Porous crystalline materials such as covalent organic frameworks and metal–organic frameworks have garnered considerable attention as promising ion conducting media. However, most of them additionally incorporate lithium salts and/or solvents inside the pores of frameworks, thus failing to realize solid-state single lithium-ion conduction behavior. Herein, we demonstrate a lithium sulfonated covalent organic framework (denoted as TpPa-SO 3 Li) as a new class of solvent-free, single lithium-ion conductors. Benefiting from well-designed directional ion channels, a high number density of lithium-ions, and covalently tethered anion groups, TpPa-SO 3 Li exhibits an ionic conductivity of 2.7 × 10–5 S cm–1 with a lithium-ion transference number of 0.9 at room temperature and an activation energy of 0.18 eV without additionally incorporating lithium salts and organic solvents. Such unusual ion transport phenomena of TpPa-SO 3 Li allow reversible and stable lithium plating/stripping on lithium metal electrodes, demonstrating its potential use for lithium metal electrodes.

The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

The hierarchical porous structure has garnered considerable attention as a multiscale engineering strategy to bring unforeseen synergistic effects in a vast variety of functional materials. Here, we demonstrate a "microporous covalent organic framework (COF) net on mesoporous carbon nanotube (CNT) net" hybrid architecture as a new class of molecularly designed, hierarchical porous chemical trap for lithium polysulfides (Li2Sx) in Li-S batteries. As a proof of concept for the hybrid architecture, self-standing COF-net on CNT-net interlayers (called "NN interlayers") are fabricated through CNT-templated in situ COF synthesis and then inserted between sulfur cathodes and separators. Two COFs with different micropore sizes (COF-1 (0.7 nm) and COF-5 (2.7 nm)) are chosen as model systems. The effects of the pore size and (boron-mediated) chemical affinity of microporous COF nets on Li2Sx adsorption phenomena are theoretically investigated through density functional theory calculations. Benefiting from the chemical/structural uniqueness, the NN interlayers effectively capture Li2Sx without impairing their ion/electron conduction. Notably, the COF-1 NN interlayer, driven by the well-designed microporous structure, allows for the selective deposition/dissolution (i.e., facile solid-liquid conversion) of electrically inert Li2S. As a consequence, the COF-1 NN interlayer provides a significant improvement in the electrochemical performance of Li-S cells (capacity retention after 300 cycles (at charge/discharge rate = 2.0 C/2.0 C) = 84% versus 15% for a control cell with no interlayer) that lies far beyond those accessible with conventional Li-S technologies.

Chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have been widely used as CER catalysts, they suffer from the concomitant generation of oxygen during the CER. Herein, we demonstrate that atomically dispersed Pt−N 4 sites doped on a carbon nanotube (Pt 1 /CNT) can catalyse the CER with excellent activity and selectivity. The Pt 1 /CNT catalyst shows superior CER activity to a Pt nanoparticle-based catalyst and a commercial Ru/Ir-based MMO catalyst. Notably, Pt 1 /CNT exhibits near 100% CER selectivity even in acidic media, with low Cl − concentrations (0.1 M), as well as in neutral media, whereas the MMO catalyst shows substantially lower CER selectivity. In situ electrochemical X-ray absorption spectroscopy reveals the direct adsorption of Cl − on Pt−N 4 sites during the CER. Density functional theory calculations suggest the PtN 4 C 12 site as the most plausible active site structure for the CER.

The development of active and stable non-precious-metal electrocatalysts for energy conversion reactions involving hydrogen and oxygen has been of pivotal importance for realizing a clean-energy-based society. As a class of non-precious-metal electrocatalysts, cobalt- and nitrogen-codoped carbon (Co–N/C) catalysts have shown promising activity for the hydrogen evolution reaction (HER). The further advancement of Co–N/C catalysts is, however, hindered by the poor understanding of their active sites; the typical preparation of Co–N/C catalysts involves high-temperature pyrolysis, yielding catalysts with a heterogeneous distribution of atomically dispersed Co–N x sites and metallic Co nanoparticles encapsulated in graphitic carbon shells (Co@C). Further, kinetic insights into the HER on Co–N/C catalysts are lacking. In this work, we prepared a series of Co–N/C catalysts with controlled Co–N x and Co@C site densities, which served as model catalysts for identifying the active sites for the HER. We found that the HER activities in both acidic and alkaline media linearly increased with the number of exposed Co–N x sites, suggesting that the Co–N x sites are the major active sites for the HER. Density functional theory (DFT) calculations suggested that hydrogen adsorption at Co–N x sites is closer to the thermoneutral state in comparison to that at Co@C sites, corroborating the HER activity results. Furthermore, pH- and temperature-dependent HER activities combined with in situ X-ray absorption spectroscopy analyses on the Co–N/C catalyst comprising only Co–N x sites provide insights into HER reaction kinetics, including the rate-determining step and spectator species in alkaline electrolytes. The Co–N/C catalyst with Co–N x sites exhibited long-term durability and stability. This work may shed light on the design of advanced Co–N/C catalysts as well as other M–N/C catalysts for promoting a diverse set of energy conversion reactions.