Tag: MXene

MXene for energy storage: More versatile than expected

MXene for energy storage: More versatile than expected

MXene materials are promising candidates for a new energy storage technology. However, the processes by which the charge storage takes place were not yet fully understood. A team at HZB has examined, for the first time, individual MXene flakes to explore these processes in detail. Using the in situ Scanning transmission X-ray microscope ‘MYSTIIC’ at BESSY II, the scientists mapped the chemical states of Titanium atoms on the MXene flake surfaces. The results revealed two distinct redox reactions, depending on the electrolyte. This lays the groundwork for understanding charge transfer processes at the nanoscale and provides a basis for future research aimed at optimising pseudocapacitive energy storage devices.

Energy storage is crucial for achieving a climate-neutral and efficient energy supply, based on renewable energy sources. Current technologies have their pros and cons. Batteries, for example, require a certain amount of time to charge but can store enormous amounts of energy, whereas electric double-layer capacitors (EDLCs) charge quickly but can only absorb a limited amount of energy. So called pseudocapacitors could combine high storage capacity with speed, due to a charge transfer process based on chemical changes without changing the phase of material. Up to now, this technology has not yet been realised due to a lack of promising materials.

The hidden talents of MXenes

This might change with MXene materials. MXenes are two-dimensional materials with a layered structure, such as titanium carbide, which form a conductive core and a highly reactive surface. The distance between layers is in the order of a few nanometers. Via aqueous electrolytes, protons and Li ions can intercalate between MXene layers and act as charge carriers. The charge carriers bind to the surface terminations on the Titanium atoms via redox reactions. Another advantage: Aqueous electrolytes are generally much more environmentally friendly than organic electrolytes used in batteries.

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Image: In a neutral electrolyte Li2SO4 the interaction of partially desolvated Li⁺ ions and water with the MXene surface results in an increased Titanium oxidation state. The two different chemical behaviours also change the interlayer spacing of the flakes.

Credit: © Energy & Environmental Science / HZB

Green hydrogen: MXenes shows talent as catalyst for oxygen evolution

Green hydrogen: MXenes shows talent as catalyst for oxygen evolution

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.

Read more on HZB website

Image: The surface of a Vanadium carbide MXene has been examined by Scanning Electron Microscopy. The beautiful structures are built by cobalt copper hydroxide molecules.

Credit: B. Schmiedecke/HZB