Intrinsic Catalytic Activity for the Alkaline Hydrogen Evolution of Layer-Expanded MoS2 Functionalized with Nanoscale Ni and Co Sulfides

Abstract

The hydrogen evolution reaction (HER) under alkaline conditions is subject to significant kinetic limitations even with the most active platinum-based catalysts, while more affordable non-noble-metal-based catalytic materials present further challenges in terms of activity and durability in operation. To improve on these aspects, we present a new microwave-assisted synthetic route to fabricate sulfides of nickel and cobalt integrated into a layer expanded molybdenum sulfide (NiSx/MoS2LE and CoSx/MoS2LE), which efficiently catalyze H2 evolution in 1 M KOH. The use of the microwave-synthesis conditions enables the formation of nanoscale Ni and Co sulfides distributed homogeneously within the highly disordered layered molybdenum sulfide, as established using a comprehensive suite of physical methods. Synthesis of FeSx/MoS2LE is also presented, but the resulting material did not exhibit promising properties. Electrocatalytic tests reveal higher activity of the Ni-based catalyst as compared to CoSx/MoS2LE and especially unmodified MoS2LE. The performance of NiSx/MoS2LE at a HER overpotential of 0.15 V at ambient temperature and 60 °C corresponds to specific H2 evolution rates of 28 ± 4 and 58 ± 10 A g-1, respectively. Analysis of the electrokinetic data indicates that the exchange current density of the HER per an electrochemically active surface area of the sulfide-based materials is not high (0.001 mA cm-2 at ambient temperature), and that the high performance per unit mass observed here is supported by the well-developed surface area of the material (corresponding to a specific capacitance of 71 F g-1). A similar conclusion likely applies to many nickel and cobalt sulfide-based alkaline hydrogen evolution catalysts reported previously. Durability in operation of NiSx/MoS2LE and CoSx/MoS2LE is also demonstrated, in particular through a 2-week-long two-electrode water splitting test.