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