Nickel Phosphide Nanomaterials for Hydrogen Evolution Reaction

Abstract

Among non-noble-metal-based materials considered for hydrogen evolution reaction (HER) electrocatalysts, metal phosphides are quite popular, as they are chemically and mechanically stable, have good conductive properties and a high number of potential active sites on the surface [1,2]. Nickel phosphide, in particular, has been investigated – with promising results – as a HER catalyst in a variety of environments [3].When it comes to the morphology of an electrocatalyst, ordered nanowire arrays seem promising. Not only do they have a large and easily accessible active surface area, but also offer a simplified electron transfer path and can be synthesized directly on a conducting substrate, which greatly reduces or removes the need for additional binders. Moreover, such a nanomaterial should be less prone to damage during usage, especially when gas evolution occurs. Nanowire arrays are often synthesized by electrodeposition into nanoporous anodic aluminum oxide (AAO) membranes. However, aqueous solutions typically used in electrosynthesis of various metal-based materials have several limitations, which may cause brittleness or sponginess of structures [5,6]. These problems can be overcome by using non-aqueous solutions, like those based on deep eutectic solvents (DESs), which offer extended potential windows and low toxicity and are easy to prepare.In this research electrodeposition of nickel in a DES-based environment is used to obtain nickel and nickel phosphide nanowire arrays of good quality, and the prepared nanostructures are then tested as HER electrocatalysts.Ordered arrays of nickel nanowires were prepared by electrodeposition of Ni into AAO templates. A non-aqueous bath based on a mixture of choline chloride and ethylene glycol in molar ratio of 1:2 was used; Ni concentration in the bath was 1.14 M. The electrodeposition was performed at 70 ºC under galvanostatic regime. Nickel phosphide nanowire arrays were obtained via phosphorization of previously prepared nickel nanowires. In order to do so, the AAO membranes with Ni nanowires inside were annealed with red phosphorus in argon atmosphere at 500 ºC for 2 h. The morphology and chemical composition of nickel and nickel phosphide nanowires were examined by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Electrochemical tests were carried out in a three-electrode cell, in 0.5 M H2SO4. For all experiments except XRD AAO templates were removed in 1 M NaOH.AcknowledgementsThis work was supported by the National Science Centre Poland (Grant No. 2017/26/M/ST5/00715). The participation in this conference was made possible by financial support from the Polish Ministry of Science and Higher Education within a conceptual non-competitive project entitled "Best of the best! 4.0." (Measure 3.3). The project is co-financed from the European Social Fund under the Operational Programme Knowledge, Education, Development 2014-2020.