A key factor in determining the effectiveness of etchants for electrochemical surface enlargement of Ta foil for electrolytic capacitor

[Display omitted] •Understanding the differences between Cl- and Br- mechanisms from multiple processes of pitting.•Although Cl- is more aggressive towards Ta, high SSA was obtained in the less aggressive NaBr solution.•The re-passivation of Ta in etchant plays a decisive role in obtaining high SSA.•Br--containing methanol solution is suitable as a candidate corrosive agent for the preparation of high specific capacitance tantalum anodes.•A point of view contrary to conventional perception is presented. For electrochemical etching, it is often assumed intuitively that the high specific surface area (SSA) is largely dependent on the aggressiveness of the corrosive agent. Today, however, we present a different perspective. In this study, we combine theoretical calculations and electrochemical experiments to fully understand the mechanisms of Cl- and Br- in the three stages of the tantalum (Ta) pitting process, including passivation film breakdown, Ta anodic dissolution and re-passivation. The results show that both Cl- and Br- cause the breakdown of the passivation film and the dissolution of Ta atoms, with the former having a more pronounced effect. However, MD simulations show that the re-passivation of Ta occurs only in Br- containing media but not in Cl- containing media. This means that Ta is protected by adsorption of dissolved oxygen while being destroyed by Br-, but is only dissolved in NaCl solution. The same result is observed in our electrochemical experiments. In addition, the specific capacity of etched Ta foil in NaBr solution is as high as 325μF/cm2 while it is only 48μF/cm2 in NaCl. It is precisely the protective rather than the destructive effect of the etchant that is more important for obtaining high SSA.