Polymer electrolyte fuel cells (PEFCs) have emerged as a promising renewable emission-free technology to solve the worldwide increasing demand for clean and efficient energy conversion. Despite large efforts in academia and automotive industry, the commercialization of PEFC vehicles still remains a great challenge. Critical issues are high material costs, insufficient catalytic activity as well as longterm durability. Especially due to the sluggish kinetics of the oxygen reduction reaction (ORR), high Pt loadings on the cathode are still necessary which leads to elevated costs. Alloys of Pt with other less precious metals (Co, Ni, Fe, Cu, etc.) show improved ORR activities compared to pure Pt catalysts. However, state-of-the-art carbon-supported catalysts suffer from severe Pt and carbon corrosion during the standard operation of PEFCs, affecting their reliability and long-term efficiency. Multimetallic aerogels constitute excellent candidates to overcome these issues. Due to their large open pores and high inner surface areas combined with electrical conductivity, they are ideal for applications in electrocatalysis. In addition, they can be employed without any catalyst support. Therefore, the fabrication of bimetallic Pt-M (M=Ni, Cu, Co, Fe) aerogels for applications in fuel cell catalysis was the focus of this thesis. Based on a previously published synthesis for Pt–Pd aerogels, a facile one-step procedure at ambient conditions in aqueous solution was developed. Bimetallic aerogels with nanochain diameters of as small as 4 nm and Brunauer-Emmett-Teller (BET) surface areas of up to 60 m2/g could be obtained. Extensive structure analysis of Pt–Ni and Pt–Cu aerogels by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDX) and electrochemical techniques showed that both metals were predominantly present in their metallic state and formed homogeneous alloys. However, metal (hydr)oxide byproducts were observed in aerogels with higher contents of non-precious metal (>25 %). Moreover, electronic and geometric structures were similar to those of carbon-supported Pt alloy catalysts. As a result, ORR activites were comparable, too. A threefold improvement in surface-specific activity over Pt/C catalysts was achieved. The mass-specific activites met or exceeded the U.S. Department of Energy (DOE) target for automotive PEFC applications. Furthermore, a direct correlation between non-precious metal content in the alloy and ORR activity was discovered. Aerogels with nonprecious metal contents >25% turned out to be susceptible to dealloying in acid leaching experiments, but there was no indication for the formation of extended surface structures like Pt-skeletons. A Pt3Ni aerogel was successfully employed as the cathode catalyst layer in a differential fuel cell (1 cm2), which is a crucial step towards technical application. This was the first time an unsupported metallic aerogel was implemented in a PEFC. Accelerated stress tests that are usually applied to investigate the support stability of fuel cell catalysts revealed the excellent stability of Pt3Ni alloyed aerogels. In summary, the Pt alloy aerogels prepared in the context of this work have proven to be highly active oxygen reduction catalysts with remarkable stability.

La criticité du réchauffement climatique incite à chercher des solutions pour réduire les émissions de dioxyde de carbone (CO₂). Le développement de catalyseurs qui peuvent aider à capturer, activer, réduire et valoriser le CO₂ est au cœur de ce défi. Cette thèse a répondu à cet appel en développant des mimétismes moléculaires inspirés de la Nature, dans le cadre plus large de la photosynthèse artificielle. Au début il s'agissait de suivre le parcours d'un photon de lumière visible et de déterminer comment il peut réduire la molécule de CO₂. Ensuite afin de réaliser des catalyseurs plus efficaces, de nouvelles molécules ont été synthétisées en s’inspirant de l’enzyme CO déshydrogénase (CODH) qui présente des performances exceptionnelles pour la réduction du CO₂. Enfin, une autre propriété du CODH a conduit à une validation de principe pour la valorisation immédiate du CO photo-produit dans la synthèse des liaisons amides marqués, une caractéristique courante des médicaments. / The criticality of global warming urges for the advancement of science to reduce carbon dioxide (CO₂) emissions in the atmosphere. At the heart of this challenge is the development of sustainable catalysts that can help capture, activate, reduce, and eventually valorize CO₂. This PhD work tried to respond to this call by developing molecular mimics inspired by natural systems in the larger scheme of artificial photosynthesis. Firstly, it involved tracking the journey of a photon of visible light and how it is transformed to a reducing power able to reduce CO₂. Secondly, in search for more efficient and stable catalysts, new mimics were synthesized inspired by the exceptional performance of CO dehydrogenase enzymes (CODH) in reducing CO₂. Lastly, further understanding of CODH also led to a proof-of-concept that directly valorizes the photo-produced CO for the synthesis of isotopically-labelled amide bonds, a common motif in pharmaceutically-relevant drugs.

