Self-adaptive amorphous CoOxCly electrocatalyst for sustainable chlorine evolution in acidic brine.

Electrochemical chlorine evolution reaction is of central importance in the chlor-alkali industry, but the chlorine evolution anode is largely limited by water oxidation side reaction and corrosion-induced performance decay in strong acids. Here we present an amorphous CoOxCly catalyst that has been deposited in situ in an acidic saline electrolyte containing Co2+ and Cl- ions to adapt to the given electrochemical condition and exhibits ~100% chlorine evolution selectivity with an overpotential of ~0.1 V at 10 mA cm-2 and high stability over 500 h. In situ spectroscopic studies and theoretical calculations reveal that the electrochemical introduction of Cl- prevents the Co sites from charging to a higher oxidation state thus suppressing the O-O bond formation for oxygen evolution. Consequently, the chlorine evolution selectivity has been enhanced on the Cl-constrained Co-O* sites via the Volmer-Heyrovsky pathway. This study provides fundamental insights into how the reactant Cl- itself can work as a promoter toward enhancing chlorine evolution in acidic brine.

Understanding Trends in Electrochemical Methanol Oxidation Reaction Activity on a Single Transition-Metal Atom Embedded in N-Coordinated Graphene Catalysts.

The lack of efficient catalysts and research on the mechanism for the methanol oxidation reaction (MOR) impedes the development of direct methanol fuel cells. In this work, based on density functional theory calculations, we systematically investigated the activity trends of electrochemical MOR on a single transition-metal atom embedded in N-coordinated graphene (M@N4C). By calculating the free energy diagrams of MOR on M@N4C, Co@N4C was screened out to be the most effective MOR catalyst with a low limiting potential of 0.41 V due to the unique charge transfers and electronic structures. Importantly, one- and two-dimensional volcano relationships in MOR on M@N4C catalysts are established based on the d-band center and the Gibbs free energy of ΔG*CH3OH and ΔG*CO, respectively. In one word, this work provides theoretical guides toward the improved activity of MOR on M@N4C and hints for the design of active and efficient MOR electrocatalysts.

Ferroelectricity and High Curie Temperature in a 2D Janus Magnet.

The breaking of the out-of-plane symmetry makes a two-dimensional (2D) Janus monolayer a new platform to explore the coupling between ferroelectricity and ferromagnetism. Using density functional theory in combination with Monte Carlo simulations, we report a novel phase-switchable 2D multiferroic material VInSe3 with large intrinsic out-of-plane spontaneous electric polarization and a high Curie temperature (Tc). The structural transition energy barrier between the two phases is determined to be 0.4 eV, indicating the switchability of the electric polarizations and the potential ferroelectricity. Carrier doping can boost the Curie temperature above room temperature, attributing to the enhanced magnetic exchange interaction. A transition from the ferromagnetic (FM) state to the antiferromagnetic (AFM) state can be induced by carrier doping in octahedra-VInSe3, while FM coupling is well-preserved in tetrahedron-VInSe3, which can be regulated to be either an XY or Ising magnet at an appropriate carrier concentration. These findings not only enrich the family of high-Tc low-dimensional monolayers but also offer a new direction for the design and multifunctional application of multiferroic materials.

Prediction of the electronic structure of single-walled GeS nanotubes.

The structure and electronic properties of puckered GeS nanotubes have been investigated using first-principles density functional theory calculation. Our results show that both the armchair and zigzag GeS nanotubes are semiconductor materials with an adjustable band gap. The band gap increases gradually with increasing the tube diameter, and slowly converges to the monolayer limit. On the application of strain, the GeS nanotubes provide interesting strain-induced band gap variation. When the compressive strain reached 20%, zigzag GeS nanotubes are completely transformed into armchair GeS nanotubes. In addition, the elastic properties of the relatively stable armchair GeS nanotubes have been studied, the Young's modulus of the armchair (11, 11), (13, 13) and (15, 15) nanotubes were calculated to be 227.488 GPa, 211.888 GPa and 213.920 GPa, respectively. Our work confirms that compared with carbon nanotubes, two-dimensional materials with a puckered structure are easier to realize phase transition by stress.

