Unraveling Electrochemical Glycine Conversion Pathways for Ammonia Recovery from Organic Waste

Abstract

Electrochemical conversion of nitrogen-containing organics in sludge offers a route for ammonia recovery but is challenged by variable sludge composition. Glycine, abundant in municipal wastewater and structurally simple, provides a model system to benchmark nitrogen/carbon product distributions and electrode stability. We report a coordinated cross-institution effort designed to elucidate electrochemical glycine conversion to ammonia and its reaction pathways. In 0.1 M KOH and 0.1 M glycine, ammonia was produced favorably under oxidative potentials (>1.60 VRHE) rather than reduction at H2 evolution potentials (< -0.40 VRHE), where Ni exhibited the lowest overpotentials than Au and Pt electrodes. Ammonia was the dominant nitrogen product (~70%) at potentials as high as 2.00 VRHE on Ni. Unfortunately, the Faradaic efficiency (FE) of ammonia remained moderate at 23.0 ± 2.5 %, while the rest can be attributed to oxidized nitrogen (NO2/3-, ~24%), substantial Ni dissolution (up to 25%), and parasitic processes (~25%) such as O2 evolution. Carbon products quantified by high-performance liquid chromatography, ion chromatography, and 13C nuclear magnetic resonance revealed a diverse mixture, including glycolate, glyoxylate, formaldehyde, cyanide and formate (FE = ~6% collectively), indicating multiple pathways involving C=N and C-C cleavage in glycine activation. Based on these data and reaction thermodynamic analyses, a unified reaction mechanism framework was proposed. The nitrogen and carbon selectivity was found to be highly sensitive to experimental parameters including stirring, electrode size, cell configuration and potential pulsing, highlighting the strong influence of local reaction environments and opportunities to steer its conversion pathways. Practices and common pitfalls were recommended to increase reproducibility and accelerate mechanistic understanding for ammonia recovery from organic waste electrolysis.