Confined water is important for energy storage performance


Figure: (a) The atomic-resolution HAADF images of three different phases with an interlayer spacing of 1.29, 1.45, and 1.57 nm, respectively. (b) The local snapshot of the Mn-MXene-N model with 3 -layer (15.8 Å), 2-layer (14.4 Å), and 1-layer (13.0 Å) water between the layer of MD simulation with the probability density profiles of water molecules along the c-lattice direction shown on the left. (c) The trends of initial phase residue and capacity increase of Mn-MXene-N during CV pre-cycles. (d) Low-field 1H time-domain nuclear magnetic resonance spectra of the indicated samples. The shaded area represents the time domain range of the interlayer water.
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Photo credit: ©Science China Press

Nanoconfined water between MXene layers provides an important way for protons to move to and from the redox surface and hence the properties of nanoconfined water will strongly affect proton transport. However, the intrinsic properties of nanoconfined water and its atomic role in electrochemical energy storage performance are still unclear.

Recently, a study led by Prof. Junliang Sun (College of Chemistry and Molecular Engineering, Peking University) proposed a simple method to manipulate the nanoconfined water by modifying the surface chemistry. By introducing nitrogen and oxygen surface groups, the interlayer distance of Ti3C2 MXene has been significantly increased with the inclusion of triple-layered nano-confined water. An exceptionally high capacity with excellent high-speed performance has been achieved. The elucidation of layer-dependent properties of nanoconfined water and pseudocapacitive charge storage at the atomic level has been extensively studied.

Since two-dimensional Ti3C2 MXene is negatively charged and metal cations can spontaneously intercalate into interlayers, the team used Mn3+ / Mn2+ as a “redox couple” to controllably oxidize the surface of Ti3C2 MXene. Ammonia annealing was also performed to introduce surface nitrogen ends and thus improve surface hydrophilicity. Finally, the interlayer water was increased from two layers to three layers.

On site XRD and ex situ XPS indicated that the introduced Ti-NO/Ti-N-OH terminals are transformed when driven by potential, which is consistent with the static DFT calculation results, suggesting that the introduction of nitrogenous terminals brings new active sites . On site XRD also showed that the interlayer spacing of modified Ti3C2 MXene changed near the 0 V potential (vs. Ag/AgCl) and reached ~2.8 Å. Molecular dynamics simulations show that such a large change comes from the intercalation/deintercalation of trapped water. Also modified Ti3C2 MXene can absorb more interlayer water during charging, which not only can store more net charge, but also can absorb a denser hydrogen bonding network to improve Ti’s capacity and rate performance3C2 MXene. Electrochemical analysis shows that modified Ti3C2 MXene has a specific capacity of up to 2000 F cm-3 (550 fg-1) in acidic electrolyte. When increasing the sampling rate from 5 mV s-1 up to 200 mV s-1, the drop in performance is no more than 10%. This performance of this material is among the best reported pseudocapacitive materials.

To better understand the role of nano-confined water in energy storage, the team observed the discontinuous water intercalation process of dried Ti3C2 MXene from on site XRD, and found three discrete interlayer spacings of modified Ti3C2 MXene at the atomic level ex situ cryo-spherical aberration electron microscope. Molecular dynamics simulations show that these three types of interlayer spacing correspond to one to three layers of trapped water, respectively. Confined water with different layers shows layer-dependent physico-chemical properties. The more layers there are, the greater the mobility of the water trapped between the layers and the greater the proton diffusion coefficient. The calculated results were confirmed by on site electrochemical impedance spectroscopy and ex situ 1H low field NMR, et al.

In summary, this study paints a complete picture of how the interactions of surface chemistry, protons, and nanoconfined water contribute to the high capacity and high-speed performance. This work could also provide new insights into other 2D and layered materials with nanoconfined liquids beyond MXenes, extending beyond energy storage to applications such as water desalination and ion-selective membranes.


See the article:

Achievement of ultra-high electrochemical performance through surface design and manipulation of nano-confined water

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