Facile electrochemical preparation of NiFeP submicron-spheres as an efficient electrocatalyst for the electrochemical hydrogenation of 4-nitrophenol at neutral pH

Electrochemical hydrogenation with water as the proton source serves as a clean and sustainable alternative to the removal and upcycling of 4-nitrophenol (4-NP), one of the major toxic pollutants commonly present in industrial wastewater effluents, into value-added chemicals (e.g., 4-aminophenol), but the development of high-performance electrocatalyst for electrochemical hydrogenation of 4-NP (e-NPR) at neutral pH remains challenging. Herein, we represent the facile electrochemical synthesis of micro-structured nickel-iron phosphide (microNiFeP), which acts as an efficient electrocatalyst in catalyzing hydrogen evolution reaction (HER) and e-NPR at neutral pH. The effects of electrosynthetic conditions on the physicochemical and electrocatalytic properties of microNiFeP were investigated and optimized. microNiFeP, prepared with optimized conditions, consisted of submicron-sized spheres (diameter: ∼200 nm) with wrinkled surface and exhibited promising HER activity, reaching a turnover frequency (TOF) of 47.18 ± 4.10 h-1 at - 0.2 V vs. reversible hydrogen electrode (RHE) at pH 7 in the absence of 4-NP. In the presence of 4-NP, however, microNiFeP exhibited high e-NPR activity and selectivity against HER. Notably, microNiFeP showed a TOF of 5.12 ± 0.48 h-1 for e-NPR at 0.0 V vs. RHE and enabled conversion of 89.6 ± 3.3 % with a Faradic efficiency of 81.98 ± 5.08 % for 4-aminophenol production after 8-h-controlled-potential electrolysis at - 0.2 V vs. RHE. Mechanistic studies reveal that e-NPR at microNiFeP involved the transfer of 5.3 ∼5.8 electrons and may involve the formation of 4,4'-dihydroxyazobenzene as one of the side products.