During the Electrochemical Energy and Water Summit, held at the ECS Fall 2014 Meeting in Cancun, Mexico, we proposed the idea for the PowerPAD, a concept for an eco-friendly, biodegradable paper-based redox power source for water monitoring in low resource settings and the idea was awarded funding in the Science for Solving Society’s Problems Challenge. The concept leverages recent advances in capillary-based paper fuel cells¹ and microfluidic co-laminar flow cells with flow-through porous electrodes², as shown in Fig. 1.

In this work we present a flow battery made on cellulose and carbon materials which is completely safe and non-toxic, yet inexpensive. The power in this device is generated by flowing organic redox pairs through porous electrodes. The organic redox substances are stored in the paper device and they start flowing upon the addition of any available liquid, such as wastewater, seawater or even urine. The mass transport within the device relies entirely on capillary flow through the different porous materials. Once the cellulose paper is completely wet and the species oxidized/reduced, the device can be disposed with no environmental impact, as it does not contain any metals or harmful substances.

Typically, flow batteries have been considered power sources that can be used for a virtually unlimited number of cycles, as long as their redox species are recharged. Lately, the use of organic species, such as quinones, in redox flow batteries has been reported^{3, 4}. These approaches exploit the high faradaic efficiency of the system to develop large-scale electrical energy storage facilities. In contrast, our approach proposes to implement a quinone-based battery designed as a single-use disposable power source in which the energy delivered is tailored to feed a particular application.

The flow battery prototype has a central inlet pad able to receive a small volume of water (500 μL). All reactant species and supporting electrolytes needed to run the cell were stored in solid form within cellulose pads. The addition of water on the inlet pad dissolves the redox species, which flow through the porous carbon paper electrodes to an absorbent pad placed at the bottom of the device. The cell utilizes co-laminar flow to separate the half-cells and thus does not require a membrane or physical separator. After evaluating different quinone species, the Hydroquinonesulfonic acid potassium salt (H2BQS) as the anode and p-Benzoquinone (pBQ) as the cathode, were selected due to their solubility and rapid kinetics on carbon electrodes in the absence of metallic catalysts.  KOH and oxalic acid were correspondently used as cell electrolytes. Provided that the flow battery uses organic reactants in a membrane-less and catalyst-free cell design it does not contain any active materials that are harmful for the environment upon disposal.

A proof-of-concept flow battery was assembled and evaluated by measuring polarization curves continuously over the course of one hour. The open circuit voltage of the cell was also recorded between the discharge periods; a maximum power output of 1.1 mW at a current output of 3 mA was measured, which is deemed sufficient to run a small signal processor for applications such as water quality monitoring. Moreover, the discharge performance of the device was tested by connecting a load of 500 Ω for three hours and recording the cell potential. The power output was then calculated and plotted against time, confirming the ability of the cell to sustain a power output higher than 0.5 mW for around 3 minutes.

Acknowledgements

The funding for this research provided by the Electrochemical Society and the Bill & Melinda Gates Foundation is highly appreciated. J.P.E. thanks support from Marie Curie International Outgoing Fellowship (APPOCS) within the 7th European Community Framework. Additional support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI) and British Columbia Knowledge Development Fund (BCKDF) is also acknowledged.

 

References

1.            J. P. Esquivel, F. J. Del Campo, J. L. Gomez de la Fuente, S. Rojas and N. Sabate, Energy & Environmental Science, 2014, 7, 1744-1749.

2.            E. Kjeang, R. Michel, D. A. Harrington, N. Djilali and D. Sinton, Journal of the American Chemical Society, 2008, 130, 4000-4006.

3.            B. Huskinson, M. P. Marshak, C. Suh, S. Er, M. R. Gerhardt, C. J. Galvin, X. Chen, A. Aspuru-Guzik, R. G. Gordon and M. J. Aziz, Nature, 2014, 505, 195-198.

4.            B. Yang, L. Hoober-Burkhardt, F. Wang, G. K. Surya Prakash and S. R. Narayanan, Journal of The Electrochemical Society, 2014, 161, A1371-A1380.