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coatings Article Design of Highly Active Electrodes for Hydrogen Evolution Reaction Based on Mo-Rich Alloys Electrodeposited from Ammonium Acetate Bath Edita Vernickaite ̇ 1, Oksana Bersirova 2, Henrikas Cesiulis 1 and Natalia Tsyntsaru 1,3,* 1 2 3 Received: 15 January 2019; Accepted: 29 January 2019; Published: 30 January 2019 Department of Physical Chemistry, Vilnius University, Naugarduko str. 24, LT-03225 Vilnius, Lithuania; edita.vernickaite@chf.vu.lt (E.V.); henrikas.cesiulis@chf.vu.lt (H.C.) V.I. Vernadsky Institute of General and Inorganic Chemistry, 32-34 Acad. Palladina ave., 03680 Kiev, Ukraine; bersirova@nas.gov.ua Institute of Applied Physics, Academiei str. 5, MD-2028 Chisinau, Moldova * Correspondence: ashra_nt@yahoo.com; Tel.: +370-672-28632 Abstract: The given research was driven by prospects to design Mo-rich coatings with iron group metals electrodeposited from a highly saturated ammonium acetate bath. The obtained coatings could be employed as prominent electrodes for the hydrogen evolution reaction (HER). It was found that the Mo content in Ni–Mo alloys can be tuned from 30 to 78 at.% by decreasing the molar ratio [Ni(II)]:[Mo(VI)] in the electrolyte from 1.0 to 0.25 and increasing the cathodic current density from 30 to 100 mA/cm2. However, dense cracks and pits are formed due to hydrogen evolution at high current densities and that diminishes the catalytic activity of the coating for HER. Accordingly, smoother and crack-free Ni–54 at.% Mo, Co–52 at.% Mo and Fe–54 at.% Mo alloys have been prepared at 30 mA/cm2. Their catalytic behavior for HER has been investigated in a 30 wt.% NaOH solution at temperatures ranging from 25 to 65 ◦C. A significant improvement of electrocatalytic activity with increasing bath temperature was noticed. The results showed that the sequence of electrocatalytic activity in alkaline media decreases in the following order: Co–52 at.% Mo > Ni–54 at.% Mo > Fe–54 at.% Mo. These peculiarities might be linked with different catalytic behavior of formed intermetallics (and active sites) in electrodeposited alloys. The designed electrodeposited Mo-rich alloys have a higher catalytic activity than Mo and Pt cast metals. Keywords: Ni–Mo; Co–Mo; Fe–Mo alloys; electrodeposition; hydrogen evolution reaction; electrocatalysis 1. Introduction Hydrogen is a clean fuel and an energy carrier that can be used for energy conversion and storage and is considered as a possible substitute for fossil fuels [1]. Electrocatalytic water splitting offers an ideal approach for highly pure hydrogen production. However, despite the multitude of on-going research, the development of an optimized, cost-effective and sustainable catalyst, which possesses a high catalytic activity for hydrogen evolution reaction (HER) is still rather appealing. Commonly, the ability of a given metal to catalyze the HER is estimated based on the exchange current density (ECD), i.e., the current density in the absence of net electrolysis at zero overpotential (at formal equilibrium potential for hydrogen evolution reaction in the particular solution). It is known, that the higher the ECD, the lower the overvoltage that must be applied to create a significant current flow. Hence, elaborated electrocatalysts should manifest exchange current densities equivalent or analogous to the ECD of polycrystalline platinum (~1 × 10−3 A/cm2 in alkaline electrolytes) [2]. Coatings 2019, 9, 85; doi:10.3390/coatings9020085 www.mdpi.com/journal/coatingsPDF Image | Electrodes for H2 Evolution Reaction Based on Mo-Rich Alloys
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