The heterointerface effect to boost the catalytic performance of single atom catalysts for sulfur conversion in lithium-sulfur batteries.

Lithium-sulfur (Li-S) batteries are considered as one of the promising next-generation energy storage devices due to their characteristics of high energy density and low cost. However, the shuttle effect and sluggish conversion of lithium polysulfide (LiPs) have hindered their commercial applications. To address these issues, in our previous works, we have screened several highly efficient single atom catalysts (SACs) (MN4@G, M = V, Mo and W) with atomically dispersed transition metal atoms supported by nitrogen doped graphene based on high throughput calculations. Nevertheless, they still suffer from low loading of metal centers and unsatisfactory capability for accelerating the reaction kinetics. To tackle such problems, based on first-principles calculations, we systematically investigated the heterointerface effect on the catalytic performance of such three MN4@G toward sulfur conversion upon forming heterostructures with 5 typical two-dimensional materials of TiS2, C3N4, BN, graphene and reduced graphene oxide. Guided by efficient descriptors proposed in our previous work, we screened VN4@G/TiS2, MoN4@G/TiS2 and WN4@G/TiS2 possessing low Li2S decomposition barriers of 0.54, 0.44 and 0.41 eV, respectively. They also possess enhanced capabilities for catalyzing the sulfur reduction reaction as well as stabilizing soluble LiPs. More interestingly, the heterointerface can enhance the capability of the carbon atoms far away from the metal centers for trapping LiPs. This work shows that introducing a heterointerface is a promising strategy to boost the performance of SACs in Li-S batteries.

High-Throughput Calculations for Screening d- and p-Block Single-Atom Catalysts toward Li2 S/Na2 S Decomposition Guided by Facile Descriptor beyond Brønsted-Evans-Polanyi Relationship.

Single-atom catalysts (SACs) are promising cathode materials for addressing issues faced by lithium-sulfur batteries. Considering the ample chemical space of SACs, high-throughput calculations are efficient strategies for their rational design. However, the high throughput calculations are impeded by the time-consuming determination of the decomposition barrier (Eb ) of Li2 S. In this study, the effects of bond formation and breakage on the kinetics of SAC-catalyzed Li2 S decomposition with g-C3 N4 as the substrate are clarified. Furthermore, a new efficient and easily-obtained descriptor Li─S─Li angle (ALi─S─Li ) of adsorbed Li2 S, different from the widely accepted thermodynamic data for predicting Eb , which breaks the well-known Brønsted-Evans-Polanyi relationship, is identified. Under the guidance of ALi─S─Li , several superior SACs with d- and p-block metal centers supported by g-C3 N4 are screened to accelerate the sulfur redox reaction and fix the soluble lithium polysulfides. The newly identified descriptor of ALi─S─Li can be extended to rationally design SACs for Na─S batteries. This study opens a new pathway for tuning the performance of SACs to catalyze the decomposition of X2 S (X = Li, Na, and K) and thus accelerate the design of SACs for alkaline-chalcogenide batteries.

Two-Dimensional Superconductivity in Air-Stable Single-Crystal Few-Layer Bi3O2S3.

The progress of unconventional superconductors at the two-dimensional (2D) limit has inspired much interest. Recently, a new superconducting system was discovered in the semimetallic ternary Bi-O-S family. However, pure-phase crystals are difficult to synthesize because of the complicated stacking sequence of multiple charged layers and similar formation kinetics among ternary polytypes, leaving several fundamental issues regarding the structure-superconductivity correlation unresolved. Herein, 2D single-crystal ultrathin Bi3O2S3 nanosheets are prepared by using low-pressure chemical vapor deposition, and their atomic arrangement is clarified. Magnetotransport measurements indicate a superconducting transition at ∼6.1 K that is thickness-independent. The transport results demonstrate 2D superconducting characteristics, such as the Berezinskii-Kosterlitz-Thouless transition, and strong anisotropy with magnetic field orientations following the 2D Tinkham formula. The difference from superconductivity of powder is demonstrated from the perspective of their corresponding microstructures. These results corroborate the superconducting behavior of Bi3O2S3, providing fresh insights into the search for other bismuth oxychalcogenides and derivative BiS2-based analogues at the 2D limit.

d- and p-Block single-atom catalysts supported by BN nanocages toward electrochemical reactions of N2 and O2.

