve the MFC performance is not only the power output but also the cost effectiveness of the materials, especially when using alginate. MPL cathode electrode requires a PTFE coated carbon veil sheet as well as a mixture of PTFE and carbon. PTFE is a highly toxic and expensive material (£138/500ml, Sigma Aldrich, 2017) compared to food grade alginate which only costs £8.76 per 500g. Moreover, the alginate electrode does not require a supporting material thus the cost was reduced further by removing the carbon veil from the assembly.
Conclusion
In conclusion, this experiment demonstrated that the development of an air-dried, extrude-able electrode material (similar to 3D printing) could successfully be incorporated in an MFC system and act as a cathode electrode. Such a development brings the field a step closer to monolithically printable MFCs, which can be made using the EVOBOT platform.
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