Highly Uniform Atomic Layer-Deposited MoS₂@3D-Ni-Foam: A Novel Approach to Prepare an Electrode for Supercapacitors

This article takes an effort to establish the potential of atomic layer deposition (ALD) technique toward the field of supercapacitors by preparing molybdenum disulfide (MoS₂) as its electrode. While molybdenum hexacarbonyl [Mo(CO)₆] serves as a novel precursor toward the low-temperature synthesis of ALD-grown MoS₂, H₂S plasma helps to deposit its polycrystalline phase at 200 °C. Several ex situ characterizations such as X-ray diffractometry (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and so forth are performed in detail to study the as-grown MoS₂ film on a Si/SiO₂ substrate. While stoichiometric MoS₂ with very negligible amount of C and O impurities was evident from XPS, the XRD and high-resolution transmission electron microscopy analyses confirmed the (002)-oriented polycrystalline h-MoS₂ phase of the as-grown film. A comparative study of ALD-grown MoS₂ as a supercapacitor electrode on 2-dimensional stainless steel and on 3-dimensional (3D) Ni-foam substrates clearly reflects the advantage and the potential of ALD for growing a uniform and conformal electrode material on a 3D-scaffold layer. Cyclic voltammetry measurements showed both double-layer capacitance and capacitance contributed by the faradic reaction at the MoS₂ electrode surface. The optimum number of ALD cycles was also found out for achieving maximum capacitance for such a MoS₂@3D-Ni-foam electrode. A record high areal capacitance of 3400 mF/cm² was achieved for MoS₂@3D-Ni-foam grown by 400 ALD cycles at a current density of 3 mA/cm². Moreover, the ALD-grown MoS₂@3D-Ni-foam composite also retains high areal capacitance, even up to a high current density of 50 mA/cm². Finally, this directly grown MoS₂ electrode on 3D-Ni-foam by ALD shows high cyclic stability (>80%) over 4500 charge-discharge cycles which must invoke the research community to further explore the potential of ALD for such applications.