Mass Transport Limitations in Electrochemical Conversion of CO2 to Formic Acid at High Pressure

Abstract

Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental data.

Carbonation in Low-Temperature CO2 Electrolyzers: Causes, Consequences, and Solutions

“…Creative GDE designs will be of paramount importance to overcome the flooding problem. , Multiscale/multiphysics modeling: CO2R at the electrode is a complex phenomenon that involves multiple phases (gas/liquid/solid) and reactions (homogeneous and heterogeneous), which are affected by the presence of electrolytes and electric field. Advanced modeling of the near-electrode and membrane environment will provide useful insights into the carbonation phenomena that is affected by mass transport, electrochemical and homogeneous reactions, and thermal effects. − Moreover, advanced thermodynamic modeling using electrolyte equations of state and molecular simulation are essential for predicting the key thermodynamic, transport, and structural properties of the relevant liquid systems. − Such properties are the mutual solubilities (e.g., gases in the aqueous electrolyte phase), transport coefficients (i.e., Maxwell-Stefan and self-diffusivities, ionic conductivities, viscosity), and partial molar properties. − Electrolyte-free electrolysis: The presence of electrolytes in CO 2 electrolyzers has several disadvantages. First, liquid products are contaminated with the electrolytes, which complicate the downstream process.…”

Perspectives of Magnetically Enhanced Electroconversion of CO2 to Formic Acid and Formate

“…So far, the alternative to overcome this limitation that has received greater attention in the literature lies in the supply of a humidified CO 2 stream at the cathode compartment. This way, the use of liquid electrolytes is prevented, since the ion exchange membrane acts as a solid polymer electrolyte. ,,, While the feed of humidified CO 2 improves the mass-transfer in the CO 2 electroreduction process, it also entails higher reactor voltages, making it difficult to operate efficiently at high current densities. Conversely, it has been reported that aqueous electrolytes operate at higher current densities and with lower reactor voltages, but they have associated the mass-transfer limitations due to the low solubility of CO 2 in aqueous electrolytes. , …”

“…However, most of the published works that employed an MEA in a two-compartment electrochemical reactor do not operate at commercially relevant current densities (200 mA·cm –2 ) . The fact that operating at higher current densities is possible when using aqueous electrolytes may promote the search for strategies that enhance the mass-transfer limitations caused by the low solubility of CO 2 in aqueous electrolytes. , In this regard, a novel approach that has recently emerged lies in the application of magnetic fields to improve ERCO 2 (Figure b). The fundamentals of this strategy as well as its reach in the ERCO 2 to formic acid/formate are discussed in the next section.…”

A comprehensive modeling for the CO2 electroreduction to CO

“…The model is validated with experimental data of current density vs. voltage at 1 sun illumination, and the effect of CO 2 flow rate is studied on the current density and efficiency of conversion. Chinnathambi et al [22] investigated the mass transport limitations across the electrolyte and the bipolar membrane for formic acid formation at different pressures, up to 40 bar. Also in this case, the model is validated through experimental data of the current density vs. the applied potential.…”