Detection of a Nickel-Oxide Nanolayer During Mild Acid Treatment of Nickel Foam and Its Effect on Alkaline Oxygen Evolution and Ammonia Oxidation Reactions

Despite nickel foam (NF) being often used as electrode support, the commonly adapted pretreatment strategy (high acid concentration and ultrasonication) may result in unnecessary surface oxidation, which may interfere with the electrochemical performance of the actual catalyst. Herein, acid-pretreatment of NF with 0.1 M HCl and ultrasonication (100 W ultrasonic power, 40 kHz frequency, 30 °C bath temperature) for a limited time (<20 min) is shown to be safe to avoid the formation of NiO nanoparticles. Although the field emission scanning electron microscope (FESEM) could not perceive any detectable corrosion on the acid-treated NFs, the Raman study has identified characteristic bands (1050-1500 cm^-1) of NiO on the NFs with a treatment time of >20 min. Ni 2p X-ray photoelectron spectra (XPS) of raw NF and NF50 (treatment time: 50 min) also depict the formation of a NiO nanolayer, which enhanced the electrochemical surface area (ECSA) from 0.09 for raw NF to 0.56 cm² for NF75 (treatment time: 75 min). During the oxygen evolution reaction (OER) study with the pretreated NFs, NF50, and NF75 electrodes remain catalytically most active after activation through ten successful cyclic voltammetric (CV) cycles within 1.0 to 1.9 V (vs RHE). Spectroscopic and microscopic studies have revealed the formation of NiO(OH) nanoparticles as the OER active species for NF50 and NF75. The NiO(OH) nanoparticles on NF50/NF75 facilitate the electrokinetics, as depicted by the low Tafel slope (51 mV dec^-1) and R_ct values. During the ammonia oxidation reaction (AOR) with the pretreated electrodes, the NF10 (treatment time: 10 min) shows the best activity with a low onset potential of 0.47 V (vs Hg/HgO). The surface of NF5/NF10, being more metallic, provides a greater scope of NH₃ adsorption, which is crucial for AOR, while the comparatively inferior AOR activity of the NF20-NF75 can be due to in situ formed NiO or NiO(OH) nanoparticles.