Continuum modeling to understand and optimize CO2 and CO reduction gas diffusion electrodes
Keywords: Humanities and Sciences.
Chemistry.

Electrolysis of carbon dioxide presents a compelling method for decarbonizing chemical and fuel production by utilizing CO₂ as a climate-friendly substitute to traditional fossil-based raw materials. Low-temperature electrolysis using Cu electrocatalysts offers the capability to generate valuable C₂₊ products from (renewable) electricity, water, and CO₂ exclusively. These products could be further transformed into carbon--neutral or even carbon--negative chemicals and fuels. Instead of directly electrolyzing CO₂ to C₂₊ products, another approach is a two-step process involving an initial CO₂--to--CO conversion followed by CO--to--C₂₊ electrolysis. CO electrolysis is considered in Chapter 2 and CO₂ electrolysis in Chapter 3, each of which pose unique design and modelling challenges. Previous studies have generally been unable to show simultaneously high synthesis rates, high carbon efficiencies, and low cell voltages in a single system at steady state. Computational continuum--scale models are presented which improve understanding and guide optimization to reach these performance goals. Improvements to CO reduction (COR) gas diffusion electrodes (GDEs) are critical for advancing CO electrolysis cells, but the low solubility of CO in electrolyte and the difficulty of experimentally probing the heterogeneous environment of a GDE pose significant barriers to a comprehensive understanding of performance. In Chapter 2, a model is constructed for COR GDEs that includes fully coupled gas and ion transport and competing electrokinetic reactions. The transport and electrokinetic equations are solved in two dimensions to calculate critical COR figures of merit across multiple operating parameters including current density, flow rate, pressure, temperature and electrochemically active surface area (ECSA). The model is validated by showing agreement with experimental data for steady-state CO electrolysis at various pressures and flow rates. Application of the model over a wide range of conditions shows how the figures of merit depend on a complex interplay of the operating parameters. Increasing cell pressure above ambient and augmenting the ECSA of the catalyst are two effective strategies to improve cathode performance. CO₂ reduction (CO2RR) performance in gas diffusion electrodes has advanced significantly over the past decade but carbon efficiency and energy efficiency are still inadequate for commercial use cases. This is primarily a result of the CO₃²⁻ problem where CO₂ and OH⁻ ions react rapidly in solution, consuming the feedstock and leading to higher cell voltages at steady state. When this reaction occurs uninhibited, the pH of the electrolyte at steady state decreases to near--neutral in a carbonate/bicarbonate buffered system. A twofold strategy of minimizing carbonate formation at the cathode and stripping CO₂ from the electrolyte stream would allow the cell to operate with high pH electrolyte at steady state. This would improve carbon efficiency by reducing the CO₂ crossing over to the anode as carbonate. In addition, raising the pH near the anode would result in a shift in the potential of the anode towards the cathode and reduce the anode overpotential. In Chapter 3, cathode GDEs are designed to perform CO2RR at high rates with an elevated carbon efficiency in basic electrolyte. This design strategy requires a model which coupled electrokinetics, ion transport, and gas transport in three dimensions and addressed a range of time and length scales in the catalyst layer microenvironment. To accomplish this, the macroscopic structure is spatially homogenized to define a unit cell and homogenized over pore-scale features, and reaction terms are solved with an efficient time--implicit scheme. The impact of a wide range of geometric configurations and operating conditions on the carbon and energy efficiency of the cathode are investigated using the model. The CO₃²⁻ problem requires controlling the rate of homogeneous reaction which can be achieved by maintaining a low local steady state OH⁻ concentration and enhancing the Faradaic reaction which is dependent on the overpotential and fundamental catalyst performance. Lastly, maintaining high single pass conversion of CO₂ requires calibrated gas delivery and gas transporting region geometry