Rational element-doping of FeOOH-based electrocatalysts for efficient ammonia electrosynthesis†

Abstract

Electrocatalysis has been intensively studied in nitrogen (N₂) reduction for its sustainable power and stable catalytic performance, but it is still limited by weak activation of N₂ at the catalytic sites, and the competition from the hydrogen evolution reaction (HER). The special d-orbital electron arrangement of transition metals and the tuning of the microenvironment provide possible strategies to enhance the activation of N₂, while improving the selectivity of the eNRR. Herein, FeO(OH, S) with high spin state and Mo–FeOOH with low spin state were designed around the FeOOH-based catalysts through elemental doping, which could achieve excellent ammonia yield performance of 80.1 ± 4.0 μg h⁻¹ mg_cat⁻¹ (FE 36.9 ± 0.5%) and 86.8 ± 4.1 μg h⁻¹ mg_cat⁻¹ (FE 29.1 ± 0.8%) in 0.1 M LiClO₄ at −0.6 V vs. RHE, respectively, coupled with polyethylene glycol (PEG) to inhibit the HER. Based on theoretical calculations to investigate the adsorption of N₂ on Fe sites, the FeO(OH, S) catalyst has stronger adsorption ability, which may originate from the high spin effect, which means that the more isolated and highly active e_g orbital electrons are more beneficial to realize the electronic feedback mechanism, promoting the d–π* orbital interaction with N₂.