Facile synthesis of silk-cocoon S-rich cobalt polysulfide as an efficient catalyst for the hydrogen evolution reaction

Abstract

Tuning the structure, morphology, and electronic state is of great significance to achieve highly efficient hydrogen evolution reaction (HER) electrocatalysts. In this work, we originally develop a silk-cocoon structured S-rich cobalt polysulfide (CoS_x, x ≈ 3.9) catalyst with extremely high HER activity through a facile hydrothermal process. The formation mechanism of the silk-cocoon structured CoS_x has been systematically studied and the key factors in the formation of this structure are discussed. Such a nanostructured catalyst has hollow spheres that are interwoven by numerous sub-10 nm nanofibers to form into a three-dimensional conductive network by connection of micrometer long nanofibers. The silk-cocoon structured CoS_x shows excellent HER activity with an onset potential of 0 V vs. a reversible hydrogen electrode and a Tafel slope of 41 mV dec⁻¹. The activity is much higher than that of the reported metal sulfide materials and is even comparable to commercial Pt/C. Moreover, the overpotential to yield a current density of 10 mA cm⁻² is only 42 mV. We ascribe the high HER activity to the S-rich sites and the conductive silk-cocoon structure, which can benefit the charge transfer of the material. Thus, this work provides a guiding strategy for synthesis of nanostructured transition metal polysulfides with enhanced HER catalytic performance.