Paradox of thiourea: A false-positive and promoter for electrochemical nitrogen reduction on nickel sulfide catalysts

Electrochemical nitrogen reduction reaction (NRR) is an environmentally friendly process for ammonia synthesis. An unusually high NH3 production rate (86.91 ± 9.29 μg h−1 cm−2) and Faradaic efficiency (F.E.) (57.17 ± 4.14 %) was observed in electrochemical tests on NiS catalysts synthesized using thiourea (TU). Through joint experiment-theory work, we demonstrate the paradoxical catalytic effect of TU as a false positive from TU-derived thiocyanate reduction and a promoter via the generation of catalytically active carbon sites after TU reduction on the NiS surface (with an effective NH3 rate of 52 ± 5.57 μg h−1 cm−2). TU reduction leads to the substitution of an S atom to a reactive C atom with fewer valence electrons, facilitating a more facile NRR pathway. This study presents a new possibility for NRR catalysts based on metal-nonmetal compounds (e.g., sulfides, chalcogenides, or nitrides) by nonmetal dopants with fewer valence electrons to create new catalytically active sites. [Display omitted] •TU-NiS catalysts show an excellent NH3 production rate and F.E. at −0.4 V vs. RHE.•Half of the produced NH3 is a false positive from surface SCN reduction.•Even without the false positives, TU-NiS significantly promotes the NRR.•DFT results show that active C sites from surface SCN reduction promotes the NRR.