A fascinating pH independent catalyst for hydrogen evolution reaction: Crystalline bimetallic hcp-Co_xRh_1-x alloy nanofibers driven by thermally induced phase transition from a single phase of CoRh₂O₄

Two distinct crystalline phases of bimetallic Co_xRh_1−x alloy nanofibers were synthesized through the thermal H₂-reduction of single-phase CoRh₂O₄ nanofibers that were prepared via calcination preceded by electrospinning. By varying the reduction temperature (T) and retention time (t), it was confirmed that the formation of a fully reduced crystalline metallic alloy required T ≥ 250 °C for t ≥ 2 h. During the reduction process, the single phase of CoRh₂O₄ was first transformed to an intermediate face centered cubic (fcc) phase where Rh element was principally reduce to Rh(0) and Co element was present as the oxide form. Then, a fully reduced Co_xRh_1−x alloy was obtained in hexagonal close-packed (hcp) phase. In particular, the hcp structured Co_xRh_1−x showed a fascinating hydrogen evolution reaction (HER) activity (e.g., the lowest overpotential at 10 mA cm⁻² and the smallest Tafel slope) pH independently that was superior to those of commercial Pt and the pure single metal (Co and Rh) nanofibers. Co_xRh_1−x alloy also showed a robust stability during 10 000-s continuous HER and 1000-repetitive potential sweeps in alkaline (1.0 M NaOH), neutral (1.0 M PBS, pH 7.2) and acidic (0.5 M H₂SO₄) media. The pH-universal HER activity was ascribed to alloying effect: Co atoms in the alloy interact O atoms in H₂O molecules and therefore assist neighboring Rh atoms in adsorbing H atoms readily in alkaline and neutral condition.