Revealing the Adsorption Behavior of Nitrogen Reduction Reaction on Strained Ti2 CO2 by a Spin-Polarized d-band Center Model.

Electrocatalytic reduction of dinitrogen to ammonia has attracted significant research interest. Herein, it reports the boosting performance of electrocatalytic nitrogen reduction on Ti2 CO2 MXene with an oxygen vacancy through biaxial tensile strain engineering. Specifically, tensile strain modified electronic structures and formation energy of oxygen vacancy are evaluated. The exposed Ti atoms with additional electron states near the Fermi level serve as active site for intermediate adsorption, leading to superior catalytic performance (Ulimit = -0.44 V) under 2.5% biaxial tensile strain through a distal mechanism. However, the two sides of the "Sabatier optimum" in volcano plot are not limited by two different electronic steps, but are induced by the diverse adsorption behaviors of intermediates. Crucially, the "Sabatier optimum" results from the different response speeds of the adsorption energy for *N2 and *NNH to strains. Moreover, the authors observe conventional d-band adsorption for *N2 and *NNH, non-linear adsorption for *NNH2 , and abnormal d-band adsorption for *N, *NH, *NH2 , and *NH3 , which can be explained by the competition between attractive orbital hybridization and repulsive orbital orthogonalization with the spin-polarized d-band model, which further clarifies the contributions of 3σ → dz2 and dxz /dyz → 2π* to the overall population of bonding and anti-bonding states.

First principles study of B7N5 as a high capacity electrode material for K-ion batteries.

Potassium-ion batteries (KIBs) are considered a promising candidate for energy storage owing to their low cost and similar "rocking chair" mechanism to lithium-ion batteries. However, there is a great lack of suitable and high working performance electrode materials for KIBs. Herein, first principles calculations based on density functional theory (DFT) are applied to evaluate the potential of the B7N5 monolayer as an anode material for KIBs. It is found that B7N5 shows negative adsorption energies for Li/Na/K on its surface. Besides, the B7N5 monolayer can effectively achieve double-layer adsorption for K atoms on both sides of the monolayer surface, while one-layer adsorption for Li and Na. Thus, B7N5 exhibits an ultra-high theoretical capacity of 1471.5 mA h g-1 for KIBs, which is almost the highest value for the anode materials reported in the literature. And the attractive capacity for KIBs is mainly contributed by the multiple empty electron orbitals of the constituting elements of B7N5 and the small distance mismatch. In addition, K atoms display high diffusivities on B7N5 with low energy barriers of 0.10 eV, and the open circuit voltages of 0.14 V for KIBs are also smaller compared with previous research. It is also shown that after adsorbing K, the semiconducting B7N5 monolayer is transformed to a metallic state with good conductivity. Furthermore, despite the large size of K+, the maximum change in the lattice constant of B7N5 is only 1.32%, indicating structural stability and ensuring good cycling stability for KIBs. The above-mentioned results suggest that B7N5 is a potential anode material for KIBs.

B5N3 as a potential high-capacity electrode material for calcium ion batteries.

Calcium ion batteries (CIBs) are considered as promising candidates for the next-generation large-scale energy storage technologies, due to the abundant resources and bivalent properties of calcium. Herein, based on first principles calculations, we systematically explore the performance of B5N3 as an electrode material for chargeable CIBs. Specifically, the adsorption of the Ca atom effectively reduces the band gap of B5N3, leading to good electrical conductivity. Additionally, the B5N3 monolayer can achieve an effective double-layered adsorption of Ca atoms on both sides of the monolayer surface, thus exhibiting an ultra-high theoretical capacity of 4463 mA h g-1 (Ca) compared with the capacities of Na (2231 mA h g-1), Li (1116 mA h g-1) and K (558 mA h g-1). The high capacity is attributed to the multiple empty electron orbitals of the constituent elements of B5N3 and low distance mismatch which can exhibit excellent adsorption properties for multivalent atoms. Furthermore, the low diffusion energy barriers and satisfactory thermal stability ensure the reliability of B5N3 as a CIB electrode material. Our work not only develops an excellent candidate for the electrode materials of CIBs, but may also inspire the rational design and synthesis of electrode materials towards high-performance CIBs.

S-functionalized 2D V2B as a promising anode material for rechargeable lithium ion batteries.

The development of novel high specific capacity anode materials is urgently needed for rechargeable metal ion batteries. Herein, S-functionalized V2B as the electrode material for Li/Na/K ion batteries are comprehensively investigated using first-principles calculations. Specifically, V2BS2 was verified with good electrical conductivity via band structure and density of states calculations. Phonon dispersion and ab initio molecular dynamic simulations were performed and confirmed the dynamic and thermal stability of V2BS2. The use of V2BS2 with a high theoretical specific capacity of 606 mA h g-1 for lithium ion batteries (LIBs) due to the bilayer adsorption of Li atoms is encouraging, which is attributed to the double empty orbitals of the S atoms and small lattice mismatch (1.5%) between the Li layers and substrate. Furthermore, dendrite formation would be well prohibited and safety issues for battery operation would be ensured for V2BS2 as electrode materials because of the low open circuit voltage with 0.37 V. The high charge/discharge rate for LIBs is also achievable owing to the high mobility of adatoms on the surface of V2BS2. Our work not only finds use as a promising material for the field of energy storage, but also provides constructive design strategies for developing high performance anode materials for rechargeable metal ion batteries.

I3 - /I- Redox Reaction-mediated Organic Zinc-Air Batteries with Accelerated Kinetics and Long Shelf Lives.

