Nickel-Based Amorphous Alloys As Electrocatalyst Materials in Alkaline Water Electrolysis
Authors: Ghobrial Samy , Kirk Donald W. , Thorpe Steven J. 
Published: February 2020

Introduction

Alkaline electrolysis with anionic exchange membranes has been shown to be a highly promising technology to produce hydrogen gas. This zero gap system does not require a liquid electrolyte and can utilize non-precious metal electrocatalyst materials. As such, it is possible to produce a low cost and energy efficient water electrolyzer [1]. Production of hydrogen gas from water electrolysis has been largely limited due to a lack of inexpensive and active electrocatalyst materials that do not degrade over time. Amorphous alloys have previously been proposed as electrocatalyst materials due to their unique atomic structure, which offer high corrosion resistance, high number of coordinatively unsaturated sites and electrochemical activity [2]. Their full potential has not been recognized as electrocatalysts largely due to their low surface area when produced and challenges in synthesis [2]. We suggest a novel process to produce high surface area amorphous alloy powders designed specifically as electrocatalysts for hydrogen evolution. This research is focused on Ni-based alloys that show high stability in alkaline solutions and high activity among non-platinum group metals [3]. Specifically, Ni-based alloys containing Nb and Y are studied in this work. Previous work has shown that Ni-Nb-Y amorphous alloys can phase separate into two distinct amorphous phases [4]. Further enhancement of the electrochemically active surface area can be achieved by selectively dealloying one of the two-phases leaving a nano-porous structure [5]. The present work assesses the electrochemical behaviour of these phase-separated Ni-Nb-Y amorphous alloys.

Experimental Methods

Ni-Nb-Y powder is produced by mechanically alloying high purity elemental powders using a high-energy ball mill. These powders are then chemically etched to selectively dealloy one of the two phases present. These powders can then be pressed into a pellet or mixed into ink and applied (by spraying, printing or brushing) onto a conductive support like carbon paper or glassy carbon rod [1]. Electrochemical characterization is assessed through steady state polarization (Tafel) curves, cyclic voltammetry, and AC impedance spectroscopy. Structural characterization is assessed through x-ray diffraction (XRD) and electron microscopy.

Results and Discussion

Mechanical alloying of nickel, niobium and yttrium elemental powders was performed, producing Ni58.2-Nb20.25Y21.25with ca 90% amorphous structure. Scanning electron microscopy (SEM) and XRD were used to analyze this alloy (see figure 1). SEM allowed analysis of particle size and morphology (average particle size is approximately 40 micron), and XRD allowed determination of microstructure, alloy formation and degree of crystallinity. The alloy produced a Ni-Nb-Y amorphous phase and a separate Ni-Y amorphous phase, indicating the presence of a phase separated alloy. The synthesized material also contains yttrium oxide, which is believed to reduce electrocatalytic activity [5]. Oxide formation can be limited by reducing exposure of the elemental powders to oxygen during the ball milling processing.

Conclusions and Future Work

The results suggest that high surface area amorphous Ni58.2-Nb20.25Y21.25 powder can be produced from high-energy ball milling. Future work will be further developing material to achieve fully amorphous structure, characterizing phase separation and evaluating electrochemical properties.
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Figure 1