Transition metal small clusters anchored on biphenylene for effective electrocatalytic nitrogen reduction

The synthesis of ammonia via an electrochemical nitrogen reduction reaction (NRR, N₂ + 6H⁺ + 6e^- → 2NH₃), which can weaken but not directly break an inert NN bond under mild conditions via multiple progressive protonation steps, has been proposed as one of the most attractive alternatives for the production of NH₃. However, the development of appropriate catalyst materials is a major challenge in the application of NRRs. Recently, single- or multi-metal atoms anchored on two-dimensional (2D) substrates have been demonstrated as ideal candidates for facilitating NRRs. In this work, by applying spin-polarized density functional theory and ab initio molecular dynamic simulations, we systematically explored the performances of nine types of transition metal multi-atoms anchored on a recently developed 2D biphenylene (BPN) sheet in nitrogen reduction. Structural stability and NRR performance catalyzed by TM (TM = V, Fe, Ni, Mo, Ru, Rh, W, Re, Ir; n = 1-4) clusters anchored on BPN sheets were systematically explored. After a strict six-step screening strategy, it was found that W₂, Ru₂ and Mo₄ clusters loaded on BPN demonstrate superior potential for nitrogen reduction with extremely low onset potentials of -0.26, -0.36 and -0.17 V, respectively. Electronic structure analysis revealed that the enhanced ability of these multi-atom catalysts to effectively capture and reduce the N₂ molecule can be attributed to bidirectional charge transfer between the d orbitals of transition metal atoms and molecular orbitals of the adsorbed N₂ through a "donation-back donation" mechanism. Our findings highlight the value of BPN sheets as a substrate for designing multi-atom nitrogen reduction reaction catalysts.