The storage time of Zn-air batteries (ZABs) for practical implementation have been neglected long-lastingly. ZABs based on organic solvents promise long shelf lives but suffer from sluggish kinetics. Here, we report a longly storable ZAB with accelerated kinetics mediated by I3 - /I- redox. In the charge process, the electrooxidation of Zn5 (OH)8 Cl2 ⋅H2 O is accelerated by I3 - chemical oxidation. In the discharge process, I- adsorbed on the electrocatalyst changes the energy level of oxygen reduction reaction (ORR). Benefitting from these advantages, the prepared ZAB shows remarkably improved round-trip efficiency (56.03 % vs. 30.97 % without the mediator), and long-term cycling time (>2600 h) in ambient air without replacing any components or applying any protective treatment to Zn anode and electrocatalyst. After resting for 30 days without any protection, it can still directly discharge continuously for 32.5 h and charge/discharge very stably for 2200 h (440 cycles), which is evidently superior to aqueous ZABs (only 0/0.25 h, and 50/25 h (10/5 cycles) by mild/alkaline electrolyte replenishment). This study provides a strategy to solve both storage and sluggish kinetics issues that have been plaguing ZABs for centuries, opening up a new avenue to the industrial application of ZABs.

A DFT study on regulating the active center of v-Ti2XT2 MXene through surface modification for efficient nitrogen fixation.

The electrochemical conversion of nitrogen to ammonia provides an encouraging method to substitute the traditional Haber-Bosch process, owing to its high efficiency and mild reaction conditions. The search for high-performance catalysts and comprehension of catalytic mechanisms remains significant challenges. Herein, we conduct a systematic theoretical calculation of the NRR performance and mechanism of 24 Ti2XT2 (X = B, C, N; T = F, Cl, Br, I, O, S, Se, Te) MXenes with a T-vacancy to explore the influence of surface functional terminations and non-metallic center elements. Our findings demonstrate that surface functionalization significantly reduces the limiting potential by altering the rate-determining step. This change ranges from -1.24 V (Ti2NF2) to -0.21 V (Ti2BSe2), signifying the remarkable efficacy of modification of the surrounding environment of the exposed transition metal active center in promoting electrocatalytic performance. Detailed investigation of the charge density difference and orbital interaction reveals that the different NRR performance originates from the surface termination and non-metallic atoms regulate the electronic properties of the active Ti atoms. We also introduce the free energy change of *NNH2 (ΔG*NNH2) as a descriptor to predict the performance of NRR, which exhibits satisfactory linear relationship with free energy change of different intermediates and displays favourable volcano plot with limiting potential. Moreover, we highlight the pivotal role of work function in tuning the energy barrier of the rate-determining step, which can be regulated through the surface modification of MXenes. Our study not only offers a comprehensive understanding of the crucial impact of surface modification on the catalytic activities of defective MXenes, but also provides a rational perspective for designing efficient NRR catalysts.

Establishing an orbital-level understanding of active origins of heteroatom-coordinated single-atom catalysts: The case of N2 reduction.

Heteroatom-coordinated single-atom catalysts (SACs) supported by porous graphene exhibit high activity in electrochemical reduction reactions. However, the underlying active origins are complex and puzzling, hindering the development of efficient catalysts. Herein, we investigate the active origins of heteroatom-coordinated Fe-XmYn SACs (X, Y = B, C, N, O, m + n = 4) toward nitrogen reduction reaction (NRR) as a model reaction, through comprehensive analysis of structural, energetic, and electronic parameters. Specifically, the number and arrangement of heteroatoms are found to significantly affect the degree of d-orbital splitting and magnetic moment of the Fe center. Moreover, d-orbital splitting energy (dSE), rather than the conventional d-band theory, explains the adsorption behavior of intermediates in multi-step electron-proton coupling (EPC) reactions. In addition, both s- and d-orbitals of Fe are found to be important for Fe-N bonding, which promotes charge transfer (CT) and N2 activation. Importantly, CT is thought to influence the Pauli repulsion and orbital interaction. Correspondingly, relationships are unveiled between limiting potential (Ulimit) and adsorption energy ΔE(*NNH), dSE, CT, Fe-N bond. In all, this work provides orbital-level insights into the active origins of Fe-XmYn SACs, contributing to the understanding of intrinsic mechanism and the design of electrocatalysts for multi-step EPC reactions.

Work function regulation of surface-engineered Ti2CT2 MXenes for efficient electrochemical nitrogen reduction reaction.

Electrochemical conversion of nitrogen to ammonia is a promising method in modern agriculture and industry due to its suitability and feasibility under mild conditions. Therefore, seeking electrocatalysts and understanding the catalytic mechanisms are of great importance. In this work, by combining the concept of the synergetic effect of the terminal vacancy and transition metal active center, we studied the whole catalytic mechanism of defective Ti2CT2 MXenes with functional groups (T = O, F, H, OH) by employing first-principles calculations. It is demonstrated that the electron transfer behavior of 2D transition metal carbides can be tuned by modifying the surface functional groups. Herein, the rarely investigated work function regulation is proved to effectively alter the electron transfer ability, thus the binding strength of key intermediates on the surface can be optimized. Besides, Ti2CO2 with an oxygen vacancy is identified as a promising candidate through a distal mechanism, where the calculated electronic properties reveal that the introduction of in-gap states is responsible for activating N2 with physical adsorption. In addition, obvious orbital splitting of the σ and π* orbitals of N2 is observed due to the hybridization of frontier orbitals. The symmetry matching rule of the frontier orbitals of π* 2p and the σ 2p orbitals of N with Ti d orbitals further illustrates the "acceptance-donation" interaction. These theoretical insights highlight the underlying mechanism of the synergetic effect of surficial vacancy and exposed transition metal atoms, and provide an alternative view of designing efficient NRR electrocatalysts.