Morphological and Electrochemical Optimization of Solid Oxide Fuel Cell (SOFC) Diffusion-Blocking Layer and Cathode Layer Fabricated By Reactive Spray Deposition Technology (RSDT)

Abstract

SOFCs typically operate up to 1000ºC [1]. This temperature is notably higher than the temperatures at which other fuel cells, such as PEM fuel cells, run. This elevated temperature offers distinct advantages, such as greater efficiency due to faster reaction kinetics, flexibility to use fuels other than hydrogen, and relatively high resistance to impurities in the fuel [1]. When running hydrogen, the faster reaction kinetics also eliminate the need for noble metal catalysts, which are an inherent component of PEM fuel cells; instead the anode and cathode in SOFCs can be made of relatively inexpensive ceramics. The cost of SOFCs can be further reduced by decreasing the operating temperature to 500-650ºC [2]. In this temperature range LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ) is a commonly used cathode materials because of its good ionic and electronic conductivity [2]. A common electrolyte used in SOFCs is YSZ (Zr0.92Y0.08O2- δ), which reacts with LSCF to form SrZrO3 [2]. The presence of SrZrO3 leads to resistive losses in the cell. To mitigate this, a diffusion-blocking layer of GDC (Ce0.9Gd0.1O1.95) can be applied between the YSZ and LSCF. Previous work suggests that GDC blocking layers applied by RSDT lead to better cell performance than GDC applied using traditional screen-printing techniques [2,3]. This work focuses on optimizing the coating of a GDC blocking layer and LSCF cathode layer on SOFC half cells with NiO,YSZ anode support, NiO,YSZ anode functional layer, and YSZ electrolyte using RSDT. RSDT is an open-atmosphere, flame-based deposition process that uses inexpensive solvents and precursors (i.e. toluene and metal acetylacetonates) and performs a direct deposition of GDC and LSCF layers onto the cell [4]. By using RSDT, a dense GDC layer with good adhesion can be achieved at ~1000°C, which is a relatively low temperature compared to the ~1400°C at which GDC is traditionally processed. This lower temperature allows for reduced production costs and limits the inter-diffusion of the YSZ and GDC. Image (a) of the attached figure shows plan-view SEM of the RSDT-fabricated GDC layer on a half cell. This thin, dense GDC layer is expected to be transparent in SEM [2]; the large grains are the underlying YSZ. Energy Dispersive X-Ray Spectroscopy indicates the presence of cerium. Gadolinium is not detected, which is likely the result of its intended low mole ratio. The cathode is fabricated by simultaneously spraying a slurry of pre-synthesized LSCF nanoparticles and spraying LSCF nanoparticles synthesized directly in the flame. Preliminary testing shows that cells with the RSDT-fabricated cathode have less mass-transfer resistance at high current densities than cells with traditional LSCF cathodes (plot (b) of attached figure). These performance tests were conducted by FuelCell Energy at 750°C with humidified hydrogen on the anode and air on the cathode. To improve the performance of a cell with RSDT-fabricated GDC blocking layer and LSCF cathode, it is important to decrease the ohmic resistance of the cell. This can be achieved by increasing the ionic and electronic conductivity of the LSCF cathode and by increasing the area of the triple phase boundary (TPB). In this work, the LSCF cathode morphology is tailored to optimize its ionic and electronic conductivity, TPB, and mass transfer resistance. The surface morphology of the dense GDC is also adjusted to optimize TPB. As part of the optimization, the morphology of the deposited layers is analyzed by X-Ray Diffraction and Scanning Electron Microscopy. Additionally, performance testing and electrochemical testing is carried out on the cells. This establishes a correlation among RSDT processing parameters, cell morphology, and cell performance.