Convertible solid oxide fuel cell/electrolyser cell with optimised composite electrodes and bi-layer electrolyte

• Main Author: Heidari, Dorna
• Other Authors: Dong Zhili
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Mechanical engineering::Alternative, renewable energy sources; DRNTU::Engineering::Materials::Ceramic materials; DRNTU::Engineering::Mechanical engineering::Energy conservation; DRNTU::Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/62228
• Institution: Nanyang Technological University

Fuel cells are electrochemical and efficient energy conversion devices that directly convert a fuel’s chemical energy into electrical energy. They have environmental advantages in reduced pollution and high energy conversion efficiency when compared with the conventional power generation systems such as internal combustion engines and gas turbines fired with fossil fuels. Among all fuel cell types, the Solid Oxide Fuel Cell (SOFC) produces high quality waste heat, which can be utilised to increase the overall energy conversion efficiency. When operating in a reverse manner, a SOFC becomes a Solid Oxide Electrolyser Cell (SOEC), which works well with the intermittent renewable energy such as solar PV and wind turbine as it converts the electrical energy generated by the renewable energy into hydrogen through electrolysis of water, thus serving as energy storage. The objective of this study is to propose a Solid Oxide Cell (SOC) system, which is optimised to operate in both SOFC and SOEC modes. In order to explore electrolytes with improved conductivity, ceria based electrolytes including gadolinia doped ceria (GDC), Gd0.2Ce0.8O1.9, and samaria doped ceria (SDC), Sm0.2Ce0.8O1.9, are synthesised and characterised. Samples sintered at 1400°C-1500°C possess relative densities as high as 97.7% suggesting that this temperature range leads to improved densification of electrolyte as reflected by the high open circuit voltages. The Arrhenius plots of GDC and SDC conductivities show that SDC electrolyte presents higher conductivity compared to GDC, especially those sintered at 1400°C and 1500°C. Moreover, Ni-GDC/GDC/Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) and Ni-SDC/SDC/BSCF cells were fabricated and polarisation curves were obtained. The thickness of the SDC and GDC electrolytes were identical and about 7.5 µm. Based on polarisation results, open-circuit voltages (OCVs) of 0.829 V, 0.817 V and, 0.792 V and maximum power densities of 0.368 W.cm-2, 0.619 W.cm-2 and, 0.790 W.cm-2 were recorded at 500°C, 550°C, and 600°C, respectively, for the Ni-GDC/GDC/BSCF cell. With humidified hydrogen (3 vol% water vapour) as fuel and air as oxidant, the Ni-SDC/SDC/BSCF cell generates OCVs at 500°C, 550°C, and 600°C as 0.848 V, 0.815 V, and 0.797 V, and maximum power densities of 0.398 W.cm-2, 0.810 W.cm-2 and, 1.1 W.cm-2, respectively. There is a potential for a bi-layer electrolyte to render high ionic conductivity without a remarkable sacrifice of open circuit voltage and lowering the operating temperature of the cell. Ni-YSZ/YSZ-SDC/BSCF-SDC cell with 7 wt.% of graphite as the pore former in the Ni-YSZ structure, dense YSZ-SDC as the electrolyte and porous 20 wt.%SDC-80 wt.% BSCF as the air electrode were the three distinguishable layers well adhered to each other in order to study the effect of blocking layer in the electrochemical performance of the SOFCs and SOECs. In order to dominate the OCV loss and enhance the chemical stability of doped ceria electrolyte in reducing atmospheres, SDC was coated with a very thin layer of YSZ film (1.5 µm, 2.5 µm, 3.5 µm, 3.8 µm, and 4.4 µm) to form a bi-layer electrolyte. In both the fuel cell and electrolyser cell mode, the blocking of electrons takes significant effect when the blocking layer is increased to 3.5 µm. Although the porosity of the Ni-YSZ anode increases by in-situ reduction of NiO available in the primary anode material, the final value of the anode porosity of the cell with Ni-YSZ structure is not high enough to provide an easy flow of the gases during the cell operation. Hence, adding a pore former enhances the gas accessibility to the anode active three-phase boundary. So, the porous structures of fuel electrodes have been proposed to fulfil the anodic and cathodic functions of SOFC and SOEC, respectively, for further improvement of the performance of the cells. The ohmic and polarisation resistances of the fuel cells and electrolyser cells were characterised over the temperature range of 650°C to 800°C. To make BSCF more thermo-mechanically compatible with the SDC electrolyte, the formation of a composite electrode by introducing SDC as the compositing material is proposed in order to decrease the mismatch of the TECs of electrode and electrolyte, hence improve the cell performance. In this study, after successfully synthesising the BSCF-SDC/YSZ-SDC/Ni-YSZ single cell with 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, and 50 wt.% of commercial SDC powder mixed with BSCF powder to make the composite air electrode, the electrochemical study was evaluated over the temperature range of 650°C to 800°C for both SOFCs and SOECs. Furthermore, for all the samples of this PhD study, the effect of 10%, 30%, 50%, and 70% hydrogen humidity levels on the performance of the electrolyser cells tested at 800°C were evaluated. As an overall conclusion, for both the BSCF-SDC/YSZ-SDC/Ni-YSZ fuel cell and electrolyser cell, 3.5 µm of YSZ as the blocking layer for the electrolyte, 7 wt.% graphite pore former for the fuel electrode, and the 20% SDC- 80% BSCF for the air electrode composition led to an optimised desired performance.