Unveiling the Role of Iron in the Nickel-Catalyzed Urea Oxidation Reaction

The electrochemical urea oxidation reaction (UOR) on Ni has recently attracted much attention as part of the global effort to produce hydrogen from waste. Fe addition to Ni (hydro)oxides results in a significant enhancement of the rate of the oxygen evolution reaction (OER), and a similar role of Fe in the UOR has often been claimed. In this work, we explore the influence of Fe3+ ions in the electrolyte on the alkaline UOR on Ni. To this end, we systematically increase the concentration of the Fe3+ ions in NaOH and incorporate them in the Ni(OH)2 layer grown on the electrode by electrochemical potential cycling. We then determine the fraction of Fe on the Ni(OH)2 surface by X-ray photoelectron spectroscopy and investigate the UOR by cyclic voltammetry and Fourier transform infrared spectroscopy (FTIRS) depending on the Fe fraction. We conclude that contrary to the OER, the presence of Fe is detrimental for the UOR. We then discuss the influence of the potential and Fe on the product distribution revealed by FTIRS.