The rise in greenhouse gas emissions over the past few decades has led to the introduction of global policies and legislation to promote carbon control. Methane contributes to (how much percent) of the total emissions and plays a critical role in rising temperatures. Technologies like dry methane reforming, steam methane reforming, and Fischer-Tropsch synthesis are common in industrial methane conversion. These technologies are expensive to initiate and maintain and re-release carbon dioxide into the atmosphere. Biological species, methanotrophs in particular, consume methane and produce commodity products such as methanol, formic acid, and formaldehyde efficiently. In this work, a manganese(oxy)hydroxide (MnOx) catalyst inspired by the functionality and components of methanotrophs is synthesized. The catalyst’s ability in activating the C-H bond in methane and its performance in the electrochemical partial oxidation of methane to methanol is investigated. The applied potentials and control of mass transport is observed to have a significant effect on the selective production of methanol. Under low and high applied overpotentials, MnOx is selective towards methanol production. At intermediate applied overpotentials, the catalyst is selective towards acetate production. Furthermore changing rotation speeds leads to changes in mass transport that affects the product selectivity between methanol and acetate. The stability of the catalyst is probed with scanning electron microscopy (SEM) and chronoamperometry control experiments and catalyst degradation after two hours of operation is indicated. Thus, tuning the effects of applied potential, mass transport, and catalyst stability is important to optimize the conversion of methane to value-added products. Our results suggest that by controlling these effects and using the bioinspired MnOx as a catalyst we electrochemically convert methane to various products.

The development of devices for the electrocatalytic transformation of carbon dioxide to fuels and chemicals at global scales is a promising alternative to store renewable electricity from intermittent sources and reduce carbon emissions. The realization of a technology that can produce C2+ liquid fuels at high rates requires a concerted effort to develop an efficient and selective electrocatalyst. However, transport phenomena inherent in aqueous electrocatalysis interfere with the observation of intrinsic reaction kinetics. The knowledge gap in the field to distinguish reaction kinetics from reactor kinetics has created discrepancies in interpreting experimental results, hindering the establishment of a fundamental design equation for the scale-up of electrolyzers.Decoupling and understanding multi-scale transport phenomena involved in the electrocatalytic transformation of small molecules is challenging, but it can be achieved using relationships between dimensionless quantities that simplify the characterization of transport properties. In the first part of this dissertation, we report the development of the gastight rotating cylinder electrode (RCE) cell and demonstrate how the mass transport characteristics of an electrochemical cell can be quantified. The gastight RCE cell enables kinetics studies under well-defined transport conditions, making it the closest experimental apparatus to differential reactors in thermal catalysis. The application of dimensional analyses on a lab-scale electrochemical reactor should enable rigorous research and development of electrocatalytic technologies. In the following sections of the dissertation, we present a large experimental dataset of electrochemical CO2 reduction on polycrystalline copper electrodes collected under a broad range of well-defined transport regimes in the gastight RCE cell. The control of relative timescales of electrode surface reactions and transport processes enables systematic parametrization of a multi-scale reaction-transport kinetics model. The model utilizes a continuous stirred-tank reactor (CSTR) volume approximation of the electrode reaction front that captures mesoscale stochastic processes of reactants and intermediates at the electrode/electrolyte interface, which determines product selectivity. Product distributions under different conditions of applied potential, mass transport, bulk electrolyte concentration, temperature, and electrode porosity are rationalized by introducing dimensionless numbers that reduce complexity and represent relative timescales of dynamic transport phenomena and electrocatalytic reactions on copper electrodes. The CSTR model demonstrates that one CO2 reduction mechanism can explain differences in selectivity reported for copper-based electrocatalysts using the residence time as a descriptor. With the insights gained from the RCE cell and the CSTR model, we propose mechanistic pathways for each CO2 reduction product. This combinative approach of experimental electrocatalysis under well-defined transport characteristics and a new framework of the reactor design equation that captures relative timescales of multi-scale processes will open up discussions from a new perspective to study kinetics and scale-up electrocatalytic processes.

