MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state (23)Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na(+) electrolyte exhibits reversible Na(+) intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na(+) is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na(+) and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na(+) intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability.

high power and high energy. Although state-of-the-art Li-ion batteries efficiently store energy by Li-ion (de)intercalation into the host electrode materials, their power is limited in part by slow ion transfer. [1,2] Furthermore, carbonaceous compounds, which are the most used negative electrode materials in Li-ion batteries, exhibit Li-ion (de)intercalation near the Li metal plating potential, hindering the charging of batteries at a high rate. [3,4] Accordingly, the development of negative electrode materials that are capable of more charge at a faster rate remains a major challenge.Electrode materials for electrochemical capacitors store the charge by surface ion adsorption, which intrinsically achieves a high power density. [5][6][7] The modest energy density of conventional double-layer capacitors owing to their insufficient capacitance can be enhanced by accumulating pseudocapacitance by surface ion adsorption accompanied with surface redox reactions. [8][9][10][11][12] However, the use of electrochemical cells composed of pseudocapacitive electrodes do not avoid the compromise between the high power and high energy densities, and a practical technical solution has been Li-ion hybrid capacitor, in which intercalation-type compounds are employed either in the cathode or anode. [13,14] One option is a Li-ion hybrid capacitor with a pseudocapacitive porous carbon cathode and an intercalation-type anode (e.g., Li 4 Ti 5 O 12 ). [15][16][17] Another example is a combination of an intercalation-type cathode (e.g., LiMn 2 O 4 ) and a pseudocapacitive anode such as a MnO 2 /carbon nanotube composite. [18,19] However, the energy and power densities of the Li-ion hybrid capacitors are not yet satisfactory for commercialization. Hence, tremendous efforts have been devoted to the development of superior pseudocapacitive electrode materials, such as nitrogendoped carbon, [20] RuO 2 ⋅nH 2 O, [21] or T-Nb 2 O 5 . [22] In particular, nanosheet compounds are of potential interest because (1) the stacked nanosheets enable a high packing density for the high volumetric capacitance;(2) open interlayer space between the nanosheets offers fast ion accessibility to the redox center, what we call "intercalation pseudocapacitance"; [22] and (3) electrically conductive nanosheets permit high power operation. [23][24][25] Among various nanosheet compounds, MXene (M n+1 X n T x ; M: Ti, V, Cr, Nb, etc.; X: C, N; n = 1-3; T: surface termination groups) is an important emerging class of electrode materials for both supercapacitors and batteries. [26][27][28][29][30][31][32] One of the advantages of MXene is its very high electronic conductivity which Pseudocapacitance is a key charge storage mechanism to advanced electrochemical energy storage devices distinguished by the simultaneous achievement of high capacitance and a high charge/discharge rate by using surface redox chemistries. MXene, a family of layered compounds, is a pseudocapacitor-like electrode material which exhibits charge storage through exceptionally fast ion accessibil...

Wet conditions in heterogeneous catalysis can substantially improve the rate of surface reactions by assisting the diffusion of reaction intermediates between surface reaction sites. The atomistic mechanisms underpinning this accelerated mass transfer are, however, concealed by the complexity of the dynamic water/solid interface. Here we employ ab initio molecular dynamics simulations to disclose the fast diffusion of protons and hydroxide species along the interface between water and ceria, a catalytically important, highly reducible oxide. Up to 20% of the interfacial water molecules are shown to dissociate at room temperature via proton transfer to surface O atoms, leading to partial surface hydroxylation and to a local increase of hydroxide species in the surface solvation layer. A water-mediated Grotthus-like mechanism is shown to activate the fast and long-range proton diffusion at the water/oxide interface. We demonstrate the catalytic importance of this dynamic process for water dissociation at ceria-supported Pt nanoparticles, where the solvent accelerates the spillover of ad-species between oxide and metal sites.

The thermodynamic, structural and electronic properties of Cu-CeO(2) (ceria) surfaces and interfaces are investigated by means of density functional theory (DFT+U) calculations. We focus on model systems consisting of Cu atoms (i) supported by stoichiometric and reduced CeO(2) (111) surfaces, (ii) dispersed as substitutional solid solution at the same surface, as well as on (iii) the extended Cu(111)/CeO(2)(111) interface. Extensive charge reorganization at the metal-oxide contact is predicted for ceria-supported Cu adatoms and nanoparticles, leading to Cu oxidation, ceria reduction, and interfacial Ce(3+) ions. The calculated thermodynamics predict that Cu adatoms on stoichiometric surfaces are more stable than on O vacancies of reduced surfaces at all temperatures and pressures relevant for catalytic applications, even in extremely reducing chemical environments. This suggests that supported Cu nanoparticles do not nucleate at surface O vacancies of the oxide, at variance with many other metal/ceria systems. In oxidizing conditions, the solid solutions are shown to be more stable than the supported systems. Substitutional Cu ions form characteristic CuO(4) units. These promote an easy and reversible O release without the reduction of Ce ions. The study of the extended CeO(2)(111)/Cu(111) interface predicts the full reduction of the interfacial ceria trilayer. Cu nanoparticles supported by ceria are proposed to lie above a subsurface layer of Ce(3+) ions that extends up to the perimeter of the metal-oxide interface.

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