Electrochemical Activation of Heterometallic Nanofibers for Hydrogen Evolution

Active catalysts play a key role in the hydrogen evolution of electrocatalytic water splitting for efficient and economical production of hydrogen. Herein a simple synthetic route to heterometallic metal–organic nanofibers (Pd–Co, Pd–Ni, and Pd–Mn, etc.) is developed via metal–organic gelation synthesized from pyridine-3,5-dicarboxylic acid, Pd2+, and various transition-metal ions at molar ratio of 2:1:2 at room temperature. These nanofiber catalysts show excellent electrochemical performance for hydrogen evolution reaction (HER). Especially, heterometallic Pd–Co nanofibers loaded on nickel foam by drop-casting showed the best HER performance at 96 mV (Pd–Ni, 107 mV; Pd–Mn, 132 mV) under the current density 10 mA cm–2, and it has a smaller Tafel slope at 63 mV dec–1. The HER performance of the heterometallic Pd–Co nanofibers is greatly improved by in situ electrochemical activation (EA) strategy (activated at the current density of 10 mA cm–2 for about 40000 s) to obtain EA Pd–Co@Pd NPs. EA Pd–Co@Pd NPs showed low overpotential of 57 mV (vs RHE) at 10 mA cm–2 and 134 mV (vs RHE) at 100 mA cm–2, and smaller Tafel slope of 55 mV dec–1 than Pd–Co nanofiber, which implies faster HER kinetics after EA. The HER performance of EA Pd–Co@Pd NPs is even better than that of commercial Pt/C at high current density, which is attributed to the formation of Pd nanoparticles (Pd NPs) grown on the fibrous network during the EA process. The valence state of Pd changes from +2 to 0 valence after the EA, and the resulting Pd NPs exhibit excellent electrocatalytic activity for HER. Meanwhile, EA Pd–Co@Pd NPs are stable at ambient temperature and exhibit excellent electrochemical stability for more than 80 h. This work provides opportunities for the development of metal–organic materials for advanced energy conversion, and a method is proposed to improve the efficiency of using precious metals as electrocatalysts for HER.