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[Display omitted] • A theoretical framework for the evaluation of flexible anode materials is established. • The mechanical property of Janus MXene is comparable with that of pristine MXene. • A constant diffusion barrier on Ti 2 COF under strain is suitable for a flexible anode. Two-dimensional transition metal carbides (MXenes) have received tremendous attention because of their great promise in flexible energy storage. Although surface engineering plays an important role in regulating the properties of MXenes with the development of synthesis technology, an atomistic design of surface diversity for desired functionalities is generally limited by purely unified surface termination. Herein, the Janus MXenes consisting of different surface terminations are proposed for flexible anodes through first-principles calculations. Taking Ti 2 C TT' (T , T' = O, F, or OH) as an illustration of Janus MXenes, critical strains, ideal strengths, Li-ion diffusion barriers, equilibrium voltages, and theoretical capacities are determined and used as descriptors to evaluate the properties of flexible anodes. These Janus MXenes exhibit high critical strains and ideal strengths, indicating their mechanical flexibility. The strain-independent Li-ion diffusion barrier of Ti 2 COF suggests its fast Li-ion transport even suffering from mechanical deformation, endowing its application potential in flexible anodes. These results establish a theoretical framework for the comprehensive evaluation of flexible anode materials, providing a theoretical scheme for designing flexible anode materials. [ABSTRACT FROM AUTHOR]

Three MXenes, namely Fe 2 N, Co 2 N, and Ni 2 N in both bare and surface-passivated forms were investigated theoretically using density functional theory. Fe 2 N(OH) 2 , Fe 2 NO 2 , Co 2 NO 2 , Ni 2 NF 2 , Ni 2 N(OH) 2 , and Ni 2 NO 2 are found to be intrinsic half-metals, while other structures have antiferromagnetic ground states. Notably, the half-metallicity of Ni 2 NT 2 (T = F, OH, and O) does not depend on the type of surface terminations. This is interesting because the synthesized MXenes usually have mixed surface terminations of F, OH, and O. The half-metallic band gaps calculated with the HSE06 hybrid density functional are 2.03, 1.81, and 1.01 eV, respectively. This implies that thermally induced spin flip should be suppressed. The ferromagnetic coupling between Ni atoms is explained by a superexchange interaction mechanism mediated by spin polarized N atoms. Based on the Heisenberg model, large intra- and inter-layer exchange parameters were obtained. Monte Carlo simulation indicates that the Curie temperatures of Ni 2 NF 2 , Ni 2 N(OH) 2 , and Ni 2 NO 2 are 1800, 2400, and 3300 K respectively, all much higher than room temperature. Ab initio molecular dynamics simulations also indicate that the ferromagnetic Ni 2 NT 2 compounds are stable at room temperature. Therefore, we predict that the half-metallicity of Ni 2 NT 2 should be relatively easy to be achieved and Ni 2 NT 2 is a good candidate for spintronics. [ABSTRACT FROM AUTHOR]

