Fe-Cu-Rh ternary alloy nanofibers as an outstanding pH-universal electrocatalyst for hydrogen evolution reaction: The catalytic roles of Fe depending on pH

Fe-Cu-Rh ternary alloy nanofibers having various elemental compositions (x-Fe@Cu-Rh) were synthesized through the thermal hydrogen reduction of Fe-substituted CuRh₂O₄ nanofibers, which were prepared by electrospinning of a solution containing Cu and Rh precursors at a constant molar ratio (Cu:Rh = 1:2) with varying Fe precursor wt% (x = 6, 12, 24, 48 and 80). A series of x-Fe@Cu-Rh nanofibers were composed of the phases of metallic (fcc) and/or spinel CuRh₂O₄ where Fe ion substitutes primarily Rh ion in the octahedral site. x-Fe@Cu-Rh ternary alloy nanofibers showed higher pH-universal HER activity than pure Rh and Cu-Rh binary counterparts; and particularly, 12-Fe@Cu-Rh composed of 7.3, 27.2 and 65.5 atomic % of Fe, Cu and Rh, respectively, presented a superb activity (e.g., the most positive onset potential and the smallest Tafel slope) along with reasonable stability, even outperforming commercial Pt. The addition of Fe to Cu-Rh binary system improved alkaline HER activity substantially, proposing the crucial role of Fe-Cu-Rh interfaces for facilitating water molecule adsorption/dissociation. In acidic condition, Fe served as an excellent diluent reducing noble Rh content without deteriorating the acidic HER activity. Surprisingly, 80-Fe@Cu-Rh having only 18.5 atomic % of noble Rh exhibited decent HER activity during 24 h, demonstrating its potential as a stable and economic catalyst particularly for acidic HER.