Date of Award

2017

Document Type

Open Access Dissertation

Department

Chemical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Xiao-Dong Zhou

Abstract

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.

Available for download on Sunday, May 06, 2018

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