Iridium-based catalysts are essential for generating hydrogen through water electrolysis. Recently, a research team demonstrated that the newly designed P2X catalyst, which uses only a quarter of the iridium compared to existing options, matches the efficiency and longevity of the top commercial catalysts. Measurements conducted at BESSY II have identified how the unique chemical conditions within the P2X catalyst enhance the oxygen evolution reaction, crucial for splitting water.
Looking ahead, hydrogen will play a crucial role in a climate-neutral energy system for energy storage, as a fuel, and as a feedstock in the chemical industry. The optimal method for producing hydrogen is through a sustainable process that utilizes electricity derived from solar or wind energy, specifically via water electrolysis. Proton Exchange Membrane Water Electrolysis (PEM-WE) is widely recognized as a pivotal technology in this regard. To facilitate the necessary reaction, both electrodes are coated with specialized electrocatalysts. Iridium-based catalysts are particularly effective for the anode, where the slow oxygen evolution reaction takes place. However, since iridium is among the rarest elements on the planet, a major challenge is to significantly decrease the reliance on this precious material. Initial analyses indicate that to satisfy global hydrogen needs for transportation using PEM-WE, the iridium content in anode materials should not exceed 0.05 mgIr/cm2. In contrast, the best currently available catalyst made from iridium oxide has about 40 times this amount.
P2X-catalyst requires less Iridium
Fortunately, new developments are underway: Within the Kopernikus P2X project, an advanced iridium-based nanocatalyst was created by the Heraeus Group. This catalyst comprises a thin iridium oxide layer on a nanostructured titanium dioxide support, known as the ‘P2X catalyst,’ which uses a minimal amount of iridium—four times less than the leading commercial catalysts.
A research team at HZB, led by Dr. Raul Garcia-Diez and Prof. Dr.-Ing. Marcus Bär, alongside colleagues from the ALBA synchrotron in Barcelona, examined the P2X catalyst. They found it to be exceptionally stable even during extended operations and compared its catalytic properties and spectroscopic characteristics to those of the benchmark commercial crystalline catalyst.
Real-time measurements at BESSY II
The HZB team performed detailed evaluations of both the commercial benchmark and the P2X catalysts at BESSY II during water electrolysis using operando measurements. “We aimed to analyze how the structural and electronic properties of the two catalyst materials evolved during the electrochemical oxygen evolution reaction utilizing operando Ir L3-edge X-ray absorption spectroscopy (XAS),” explains Marianne van der Merwe, a researcher in Bär’s team. They also established a new experimental method to ensure that both samples were tested under the same oxygen production rates, allowing for a fair comparison of the two catalysts under identical conditions.
Exploring Different Chemical Environments
“From the data collected, we could determine that the mechanisms for the oxygen evolution reaction (OER) differ between the two types of iridium oxide catalysts, driven by their distinct chemical environments,” notes van der Merwe. The findings also highlight why the P2X catalyst outperforms its more crystalline counterpart: in the P2X sample, the distances between iridium and oxygen atoms decrease much more significantly at relevant OER potentials. This contraction in Ir-O bond lengths is linked to the involvement of defect sites that are believed to play an essential role in the highly active pathways of the oxygen evolution reaction.
“Moreover, the observations regarding the electronic states correspond with local geometric data,” van der Merwe emphasizes. “Our research provides important insights into the different mechanisms of iridium oxide-based electrocatalysts during the oxygen evolution reaction, enhancing our understanding of catalyst performance and stability. Furthermore, our newly introduced in situ spectroscopic electrochemical protocol is broadly applicable across all anode materials analyzed under pertinent OER conditions.”
