Revealing unusual rigid diamond net analogues in superhard titanium carbides

Electrochemical Capacitors with Confined Redox Electrolytes and Porous Electrodes

Electrochemical capacitors (ECs), including electrical-double-layer capacitors and pseudocapacitors, feature high power densities but low energy densities. To improve the energy densities of ECs, redox electrolyte-enhanced ECs (R-ECs) or supercapbatteries are designed through employing confined soluble redox electrolytes and porous electrodes. In R-ECs the energy storage is based on diffusion-controlled faradaic processes of confined redox electrolytes at the surface of a porous electrode, which thus take the merits of high power densities of ECs and high energy densities of batteries. In the past few years, there has been great progress in the development of this energy storage technology, particularly in the design and synthesis of novel redox electrolytes and porous electrodes, as well as the configurations of new devices. Herein, a full-screen picture of the fundamentals and the state-of-art progress of R-ECs are given together with a discussion and outlines about the challenges and future perspectives of R-ECs. The strategies to improve the performance of R-ECs are highlighted from the aspects of their capacitances and capacitance retention, power densities, and energy densities. The insight into the philosophies behind these strategies will be favorable to promote the R-EC technology toward practical applications of supercapacitors in different fields.

Superconductive Sodium Carbides with Pentagon Carbon at High Pressures

The design of metal-bearing carbon-based materials with unique structures and intriguing properties is highly desirable in the fields of physics, chemistry, and materials science. Here, within swarm-intelligence structure search and first-principles computations, we uncovered several hitherto unknown sodium carbides (i.e., Na₄C, Na₃C₂, NaC, Na₂C₃, and NaC₂) under high pressure. Intriguingly, the C atom arrangement reveals multiple structure evolution behavior with increased carbon content, from isolated anions in Na₄C, tetramers in Na₃C₂, extended chains in NaC, pentagonal rings in Na₂C₃, to eventually hexagonal rings in NaC₂. Among predicted phases, the superconducting critical temperature T_c of NaC₂ could approach ∼42 K at 80 GPa, which is slightly higher than the T_c of 39 K in the highest phonon-mediated superconductivity of MgB₂ at ambient pressure. This work offers insights into the reaction of carbides containing alkali metals and paves the way for the future investigation of high superconductivity in metal carbide systems.

Role of W-site substitution on mechanical and electronic properties of cubic tungsten carbide

In order to understand the role of W-site substitution on properties of cubic tungsten carbide ([Formula: see text]-WC), we have investigated the structural, mechanical, and electronic properties of WXC₂ (X  =  Si, Sc, Ti, V, Cr, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, Hf, Ta, Re, Os, Ir, Pt, Th, U) using first principles calculations based on density functional theory, within generalized gradient approximation. The structural optimization has carried out for all these compounds using force as well as stress minimization. The optimized structural parameters for experimentally known compounds are in good agreement with the available x-ray diffraction measurements and structural parameters for nineteen WXC₂ compounds are newly predicted. The W-site substitution of the above-listed elements into [Formula: see text]-WC reduces the symmetry of the primitive lattice to tetragonal structure. The heat of formation ([Formula: see text]) and the mechanical stability studies are carried out to investigate the stability of these systems. The single-crystal elastic constants c _ij , elastic moduli of the polycrystalline aggregates, anisotropy in elastic constants and related properties of the WXC₂ materials have calculated and discussed in detail. The hardness of the above materials is predicted using two different criteria, based on the softest elastic mode as well as the Pugh's modulus ratio. There is a correlation in the hardness predicted from these two approaches except in the case of [Formula: see text]-WC. The chemical bonding interaction between the constituents is analysed using the density of states, crystal orbital Hamiltonian population, and charge density for selected systems. All these compounds are predicted to be metal and our calculations suggest that W-site substitutions do not improve the hardness of [Formula: see text]-WC. However, from the heat of formation studies, we have identified five new stable compounds such as CrWC₂, NbWC₂, ScWC₂, YWC₂, and UWC₂ with reasonably good hardness and those need experimental verifications.

Role of TM-TM Connection Induced by Opposite d-Electron States on the Hardness of Transition-Metal (TM = Cr, W) Mononitrides

Recent reports exposed an astonishing factor of high hardness that the connection between transition-metal (TM) atoms could enhance hardness, which is in contrast to the usual understanding that TM-TM will weaken hardness as the source of metallicity. It is surprising that there are two opposite mechanical characteristics in the one TM-TM bond. To uncover the intrinsic reason, we studied two appropriate mononitrides, CrN and WN, with the same light-element (LE) content and valence electron concentration. The two high-quality compounds were synthesized by a new metathesis under high pressure, and the Vickers hardness is 13.0 GPa for CrN and 20.0 GPa for WN. Combined with theoretical calculations, we found that the strong correlation of d electrons in TM-TM could seriously affect hardness. Thus, we make the complementary suggestions of the previous hardness factors that the antibonding d-electron state in TM-TM near the Fermi level should be avoided and a strong d covalent coupling in TM-TM is very beneficial for high hardness. Our results are very important for the further design of high-hardness and multifunctional TM and LE compounds.