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

• Published in: Applied catalysis. B, Environmental Vol. 328; p. 122485
• Main Authors: Kim, Min-Cheol, Chung, Jiyong, An, Tae-Yong, Lee, Jaeyoung, Han, Mi-Kyung, Lee, Shinbi, Choi, Wonyong, Kim, Jung Kyu, Han, Sang Soo, Sim, Uk, Yu, Taekyung
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.07.2023
• Subjects: Carbon doping; Electrochemical ammonia production; Nickel sulfide; Nitrogen reduction reaction; Thiourea; Thiourea; Carbon doping; Nitrogen reduction reaction; Nickel sulfide; Electrochemical ammonia production
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2023.122485

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.