A fascinating pH independent catalyst for hydrogen evolution reaction: Crystalline bimetallic hcp-CoxRh1-x alloy nanofibers driven by thermally induced phase transition from a single phase of CoRh2O4

Abstract

Two distinct crystalline phases of bimetallic CoxRh1−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 CoxRh1−x alloy was obtained in hexagonal close-packed (hcp) phase. In particular, the hcp structured CoxRh1−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. CoxRh1−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. © 2021