Microfluidic Redox Battery with Symmetric, Dual-Pass Architecture

Microfluidic fuel cells and microfluidic redox batteries hold promises for future power generation of portable devices. In these membrane-less cells, a co-laminar interface in the micro-channel provides the separation between the reactants whilst permitting ionic charge transfer [1]. The use of flow-through porous electrodes allowed higher power densities and increased fuel utilization [2]. Recently, a dual-pass architecture utilizing flow-through porous electrodes was reported [3]. The cell achieved power densities up to 0.3 W/cm²using vanadium redox species at high flow rates and enabled in-situ recharging. Vanadium is a special element that has four different oxidation sates. Hence, it is commonly used in both electrolytes of a redox flow battery as well as in recent microfluidic electrochemical cells [4-5]. 

In this study, a microfluidic redox battery (MRB) with flow-through porous electrodes and a symmetric, dual-pass design is presented. The design of the cell is capable of both recharging and regenerative use of the reactant species and therefore, maximizing the fuel utilization. The architecture presented is an extension of our previously reported work [3], with the added modification of splitting each of the two electrodes into two sections, as shown in Fig. (1a). This modification essentially divides the original MRB into a full upstream cell and a full downstream cell which can be in-situ characterized independently.  The advantage of this analytical cell design is a more detailed understanding of reactant conversion in the dual-pass architecture of the MRB which will help with device optimization.

The cell was fabricated using soft lithography techniques in polydimethylsiloxane (PDMS) using an SU-8 master and bonded to a glass substrate. Micro-porous carbon paper is used to make rectangular electrode strips for the device. The vanadium redox electrolytes were prepared as described elsewhere [6]. 

First, preliminary experiments are conducted to test the performance of the upstream cell in order to benchmark the new cell design against regular cells. The cell has an open circuit voltage around 1.51 V at a high flow rate of 100 μL/min. The full polarization and power curves for the upstream cell operated at the same flow rate is presented in Fig. (1b). These results demonstrate the successful operation of the cell in the discharge mode, where it produced up to 2.5 mW at a current of 4 mA. Moreover, the power output and the fuel utilization of the device are evaluated for both series and parallel connections of the upstream and downstream cells. These comparisons will enable quantification of the coupling between the upstream and downstream cells and evaluation of asymmetric flow separation at the downstream stagnation point of the device.

Acknowledgements

 Funding for this research provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) is highly appreciated.

 References

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[3]     J. W. Lee, M.-A. Goulet, and E. Kjeang, Lab on a Chip, 13, 2504-2507 (2013).

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[5]     J. W. Lee, J. K. Hong, and E. Kjeang, Electrochimica Acta, 83, 430–438 (2012).

[6]     J. W. Lee, and E. Kjeang, Journal of Power Sources, 242, 472-477, (2013).