Modelling and Experimental Validation of the Flow and Mixing Processes in a Microfluidic Membraneless X-Cell with Flowing Vanadium Electrolytes

Abstract

Microfluidic devices are characterized by submillimeter channel cross-sections and tiny flow rates, typically below 100 µL/min. This results in rather small Reynolds numbers that maintain the flow laminar and steady, preventing chaotic mixing from turbulence [1-4]. This orderly flow can be used to remove the ionomeric membrane in redox flow batteries, generating membraneless microfluidic cells with low internal electric resistance. These devices could be manufactured at a lower cost than their macro counterparts. However, without a membrane, the mixing region where the electrolytes come into contact must be accurately controlled to avoid unwanted interface deflections leading to increased cross-contamination of liquid and species.In this work, we compare experimental and CFD realizations of the flow in a membraneless microfluidic flow channel [5], including in the simulations the effect of viscosity variations across the positive and negative vanadium electrolytes. The apparatus includes a microfluidic control system that uses piezoelectric pumps and valves for flow regulation, an X-cell as flow channel, and small tanks with discharged positive (VIV) and negative (VIII) electrolytes. We use a UV/Visible spectrometer to measure the amount of mixing at the outlet of the cell [6]. This allows us to validate the CFD model against the experimentally measured cross-contamination and liquid transfer. To our knowledge, these results are unique in using spectrometry data to quantify the mixing ratio of VIII/VIV, and provide valuable data to validate CFD models that could later be used to engineer the flow in membraneless microfluidic vanadium redox flow batteries. Acknowledgments This work has been partially funded by FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación Projects PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033, and by Grant IND2019/AMB-17273 of the Comunidad de Madrid.