There is widespread interest in determining the structural features of redox-active electrochemical energy storage materials that enable simultaneous high power and high energy density. Here, we present the discovery that confined interlayer water in crystalline tungsten oxide hydrates, WO3·nH2O, enables highly reversible proton intercalation at subsecond time scales. By comparing the structural transformation kinetics and confined water dynamics of the hydrates with anhydrous WO3, we determine that the rapid electrochemical proton intercalation is due to the ability of the confined water layers to isolate structural transformations to two dimensions while stabilizing the structure along the third dimension. As a result, these water layers provide both structural flexibility and stability to accommodate intercalation-driven bonding changes. This provides an alternative explanation for the fast energy storage kinetics of materials that incorporate structural water and provides a new strategy for enabling high power and high energy density with redox-active layered materials containing confined fluids.

The redox reaction of intercalated protons is key to the pseudocapacitance of MXenes (two-dimensional (2D) carbides and nitrides) in H 2 SO 4 . However, an atomistic understanding of proton redox and transfer in water confined between MXene layers is still lacking. Here, we use firstprinciples molecular dynamics (FPMD) simulations to reveal the protontransfer mechanism in MXene-confined water layers of different thicknesses by using O-terminated Ti 3 C 2 as a prototypical MXene. We found that the proton redox process takes place reversibly between surface −O sites and interfacial water molecules, intermitted by the more frequent in-water proton-transfer events. The surface redox rate is much higher in the highly confined one-layer water than in the two or three layers of water. Proton mobility increases with the water-layer number and already approaches the bulk value in the threelayer water. The proton transfer still follows the Eigen−Zundel−Eigen mechanism in the 2D-like confined water (regardless of the thickness) as in the 3D bulk water via the special pair dance. Our model in the case of two layers of water is in excellent agreement with the experimental interlayer spacing after charging the Ti 3 C 2 O 2 electrode in H 2 SO 4 . Our finding from FPMD of fast surface redox and in-water transfer for the intercalated protons implies that other processes such as the intercalating step are likely the bottleneck for the ionic transport.

Identifying and understanding charge storage mechanisms is important for advancing energy storage. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradaic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti3C2T x MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.

Interlayer structural protons in H2Ti3O7 are identified as the key structural feature to enable electrochemical proton intercalation beyond the near-surface because they effectively reduce interconnections of the titanate layers.

Since the discovery of two-dimensional transition metal carbides, referred to as MXenes, research efforts targeted their applications in energy storage, as lithium-ion batteries and supercapacitors. This interest is attributable to MXenes' large volumetric capacitance, high rate handling capability and stable cycling performance, which largely rely on surface chemistry provided by the terminating groups, such as -OH, -O, and -F. However, the atomic-scale characterization of these surface terminations challenges the diffraction methods. Solid-state (SS)NMR spectroscopy, especially 1 H SSNMR, is a promising approach for scrutinizing the surface terminations and the intercalated water in an atomistic-scale, yet only a few SSNMR studies in MXenes have been reported to date offering conflicting results and limited understanding of -OH terminations. Here, we used 1 H SSNMR experiments in concert with the DFT calculations of NMR parameters to identify multiple types of -OH groups, residing on the external and internal surfaces, in a commonly studied MXene, Ti3C2Tx. The study also identifies bulk-like water trapped between the MXene flakes and interfacial water stranded on the surface. Lastly, two-dimensional 1 H-1 H correlation spectra elucidated the water-surface interactions and the mechanism of de-intercalation of water upon annealing.