Electrocatalysis is involved in many energy storage and conversion devices, triggering research and development of electrocatalysts, particularly single-atom catalysts (SACs). The introduction of the strain effect to enhance the performance of SACs has drawn ever-increasing research attention, which can tailor the local atomic and electronic structure of active sites. Herein, via high throughput calculations, we have explored the effects of strain on the catalytic performance of SACs with MN4 configuration for electrochemical reactions of N2 and O2 by incorporating d- and p-block single metal atoms into BN nanocages (BNNCs). The calculations demonstrate that Os@BNNC exhibits the highest catalytic activity for the nitrogen reduction reaction (NRR) with a limiting potential of -0.29 V. Co@BNNC can serve as an excellent bifunctional SAC for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), with overpotentials of 0.32 and 0.37 V, respectively. In particular, Sn@BNNC with a p-block metal as the active center is a competitive SAC for the ORR with an overpotential of 0.64 V. More interestingly, the NRR and ORR performances of SACs supported by BNNCs have a close correlation with the structural and electronic properties of adsorbed N2 and O2 molecules, which proves that controlling the adsorption energy of N2 and O2 molecules is crucial to improving the catalytic activity of BNNC. The current investigation opens up an avenue for designing SACs embedded in nanocages possessing intrinsically curved surfaces for electrochemical reactions.

Coexistence of Anisotropic Large Magnetoresistance and Ferroelectricity in Two-Dimensional Narrow-Bandgap Bi2O2Te.

Characteristics like air-stability and high carrier mobility make non-van-der-Waals layered Bi2O2Se a good prospect for planar integrated nanosystems. However, experimental investigation about its analogue Bi2O2Te is rather rare due to difficulty in synthesis. Herein, a low-pressure CVD process is proposed that is adjusted to the rigorous growth condition required, with large-scale Bi2O2Te ultrathin film obtained. Magneto-transport behavior reveals a very large anisotropic nonsaturating low-temperature magnetoresistance (∼1133% under 9 T magnetic field). Despite the contradiction between high conductivity and ferroelectricity in principle (mobile electrons screen electrostatic forces between ions), the high-conductive Bi2O2Te film here is revealed experimentally as another intrinsic ferroelectric with the polarization switchable by external electric field (predicted in Nano Lett. 2017, 17, 6309). These results prove that Bi2O2Te possesses a very narrow bandgap (∼0.15 eV), high conductivity, large magnetoresistance, and room-temperature ferroelectricity, displaying great potential as a high-performance nanoelectronic two-dimensional semiconductor and, in advanced functional devices, working in the mid-infrared region.

Simultaneously Enhancing Catalytic Performance and Increasing Density of Bifunctional CuN3 Active Sites in Dopant-Free 2D C3N3Cu for Oxygen Reduction/Evolution Reactions.

Atomically dispersed M-N-C has been considered an effective catalyst for various electrochemical reactions such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which faces the challenge of increasing metal load while simultaneously maintaining catalytic performance. Herein, we put forward a strategy for boosting catalytic performances of a single Cu atom coordinated with three N atoms (CuN3) for both ORR and OER by increasing the density of connected CuN3 moieties. Our calculations first show that a single CuN3 moiety exhibiting no catalytic performance for ORR and OER can be activated by increasing the density of metal centers, which weakens the binding affinity to *OH due to the lowered d-band center of the metal atoms. These findings stimulate the further theoretical design of a two-dimensional compound of C3N3Cu with a high concentration of homogeneously distributed CuN3 moieties serving as bifunctional active sites, which demonstrates efficient catalytic performance for both ORR and OER as reflected by the overpotentials of 0.71 and 0.43 V, respectively. This work opens a new avenue for designing effective single-atom catalysts with potential applications as energy storage and conversion devices possessing high density of metal centers independent of the doping strategy and defect engineering, which deserves experimental investigation in the future.

Computational Design of Two-Dimensional Boron-Containing Compounds as Efficient Metal-free Electrocatalysts toward Nitrogen Reduction Independent of Heteroatom Doping.

As metal-free carbon based catalysts, boron (B)-doped carbonaceous materials have proved to exhibit superior catalytic performance toward nitrogen reduction reaction. However, this strategy of heteroatom doping encounters the synthesis challenges of precise control of the doping level and homogeneous distribution of the dopants, and in particular, these materials cannot be utilized in electrochemical N2 reduction because of poor electrical conductivity. Accordingly, via first-principles calculations, we here predicted two stable two-dimensional crystalline compounds: BC6N2 and BC4N, which have small band gaps and uniform distribution of NRR active sp2-B species and holey structures. Between them, the BC6N2 monolayer originally possesses nice NRR activity with limiting potentials of -0.47 V. In the proton-rich acid medium, the electronic properties of these two B-C-N monolayers could be further tailored to exhibit a metallic characteristic by H pre-adsorption. This drastically improves the conductivity and enhances their NRR performances as reflected by the limiting potentials of -0.15, -0.34, and -0.34 V for BC6N2 via enzymatic, distal, and alternating mechanisms, respectively. Besides, NRR on BC4N through enzymatic mechanism proceeds as the limiting potential moderated from -1.20 to -0.90 V. More than that, the competing hydrogen evolution reaction can be effectively suppressed. The current investigation opens an avenue of designing a 2D crystalline phase of MFC catalysts independent of heteroatom doping and gives insightful views of surface functionalization as an impactful strategy to improve the electrocatalytic activity of metal-free catalysts.