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To improve the dimensional accuracy of 3D printed parts produced by vat photopolymerization, a controlled light penetration through the photopolymer resin is necessary to avoid excessive unwanted curing caused by overexposure. Although photoabsorbing additives like Sudan I can attenuate light to decrease excessive light penetration, they also require longer exposure time to solidify the resin. In this work, photocatalytic zinc oxide nanoparticles (ZnONP) were used as resin additives substituting photoabsorbers to control overexposure that causes loss in dimensional accuracy. Compared to adding 0.1 wt/wt% of Sudan I, adding 2.5 wt/wt% of ZnONP not only showed similar level of control in light penetration, but reduced the required curing time of the resin. Significant improvement in preventing unwanted curing was also observed in resin with ZnONP, as evident by our printing tests on open channels and overhanging structures. Unlike Sudan I which deteriorated the mechanical properties of printed parts, ZnONP additives have rendered more effective curing that resulted in improved tensile strength, fracture strain and Young's modulus. National Research Foundation (NRF) This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme.

Low-temperature solid oxide fuel cells (LT-SOFCs) have attracted significant attention because of their better long-term stability, shorter start-up times and potential applications in small-scale portable devices. Proton-conducting ceramics are regarded as promising electrolyte materials for LT-SOFCs since they show smaller activation energy for proton conduction and larger ionic conductivities at low temperature range than conventional oxygen-ion conducting electrolytes. However, the high processing temperatures and the lack of economical and scalable fabrication method for proton-conducting electrolytes have restricted the commercial development of proton-conducting SOFCs. To address these problems, this thesis aims at developing economical and scalable methods to fabricate BaZrO3-BaCeO3 based proton-conducting electrolytes at reduced processing temperatures. In the first part, a single-step slurry spray coating method has been developed to prepare proton-conducting BaCe 0.7Zr0.1Y0.2O3 (BCZY) membranes on the pre-annealed porous anode substrates. The effects of sintering temperature on the electrolyte membrane and anode microstructure, proton conductivity of the resulted films, and their fuel cell performances have been investigated. Dense BCZY membranes with improved anode microstructure and high proton conductivity have been obtained at reduced temperature. SOFCs using spray-coated electrolyte films gave stable OCV and good performance. In the second part, chemical solution deposition (CSD) method has been proposed to realize a scalable and cost-effective fabrication of sub-micrometer scale proton-conducting electrolyte thin film. Different chemical solution synthesis routes and heat treatment processes have been investigated to obtain dense and crack-free BaZr0.8Y0.2O3 (BZY) thin film. The crystallinity, microstructure and gas tightness of BZY thin films have been studied. Compared to the sophisticated vacuum-based thin film deposition techniques, this cost effective and scalable CSD method is a promising way for mass production of electrolyte thin films for micro-SOFCs. Doctor of Philosophy (MAE)

