Solid oxide cells (SOCs) are electrochemical energy conversion devices which can work in either fuel cell mode to directly convert fuel into electrical power or vice versa when working in electrolysis mode. SOCs are ceramic-based devices with a dense solid oxide electrolyte, able to conduct negative oxygen ions, sandwiched between two electrodes.
This thesis focuses on the oxygen electrode optimization using thin films (≤1000 nm) deposited by Pulsed Injection-Metal Organic Chemical Vapor Deposition (PI-MOCVD) and on their advanced structural and electrochemical characterization. La₂NiO_4+δ (L2NO4) is an oxide with a Ruddlesden-Popper phase layered structure consisting of alternated rock salt and perovskite layers.
It is a promising oxygen electrode material for intermediate (500- 700  °C) and low temperature (< 500 °C) operation due to its high oxygen surface exchange and diffusion coefficients, and thermal expansion coefficients close to the commonly used electrolytes. This study is aimed at tailoring and optimizing the nanostructure of L2NO4 thin films for high performance reversible solid oxide cells (R-SOCs) and micro-solid oxide cells (μ-SOCs). Kinetic studies have been performed by Electrical Relaxation Conductivity (ECR) and Electrochemical Impedance Spectroscopy (EIS). Advanced characterization tools such as in situ Raman spectroscopy have been utilized to understand the phase transitions of L2NO4.
An innovative electrochemical device was fabricated to control and measure the oxygen content on L2NO4 thin film.
This was combined with advanced characterization tools such as in situ X-ray diffraction, in situ Raman spectroscopy and in situ spectroscopic ellipsometry to study the structural and optical properties of L2NO4 when varying the oxygen content. Finally, full cell measurements and stability tests in SOFC and SOEC modes have been carried out on fuel electrode-supported and electrolyte-supported cells.