Enhanced catalytic activity of MXene for nitrogen electoreduction reaction by carbon doping

Machine learning-driven shortening the screening process towards high-performance nitrogen reduction reaction electrocatalysts with four-step screening strategy

The urgent need to prepare clean energy by environmentally friendly and efficient methods, which has led to widespread attention on electrocatalytic nitrogen reduction reaction (NRR) for ammonia production. At present, single atom catalytic nitrogen reduction has become the earliest promising method for industrial production due to its high atomic utilization rate, high selectivity, high controllability, and high stability. However, how to quickly screen catalysts with high catalytic efficiency and selectivity in single-atom catalysts (SACs) remains a challenge. Herein, the 29 SACs are constructed from C6N2 nanosheets doped with transition metals (TM@C6N2), which are analyzed for stability, adsorption performance, NRR catalytic activity, electronic properties, and competitiveness using first-principles calculations. The results show that Mo@C6N2 and Re@C6N2 exhibit the most outstanding catalytic performances, with limiting potentials (UL) of -0.29 and -0.31 V, respectively, in the solvent model. Machine learning is used to derive descriptors from the intrinsic features to predict the free energy changes for the potential-determining step. The importance of features is calculated, with the first ionisation energy (IE1) being the most significant influencing factor. Based on the guidance of machine learning and considering that IE1 is related to the ability of metal atoms to donate electrons, a four-step screening strategy using the Integrated Crystal Orbital Hamilton Populations (ICOHP) to screen catalysts instead of the traditional five-step screening not only improves the screening efficiency but also obtains completely consistent screening results. This work presents a new approach to predicting the catalytic performance of SACs and provides new insights into the influence of intrinsic properties on catalytic activity.

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.

Transition metals anchored on two-dimensional p-BN support with center-coordination scaling relationship descriptor for spontaneous visible-light-driven photocatalytic nitrogen reduction

Solar energy has the potential to revolutionize the production of ammonia, as it could provide a reliable and uninterrupted source of energy for the chemical reaction involved. However, improving the catalytic performance of catalysts often leads to a reduction in their band gaps, which results in insufficient photogenerated electron potential to realize the nitrogen reduction reaction (NRR), and thus the development of NRR efficient photocatalysts remains a great challenge. Herein, based on the density functional theory (DFT), a series of single-atom photocatalysts with transition metals (TMs) doped on porous boron nitride (p-BN) nanosheet are proposed for NRR. Among them, Re-B3@p-BN could effectively catalyze gas-phase N2 through the corresponding pathways with limiting potentials of 0.31 V. Meanwhile, it exhibits excellent light absorption efficiency under illumination and could spontaneously catalyse nitrogen fixation reactions due to the suitable forbidden band and high photogenerated electron potential. Moreover, a linear relationship descriptor based on the intrinsic properties has been established, using a machine learning approach by considering the combined effects of the central metal atom and the coordination atoms. This descriptor could help accelerate the development of rational and improved 2D NRR photocatalysts with high catalytic activity and high selectivity.

Single silicon-doped CNT as a metal-free electrode for robust nitric oxide reduction utilizing a Lewis base site: an ingenious electronic "Reflux-Feedback" mechanism

The electrocatalytic reduction of nitric oxide (NO) has become the most charming approach for the sustainable synthesis of ammonia (NH₃), however, the development of a valid catalyst endowed with low cost, high efficiency, and long-term endurance still faces an enormous challenge. In view of the famous concept of "donate and accept", various transition metal-based electrodes have been predicted and brought into production for electrocatalysis, but metal-free materials or novel activation mechanisms are rarely reported. Here, metal-free electrocatalysts, namely individual silicon (Si) atom-embedded single-walled carbon nanotubes (CNTs), for the NO reduction reaction (NORR) were put forward by performing first-principles calculations. The results disclose that the discarded NO can be converted into value-added NH₃ on Si-CNT(10, 0) with a limiting potential of -0.25 V. Importantly, the doped Si atom acts as a Lewis base site that drives some of the p-orbital electrons to return to the surrounding carbon atoms and then feed adequate electron back to intermediates, rendering it more flat for the electroreduction progress. In summary, the designed carbon-based electrode holds great promise for experimental trial and offers a certain degree of theoretical guidance.

The Rise of MXene: A Wonder 2D Material, from Its Synthesis and Properties to Its Versatile Applications-A Comprehensive Review

MXene, a new member of 2D material, unites the eminence of hydrophilicity, large surface groups, superb flexibility and excellent conductivity. Because of its prodigious characteristics, MXene has gained much approbation among researchers worldwide. MXene's noteworthy features, such as its electrical conductivity, structural property, magnetic behaviour, etc., manifest a broad spectrum of applications, including environment, catalytic, wireless communications, electromagnetic interference (EMI) shielding, drug delivery, wound dressing, bio-imaging, antimicrobial and biosensor. In this review article, an overview of the latest advancements in the applications of MXene has been reported. First, various synthesis strategies of MXene will be summarized, followed by the different structural, physical and chemical properties. The current advances in versatile applications have been discussed. The article aims to incorporate all the possible applications of MXene, making it a versatile material that juxtaposes it with other 2D materials. A separate section is dedicated to the bottlenecks for future developments and recommendations.

