Less can be more: Insights on the role of electrode microstructure in redox flow batteries from 2D direct numerical simulations

TL;DR

This work addresses how electrode microstructure influences transport and performance in redox flow batteries by performing pore-scale direct numerical simulations in 2D using an AMReX-based, embedded-boundary framework. By systematically varying lattice order, vacancy placement, and density gradients, the authors demonstrate that reducing reactive surface area via well-placed vacancies can paradoxically increase current density through improved species mixing and diminished shadowing, especially under higher applied voltages and flow rates. The key contribution is the identification of concrete design rules—vacancy location and density-gradient strategies—that outperform both regular and disordered microstructures, offering a path toward optimized, high-performance RFB electrodes. The work also provides a scalable computational tool to explore microstructure–transport relationships, with potential extensions to 3D and optimization-driven electrode design that can guide experimental fabrication efforts. In sum, the paper delivers both methodological advancements in pore-scale simulations and practical insights into how

Abstract

Understanding how to structure a porous electrode to facilitate fluid, mass, and charge transport is key to enhance the performance of electrochemical devices such as fuel cells, electrolyzers, and redox flow batteries (RFBs). Using a parallel computational framework, direct numerical simulations are carried out on idealized porous electrode microstructures for RFBs. Strategies to improve electrode design starting from a regular lattice are explored. We observe that by introducing vacancies in the ordered arrangement, it is possible to achieve higher voltage efficiency at a given current density, thanks to improved mixing of reactive species, despite reducing the total reactive surface. Careful engineering of the location of vacancies, resulting in a density gradient, outperforms disordered configurations. Our simulation framework is a new tool to explore transport phenomena in RFBs and our findings suggest new ways to design performant electrodes.

Figures (10)

• Figure 1: Schematic of (a) full redox flow battery and (b) half-cell with an example of electrode microstructure (experimentally realizable with direct ink printing method). (c) Boundary conditions employed in this study. At the inlet, pressure $p$ and concentrations $C_j$ are fixed, while no flux boundary conditions are assumed for the electrolyte potential $\phi_L$. No-slip walls with no concentration flux are assumed at the bottom and top of the system. The bottom wall, where the membrane is located, is also the reference value (i.e. the ground) for the electrolyte potential; whereas no flux condition for $\phi_L$ is imposed at the top wall. At the outlet, fixed pressure and no flux conditions for the other variables are imposed.
• Figure 2: (a) Mesh convergence study for regular (ordered) and random (disordered) configurations ($L_x=300\um$) simulated at various grid resolution $\Delta x$ under different operating conditions. (b) Relative current density with respect to the finest grid resolution. We observe that a resolution of $\Delta x \simeq 0.58\um$ is adequate for this study.
• Figure 3: Typical simulation output for a regular lattice configuration ($L_x=600\um, \Delta P/L_x=0.5\MPa/\m, V_{\text{app}}=25mV$). The lattice geometry is discretized according to the embedded boundary method (a) and the flow velocity at steady-state is shown in (b). The concentration profiles (c-d) and the associated state of charge (e) show how the material upstream creates trails of product investing the subsequent cylinders (shadowing effect). In (f) the potential gradient that develops from the membrane (bottom) to the top of the half-cell is shown.
• Figure 4: (a) Current density as a function of applied potential for regular configurations and ones with vacancies in the middle row at different flow rates. (b) Associated snapshots of the simulated configuration with vacancies ($\Delta P/L_x=0.5\MPa/\m$, $V_{\text{app}}=50\mV$) showing velocity, state of charge and electrolyte potential. The configuration with vacancies outperforms the regular intact material, because of better mass transport properties despite featuring less reactive surface.
• Figure 5: Role of system size. (a) Current density as a function of system length for configurations with and without vacancies for four applied potentials $V_\text{app} = 25mV, 50mV, 75mV, 100mV$---see labels in panel (b) for a guide to the colors. (b) Increase in performance of configurations with vacancies with respect to regular ones as a function of $L_x$. Vacancies are more beneficial in longer systems where mass transport limitations are more severe for the same imposed pressure difference per unit length $\Delta P/L_x$.