Single copper sites dispersed on defective TiO2-x as a synergistic oxygen reduction reaction catalyst.

Catalysts containing isolated single atoms have attracted much interest due to their good catalytic behavior, bridging the gap between homogeneous and heterogeneous catalysts. Here, we report an efficient oxygen reduction reaction (ORR) catalyst that consists of atomically dispersed single copper sites confined by defective mixed-phased TiO2-x. This synergistic catalyst was produced by introducing Cu2+ to a metal organic framework (MOF) using the Mannich reaction, occurring between the carbonyl group in Cu(acac)2 and the amino group on the skeleton of the MOF. The embedding of single copper atoms was confirmed by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and x-ray absorption fine structure spectroscopy. Electronic structure modulation of the single copper sites coupling with oxygen vacancies was further established by electron paramagnetic resonance spectroscopy and first-principles calculations. Significantly enhanced ORR activity and stability were achieved on this special Cu single site. The promising application of this novel electrocatalyst was demonstrated in a prototype Zn-air battery. This strategy of the stabilization of single-atom active sites by optimization of the atomic and electronic structure on a mixed matrix support sheds light on the development of highly efficient electrocatalysts.

Tunable Redox Chemistry and Stability of Radical Intermediates in 2D Covalent Organic Frameworks for High Performance Sodium Ion Batteries.

Radicals are inevitable intermediates during the charging and discharging of organic redox electrodes. The increase of the reactivity of the radical intermediates is desirable to maximize the capacity and enhance the rate capability but is detrimental to cycling stability. Therefore, it is a great challenge to controllably balance the redox reactivity and stability of radical intermediates to optimize the electrochemical properties with a good combination of high specific capacity, excellent rate capability, and long-term cycle life. Herein, we reported the redox and tunable stability of radical intermediates in covalent organic frameworks (COFs) considered as high capacity and stable anode for sodium-ion batteries. The comprehensive characterizations combined with theoretical simulation confirmed that the redox of C-O· and α-C radical intermediates play an important role in the sodiation/desodiation process. Specifically, the stacking behavior could be feasibly tuned by the thickness of 2D COFs, essentially determining the redox reactivity and stability of the α-C radical intermediates and their contributive capacity. The modulation of reversible redox chemistry and stabilization mechanism of radical intermediates in COFs offers a novel entry to design novel high performance organic electrode materials for energy storage and conversion.

Lamellarly Stacking Porous N, P Co-Doped Mo2 C/C Nanosheets as High Performance Anode for Lithium-Ion Batteries.

Layered stacking and highly porous N, P co-doped Mo2 C/C nanosheets are prepared from a stable Mo-enhanced hydrogel. The hydrogel is formed through the ultrafast cross-linking of phosphomolybdic acid and chitosan. During the reduction of the composite hydrogel framework under inert gas protection, highly porous N and P co-doped carbon nanosheets are produced with the in situ formation of ultrafine Mo2 C nanoparticles highly distributed throughout the nanosheets which are entangled via a hierarchical lamellar infrastructure. This unique architecture of the N, P co-doped Mo2 C/C nanosheets tremendously promote the electrochemical activity and operate stability with high specific capacity and extremely stable cycling. In particular, this versatile synthetic strategy can also be extended to other polyoxometalate (such as phosphotungstic acid) to provide greater opportunities for the controlled fabrication of novel hierarchical nanostructures for next-generation high performance energy storage applications.

Toward Two-Dimensional π-Conjugated Covalent Organic Radical Frameworks.

Reported is the synthesis, characterization, and material properties of the first π-conjugated two-dimensional covalent organic radical framework (CORF), PTM-CORF, based on the stable polychlorotriphenylmethyl (PTM) radical. The covalent organic framework (COF) precursor (PTM-H-COF) was first synthesized by liquid/liquid interfacial acetylenic homocoupling of a triethynylpolychlorotriphenylmethane monomer, and showed crystalline features with a hexagonal diffraction pattern matching that of A-B-C stacking. Subsequent deprotonation and oxidation of the PTM units in PTM-H-COF gave PTM-CORF. Magnetic measurements revealed that the neighboring PTM radicals in the PTM-CORF are anti-ferromagnetically coupled each other, with a moderate exchange interaction (J=-375 cm-1 ). The PTM-CORF has a small energy gap (ca. 0.88 eV) and a low-lying LUMO energy level (-4.72 eV), and exhibits high electrocatalytic activity and durability toward the oxygen reduction reaction.

Failure Mechanism and Interface Engineering for NASICON-Structured All-Solid-State Lithium Metal Batteries.

