Synergistic nickel phthalocyanine integration on carbon nitride for improved selective nitrogen reduction to ammonia

Abstract

As a sustainable alternative to the energy-intensive ammonia synthesis through Haber–Bosch method, the electrochemical nitrogen reduction reaction (eNRR) offers a promising sustainable green approach for converting abundant atmospheric N₂ into ammonia. The design and development of transition metal-based composite systems have received significant attraction for eNRR applications due to their abundant active sites, enhanced charge transfer capabilities, and high selectivity. Herein, we report a two-dimensional carbon nitride (C₃N₄)-supported nickel phthalocyanine (NiPc) composite system (NiPc_C₃N₄) that exhibits outstanding performance for the nitrogen reduction reaction (NRR), achieving an ammonia yield rate of 460 ± 14 μg h⁻¹ mg⁻¹_cat and a faradaic efficiency of 43 ± 1% at −0.3 V vs. RHE under ambient conditions. Our work demonstrates, through density functional theory (DFT) calculations and experimental studies, that NiPc anchored on C₃N₄ significantly facilitates the adsorption and activation of chemically inert nitrogen molecules. The enhanced catalytic activity of the composite system may be attributed to the re-distribution of charges across the NiPc, which adjusts the valence orbital of the Ni center due to the presence of the 2D layer of C₃N₄. This process substantially reduces the energy barrier necessary for the crucial dissociation of inert N₂, hence enhancing N₂ reduction efficiency markedly.