Development of highly active ceria-based oxygen-electrode for thin-film solid oxide fuel cells

Abstract

Thin-film solid oxide fuel cells (TF-SOFCs), manufactured using micro-fabrication techniques, have attracted much interest given their capability to lower the operating temperature to less than 650 $^\circ C$ and their resulting extended lifetimes and reduced material and system costs compared to those of conventional SOFCs. Pt thin films and Sr/Co-based perovskite oxides, which are commonly used as an oxygen electrode in TF-SOFCs, have been studied due to their high electrical conductivity and excellent catalytic activity for the oxygen reduction reaction (ORR). However, in Pt a thin-film electrode, the ORR occurs at the limited reaction site of the triple-phase boundary, where Pt, the electrolyte and oxygen gas come into contact, and the electrode performance is degraded due to the inherent thermal instability of Pt in the operating environment. On the other hand, Sr/Co-based perovskite electrodes undergo not only chemical instability related to the Sr segregation phenomenon but also structural instability due to the volatility of Co, degrading the electrode performance. For these reason, research on simultaneously improving the ORR activity and thermal stability of Pt electrodes as well as research on the development of a Sr/Co-free electrode with favorable ORR activity and high chemical stability is in great demand. To address these issues, I attempted to apply a fluorite-based oxide, $Pr-doped CeO_{2}$ (PCO) in this work, to both Pt and perovskite-based electrodes, as PCO films offer several advantages of mixed ionic electronic conducting capabilities in an oxidizing atmosphere, favorable electro-catalytic activity for the ORR, and excellent chemical stability. First, I fabricated PCO-Pt composite symmetric cells as a model system to evaluate the impact of PCO overcoats quantitatively on the electrode activity and thermal stability of Pt thin-film oxygen electrodes at a reduced temperature. I overcoated Pt thin film with PCO by means of a simple, cost-effective and scalable cathodic electrochemical deposition process that produced nanostructured oxide layers with a high specific surface area and uniform metal coverage. The resulting structures were examined by SEM, BET and ICP-MS. The electrode activity of the symmetric cells was then analyzed by electrochemical impedance spectroscopy. The combination of excellent conductivity, reactivity, and durability of the PCO dramatically enhances the ORR by more than 1000 times while maintaining the nanoscale architecture of the PCO layers and thus the performance of the PCO-coated Pt electrodes at high temperatures. This occurs because the predominant reaction sites expand from triple-phase boundary to double-phase boundary. Furthermore, the resulting films exhibit good thermal stability for 60 h at a relatively high temperature of 650 $^\circ C$. Next, as part of the process of substituting Sr/Co-based perovskite oxide with PCO, I fabricated model PCO thin films with four different Pr concentrations (5, 10, 20 and 40 mol% in this work) on $c-Al_{2}O_{3}$ (0001) single-crystal substrates via pulsed laser deposition. The resulting films were used to investigate the oxygen exchange kinetics on the PCO surface by means of electrical conductivity relaxation measurements. In-plane conductivity measurements of the PCO films indicate that the total electrical conductivity increases with the Pr concentration. Furthermore, the oxygen exchange coefficient ($k_{chem}$) was examined by electrical conductivity relaxation as a function of the Pr concentration. I found that the higher the Pr concentration in the PCO films was, the higher the $k_{chem}$ value became, with the highest value found at 40 mol% Pr. The activation energy ($E_{a}$) values as obtained through an Arrhenius plot decrease with the Pr concentration. Interestingly, this trend of $E_{a}$ is similar to that of the Pr ionization enthalpy, $H_{Pr}$, implying that there is a close correlation between the oxygen exchange kinetics on the PCO surface and the electronic structures of PCO films. These observations provide a guideline for the design and fabrication of high-performance oxygen electrodes for TF-SOFCs. Finally, based on the surface activity and electrical conductivity, I prepared symmetric electrochemical cells with vertically oriented columnar PCO films by pulsed laser deposition in an effort to realize a stable and high-performance electrode for TF-SOFCs. To maximize the electrode performance of vertically oriented columnar PCO films, various control variables, in this case the film thickness, working pressure for the deposition, and Pr composition, were appropriately controlled. As a result, as the thickness of the films increases, the electrode resistance decreases significantly. Furthermore, it was found that the higher the working pressure is for the deposition and Pr concentration, the better the electrode performance becomes. Taken together, the columnar PCO40 films exhibit low electrode resistance of $\sim 0.05 \Omega cm^{2}$ even at the low temperature of 550 $^\circ C$ with $pO_{2}$ = 0.21 atm, which is much better to that of the reference $1-{\mu}m-thick La_{0.6}Sr_{0.4}CoO_{3-\delta}$ $(\sim 1 \Omega cm^{2})$, considered to be the state-of-the-art perovskite oxide. It shows superior long-term stability for 330 h at 550 $^\circ C$ with degradation of 0.2 %/h compared to 3.1 %/h for the perovskite oxide. Based on these observations, for the first time, we designed and fabricated an anode-supported cell with Sr/Co-free columnar PCO films and succeeded in achieving stable, high performance outcomes with a peak power density of $0.92 W cm^{-2}$ at 600 $^\circ C$. These findings present a new concept of electrode material design and fabrication for stable and high-performance TF-SOFCs.