Composites of Nickel Foam and Transition Metal‐Loaded Carbon Nanotubes as Binder‐Free Electrocatalysts for Urea Electro‐Oxidation

Bifunctional electrodes that catalyze both the urea oxidation reaction (UOR) as well as the oxygen evolution reaction (OER) are particularly relevant for integrated systems that simultaneously produce hydrogen and remediate urea‐rich wastes. To enhance the OER and UOR performance, Ni foam (NF) electrodes were modified by binder‐free deposition of acid‐modified multiwalled carbon nanotubes (MWCNTs) and subsequent electrodeposition of transition metals (Ni, Fe, and Mn). At 10 mA/cm ² , the lowest required potential for UOR was found for NiFeMn/NF (+1.39 V vs RHE) followed by NiFeMn‐CNT/NF (+1.41 V) and NF (+1.45 V). The electrodes contained higher Fe and lower Mn contents than expected, which negatively affected the electrode activity. While the incorporation of MWCNTs increased the electrocatalytically active surface area, any synergistic effects between MWCNTs and Ni on UOR were absent. This was ascribed to the poor contact between MWCNTs and the Ni foam surface and led to a negative synergy between the MWCNTs and Ni. The decreased long‐term stability was ascribed to a higher charge transfer resistance towards UOR as confirmed by electrochemical impedance spectroscopy. While the performance of the NiFeMn/NF and NiFeMn‐CNT/NF electrocatalysts is promising, further improvements in activity and stability are necessary to warrant their industrial application.