Effects of pressure on a sequential cascade system for electrochemcial CO2 reduction

Abstract

Within the electricity driven conversion methods, electrocatalysis has been assessed to be the closest to commercialisation. This can only be realised, if the system efficiency is increased in terms of the reaction rate, onset potential and/or selectivity. Current research has shown that, apart from the more widely investigated routes to improve these factors (GDEs, catalyst, etc.), cascade electrode systems and high pressure reactors could prove to be effective for increasing the system efficiency. By splitting the CO2 reduction into two separate steps, CO2 reduction to CO and the sequential reduction to C2+ products, both steps can be optimised in terms of operating conditions. Applying a cascade system has, therefore, shown to increase the selectivity and reaction rates in the system. Additionally, the advantage of the high pressure reactor originates from the fact that increasing the pressure, will result in an increase in the solubility of the reactant. By increasing the solubility, the mass transport to the electrode surface will subsequently be enhanced. The low solubility of reactants is often identified as a main limiting factor in system efficiency, therefore, increasing solubility has proven to increase reactant transport (current density) and selectivity in high pressure systems. Even though both of these advancements have shown promising results, technoeconomic studies indicate that their feasibility is still too low to become commercially attractive at this point. Therefore, this research proposes to combine both technologies to increase the overall system efficiency in terms of: increasing the current density, increasing the Faradaic efficiency and decreasing the overpotential losses for the production of C2+ products. Since this combination has not been investigated before, and both technologies are still rather new, there will be a lot to investigate in order to demonstrate the potential of this new combination. Therefore, in addition to an extensive literature study to uncover the relevant unanswered research questions regarding this field of research, a mathematical model was developed. A model is a valuable resource in determining the potential for a novel system, as it enables instantaneous control over system parameters and, therefore, can provide a lot of insight into its relations and limitations. However, as the accuracy of a model strongly depends on the quality of its input data, this research will also provide a design approach leading to a novel high pressure cascade reactor design. Eventually, the model can, therefore, provide the insight required for extensive experimental research, while the design can simultaneously aid in the improvement of the model. The results evaluated by the model demonstrate both sequential reduction steps are positively affected by increasing the pressure. In addition, the otherwise poor CO solubility, can be dramatically increased by applying the combined system. The reaction rates are also evaluated to increase with the higher reactant concentration of CO and CO2. In addition, since the hydrogen evolution reaction is not affected by the pressure, as it does not present mass transfer limitations, the selectivity has been shown to also increase with increasing the system pressures. Additionally, the presence of the high CO concentration in the reduction towards C2+ products affects the selectivity of the system as well. The CO reduction reactions possess different behaviour from the CO2 reduction reactions. Therefore, by indicating the share of both separate reduction reactions in the generated products, operating conditions for the maximum C2+ selectivity can be identified. This way, the combination of high pressure on a cascade system, has been demonstrated to possess a lot of potential for increasing the system efficiency towards C2+ products.