Among the investigated systems in this study, W C 2 exhibited the lowest overpotential as 0.42 V, indicating that it can be potentially promising electrocatalyst for oxygen reduction reactions (ORR). [Display omitted] • Oxygen reduction reaction performance of CrC 2 , MnC 2 , FeC 2 , TcC 2 , HfC 2 , and WC 2 was explored. • The overpotential values of CrC 2 , MnC 2 , FeC 2 , TcC 2 , and HfC 2 were 1.04, 1.16, 1.56, 1.39, and 1.69 V, respectively. • WC 2 showed the lowest overpotential as 0.42 V among the other investigated systems. • Dual-active-site mechanism is found to be important for WC 2. Investigating highly efficient electrocatalysts for fuel cells is urgent. Herein, the oxygen reduction reaction performance of a group of MXene-like carbide two-dimensional materials, including C r C 2 , M n C 2 , F e C 2 , T c C 2 , H f C 2 , and W C 2 , was investigated through first-principles calculations. In these systems, transition metals are sandwiched between C 2 dimers at the top and those at the bottom of a M C 2 structure. Among the investigated systems, W C 2 exhibits the low overpotential of 0.42 V compared with the other systems. The overpotential values of C r C 2 , M n C 2 , F e C 2 , T c C 2 , and H f C 2 are 1.04, 1.16, 1.56, 1.39, and 1.69 V, respectively. In addition, two different catalytic mechanisms based on single and double active sites were compared to each other for T c C 2 and W C 2 , revealing that the mechanism based on dual-active-sites is more favorable than the mechanism based on a single-active-site for these two systems. Our study suggests that a W C 2 monolayer can be a potential electrocatalyst for oxygen reduction. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Zr 2 CCl 2 -III monolayer has a Dirac-like cone near the Fermi level. • By considering the spin-orbit coupling, the splitting value of Zr 2 CCl 2 -III becomes larger. • Electronic structure of Zr 2 CCl 2 -III can be tunable by strain. Two dimensional MXenes is a general term for transition metal carbides and their derivatives. Different MXenes materials have attracted much attention due to their unique electronic properties, which have the potential applications in spintronic devices. With the synthesis of two dimensional Zr-containing materials, the related two dimensional Zr MXenes materials have gradually been paid more attention, so theoretical research based on first principles has also been enriched. In this paper, the electronic structure of the two dimensional MX Zr 2 C material after adsorption of Cl functional groups at different symmetrical positions on both sides is studied systematically by using the density functional theory. The bare Zr 2 C is a metal with magnetism, while the Zr 2 CCl 2 structure becomes a non-magnetic metal. Three adsorption structures of Zr 2 CCl 2 have been studied. The Zr 2 CCl 2 -I monolayer is the most stable structure, but the Zr 2 CCl 2 -II monolayer is the only unstable structure among the three structure. Additionally, a Dirac-like structure with a tiny gap appears in Zr 2 CCl 2 -III monolayer, where the position of gap can be further tailored by biaxial strain. As the compressive strain increases, the Dirac-like will move up. Moreover, at a compressive strain of −0.9%, the Dirac-like structure of Zr 2 CCl 2 -III will move to the Fermi level with an indirect-band-gap of 7.3 meV. At the same time, considering the influence of SOC, its band gap value will become 29.3 meV. The Dirac-like structure with a band gap also makes up for the shortcoming of graphene's zero band gap. And Zr 2 C with different functional groups has different electronic properties, since Zr 2 C MXenes have potential applications in electronic devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • Five kinds of Ti 3 C 2 T x models with various terminations ratio are particular built. • Roles of terminations are studied by electronic properties and storage performance. • Optimal termination ratio for Ti 3 C 2 T x is screened out as anode for Na-ion battery. 2D transition metal carbides MXenes stand out on energy storage application due to superior electrochemical performance and encouraging capacitances, and the energy storage properties strongly depend on the nature of terminations on the surface. Here, five different terminal ratios of Ti 3 C 2 T x (O: F: OH = 9:0:0, 6:2:1, 4:4:1, 3:5:1, 0:9:0, respectively) are designed to explore the influence of different terminations on electronic properties and Na storage capability using density functional theory, moreover the Ti 3 C 2 T x with O: F: OH = 3:5:1 has been synthesized to verify some calculation results. The results show that the Ti 3 C 2 T x MXene, with terminal ratios of 6:2:1, is thermally stable when 36Na atoms (26.67%) intercalate. Meanwhile, it also exhibits superior conductivity, lower open circuit voltages, especially the higher theoretical capacity. The theoretical capacity is more than 6 times than that of pure F termination's structure and 3 times than that of the system with the O: F: OH = 3:5:1. The presence of F can ensure good conductivity and O provide larger capacity. The finding is one to establish interlayer models of Ti 3 C 2 T x -MXene with different terminal ratios and provides a basic understanding about the influence of terminates on electronic properties and Na storage performance of Ti 3 C 2 T x (T = O, F and OH). [ABSTRACT FROM AUTHOR]

• We theoretically report the low-miller-index surfaces of G VB -MCs for HER performance for the first time. • VC has a small absolute ΔG H* value of 0.011 eV, which is better than Pt −0.085 eV. • The (1 1 0) surfaces of G VB -MCs exhibit outstanding HER performance due to the proper d-band center and low work function. • Electronic transfer and bond length analysis indicate that metal–metal bridge site serves as the best active site. Transition metal carbides have received increasing attention for the application in hydrogen evolution reaction (HER), especially the group VB transition metal monocarbides (G VB -MCs). G VB -MCs display excellent HER activity in electrochemical water splitting which can even be favorably comparable with Pt in acidic condition, whereas it still remains as a challenge to illustrate their inherent catalytic mechanism. Herein, we use first-principle study to get insight into the HER activity of G VB -MCs and ascertain their catalytic mechanism. The resultant density functional theory (DFT) calculations suggest that the (1 1 0) surfaces of G VB -MCs have high stability and good conductivity. More importantly, we discover the highly active sites of the metal–metal bridge for the first time, which play an important role in guaranteeing the excellent HER activity on the (1 1 0) surfaces of G VB -MCs. Specifically, the Gibbs free energy of hydrogen adsorption (ΔG H*) of VC (1 1 0) surface (0.011 eV) is superior to that of precious metal Pt, and the calculated activation energy (0.667 eV) for the Tafel reaction on VC (1 1 0) surface is only slightly lower than that on the Pt (1 1 1) surface. Therefore, this work emphasizes the importance of active sites and provides reliable DFT guidelines for the further design of effective G VB -MCs electrocatalysts. [ABSTRACT FROM AUTHOR]