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.

Descriptors and graphical construction for in silico design of efficient and selective single atom catalysts for the eNRR

Outline a screening protocol that uses density functional theory calculations to simultaneously optimize with respect to multiple criteria, thereby successfully identifying catalysts that are highly selective and also result in low overpotentials for ammonia production through eNRR.

Design strategies of two-dimensional metal-organic frameworks toward efficient electrocatalysts for N2 reduction: cooperativity of transition metals and organic linkers

Two-dimensional (2D) metal-organic frameworks (MOFs) serve as emerging electrocatalysts due to their high conductivity, chemical tunability, and accessibility of active sites. We herein proposed a series of 2D MOFs with different metal atoms and organic linkers with the formula M₃C₁₂X₁₂ (M = Cr, Mo, and W; X = NH, O, S, and Se) to design efficient nitrogen reduction reaction (NRR) electrocatalysts. Our theoretical calculations showed that metal atoms in M₃C₁₂X₁₂ can efficiently capture and activate N₂ molecules. Among these candidates, W₃C₁₂X₁₂ (X = O, S, and Se) show the best NRR performance due to their high activity and selectivity as well as low limiting potential (-0.59 V, -0.14 V, and -0.01 V, respectively). Moreover, we proposed a d-band center descriptor strategy to screen out the high activity and selectivity of M₃C₁₂X₁₂ for the NRR. Therefore, our work not only demonstrates a class of promising electrocatalysts for the NRR but also provides a strategy for further predicting the catalytic activity of 2D MOFs.

Phosphorus modulation of a mesoporous rhodium film for enhanced nitrogen electroreduction

Electrochemical reduction of nitrogen to ammonia has received considerable attention for sustainable nitrogen fixation, but the sluggish kinetics results in unsatisfactory activity and efficiency. Designing electrocatalytic active centers for nitrogen adsorption and activation is highly desired. Herein, we present an electrodeposition method for the synthesis of a phosphorus-doped mesoporous rhodium film on nickel foam for the electrochemical synthesis of ammonia. Due to the unique combination of components and structure, the obtained catalyst not only shows excellent catalytic performance (NH₃ yield: 32.57 μg h^-1 mg^-1_cat.; faradaic efficiency: 40.86%), but also exhibits almost no decrease in activity after the durability test. This research work can provide a facile synthesis strategy for non-metal-doped porous metal based catalysts, which would be promising for the electrochemical synthesis of ammonia.

Termination effects of single-atom decorated v-Mo2CTx MXene for the electrochemical nitrogen reduction reaction

The lack of the green, economical and high-efficient catalysts restrict the development of electrochemical nitrogen reduction reaction (NRR). By means of density functional theory (DFT) calculations, we have systematically investigated the NRR catalytic performance of single atoms decorated v-Mo₂CT₂ (T = O, F, OH, Cl, and Li) MXene (TM@v-Mo₂CT₂). Our calculation results reveal the introduction of single atom can significantly improve the NRR activity and selectivity on v-Mo₂CO₂, and Ir@v-Mo₂CO₂ system possesses the lowest limiting potential of only -0.33 V among all studied systems. The termination effects of TM@v-Mo₂CT₂ are further discussed and a descriptor of the adsorption energy of *NNH species (ΔE(*NNH)) is proposed to establish the relationship with NRR limiting potential (U_L(NRR)), in which a moderate (ΔE(*NNH)) is required for high NRR activity. Moreover, a good linear relationship between the ΔE(*NNH) and the excess electrons on Ir atom shows that different ΔE(*NNH) originates from the difference of valence state of Ir atom, which is due to the change of coordination environment. Importantly, the synergistic effects of Ir atom and the surface O-terminations during the first hydrogenation step lead to a promoted NRR performance. Our study might provide new possibilities for rational design of cost-effective MXene-based NRR electrocatalysts.

High conductivity and stability of polystyrene/MXene composites with orientation-3D network binary structure

Recently, two-dimensional transition metal carbide/nitride (MXene) and its composites with polymers have attracted great interest from researchers due to their potential applications in flexible electronics, electromagnetic shielding, catalysis, and energy storage. However, the easy oxidation of MXene and the low efficiency of traditional composites preparation methods have brought great challenges to the practical application of polymer/MXene composites. Here, we prepared polystyrene/Mxene (PS/MXene) composites with a 3D conductive network structure through particle construction strategy. Because of the compact and ordered structure, the conductivity of the material reached 3846.15 S/m when the filler content was only 1.81 vol%, and it can retain 53.4% of the initial value after 180 days. Furthermore, based on the 3D network, we orientated the MXene nanosheets in the matrix to form the MXene orientated 3D network binary structure. This unique structure design further increased the utilization rate of MXene and made the material conductivity reach to 4471.13 S/m, with the percolation threshold as low as 0.175 vol%. We believe that this research can provide a feasible way for the practical application of MXene composite materials.