The Development of Three Dimensional Porous Nickel Materials and their Catalytic Performance towards Oxygen Evolution Reaction in Alkaline Media

Abstract

As the global energy crisis and environmental pollution problem continues, there is an increasing demand for clean and sustainable energy storage and conversion technologies, such as water-splitting electrolysis. Water electrolysis is a process of running an electrical current through water in separating the hydrogen and oxygen. Oxygen evolution reaction (OER) is a key reaction in this electrochemical process, and the electrochemical performance of these systems is usually hindered by the slow OER reaction kinetics. In order to achieve high energy conversion efficiency, the development of efficient OER catalysts is the key. To achieve that, abundant research is done by using noble metal oxides as catalyst, such as IrO2 and RuO2. However, considering their high cost, a cheap earth-abundant material with a high OER catalytic activity is required. Accordingly, this study has been focused on the synthesis of three dimensional porous structured Ni-based OER catalysts. First, a 3D porous Ni meso-foam was developed through a facile high-temperature one-pot synthesis method, and its catalytic activity towards OER was explored. Specifically, the as-synthesized Ni meso-foam material, referred to as raw NMF, has a wire-linked structure and high surface area. A reduction procedure was introduced to obtain reduced Ni meso-foam materials, referred to as NMF-H2. It was also oxidized in air at 600 ℃ to form a semi-hollow NiO crosslinking phase and subsequently reduced in H2 at 300℃, forming a regenerated porous Ni foam material, referred to as NMF-O2/H2. The composition and morphology of all materials were investigated by XRD and SEM, respectively. The SEM image reveals that, in the porous NMF-O2/H2, the cross-linked meso-wire structure was maintained, and the average pore size is between 0.5-5 μm. Electrochemical analysis show that the OER activity of the Ni foam catalysts follows NMF-O2/H2 > NMF-H2 > raw NMF. In addition to the NMF-based materials, a Ni/Ni(OH)2 layer-structured electrocatalyst, referred to as NiDHBT, was also developed using a dynamic hydrogen bubble templating (DHBT) method. First, the 3D-porous micro Ni/Zn nanoplatelets were constructed in a two-step DHBT deposition method. The Ni/Zn foil was used as a scaffold, featured with the open porous structure and high surface area, for the subsequent electrodeposition of Ni(OH)2. Then, the Zn was etched from the as-prepared Ni/Zn/Ni(OH)2 nanocomposite to obtain the NiDHBT. The catalytic performance of the NiDHBT toward OER reaction was evaluated, and the optimal catalysts developed from different electro deposition potentials were determined. On the recognition of the high catalytic activity of NMF-O2/H2 and NiDHBT, porous structured FeOx-Nickel meso-foam, referred to as Fe@NMF-O2/H2, and FeOx- Ni/Ni(OH)2 layered-structure materials, referred to as Fe@NiDHBT, was further developed to explore the benefits of FeOx deposition for its OER catalytic performance. The deposition of FeOx is achieved by physical mixing FeOx colloid with NMF-O2/H2 and NiDHBT, and the electrochemical performance of these materials was examined in 1 M KOH. Among the developed materials, the best performing catalyst is Fe@NiDHBT synthesized by loading FeOx colloid onto the NiDHBT support. The overpotential for Fe@NiDHBT to reach 10 mA·cm-2 is 247mV, and the corresponding Tafel slope is 48.10mV·dec-1. Therefore, it was concluded that the FeOx¬¬ loading modification is an effective strategy to improve the OER activity of Ni foam-based catalysts.