Nanoreactor Based on Cyclodextrin for Direct Electrocatalyzed Ammonia Synthesis

Abstract

The high-efficiency transition metal-free electrocatalytic nitrate reduction reaction (NO₃^-RR) for ammonia synthesis has received more attention because of its green and environmentally friendly characteristics. Here, we report an efficient electrochemical NH₃ synthesis directly from purely organic macrocyclic compounds α-, β-, and γ-cyclodextrins (CDs)-catalyzed transition metal-free electroreduction of nitrate under ambient conditions. In comparison with α-, and β-CDs, parent γ-CD presented uncommon catalytic performance with a relatively higher NH₃ yield that can reach up to 2.28 mg h^-1 cm^-2 with a Faradaic efficiency (FE) of 63.2% at -0.9 V versus a reversible hydrogen electrode in alkaline medium, and the potassium ion-coordinated γ-CD complex can achieve a maximum NH₃ production rate up to 4.66 mg h^-1 cm^-2 with an NH₃ FE of 79.3%. Further comparison with permethyl-γ-CD, d-glucose, and poly(vinyl alcohol) for the NO₃^-RR indicated that the typical torus-shaped cyclic conformation and edge hydroxyl groups of parent CDs play important roles in the electrocatalytic process. The K⁺-mediated 3D γ-CD-K⁺ frameworks containing six CDs as nanoreactors greatly strengthen the enrichment effect of nitrate through hydrogen-bonding interaction and electrostatic interaction and promote the mass transfer, thus leading to the efficient NO₃^-RR in an alkaline electrolyte. This work provides a convenient, green, and economic method for high-performance NO₃^-RR, which has potential applications in the fields of environment, energy, and industry.