# Controllable synthesis of nickel sulfides integrated with carbon fibers towards enhanced hydrogen evolution reaction kinetics

## TL;DR

Researchers developed a method to control the synthesis of nickel sulfides on carbon fibers, improving their efficiency in producing hydrogen through electrocatalysis.

## Contribution

A controllable synthesis method for nickel sulfides with tailored crystal phases to enhance hydrogen evolution reaction (HER) performance.

## Key findings

- NiS-800@CNFs showed an overpotential of 119 mV at 10 mA cm−2, indicating excellent HER activity.
- Sulfur-deficient Ni9S8 and coexisting NiO phases synergistically enhance alkaline HER performance.
- Sulfur-rich NiS crystals hinder hydrogen desorption, as revealed by electrochemical impedance spectroscopy.

## Abstract

Nickel sulfides are considered as one of the promising electrocatalysts for the hydrogen evolution reaction (HER). Herein, the monolith NiSx@CNFs was constructed as a HER working electrode via in situ electrospinning, and the crystal phase of Ni9S8 and NiS could be controlled by adjusting the annealing temperature and S source. Specifically, NiS-800@CNFs exhibits excellent activity with the required overpotential of 119 mV at 10 mA cm−2. This is attributed to the sulfur-deficient Ni9S8 crystal that provides sufficient hydrogen adsorption sites, with the coexistence of the NiO phase that is beneficial for water dissociation, which synergically promotes the alkaline HER process. Based on the electrochemical impedance spectroscopy (EIS) characterization, the HER mechanisms of the NiSx@CNFs series were systematically explored. It is revealed that the sulfur-rich NiS crystal surface is not conducive to the desorption of adsorbed hydrogen to produce hydrogen. This work provides a valuable reference for the regulation of phase structure and the HER mechanism of nickel sulfides.

Nickel sulfides were controllably synthesized via electrospinning, and their structure activity relationships and kinetic mechanisms of the HER were systematically discussed.