Atomic-Layer-Deposited MoNx Thin Films on Three-Dimensional Ni Foam as Efficient Catalysts for the Electrochemical Hydrogen Evolution Reaction

Abstract

Future realization of a hydrogen-based economy requires a high-surface-area, low-cost, and robust electrocatalyst for the hydrogen evolution reaction (HER). In this study, the MoNx thin layer is synthesized on to a high-surface area three-dimensional (3D) nickel foam (NF) substrate using atomic layer deposition (ALD) for HER catalysis. MoNx is grown on NF by the sequential exposure of Mo(CO)(6) and NH3 at 225 degrees C. The thickness of the thin film is controlled by varying the number of ALD cycles to maximize the HER performance of the MoNx/NF composite catalyst. The scanning electron microscopy and transmission electron microscopy (TEM) images of MoNx/NF highlight that ALD facilitates uniform and conformal coating. TEM analysis highlights that the MoNx film is predominantly amorphous with the nanocrystalline MoN grains (4 nm) dispersed throughout it. Moreover, the high-resolution (HR)-TEM analysis shows a rough surface of the MoNx film with an overall composition of Mo0.59N0.41. X-ray photoelectron spectroscopy depth-profile analysis reveals that oxygen contamination is concentrated at the surface because of surface oxidation of the MoN film under ambient conditions. The HER activity of MoNx is evaluated under acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) conditions. In an acidic electrolyte, the sample prepared with 700 ALD cycles exhibits significant HER activity and a low overpotential (eta) of 148 mV at 10 mA cm(-2). Under an alkaline condition, it achieves 10 mA cm(-2) with eta of 125 mV for MoNx/NF (700 cycles). In both electrolytes, the MoNx thin film exhibits enhanced activity and stability because of the uniform and conformal coating on NF. Thus, this study facilitates the development of a large-area 3D freestanding catalyst for efficient electrochemical water-splitting, which may have commercial applicability.