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.

No abstract

Reactions of reduced cerium oxide CeO x with water are fundamental processes omnipresent in ceria-based catalysis. Using thin epitaxial films of ordered CeO x , we investigate the influence of oxygen vacancy concentration and coordination on the oxidation of CeO x by water. Upon changing the CeO x stoichiometry from CeO2 to Ce2O3, we observe a transition from a slow surface reaction to a productive H2-evolving CeO x oxidation with reaction yields exceeding the surface capacity and indicating the participation of bulk OH species. Both the experiments and the ab initio calculations associate the effective oxidation of highly reduced CeO x by water to the next-nearest-neighbor oxygen vacancies present in the bixbyite c-Ce2O3 phase. Next-nearest-neighbor oxygen vacancies allow for the effective incorporation of water in the bulk via formation of OH– groups. Our study illustrates that the coordination of oxygen vacancies in CeO x represents an important parameter to be considered in understanding and improving the reactivity of ceria-based catalysts.

Discontinuous ceria layers on Cu(111) represent heterogeneous catalysts with notable activities in water− gas shift and CO oxidation reactions. Ultrathin ceria islands in these catalysts are composed of monolayers of ceria exhibiting CeO 2 (111) surface ordering and bulklike vertical stacking (O−Ce−O) down to a single ceria monolayer representing the oxide-metal interface. Scanning tunneling microscopy (STM) reveals marked differences in strain buildup and the structure of oxygen vacancies in this first ceria monolayer compared to thicker ceria layers on Cu(111). Ab-initio calculations allow us to trace back the distinct properties of the first ceria monolayer to pronounced finite size effects when the limiting thickness of the oxide monolayer and the proximity of metal substrate cause significant rearrangement of charges and oxygen vacancies compared to thicker and/or bulk ceria. ■ INTRODUCTIONIn heterogeneous catalysis, oriented thin films of oxides on metal substrates have traditionally been used as model systems mimicking the surfaces of bulk oxides. 1 Decreasing the thickness of the oxide films causes emerging of a broad range of effects that significantly influence the catalytic properties. 2,3 When the oxide thickness reduces to few monolayers (ML) oxide stoichiometry, 4 electronic and crystalline structure, 5−7 and charge of molecular adspecies 8 start to differ significantly from thicker films or bulk-truncated surfaces. Ultrathin oxide films are often discontinuous representing inverse model catalysts with unique catalytic properties. 9,10 Cerium oxide (ceria, CeO 2 ) is a broadly studied oxide with a vast application potential in catalysis and energy conversion. 11−13 Continuous thin films of ceria have been prepared on Ru (0001) 14,15 and Cu(111). 16,17 These films have served numerous model studies evaluating the catalytic properties of bare ceria surfaces 18,19 as well as ceria surfaces activated with metal clusters. 20−23 Discontinuous ultrathin films of ceria have been prepared on various metal substrates. Some of these ceriabased inverse model catalysts show exceptional activity in technologically relevant reactions such as water−gas shift (WGS) 24,25 and CO oxidation. 26−28 Particularly, ceria on Cu (111) is a featured inverse model catalyst inspiring design of novel noble-metal-free industrial catalysts. 25,29 The high activity of inverse model catalysts is explained by a cooperative action of oxide and metal at the perimeter of oxide islands. 24−28 However, as first pointed out by Castellarin-Cudia et al., 30 the surface of ultrathin oxide islands in inverse model catalysts may itself incorporate catalytically active sites, which are not stable on thicker films and/or bulk oxide surfaces. Indeed, microscopic studies of 1−3 ML thick ceria islands on various metal substrates confirm that ultrathin ceria is strained, 30−32 influenced by coincidence effects between substrate and oxide lattices, 15,17,30 and shows several characteristic phenomena regarding oxygen vacancies, such as their ordering...

The chemistry of several catalytic processes can be controlled by tuning metal−oxide interfaces, as demonstrated by fundamental studies on inverse model catalysts. We investigate the effects of the metal−oxide interface on the surface reactivity of ceria (CeO 2 ) thin films supported by a copper metal surface. Our density functional theory (DFT+U) calculations reveal that the interface has impact on the surface water adsorption and dissociation when the thickness of the ceria film is below ≈9 Å. On thinner films, the energetics of adsorption and dissociation display a significant variation, which arises from a combination of thickness and interface-proximity effects, and which we rationalize in terms of charge-density response at the adsorbate-oxide and oxide-metal interfaces. The adsorption energy is maximized for film thicknesses of 5.5 Å (corresponding to two O−Ce−O trilayers), while thinner films affect primarily the relative stability between molecular, semidissociated, and dissociated water adsorption. These results provide useful insights into the effect of low-dimensional ceria species in Cu/CeO 2 catalysts.