Hydrophobicity modulation on a ferriporphyrin-based metal–organic framework for enhanced ambient electrocatalytic nitrogen fixation

Abstract

Electrocatalytic N2 fixation represents an energy-efficient and long-term sustainable approach, which can convert N2 to NO3– or NH3 via the electrochemical N2 oxidation reaction (NOR) or N2 reduction reaction (NRR). However, the inert N2 molecule, low activity of electrocatalysts, and predisposed competitive reactions result in the poor yields and Faradaic efficiencies of N2 fixation reactions, which greatly restrict the application of such green synthesis technology. In this work, a molecular-level post-modification strategy has been explored to integrate diverse alkyl chains on a ferriporphyrin-based metal–organic framework (MOF) PCN-222(Fe), which provides adjustable hydrophobicity and highly dispersed active sites. The increased lengths of alkyl groups can gradually improve the hydrophobicity of decorated MOFs, which effectively suppress the competitive reactions and boost the electrocatalytic NOR and NRR performances. Significantly, the highest Faradaic efficiency of 70.7% so far and a state-of-the-art NO3− yield of 110.9 μg h–1 mgcat.–1 can be achieved for NOR, which are attributed to the synergistic effect of FeN4 active sites, high porosity, and strong hydrophobicity for n-octadecylphosphonic acid (OPA) decorated PCN-222(Fe).