Atomically Dispersed Co2 -N6 and Fe-N4 Costructures Boost Oxygen Reduction Reaction in Both Alkaline and Acidic Media.

Polynary transition-metal atom catalysts are promising to supersede platinum (Pt)-based catalysts for oxygen reduction reaction (ORR). Regulating the local configuration of atomic catalysts is the key to catalyst performance enhancement. Different from the previously reported single-atom or dual-atom configurations, a new type of ternary-atom catalyst, which consists of atomically dispersed, nitrogen-coordinated Co-Co dimers, and Fe single sites (i.e., Co2 -N6 and Fe-N4 structures) that are coanchored on highly graphitized carbon supports is developed. This unique atomic ORR catalyst outperforms the catalysts with only Co2 -N6 or Fe-N4 sites in both alkaline and acid conditions. Density functional theory calculations clearly unravels the synergistic effect of the Co2 -N6 and Fe-N4 sites, which can induce higher filling degree of Fe-d orbitals and favors the binding capability to *OH intermediates (the rate determining step). This ternary-atom catalyst may be a promising alternative to Pt to drive the cathodic ORR in zinc-air batteries.

Prediction of superconductivity in bilayer borophenes.

Borophenes and related two-dimensional materials have exhibited many exotic properties, especially for superconductivity, although the superconductivity of single-layer borophene is suppressed by the strains or doping from its substrates. Intriguingly, bilayer (BL) borophenes can be stabilized by appropriate pillar density and hexagonal holes density, rather than being supported by Ag(111) or Cu(111) substrates. Thus, we studied the two most stable structures, namely BL-B8 and BL-B30, stabilized by the above-mentioned two methods. Within density functional theory and Bardeen-Cooper-Schrieffer theory framework, their stability, electron structures, and phonon properties, as well as possible superconductivity are systematically scrutinized. The metallic BL-B8 and BL-B30 exhibit intrinsic superconducting features with superconductivity transition temperatures (T c) of 11.9 and 4.9 K, respectively. The low frequency (below 400 cm-1) consisting of out-of-plane vibrations of boron atoms plays crucial rule in their superconductivity. In particular, a Kohn anomaly appears at the Γ point in BL-B8, leading to substantial electron-phonon coupling. Here, our findings will provide instructive clues for experimentally determining the superconductivity of borophene and will broaden the two-dimensional superconductor family.

Embedding tetrahedral 3d transition metal TM4 clusters into the cavity of two-dimensional graphdiyne to construct highly efficient and nonprecious electrocatalysts for hydrogen evolution reaction.

On the basis of density functional theory (DFT) calculations, we have systematically investigated the structures and hydrogen evolution reaction (HER) catalytic activities for a series of new composite systems TM4@GDY (TM = Sc, Ti, Mn, Fe, Co, Ni and Cu), which are constructed by embedding tetrahedral 3d transition metal TM4 clusters in the in-plane cavity of two-dimensional (2D) π-conjugated graphdiyne (GDY). Our computed results reveal that compared with the constituent subunits, namely the sole TM4 cluster and GDY, all these composite TM4@GDY nanostructures can uniformly exhibit considerably high HER catalytic activity over a wide range of hydrogen coverage, and especially the Fe4@GDY and Co4@GDY systems can possess higher HER activity, in view of their higher number of active sites. The high HER catalytic activity for TM4@GDY can be mainly due to the occurrence of obvious electron transfer from TM4 cluster to GDY, significantly activating the correlative C and TM atoms. Moreover, all these composite TM4@GDY systems can also exhibit high structural stability and good conductivity. Therefore, all of them can be considered as a new kind of promising HER catalyst, and this study can provide new strategies for designing low-cost and high-performance 2D carbon-based electrocatalysts.