Double-exchange interaction modulated e_g orbital occupancy in heteronuclear metal-organic framework toward efficient electrochemical N₂ fixation

Establishing well-defined electrocatalysts to realize the efficient nitrogen reduction reaction (NRR) is highly desirable, but facing poor efficiency limitations due to the difficult cleavage of inert N=N triple bond. Modulating the spin-dependent electronic configuration shows great promise for facilitating the interplay between catalytic active sites and reactant molecules. Herein, we propose a concept for manipulating the e_g orbital occupancy in Fe₂Co heteronuclear metal-organic frameworks (MOFs) to dramatically enhance the activation and conversion of N₂. Experimental and theoretical calculations corroborate that the intense double-exchange interaction between adjacent FeO₆ and CoO₆ octahedrons readily triggers vibronic Co³⁺-O-Fe²⁺ effect and further regulates the e_g orbital occupation state of active sites. The resulting intermediate spin states of Co³⁺ (t_2g⁵e_g¹) promotes its lone-paired e_g electrons and empty orbital concerted interplay with the antibonding π-orbital and bonding σ-orbital of N₂, which significantly weakens N=N triple bond via “acceptance-donation” mechanism and lowers the energy barrier of *NNH to *NNH₂. Consequently, Fe₂Co heteronuclear MOFs achieves a Faraday efficiency of 34.10% and a NH₃ yield rate of 107.5 μg h⁻¹ mg_cat.⁻¹, which is about 3-fold higher than high-spin homonuclear MOFs. This orbital regulation strategy provides new fundamental insights into the rational design of high-activity electrocatalysts.