Interlayer Space Confined-S Vacancy and Single Atom within Graphene/MoS2 Bilayers: Synergistic Adsorption and Active Site Switching for Enhanced Nitrogen Fixation

Transition metal (TM) single-atom catalysts (SACs) embedded in nitrogen-doped carbon materials (e.g. M-N4-C structures) have emerged as a promising class of electrocatalysts for electrochemical nitrogen reduction reaction (eNRR). However, the unfavorable adsorption and insufficient activation of N2 limits the improvement of their activity. Here, we combined the advantages of M-N4-C site and sulfur vacancies (VS) on the MoS2 basal plane (MoS2-VS) to investigate the eNRR catalytic behavior within the interlayer space of Mn-N4-C/MoS2-VS bilayers, where TM atoms are doped at the S vacancy sites (denoted as M-N4-C/MoS2-TM), using first-principles simulation. Our results show that the interlayer space of M-N4-C/MoS2-TM provides an excellent reactor, in which the S vacancy and Mn-N4 structure are served as the dual single-atom sites optimizing the *N2 adsorption and activation via synergistic effect. More importantly, during the catalytic process, switching of the active sites between the S vacancy and Mn-N4 within the interlayer space for key intermediate binding significantly lowers the energy barrier of potential-determining step, thereby facilitating N2 reduction to NH3. The detailed reaction mechanism exploration shows that the Mn-N4/MoS2-TM (TM = Mn, Tc and Ag) are screened as promising electrocatalysts for NRR, with a low limiting potential of -0.3 V and maximum kinetic barrier of 0.75 eV. Furthermore, by relying solely on intrinsic material properties such as the number of d electrons (θd) and the electronegativity (XTM) of the doped TM atoms, we constructed a novel descriptor, φ=√(θ_d*X_TM ), that accurately predicts catalytic activity. This work presents a unique strategy for the rational design of bimetallic single-atom site catalysts.