Semester

Summer

Date of Graduation

2025

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Edward M. Sabolsky

Committee Member

Wenyuan Li

Committee Member

Kostas Sierros

Abstract

As the world seeks to reduce its reliance on hydrocarbons, the demand for sustainable hydrogen production methods has become increasingly critical. Hydrogen is a versatile energy carrier that can be produced from various sources, including water, natural gas, and biomass. Replacing the burning of hydrocarbons with hydrogen could significantly reduce greenhouse gas emissions, contributing to global climate goals like those set by the US H2NEW program. The joint technologies of solid oxide fuel and electrolysis cells present the ability to both produce and process clean hydrogen to meet these goals. Development into solid oxide fuel cells has been met with success, with several on-market fuel cell stacks having been developed and utilized. However, the development of solid oxide electrolysis cells has lagged, leaving a critical gap in the production of clean hydrogen gas. Though SOECs offer several advantages over traditional low-temperature electrolysis methods, including higher efficiency and the potential for integration with renewable energy sources and industrial processes, the technology faces challenges. The presence of steam and the common use of stainless-steel interconnects results in the known issue of Cr migration, where high-temperature gaseous chromium hydroxide’s high vapor pressure enables it to react with many components within the cells. This causes insulative solid phases and altered electrode chemistries, as well as blockage of active sites and Cr migration to the electrode-electrolyte interface, ultimately leading to delamination of the cells and long-term degradation of the system. This issue and others result in the need for chromium-resistant electrode materials that must have strong electrochemical properties. Commonly used SOFC materials have been shown to struggle with chromium poisoning in part due to strontium content, while newer developed SOEC materials may have suboptimal electrochemical properties.

In this work, the process of surfactant-enhanced wet impregnation of nanoparticulate oxides was studied as a potential nano-coating method for the SOEC material La2NiO4+δ. The overarching goal of the research was to improve the surface oxygen exchange coefficient by addition of an ionically conductive oxide surface layer, with the effect being the improvement of the overall electrochemical performance of the cell. Three nanocoating compositions were studied, including ternary oxide LaCoO3 (LCO), quaternary oxide La2MnNiO6 (LMNO), and high-entropy particle (HEP) La0.2Sr0.2Pr0.2Y0.2Ba0.2Co0.2Fe0.8 (LSPYB). Initially, the work studied the LCO composition, and the information from these experiments was then applied to the infiltration of the chosen quaternary and HEP compositions.

Investigation into the LCO composition as a coating candidate began with the selection of a variety of surfactant molecules as chelating agents for the infiltration process. A powder study was completed to determine the most suitable candidate for chelation of the oxides in aqueous solution and enabling correct phase formation when calcined at temperatures ≤900 °C. This low temperature was chosen to maintain the high surface area of the nanocoating by reducing nanoparticle densification, a property that contributes to more efficient oxygen exchange properties. The molecules studied were catechol and catechol-like chemistries, including poly-norepinephrine (pNE), caffeic acid, dihydroxybenzoic acid (DHBA), and gallic acid. Previous work from Ozmen et al. and Wang et al. showed the success of bio-inspired catechols such as pNE, caffeic acid, and DOPA to deposit single-component nanoparticles [1], [2]. This work expands on this premise to include both new catechol-family molecules as chelating agents, and to attempt deposition of more complex oxides (≥3 different cations within the perovskite). For the purposes of nanocoating, a successful chelating agent will be defined to serve three purposes: 1) act as a

complexing agent and in the formation of complex, high-order oxides, 2) assist in the deposition of these chemistries in distinct, nano-range structures, and 3) assist in controlling the homogeneity of these coatings, with optimal coverage statistics.

Initial experiments focused on the formation of the binary LCO composition. Complexing properties of the chosen chelating agents were analyzed using X-ray diffractometry (XRD) to determine the impact of the molecules on the formation phase pure nanoparticles. LCO nanoparticles were deposited on highly polished and flat single-crystal YSZ substrates, then analyzed by atomic force microscopy (AFM) to determine the impact of deposition parameters on the nucleation and growth rate of the samples and the final microstructure of the nano-coatings. These samples were then evaluated using X-ray photoelectron spectroscopy (XPS) for a second level of validation for the results of the phase-purity testing by analysis of binding energies. Results from these experiments were implemented on the heterostructured LNO – LNO/GDC porous electrode microstructures to determine their impact on the performance of the coated cell. The infiltrated LNO microstructures were studied by scanning electron microscopy (SEM) and electron diffractometry spectroscopy (EDX), and the polarization resistance of the LCO-coated LNO was evaluated by symmetrical cell electrochemical impedance spectroscopy (EIS) testing and compared to baseline LNO performance.

Using the methods and understanding gleaned from the LCO nano-deposition study, selected experiments were completed for higher complexity solid solution nano-catalysts. For both LMNO and LSPYB, a representative powder study (with samples analyzed using XRD) was completed with each of the previously attempted chelating agents to determine any changes in effectiveness, after which infiltration into symmetrical LNO cells was completed. LMNO samples were analyzed using EIS and SEM to determine effectiveness of nanocoating. The same methods and understanding were utilized to successfully deposit the more complex high-entropy perovskite (HEP) LSPYB. Despite success in ex-situ analysis, low pH as a complication from Fe-nitrate in solution resulted in destruction of the LNO backbone. Alternate infiltration procedures and materials were investigated and implemented, resulting in a high phase purity nano-coating that provided electrochemical improvements at lower infiltration concentration and deposition times.

From all tested nanocoatings, all three showed at least some performance improvements, with the most successful LCO nano-catalyst improving the performance across all infiltration times and concentrations.

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