Structural, Interfacial, and Electrochemical Properties of Pr2NiO4+δ – Based Electrodes for Solid Oxide Fuel Cells

Currently, the electrochemical performance and performance durability of solid oxide fuel cells (SOFCs) are limited by cathode materials. The high polarization resistance and phase instability of the cathode are two major challenges to hinder the commercialization of SOFC systems. Two families of oxides are presently known as potential cathode materials for SOFCs: (1) the perovskite family of oxides with a general formula of ABO3, and (2) the Ruddlesden-Popper (RP) family of oxides (e.g. nickelates) with a general formula of A2BO4. The electron-hole conduction in these materials occurs simultaneously with oxygen ion conduction based on either oxygen vacancies (e.g. La1-xSrxCo1-yFeyO3-δ , LSCF), cation vacancies (e.g. La1-xSrxMnO3+δ, LSM), or oxygen interstitials (Ln2NiO4+δ, where Ln=La, Pr, Nd). Among these candidates, the Pr2NiO4+δ (PNO) shows the highest surface exchange and diffusion coefficients, lowest activation energy for oxygen reduction reaction, and lowest electrode polarization, making it a potential candidate for the next generation SOFC systems (Chapter 1). However, the phase transformation in PNO is of a concern as the structural instability has been linked to the long-term performance degradation (Chapters 3-4). Therefore, it is of a great scientific interest to find ways to stabilize the phase while retaining the activity in PNO. In this thesis, a new series of compositions (Pr1-xNdx)NiO4+δ and (Pr1-xNdx)2Ni1-yCuyNiO4+δ are introduced as phase and performance stable cathodes (Chapters 5-8). Detailed x-ray diffraction and in situ synchrotron studies showed that combination of doping on A- and B-sites provides structural rigidity, which in turn leads to suppressed phase transformations and stabilized performance, as evaluated via long-term durability studies in powders, electrodes, and full cells. This thesis also presents an in-depth comparison between phase transformation in thermal vs. electrochemical systems (Chapters 9-10). A discrepancy was found between the rates of phase transformation in thermally annealed nickelates when compared to their operation in full cells. Therefore, the thermodynamics and electrochemical potential driving forces were addressed respectively. Furthermore, the accelerated tests protocols were developed (Chapter 11) which can simulate the long-term cell operation (10,000-20,000 hours) within a fraction of time (1,000-2,000 hours). Finally, a deeper understanding behind the use of an interlayer (a buffer layer between the cathode and the electrolyte) was obtained (Chapter 12). It was found that by manipulating the interlayer chemistry the phase transformation in nickelates can be fully suppressed with a remarkable performance improvement of 48%. These combined studies provide deeper fundamental understanding behind structure - phase stability - electrochemical property relationship and can serve as a platform for future cathode and interlayer development

Activity and Stability of (Pr1-xNdx)(2)NiO4 as Cathodes for Solid Oxide Fuel Cells

Single phase (Pr1-xNdx)(2)NiO4 cathode powders (x = 0, 0.25, 0.50, 0.75, and 1.0) were synthesized via a glycine-nitrate combustion and high temperature calcination. Anode supported cells were used to investigate the cathode property. A reproducible performance, within 9% for each cathode composition, was observed providing a wealth of data for quantitative studies. Area specific resistance analysis and i-V measurements between 650 and 850 degrees C showed a decrease in the cell performance with increasing Nd content. Impedance spectrum analysis suggests that the decline in performance results from an increase in electrode polarization. While Pr2NiO4 cells showed significant performance degradation of 6.40%/1,000 hours, the degradation rate for (Pr0.75Nd0.25)(2)NiO4 cells was reduced by an order of magnitude (0.56%/1,000 hours) with a 7% lower power output. Likewise, the cathodes with a higher Nd content showed further improvement in performance stability with a marginal degradation rate of 0.06%/1,000 hours. (C) The Author(s) 2016. Published by ECS. All rights reserved

Activity and Stability of (Pr1-xNdx)(2)NiO4 as Cathodes for Solid Oxide Fuel Cells III. Crystal Structure, Electrical Properties, and Microstructural Analysis

This study is to complement an early article (Dogdibegovic et al., J. Electrochem. Soc., 163(13), F1344 (2016)) on the electrochemical activity and performance stability of (Pr1-xNdx)(2)NiO4+delta (PNNO) electrodes. Here, we report the crystal structure, electrical properties, and microstructures of PNNO series as the cathodes for solid oxide fuel cells. Rietveld refinements on powders (x = 0, 0.25, 0.50, 0.75, and 1) show that the unit cell volume decreases with an increase in x, primarily due to a decrease in the c lattice parameter. Larger cell volume (similar to 1.50%) and higher total electrical conduction (40%) in Pr2NiO4+delta are in favor with its mixed conducting properties during operation, but Pr2NiO4+delta cathode exhibits a severe phase evolution. Substitution of Pr with Nd shows the suppression of phase evolution in both thermally annealed powders and electrodes. An increase in Nd content leads to a full preservation of the parent phase in both (Pr0.25Nd0.75)(2)NiO4+delta and Nd2NiO4 after 2,500 hour annealing at elevated temperatures. Reaction with GDC buffer layer was also suppressed with the presence of Nd, which was shown by a reduction of Pr and Ni elemental diffusion into GDC bulk. STEM analysis confirms multiple phases present in an operated Pr2NiO4+delta electrode, while suppressed phase transition was observed in electrodes with high Nd content. (c) 2016 The Electrochemical Society

Activity and Stability of (Pr1-xNdx)(2)NiO4 as Cathodes for Solid Oxide Fuel Cells: Part V. In Situ Studies of Phase Evolution

This study is to complement an early report (the manuscript is attached for review purpose) on the role of interlayer on activity and performance stability in praseodymium nickelates. The aforementioned report showed a remarkable 48% increase in power density while switching from common GDC interlayer to a new interlayer chemistry (PGCO). Furthermore, a stable long-term performance was linked with suppressed reaction between the cathode and PGCO interlayer. In this article, we report in situ studies of the phase evolution. The high energy XRD studies at a synchrotron source showed fully suppressed phase transition in praseodymium nickelates with PGCO interlayer, while the electrodes on the GDC interlayer undergo substantial phase transformation. Furthermore, in operando and post-test XRD analyses shown fully suppressed structural changes in electrodes operated in full cells at 750 degrees C and 0.80 V for 500 hours. SEM-EDS analysis showed that the formation of PrOx at the cathode-interlayer interface may play a role in a decrease of mechanical integrity of the interfaces, due to thermal expansion mismatch, leading to a local stress between the two phases. Consequently, phase evolution at a narrow interface may propagate toward the electrode bulk, leading to structural changes and performance degradation. (C) The Author(s) 2017. Published by ECS. All rights reserved

The Role of Interlayer on the Catalytic Activity and Performance Stability of (Pr1-xNdx)(2)NiO4 as Cathodes for Solid Oxide Fuel Cells

Design optimization; reaction sphere; support vector machines; finite element metho