Effect of Fe on Ni-based catalysts for electrochemical glycine oxidation to ammonia.

Abstract

Management of excess organic nitrogen in wastewater-derived sludge is increasingly important due to the high cost of sludge disposal. Recovering ammonia from organic nitrogen in sludge through electrolysis offers a promising approach to managing this nitrogen waste stream while generating economic value. Despite the abundance of high-molecular-weight nitrogen compounds in sludge, limited progress has been made in understanding how organic nitrogen can be directly and efficiently transformed into ammonia without microorganisms. Here, we investigate the abiotic reaction pathways for converting glycine into ammonia on nickel-based electrodes. Glycine was chosen as a model sludge-based organic molecule due to glycine's prevalence in activated sludge. We demonstrate, through cyclic voltammetry and X-ray photoelectron spectroscopy, that trace iron present in the electrolyte forms FeNi(OH)2 at the surface of the Ni electrode. As the Fe concentration increases, the product selectivity shifts. Formate selectivity increases from 2.5% to 18% when Fe is present, while glycolate is only measured when Fe is absent (ca. 30% selectivity). Ammonia selectivity increases from 70% to 82% when Fe is present in the electrolyte. Possible pathways for formate, acetate, glycolate, glyoxylate, ammonia, nitrite, and nitrate are discussed. This work aims to inform future catalyst design strategies to steer organic nitrogen conversion to ammonia.