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With the unique-layered structure, MXenes show potential as electrodes in energy-storage devices including lithium-ion (Li ) capacitors and batteries. However, the low Li -storage capacity hinders the application of MXenes in place of commercial carbon materials. Here, the vanadium carbide (V C) MXene with engineered interlayer spacing for desirable storage capacity is demonstrated. The interlayer distance of pristine V C MXene is controllably tuned to 0.735 nm resulting in improved Li-ion capacity of 686.7 mA h g at 0.1 A g , the best MXene-based Li -storage capacity reported so far. Further, cobalt ions are stably intercalated into the interlayer of V C MXene to form a new interlayer-expanded structure via strong V-O-Co bonding. The intercalated V C MXene electrodes not only exhibit superior capacity up to 1117.3 mA h g at 0.1 A g , but also deliver a significantly ultralong cycling stability over 15 000 cycles. These results clearly suggest that MXene materials with an engineered interlayer distance will be a rational route for realizing them as superstable and high-performance Li capacitor electrodes.

Developing highly efficient catalysts for oxygen evolution reaction (OER) in neutral media is extremely crucial for microbial electrolysis cells and electrochemical CO reduction. Herein, a facile one-step approach is developed to synthesize a new type of well-dispersed iridium (Ir) incorporated cobalt-based hydroxide nanosheets (nominated as CoIr) for OER. The Ir species as clusters and single atoms are incorporated into the defect-rich hydroxide nanosheets through the formation of rich Co-Ir species, as revealed by systematic synchrotron radiation based X-ray spectroscopic characterizations combining with high-angle annular dark-field scanning transmission electron microscopy measurement. The optimized CoIr with 9.7 wt% Ir content displays highly efficient OER catalytic performance with an overpotential of 373 mV to achieve the current density of 10 mA cm in 1.0 m phosphate buffer solution, significantly outperforming the commercial IrO catalysts. Further characterizations toward the catalyst after undergoing OER process indicate that unique Co oxyhydroxide and high valence Ir species with low-coordination structure are formed due to the high oxidation potentials, which authentically contributes to superior OER performance. This work not only provides a state-of-the-art OER catalyst in neutral media but also unravels the root of the excellent performance based on efficient structural identifications.

Because the reconstruction of catalysts is generally observed during oxidation reactions, understanding the intrinsic structure-related reconstruction ability of electrocatalysts is highly desirable but challenging. Herein, a controllable hydrolysis strategy is developed to obtain nickel hydroxide electrocatalysts with controllable nickel vacancy (V Ni ) concentrations, as confirmed by advanced spectroscopic characterization. Electrochemical measurements show that the reconstruction can be promoted with the increase of V Ni concentration to generate true active components, thereby boosting activities for both oxygen evolution reaction (OER) and urea oxidation reaction (UOR). Density functional theory calculations confirm that the increased V Ni concentration yields decreased formation energies of the true active components during reactions. This work provides fundamental understanding of the reconstruction ability of electrocatalysts in anodic oxidation reactions from the view of intrinsic defects.

ideal performance toward ORR or OER, the high price, scarcity, and instability still hampers their large-scale generalization. At present, developing efficient and nonnoble-metal catalysts has attracted extensive interest. [11][12][13][14][15][16][17][18] For ORR, defective carbon-based materials, typically heteroatom-doped carbon, are extensively demonstrated as efficient electrocatalysts. [19][20][21][22] For OER, in addition to common transition metal oxides or (oxy)hydroxides, transition metal phosphides (TMPs) have achieved considerable research and development attention due to superior performance. [23][24][25][26][27] In this regard, the composites of the TMPs and defective carbon are considered as promising candidates for both ORR and OER. More recently, some works reported that the composites of the TMPs and defective carbon compared to the single component displayed enhanced catalytic performance, which was probably attributed to the increased electronic conductivity due to the introduction of conductive carbon. [28][29][30] However, the promoting factor was not well understood. For the composites, undoubtedly, the interfacial properties, especially the interfacial charge states, are important parameters that could influence the catalytic performance. [31,32] Therefore, in order to overcome high catalytic reaction barrier, designing the hybrids of the TMPs and defective carbon and probing the interfacial charge distribution behavior are highly desirable to realize bifunctional oxygen electrocatalysis.Herein, we constructed a new type of hybrids of the CoP and defective carbon (marked as CoP-DC). We revealed the interfacial charge transfer process of the hybrids by multiple synchrotron-based X-ray absorption structure, ultraviolet photoelectron spectra (UPS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The interfacial charge redistribution was observed, which subsequently contributed to enhanced ORR activity on the defective carbon and enhanced OER activity on the CoP.The CoP-DC hybrids were synthesized through a simple phosphorization reaction toward the Co 2+ -contained polymer hydrogel. Typically, the polymer hydrogel was obtained by inserting Co 2+ into polymer hydrogel framework under alkaline condition according to previous reports, and then was phosphorized The development of efficient catalysts for both oxygen reduction and evolution reactions (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)) is central to regenerative fuel cells and rechargeable metalair batteries. It is highly desirable to achieve the efficient integration of dual active components into the catalysts and to understand the interaction between the dual components. Here, a facile approach is demonstrated to construct defective carbon-CoP nanoparticle hybrids as bifunctional oxygen electrocatalysts, and further probe the interfacial charge distribution behavior. By combining multiple synchrotron-based X-ray spectroscopic characterizations with density functional theo...

