Ion Transport in an Electrochemical Cell: A Theoretical Framework to Couple Dynamics of Double Layers and Redox Reactions for Multicomponent Electrolyte Solutions

Abstract

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.

Mass Transport Modifies the Interfacial Electrolyte to Influence Electrochemical Nitrate Reduction

“…To simulate interfacial electrolyte properties under NO3RR reaction conditions, we adapted the GMPNP originally developed for CO2RR, 29 because of several advantages: (1) the continuum treatment of the electrolyte circumvents the prohibitively high computational cost of ab initio simulations for regions larger than the nanometer scale, (2) the inclusion of migration enables the reconstruction of the EDL and illustrates how different time scales govern bulk and interfacial phenomena, which the reaction-diffusion model fails to capture, and (3) the inclusion of solvated species sizes to the PNP model facilitates derivation of physically relevant concentration profiles (i.e., below the steric limit). [29][30][31][32] The physical regions simulated in the GMPNP model are depicted in Fig. 2a.…”

“…15,16,22 To this end, computational simulations using continuum models have shown promise as a high-fidelity, computationally efficient approach to describe interfacial electrolyte properties. [29][30][31][32] Although their physicochemical properties differ, the interfacial and bulk electrolytes are bridged by mass transport phenomena. During electrochemical NO3RR, the anionic reactant nitrate must travel against the electric field generated by the often negatively charged electrode and specifically adsorb to the electrode surface.…”