In operando visualization of redox flow battery in membrane-free microfluidic platform

Abstract

Redox flow batteries (RFBs) are attractive large-scale energy storage techniques, achieving remarkable progress in performance enhancement for the last decades. Nevertheless, an in-depth understanding of the reaction mechanism still remains challenging due to its unique operation mechanism, where electrochemistry and hydrodynamics simultaneously govern battery performance. Thus, to elucidate the precise reactions occurring in RFB systems, an appropriate analysis technique that enables the real-time observation of electrokinetic phenomena is indispensable. Herein, we report in operando visualization and analytical study of RFBs by employing a membrane-free microfluidic platform, that is, a membrane-free microfluidic RFB. Using this platform, the electrokinetic investigations were carried out for the 5,10-bis(2-methoxyethyl)-5,10-dihydrophenazine (BMEPZ) catholyte, which has been recently proposed as a high-performance multiredox organic molecule. Taking advantage of the inherent colorimetric property of BMEPZ, we unravel the intrinsic electrochemical properties in terms of charge and mass transfer kinetics during the multiredox reaction through in operando visualization, which enables theoretical study of physicochemical hydrodynamics in electrochemical systems. Based on insights on the electrokinetic limitations in RFBs, we verify the validity of electrode geometry design that can suppress the range of the depletion region, leading to enhanced cell performance.

Advances in Microfluidic Technologies for Energy Storage and Release Systems

“…The modeling results revealed bottlenecks in the depletion region that hindered the electrochemical redox reactions and based on that, the redesign of the electrode geometry to a tapered design was proposed for enhanced battery performance (Figure 12f ). [159] For microfluidic microbial fuel cells, the in situ visualization of biofilm formation was investigated as well. Differing from batteries, in microbial fuel cells there is a start-up process whereby bacteria will form a biofilm on the anode and this formation of biofilm has a direct influence on the performance.…”

“…The modeling results revealed bottlenecks in the depletion region that hindered the electrochemical redox reactions and based on that, the redesign of the electrode geometry to a tapered design was proposed for enhanced battery performance (Figure 12f). [ 159 ]…”

“…Reproduced under the terms of the CC‐BY license. [ 159 ] Copyright 2022, The Authors. Published by United States National Academy of Sciences.…”

“…f ) Comparison of the battery's performance when using different electrode geometries. Reproduced under the terms of the CC-BY license [159]. Copyright 2022, The Authors.…”