Renewable energy production from waste using microbial fuel cell (MFC) technology is attracting increasing attention. MFCs are bio-electrical devices that use microorganisms as biocatalysts to convert chemical energy (stored in organic matter) into electrical energy. MFCs consist of a positive cathode and a negative anode, which are separated by a semi-permeable membrane. Microorganisms are inoculated in the anodic compartment and through substrate oxidation, release electrons to the anode electrode. The two electrodes are connected by an external circuit, which facilitates the flow of electrons from the anode to the cathode. One of the main contributors affecting the cost and performance of MFCs is the membrane, since these tend to be quite expensive, even though they are commercially available. To overcome this, alternative materials and configurations need to be identified. One design is the membrane electrode assembly (MEA) that improves power output by reducing the internal resistance. This study looks at 3D printing MFCs using novel extrude-able materials that can emerge from the Evobot platform (Figure 1). The focus is on the development of cost-effective MEA using extrude-able air-dry membranes painted with conductive paint.
Materials and Methods:
Twelve cubic analytical size MFCs were assembled with only one chamber forming the 25mL anode, so as to have an oxygen-diffusion cathode, whilst the membranes were glued to the anode chamber. For this experiment, three types of potentially extrude-able membranes were tested against a conventional CEM. These materials were two air-dry clays; Fimo and terracotta and standard terracotta clay (Figure 2A). The latter was kilned at a temperature of 1070 oC prior to use, to allow the structural bonding of the clay and ensure durability, whereas the rest were dried overnight at room temperature. The thickness of the tested membranes was consistent for all the custom made membranes (2.5mm). The control membrane required activation in 5% NaCl prior to use. A conductive graphite coating was applied to each membrane and formed the cathode electrode (Figure 2B). The coating was fabricated using polyurethane rubber coating (PlastiDip), white spirit and graphite powder. The membranes were coated with the conductive cathode mixture and the surface resistance was measured for each coating, until the lowest value was achieved (100-200 Ohms). After the membrane electrode assembly had dried, a cable was attached to the cathode using conductive wire glue, to form the cathodic current collector. The MFCs were then partially wrapped with Parafilm® to ensure moisture retention in the open-to-air cathode side (Figure 2C). All the cells were inoculated with activated sludge and fed with neat human urine collected anonymously from healthy individuals.
Results and Discussion:
Initially, the air-dry terracotta outperformed the other materials (70 µW), whereas the commercially available and most commonly used CEM was the least performing (30 µW). Air-dry Fimo and kilned terracotta were almost identical in terms of power output (50 µW). The experiment started with a 2.7 kΩ load, and although initially the air-dry clay was outperforming the rest, after fourteen days, both air-dry clay and fimo were identical. Following electrochemical analysis, the optimal external resistance was identified (1 kΩ). Once the MFCs were run at this resistance value the performance levels had clearly diverged and Fimo outperformed the other materials. The results from the polarisation experiment showed a difference with the real-time data, suggesting that the air-dry clay was the best performing with 123 μW, followed by Fimo with an output of 79 μW. However, in all cases, the soft materials were operating better than the conventional cation exchange membrane. The materials tested as alternative membranes come in the form of soft modelling clay, which makes these suitable for extrusion from the EVOBOT platform. As the original form of the electrode material is fluid, it can also be applied using EVOBOT by incorporating a brush/roller on the actuation layer of the robot. This will apply the conductive coating onto the dried extruded membranes, and help produce a uniform layer on the surface.
Conclusions:
The findings presented in this study demonstrate for the first time that soft materials cured in air can be used as membranes for MFCs, and in addition, even improve power output. This offers a great advantage over the conventional and expensive CEMs, and is a novelty in the MFC field. The EVOBOT robotic platform is flexible and can be modified to extrude such membranes. This is an exciting development and a step towards the overall goal of the EVOBLISS project, which is to monolithically 3D-print MFCs using EVOBOT.