Ultrasmall Hematite Nanocrystals Interfaced Multilayered Carbon Onions To Synergistically Promote Selective Nitrogen Reduction to Ammonia under Ambient Conditions

Simultaneous adsorption and activation of N 2 molecules on the surface of an electrocatalyst are crucial for electrochemical N 2 fixation. To harness rapid electron transfer, a higher surface area characteristic of onion-structured carbon (OLCs), and the inherent oxygen deficiency in Fe 2 O 3 nanocrystals that provide optimal active sites for N 2 activation, ultrasmall Fe 2 O 3 nanocrystals (3.57 ± 0.03 nm) decorated OLCs (45.98 ± 1.08 nm) with varied loading concentrations have been synthesized. The FTIR peak detected at 546 cm −1 signifies the formation of an Fe−O−C bond, suggesting a chemical interaction between α-Fe 2 O 3 and OLC. Furthermore, the increased I D / I G ratio arising from the enhanced sp 3 hybridization, along with HRTEM analysis, confirms their synergistic coupling. The optimized α-Fe 2 O 3 coupled OLC exhibits N 2 reduction to NH 3 (NRR) activity with 2- and 10-fold enhancement than that of bare OLC and bare α-Fe 2 O 3, respectively. The NRR proceeds via an associative pathway, as revealed by the formation of N−H bonds. Moreover, isotopic nitrogen ( 15 N) labeling experiments confirm that the produced NH 3 originates solely from the purged N 2 gas while ruling out N 2 contributions from the catalyst or any other sources. Further, there is no significant change in current density over 24 h, indicating good stability of the catalyst. The boost in the NRR activity is attributed to synergetic properties: increased electrochemical surface area (0.24 mF cm −2 ), electron density (1.25 × 10 25 cm −3 ), reduced electron transfer resistance, and negative flat band potential enabling downward bending closer to the conduction band for improved charge separation, arising upon coupling Fe 2 O 3 with OLC.