Investigation of the Corrosion and Electrochemical Behaviour of Nickel Electrodes in Alkaline Medium and Preparation of Nickel-based Electrocatalysts for Efficient Alkaline Water Electrolysis

Abstract

Nickel (Ni) and Ni-based materials are of great interest to the development of renewable energy technologies, including fuel cells and water electrolysers, due to their good stability and catalytic activity, and lower cost when compared to currently used materials (e.g platinum). To increase the utilisation of Ni in such technologies, it is crucial to (i) expand our knowledge on interfacial Ni electrochemistry and (ii) prepare and characterize new efficient and durable Ni electrocatalysts. This work attempts to face both challenges in a systematic manner. The corrosion and electrochemical behaviour of polycrystalline Ni in alkaline media saturated with either N2(g), H2(g) or O2(g) was investigated using cyclic voltammetry (CV) and potentiodynamic polarization (PDP) measurements. The CV measurements were performed using different potential windows and potential scan rates (s) (5.00 – 100 mV s–1), while the PDP measurements were performed in both the anodic and cathodic directions using s = 0.10 mV s–1. The effect of the state of the electrode (metallic or oxidized) was also investigated in the PDP measurements. The CV and PDP results show that the nature of the dissolved gas and the state of the electrode’s surface can significantly affect the corrosion and electrochemical behaviour of polycrystalline Ni. The electrochemical reduction of surface Ni hydroxide species was investigated by means of CV and linear sweep voltammetry measurements. The results indicate that, contrary to popular belief, surface β-Ni(OH)2 layers may be reduced electrochemically under specific experimental conditions. Finally, this work presents a novel procedure for the preparation of Ni, NiCo and NiFe electrocatalysts for renewable technologies. Nickel-based (NiX) layered double hydroxide nanoparticles (LDHNPs) were prepared by a one-step hydrothermal method using Tris-NH2 to control particle growth. The NiX LDHNPs were mixed with a solution of a polymer template (Pluronic F127) to prepare mesoporous mixed metal oxides (MMOs) after a thermal treatment. Both NiX LDHNPs and MMOs were characterized using standard materials science and electrochemical techniques. All samples showed excellent catalytic activity and long-term cycling stability. The synthesis procedure reported herein can be easily scaled-up or tailored to different reactions, highlighting the importance of this work.