The widespread use of fossil fuels since the Industrial Revolution has caused CO2 concentrations in the atmosphere to rise exponentially, leading to severe ecological and social challenges. Large-scale renewable energy adoption is key to reducing carbon emissions, while sources like solar and wind are intermittent and variable. As a result, they require large-scale, long-duration energy storage systems. The low-temperature CO2 electrolyser addresses this issue by using renewable electricity to convert CO2 into fuels and chemicals, enabling both energy storage and CO2 valorization, and closing the loop on anthropogenic CO2 emissions...

Our world has witnessed the devastating effects of fossil fuels extraction; the primary energy source for modern society. Transitioning to sustainable energy sources requires exploring alternative fuel production methods, focusing on utilizing captured anthropogenic CO2. Electrochemical reduction of CO2 (CO2RR) using renewable electricity presents an attractive option for storing energy in chemical bonds. While many electrocatalysts efficiently reduce CO2 to CO, only a few, such as copper, can produce high energy-dense molecules like ethanol or methanol with selectivities exceeding 60%. Molecular catalysts (MCs) hold promise as alternatives to metallic electrocatalysts, particularly when immobilized on the surface of gas diffusion electrodes (GDEs), where state-of-the-art CO2RR takes place. However, their efficiency and scalability must be addressed for industrial applications. Our work focuses on evaluating the impact of engineering the micro-environment at the interface where CO2RR occurs, for the purposes of optimizing the performance of MCs systems.

Specifically, the electrode active-site interaction for the CO2-to-ethanol pathway was investigated using a nickel iron-tetraphenylporphyrin (Ni-FeTPP) electrocatalyst. We explored the challenges of translating a molecular catalyst system from an unconventional 3D electrode design to a GDE-based electrolyzer. The difficulty lies in replicating the strong electronic coupling between Ni and FeTPP necessary for enabling the CO2-to-ethanol pathway; which led to two methods of coupling Ni to FeTPP: drop-casting and spray-coating. Although our system did not achieve CO2 reduction to ethanol, it provided significant insights into the impact of subtle micro-environment changes. These changes uncovered possible unintended interactions, suggesting the formation of nickel hydride species that could greatly influence the CO2-to-ethanol pathway catalyzed by Ni-FeTPP. Moreover, the competing behavior between CO2 and CO to enhance the CO2-to-methanol pathway was adressed by using cobalt phthalocyanine supported on multi-walled carbon nanotubes (CoPc/MWCNT). We engineered the local environment where molecular catalysts interact by introducing silver nanoparticles and PTFE particles to increase local CO availability near the active sites and promote further CO reduction. This approach resulted in a remarkable 4-fold and 17-fold increase in the Faradaic efficiency of methanol, from the addition of silver nanoparticles and PTFE particles, respectively. Our results demonstrate the CO2-to-methanol pathway is enhanced through the modification of the catalyst surface microenvironment of the GDE. Our research contributes to a deeper understanding of the prospect for improving the challenging electrochemical reduction of CO2 beyond CO.

The shift towards a carbon-neutral economy represents a considerable challenge in the search of sustainable carbon-sources. The electrolysis of CO2 presents an attractive, direct pathway to produce sustainable fuels using only CO2, water, and electricity. The rapid shift towards high reaction rates and industrially relevant process conditions in just a few short years has opened new research directions towards understanding the intricacy of the complex interfaces involved in this reaction. The two key enabling technologies: gas-diffusion electrodes and membrane electrode assemblies, have greatly increased efficiency metrics of CO2 electrolyzers, but many questions on the homogeneity and activity of these electrodes remain unanswered....

Currently, hydrogen consumption is heavily concentrated within larger inflexible industrial applications, where it is produced onsite for further downstream processing. Considering traditional production pathways have a high emission intensity, electrolytic hydrogen production could prove an essential decarbonization pathway for existing, unabated consumers and additional hard-to-abate industrial applications. While legislative frameworks limiting unwanted effects of the electrolytic hydrogen transition are being drafted, it is important to provide policymakers with quantitative data on the effects of this legislature. Other than emissions intensity standards, additionality principles are being considered in a number of these frameworks to further minimize unwanted effects. One of the central principles of additionality is a temporal-correlation requirement, which synchronizes renewable electricity generation with electrolyzer consumption over a predetermined period. The length of this period could significantly affect the intermittency of electrolyzer operations. Although this has been subject to debate, it has seen little attention in literature within the context of industrial applications, which bring substantial additional downstream constraints. It is imperative to understand the effects of these additionality principles quantitatively within the context of their dominant application, heavy industry. This could aid policymakers to arrive at a framework that minimizes the adverse effects of electrolytic hydrogen production while preventing cost increases that hinder widespread adoption.

