MXenes are found to be catalysts for the oxygen evolution reaction

The MXene class of materials has many strengths. An international team led by chemist Michelle Browne from HZB has now demonstrated that MXenes, when properly functionalized, are excellent catalysts for the oxygen evolution reaction in electrolytic water splitting. They are more stable and efficient than the best metal oxide catalysts currently available. The team is currently extensively characterizing these MXene catalysts for water splitting at the Berlin X-ray source BESSY II and at the Soleil synchrotron in France.

The results are published in the Journal of Materials Chemistry A.

Green hydrogen is considered one of the energy storage solutions of the future. The gas can be produced in a climate-neutral manner from electricity generated by the sun or wind by electrolytic water splitting. While hydrogen molecules are produced at one electrode, oxygen molecules are formed at the other. This oxygen evolution reaction (OER) is one of the limiting factors in electrolysis. Special catalysts are required to facilitate this reaction.

Nickel oxides, for example, are among the best candidates for OER catalysts. They are inexpensive and widely available. However, they corrode quickly in the alkaline water of an electrolyzer and their conductivity also leaves much to be desired. This currently prevents the development of low-cost, high-performance electrolyzers.

MXenes as catalysts

A new class of materials could offer an alternative: MXenes, layered materials made of metals, such as titanium or vanadium, combined with carbon and/or nitrogen. These MXenes have a huge internal surface area that can be used in fantastic ways, whether for storing charges or as catalysts.

Doctoral student Bastian Schmiedecke chemically “functionalized” the MXenes by attaching copper and cobalt hydroxides to their surface. In preliminary tests, the catalysts produced in this way proved to be significantly more efficient than pure metal oxide compounds. In addition, the catalysts showed no degradation and even improved their efficiency in continuous operation.

Measurements at BESSY II

Measurements at the BESSY II X-ray source, with Namrata Sharma and Tristan Petit, showed why this process works so well: “We were able to use the Maxymus beamline to find out how the outer surfaces of MXene samples differ from the inside,” explains Schmiedecke. The researchers combined scanning electron microscopy (SEM/TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray transmission microscopy (STXM) and X-ray absorption near-edge structure (XANES) to gain deeper insights into the material.

“We were able to show that MXenes have great potential for use as catalysts in electrolysers,” says Michelle Browne. The collaboration with partner teams from Trinity College Dublin, Ireland, and the University of Chemistry and Technology in Prague will continue. In addition to other chemical variants of MXene catalysts, the team also plans to test these catalysts in conventional electrolysers in continuous operation.