UNDERSTANDING THE SOLID/SOLID AND LIQUID/SOLID INTERFACE PHENOMENA FOR ALTERNATIVE ENERGY APPLICATIONS

In many alternative energy technologies, interfacial phenomena are critical in determining process efficiency and feasibility. Previously, a complete application of density functional theory (DFT) as a tool to study interfaces has not only led to fundamental insights into the system, but also predictive models and recommendations for materials that were experimentally validated. In this thesis, it is first demonstrated that such an approach can also be used to solve a well-characterized interface problems related to sustainable energy technologies. This first problem is a solid/solid interface between the solid oxide fuel cell (SOFC) and its sealant, specifically an interface between SOFC’s exposed electrolyte, which is typically made of yttria-stabilized zirconia (YSZ), and the commercial Ag-CuO sealing braze. The current material, CuO, which helps molten Ag to wet on YSZ surfaces and later adhere once the Ag is solidified, has been previously characterized to form a CuO-rich layer between Ag and YSZ. However, over time it is reduced by the SOFC’s reductive conditions and results in formation of pores. Therefore, alternative oxides are needed that are thermodynamically stable in SOFC’s high operating temperatures and also provide similar adhesion properties to CuO. To that end, this thesis has identified two mechanisms that enable only CuO to work so far. First, it was found that, unlike most other Ag/oxides, the Ag/CuO interface has a particularly strong adhesion ‒ five times that of Ag/YSZ. Second, the dissolved oxygen in molten Ag from air during the brazing process diffuses to the interface and partially improves adhesion. The high adhesion of Ag/CuO was proposed to be from CuO’s unique atomic structure. Therefore, a descriptor based on the oxide’s structural and chemical features was developed to predict Ag/oxide adhesion. The descriptor expedites screening of new oxides as no expensive calculation is needed, leading to several recommendations such as CuAlO2 and Cu3TiO4.In the second problem, a more complex and less well-characterized interface in sustainable energy technologies is investigated. It should be noted that due to the complexity, the screening model cannot be developed yet. This second interface is a liquid/solid interface between solvated lignin and its catalyst. Lignin is a highly heterogeneous polymer in lignocellulosic biomass with many functional groups. Its valorization is essential to the economic feasibility of biorefinery, and its catalytic hydrogenolysis in a liquid phase is well-studied technology route to add value to the lignin. Many experiments have shown that solvent choice can have large impacts on product types and yields. However, the solvent effect on the reaction is still unclear. An understanding of the solvated lignin/catalyst interface would be valuable. For this, we are interested in the adsorption. To achieve this, spectroscopic and wet chemistry techniques are first used to help characterize the system by identifying that the quantity of ether linkages in lignin is the most critical functional group that determines the hydrogenolysis yields. Using a lignin dimer that contains a characteristic ether linkage, DFT shows that, in vacuum, the adsorption of the dimer is much stronger on Ni(111) than Cu(111). Upon solvation with ethanol, it was found that the dimer-metal interactions weaken so significantly that for Cu(111), it no longer adsorbs onto the metal surface. This implies that Cu may not provide high catalytic activity, which agrees with hydrogenolysis experiments performed in this thesis, although there could be other contributing factors. Lastly, to circumvent large DFT calculations, a model based on a thermodynamic cycle was developed to predict adsorption energy of a solvated lignin dimer for a given pair of solvent and catalyst. Although the model is not suitable for screening purposes, it provides a valuable, quantitative insight to the solvent effects.