Self-Assembled Monolayers of Alkyl and Aryl Thiols on Copper for the Selective Electrochemical Reduction of Nitrogen to Ammonia

Electrochemical nitrogen reduction (eNRR) offers a sustainable alternative to the energy-intensive Haber-Bosch process, yet it faces challenges related to selectivity and efficiency due to the inertness of N2 and competition with the hydrogen evolution reaction. In this work, we investigate the use of thiol-based self-assembled monolayers (SAMs) as molecular overlayers on Cu catalysts to improve ammonia production via eNRR. Ten thiol SAMs, six alkyl and four aryl, were systematically evaluated for their influence on Faradaic efficiency (FE) and NH3 production rate. Among the SAMs tested, thiophenol-modified Cu exhibits the highest FE (∼40%) and stability over multiple chronoamperometry cycles. Cyclic voltammetry results demonstrate that aromatic SAMs, potentially through cation–π interactions, promote electron transfer during the eNRR. Additionally, the eNRR was performed using 15N-enriched gas and the resulting 15NH3 was detected using a GC–MS derivative method, which confirmed that the NH3 originates from the N2. This study highlights the potential of SAM engineering to tailor catalytic surfaces and improve performance for ambient NH3 synthesis.