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We are facing an energy crisis because of the limitation of the fossil fuel and the pollution caused by burning it. Clean energy technologies, such as fuel cells and metal-air batteries, are studied extensively because of this high efficiency and less pollution. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential in the process of energy storage and conversion, and noble metals (e.g. Pt) are needed to catalyze the critical chemical reactions in these devices. Functionalized carbon nanomaterials such as heteroatom-doped and molecule-adsorbed graphene can be used as metal-free catalysts to replace the expensive and scarce platinum-based catalysts for the energy storage and conversion. Traditionally, experimental studies on the catalytic performance of carbon nanomaterials have been conducted extensively, however, there is a lack of computational studies to guide the experiments for rapid search for the best catalysts. In addition, theoretical mechanism and the rational design principle towards ORR and OER also need to be fully understood. In this dissertation, density functional theory calculations are performed to calculate the thermodynamic and electrochemical properties of heteroatom-doped graphene and molecule-adsorbed graphene for ORR and OER. Gibb's free energy, overpotential, charge transfer and edge effect are evaluated. The charge transfer analysis show the positive charges on the graphene surface caused by the heteroatom, hetero-edges and the adsorbed organic molecules play an essential role in improving the electrochemical properties of the carbon nanomaterials. Based on the calculations, design principles are introduced to rationally design and predict the electrochemical properties of doped graphene and molecule-adsorbed graphene as metal-free catalysts for ORR and OER. An intrinsic descriptor is discovered for the first time, which can be used as a materials parameter for rational design of the metal-free catalysts with carbon nanomaterials for energy storage and conversion. The success of the design principle provides a better understanding of the mechanism behind ORR and OER and a screening approach for the best catalyst for energy storage and conversion.

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Atmospheric carbon dioxide (CO₂) concentrations have increased rapidly in recent decades due to the burning of fossil fuels, deforestation, and other industrial practices. The excessive accumulation of CO₂ in the atmosphere leads to global warming, ocean acidification, and other environmental imbalances, which may ultimately have wider societal implications. One potential solution to closing the carbon cycle is utilizing CO₂, rather than fossil fuels, as the carbon source for fuels and chemicals production. This lowers atmospheric CO₂ levels while simultaneously providing an economic incentive for capturing and converting CO₂ into more valuable products. This dissertation includes studies on three hybrid catalytic reactor systems coupling electrochemistry, thermochemistry, and plasma chemistry for the conversion of CO₂ into value-added oxygenates, such as methanol and C3 oxygenates (propanal and 1-propanol). First, a tandem two-stage system is described where CO₂ is electrochemically reduced into syngas followed by the thermochemical methanol synthesis reaction. The work here specifically focuses on the electrochemical CO₂ reduction reaction to produce syngas with tunable H₂/CO ratios. Using a combination of electrochemical experiments, in-situ characterization, and density functional theory calculations, palladium-, gold-, and silver-modified transition metal carbides and nitrides were found to be promising catalysts for enhancing electrochemical activity while reducing the overall precious metal loading. Second, another tandem two-stage system is demonstrated where CO₂ is electrochemically reduced into ethylene and syngas followed by the thermochemical hydroformylation reaction to produce propanal and 1-propanol. The CO₂ electrolyzer was evaluated with Cu catalysts containing different oxidation states and with modifications to the gas diffusion layer hydrophobicity, while the hydroformylation reactor was tested over a Rh₁Co₃/MCM-41 catalyst. The tandem configuration achieved a C₃ oxygenate selectivity of ~18%, representing over a 4-fold improvement compared to direct electrochemical CO₂ conversion to 1-propanol in flow cells. Third, a hybrid plasma-catalytic system is investigated where CO₂ and ethane are directly converted into multi-carbon oxygenates in a one-step process under ambient conditions. Oxygenate selectivity was enhanced at lower plasma powers and higher CO₂ to C₂H₆ ratios, and the addition of a Rh₁Co₃/MCM-41 catalyst increased the oxygenate selectivity at early timescales. Plasma chemical kinetic modeling, isotopically-labeled CO₂ experiments, and in-situ spectroscopy were also used to probe the reaction pathways, revealing that alcohol formation occurred via the oxidation of ethane-derived activated species rather than a CO₂ hydrogenation pathway. It is critical to assess whether the proposed CO₂ conversion strategies consume more CO₂ than they emit. A comparative analysis of the energy costs and net CO₂ emissions is conducted for various reaction schemes, including four hybrid pathways (thermocatalytic-thermocatalytic, plasma-thermocatalytic, electrocatalytic-thermocatalytic, and electrocatalytic-electrocatalytic) for converting CO₂ into C₃ oxygenates. The hybrid processes can achieve a net reduction in CO₂ provided that low-carbon energy sources are used, however further catalyst improvements and engineering optimizations are necessary. Hybrid catalytic systems can provide an alternative approach to traditional processes, and these concepts can be extended to other chemical reactions and products, thereby opening new opportunities for innovative CO₂ utilization technologies.

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