Continuum Modeling of Porous Electrodes for Electrochemical Synthesis

Abstract

Electrochemical synthesis possesses substantial promise to utilize renewable energy sources to power the conversion of abundant feedstocks to value-added commodity chemicals and fuels. Of the potential system architectures for these processes, only systems employing 3-D structured porous electrodes have the capacity to achieve the high rates of conversion necessary for industrial scale. However, the phenomena and environments in these systems are not well understood and are challenging to probe experimentally. Fortunately, continuum modeling is well-suited to rationalize the observed behavior in electrochemical synthesis, as well as to ultimately provide recommendations for guiding the design of next-generation devices and components. In this review, we begin by presenting an historical review of modeling of porous electrode systems, with the aim of showing how past knowledge of macroscale modeling can contribute to the rising challenge of electrochemical synthesis. We then present a detailed overview of the governing physics and assumptions required to simulate porous electrode systems for electrochemical synthesis. Leveraging the developed understanding of porous-electrode theory, we survey and discuss the present literature reports on simulating multiscale phenomena in porous electrodes in order to demonstrate their relevance to understanding and improving the performance of devices for electrochemical synthesis. Lastly, we provide our perspectives regarding future directions in the development of models that can most accurately describe and predict the performance of such devices and discuss the best potential applications of future models.

Anion-exchange membranes with internal microchannels for water control in CO2 electrolysis

“…17 Thus, MEAs for CO 2 RR will need to manage salt fluxes. 34 Fig. 8b shows the K + crossover to the cathode GDE as a function of the KHCO 3 concentration in the internal microchannels.…”

“…To further highlight the role of hydration in these ionomers, it is instructive to calculate the average tortuosity of these modified AEMs. These tortuosities are calculated using power loss analysis 34,35 for the fully hydrated AEM at low applied potentials to calculate an effective conductivity of the AEM that accounts for the increased average path length required to traverse around the water channel (See Supporting Information). The ratio of the conductivity of the membrane without the water channels to the effective conductivity of the membrane with the water channels represents the increase in the tortuosity of the ion conduction pathways.…”

“…17 Thus, MEAs for CO 2 RR will need to manage salt fluxes. 34 Figure 8b shows the K + crossover to the cathode GDE as a function of the KHCO 3 concentration in the internal microchannels. Consistent with prior CO 2 RR literature, 15,17,23 a significant amount of K + crosses over through the AEM from the anolyte (100 mM KOH) in the absence of internal channels.…”

Electrochemical Reduction of CO2 in Tubular Flow Cells under Gas–Liquid Taylor Flow

“…The faradaic efficiency toward CO (FE CO ) gives the amount of current driving the desired reduction toward CO over the overall current 25 − 27 with i CO and being the current densities for CO and H 2, respectively. The current density for a mass transfer-limited species (CO 2 in the here considered exemplary system) equals 28 with Faraday’s coefficient F , mass transfer coefficient under Taylor flow, and the saturation concentration of CO 2 in the catholyte , which can be determined based on Henry’s law and the Sechenov equation.…”

Progress and Understanding of CO2/CO Electroreduction in Flow Electrolyzers

“…Therefore, developing advanced theoretical models that can simulate the microenvironment of these cell components under the CO (2) RR conditions is a viable approach to gain mechanistic insights into the problems that develop during the CO (2) RR. Modeling studies of the CO (2) RR in the flow cell configuration are typically based on finite element analysis, in which a system of equations that describe the multiple processes involved (e.g., Nernst–Planck equation for mass-transfer and Butler–Volmer equation for charge-transfer) is constructed. , This system of equations is subsequently solved numerically with software including MATLAB and COMSOL Multiphysics. , By adjusting the parameters of interest, the impact on the reaction microenvironment can be conveniently reflected by the computational results.…”