Experimental and computational study of determining mass transport parameters in vanadium redox flow batteries

Vanadium redox flow batteries are a promising large-scale energy storage technology, but a number of challenges must be overcome for commercial implementation. At the cell level, mass transport contributes significantly to performance losses, limiting VRFB performance. Therefore, understanding mass transport mechanisms in the electrode is a critical step to mitigating such losses and optimizing VRFBs.

In this study, mass transport mechanisms (e.g. convection, diffusion) are investigated in a VRFB test bed using a strip cell architecture, having 1 cm² active area. It is found that diffusion-dominated cells have large current gradients; convection-dominated cells have relatively uniform current distribution from inlet to outlet under a mass transport limited condition. This behavior is attributed to convective mass transport in the electrode.

Computational flow simulation is utilized to assess velocity and pressure distributions; experimentally measured in-situ current distribution is quantified for the cell. CFD simulation has shown that the total current in the cell is directly proportional to electrolyte velocity in the electrode. However, maximum achievable current is limited by diffusion mass transport resistance between the liquid electrolyte and the electrode surfaces. The pressure drop arising due to any fluid path outside the channel-electrode region is found to be ineffective and must be minimized to improve overall system efficiency of the VRFB.

A three-dimensional, steady-state multiphysics model for VRFB strip cell architecture is further developed to investigate mass transport more fundamentally. Numerical predictions are validated by experimental measurements (polarization curve and current distribution). Diffusion coefficient of the vanadium active species and electrode permeability are found to be the most important parameters affecting electrochemical performance and performance distribution.

Carbon paper electrode permeability is investigated both computationally and experimentally. While three-dimensional pore-level Lattice Boltzmann model is adopted to predict electrode permeability, a permeability cell experimental setup is designed to measure carbon paper electrode permeability under different compressions. It is found that permeability is directly proportional to the electrode porosity. While a simulated solid domain considering only the fibers does not predict experimentally measured permeabilities for higher electrode porosities, a composite domain considering both fibers and filler material successfully simulates carbon paper electrode macropore structure.