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 NO 3 RR reaction conditions, we adapted the GMPNP originally developed for CO 2 RR 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). − …”

“…In terms of ionic solutes, coexisting anions (e.g., Cl – , SO 4 2– ) can compete with reactant nitrate for surface sites and affect activity; cations (e.g., Na + , K + ) are known to interact with reaction intermediates as well as modify the interfacial electric field and pH, thus influencing activity and selectivity. ,, Although the effects of bulk electrolyte properties on electrochemical NO 3 RR have been reported, ,− few studies have explicitly investigated the interfacial electrolyte environment during reaction, much less its impacts on NO 3 RR activity and selectivity. This gap in understanding is largely due to the micron-scale and dynamic nature of the interfacial electrolyte environment, which makes it inherently challenging to probe experimentally. ,, To this end, computational simulations using continuum models have shown promise as a high-fidelity, computationally efficient approach to describe interfacial electrolyte properties. − …”

Asymmetric rectified electric and concentration fields in multicomponent electrolytes with surface reactions

“…The sinusoidal potential boundary conditions are given as Φ (± L , τ ) = ± Φ D sin( Ωτ ).The above equation ignores any native zeta potential of the electrodes, similar to Hashemi et al 17 We consider a surface reactive flux condition ( i.e. , non-ideally blocking) at the two electrodes N i (± L , τ ) = N i 0 sin( Ωτ ).We note that the flux amplitude N i 0 may not be identical at the two electrodes, 37 but is assumed to be the same and time-independent for simplicity. Typically, N i 0 is dependent on applied potential and ion concentrations.…”

Incorporating ion-specific van der Waals and soft repulsive interactions in the Poisson–Boltzmann theory of electrical double layers

“…The accumulation of excess counter-ions on charged surfaces, when placed in the vicinity of electrolytes, gives rise to an electrical double layer (EDL). 1 The EDL phenomenon lies at the heart of several applications such as supercapacitors, [2][3][4][5] porous electrodes, 6 electrocatalytic reactors, 7 membranes for water desalination, 8 and nanofluidic transport devices. [9][10][11] It also has important implications in determining colloidal stability, [12][13][14] cellular integrity, 15 the structure of ionic liquids, [16][17][18] pH effects close to electrodes, 19 and nanoconfined transport phenomena.…”