Abstract:

The development of efficient synthetic routes for ammonia (NH₃) production is the cornerstone of the modern industrial processes and human survival. Owing to the chemical inertness of nitrogen, the current ammonia industry suffers from high energy consumption and high CO₂ emission. Electrochemical nitrogen reduction reaction (NRR) provides a promising alternative to the energy-intensive Haber-Bosch (HB) process, enabling green and sustainable NH₃ production. However, a low NH₃ yield and limited energy conversion efficiency due to the chemical inertness of N₂ and competitive hydrogen evolution reaction (HER) are still critical challenges in artificial nitrogen fixation using the electrochemical NRR. Herein, we report a hole-enriched P-doped carbon (PC)-supported Zn₃(PO₄)₂/Zn₂P₂O₇ nanocomposite (h-PC/Zn₃(PO₄)₂/Zn₂P₂O₇) for efficient electrocatalytic conversion of N₂ to NH₃ in both acidic and neutral media. Remarkably, the unique hierarchical porous structure of the h-PC/Zn₃(PO₄)₂/Zn₂P₂O₇ catalyst improves the surface roughness and facilitates the diffusion of N₂ within the catalyst layer, thereby prolonging the residence time of N₂ and improving the utilization of active sites. The uniform distribution of multiple components modulates the electronic structure of the active sites and optimizes the adsorption behavior of various reaction intermediates, enhancing the intrinsic activity of the catalyst. Benefiting from the porous structure and multicomponent active sites, including the Zn species and PC, the h-PC/Zn₃(PO₄)₂/Zn₂P₂O₇ achieves an excellent NRR performance with an NH₃ yield rate of 38.7 ± 1.2 μg∙h⁻¹∙mg_cat⁻¹ and Faradaic efficiency (FE) of 19.8% ± 0.9% at −0.2 V vs. reversible hydrogen electrode (RHE) in 0.1 mol∙L⁻¹ HCl electrolyte. Moreover, it delivers a high NH₃ yield rate of 17.1 ± 0.8 μg∙h⁻¹∙mg_cat⁻¹ with an FE of 15.9% ± 0.6% at −0.2 V vs. RHE in 0.1 mol∙L⁻¹ Na₂SO₄ solution, which is superior to those of PC/Zn₃P₂, C/ZnO, and many other non-noble-metal-based electrocatalysts. Ex situ X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) studies were conducted to monitor the changes in the composition and structure of h-PC/Zn₃(PO₄)₂/Zn₂P₂O₇ after being used in NRR. In particular, a new signal of N appeared in the XPS profile after NRR, confirming the occurrence of NRR. This work provides a new strategy for synchronously constructing mass transfer channels and coupling different active sites to synergistically enhance the NRR activity and selectivity of a catalyst, which is of great significance in progressing the industrialization of green ammonia production.

Key words: Nitrogen reduction reaction, Electrocatalyst, Porous structure, Multiple active center, Synergistic effect