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 NH3 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 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 x film under ambient conditions. The HER activity of MoN x 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 (η) of 148 mV at 10 mA cm–2. Under an alkaline condition, it achieves 10 mA cm–2 with η of 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.

Hydrogen Evolution Reaction by Atomic Layer‐Deposited MoNx on Porous Carbon Substrates: The Effects of Porosity and Annealing on Catalyst Activity and Stability

“…This observation was in good agreement with MoN x films that were deposited under similar conditions on either SiO 2 /Si or Ni-foam substrates. [21][22][23] Furthermore, the annealing in a H 2 atmosphere at 400 8C had a minimal influence on the phase of ALD-MoN x . However, the minor hump observed at 35-408 (Mo 2 N 111) was much clearer and apparent.…”

“…The ALD technique is extensively used to grow various active compounds on different substrates for applications such as Cu diffusion barrier, energy storage devices, and electrocatalysis. [21][22][23][24][25][26][27][28] Recently, ALD has been explored as a fabrication technique for the preparation of various active materials on a highly porous substrate for application in fields such as Li-ion batteries and supercapacitor. [29][30][31][32][33] However, the deposition of MoN x on CC or NCCC has not been reported and studied as a catalyst for HER.…”

“…The self-limiting growth kinetics, the linearity of growth with ALD cycles and the temperature window for ALD-MoN x using Mo(CO) 6 and NH 3 has been previously established by our group: The results indicated that the ideal ALD conditions occurs only in a very narrow temperature range of 200-215 8C on a SiO 2 /Si substrate. [21][22][23] The number of ALD cycles for the coating of a MoN x thin film on CC and NCCC substrates was chosen to be 700 cycles to obtain a uniform thin film with a thickness of approximately 25-30 nm based on our previous study. [21][22][23] A schematic representation of the synthesis of the electrode is shown in Figure S1 (Supporting Information), and the conditions used during NCCC preparation, ALD-MoN x coating, and heat-treatment are tabulated in Table S1.…”

“…The ALD-growth kinetics, growth rate, and the composition of the ALD-MoN x film have been extensively investigated and are described in our previous work. [21][22][23] The structure and phase of the as-deposited MoN x film on NC-powder-coated carbon cloth (MoN x /NCCC), and that of the sample annealed in a hydrogen atmosphere at 400 8C (MoN x /NCCC-400) and 600 8C (MoN x /NCCC-600) were analyzed by XRD spectroscopy (Figure 2 a). The post-annealing temperatures were chosen according to two important criteria; the Mo 2 C formation temperature (by the reaction of carbon and molybdenum compounds, which is ca.…”

“…The challenges associated with the synthesis of high-surface-area and porous molybdenum-based carbide and nitride electrodes can be overcome by using atomic layer deposition (ALD), which can effectively coat a thin film with excellent uniformity and conformality on a high-surface-area porous substrate at relatively low temperatures. [21][22][23] Furthermore, ALD can be used for precise control of the thickness of the coating, which is beneficial for electrode fabrication because an unnecessarily thicker active material not only adversely effects the electrical conductivity of the electrode but might also influences the catalytic activity as the electrocatalytic process is a surface-restricted phenomenon. Therefore, ALD is beneficial for the fabrication of free-standing large area electrodes for practical implementation.…”