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

• TiHfZrNb x HEAs resist corrosion 600 x than Ti in 0.5 m H 2 SO 4 + 5 ppm F – solution at 70 °C. • Hf, Zr, Nb synergistically form a dense passive film consisted of HfO 2 , ZrO 2 , TiO 2 , Nb 2 O 5 . • The passive film is an n-type semiconductor with low electron-donor concentration. • The lower the Nb content in the HEAs the lower the corrosion current density. • Nb has a small role toward passivation in the aggressive acidic solution. 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 H 2 SO 4 + 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.cm 2 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 HfO 2 > ZrO 2 > TiO 2 , with a presence of Nb 2 O 5 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 Nb 2 O 5 on the top surface of TiHfZrNb 0.2 and TiHfZrNb 0.3 indicated a small role of Nb toward passivation.