In the longer term, hydrogen will probably be needed in a climate-neutral energy system to store energy, as a fuel, and a raw material for the chemical industry. Ideally, it must be produced in a climate-neutral way, using electricity generated from harnessing the sun’s or wind energy, via the electrolysis of water. In that respect, Proton Exchange Membrane Water Electrolysis (PEM-WE) is currently considered a key technology. Each electrodes are coated with special electrocatalysts to speed up the specified response. Iridium-based catalysts are best fitted to the anode, where the sluggish oxygen evolution response occurs. Nonetheless, iridium is considered one of the rarest elements on earth, and considered one of the key challenges is to significantly reduce the demand for this precious metal. A rough evaluation showed that to fulfill the world’s hydrogen demand for transport using PEM-WE technology, iridium-based anode materials should contain not more than 0.05 mgIr/cm2. The present, best commercially available catalyst produced from iridium oxide accommodates about 40 times as much as this goal value.
P2X-catalyst needs less Iridium
But latest options are already within the pipeline: Throughout the Kopernikus P2X project, a brand new efficient iridium-based nanocatalyst was developed by the Heraeus Group, consisting of a skinny layer of iridium oxide deposited on a nanostructured titanium dioxide support. The so-called ‘P2X catalyst’ requires only an especially small amount of iridium, reducing precious metal loading substantially (4 times lower than in the present best business material).
A team at HZB led by Dr. Raul Garcia-Diez and Prof. Dr.-Ing. Marcus Bär, along with colleagues from the ALBA synchrotron in Barcelona, have studied the P2X catalyst, which shows remarkable stability even in long-term operation, and compared its catalytic and spectroscopic signature with the benchmark business crystalline catalyst.
Operando measurements at BESSY II
The HZB team has thoroughly investigated the business benchmark catalyst in addition to the P2X catalyst at BESSY II during water electrolysis (operando measurements). “We wanted to look at how the 2 different catalyst materials change structurally and electronically throughout the electrochemical oxygen evolution response using operando Ir L3-edge X-ray absorption spectroscopy (XAS),” says Marianne van der Merwe, a researcher in Bär’s team. In addition they developed a brand new experimental protocol to be certain that the outcomes are measured in each samples under the exact same oxygen production rate. This made it possible to match the 2 catalysts under equivalent conditions.
Different chemical environments explored
“From the measurement data, we were in a position to conclude that the mechanisms for OER within the two classes of iridium oxide catalysts are different, and that is driven by the various chemical environments of the 2 materials,” says van der Merwe. The measurement data also show why the P2X catalyst performs even higher in comparison with its more crystalline benchmark: within the P2X sample, the bond lengths between iridium and oxygen decrease significantly greater than within the reference catalyst at OER relevant potentials. This reduction in Ir-O bond lengths may be associated to the participation of defective environments which might be proposed to be key players in highly lively pathways of the oxygen evolution response.
“As well as, the electronic state observations also correlate with local geometric information,” van der Merwe points out. “Our work provides invaluable key details about the various mechanisms of iridium oxide-based electrocatalysts throughout the oxygen evolution response and deepens our understanding of catalyst performance and stability, while our newly proposed in situ spectroscopic electrochemical protocol approach is usually applicable to all anode materials studied under relevant OER conditions.”