Electrochemical conversion of greenhouse gases to value-added products has emerged as a promising strategy to mitigate climate change and expand the penetration of renewable energy sources into the various sectors of our economy. As the accumulation of greenhouse gases in the atmosphere continues to increase, researchers have been exploring ways to utilize electrochemistry to capture and convert these gases into useful chemicals and fuels. This approach not only reduces the accumulation of greenhouse gases in the atmosphere but also provides a sustainable solution to the production of high-value chemicals and fuels. CH4, CO2 and other greenhouse gases can be converted into a range of valuable products, such as methanol, ethanol, formic acid, and hydrocarbons. The use of renewable electricity sources, such as solar and wind energy, to drive the electrochemical conversion process further enhances its sustainability. The development of efficient and selective electrochemical conversion strategies for greenhouse gas utilization holds great potential for meeting the energy demands of the future while addressing the challenges of climate change starting today.In this doctoral dissertation work, a series of systematic studies are carried out to elucidate the mechanisms of electrochemical CH4 partial oxidation to methanol (Chapter 2), and the reactive capture of CO2 to CO in amine and carbonate capture solutions (Chapter 3). Among the catalysts for partial oxidation of methane studied in Chapter 2, electrochemically deposited transition metal (oxy)hydroxides are found to be active even without the application of a bias potential. Taking CoOx as a prototypical methane partial oxidation electrocatalyst and combining systematic experiments in a rotating cylinder electrode (RCE) cell with DFT calculations, optimal conditions of low catalyst film thickness, intermediate overpotentials, intermediate temperatures, and fast methanol transport are identified to favor methanol selectivity. CO2 reactive capture and conversion (RCC) is also investigated with the RCE cell in Chapter 3. In the RCE cell, the transport properties are well-defined in the gas, liquid, and solid phases, which allows the elucidation of the origin of carbon sources during the electrochemical reduction of bicarbonate and amine-based CO2 capture solutions on a silver catalyst electrode. In this study, dissolved CO2 is revealed to be the primary carbon species being consumed while the CO2-absorber complex appears to serve as a secondary carbon source only at highly negative potentials. Through the development of experimental methods for the differentiation of carbon species in solution during electrochemical carbon upgrading, the work presented here allows the comparison of experimental results to density function theory (DFT) calculations and contributes to the acceleration of catalyst discovery and reactor design for RCC technologies. This thesis work stablishes new methodological approaches to catalyst and reactor design in the Morales-Guio group, and contributes broadly to the development of electrocatalytic technologies for the capture and conversion of greenhouse gases.

With increasing concern over the environmental impact of chemical manufacturing processes, a transition to synthesis methods that replace fossil fuel based processes with renewable electricity based processes is eminent. This work highlights the development of a high temperature proton conducting membrane reactor based on BZCY tubular membrane architecture which was designed and built to test the electrochemical SMR performance of BZCY membrane systems. Alongside this, an existing reactor system was retrofit to utilize Joule heating by applying an electric current directly through the FeCrAlloy reactor tube. CO2 methanation and SMR tests were performed, and results was compared between Joule heating and external heating by a furnace. Joule heating showed improved methane conversion and reaction rates up to twice that observed when externally heated by the furnace. Together, these systems provide new capabilities for testing electric based SMR that will aid future research in the Morales-Guio group.

Reactive capture of CO2 (RCC) is an innovative and integrated approach to carbon capture and utilization (CCU) that bypasses the inefficient thermal release of CO2 from capture agents by using renewable electricity. RCC can offer significant capital and operational cost savings over the traditional multi-step process of CO2 capture, release, concentration, and conversion, as it directly converts captured CO2 under the same low temperature and high pressure conditions used in the CO2 absorption unit. However, RCC systems introduce an additional layer of complexity due to interactions between the capture agent and the catalyst, which must be thoroughly understood to advance RCC technology. We selected amine-based capture agents as a practical starting point for RCC systems, given their extensive industrial application and well-documented capture mechanisms. Amines can capture CO2 from flue gas and other diluted CO2 streams with high recovery rates and at high CO2 loadings. Our goal is to incorporate industrially mature and technologically advanced amine-based carbon capture process, into the CO2 conversion step by increasing our understanding of the interactions of capture agents with catalysts that ultimately could lead to further advancement of the RCC systems.

