Temperature-Controlled Molten-Salt Treatment of Co-Coated Ni Substrates to Form Porous Electrodes for Alkaline Hydrogen and Oxygen Evolution Reactions

In this work, porous coatings were obtained by Co electrodeposition on Ni, followed by Al deposition and anodic dissolution in a NaCl–KCl–AlF3 melt at 750, 825 and 900 °C. The same Co-coated Ni precursor, applied potentials and 3600 s Al-deposition time were used at all temperatures; the recorded anodic durations and transferred charges differed. SEM observations showed a change from a fine sponge-like surface to a coarser porous network. The estimated layer thicknesses were 8–12, 32–55 and 50–65 µm, respectively. The main XRD reflections were consistent with an fcc metallic phase in Co-rich material, with possible overlapping signals from Ni. Weak additional reflections were tentatively associated with Al-containing phases, but their identity was not confirmed. The highest apparent capacitance estimate was obtained at 825 °C. In 1 M NaOH, the lowest HER overpotentials among the treated electrodes were registered for the 900 °C sample: 109 and 494 mV after 2 h at −10 and −100 mA cm−2. For OER, the lowest apparent Tafel slope was obtained at 750 °C, while the lowest end-of-test overpotentials were obtained at 900 °C. Unmodified Ni showed higher anodic currents in part of the polarisation range, so the treatment did not give a general improvement in OER compared with Ni. No post-measurement iR correction was applied. The measured electrodes showed different trends in morphology, apparent capacitance and HER/OER response; statistical significance and between-electrode reproducibility were not established.