In an attempt to solve the energy trilemma, the solid oxide fuel cells (SOFCs) have attracted worldwide interest due to the high efficiency, fuel flexibility, sustainable electricity generation on demand and low pollutant emissions. Conventional SOFCs which operate at high temperatures (700 – 1000°C) and therefore has resulted in several problems, especially with regard to cost, durability and, choice of functional and structural materials. Therefore, reducing the operating temperatures to intermediate temperatures solid oxide fuel cells (IT)-SOFCs (500 – 700°C) is desirable for decreasing operating costs, widening the choice of materials, increasing sustainability, and prolonging durability. However, reducing the operating temperature can limit the oxide ion conductivity of the electrolyte, which can impede the performance of IT-SOFCs. Accordingly, finding the optimum electrolyte materials with high ionic conductivity at intermediate temperatures becomes a factor for the future impact and success of the technology. This research will focus on the development of the electrolyte material for IT-SOFCs with high ionic conductivity.
Two rare-earth substitutional aliovalent cations (co-doped) in MO2 type of fluorite materials are considered as a promising material for use as the solid electrolyte in solid oxide fuel cells because two dissimilar trivalent substitutional cations (co-doping) can be used to reduce the activation energy, increase bulk conductivity and optimise the total conductivity (bulk + grain boundary) at intermediate temperatures. In this research, three different combinations of co-doped materials with four different concentrations using the novel Sodium Alginate (SAL) method also known as Leeds Alginate Process (LAP) in some recent literature have been investigated for IT-SOFCs electrolyte application. A detailed investigation using thermal analysis, scanning and transmission electron microscopy, ambient and high-temperature X-ray diffraction, Raman spectroscopy, and impedance spectroscopy is carried out to optimise the synthesis and characterisation of the new solid electrolyte compositions for future applications.
Firstly, the development of the optimum process to produce nanoparticles is reported in this research. The novel sol-gel sodium alginate bead and granule method are used to prepare the nanoparticle complex metal oxide. Initially, these methods have been successfully applied in this research to produce cubic single-phase nanoparticle of Ce0.8Gd0.2O1.9, Ce0.8Dy0.1Gd0.1O1.9, Ce0.8Ho0.1Gd0.1O1.9 and Ce0.8Er0.1Gd0.1O1.9 having homogeneous composition. The calcination temperature at 500°C for 2h showed the optimum temperature to obtain the particles size ~10nm and the sintering temperature at 1500°C for 2h showed the fully dense ceramics with higher relative density (> 96%) obtained.
Subsequently, the alginate method has also been applied in this research to synthesise successfully thirteen co-doped cerium rear-earth oxide solid solution series Ce0.8RExGd0.2-xO1.9 (RE = Dy, Ho, and Er; 0 ≤ x ≤ 0.2). The composition of the solid solution in each series has been varied at an incremental interval of x=0.05. X-ray diffraction (XRD) data shows a single-phase fluorite structure in the entire solid solution series and obey the Vegard’s Law. The crystallite size calculated from XRD using the Rietveld refinement method is in good agreement with those observed from transmission electron microscopy (TEM) (~10nm). The fluorite and defect structure were observed for the entire series by Raman Spectroscopy. The co-doped materials do not change the coefficient of thermal expansion (CTE) significantly.
Co-doping as well as the synthesis method has helped to improve the relative density, which has been confirmed with the microstructure analysis by scanning electron microscopy (SEM) and also leads to an anomalous increase in the ionic conductivity above the critical temperatures in all the three solid solution series. The Ionic conductivity observed is almost 100 times higher than the single doped at a typical temperature of 500°C from 0.00351 S cm-1 for Ce0.8Gd0.2O1.9 to 0.3169 S cm-1, 0.2871 S cm-1 and 0.3573 S cm-1 for Ce0.8Dy0.1Gd0.1O1.9, Ce0.8Ho0.1Gd0.1O1.9, Ce0.8Er0.15Gd0.05O1.9, respectively.
Hence, these novel sol-gel methods are the promising processes to produce nanoparticles of complex metal oxide compounds at low temperatures, and co-doped material can be an alternative solid electrolyte material for future IT-SOFCs due to the high ionic conductivity.
