In-situ synthesis of NiS2 nanoparticles/MoS2 nanosheets hierarchical sphere anchored on reduced graphene oxide for enhanced electrocatalytic hydrogen evolution reaction.

As an important energy storage and transportation carrier, hydrogen has the advantages of high combustion heat, non-toxic, and pollution-free energy conversion process. Bimetallic sulfide composites are one of the emerging catalysts for hydrogen evolution reactions (HER) during water splitting. Herein, a hydrothermal method has been employed for the in-situ synthesis of NiS2 nanoparticles/MoS2 nanosheets (NiS2/MoS2) hierarchical sphere anchored on reduced graphene oxide (RGO) for enhanced electrocatalytic HER activity. The NiS2/MoS2/RGO composite displays improved HER activity compared to MoS2/RGO and NiS2/RGO. The optimized NiS2/MoS2/RGO-9 requires only an overpotential of 136 mV at a current density of 10 mA cm-2, a small Tafel slope of 53.4 mV dec-1, and good stability in acid solution. The synergetic effect between NiS2 nanoparticles and MoS2 nanosheets is responsible for enhanced HER performance. Moreover, RGO provides the substrate for NiS2/MoS2 species and maintains the overall conductivity of NiS2/MoS2/RGO composites. Finally, density functional theory (DFT) calculations justify and approve the efficient HER activity of NiS2/MoS2/RGO in terms of lower Gibbs free energy (0.07 eV) and lower work function (3.98 eV) that subsequently enhance the dissociation of H2O.

Ni3S2 nanostrips@FeNi-NiFe2O4 nanoparticles embedded in N-doped carbon microsphere: An improved electrocatalyst for oxygen evolution reaction.

The designing and preparing of low-cost and easily available electrocatalyst for oxygen evolution reaction (OER) are crucial for many advanced energy technologies. Herein, the Ni3S2 nanostrips@FeNi-NiFe2O4 nanoparticles embedded in N-doped carbon (Ni3S2@FeNi-NiFe2O4/C) microspheres were synthesized as improved electrocatalyst for OER, using a facile heat-treatment method. The optimized Ni3S2@FeNi-NiFe2O4/C-3 sample exhibits enhanced electrocatalytic activity toward OER performance with an overpotential of 280 mV at 10 mA cm-2 and a small Tafel slope of 33.9 mV dec-1. Furthermore, Ni3S2@FeNi-NiFe2O4/C-3 composite shows good stability in alkaline media. The outstanding electrocatalytic OER performance of composites was attributed due to the synergetic effect between Ni3S2 nanostrips and FeNi-NiFe2O4 nanoparticles and it is believed that the heterointerfaces between them act as active centers for OER. Additionally, N-doped carbon prevents the aggregation of Ni3S2@FeNi-NiFe2O4 species and enhances the conductivity of composites during the OER process.