Potential distribution and anode dissolution in gel electroplating of Ni films using chloride- and sulfate-based electrolytes

Abstract

While electroplating is widely used for preparing soft magnetic films, reducing its environmental impact remains a key challenge. As an eco-friendly alternative, we recently developed a gel plating method for Ni films. In the present study, chloride- and sulfate-based gel electrolytes were compared to clarify the origin of the high cell voltage and low current efficiency (C.E.) observed during gel plating. The chloride-based bath reduced the cell voltage from over 3.0 V to approximately 1.5 V and generally exhibited higher current efficiency, particularly at lower Ni salt concentrations and during prolonged plating. To investigate the origin of this voltage difference, the local potentials and potential difference within the gel were evaluated together with the mass changes of the Ni anode and plated film. The measured potential difference between two positions in the gel remained approximately 0.07–0.08 V in the chloride bath, whereas it increased to approximately 0.25 V in the sulfate bath. Even in the sulfate-based bath, the measured potential difference within the gel was at most approximately 0.25 V, which was much smaller than the total cell voltage of over 3 V. After 30 min of plating, the anode mass loss and deposit mass were 2.62 and 2.61 mg, respectively, in the chloride bath, whereas they were 0.60 and 1.40 mg in the sulfate bath. These results indicate that the high cell voltage of the sulfate-based gel bath cannot be explained solely by the measured potential difference across the probed gel region and suggest that inhibited Ni anode dissolution and electrode/gel interfacial phenomena may make important contributions to the high cell voltage. XRD and scanning electron microscope analyses further confirmed the formation of crystalline Ni films from both baths without a qualitative change in the overall surface morphology. These findings clarify the coupled influence of Ni-salt chemistry, Ni anode dissolution, and the confined gel environment on Ni gel electroplating.