Electrodeposition of Amorphous and Nanocrystalline Ni-Co Alloys of Controlled Composition from Protic Ionic Liquid and Their Use for the Hydrogen Evolution Reaction

Abstract

In recent years, a growing interest has been paid in developing new and more efficient electrocatalysts for water splitting [1]. This study focuses on the synthesis of amorphous and nanocrystalline Ni-Co alloys through a one-step electrodeposition technique in pure ethylammonium nitrate (EAN), a protic ionic liquid. The use of ionic liquid prevents from the formation of oxide film during the electrodeposition of the metal [2]. However, only few studies involve protic ionic liquids and the use of EAN, in particular, the electrodeposition of Ni/Co-based materials is still scarce. In this work, the physicochemical properties of EAN with different concentrations of Ni2+ and Co2+ salts were investigated, along with the electrochemical behavior of these metal cations in the protic ionic liquid in an aerated atmosphere. Moreover, the redox reversibility of electrochemical systems involving Ni and Co ions were examined by cyclic voltammetry, revealing the formation of an oxide film or side-reactions in the presence of air during electrodeposition, showing the need to prepare these materials in an oxygen-free environment. The deposits have then been characterized by TEM (Figure 1). Nanocrystalline Ni-Co alloys with different atomic ratios were obtained depending on the deposition potential and ion concentrations. We showed that when a higher cathodic potential is applied (-1.3 V/Ag/Ag+), the amorphous Ni51.9Co48.1 alloy could be electrodeposited, instead of nanocrystalline alloys. The XRD and SEM analyses exhibited different morphologies and atomic structures of the synthesized alloys.The electrocatalytic properties of the synthesized materials were investigated towards the hydrogen evolution reaction (HER). The results showed that the addition of Co in Ni-based alloys could enhance the HER, and notably, amorphous Ni-Co alloys exhibited lower overpotential, indicating better reaction abilities and faster reaction rates. The deposited Ni-Co alloys on Cu board also demonstrated good stability during the recycle measurement. This study provides a new pathway for preparing nanocrystalline and amorphous Ni-Co alloys with different ratios using protic ionic liquid, which can have practical applications and good performances in electrocatalysis such as water splitting.[1] M. Zhao, K. Abe, S.-I. Yamaura, Y. Yamamoto, N. Asao, Chem. Mater. 2014(26) 1056-1061.[2] M. Asnavande, C. Zhao, Chem. Mater. 26 (2014) 1056-1061. ACS Sustain. Chem. Eng. 2017 5 (1) 85-89. Figure 1