Abstract
We report on platinum oxide formation during electrochemical anodic polarization of a platinum film on yttria-stabilized zirconia (YSZ) under electrochemical oxygen potential control. The electrochemical potential drives oxygen through the YSZ electrolyte towards a nominally 175 nm thin Pt film, which we found to locally delaminate from the substrate by forming nano-scale blisters. High resolution scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) mapping of focused-ion beam (FIB)-prepared cross-sections of single bubbles of a few micrometers in diameter reveal them to be hollow and enclosed by a Pt outer and a few tens of nanometers thick PtOx inner shell. The oxide shell presumably formed due to the increase of local oxygen chemical activity under the applied process conditions (723 K, 500 mbar O2, bias voltage +100 mV). Interface X-ray diffraction indicates that the solid electrolyte surface morphology is largely unaffected by the process suggesting that the YSZ surface is stable on the atomic scale under application relevant oxygen transport conditions. Platinum is known to be rather stable towards oxidation, even at elevated oxygen pressure, leading to oxide-scale thicknesses of the order of 1 nm. Our results however indicate that many of the kinetic barriers for oxidation during the nano-confined blistering process are lowered. This may have implications in general for the mechanism how oxygen is stored in an electrode at such an internal metal - oxide/metal - gas interface, which is important for the functionality of many solid-state electrochemical and memresistive devices.
Highlights
Oxygen sensors, such as the so-called lambda probe, are devices which play an important role in automotive exhaust regulation
Under anodic polarization oxygen ions migrate towards the Pt thin film electrode where bubbles may be formed by mechanical deformation, as it was shown by in situ scanning electron microscopy (SEM) [12]
The as-grown Pt-yttria-stabilized zirconia (YSZ)(111) electrode interface was characterized by high energy X-Ray Reflectivity (XRR) and Crystal Truncation Rod (CTR) analysis
Summary
Oxygen sensors, such as the so-called lambda probe, are devices which play an important role in automotive exhaust regulation. Under anodic polarization oxygen ions migrate towards the Pt thin film electrode where bubbles may be formed by mechanical deformation, as it was shown by in situ scanning electron microscopy (SEM) [12]. Such a gas formation underneath gas-tight electrodes can occur more generally at different electrode/electrolyte interfaces during OER or hydrogen evolution reactions (HER) [13] and plays a major role for device stability because of potential electrode delamination. We are addressing the following open questions: i) Does Pt oxidize at the internal Pt surface of bubbles formed by lifting-off from the YSZ substrate, and under which conditions? ii) How thick is the Pt oxide shell that has formed under the here applied conditions? iii) Which Pt oxide has formed? iv) Which kinetic and thermodynamic model can describe the formation of the observed Pt oxide?
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