N-Doped Porous Carbon Self-Generated on Nickel Oxide Nanosheets for Electrocatalytic N₂ Fixation with a Faradaic Efficiency beyond 30%

The electrocatalysis conversion of N₂ to NH₃ at ambient conditions is promising for achieving clean and sustainable NH₃ production with low energy consumption. However, this process suffers from the low yield rate of NH₃ and low Faradaic efficiency (FE) by the previously reported electrocatalysts. In this work, a nanocomposite of nickel oxide coated with nitrogen-doped porous carbon distributed on graphite paper (N-C@NiO/GP) with remarkable electrocatalytic activity for nitrogen reduction reaction (NRR) is reported. N-C@NiO/GP attains an impressive Faradaic efficiency of 30.43% for NH₃ production, and the NH₃ yield rate reaches 14.022 μg h^-1 mg_cat. ^-1 (1.15 × 10^-10 mol s^-1 cm^-2) at -0.2 V vs the reversible hydrogen electrode. The composite also shows excellent electrocatalytic activity and structure stability with an electrocatalytic period of up to 20 h. When the ¹⁵N₂ is used as the feeding gas, the produced species are ¹⁴NH₄ ⁺ and ¹⁵NH₄ ⁺, which suggests that the reaction follows a Mars-van Krevelen mechanism. The synthesized NH₃ is determined from the introduced N₂ as indicated by no obvious change in N/Ni ratio. The microstructure before and after the durability test is similar and the catalytic performance during the 2 h NRR process, which is repeated six times, is stable. This work not only exploits an excellent electrocatalyst for N₂ fixation to NH₃ but also provides a direction for the inexpensive preparation of highly active and stable transition metal oxide catalysts.