Hydrogen fuel cells and hydrogen production stand at the forefront of efforts to achieve Net-zero emissions. Among these technologies, solid oxide fuel cells (SOFCs) and electrolysers (SOEs) are distinguished as promising devices for the broad practical application of electricity and clean fuel generation, respectively. Lowering their operating temperatures can significantly facilitate their commercialisation by improving the stability and reducing the costs associated with electrodes and the fabrication process. Decreasing the operating temperatures, SOFCs and SOECs' performance heavily relies on electrolyte ionic conductivity. Therefore, this PhD project aims to develop electrolyte materials with suitable ionic conductivity at intermediate temperatures for SOFCs and SOECs.

The first approach was to find a proper electrolyte candidate and develop a low-cost, efficient synthesis method to obtain the targeted electrolyte powders. The apatite phase of La_9.33+xSi₆O_26+3x/2 (LSO) was chosen as the basis electrolyte material, due to its high ionic conductivity and stability at intermediate temperatures. A novel method of co-precipitation was used to fabricate the LSO-based electrolyte powders. Mg and Mo ions were first introduced into the LSO structure which can increase the number of interstitial oxide ions and improve the degree of densification at lower sintering temperatures. As a result, the relative density of the fabricated Mo/Mg-LSO electrolytes pellets exceeded 98% after sintering at 1500 °C for 4 hours. Moreover, the ionic conductivity of these pellets increased significantly, from 0.78 mS·cm^-1 for the pristine LSO to 33.94 mS·cm^-1 for the doped sample La_9.5Si_5.45Mg_0.3Mo_0.25O_26+δ at 800 °C. The second step was to substitute three dopants (Nd/Cu/Fe) into the LSO structure at controlled concentrations, allowing the sintering temperature to be reduced to 1400 °C. This modification achieved a maximum ionic conductivity of approximately 20.52 mS· cm^-1 at 800 ℃ for Nd_0.1Fe_0.25Cu_0.5-LSO, attributed to the enlargement of migration channels and distortion of tetrahedral structures.

Composite electrolytes were investigated in this thesis since the advantages of single-component electrolytes can be combined to improve conductivity and stability. In this thesis, the composite electrolyte Ce_0.8Gd_0.2O_2-δ-LSO (GDC-LSO) was synthesised with different percentages and can obtain the densified layer with binder after sintering at 1500 ℃. The maximum conductivity was about 25.36 mS·cm^-1 at 800 ℃ for the composite electrolyte 40GDC-60LSO. The factors contributing to this enhancement in ionic conductivity were explored through detailed physicochemical characterisations. The conductivity of the GDC-modified LSO electrolyte improved dramatically compared to pure LSO since the cations of Ce⁴⁺ and Gd³⁺ were substituted into the La site. This substitution enlarged the channel of oxygen ions through the tilting or rotation of [SiO₄]^4- units within the LSO phase.

With regards to the fabrication of single cells, electrolyte-supported and anode-supported cells were manufactured. All synthesised electrolyte powders illustrated high sinterability, achieving optimal results after being pressed at 10 MPa and sintered at 1500 ℃. The electrolyte-supported cell (NiO + 40GDC-60LSO // 40GDC-60LSO // LSCF + 40GDC-60LSO) possessed good open circuit voltage (OCV) of 1 V, compared to 0.75 V when using pure GDC as electrolyte at 800 ℃. This improvement significantly enhances the stability of the cell in a hydrogen atmosphere. Besides, the electrolyte-supported SOFCs with a dense Mo_0.3/Mg_0.25-LSO electrolyte layer, 500 μm in thickness, achieved a maximum power density of 103 mW/cm² at 800 ℃ and exhibited a relatively stable OCV (0.9-1 V) at 600 ℃-800 ℃. Further improvement in power density is anticipated when the electrolyte layer is made thinner.

Overall, this thesis is based on the development of electrolyte materials suitable for intermediate temperatures applications, offering for good stability and ionic conductivity for use in SOFCs or SOECs.