  Electrochemical devices often consist of multicomponent electrolyte
solutions. Two processes influence the overall dynamics of these devices: the
formation of electrical double layers and chemical conversion due to redox
reactions. However, due to the presence of multiple length and time scales, it
is challenging to simulate both processes directly from the
Poisson-Nernst-Planck equations. Therefore, common modeling approaches ignore
one of the processes, assume the two are independent, or extrapolate the
results from reaction-free systems. To overcome these limitations, we formulate
and derive an asymptotic model by solving the Poisson-Nernst-Planck equations
for an arbitrary number of ions in the thin-double-layer limit. Our analysis
reveals that there are two distinct timescales in the system: double-layer
charging and bulk diffusion. Our model displays excellent quantitative
agreement with direct numerical simulations. Further, our approach is
computationally efficient and numerically stable, even for large potentials. We
investigate the dynamics of charging for a binary electrolyte and three-ion
system, and find that redox reactions impact the double-layer charging process
at short times whereas they modify the double-layer capacitance at long times.
Overall, the proposed theoretical framework advances our ability to simulate
electrochemical devices that contain multiple ions and widens opportunities for
future research in the field.
