Formation of a Porous Nickel Electrode in Zinc Chloride-Alkali Chloride Melts

When a nickel electrode was kept at a low potential and then at a high potential in a ZnCl2-NaCl (6040 mol %) melt at 450°C, -NiZn alloy was first formed which was then dealloyed to give a porous layer of -NiZn alloy [1]. The present paper describes the mechanism of formation of the porous structure in the anodic dealloying as studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Cathodic and anodic treatments of a nickel electrode were performed in ZnCl2-NaCl (60-40 mol %) and ZnCl2-KCl (60-40 mol %) melts at 300-450°C in a Pyrex cell. Nickel foils (0.1 mm thick) were used as the starting material. The reference electrode was zinc metal in ZnCl2-NaCl (saturated with NaCl) which was separated from the main compartment by a small piece of sodium alumina. Potential E or current i was controlled, and quantities of electricity in cathodic (QC) and anodic (QA) treatments were measured. In order to identify the alloy phases in different stages of dealloying, X-ray diffraction measurements were performed on samples which were cathodically treated at QC = 35 C/cm 2 and then anodically treated at QA = 1-29 C/cm. -, 1-, and -NiZn alloys were found depending upon QA. This suggests that the dealloying occurs stepwise from to 1-, and from 1to -NiZn alloy. An SEM image of a sample containing 1and -NiZn alloys (prepared by QC = 35 C/cm 2 and QA = 20 C/cm 2 at 400°C) exhibited a porous structure which was relatively uniform. EPMA measurement (Fig. 1) also indicated that the Zn and Ni contents were almost constant in the porous layer. These observations suggest that the 1and -phases were uniformly dispersed. It seems that deep pores or crevices are formed in an early stage of dealloying, and therefore the anodic dissolution of zinc occurs easily even from the deepest point of the alloy layer.