Spatially non-uniform reactions are a pervasive yet unresolved feature of redox flow battery (RFB) electrodes, limiting their utilization and scalability. For decades, its mechanistic origins have been obscured by the inability to directly probe pore-scale transport under operating conditions. As a result, electrode design has relied largely on bulk performance metrics, overlooking the decisive role of internal flow accessibility in governing reaction patterns.
Here, we deliver the direct, operando mapping of coupled flow, mass transport, and electrochemical reaction inside working RFB electrodes with pore resolution. We fuse high-speed fluorescence microscopy with a combined framework—lattice Boltzmann (LBM) to reconstruct three-dimensional flow topology, and finite volume method (FVM) to simulate reactive species transport and kinetics under realistic operating regimes. Using a fluorescent probe, we resolve the spatiotemporal concentration fields in carbon felt, cloth, and paper electrodes, linking observed reaction heterogeneity directly to internal convective pathways predicted by simulation. Under comparable operating conditions, carbon felt—with its broader pore size distribution and more heterogeneous network—facilitated more uniform reactant distribution and reaction. In contrast, carbon cloth and carbon paper, possessing narrower pore size distributions and more regular microstructures, displayed pronounced concentration gradients and stronger spatial heterogeneity in reaction.
This combined experimental–computational framework provides a quantitative link between electrode structure, internal flow topology, and reaction distribution, offering guidance for the design of RFB electrodes with improved transport characteristics and utilization. It can be readily extended to other porous electrochemical systems to elucidate structure–transport–reaction interactions under realistic operating conditions.
 

Flow battery; electrode kinetics; porous electrode,porescale