Probing Electrocatalytic Synergy in Graphene/MoS2/Nickel Networks for Water Splitting through a Combined Experimental and Theoretical Lens.

ABSTRACT

The development of low-cost and active electrocatalysts signifies an important effort toward accelerating economical water electrolysis and overcoming the sluggish hydrogen or oxygen evolution reaction (HER or OER) kinetics. Herein, we report a scalable and rapid synthesis of inexpensive Ni and MoS2 electrocatalysts on N-doped graphene/carbon cloth substrate to address these challenges. Mesoporous N-doped graphene is synthesized by using electrochemical polymerization of polyaniline (PANI), followed by a rapid one-step photothermal pyrolysis process. The N-doped graphene/carbon cloth substrate improves the interconnection between the electrocatalyst and substrate. Consequently, Ni species deposited on an N-doped graphene OER electrocatalyst shows a low Tafel slope value of 35 mV/decade at an overpotential of 130 mV at 10 mA/cm2 current density in 1 M KOH electrolytes. In addition, Ni-doped MoS2 on N-doped graphene HER electrocatalyst shows Tafel slopes of 37 and 42 mV/decade and overpotentials of 159 and 175 mV, respectively, in acidic and alkaline electrolytes at 10 mA/cm2 current density. Both these values are lower than recently reported nonplatinum-group-metal-based OER and HER electrocatalysts. These excellent electrochemical performances are due to the high electrochemical surface area, a porous structure that improves the charge transfer between electrode and electrolytes, and the synergistic effect between the substrate and electrocatalyst. Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations demonstrate that the Ni hydroxide species and Ni-doped MoS2 edge sites serve as active sites for OER and HER, respectively. Finally, we also evaluate the performance of the HER electrocatalyst in commercial alkaline electrolyzers.