Boosting electrochemical nitrogen reduction reaction performance of two-dimensional Mo porphyrin monolayers via turning the coordination environment

Electrocatalytic Dinitrogen Reduction to Ammonia Using Easily Reducible N-Fused Cobalt Porphyrins

Single-site molecular electrocatalysts, especially those that perform catalytic conversion of N2 to NH3 under mild conditions, are highly desirable to derive fundamental structure-activity relations and as potential alternatives to the current energy-consuming Haber-Bosch ammonia production process. Combining theoretical calculations with experimental evidence, it has been shown that easily reducible cobalt porphyrins catalyze the six-electron, six-proton reduction of dinitrogen to NH3 at neutral pH and under ambient conditions. Two easily reducible N-fused cobalt porphyrins - CoNHF and CoNHF(Br)2 - reveal NRR activity with Faradic efficiencies between 6-7.5 % with ammonia yield rates of 300-340 μmol g-1 h-1. Contrary to this, much harder-to-reduce N-fused porphyrins - CoNHF(Ph)2 and CoNHF(PE)2 - reveal no NRR activity. The present study highlights the significance of tuning the redox and structural properties of single-site NRR electrocatalysts for improved NRR activity under mild conditions.

Atomic Molybdenum Nanomaterials for Electrocatalysis

As a sustainable energy technology, electrocatalytic energy conversion requires electrocatalysts, which greatly motivates the exploitation of high-performance electrocatalysts based on nonprecious metals. Molybdenum-based nanomaterials have demonstrated promise as electrocatalysts because of their unique physiochemical and electronic properties. Among them, atomic Mo catalysts, also called Mo-based single-atom catalysts (Mo-SACs), have the most accessible active sites and tunable microenvironments and are thrivingly explored in various electrochemical conversion reactions. A timely review of such rapidly developing topics is necessary to provide guidance for further exploration of optimized Mo-SACs toward electrochemical energy technologies. In this review, recent advances in the synthetic strategies for Mo-SACs are highlighted, focusing on the microenvironment engineering of Mo atoms. Then, the representative achievements of their applications in various electrocatalytic reactions involving the N2, H2O, and CO2 cycles are summarized by combining experimental and computational results. Finally, prospects for the future development of Mo-SACs in electrocatalysis are provided and the key challenges that require further investigation and optimization are highlighted.

Regulating the Coordination Environment of Single-Atom Catalysts Anchored on Thiophene Linked Porphyrin for an Efficient Nitrogen Reduction Reaction

The electrochemical nitrogen reduction reaction (NRR) offers a promising strategy to resolve high energy consumption in the nitrogen industry. Recently, the regulation of the electronic structure of single-atom catalysts (SACs) by adjusting their coordination environment has emerged as a rather promising strategy to further enhance their electrocatalytic activity. Herein, we design novel SACs supported by thiophene-linked porphyrin (TM-N₄/TP and TM-N_4-xB_x/TP, where TM = Sc to Au) as potential NRR catalysts using density functional theory calculations. Among these catalysts, TM-N₄/TP (TM = Ti, Nb, Mo, Ta, W, and Re) and TM-N₄/TP with a water bilayer (TM = Nb, Mo, W, and Re) show excellent activity (low limiting potential) but low selectivity. Encouragingly, we find that Mo-N₃B/TP, Mo-N₂B₂-2/TP, W-N₃B/TP, W-N₂B₂-2/TP, Re-N₃B/TP, Re-N₂B₂-2/TP, and Re-N₂B₂-1/TP serve as the most efficient NRR electrocatalysts, as they present stability, superior activity, better selectivity with low limiting potentials (-0.18 ∼ -0.90 V), and high Faradaic efficiencies (>99.80%). Based on microkinetic modeling, kinetic analysis of the NRR is performed and shows that the Re-N₂B₂-1/TP catalyst is more efficient for NH₃ formation. Additionally, multiple-level descriptors provide insight into the origin of NRR activity and enable fast prescreening among numerous candidates. This work provides a new perspective to design highly efficient catalysts for the NRR under ambient conditions.

Catalytic Role of Adsorption of Electrolyte/Molecules as Functional Ligands on Two-Dimensional TM-N4 Monolayer Catalysts for the Electrocatalytic Nitrogen Reduction Reaction

Two-dimensional single-atom catalysts (2D SACs) have been widely studied on the nitrogen reduction reaction (NRR). The characteristics of 2D catalysts imply that both sides of the monolayer can be catalytic sites and adsorb electrolyte ions or molecules from solutions. Overstrong adsorption of electrolyte ions or molecules on both sides of the catalyst site will poison the catalyst, while the adsorbate on one side of the catalytic site will modify the activity and selectivity of the other side for NRR. Discovering the influence of adsorption of electrolyte ions or molecules as a functional ligand on catalyst performance on the NRR is crucial to improve NRR efficiency. Here, we report this work using the density functional theory (DFT) method to investigate adsorption of electrolyte ions or molecules as a functional ligand. Among all of the studied 18 functional ligands and 3 transition metals (TMs), the results showed that Ru&F, Ru&COOH, and Mo&H₂O combinations were screened as electrocatalysis systems with high activity and selectivity. Particularly, the Mo&H₂O combination possesses the highest activity with a low ΔG_MAX of 0.44 eV through the distal pathway. The superior catalytic performance of the Mo&H₂O combination is mainly attributed to the electron donation from the metal d orbital. Furthermore, the functional ligands can occupy the active sites and block the competing vigorous hydrogen evolution reaction. Our findings offer an effective and practical strategy to design the combination of the catalyst and electrolyte to improve electrocatalytic NRR efficiency.