Liu, Tian-Hui
(2020)
Developing cathode and electrolyte materials for intermediate temperature solid oxide fuel cells.
PhD thesis, University of Nottingham.
Abstract
Solid Oxide Fuel Cells (SOFCs) are one of the most promising energy-conversion technologies due to their high work efficiency (50 - 60%) and fuel flexibility (hydrogen, syngas, biogas etc.). However, the high operating temperature (800 - 1000 °C) of most existing commercial SOFCs induces a number of issues including high manufacturing and operation cost, fast degradation of cell performance, slow start-up and shut-down cycles. Lowering the operating temperature to an intermediate temperature (550 °C - 750 °C) is an effective way to solve these issues. However, reducing operating temperature reduces the electrocatalytic performance of cathode and the oxide ion conductivity for the electrolyte, leading to increased cell internal resistance and reduced power density.
This project focused on developing new cathode and electrolyte materials that exhibit sufficiently high oxide ion conductivity and electrocatalytic activity at 550 - 750 °C, and high stability under cell operation conditions. In particular, the oxide ion conductivity and electrocatalytic activity of established cathode materials La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and La0.8Sr0.2MnO3+δ (LSM) were improved by doping bismuth into the La-site. Additionally, the oxygen ion conductivity of Mg-doped Na0.50Bi0.5TiO3 (NBT) system, which is a new electrolyte material, was improved through a wet chemistry synthesis method. Furthermore, the La0.8Sr0.2MnO3+δ/Na0.50Bi0.5Ti0.98Mg0.02O3 composite cathode material was developed by mixing the ionically conducting material Na0.50Bi0.5Ti0.98Mg0.02O3 and an electronically conducting material La0.8Sr0.2MnO3+δ, which exhibit improved catalytic activity as compared to La0.8Sr0.2MnO3+δ.
The La0.6-xBixSr0.4Co0.2Fe0.8O3-δ (LBSCF) powders were synthesised by the solid-state reaction and citrate gel methods, and the phase purity, crystal structure, composition were examined using XRD, SEM/EDX. Single phase LBSCF can be obtained when x ≤ 0.2 via the solid-state reaction method and when x ≤ 0.3 via the citrate gel method. The oxide ion conductivity and electrocatalytic performance were evaluated by oxygen permeation tests and area specific resistance (ASR) tests. LBSCF exhibits higher oxide ion conductivity, lower ASR and higher stability under CO2 as compared to LSCF. For example, the composition of x = 0.2, La0.4Bi0.2Sr0.4Co0.2Fe0.8O3-δ (LBSCF 42428) exhibits oxygen ion conductivity of 0.091 S cm-1 at 800°C as compared to 0.029 S cm-1 for undoped LSCF (x = 0.0, LSCF 6428). The ASR at 600 °C of LBSCF 42428 (0.28 Ω·cm2) is nearly one order of magnitude of lower than that of LSCF 6428 (2.56 Ω·cm2). After the 120 h long term test under the 5 % CO2, the ASR at 600 °C of LBSCF 42428 and LSCF 6428 increased to 0.37 Ω·cm2 and 3.22 Ω·cm2, respectively.
For the Na0.51Bi0.5-xTi1-yMgyO3 (x = 0.0, 0.04, 0.06 and 0.10, y = 0.02 and 0.05) system, the oxide ion conductivity at 350 °C was increased to 1.58×10-3 S cm-1 in Na0.51Bi0.44Ti0.98Mg0.02O3 by utilizing the sol-gel method. This is 3 times that of the reported data by Li et al. from solid-state reaction method. However, secondary phases (Na-Ti rich or Mg-Ti rich) were found in this system which are due to the losses of Na, Bi, Ti during the synthesis process.
The Bi doping limit in the La0.8-xBixSr0.2MnO3+δ system was found to be x = 0.2. The catalysis activity of LSM was improved by doping Bi or mixing with Na0.51Bi0.5-xTi1-yMgyO3, presumably associated with increased oxide ion conductivity.
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