Reconsidering Water Electrolysis: Producing Hydrogen at Cathodes Together with Selective Oxidation of n-Butylamine at Anodes

Electrocatalysis for the oxygen evolution reaction (OER) is of great interest for improving the effectiveness of water splitting devices. Decreasing the anodic overpotential and simultaneously changing the anodic reaction selectively to produce valuable chemicals instead of O₂ would be a major improvement of the overall cost efficiency. Some amines, when present in aqueous electrolytes, were recently shown to change the selectivity of the anodic process to generate H₂O₂ rather than O₂ on MnO_x at pH 10. This results in unusually high apparent “anodic activities”. In this work, industrially relevant OER catalysts, oxyhydroxides of cobalt (CoO_x), nickel–iron (NiFeO_x), and nickel (NiO_x) all show more pronounced effects. Moreover, as anodes they also selectively catalyzed the production of nbutyronitrile from n-butylamine at higher pH as an easily retrievable valuable product. The pH dependence of the activity was investigated at pH values closer those at which alkaline electrolyzers operate. The highest activities were observed for NiO_x thin-film electrodes at pH 12 in the presence of 0.4 m n-butylammonium sulfate, without poisoning the active sites of Pt electrocatalysts at the hydrogen evolution electrode. ¹H NMR spectroscopy showed that n-butylamine is selectively oxidized to n-butyronitrile, an organic chemical with numerous applications. However, measurements using rotating ring-disk electrodes indicated that some H₂O₂ is also generated at the surface of the oxide anodes.