Encoding Enantiomeric Molecular Chiralities on Graphene Basal Planes.

Graphene has demonstrated broad applications due to its prominent properties. Its molecular structure makes graphene achiral. Here, we propose a direct way to prepare chiral graphene by transferring chiral structural conformation from chiral conjugated amino acids onto graphene basal plane through π-π interaction followed by thermal fusion. Using atomic resolution transmission electron microscopy, we estimated an areal coverage of the molecular imprints (chiral regions) up to 64 % on the basal plane of graphene (grown by chemical vapor deposition). The high concentration of molecular imprints in their single layer points to a close packing of the deposited amino acid molecules prior to "thermal fusion". Such "molecular chirality-encoded graphene" was tested as an electrode in electrochemical enantioselective recognition. The chirality-encoded graphene might find use for other chirality-related studies and the encoding procedure might be extended to other two-dimensional materials.

Single-Atom Catalyst Aggregates: Size-Matching is Critical to Electrocatalytic Performance in Sulfur Cathodes.

Electrocatalysis is critical to the performance displayed by sulfur cathodes. However, the constituent electrocatalysts and the sulfur reactants have vastly different molecular sizes, which ultimately restrict electrocatalysis efficiency and hamper device performance. Herein, the authors report that aggregates of cobalt single-atom catalysts (SACs) attached to graphene via porphyrins can overcome the challenges associated with the catalyst/reactant size mismatch. Atomic-resolution transmission electron microscopy and X-ray absorption spectroscopy measurements show that the Co atoms present in the SAC aggregates exist as single atoms with spatially resolved dimensions that are commensurate the sulfur species found in sulfur cathodes and thus fully accessible to enable 100% atomic utilization efficiency in electrocatalysis. Density functional theory calculations demonstrate that the Co SAC aggregates can interact with the sulfur species in a synergistic manner that enhances the electrocatalytic effect and promote the performance of sulfur cathodes. For example, Li-S cells prepared from the Co SAC aggregates exhibit outstanding capacity retention (i.e., 505 mA h g-1 at 0.5 C after 600 cycles) and excellent rate capability (i.e., 648 mA h g-1 at 6 C). An ultrahigh area specific capacity of 12.52 mA h cm-2 is achieved at a high sulfur loading of 11.8 mg cm-2 .

Regulating Lithium Plating and Stripping by Using Vertically Aligned Graphene/CNT Channels Decorated with ZnO Particles.

Lithium (Li) metal is regarded as the ultimate anode material for use in Li batteries due to its high theoretical capacity (3860 mA h g-1 ). However, the Li dendrites that are generated during iterative Li plating/stripping cycles cause poor cycling stability and even present safety risks, and thus severely handicap the commercial utility of Li metal anodes. Herein, we describe a graphene and carbon nanotube (CNT)-based Li host material that features vertically aligned channels with attached ZnO particles (designated ZnO@G-CNT-C) and show that the material effectively regulates Li plating and stripping. ZnO@G-CNT-C is prepared from an aqueous suspension of Zn(OAc)2 , CNTs, and graphene oxide by using ice to template channel growth. ZnO@G-CNT-C was found to be mechanically robust and capable of guiding Li deposition on the inner walls of the channels without the formation of Li dendrites. When used as an electrode, the material exhibits relatively low polarization for Li plating, fast Li-ion diffusion, and high Coulombic efficiency, even over hundreds of Li plating/stripping cycles. Moreover, full cells prepared with ZnO@G-CNT-C as Li host and LiFePO4 as cathode exhibit outstanding performance in terms of specific capacity (155.9 mA h g-1 at 0.5 C), rate performance (91.8 mA h g-1 at 4 C), cycling stability (109.4 mA h g-1 at 0.5 C after 800 cycles). The methodology described can be readily adapted to enable the use of carbon-based electrodes with well-defined channels in a wide range of contemporary applications that pertain to energy storage and delivery.