Corrosion of TiHfZrNb_x high-entropy alloys in a simulated condition of proton exchange membrane water electrolyser

In the present work, the corrosion behavior and passive film properties of TiHfZrNb_x (x = 0.2, 0.3 and 0.4) high-entropy alloys (HEAs) were evaluated under a simulated condition of a proton exchange membrane water electrolyser (PEMWE) in view of their application as bipolar plates. Results from electrochemical-impedance spectroscopy, cyclic- and static-polarizations evaluation revealed that in a 0.5 M H₂SO₄ + 5 ppm F^– solution at 70 °C the HEAs exhibited about 600 times higher polarization resistance than that of CP-Ti, with the highest achieved at 79.57 kΩ.cm² for TiHfZrNb_0.2, leading to a sharp contrast in the corrosion-current density, 805.11 µA.cm^-2 for the CP-Ti vs. 0.92 µA.cm^-2 for the TiHfZrNb_0.2, reflecting a far superior corrosion resistance of the HEAs. X-ray photoelectron spectroscopy analysis confirmed the formation of a multi-oxide passive film, predominantly by HfO₂ > ZrO₂ > TiO₂, with a presence of Nb₂O₅ only in TiHfZrNb_0.4, all possess an n-type semiconducting characteristic and a much lower electron-donor concentration in the HEAs than in the CP-Ti. The complementary analyses of scanning electron microscopy, atomic force microscopy and solution chemistry highlighted the synergistic effects of Hf, Zr, and Nb in enhancing protectiveness of the passive film, but the absence of Nb₂O₅ on the top surface of TiHfZrNb_0.2 and TiHfZrNb_0.3 indicated a small role of Nb toward passivation.