Electrodeposition of Ni-Co alloys from neat protic ionic liquid: Application to the hydrogen evolution reaction

Abstract

Amorphous and nanocrystalline Ni-Co alloys have been deposited on FTO glass in ethylammonium nitrate (EAN), a protic ionic liquid (PIL). The physicochemical properties of EAN with Ni2+ and Co2+ salts have been studied as well as the electrochemical behavior of these metal cations in the PIL in aerated atmosphere. Electrochemical studies showed that the redox reactions of Ni2+ and Co2+ leading to metal deposit on glassy carbon are not reversible in those conditions due to the formation of an oxide film. Conversely, when electrodeposition of Co, Ni and Ni-Co alloys on FTO glass in EAN was performed at 60 °C in an oxygen-free environment, nanocrystalline Ni-Co alloys with different atomic ratios were obtained depending on both the deposition potential and ion concentration in solution. The composition and the morphology of the electroplated alloys were determined by several techniques such as SEM, TEM, XRD and XPS. The electrocatalytic properties of these materials were investigated towards the hydrogen evolution reaction (HER). It was shown that the addition of Co in Ni-based alloys can promote the HER, and most notably, amorphous Ni-Co alloys shows lower overpotential or reaction rate. Up to now, the better electrocatalytic results were obtained with the amorphous Ni51.9Co48.1 alloy, electrodeposited in EAN at high potential (-1.3 V/Ag/Ag+). This work provides a new route to prepare nanocrystalline and amorphous Ni-Co alloys with different ratio in PIL for practical applications in electrocatalysis such as water splitting.