Solid oxide fuel cell (SOFC) is a high-efficient energy conversion device, which can transform chemical energy into electricity directly. With the effort input by numerous researchers in the past decade, the operating temperature of SOFCs had been successfully decreased to below 500 ˚C with more than 1 W·cm-2 of output power density. At low operating temperature, one of the major issues is the sluggish kinetics of oxygen reduction reaction, which causes the major loss. To achieve high performance at a low operating temperature of 300 to 500 ˚C for SOFC, this dissertation is dedicated to developing metal-based cathodes that can meet the criteria of long-term stability, low area-specific resistance, easy to fabricate, and cost effective. To study the stability of metallic cathode, an interfacial structure characterization method, double cantilever beam delamination, was developed to determine the triple phase boundary structure for the thin-film electrode. By this delamination technique, the thermal-driven morphological evolution between the electrode top surface and the substrate contact interface were compared. For the nanoporous platinum electrode, the temperature required for significant agglomeration to occur was approximately 100 °C higher at electrolyte contact interface than at the top surface. In the next stage, inkjet printing technique was employed to fabricate Ag cathode. Comparing to the conventional thin-film electrode that was fabricated by sputtering, the cathode fabricated by inkjet printing showed improved performance and structural stability. In a 45-hour test on fuel cell operation, the cell with an inkjet-printed cathode had a current output degradation of 12.1 % at 400 ˚C; while the degradation of sputtered Ag cathode was 68.1 % after a 20-hour test. For a high-performance fuel cell, a porous silver cathode is required, and this simple inkjet printing technique provides an effective way to achieve such desirable porous structure with required thermal morphological stability. To further enhance the stability of Ag cathode, the surface modification was employed by sputtering a 2 nm-thick of gadolinium-doped ceria (GDC) on the Ag cathode. With the GDC capping layer, the thermal stability of Ag cathode was improved. The GDC-modified cathode could be operated at 400 ˚C for 24 h with 7.8 % of output degradation. To study the effect of GDC coating on surface O-exchange properties, a “porous and dense” bi-layered Ag cathode and a “porous” Ag cathode were used for surface modification. When GDC capping was applied, the ohmic resistance reduced from 1186.0 to 1052.0 Ωcm2 while the polarization resistance increased from 767.3 to 2541.4 Ωcm2 at 400 ˚C for the bi-layered Ag cathode; for the porous Ag cathode, the ohmic resistance reduced from 1190.0 to 963.5 Ωcm2 while the polarization resistance increased from 325.8 to 421.2 Ωcm2. This GDC capping layer could reduce the ohmic resistance due to the extra ion conduction pathways, while the polarization resistance increased since the GDC capping impeded oxygen surface exchange on Ag. These results provide engineering strategies for applying Ag cathode which operating at a temperature around 400 °C. Finally, the integration of Ag cathode on a micro-fabricated SOFC (or µ-SOFC) was explored. The Ag cathode penetrated the electrolyte during the operation and caused a short circuit of cathode and anode. Due to the diffusivity of Ag in the electrolyte, the design of µ-SOFC needs to be reconsidered. To employ Ag on a µ-SOFC without the short-circuit failure, further investigation on Ag diffusion-blocking layer and the thickness requirement of the electrolyte is needed. Based on these results, the opportunities and the strategies of future work are discussed. Doctor of Philosophy (IGS)

Solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices which can convert chemical energy directly into electricity without combustion. The major hurdle to SOFCs is the requirement of high operating temperature (>800 °C), which leads to a technological challenge for materials development and thermal management. Thus reducing the operating temperature of SOFCs to intermediate temperature (500-800 °C) range or even low temperature range (

Solid oxide fuel cells (SOFCs) are electrochemical conversion devices which directly convert chemical energy of the fuel into electrical energy with high efficiency. SOFC comprises of an anode, electrolyte and cathode layer that are fabricated individually upon each other, usually one at a time. Additive manufacturing (AM) is a new fabrication technique that utilizes a layer by layer fabrication technique, which can potentially be suitable for use in SOFC, since it is also building different materials in a layer-wise manner. Furthermore, the use of laser-based AM techniques can eliminate the issue of long sintering hours by directly densifying the materials using laser as the heat source. In addition, SOFCs possessing newer designs can be fabricated with ease using this technique, gradient-structure electrodes can also be created either by design or varying the laser parameter and material composition. Therefore, this thesis aims to develop a process as well as study the feasibility for fabricating SOFC using laser-based AM techniques. First, a step-by-step process was conducted to identify significant factors affecting the building of Nickel metal support structures using Selective Laser Melting (SLM) process. Laser power, scanning speed and hatch spacing were found to be the dominant factors affecting the printability of the parts. Next, a porous structure was designed and printed which acted as a metal support for SOFC. A functional cell was further built upon the metal support and the electrochemical performance was investigated. Although a low open circuit voltage (OCV) of 0.53 V was obtained with a maximum power density of 11.0 mW/cm2 from the cell, it demonstrated potential of this technique as a new fabrication process. In order to realize the fabrication of the entire SOFC by laser-based AM, the influence of laser on SOFC materials were conducted. The effect of two different laser sources, Nd:YAG fibre laser and CO2 laser was studied on NiO-YSZ anodic material, which showed that CO2 xvi laser was better absorbed by the oxide material. In addition, dendrite features were formed on the surface, which were influenced by the rapid solidification process and could be modified by adjusting laser processing parameters. Although the laser-modified SOFC did not perform as well as the conventional one, it was found that significant improvement was made on the charge transfer kinetics. Lastly, the influence of laser on YSZ electrolyte surface showed that the polarization resistance of the cell was reduced due to better oxygen adsorption/dissociation process. These experiments demonstrated that laser-based AM is a viable fabrication tool for SOFC. Master of Science