GEORGIA INSTITUTE OF TECHNOLOGY
Under the provisions of the regulations for the degree
DOCTOR OF PHILOSOPHY
on Monday, June 20, 2022
10:00 AM
via
Teams Video Conferencing
will be held the
DISSERTATION DEFENSE
for
Weilin Zhang
“Development of Novel Electrode and Catalyst Materials for Solid Oxide Electrochemical Cells”
Committee Members:
Prof. Meilin Liu, Advisor, MSE
Prof. Preet Singh, MSE
Prof. Hamid Garmestani, MSE
Prof. Faisal Alamgir, MSE
Prof. Thomas Fuller, CHBE
Abstract:
Solid oxide electrochemical cell (SOC), which offers a promising solution to a sustainable energy future, is a class of solid-state electrochemical devices for efficient energy storage and conversion. A SOC can operate on the fuel cell mode (solid oxide fuel cell, SOFC) to generate electricity from hydrogen or hydrocarbon fuels. It can also operate on the electrolysis cell mode (solid oxide electrolysis cell, SOEC) to produce valuable fuels by electrolyzing water or carbon dioxide. This dissertation focuses on the development of novel electrode, catalyst, and fabrication techniques for SOCs. The overall study can be separated into four parts.
The first study focuses on the development of active and durable air electrode material for intermediate-temperature reversible solid oxide cells. To achieve high round-trip efficiency of SOCs, highly efficient and durable air electrode materials are needed to minimize energy loss associated with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here we report a bifunctional air electrode material, PrBa0.9Co1.96Nb0.04O5+δ, demonstrating outstanding electrochemical performance (e.g., achieving peak power densities of over 1.5 and 1 W cm−2, respectively, for Gd0.1Ce0.9O1.95 and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ based fuel cells at 600 ºC) while maintaining excellent stability (e.g., having a degradation rate of 40 mV per 1,000 h for H2O electrolysis cells). The excellent property of the new electrode is attributed to the improved stability from Nb doping and the enhanced electrocatalytic activity from tuning Ba deficiency, as confirmed by experimental results and computational analysis.
Following the first study, to lower the operating temperature of SOC, the second study focuses on the development of triple conducting air electrode materials for low-temperature dual-ion conducting fuel cells. A series of materials candidates were designed by heavily doping transition metal and rare earth metal into BaHf0.8Y0.2O3-δ-based electrolyte material. The optimized composition Ba0.9Pr0.1Hf0.1Y0.1Co0.8O3-δ demonstrates an outstanding electrochemical activity on both BaZr0.1Ce0.7Y0.1Yb0.1O3-δ based symmetrical cells and single cells. X-ray diffraction and transmission electron microscopy results confirm that BPHYC is a mixture of three different phases, consisting of Y doped BaCoO3-δ (BYC), PrBaCo2O5+δ (PBC), and Y doped BaHfO3-δ (BHY). Detailed electrochemical analysis unravels that BYC and PBC phases play a synergistic effect on the ORR kinetics on oxygen-ion conducting cells, and BHY contributes to the catalytic activity on proton-conducting cells. The triple conductivity of BPHYC was evaluated by the electrical conductivity relaxation measurement and the isotope exchange diffusion profile measurement. The long-term stability of BPHYC was also confirmed on both symmetrical cells and single cells under typical operating conditions.
For the fuel electrode development, the third study focuses on the development of anode catalyst materials for SOFCs operated on hydrocarbon fuels. Conventional Ni-based cermet anode suffers from the coking issue when the hydrocarbon is used as the fuel. In this study, I designed and synthesized Ni and Ru co-doped BaZr0.8Y0.2O3-δ (BZYNR) as a promising anode catalyst material for SOFCs operated on various hydrocarbon fuels. When applied BZYNR on button cells with a conventional Ni-based anode, the single cells can operate on wet iso-octane (with 3 vol% H2O) for over 1,000 hours without obvious coking behavior. BZYNR was further applied on large-scale tubular cells with an effective area of about 38 cm2. World-record power output and durability were demonstrated on iso-octane, ethanol, and methane fuels. Two key properties of BZYNR contributed to its outstanding performance. First is that Ni and Ru cations are highly active sites for the hydrocarbon reforming process. The second is that BZYNR shows good water absorption capability, which benefits the water-mediated carbon removal process.
The fourth study focuses on the fabrication techniques to create nanostructured electrodes for SOFCs. Durable, nanostructured electrodes fabricated via a simple, cost-effective method is effective to solve the performance and durability issues for SOFCs. In this work, both the nanostructured PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) cathode and Ni−Ce0.8Sm0.2O1.9 (SDC) anode are fabricated on porous yttria-stabilized zirconia (YSZ) backbone via solution infiltration. Symmetrical cells with a configuration of PBSCF|YSZ|PBSCF show a low interfacial polarization resistance of 0.03 Ω cm2 with minimal degradation at 700 °C for 600 h. Ni-SDC|YSZ|PBSCF single cells exhibit a peak power density of 0.62 W cm−2 at 650 °C operated on H2 with good thermal cycling stability for 110 h. Single cells also show excellent coking tolerance with stable operation on CH4 for over 120 h. This work offers a promising pathway toward the development of high-performance and durable SOFCs to be powered by natural gas.
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