(187c) 3D Flow Cell Simulation Via Accelerated Buffer Chemistry

To date the literature on flow cell reactors for CO₂ electrochemical reduction buffered by bicarbonate solutions has been dominated by 1D film models. These models assume a static boundary layer adjacent to the electrode of finite width, through which the CO2, HCO3-, CO32-, OH- and H+ species diffuse and react according to their own diffusion-reaction equations. These models are very popular, but are difficult to solve in higher dimensions, as the ~10 orders of magnitude variation among the reaction rate constants leads to a stiff system of differential equations which scales with computational mesh size. In this work, we reduce these equations to an alternative set of non-stiff PDE’s via a quasi-steady approximation as derived from a two-variable singular perturbation expansion. This reduced set of PDE’s accelerates the computation by over 100x in 1D while typically introducing errors <1% (Figure 1a-b.).

The acceleration of the bulk carbonate chemistry allows the rigorous simulation of a 3D flow reactor (Figure 1c-d) in which a CO₂-saturated bicarbonate solution flows past a silver cathode at which COER and HER occur. We implement the accelerated reactive model in the commercial tool StarCCM+ and simulate the coupled fluid flow, electrostatics, and reactive mass transport in the system, including the effects of electromigration and electroneutrality. Surface kinetics are modeled via Tafel expression for COER, HER, and OER. Steady-state, 3D simulations on computational meshes with 2.65 M cells converge in ~ 5 core-hours. This simplified approach to modeling the reactor chemistry may allow the rigorous simulation of more complex 3D geometries (such as minimal surfaces or structures for convective transport enhancement) which have been shown to increase Sherwood and Nusselt numbers significantly in the heat-exchanger and membrane-reactor literature.

Prepared by LLNL under Contract DE-AC52-07NA27344. This work was supported via the laboratory directed research and development project 19-SI-005.