Non-Precious Metal-Based Mesoporous Electrocatalysts for Enhanced Oxygen Evolution Reactions

Abstract

The depletion of fossil fuels and the climate change have spurred tremendous research interest in the development of electrochemical technologies such as water splitting for conversion and storage of electricity produced from renewable sources such as solar and wind. However, the sluggish kinetics of the anodic oxygen evolution reaction (OER) in water splitting remains a major challenge for widespread application of the technology. Catalysts play a key role in reducing the OER energy barrier, and improving the overall efficiency. Currently, the most efficient OER catalysts are noble-metal based materials including RuO2 and IrO2. However, these noble metals are expensive and the supply is unsustainable, and is not suitable for large-scale application. Therefore, the development of highly efficient and cost-effective electrocatalysts is highly demanded for substitution of these noble metal-based catalysts. In this thesis, two novel non-precious metal-based mesoporous multi-metallic catalysts are developed for enhanced OER. Specifically, i) Fe embedded mesoporous Co3O4 (Fe/mCo3O4) is prepared via hard-template direct nanocasting method for OER. Attributed to an unusual synergistic effect between mCo3O4 and Fe, Fe/mCo3O4 is capable of delivering current density of 30 mA cm-2 at the overpotential of 440 mV showing higher catalytic activity than conventional Co3O4 nanoparticles. ii) A freestanding, hierarchically structured 3D electrode comprising of topmost mesoporous NiFe nanosheets, in-between nanoporous NiCo2O4 nanoflakes and bottom macroporous Ni foam (NiFe/NiCo2O4/NF) is achieved for highly efficient OER at high current densities. The electrode can deliver a current density of 1200 mA cm-2 at 1.57 V vs. RHE, which is comparable to the benchmark IrO2 and RuO2 catalysts. The physical properties of the catalysts are characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and N2 adsorption-desorption isotherms. The electrochemical performances are evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry. The results obtained in this thesis highlight that by proper design of nanoporous structures of the catalysts and smart utilisation of metal-metal synergistic interactions, multi-transition metal-based catalysts offer promising substitutes for the noble metal-based catalysts for use in commercial water splitting systems.