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Increasing the surface area of iridium oxide as an oxygen evolution reaction catalyst
Author(s)
Jabe, Ziyanda
Date Issued
2024
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
Hydrogen is a promising clean energy carrier for transportation and chemical industries,
helping reduce fossil fuel emissions and combat climate change. Proton exchange membrane
water electrolysis (PEMWE) efficiently produces high-purity hydrogen with minimal CO₂
emissions when powered by renewable energy sources like solar and wind. However, the
oxygen evolution reaction (OER) at the PEMWE anode is kinetically challenging and requires
significant amounts of noble metal oxide electrocatalysts. Iridium oxide (IrO₂) is the preferred
OER catalyst due to its high activity and corrosion resistance, but its high cost and scarcity
hinder widespread use. To overcome this, researchers are exploring ways to reduce iridium
content, such as using support materials to improve efficiency and extend the catalyst duration.
This study focused on exploring different pathways to increase the surface area of iridium
oxide-based and mixed oxide catalysts from metallic iridium supplied by local mines.
Modifications were applied using various oxidising agents in the Adams’ fusion method to
adjust the catalyst’s oxidation state, porosity, surface area, and morphology. Post-treatment
processes involving different cooling rates were applied to modify the phase and crystallinity
of metal oxides. Dispersing IrOx on high-surface-area support material and mixed oxide to
increase the surface area and OER performance.
The physicochemical properties of the in-house nanomaterials were confirmed using various
analytical techniques. The X-ray spectroscopy (XRD) and high-resolution-scanning
transmission electron microscopy (HR-STEM) analyses revealed that the synthesised catalysts
were amorphous/low crystalline nanomaterials with an average particle size range ± 2 - 5 nm.
X-ray photoelectron spectroscopy (XPS) showed that the in-house are in the Ir3+/ Ir4+ oxidation
state. According to the scanning electron microscopy-energy dispersive X-ray spectroscopy
(SEM-EDX), the nanomaterials exhibit different iridium loading. The stability and activity of
the iridium oxide-based nanomaterials for oxygen evolution reaction (OER) were evaluated
using the rotating disk electrode (RDE) technique in an acidic electrolyte.
The highest OER performance was obtained from the catalyst produced by using sodium
peroxide oxidising agent compared to barium peroxide. Different cooling rates did not result
in a significant increase in catalyst surface area. A notable increase in surface area was achieved
due to the usage of barium peroxide from 11.0 m2
/g to 31.7 m2
/g, while for mixed-oxide
catalyst 18.4 m2
/g was obtained. Furthermore, the electrochemical stability of the catalyst was
achieved for IrOx-(Ba) and TaIrOx catalysts, however at the expense of OER activity. The IrOx-(Na) catalyst outperformed both IrOx-(Ba) and TaIrOx mixed oxide with greater mass-specific
activity at 1.525 V. Even though the IrOx-(Na)-(R) catalyst had the highest OER compared to all
catalysts it was the least stable. For TaIrOx, XPS revealed that Ta is present as Ta5+ (Ta2O5) at
the catalyst surface, this likely affected OER activity since it is a non-conductive material.
helping reduce fossil fuel emissions and combat climate change. Proton exchange membrane
water electrolysis (PEMWE) efficiently produces high-purity hydrogen with minimal CO₂
emissions when powered by renewable energy sources like solar and wind. However, the
oxygen evolution reaction (OER) at the PEMWE anode is kinetically challenging and requires
significant amounts of noble metal oxide electrocatalysts. Iridium oxide (IrO₂) is the preferred
OER catalyst due to its high activity and corrosion resistance, but its high cost and scarcity
hinder widespread use. To overcome this, researchers are exploring ways to reduce iridium
content, such as using support materials to improve efficiency and extend the catalyst duration.
This study focused on exploring different pathways to increase the surface area of iridium
oxide-based and mixed oxide catalysts from metallic iridium supplied by local mines.
Modifications were applied using various oxidising agents in the Adams’ fusion method to
adjust the catalyst’s oxidation state, porosity, surface area, and morphology. Post-treatment
processes involving different cooling rates were applied to modify the phase and crystallinity
of metal oxides. Dispersing IrOx on high-surface-area support material and mixed oxide to
increase the surface area and OER performance.
The physicochemical properties of the in-house nanomaterials were confirmed using various
analytical techniques. The X-ray spectroscopy (XRD) and high-resolution-scanning
transmission electron microscopy (HR-STEM) analyses revealed that the synthesised catalysts
were amorphous/low crystalline nanomaterials with an average particle size range ± 2 - 5 nm.
X-ray photoelectron spectroscopy (XPS) showed that the in-house are in the Ir3+/ Ir4+ oxidation
state. According to the scanning electron microscopy-energy dispersive X-ray spectroscopy
(SEM-EDX), the nanomaterials exhibit different iridium loading. The stability and activity of
the iridium oxide-based nanomaterials for oxygen evolution reaction (OER) were evaluated
using the rotating disk electrode (RDE) technique in an acidic electrolyte.
The highest OER performance was obtained from the catalyst produced by using sodium
peroxide oxidising agent compared to barium peroxide. Different cooling rates did not result
in a significant increase in catalyst surface area. A notable increase in surface area was achieved
due to the usage of barium peroxide from 11.0 m2
/g to 31.7 m2
/g, while for mixed-oxide
catalyst 18.4 m2
/g was obtained. Furthermore, the electrochemical stability of the catalyst was
achieved for IrOx-(Ba) and TaIrOx catalysts, however at the expense of OER activity. The IrOx-(Na) catalyst outperformed both IrOx-(Ba) and TaIrOx mixed oxide with greater mass-specific
activity at 1.525 V. Even though the IrOx-(Na)-(R) catalyst had the highest OER compared to all
catalysts it was the least stable. For TaIrOx, XPS revealed that Ta is present as Ta5+ (Ta2O5) at
the catalyst surface, this likely affected OER activity since it is a non-conductive material.
Additional information
Thesis (Master of Applied Science: Chemistry)--Cape Peninsula University of Technology, 2024
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Sponsor(s)
HySA (Hydrogen South Africa) Catalysis