The nanoconfined water between the layers of MXene allows protons to move to and from the redox surface, and therefore the properties of the nanoconfined water will influence proton transport.
(a) Atomic resolution HAADF images of three distinct phases with interlayer spacing of 1.29, 1.45 and 1.57 nm, respectively. (b) the local snapshot of the Mn-MXene-N model with 3 layers (15.8 Å), 2 layers (14.4 Å) and 1 layer (13.0 Å) of water between the MD simulation layers , with the probability density profiles of water molecules along the lattice direction c displayed on the left side. (c) Trends of initial phase residue and Mn-MXene-N capacity enhancement during CV pre-cycles. (d) Low-field 1H time-domain nuclear magnetic resonance spectra of the indicated samples. The shaded region represents the time domain region of the interlayer water. Image credit: © Science China Press.
But the intrinsic properties of nanoconfined water and its atomic role on the performance of electrochemical energy storage remain to be known.
Recently, a study led by Professor Junliang Sun (College of Chemistry and Molecular Engineering, Peking University) suggested an easy technique to deal with nanoconfined water via modification of surface chemistry.
By introducing oxygen and nitrogen surface groups, the interlayer spacing of Ti3VS2 MXene has been greatly increased by housing three-layered nanoconfined water. Unusually high capacity has been achieved with excellent high throughput performance.
The atomic-scale explanation of the layer-dependent properties of nanoconfined water and pseudocapacitive charge storage has been probed extensively.
Like two-dimensional Ti3VS2 MXene was negatively charged and metal cations can be simultaneously interposed in the interlayers, the researchers used Mn3+/Mn2+ as the so-called “redox pair” to manageably oxidize Ti3VS2 Surface MXene.
In addition, ammonia annealing was performed to initiate surface nitrogen terminations and thus improve surface hydrophilicity. Eventually, the interlayer water went from two layers to three layers.
In situ XRD and ex-situ XPS indicated that the initiated Ti–N–O/Ti–N–OH terminals must be converted while being driven by the potential. This was consistent with the results of the static DFT evaluation. This implies that the initiation of nitrogen terminals provides new active sites.
Otherwise, on the spot XRD showed that the interlayer spacing of modified Ti3VS2 Modified MXene next to the 0 V potential (vs. Ag/AgCl), obtaining about 2.8 Å. Molecular dynamics simulation shows that such a huge change came from the intercalation or deintercalation of confined water.
Moreover, modified Ti3VS2 MXene has the potential to hold more interlayer water upon loading. This can not only store a high net charge, but also harbor a denser hydrogen bond network. This is to improve the capacity and throughput performance of Ti3VS2 MXene.
Electrochemical analysis suggests that altered Ti3VS2 MXene displays a particular capacity of up to 2000 F cm-3 (550 Fg-1) in an acidic electrolyte.
As the slew rate changes from 5 mV s-1 at 200 mV s-1, the performance degradation is not greater than 10%. The performance of this material is the best among the pseudocapacitive materials that have been reported.
To better understand the role of nanoconfined water on energy storage, the team noted the intercalation process of broken water from dried Ti3VS2 MXene by on the spot XRD and discovered three distinct interlayers of altered Ti spacing3VS2 MXene at the atomic level by ex-situ spherical aberration electron cryomicroscope.
The molecular dynamics simulation highlights the fact that these three types of layer spacing correspond respectively to one to three layers of confined water. Confined water, as well as the different layers, exhibit layer-dependent physico-chemical properties.
The higher the number of layers present, the higher the diffusion coefficient of the protons and the higher the mobility of the interlayer confined water. Estimated results have been verified by on the spot electrochemical impedance spectroscopy and ex-situ 1H NMR low field, et al.
In the end, this study paints a complete picture of how the interactions of surface chemistry, proton and nanoconfined water contribute to high performance rate and high capacity.
Additionally, this work may provide new insights into other 2D and layered materials as well as nanoconfined fluids on MXenes, and extend beyond energy storage to applications such as selective membranes. ions and water desalination.
Li, H. et al. (2022) Achieving ultra-high electrochemical performance through surface design and manipulation of nanoconfined water. National Science Review. doi.org/10.1093/nsr/nwac079.