Green hydrogen: MXenes shows talent as catalyst for oxygen evolution
The surface of a Vanadium carbide MXene has been examined by Scanning Electron Microscopy. The beautiful structures are built by cobalt copper hydroxide molecules. © B. Schmiedecke/HZB
The MXene class of materials has many talents. An international team led by HZB chemist Michelle Browne has now demonstrated that MXenes, properly functionalised, 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 now extensively characterising these MXene catalysts for water splitting at the Berlin X-ray source BESSY II and Soleil Synchrotron in France.
Green hydrogen is seen as one of the energy storage solutions of the future. The gas can be produced in a climate-neutral way using electricity from 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 needed to facilitate this reaction. Among the best candidates for OER catalysts are, for example, nickel oxides, which are inexpensive and widely available. However, they corrode quickly in the alkaline water of an electrolyser and their conductivity also leaves much to be desired. This is currently preventing the development of low-cost, high-performance electrolysers.
MXene 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 put to fantastic use, whether for storing charges or as catalysts.
An international team led by Dr Michelle Browne has now investigated the use of MXenes as catalysts for the oxygen evolution reaction. PhD student Bastian Schmiedecke chemically 'functionalised' the MXenes by docking copper and cobalt hydroxides onto their surfaces. In preliminary tests, the catalysts produced in this way proved to be significantly more efficient than the pure metal oxide compounds. What's more, the catalysts showed no degradation and even improved 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 works so well: “We were able to use the Maxymus beamline there to find out how the outer surfaces of the 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 further insights into the material.
Outlook: observation under continuous load
"We have been 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, Prague will continue. In addition to further chemical variations of MXene catalysts, the team also plans to test such catalysts in conventional electrolysers in continuous operation.
- Iridium-free catalysts for acid water electrolysis investigatedHydrogen will play an important role, both as a fuel and as a raw material for industry. However, in order to produce relevant quantities of hydrogen, water electrolysis must become feasible on a multi-gigawatt scale. One bottleneck is the catalysts required, with iridium in particular being an extremely rare element. An international collaboration has therefore investigated iridium-free catalysts for acidic water electrolysis based on the element cobalt. Through investigations with various methods, among them experiments at the LiXEdrom at the BESSY II X-ray source in Berlin, they were able to elucidate processes that take place during water electrolysis in a cobalt-iron-lead oxide material as the anode. The study is published in Nature Energy.
- MXene as a frame for 2D water films shows new propertiesAn international team led by Dr. Tristan Petit and Prof. Yury Gogotsi has investigated MXene with confined water and ions at BESSY II. In the MXene samples, a transition between localised ice clusters to quasi-two-dimensional water films was identified by increasing temperature. The team also discovered that the intercalated water structure drives a reversible transition from metallic to semiconducting behaviour of the MXene film. This could enable the development of novel devices or sensors based on MXenes.
- Lithium-sulphur batteries with lean electrolyte: problem areas clarifiedUsing a non-destructive method, a team at HZB investigated practical lithium-sulphur pouch cells with lean electrolyte for the first time. With operando neutron tomography, they could visualise in real-time how the liquid electrolyte distributes and wets the electrodes across multilayers during charging and discharging. These findings offer valuable insights into the cell failure mechanisms and are helpful to design compact Li-S batteries with a high energy density in formats relevant to industrial applications.