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More green hydrogen with less iridium

Metal Tech News - May 13, 2024

Riken researchers have developed a green hydrogen catalyst that reduces the amount of rare and expensive iridium needed by 95%.

One of the most elementary chemistry lessons taught to schoolchildren is that water is a molecule made up of two parts hydrogen and one part oxygen. Now, scholars at the highest level of academics are working to find the most efficient and sustainable means of breaking those hydrogen atoms off of water to produce a green fuel that turns back into water as it generates energy for a zero-carbon future.

Toward this lofty goal, scientists at the Riken Center for Sustainable Resource Science (CSRS) in Japan have developed new catalysts that could help overcome a major bottleneck in the production of green hydrogen – the need for an extremely rare platinum group metal called iridium to split the H2O bond.

While iridium has proven its mettle when it comes to breaking water's molecular bond to create hydrogen, it costs around $4,750 to buy just one ounce of this metal, which is one of six platinum group elements. This hefty price tag is based on the metal's scarcity, which is the real holdback – there isn't enough iridium mined to produce enough green hydrogen to meet the world's clean energy needs, not even close.

On average, around 18 terawatts (trillion watts) of energy are currently produced on Earth at any given time. More than 80% of this energy comes from fossil fuels. This means that nearly 15 trillion watts of energy would need to be produced by hydrogen and other clean energy methods to completely replace fossil fuels.

For green hydrogen to be a significant factor in this equation, either a massive deposit of iridium or an alternative catalyst for splitting hydrogen off water needs to be discovered.

"Iridium is so rare that scaling up global hydrogen production to the terawatt scale is estimated to require 40 years' worth of iridium," says Shuang Kong, the first author of two scientific studies published by Riken (CSRS) researchers last week that outline a potential path to eliminating iridium from the green hydrogen equation.

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Nearly 15 terawatts of energy produced with hydrogen and other clean energy methods is needed to completely replace the fossil fuels currently supplying the world's energy.

Promising manganese catalyst

Almost two years ago, a Riken (CSRS) team of researchers led by Ryuhei Nakamura developed a catalyst for proton exchange membrane (PEM) electrolyzers – devices that split water into hydrogen and oxygen – made of cobalt and manganese. While both metals are much more abundant and less expensive than iridium, the green hydrogen electrolysis process using these metals was not as stable as it needs to be.

Since that time, Nakamura and his team have developed a new manganese oxide catalyst that is 40 times more stable and does not need cobalt.

The secret to increasing the stability was in the type of oxygen used.

Oxygen in the 3D lattice structure of manganese oxide comes in two configurations: planar and pyramidal. The planar version forms stronger bonds with manganese, and the researchers discovered that increasing the amount of planar oxygen in the lattice significantly enhanced catalytic stability.

The total quantities of hydrogen produced with this modified manganese oxide over a six-week test period was 10 times more than has been achieved with previous catalysts that did not involve the use of rare metals such as iridium.

Manganese, used for steel production and lithium-ion batteries, is an abundant metal that is showing promise as a green hydrogen catalyst.

While the manganese oxide catalyst has proved to be stable, the rate of hydrogen production is not high enough for commercial consideration, and the stability needs to be measured in years, not weeks or months.

The Japanese research team, however, believes it is closing in on a non-iridium green hydrogen catalyst with real-world applications that will eventually contribute to carbon neutrality.

"We will continue to modify catalyst structure to increase both current density and catalyst lifetime," says Nakamura.

The Riken (CSRS) research team published the results of its study on stable manganese oxides for PEM electrolysis in the scientific journal Nature on May 7.

Building a bridge catalyst

While Nakamura and his team continue to perfect a green hydrogen catalyst that eliminates iridium from the equation but still meets industry criteria for commercial applications, they understand that the need for clean energy solutions is urgent.

"We need a way to bridge the gap between rare metal- and common metal-based electrolyzers, so that we can make a gradual transition over many years to completely sustainable green hydrogen," says Nakamura.

A study published by the Riken (CSRS) team in Science on May 10 provides the blueprint for a bridge that is built with a manganese oxide framework that is reinforced by iridium.

Iridium, one of six platinum group metals, is an excellent hydrogen catalyst but is extremely rare.

By inserting individual iridium atoms into a piece of manganese oxide so that they didn't touch, the research team created a hydrogen catalyst that is as effective and stable as pure iridium but with 95% less of the rare metal.

The scientists achieved more than 3,000 hours (four months) of continuous hydrogen production at 82% efficiency without degradation.

"The unexpected interaction between manganese oxide and iridium was key to our success," said Ailong Li, a co-author of the study.

Nakamura believes this iridium-enhanced manganese oxide catalyst could be immediately put to use for commercial green hydrogen production.

"We expect our catalyst to be easily transferred to real-world applications, which will immediately increase the capacity of current PEM electrolyzers," he said.

The Riken (CSRS) research team has already collaborated with industry partners that have found ways to make this manganese oxide-iridium catalyst even better.

With the bridge catalyst on its way to commercial applications, the Nakamura-led team continues to investigate ways to further reduce the quantities of iridium needed – with the ultimate goal of getting that number to zero.

Author Bio

Shane Lasley, Metal Tech News

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With more than 16 years of covering mining, Shane is renowned for his insights and and in-depth analysis of mining, mineral exploration and technology metals.


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