1. Introduction
Ammonia (NH3) plays an essential role in human society for fertilizer, industry chemistry, dyes, hydrogen carriers, etc.1, 2 Currently, the primary industrial method to produce ammonia is the century-wide Haber-Bosch process3, 4, where the massive energy is required to break the triple bond of nitrogen (N2), leading to nearly 2% energy cost annually. Meanwhile, this process releases 1% CO2 of the whole emission every year, accelerating the greenhouse effect and climate warming5, 6. As an alternative strategy, electrocatalytic nitrogen reduction reaction (eNRR) for NH3 synthesis has attracted more attention for its multichannel energy input and CO2-free emmosion7, 8.
Nowadays, various electrocatalysts have been investigated for eNRR, including bulk catalysts, two-dimensional catalysts, atomic cluster catalysts, single-atom catalysts, dual-atom catalysts, MOF-based catalysts, and three-atom cluster catalysts. For example, the PdCu nanoparticle9, MXenes10, Fe3 cluster11, W/g-CN12, Mo@C9N413, NiCo@GDY14 and so on15, 16, 17, 18. Among them, dual atom catalyst (DAC) shows excellent electrocatalytic performance for eNRR since the synergistic catalytic process among two active sites 19, 20. Detailly, the DAC can be divided into two categories, the homonuclear 21, 22, 23 and heteronuclear ones24, 25. For the homonuclear DAC, the two active sites show the same properties, while the same characters of the dimers make the activity of DAC very limited26. Compared to homonuclear DAC, heteronuclear DAC with various properties in both electronic and structural characters, leading the active sites not only play different role in catalytic process, but also break the scaling relationship for eNRR, which effectively enable highly activity and selectivity during the eNRR process19, 27. For example, He et al, demonstrated that B site and Si site take different roles in electrocatalytic eNRR process and put forward ‘capture-backdonation-recapture’ mechanism for metal-free dual atom-based catalyst19. Hu et al. verified that the scaling relationship is broken through adding additional atom in M/C3N system, leading to significantly decreased in limiting potential in M1M2/C3N system28. Nevertheless, for the widely studied transitional metal (TM) based heteronuclear dual atom catalyst, the underlying mechanism is still unclear. On one hand, the roles of each TM atoms could not sensitively detect and efficiently differentiate during the eNRR process due to high activity and low distinction of them. On the other hand, heteronuclear induced atomic diversity could enlarge this dilemma and pollute the intrinsic electronic influence during the underlying electronic mechanism finding process, thus leading people could not make deep understanding to the intrinsic electronic determined mechanism finding exactly. Therefore, how to remove the heteronuclear atomic influence and enhance the intrinsic properties’ sensitivity for investigating the underlying mechanism is extremely urgent for TM-based dual atom catalyst.
To solve this dilemma, an efficient strategy is to enlarge the intrinsic properties’ variety to minimize the diversity of two TM atoms. For example, turning dual heteronuclear TM atoms into single active atoms and spontaneously removes this influence, while this strategy violates the purpose of DAC’s investigation, such as the synergistic effect among two active sites. It should be noted that maintaining dual TM atoms homogeneity29, 30 could remove the atomic diversity effect as well, but it makes the intrinsic properties can’t be detected efficiently at the same time due to the same intrinsic properties they owned. Comparatively, it is impossible to answer how to remove the heteronuclear induced atomic diversity for heteronuclear by uniting their atomic size properties. However, it is easy to make the intrinsic properties different when atomic properties remain the same for homonuclear dual TM atom catalysts. Therefore, changing the strategy to keep structural properties the same but make the intrinsic properties vary could be an efficient method for sensitively detecting and differentiating the roles of each TM atom in the catalytic process. In fact, this phenomenon is noticed in some experiments and is also initially investigated for organic synthesis31 and CO2RR32. Meantime, the complicated coordinating environment of the substrate (especially for the porous materials) further guarantees the feasibility of this proposal33, 34. However, a general and systematic investigation of the intrinsic mechanism is rare, especially since this strategy can remove the atomic diversity effect. Hence, by considering the different configuration properties as proved in the experiment, it is evident to study the same atomic properties but intrinsic properties’ variety homonuclear DAC and dig out the intrinsic mechanisms for dual TM atoms catalyst.
Combining the advantage of homonuclear DAC’s atomic similarity and the configuration difference induced heteronuclear-like DAC, as well as the high sensitivity of N2 molecule, we designed 23 kinds of dual transition metals doping on four asymmetric defective C3N (total of 92 systems) to investigate the homonuclear induced atomic size-constant DAC for electrocatalytic NRR by density functional theory. After ascertaining the stability of the doped systems, the homonuclear twain TM atom catalyst systems exhibit differential intrinsic properties identified by analyzing the intrinsic properties like Bader charge analysis, magnetic analysis, and d-band center analysis, demonstrating the feasibility of our proposal. Then, a novel and extended ’capture-charge distribution-recapture-charge redistribution’ mechanism was developed by performing the N2 capture, activation, and hydrogenation and analyzing the capture capacity charge distribution as well as intrinsic electronic properties combined with machine learning. Finally, a total 5 candidates are selected as high active, high selective, and high stable electrocatalysts for NRR. Our work not only makes a deep understanding of the catalytic behavior of NRR but also develops a new method for designing heteronuclear-like catalysts.