In brief

A review of existing iridium catalyst structures and activity provides valuable insights on ways of enhancing and optimising their activity to spur future studies and industry adoption.

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Making the most of rare metal catalysts

22 Jun 2023

Researchers take a deep dive into the current state of the art of iridium-based catalysts for green hydrogen production.

Say goodbye to plumes of dirty, carbon-filled smoke. When green hydrogen is burned as a fuel, it combines with oxygen from the air to produce energy, leaving only harmless water vapour as a byproduct. Unsurprisingly, sustainability experts have pinned their hopes on green hydrogen as a clean, renewable alternative fuel to serve carbon-intensive industries that are difficult to electrify, such as maritime shipping and steel production.

Creating green hydrogen involves the splitting of water molecules (H2O) into hydrogen (H2) and oxygen (O2) using electricity in a process consisting of a hydrogen evolution reaction (HER) and an oxygen evolution reaction (OER). OER is typically considered the bottleneck of the whole process. In electrolysers, iridium-based catalysts have been shown to boost OER’s efficiency which can benefit in large-scale commercial applications.

Jiajian Gao, a Research Scientist at A*STAR's Institute of Sustainability for Chemicals, Energy and Environment (ISCE2), said that despite iridium catalysts’ potential, unresolved issues have stalled their widespread industrial adoption.

“The inadequate long-term stability and limited global reserves of the iridium metal are the crux of the issue,” elaborated Gao. “To overcome these obstacles, it is essential to first step back and develop a holistic understanding of the current state of iridium catalysts, which will enable us to steer future research more effectively.”

Collaborating with colleagues from ISCE2 and Nanyang Technological University, Singapore, Gao explored the latest developments in the field of iridium catalyst research. Their goal was to showcase the diversity in emerging iridium catalyst formulations and understand how their chemical structures influence their activity and kinetics to steer the course towards better catalysts for OER.

Given the rarity of iridium, Gao predicted that catalyst formulations with significantly smaller proportions thereof will dominate the field in the coming years. “To achieve this, we must innovate designs that optimise the interactions between the catalyst, liquid solution, and reacting intermediates during electrolyser operation,” said Gao.

The researchers discovered that a plethora of factors impacts the properties and performance of iridium catalysts. “Their shape, arrangement of atoms, and the electric charge can influence their ability to conduct electricity, maintain stability and interact with water molecules during OER,” noted Gao.

Drawing on their insights from the review, the researchers suggested that a strategic choice of initial ingredients during the preparation stage is critical for optimising the catalyst’s efficiency. “We recommend materials that excel in conducting electricity and maintaining their structural stability,” added Gao.

In addition, they proposed that fine-tuning a catalyst’s internal environment where reactions occur, through the introduction of ‘dopants’, can also supercharge the OER. Together, these strategies may help create stable, durable and effective iridium catalysts for greener industrial practices.

Moving forward, Gao’s research team is taking a two-fold approach to transforming current practices for green hydrogen production. Firstly, they plan to design iridium catalysts that can be produced on a large scale. Secondly, they are working towards optimising the utilisation of the precious metal to minimise the amount needed, thereby making the entire process more efficient, sustainable and commercially viable.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2) and the Institute of Materials Research and Engineering (IMRE).

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Gao, J., Liu, Y., Liu, B. and Huang, K,-W. Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts. ACS Nano 16 (11), 17761-17777 (2022)│article

About the Researcher

Jiajian Gao is currently a Research Scientist at A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2). He obtained his PhD degree from the Institute of Process Engineering, Chinese Academy of Sciences in 2014, and subsequently worked as a research fellow at Nanyang Technology University, Singapore. His current research interests mainly include sustainable energy-related processes such as electrocatalysis (water splitting, CO2 conversion) and thermocatalysis (CO/CO2 hydrogenation).

This article was made for A*STAR Research by Wildtype Media Group