Tag: MXenes

MXenes for energy storage: Chemical imaging more than just surface deep

MXenes for energy storage: Chemical imaging more than just surface deep

A new method in spectromicroscopy significantly improves the study of chemical reactions at the nanoscale, both on surfaces and inside layered materials. Scanning X-ray microscopy (SXM) at MAXYMUS beamline of BESSY II enables the investigation of chemical species adsorbed on the top layer (surface) or intercalated within the MXene electrode (bulk) with high chemical sensitivity. The method was developed by a HZB team led by Dr. Tristan Petit. The scientists demonstrated among others first SXM on MXene flakes, a material used as electrode in lithium-ion batteries.

Since their discovery in 2011, MXenes have gathered significant scientific interest due to their versatile tunable properties and diverse applications, from energy storage to electromagnetic shielding. Researchers have been working to decipher the complex chemistry of MXenes at the nanoscale.

The team of Dr. Tristan Petit now made a significant progress in MXene characterization, as described in their recent publication. They utilized SXM to investigate the chemical bonding of Ti3C2Tx MXenes, with Tx denoting the terminations (Tx=O, OH, F, Cl), with high spatial and spectral resolution. The novelty in this work is to combine simultaneously two detection modes, transmission and electron yield, enabling different probing depths.

SXM provided detailed insights into the chemical composition and structure of MXenes. According to Faidra Amargianou, first author of the study: “Our findings shed light on the chemical bonding within MXene structure, and with surrounding species, offering new perspective for their utilization across various applications, especially in electrochemical energy storage.”

Read more on HZB website

Image: Scanning X-ray images of a dismounted Li-ion battery with cycled MXene electrode (green), electrolyte/ carbonate species (red) and separator (yellow). The Transmission (bulk-sensitive) image is on the left, the electron yield (surface-sensitive) image on the right.

Credit: HZB

Superstore MXene: New proton hydration structure determined

Superstore MXene: New proton hydration structure determined

MXenes are able to store large amounts of electrical energy like batteries and to charge and discharge rather quickly like a supercapacitor. They combine both talents and thus are a very interesting class of materials for energy storage. The material is structured like a kind of puff pastry, with the MXene layers separated by thin water films. A team at HZB has now investigated how protons migrate in the water films confined between the layers of the material and enable charge transport. Their results have been published in the renowned journal Nature Communications and may accelerate the optimisation of these kinds of energy storage materials.

One of the biggest challenges for a climate-neutral energy supply is the storage of electrical energy. Conventional batteries can hold large amounts of energy, but the charging and discharging processes take time. Supercapacitors, on the other hand, charge very quickly but are limited in the amount of stored energy. Only in the last few years has a new class of materials been discussed that combines the advantages of batteries with those of supercapacitors, named pseudocapacitors.

Promising materials: Pseudocapacitors

Among pseudocapacitive materials, so-called MXenes consisting of a large family of 2D transition metal carbides and nitrides appear particularly promising. Their structure resembles a puff pastry, with the individual layers separated by a thin film of water that enables the transport of charges. Titanium carbide MXenes, especially, are conductive and their layered structure combined with highly negatively-charged hydrophilic surfaces offers a unique material in which positively charged ions such as protons can diffuse very efficiently. The MXenes used in this study were synthesized in the group of Prof. Yury Gogotsi in Drexel University, USA.

Charge transport examined

Over the last years, this property has been used to store and release energy from protons at unprecedented rates in acidic environment. It remains though unclear if the charges are mostly stored based on proton adsorption at the MXene surface or through desolvation of proton in the MXene interlayer.

Confinement effect expected

Due to its two-dimensional geometry, the 2-3 layer thick water film trapped between the MXene layers is expected to solvate protons differently from bulk water that we classically know. While this confinement effect is supposed to play a role in the fast diffusion of protons inside MXene materials, it has been impossible until now to characterise protons inside a MXene electrode during charging and discharging.

Vibrational modes analysed

The team led by Dr. Tristan Petit at HZB has now succeeded in doing this for the first time by analysing vibrational modes of protons excited by infrared light. Postdoctoral researcher Dr Mailis Lounasvuori has developed an operando electrochemical cell that she used to analyse protons and water inside titanium carbide MXenes at BESSY II during the charging and discharging processes. In the process, she also succeeded in distilling out the special signature of the protons in the confined water between the MXene layers.

Read more on the HZB website

Image: The experiment: Infrared light excites protons in the water film, which move between the Ti3C2-MXene layers. Their oscillation patterns show that they behave differently than in a thicker film of water.

Credit: © M. Künsting /HZB