Sapphire-Supported Ta-Pt Thin Films for Electrochemical Studies under Hydrothermal Conditions

High-temperature platinum (Pt) thin films have been widely used in gas-phase devices such as temperature sensors and microheaters, but little has been done in the field of high-temperature electrochemistry, particularly under hydrothermal conditions. In this work, we explored the chemical and mechanical stability of sapphire-supported tantalum-platinum (sapphire/Ta/Pt) electrodes capped at their edges with a conformal Ta/SiO₂ layer in 0.1 M H₂SO₄ at 200 °C under autogenous pressure. A subset of samples was annealed for 10 min in flowing O₂ at 650 °C to gauge the impact of the Ta/SiO₂ layer on the overall performance of the Pt electrodes in acidic solutions. The temperature and pressure investigated in this work are well above those reported in previous studies on Ta, Pt, and Pt-alloy thin films, particularly those focused on catalyst materials for energy storage and conversion devices. The electrochemically (or electrochemical) active surface area (ECSA) of the Pt electrodes was measured before and after immersion in H₂SO₄ at 200 °C and used as the primary durability metric, while X-ray photoelectron spectroscopy (XPS), grazing-incidence X-ray diffraction (GI-XRD), and scanning electron microscopy (SEM) tracked changes in surface chemistry and morphology. Results show that both unannealed and annealed electrodes perform well, retaining more than 80% of their initial ECSA after cycling the potential between 0 and 1.4 V at 500 mV/s in 0.1 M H₂SO₄ at room temperature, constituting a significant first step toward the development of reusable/low-cost sensors for electrochemical studies in hydrothermal systems.