Developing metallic cathode for low-temperature solid oxide fuel cell applications
Date of Issue2017-01-31
Interdisciplinary Graduate School (IGS)
Energy Research Institute @NTU
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.