A mixed-integer linear-programming problem is formulated to model an onsite electrolytic hydrogen production facility for a larger industrial downstream process. The downstream flexibility and temporal correlation constraints in this model are generalized to study their potential antagonistic effects abstractly. The downstream flexibility constraints considered are the minimum partial-load and the period over which production has to match the desired output, mimicking further downstream supply chain constraints. The model employs integrated design and operations optimization, considering the cost-optimal production facility will vary depending on the legislature and downstream process.

The results indicate that temporal correlation requirements affect the production costs of hydrogen as a consequence of limiting the operational flexibility. Additionally, strict temporal correlation requirements exacerbate the escalation of these costs. The availability of a geological storage site reduces the effects of temporal correlation requirements and DSP inflexibility on production costs. Regarding emissions, at current allowance prices, the ETS is not sufficient for emissions abatement of onsite electrolytic hydrogen production. On the other hand, temporal correlation requirements are an effective tool for reducing the attributable emissions intensity. However, a focus on emissions abatement for onsite electrolytic hydrogen production, without adjustments to the ETS, risks cost inefficient sectoral emissions reduction without reducing system emissions, due to leakage to other sectors.

Electrochemical CO2 reduction (CO2R) provides the opportunity to mitigate our fossil-carbon dependence, by using CO2 as an alternative source to produce value-added chemicals. Especially multi-carbon (C2+) products are industrially of high value and hence a promising candidate for CO2R products. Copper is uniquely capable of producing these value-added C2+ products. Currently, the application of CO2R technology is hampered by low selectivity and rates of C2+ products. Ionomers, or ion-conducting polymers, are used in the catalyst layer to optimize the local reaction environment. In particular Nafion, a cation exchange ionomer, has proven to promote C2+ formation, but the mechanism behind it remains unclear. For this thesis, we focused on elucidating the role of the Nafion ionomer in altering product distribution on copper. A planar Cu electrode was used, onto which the Nafion was drop-casted. Atomic force microscopy was used to observe the restructuring of the ionomer during CO2R, while attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS) enabled us to observe adsorbed chemical intermediate species during electrochemical CO2R. Flow cell experiments were performed to study the effect of the ionomer on product distribution and activity. The addition of Nafion significantly increased formation towards ethylene, due to stabilization of the atop-CO intermediate. The ionomer layer underwent restructuring during CO2R, where it is expected that the hydrophilic domain of the ionomer takes over the surface interactions from the hydrophobic backbone, due to electrowetting of the catalyst. With the insights gained in this thesis we elucidated the interactions between the ionomer and catalyst during electrochemical CO2R, which relates to the fundamental understanding required for designing advanced catalyst layers in the gas diffusion electrode.

Electroreduction of CO2 into high-valued chemicals is a promising way to reduce CO2 emissions while simultaneously producing bulk chemicals currently produced from fossil-fuel feedstocks. The downside of this process is that conversion rates are low, meaning the resulting product stream is a complex gas mixture consisting primarily of reactants and by-products and a relatively small amount of product. This study focuses on the development of a new downstream separation process to capture ethylene from a mock-up reaction mixture (mole fractions C2H4/CO2/CO/H2/H2O : 20/55/15/15/5), based on low driving forces and suitable for application in a 100kW test case within the e-Refinery. An extensive literature study of numerous separation techniques for gases was conducted and adsorption was chosen as the most suitable option. After screening of various adsorbents, active carbon was selected as the most potential sorbent. Based on a selectivity analysis, the primary focus was on the behaviour of C2H4/CO2 on active carbon. Using a simple, custom-build set-up, transient breakthrough experiments were performed for this gas mixture and the resulting selectivity for an equivolume feed, yielded a lower separation performance than expected based on the ideal adsorption solution theory, respectively a selectivity of 1.5–1.7 versus 3.2–3.5. Additionally a theoretical model was developed using MATLAB, which described the velocity profile inside the adsorber column and could qualitatively predict breakthrough behaviour. Further analysis led to the conclusion that for a more accurate quantitative match between experimental and numerical results, isotherm parameters should be obtained from the same type of active carbon. Ultimately this technique could be used to increase the ethylene content in a CO2-bearing stream and pave the way for a new, energy-efficient method to obtain hydrocarbons, ethylene in this case, from an electrolyzer cell.