Date of Award

2018

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Fanglin (Frank) Chen

Abstract

Solid oxide cells (SOCs) can convert chemical energy to electricity in the fuel cell mode and store electricity to chemicals in the electrolysis mode. However, there are still critical barriers, such as energy efficiency and durability, for the development and commercialization of SOCs. The objective of this dissertation is to apply system optimization and materials design to address the critical barriers for solid oxide cells in energy conversion and energy storage applications. One major focus of the dissertation is related to improve energy efficiency, enhance the cell performance and achieve multifunctionality in solid oxide electrolysis cells (SOECs). In addition, development of robust air electrode and mitigation of Cr poisoning in the air electrode of solid oxide fuel cells (SOFCs) is also pursued.

Electrolysis of steam or carbon dioxide using SOECs is a promising energy storage method that can efficiently convert electrical energy into chemicals. In conventional SOECs, a significant portion of electricity input is consumed to overcome a large oxygen potential gradient between the electrodes. Therefore, to reduce the electricity consumption and improve the system efficiency, a novel and efficient syngas generator, integrating carbon gasification and solid oxide co-electrolysis, is presented and evaluated in the first part of this dissertation. The feasibility of this new system is demonstrated in La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte-supported SOECs. Both thermodynamic calculation and experimental results show that the potential barrier for co-electrolysis can be reduced by about 1 V and the electricity consumption can be reduced by more than 90% upon integration SOECs with carbon gasification. On the anode side, “CO shuttle” between the electrochemical reaction sites and solid carbon is realized through the Boudouard reaction (C+CO2=2CO). Simultaneous production of CO on the anode side and CO/H2 on the cathode side generates syngas that can serve as fuel for power generation or feedstock for chemical plants. The integration of carbon gasification and SOECs provides a potential pathway for efficient utilization of electricity, coal/biomass, and CO2 to store electrical energy, produce clean fuel, and achieve a carbon neutral sustainable energy supply.

In the second part of this dissertation, to achieve multifunctionality and regulate product in SOECs, a novel micro-tubular electrochemical reactor is studied, in which high temperature co-electrolysis of H2O-CO2 and low temperature methanation processes are synergistically integrated. The temperature gradient along the micro-tubular reactor provides favorable conditions for both the electrolysis and methanation reactions. Moreover, the micro-tubular reactor can provide high volumetric factor for both the electrolysis and methanation processes. When the cathode of the micro-tubular reactor is fed with a stream of 10.7% CO2, 69.3% H2 and 20.0% H2O, an electrolysis current of -0.32 A improves CH4 yield from 12.3% to 21.1% and CO2 conversion rate from 64.9% to 87.7%, compared with the operation at open circuit voltage. Furthermore, the effects of the inlet gas composition in the cathode on CO2 conversion rate and CH4 yield are systematically investigated. Higher ratio of H:C in the inlet results in higher CO2 conversion rate. Among all the cases studied, the highest CH4 yield of 23.1% has been achieved when the inlet gas in the cathode is consisted of 21.3% CO2, 58.7% H2 and 20.0% H2O with an electrolysis current of -0.32 A.

Furthermore, the state-of-the-art SOECs are based on oxygen-ion conducting electrolyte (O-SOECs), but SOECs using proton-conducting electrolyte (H-SOECs) offer the advantages of producing dry and pure hydrogen. However, the development of H-SOECs falls far behind that of O-SOECs, mainly due to technical challenges such as the stability of the electrolyte and electrode in H2O-containing atmosphere at operating conditions and the fabrication of thin electrolyte layer. Therefore, in the third part of the dissertation, BaZr0.8Y0.2O3-δ (BZY) electrolyte and Sr2Fe1.5Mo0.5O6-δ (SFM) air electrode, both are stable in H2O-containing atmosphere at operating conditions, are evaluated in H-SOECs. In addition, in order to improve the performance of H-SOECs, thin BZY electrolyte layer (about 16 μm in thickness) and nano-scaled SFM-BZY air electrode are fabricated successfully, showing excellent SOEC performance (-0.21 A cm-2 at 600 oC) and achieving faradaic efficiency of 63.6% at intermediate temperature.

Solid oxide fuel cell (SOFC) is the reverse operation of SOEC and can directly convert the chemical energy in fuels to electricity with high efficiency and is fuel flexible. The durability and performance of SOFCs are highly related to the reaction kinetics and stability of the air electrode. To enhance the reaction kinetics of the air electrode, a novel hybrid catalyst consisting of PrNi0.5Mn0.5O3 and PrOx is impregnated in the conventional (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x (LSCF) air electrode of H-SOFCs for the first time. The effects of this impregnation on the electrochemical performance and durability of H-SOFCs are investigated in the forth part of the dissertation. Single cells with impregnated LSCF cathode and BZY electrolyte yield a maximum power density (MPD) of 0.198 W cm-2 at 873 K, more than doubled than that with blank LSCF cathode (0.083 W cm-2) at the same operating conditions. Electrical conductivity relaxation (ECR) and electrochemical impedance spectroscopy (EIS) studies reveal that the hybrid catalyst can substantially accelerate the oxygen-ion transfer and oxygen dissociation-absorption processes in the cathode, resulting in significantly lower polarization resistance and higher MPD. In addition, the hybrid catalyst possesses good chemical and microstructural stability at 600 oC. Consequently, the single cells with impregnated LSCF cathode show excellent durability. This study shows that the impregnation of this novel hybrid catalyst in the cathode could be a promising approach to improve the performence and stability of H-SOFCs.

Moreover, the state-of-the-art SOFC air electrode is suffering from chromium-poisoning, leading to high SOFC cell performance degradation. To mitigate the effect of Cr poisoning, placing Cr getter between the air electrode and the Cr sources is an efficient approach. However, the stability of the state-of-the-art Cr getter materials in the SOFC cathodic operation conditions remains challenging. For the first time, SFM is investigated as Cr getter material in the fifth part of the dissertation. The reactivity of SFM, which is stable in an atmosphere containing H2O and CO2, with Cr species (Cr2O3) is evaluated. Subsequently, the feasibility of SFM as Cr getter material is investigated. The experiment results show that SFM, which is stable in an atmosphere containing H2O and CO2, possesses high reactivity and selectivity with Cr species (Cr2O3). Consequently, the application of SFM layer can efficiently capture the gaseous Cr species, resulting in significant mitigation of Cr poisoning for the air electrode.

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