The versatility of Microbial fuel cells (MFCs) has meant that the technology can now branch towards a growing variety of applications. One of the big challenges however, has not been the microbial community but the identification of materials that allow the organisms to thrive and ultimately produce power. Paper has recently been adopted as a viable material that functions both structurally and as the medium for proton exchange. To date; the most reported use for paper-based MFCs has been as a diagnostics tool, however it is highly desirable to develop paper MFCs as lightweight, portable power supplies. In a diagnostic role, the paper MFCs reported have been low-power and short-lived (lasting just minutes) which is not suitable if the role were as power supply.

There have been reports of longer-term electrical outputs using paper-based MFCs (with air-cathodes) such as origami stacks¹ and 3D-tetrahedron MFCs². However, in both cases the method for feeding was via careful liquid injection. This is all very well in a lab-based environment but in a real-world scenario MFCs might need activating quickly and without delicate feeding requirements. Here it would be advantageous to simply distribute onto pools of liquid and have them passively intake the fuel. The goal of the current study therefore was to look at paper-based MFCs and investigate whether they might be able to passively intake nutrients from the surrounding environment.

Two MFC designs were looked at; the first being flat 2D-MFCs with the anode on the underside of the paper, directly in contact with the liquid and the cathode on top, open to air. The second design was 3D-tetrahedron MFCs constructed from standard copier paper but with an absorbent cellulose material incorporated into the base. For all MFCs, air-cathodes were used without platinum or ferricyanide.

For the 2D-flat MFCs, two methodologies were tested; the first was to print electrodes on either side of the paper and the second were hand-made, three-layered structures with conductive latex cathodes painted on one side and a sheet of carbon fibre adhered to the other. Each MFC was approximately 2cm x 4cm. In all experiments, there was no inoculation prior to the MFC being placed on puddles of enriched wastewater.

The MFCs with printed electrodes reached peak OCV of 300mV which quickly dropped. In closed circuit the current peaked at 2.4 µA before rapidly declining which is clearly unsuitable for real-world use. The reason for the poor performance was the dissolution of the electrode as the liquid displaced the conductive elements as verified by the significant increase in resistance after use. Further work with printed MFCs will investigate incorporating a stabilising material to prevent the electrodes dissolving.

The flat hand-made MFCs fared much better. Interestingly the conductive-latex cathodes were more resistive than the printed ones yet the MFCs were superior and more stable over time. These MFCs climbed and stabilised for 4 days at 11 µW (185mV). Different types of paper were trialled including baking, greaseproof and copier and all performed comparably. Further work will investigate stacking multiple flat-MFCs on single sheets of paper.

The simple flat-MFCs are promising but for MFCs operating outside and tapping into nutrients in puddles they should ideally have enclosed chambers housing the anode. To trial this, 3D-tetrahedron MFCs with 15mL volume were set up to sit on pools of liquid with the only method of feeding via capillary motion of the absorbent base.

The 3D-MFCs with absorbent bases were compared to MFCs with a waterproof coating over their base. Those with the absorbent bottoms immediately generated a current and continued increasing in power for over 2 weeks peaking and stabilising at 40µW (2.7 W/m³). In addition, when fresh nutrient was added to the reservoir (not directly to the MFCs), they responded almost immediately, a factor that could be advantageous if the role were biosensor. The MFCs with plastic coated bases showed no working voltage throughout the period. The output generated by the MFCs relying on a passive feeding mechanism is comparable to that produced by the same MFCs with sealed bases from a previous study (where injection feeding took place²). This output is sufficient to initiate a power management system and broadcast radio signals. These findings are an exciting development because lightweight paper-MFCs could potentially be dropped onto puddles of organic liquid, passively sucking up nutrients from the environment and subsequently broadcasting distress signals.

References

¹Fraiwan et al., (2016) Biosens Bioelectron 85: 190-197

²Winfield et al., (2015) J Mater Chem A 13: 7058-7065