
Iridium is about 4 times rarer than gold [1], making it one of the most expensive metals with a current price of more than € 150 000 per kg [2]. The total yearly production of iridium is only about 8-9 tons since there is no direct mining of iridium ore. Most of the iridium, 95%, is obtained as a by-product of platinum mining, namely in southern Africa [3].
What makes Iridium special are its physical and chemical properties. Its hardness on the Mohs scale is 6.5, harder than titanium with 6. And, most important for its use in electrolysis, it is unique in its resistance towards oxidization: even aqua regia does not dissolve it. It is a rare metal which cannot easily be substituted by alternatives, making it a so-called critical commodity.
PEM electrolyzers require iridium as an integral component of the anode catalyst. This is an issue since the scarcity of iridium significantly limits the potential of water electrolysis. How tight the iridium market is, was recently demonstrated when iridium was made obligatory for use in electrolytic chlorination systems for ship ballast water treatment. The heightened demand caused iridium prices to increase by a factor of 4 within a few months [4].
Approximately 400 kg of iridium are required per GW power of PEM electrolyzers [5]. In other words, the total current annual production of iridium corresponds to a little more than 20 GW of PEM electrolyzers. Following a study of German Raw Materials Agency (DENRA) the global Iridium demand could already increase to up to 34 tons annually by 2040 [6]. As demand for hydrogen production scales up, concerns about Iridium scarcity have grown. Industry experts warn that a surge in demand, without alternative solutions, could lead to significant supply bottlenecks and skyrocketing costs, potentially slowing down the green hydrogen ramp-up. Therefore, research activity towards lowering the iridium demand or enhancing recycling efficiency and capabilities is high. Regardless, it remains unclear if and to what extent it is possible to avoid the iridium supply bottleneck.
There are alternatives, however. Traditional alkaline water electrolysis does not require iridium. It is, however, not well suited for the fluctuating demand put on the electrolyzers by photovoltaic and wind parks. AEM electrolysis on the other hand, combines the iridium-free approach of alkaline water electrolysis with the flexibility of PEM. Without the need for iridium, AEM electrolyzers have the potential to strongly decrease the cost of hydrogen production. Additionally, as the stack production does not depend on the availability of a rare metal such as iridium, supply and cost can be expected to be steadier and more predictable.
Enapter offers a 2.4 kW AEM stack that is completely iridium-free. It is highly modular and is available in single-core configuration as well as in multi-core configuration where up 420 individual stacks are combined to a 1 MW AEM electrolyzer, the AEM Nexus. It produces up to 210 Nm³/h of hydrogen or 450 kg in 24 hours. Intermediate solutions are available in the form of the 500 kW AEM Nexus and the AEM Flex which comes in 70 to 480 kW configurations.
Due to the underlying AEM technology, all products are highly flexible and perform well with the fluctuating demand of renewable energy sources.
Future outlook
As demand for green hydrogen increases, it is crucial that iridium supply does not become a bottleneck. While research is ongoing to reduce the iridium reliance of PEM, with AEM there is a technology that is commercially available today that does not require any iridium at all. Due to this advantage, AEM has the potential to strongly decrease the levelized cost of hydrogen (LCOH). A recent study showed that 1.29 €/kg are feasible which is close to the US DoE cost target [7].
Conclusion
- Iridium is rare and expensive, and production rates strongly depend on platinum mining
- Commercial PEM electrolyzers require iridium for the anode catalyst
- Alkaline electrolysis does not require iridium, but does not perform well with frequent on/off-cycles
- AEM combines non-iridium electrolysis with the ability to quickly react to fluctuating hydrogen production rates
- LCOH of 1.29 €/kg projected
References
[1] CRC Handbook of Chemistry and Physics, 97th ed. 2016–2017, 14-17
[2] https://pmm.umicore.com/en/prices/iridium/
[3] https://matthey.com/en/science-and-innovation/expert-insights/2022/recycling-and-thrifting-the-answer-to-the-iridium-question-in-electrolyser-growth
[4] https://www.dailymetalprice.com/metalpricecharts.php?c=ir&u=oz&d=120
[5] Energy Fuels 37 (2023) 12558
[6] https://www.wto.org/english/res_e/booksp_e/green_hydrogen_e.pdf
[7] Int J Hydrogen Energy 49 (2024) 518
Authors
Florian Baur, Enapter, Labmanager
Vito Pinto, Enapter, VP R&D