Asymmetrical π back-donation of hetero-dicationic Mo⁴⁺–Mo⁶⁺ pairs for enhanced electrochemical nitrogen reduction

The breaking of nonpolar N≡N bond of dinitrogen is the biggest dilemma for electrocatalytic nitrogen reduction reaction (NRR) application, driving electron migration between catalysts and N≡N bond (termed “π back-donation” process) is crucial for attenuating interfacial energy barrier but still remains challenging. Herein, using density functional theory calculations, we revealed that constructing a unique hetero-dicationic Mo⁴⁺–Mo⁶⁺ pair could effectively activate N≡N bond with a lying-down chemisorption configuration by an asymmetrical “π back-donation” process. As a proof-of-concept demonstration, we synthesized MoO₂@MoO₃ heterostructure with double Mo sites (Mo⁴⁺–Mo⁶⁺), which are embedded in graphite, for electrochemical nitrogen reduction. Impressively, this hetero-dicationic Mo⁴⁺–Mo⁶⁺ pair catalysts display more excellent catalytic performance with a high NH₃ yield (60.9 µg·h⁻¹·mg⁻¹) and Faradic efficiency (23.8%) as NRR catalysts under ambient conditions than pristine MoO₂ and MoO₃. Operando characterizations using synchrotron-based spectroscopic techniques identified the emergence of a key *N₂H_y intermediate on Mo sites during NRR, which indicates that the Mo sites are active sites and the NRR process tends to follow an associative mechanism. This novel type of hetero-dicationic catalyst has tremendous potential as a new class of transition metal-based catalysts with promising applications in electrocatalysis and catalysts for energy conversion and storage.