Regulation of Electronic Structures to Boost Efficient Nitrogen Fixation: Synergistic Effects between Transition Metals and Boron Nanotubes.

Borophene possesses outstanding physical and chemical properties and thus demonstrates great application potential in catalysis. However, the lack of a controllable strategy for regulating the electronic structures of borophene for efficient catalysis limits the exploration of this material for a "black-box" model. Herein, taking advantage of the synergistic effects between metals and boron nanotubes (BNT), we report a core-shell structure that encapsulates early transition-metal nanowires into BNT (TMs@BNT) to improve the inherent electronic structures of primitive borophene for an efficient electrochemical nitrogen reduction reaction (eNRR). These filled BNT with disconnected π conjugation and vacant boron (B) pz orbitals enable the regulation of electronic states of B atoms in spatial extent and occupancy that has a great effect on the adsorption strength of intermediates. Using first-principles calculations, we demonstrate that the *N2H adsorption energy (ΔE*N2H) is strongly correlated with the intrinsic activity trends and that the variation of ΔE*N2H is attributed to the distribution of 2p states and charge of B atoms. Finally, we utilize the coupling of the d 2p states between B atoms and metals to obtain a quantitative explanation for synergistic effects and conclude that metals with a lower d-band center (εTM d) raise the average 2p state energy (ε̅2p) of B through two-level quantum coupling, which is the physical origin of this interaction. Therefore, two candidates (Mo@BNT and W@BNT) with lower εTM d are screened, benefiting from their high eNRR activity (limiting potentials of -0.75 and -0.77 V, respectively) and high selectivity. This work explores the activity origin, constructs a bridge between electronic structures and activity trends, and paves the way for future eNRR studies.