Nanoscale Hexagonal Oxide-ion Conductors for Low-Temperature Applications

Renewable energy conversion technologies are crucial for achieving a sustainable future as they play an essential role in reducing CO₂ emissions. Among them, electrochemical cells, including electrolysis, fuel cells, and batteries, are particularly considered to be integral for future generations across the vehicular applications, stationary power systems, and portable electronic devices by their high efficiency in converting chemical energy into electrical energy. This interesting feature highlights the potential importance of electrochemical energy storage and conversion, which has thereby emerged to meet more demands in future energy solutions. Solid Oxide Cells (SOCs) remain a promising technology for stationary and power to chemical synthesis (P to X) among the electrochemical cells, as it provides numerous advantages such as flexibility for operating, cost-effective catalysts, combined heat and power generation, and reduced electrolyte utilization, which all collectively lead to an increase in overall efficiency. Due to these notable advantages of SOCs, research has focused on oxygen-ion conductors, which have attracted substantial interest as electrolytes in various technologies, including Solid Oxide Fuel Cells (SOFCs), membranes, Oxygen sensors, and battery applications. However, high operating temperatures, typically above 700°C, are required to fulfil the optimal performance. The high operating temperature influences the device’s performance, including incompatibility with cell components, reduced stability, high costs, and prolonged startup times. The traditional oxide-ion conductor, Yttria-Stabilized Zirconia (YSZ), has been the predominant in SOCs Applications. However, the demand has been increasing for materials in solid-state Ionics, which is the main reason that drives the exploration of advanced innovative electrolytes and electrode materials for enhancing electrochemical performance. Significant efforts are dedicated to developing Solid-state electrolytes for intermediate to low temperature (300 to 600°C) Applications. A wide range of materials includes LAMOX (lanthanum molybdate), Silicon and germanium-based apatite, bismuth-based compounds, and Arivillius-based oxides such as BIMEVOX, BICUVOX, and perovskite oxides, where the last one exhibits high oxide-ion conductivity. Recent studies have revealed substantial oxide-ion conduction in cation-deficient hexagonal perovskite derivatives. One of them is Ba₃M"M"O_8.5, a disordered hybrid 9R-palmierite structure, which has gained attention for exploring this family as novel oxide-ion conductors. The notable oxide-ion conductor Ba₃NbMO_8.5 (M=Mo,W) adopts a hybrid structure intermediate between 9R perovskite and palmierite crystal system. Based on these insights, our work aims to investigate the transport properties and interface effects at both bulk and nano scale of Ba₇Nb₄MoO₂₀, a cation-deficient 7H hexagonal perovskite derivative. The remarkable oxide-ion conductivity at low temperature makes this material a promising candidate with its distinct disordered crystal structure. Numerous factors are essential for enhancing the performance and stability of operation, including microstructure engineering, optimizing suitable electrode material, operating conditions, and understanding the electrode/electrolyte interface effects. Exploring the electrochemical properties at the nanoscale for this ionic conductor can open a platform for designing advanced solid-state electrolytes for low-temperature applications in many technologies. This work primarily explores the understanding of the interfacial effects of this ionic conductor at the bulk level with different electrodes and the impacts of processing methods, providing a strong insight into the system. It mainly aims to address the challenge through the systematic approach from bulk to nanoscale thin-film engineering by adopting various techniques and characterization including Pulse laser deposition (PLD) method for fabrication of thin films, X-ray diffraction, Reciprocal space mapping (RSM), Microscopic techniques (SEM and TEM) to understand the growth mechanism of the films, Spectroscopic methods such as Raman and XPS to analyze the composition of the films, Electrochemical Impedance Spectroscopy as a primary tool to understand the different conductivity contributions (ionic, protonic and electronic) for this Ba7Nb4MoO20 - system both at bulk and thin film level.

In summary, this PhD thesis aims to understand the transport mechanisms of Ba₇Nb₄MoO₂₀ oxide-ion conductor, focusing on the transition from bulk to thin film, highlighting this hexagonal perovskite system as a suitable solid-state electrolyte for intermediate and low temperature applications. The work encompasses the optimization of interfacial features between electrodes and electrolyte at the bulk level, as well as a detailed exploration of strain effects and diffusion mechanisms influenced by different substrates at nanoscale thin films. These findings underscore the potential of thin-film oxide conductors to improve efficiency and versatility, making this system a promising candidate for integration into next-generation energy devices.