Vertical 1T-MoS nanosheets with an expanded interlayer spacing of 9.8 Å were successfully grown on a graphene surface via a one-step solvothermal method. Such unique hybridized structures provided strong electrical and chemical coupling between the vertical nanosheets and graphene layers by means of C-O-Mo bonding. The merits are very beneficial for a high-efficiency electron/ion transport pathway and structural stability. As a proof of concept, the lithium ion battery with the as-obtained hybrid's electrode exhibited excellent rate performance with a 666 mA h g capacity at a high current density of 3500 mA g. We can extend this method to produce various metallic 1T-MX (M = transition metal; X = chalcogen) vertically edged on a graphene frame as one of the promising hetero-structures for several specific applications in the fields of electronics, optics and catalysis.

Two-dimensional stable metallic 1T-MoSe with expanded interlayer spacing of 10.0 Å in situ grown on SWCNTs film is fabricated via a one-step solvothermal method. Combined with X-ray absorption near-edge structures, our characterization reveals that such 1T-MoSe and single-walled carbon nanotubes (abbreviated as 1T-MoSe/SWCNTs) hybridized structure can provide strong electrical and chemical coupling between 1T-MoSe nanosheets and SWCNT film in a form of C-O-Mo bonding, which significantly benefits a high-efficiency electron/ion transport pathway and structural stability, thus directly enabling high-performance lithium storage properties. In particular, as a flexible and binder-free Li-ion anode, the 1T-MoSe/SWCNTs electrode exhibits excellent rate capacity, which delivers a capacity of 630 mAh/g at 3000 mA/g. Meanwhile, the strong C-O-Mo bonding of 1T-MoSe/SWCNTs accommodates volume alteration during the repeated charge/discharge process, which gives rise to 89% capacity retention and a capacity of 971 mAh/g at 300 mA/g after 100 cycles. This synthetic route of a multifunctional MoSe/SWCNTs hybrid might be extended to fabricate other 2D layer-based flexible and light electrodes for various applications such as electronics, optics, and catalysts.

1100 mA h g −1 Li + storage capacity with good cyclic stability for V 2 C MXene after cobalt ion intercalation. On the other side, although many works have been recently devoted to improving MXene's energy storage ability, rare operando studies are reported for probing the work mechanism during the charging and discharging process. Recently, in situ X-ray absorption fine structure (XAFS) [18,19] becomes an appropriate technique to study dynamic lithiation/delithiation mechanism. In particular, XAFS can determine dynamic valance change of metal atoms, as well as the real structural evolution of materials during the electrochemical reaction. [20] Herein, we demonstrated, for the first time, the fabrication of Sn 4+ -intercalated V 2 C MXene (V 2 C@Sn) through ion exchange process with outstanding specific capacity of 1284.6 mA h g −1 under 0.1 A g −1 so far. Notably, operando XAFS characterizations, combining with ex situ tests, were carefully performed to explore the dynamic lithiation/delithiation mechanism of V 2 C@Sn MXene electrode. Our operando spectroscopic analysis clearly confirmed the electrochemical activity of Sn and V atoms, along with the positive contribution of O atoms and the hindrance of F atoms.The schematic for fabricating Sn-intercalated V 2 C MXene is shown in Figure 1a. V 2 C@Sn MXene was obtained after the ion-exchange interaction between Sn 4+ and the preintercalated K + in V 2 C layers. The structure of each step is confirmed by the X-ray diffraction (XRD) patterns ( Figure S1a, Supporting Information). The c lattice parameter (c-LP) of V 2 C@Sn MXene is enlarged to 19 from 14.7 Å of V 2 C MXene. Additionally, there is a little increase of (002) peak from V 2 C-KOH to V 2 C@Sn ( Figure S1b, Supporting Information), indicating a decrease of interlayer spacing after Sn 4+ intercalation. This change is mainly due to the stronger electrostatic interactions of Sn ion than K ion. The morphology of V 2 AlC MAX (Figure 1b) is bulk with blocked multilayer, while V 2 C MXene shows the typical accordion-like multilayer nanostructure (Figure 1c). However, V 2 C@Sn MXene exhibits no obvious layer stripes (Figure 1d), and the signal intensity of Sn, as shown in Figure S2 (Supporting Information), is much stronger in the interlayer than other places, confirming that Sn 4+ is mainly intercalated between V 2 C MXene layers. High-resolution transmission electron microscopy (HRTEM) image in Figure 1e clearly