The first chapter aims to develop descriptors of activity and stability for RCC systems from a comprehensive understanding of the complex interactions between molecular species in solution and their interactions with metal surfaces in the direct conversion of captured CO2-adducts within amine solutions, both during operation and under resting conditions. To develop effective descriptors of activity for RCC, the first part of this dissertation proposes to validate whether the well-established descriptors of activity and selectivity for electrochemical CO2 reduction (CO2R), based on the binding energy of intermediate species, can be effectively extended to the RCC system. We reasoned that a suitable starting point would be to compare the electrochemical reduction reactions of CO2 in a bicarbonate solution, which has been widely studied, with its reduction in an RCC system using the ammonia which is the simplest amine-based capture agent. This chapter systematically explores RCC systems to identify stability and activity descriptors by integrating experimental observations with theoretical insights. Findings reveal that the binding energy descriptors commonly used to determine product selectivity and catalytic activity of transition metals in CO2R are inadequate to fully interpret the electrochemical performance of transition metals as catalysts in RCC systems. Additionally, we observed the corrosion of Cu and Sn catalysts in ammonia carbon capture solutions highlighting the need for stability descriptors that account for compatibility between amine-based capture agents and metal catalysts in RCC applications.

In the following chapter of this dissertation, we present an extensive experimental dataset on the electrocatalytic performance and stability of polycrystalline Cu under RCC operating conditions with various amines. This systematic evaluation assesses the viability of primary and secondary amines as capture agents in RCC systems. The hydrogen evolution reaction is shown to dominate catalysis in all amine capture solutions, with both the pKa and steric effects of the amines correlating directly to the corrosion rates of the Cu films. This thesis highlights challenges inherent in amine-based RCC systems, emphasizing the need for comprehensive investigation and further efforts in system design and optimization to advance RCC technologies.

As of 2023, atmospheric CO2 concentrations have surged to unprecedent levels, compelling urgent action for large-scale carbon removal to combat climate change. Amidst this pressing need, exploring alternative technologies such as electrocatalytic CO2 reduction to generate liquid fuels emerges as a promising avenue. This approach not only offers the potential to store renewable electricity but also significantly mitigate carbon emissions. However, the scalability of this technology presents many challenges, notably in controlling reaction kinetics and mass transport within electrochemical reactors. Overcoming these obstacles requires efforts towards enhancing catalyst design, refining mass transport mechanisms, and implementing scalable manufacturing methods. In this context, the following research focuses on the development and refinement of a 100 cm2 active electrode area electrocatalytic unit tailored for sustainable CO2 reduction. Its aim is to provide a scalable solution for producing synthetic fuels by converting CO2 exhaust gases from power plants into ethanol, propanol, and butanol using electricity from the grid. This technology would be applied directly at the source of flue gases, contributing to global efforts to combat climate change and transition to a sustainable energy future, while circumventing the land requirement issues often faced when applying similar technologies.

The conversion of methane to valuable chemicals via electrochemical approaches is of great interest in the field of catalysis. Conventional catalytic processes utilize extreme conditions (high temperatures or pressures) to provide the energy required to achieve methane activation and require sophisticated heat integration networks to be economically viable. In contrast, catalytic processes via electrification or electrocatalysis offers direct routes of methane activation under ambient conditions, with lower energy requirements and simplified configurations. However, electrochemical oxidation of methane using current electrocatalysts remains challenging due to low energy efficiencies and a seemingly unavoidable trade-off between conversion and selectivity. In this regard, many research efforts have been devoted to the development of efficient and selective electrocatalysts for the activating and transformation of methane into valuable chemicals. Over the last decade, researchers have shown that composite transition metal oxides such as NiO/ZrO2 and Co3O4/ZrO2 can catalyze the electrochemical partial oxidation of methane to value-added chemicals such as methanol, ethanol, and propanol in a carbonate electrolyte. Chemical co-precipitation has been utilized predominantly for the preparation of metal oxide catalysts which involves multiple steps such as centrifugation, collection, drying, and annealing, and result in oxide materials with poor conductivity which are not amenable to electrocatalysis. In this work, a one-step electrodeposition method has been developed for the preparation of CoZrOx electrocatalysts. The electrodeposited CoZrOx material was found to be an active electrocatalyst for the partial oxidation of methane with a simple fabrication method. Furthermore, different electrodeposited unary transition metal oxides (CoOx, NiOx, MnOx, FeOx, and CuOx) were prepared through the same electrodeposition method, and were also studied for the electrochemical oxidation of methane. CoOx, NiOx, CuOx, and the CoZrOx electrocatalysts have been discovered to catalyze the conversion of methane to methanol. The preliminary results in this work demonstrate an additional approach among the available strategies for catalyst fabrication and may provide an efficient strategy of catalyst preparation for further studies of the electrochemical oxidation of methane under ambient conditions.