Electrochemical and surface characterization of TiHfZrNbx high-entropy alloys as bipolar plates for proton exchange membrane water electrolyser

One of the core components of a proton exchange membrane water electrolyser (PEMWE) is bipolar plate, critical for providing mechanical support, gas circulation, thermal and electrical conduction, and accounted for ~50% of PEMWE’s construction cost. Titanium is often used as the plate material, but the stability of its oxide film is prone to fluoride attack, thus reducing corrosion resistance and service life of the plate. With an aim of defining alternative materials, the present work studied the corrosion resistance of TiHfZrNb_x high-entropy alloys (HEAs) in 0.5 M H₂SO₄ + 5 ppm F^- solution at 70 °C, employing electrochemical-impedance spectroscopy (EIS), cyclic- and static-polarizations (CPP, PSP), SEM, AFM, XPS and MP-AES. The EIS and CPP evaluation revealed that the HEAs exhibited ~600 times higher polarization resistance than CP-Ti, with the highest achieved at 79.57 kΩ.cm² for TiHfZrNb_0.2 vs. 0.1 kΩ.cm² for the CP-Ti, 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. The XPS analysis confirmed a formation of multi-oxide passive film, predominated 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 PSP, SEM, AFM and MP-AES 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. The findings demonstrate the potential of TiHfZrNb_x HEAs as alternative materials to titanium for durable and efficient bipolar plates in a PEMWE system.