Current oscillations in the course of birnessite electrodeposition as related to resulting microstructure: experimental manifestations and qualitative hypotheses

We report oscillations of current which accompany anodic deposition from Mn(II) solutions in a neutral acetic buffer and also cathodic birnessite deposition from alkaline permanganate solutions. We demonstrate that for both processes, oscillations appear at high enough overpotentials. The current oscillations are affected by solution convection. The results of rotating disc electrode experiments favor mixed control, with higher diffusion contribution in case of cathodic deposition. Electron microscopy combined with electron and X-ray diffraction and coulometric analysis is applied to assign oscillations to certain morphological features of the deposits. A pronounced difference in microstructure is found for anodic and cathodic deposits formed under oscillating growth conditions: "anodic" birnessite consists of parallel layers of small crystals, whereas "cathodic" birnessite is globular. This difference is interpreted with account for specific crystallographic features. Namely, the more ordered birnessite lattice formed by reduction of permanganate favors a preferentially lateral growth of thin lamellas, in contrast to birnessite having a pronounced interplane distortion, formed in the course of anodic deposition. We assign the periodic current decrease to diffusion limitations in growing porous layers and assume that the subsequent current increase in each period corresponds to dendrite-like growth of a low number of crystals located in the outer diffusion layer.