Nanoconfined water in-between MXene layers provides a major 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 on 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 facile method to manipulate the nanoconfined water through surface chemistry modification. By introducing nitrogen and oxygen surface groups, the interlayer spacing of Ti3C2 MXene was significantly increased by accommodating three-layer nanoconfined water. Exceptional high capacitance was obtained with outstanding high-rate performance. The atomic scale elucidation of layer-dependent properties of nanoconfined water and pseudocapacitive charge storage was deeply probed.
Since two-dimensional Ti3C2 MXene is negatively charged and metal cations can be spontaneously intercalated into interlayers, the team used Mn3+ / Mn2+ as the "redox pair" to controllably oxidized the surface of Ti3C2 MXene. Further, ammonia annealing was conducted to introduce surface nitrogen terminals, and hence improve the surface hydrophilicity. Finally, the interlayer water was increased from two layers to three layers.
In-situ XRD and ex-situ XPS indicated that the introduced terminals Ti-N-O / Ti-N-OH will be transformed when driven by potential, which is consistent with the calculation results of static DFT, suggesting that the introduction of nitrogen-containing terminals brings new active sites. In-situ XRD also showed that the interlayer spacing of modified Ti3C2 MXene changed near the potential 0 V (vs. Ag / AgCl), reaching ~ 2.8 Å. Molecular dynamics simulation shows that such a large change originated from the intercalation/de- intercalation of confined water. Moreover, modified Ti3C2 MXene can accommodate more interlayer water during charging, which can not only store more net charge, but also accommodate a more dense hydrogen bond network, so as to improve the capacitance and rate performance of Ti3C2 MXene. Electrochemical analysis shows that modified Ti3C2 MXene exhibits a specific capacitance of up to 2000 F cm-3 (550 F g-1) in acidic electrolyte. When the scanning rate increases from 5 mV s-1 to 200 mV s-1, the performance decay is no more than 10%. This performance of this material is among the best pseudocapacitive materials reported.
To deeply understand the role of nanoconfined water on energy storage, the team observed the discontinuous water intercalation process of dried Ti3C2 MXene by in-situ XRD, and found three discrete interlayer spacing of modified Ti3C2 MXene at the atomic level by ex-situ cryo spherical aberration electron microscope. Molecular dynamics simulation shows that these three kinds of layer spacing correspond to one to three layers of confined water respectively. Confined water with different layers shows layer- dependent physicochemical properties. The more layers, the stronger the mobility of interlayer confined water and the greater the proton diffusion coefficient. The calculated results were confirmed by in-situ electrochemical impedance spectroscopy and ex-situ 1H low field NMR, et al.
In conclusion, this study depicts a complete picture of how the interactions of surface chemistry, proton and nanoconfined water contribute to the high capacitance and high-rate performance. This work might also provide new insights into other 2D and layered materials with nanoconfined fluids beyond MXenes, and extend beyond energy storage to applications such as water desalination and ion-selective membranes.
See the article:
Achieving ultrahigh electrochemical performance by surface design and nanoconfined water manipulation
National Science Review