In-Situ Characterization of Microfluidic Redox Battery with Dual-Pass Architecture

Microfluidic co-laminar flow cells is a growing field of energy research due to their inherent simplicity and low cost. In these cells, the two reactant containing electrolytes flow in a co-laminar manner that is dominated by slow diffusive mixing, which in turn forms an interface that provides the necessary separation of the reactant streams whilst allowing ion transfer between the electrodes [1]. Introducing the flow-through porous electrodes allowed higher performance in terms of power output and fuel utilization [2]. Furthermore, a unique dual-pass architecture was recently demonstrated which enabled in-situ recharging, fuel recirculation, and significant performance improvements [3-4]. The dual-pass cell architecture was further modified by splitting the electrodes, thus forming two symmetric cell portions, in order to experimentally measure and analyze the parasitic shunt current that appears in microfluidic electrochemical cell arrays with a shared electrolyte manifold [5-6]. In the present work, the same cell array design is used as an analytical device in order to analyze the performance characteristics of the dual-pass architecture and its application in co-laminar flow cells.

The cell is fabricated using soft lithography techniques in polydimethylsiloxane (PDMS) using an SU-8 master and then bonded to a glass substrate. Porous carbon paper is cut into rectangular strips and placed as the electrodes for the device. Details about the vanadium redox electrolytes preparation and the device fabrication are described elsewhere [6].

Each cell portion is tested individually with vanadium redox species delivered at low and high flow rates and the results are used to quantify the species crossover losses, which result in a mixed potential and thus causes a drop in open circuit potential and overall performance. The crossover losses at the downstream portion are reduced from 41 mV at 10 μL/min to 13 mV at 100 μL/min. The upstream cell portion demonstrates maximum power density of 744 mW/cm² at a high current density around 1000 mA/cm². This compares favorably to all previously reported conventional counterparts.

Moreover, the two cell portions are connected in parallel to resemble the original cell with dual-pass architecture [3] which allows assessing the contribution of the inlet and outlet passes of the dual-pass architecture in-situ. The fuel utilization is estimated from the current outputs of the two portions at low and high flow rates, and the contribution of the downstream cell portion is found to be on the same order as that of the upstream portion. Overall, the results of this study are expected to provide a deeper understanding of the reactant conversion and reactant crossover phenomena in co-laminar flow cells which will be useful for future device optimization.

Acknowledgements

Funding for this research provided by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), and British Columbia Knowledge Development Fund is highly appreciated.

References:

[1] E. Kjeang, Microfluidic Fuel Cells and Batteries, Springer (2014).

[2] E. Kjeang, R. Michel, D. A. Harrington, N. Djilali, and D. Sinton, J. Am. Chem. Soc., 130, 4000-4006 (2008).

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

[4] M.-A. Goulet and E. Kjeang, Electrochim. Acta., 140, 217-224 (2014).

[5] O. Ibrahim, M.-A. Goulet and E. Kjeang, In the 226^th Meeting of the Electrochemical society, Cancun, Mexico, October 2014.

[6] O. Ibrahim, M.-A. Goulet and E. Kjeang, J. Electrochem. Soc., 162, F639-F644 (2015).