All-solid-state lithium metal batteries (ASSLiMB) have been considered as one of the most promising next-generation high-energy storage systems that replace liquid organic electrolytes by solid-state electrolytes (SSE). Among many different types of SSE, NASICON-structured Li1+ xAl xGe2- x(PO3)4 (LAGP) shows high a ionic conductivity, high stability against moisture, and wide working electrochemical windows. However, it is unstable when it is in contact with molten Li, hence largely limiting its applications in ASSLiMB. To solve this issue, we have studied reaction processes and mechanisms between LAGP and molten Li, based on which a failure mechanism is hence proposed. With better understanding the failure mechanism, a thin thermosetting Li salt polymer, P(AA- co-MA)Li, layer is coated on the bare LAGP pellet before contacting with molten Li. To further increase the ionic conductivity of P(AA- co-MA)Li, LiCl is added in P(AA- co-MA)Li. A symmetric cell of Li/interface/LAGP/interface/Li is prepared using molten Li-Sn alloy and galvanically cycled at current densities of 15, 30, and 70 µA cm-2 for 100 cycles, showing stable low overpotentials of 0.036, 0.105, and 0.257 V, respectively. These electrochemical results demonstrate that the interface coating of P(AA- co-MA)Li can be an effective method to avoid an interfacial reaction between the LAGP electrolyte and molten Li.

Synergistic Effects of C/α-MoC and Ag for Efficient Oxygen Reduction Reaction.

It remains challenging to prepare highly active and stable catalysts from earth-abundant elements for the oxygen reduction reaction (ORR). Herein we report a facile method to synthesize cost-effective heterogeneous C/α-MoC/Ag electrocatalysts. Rotating disc electrode (RDE) experiments revealed that the obtained C/α-MoC/Ag exhibited much superior catalytic performance for ORR than that of C/Ag, C/α-MoC, or even the conventional Pt/C. First-principles calculations indicated that the enhanced activity could be attributed to the efficient synergistic effects between Ag and α-MoC/C by which the energy barrier for O2 dissociation has been substantially reduced. Furthermore, Li-air and Al-air cells were assembled to demonstrate the unprecedented electrochemical performance of C/α-MoC/Ag nanocomposites surpassing the Pt/C. Thus experimental results and theoretical calculations together showed that the heterogeneous C/α-MoC/Ag nanocomposites are a promising alternative to platinum for applications in industrial metal-air batteries.

Supramolecular hydrogel directed self-assembly of C- and N-doped hollow CuO as high-performance anode materials for Li-ion batteries.

A novel hollow-structured CuO derived from chitosan-coated Cu/Cu2O composite hollow spheres was mass produced via a supramolecular hydrogel templating method and homogeneously co-doped with C and N. Evaluation of the as-produced co-doped CuO hollow spheres as anode materials for Li ion batteries showed that they had superior electrochemical properties.

Low-Cost and Novel Si-Based Gel for Li-Ion Batteries.

Si-based nanostructure composites have been intensively investigated as anode materials for next-generation lithium-ion batteries because of their ultra-high-energy storage capacity. However, it is still a great challenge to fabricate a perfect structure satisfying all the requirements of good electrical conductivity, robust matrix for buffering large volume expansion, and intact linkage of Si particles upon long-term cycling. Here, we report a novel design of Si-based multicomponent three-dimensional (3D) networks in which the Si core is capped with phytic acid shell layers through a facile high-energy ball-milling method. By mixing the functional Si with graphene oxide and functionalized carbon nanotube, we successfully obtained a homogeneous and conductive rigid silicon-based gel through complexation. Interestingly, this Si-based gel with a fancy 3D cross-linking structure could be writable and printable. In particular, this Si-based gel composite delivers a moderate specific capacity of 2711 mA h g-1 at a current density of 420 mA g-1 and retained a competitive discharge capacity of more than 800.00 mA h g-1 at the current density of 420 mA g-1 after 700 cycles. We provide a new method to fabricate durable Si-based anode material for next-generation high-performance lithium-ion batteries.

Highly durable organic electrode for sodium-ion batteries via a stabilized α-C radical intermediate.

It is a challenge to prepare organic electrodes for sodium-ion batteries with long cycle life and high capacity. The highly reactive radical intermediates generated during the sodiation/desodiation process could be a critical issue because of undesired side reactions. Here we present durable electrodes with a stabilized α-C radical intermediate. Through the resonance effect as well as steric effects, the excessive reactivity of the unpaired electron is successfully suppressed, thus developing an electrode with stable cycling for over 2,000 cycles with 96.8% capacity retention. In addition, the α-radical demonstrates reversible transformation between three states: C=C; α-C·radical; and α-C- anion. Such transformation provides additional Na+ storage equal to more than 0.83 Na+ insertion per α-C radical for the electrodes. The strategy of intermediate radical stabilization could be enlightening in the design of organic electrodes with enhanced cycling life and energy storage capability.