Less is more: Why economical iridium catalysts work so well

Producing hydrogen using water electrolysis requires an iridium-based catalyst. Now, the HZB team has shown that a newly developed P2X catalyst that requires only a quarter of the amount of iridium is as efficient and long-term stable as the best commercially available catalysts. BESSY II measurements reveal how the special chemical environment within the P2X catalyst during electrolysis facilitates the oxygen evolution reaction during water splitting.

In the future, hydrogen will be needed for climate-neutral energy systems, for energy storage, as a fuel and as a feedstock for the chemical industry. Ideally, it should be produced in a climate-neutral manner, using electricity generated by electrolysis of water using solar or wind energy.

In this regard, polymer exchange membrane water electrolysis (PEM-WE) is currently considered an important technology. Both electrodes are coated with a special electrocatalyst to promote the desired reaction. Iridium-based catalysts are ideal for anodes where the oxygen evolution reaction is slow. However, iridium is one of the rarest elements on earth, and one of the big challenges is to significantly reduce the demand for this precious metal.

A rough analysis shows that the content of iridium-based anode material needs to be below 0.05 mgIr/cm2 to meet the world’s hydrogen demand for transport using PEM-WE technology. Ta. The best iridium oxide catalysts currently available on the market contain approximately 40 times this target.

Less iridium is required for P2X catalysts

But new options are already in the pipeline. Within the Copernicus P2X project, a new efficient iridium-based nanocatalyst was developed by the Heraeus group, consisting of a thin layer of iridium oxide deposited on a nanostructured titanium dioxide support. The so-called “P2X catalyst” requires only extremely small amounts of iridium, significantly reducing the amount of precious metal added (a quarter compared to currently the best commercially available materials).

The HZB team led by Dr. Raul Garcia-Diez and Professor Dr.-Ing. Together with his colleagues at the ALBA synchrotron in Barcelona, ​​Marcus Bär studied a P2X catalyst that shows remarkable stability even in long-term operation and compared its catalytic properties and spectroscopic characteristics to benchmark commercially available crystalline catalysts.

This paper will be published in the journal ACS Catalysis.

Operand measurements on BESSY II

The HZB team thoroughly investigated commercially available benchmark catalysts and P2X catalysts during water electrolysis (operando measurements) on BESSY II.

“We use operando Ir L3-edge X-ray absorption spectroscopy (XAS) to observe how two different catalytic materials change structurally and electronically during an electrochemical oxygen evolution reaction. we wanted to,” says Marianne van der Merwe, a researcher on the Bär team.

They also developed a new experimental protocol to ensure that the results were measured in both samples under exactly the same oxygen production rate. This made it possible to compare the two catalysts under comparable conditions.

Investigate various chemical environments

“From our measured data, we were able to conclude that the mechanisms of OER in the two classes of iridium oxide catalysts are different and that this is caused by the different chemical environments of the two materials,” says van der Merwe.

The measured data also show why the P2X catalyst performs even better compared to more crystalline benchmarks. In the P2X sample, the bond length between iridium and oxygen is significantly reduced compared to the reference catalyst at OER-related potentials. This decrease in Ir–O bond length may be related to the involvement of a defective environment, which has been proposed to play a key role in the highly active pathway of oxygen evolution reactions.

“Furthermore, observations of electronic states are also correlated with local geometric information,” van der Merwe points out.

“Our study provides valuable and important information about the different mechanisms of iridium oxide-based electrocatalysts during oxygen evolution reactions and improves our understanding of catalyst performance and stability. The newly proposed in situ spectroelectrochemical protocol approach is generally applicable to all anode materials that have been studied under relevant OER conditions. ”