Enhancement of Thermal Stability of Sputtered Nanoporous Pt Thin Film Using Oxygen Plasma Treatment in Low Temperature Solid Oxide Fuel Cells

Abstract

Sputtered nanoporous platinum (Pt) thin films are common choices as catalytic cathodes for low temperature (300~500°C) solid oxide fuel cells (LT-SOFCs) for its high catalytic activity towards oxygen reduction reaction. However, the nanoporous pure Pt is vulnerable to thermal coagulation during high temperature operation, which often causes irreversible deterioration to the cell performance. In our earlier work, physical confinement of the nanoporous Pt with a few nanometers of zirconia (ZrO2) conformal coating by atomic layer deposition (ALD) improved the thermal stability of Pt significantly, with more than 50% of the cell power density retained after 80 hours of operation. Still, around 29.5% of cell power density degradation was observed in cells using ALD-ZrO2 capped nanoporous Pt cathode. It is likely that the poor adhesion between the Pt cathode and the yttria-stabilized zirconia (YSZ) electrolyte that caused the triple phase boundaries to be unstable under high temperature operation and therefore a significant thermal degradation is still observed. In this work, we studied the effect of adhesion between Pt and YSZ on the morphological stability of nanoporous Pt at the Pt/YSZ interface. The surface of YSZ electrolyte was treated with oxygen plasma to modify the wettability between ALD-ZrO2 capped Pt and YSZ. The contact angle of a water droplet on the untreated YSZ surface was 80˚, while it dropped to 16˚ on the oxygen-plasma treated YSZ surface, meaning a more hydrophilic YSZ surface was obtained. The oxygen-based functional groups may have formed after the oxygen plasma treatment and increased the YSZ-water interfacial energy. The additional chemical bonding may exist if the nanoporous Pt films are deposited on surface-treated YSZ to stabilize the morphology of nanoporous Pt at the Pt/YSZ interface. The surface confined nanoporous Pt thin film cathodes were annealed at 600°C to induce the agglomeration, followed by delamination of the Pt cathode film from the YSZ substrate using double cantilever beam (DCB), in order to investigate the morphological stability of the nanoporous Pt at the Pt/YSZ interface after oxygen plasma treatment. The interfacial morphology observation showed that the Pt agglomeration at the Pt/YSZ interface remained significantly severe for Pt cathode with only ALD-ZrO2 surface capping. For Pt cathode deposited on YSZ substrate treated with 5 minutes of oxygen plasma at 18 watts showed improved interfacial thermal stability. A fuel cell with ALD-ZrO2 capped nanoporous Pt cathode thin film deposited on the oxygen-plasma treated YSZ electrolyte was tested. It was observed that the performance degradation was effectively hindered by the oxygen plasma treatment after 75 hours of operation at 450°C. Figure 1