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 compositional complexity. Glycine, abundant in municipal wastewater and structurally simple, provides a model system to benchmark nitrogen and carbon product distributions and electrode stability. Here, we report a coordinated cross-institutional study to elucidate glycine electro-oxidation pathways to ammonia. In alkaline electrolyte, ammonia was produced preferentially under oxidative potentials (>1.60 V_RHE), rather than reductive conditions (<−0.40 V_RHE), with Ni exhibiting lower overpotentials than Au and Pt. At 2.00 V_RHE, ammonia was the dominant nitrogen product (∼70%), but with moderate Faradaic efficiency (23.5 ± 2.5%), accompanied by NO₂⁻/NO₃⁻ (∼24%), Ni dissolution (∼12%), and O₂ evolution (∼40%), collectively closing the charge balance. Carbon analysis using HPLC, IC, and ¹³C NMR revealed a mix of glycolate, glyoxylate, formaldehyde, cyanide, and formate (∼20% carbon, 6% Faradaic efficiency), with the remainder as CO₂, indicating concurrent C–N and C–C cleavage pathways. These data, combined with thermodynamic analysis, inform a unified reaction framework and reveal C–N cleavage as the rate-limiting step. Furthermore, the ammonia-dominated production and coupled Ni²⁺ dissolution are correlated across different amino acids, highlighting Ni-complexation as a possible underlying mechanism favoring ammonia production. This work establishes a product-resolved framework and assesses experimental parameters (stirring, cell geometry, potential pulsing) to improve reproducibility and advance mechanistic understanding of ammonia recovery from organic nitrogen electrolysis.