Synthesis and Evaluation of Layered Ni–Co and Ni–Co–Ni Electrodes Modified by Molten–Salt Al Deposition/Dissolution Technique for Electrochemical Applications

Porous bilayer Ni–Co and sandwiched Ni–Co–Ni electrodes were fabricated by combining aqueous electrodeposition with high-temperature molten-salt Al deposition and subsequent electrochemical dissolution in NaCl–KCl–AlF3 melt at 750 °C. The study aimed to determine how the initial layer architecture controls phase evolution, porous structure formation, and hydrogen evolution performance in alkaline media. SEM/EDS and XRD analyses showed that the two electrode designs followed different reaction pathways during molten-salt treatment. In the Ni–Co system, Al reacted predominantly with Co, leading mainly to Co–Al intermetallic formation and, after dissolution, to a highly open coral-like porous network. In contrast, the Ni–Co–Ni architecture promoted mainly Ni–Al phase formation and produced a more compact porous surface with a Ni-rich outer layer. Despite these morphological differences, both layered porous electrodes outperformed untreated Ni and porous Ni in 1 M NaOH. At −0.6 V vs. RHE, porous Ni–Co and NiCo–Ni reached current densities of −162 and −141 mA·cm−2, respectively, compared with −87 mA·cm for porous Ni and −45 mA·cm for flat Ni. The Ni–Co–Ni sandwiched electrode showed the most favourable HER kinetics and benchmark performance, with the lowest Tafel slope (111 mV·dec) and the lowest potentials at −10 and −100 mA·cm (−0.132 and −0.556 V, respectively). These results demonstrate that the electrocatalytic response of molten-salt-derived porous Ni-based electrodes is governed not only by porosity development but also by the spatial arrangement of metallic layers prior to Al infiltration and dealloying.