Parallel-flow reactant separation for high-performance membrane-free zinc–manganese microfluidic flow batteries

Microfluidic redox flow batteries (μRFBs) have emerged as promising membrane-free energy storage systems due to their enhanced mass transport characteristics and precise control of microscale fluid dynamics; however, their performance remains strongly constrained by electrolyte crossover and flow instability. Here, we systematically investigate the coupled effects of microchannel geometry, flow rate, and electrode spacing on the hydrodynamic and electrochemical behavior of a graphene-based μRFB employing a Zn/Mn redox couple. A three-dimensional computational fluid dynamics model was developed to analyze co-laminar flow and species transport within rectangular microchannels, revealing that channel dimensions and operating conditions critically govern micromixing, reactant utilization, and energy conversion efficiency. Excessive mixing, driven by longer residence times and specific channel aspect ratios, was found to accelerate electrolyte crossover and reduce electrochemical performance. Using a Taguchi design-of-experiments approach, an optimized microchannel geometry was identified and subsequently validated through fluorescence-based micromixing measurements. The electrode spacing plays a decisive role in device performance; although reducing the inter-electrode gap lowers ohmic resistance, excessively narrow spacing enhances cross-stream species transport and activation losses. The findings establish fundamental design principles for optimizing μRFB architectures and provide a robust framework for developing high-performance, scalable, and flexible microfluidic energy storage systems.