Atomic-Layer-Deposited MoN x 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 MoN x 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. MoN x is grown on NF by the sequential exposure of Mo(CO) 6 and NH 3 at 225 °C. The thickness of the thin film is controlled by varying the number of ALD cycles to maximize the HER performance of the MoN x /NF composite catalyst. The scanning electron microscopy and transmission electron microscopy (TEM) images of MoN x /NF highlight that ALD facilitates uniform and conformal coating. TEM analysis highlights that the MoN x 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 MoN x film with an overall composition of Mo 0.59 N 0.41 . X-ray photoelectron spectroscopy depth-profile analysis reveals that oxygen contamination is concentrated at the surface because of surface oxidation of the MoN x film under ambient conditions. The HER activity of MoN x is evaluated under acidic (0.5 M H 2 SO 4 ) 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 (Ε) of 148 mV at 10 mA cm -2 . Under an alkaline condition, it achieves 10 mA cm -2 with βof 125 mV for MoN x /NF (700 cycles). In both electrolytes, the MoN x 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. © 2019 American Chemical Society.