Modeling Vapor-fed Electrolyzers for CO2 Reduction

The electrochemical reduction of CO2 (CO2R) to value-added products is an attractive route for tackling the rising atmospheric CO2 levels and storing intermittent renewable energy. Fundamental understanding of CO2R has progressed significantly in recent years and is critical in the development of industrial-scale CO2 electrolyzers. However, most of our understanding has been drawn from experiments optimized for aqueous-phase CO2R systems. These systems are limited by mass transfer at low current densities (in the order of 10 mA/cm2) due to unfavorable CO2 solubility and transport in aqueous media, whereas technoeconomic analysis emphasizes the need to operate CO2R at current densities above 100 mA/cm2 for industrial viability./nTo achieve commercially-relevant CO2R rates, gas-diffusion electrodes (GDEs) play a critical role as they can decrease the diffusion length of CO2 from 100 µm seen in aqueous systems to as small as 10 nm, and increase the local concentration of CO2. These architectures enable current densities that are almost two orders of magnitude greater than what is achievable with planar electrodes in an aqueous electrolyte at the same applied cathode overpotential. The thin diffusion layer in GDEs allows access to higher CO2R current densities, as well as environments with higher OH concentrations. The addition of porous cathode layers, however, introduces several parameters that require tuning in order to optimize GDE cell performance. Because the structure of GDEs is complex and CO2R in such systems involves the simultaneous occurrence of many physical processes, it is very hard, if not impossible, to assess the impact of a particular change in the composition and structure of the GDE without a detailed model that accounts for the convoluted chemistry and physics interrelationships. An overview of such model is presented in Chapter 2, and the complex interplays are probed through the model in Chapter 3 and used to shed light on the optimization of GDEs./nWith the order-of-magnitude increase in current density obtained by GDEs, conventional cell designs for planar electrodes become severely limited by the ohmic drop across the cell, making membrane-electrode assemblies (MEAs) an attractive alternative. MEAs have smaller cell resistances as they do not have aqueous electrolyte compartments and can greatly reduce the distance between the two electrodes. These MEA designs are investigated through a complete electrochemical cell model in Chapter 4 and Chapter 5; cell design considerations such as the distribution of the applied voltage, water management, and CO2 utilization efficiencies are explored./nChapter 4 uses silver-based MEAs (Ag-MEAs) as a model system and studies the performance and limitations of two MEA designs: one with gaseous feeds at both the anode and cathode, and the other with an aqueous anode feed (KHCO¬3 or KOH exchange solution, or liquid H2O) and a gaseous cathode feed. Copper-based MEAs (Cu-MEAs) are examined in Chapter 5. Copper has been shown to be the most effective CO2R catalyst to yield high selectivity towards hydrocarbons and alcohols; Cu-MEAs hold promise for increasing the energy efficiency for electrochemical reduction of CO2 to C2+ products while maintaining high current densities. However, fundamental understanding of Cu-MEAs is still limited compared to the wealth of knowledge available for aqueous electrolyte Cu systems. Sensitivity analysis shows that catalyst layer properties can be optimized to increase the energy efficiencies of C2+ products. Issues associated with water management and tradeoffs associated with elevated operating temperatures are also discussed to guide the optimization of Cu-MEAs./nFinally, Chapter 6 summarizes the finding in relation to the field and proposes future directions to progress the development of practical CO2 electrolyzers. Overall, this dissertation demonstrates how physics-based modeling can assist in the transfer of knowledge from aqueous to vapor-fed systems by deconvoluting the impacts of the multiple physical processes occurring in the device.

Advisor: Weber, Adam Z
Bell, Alexis T