In this study, branched layers formed under various conditions of the electro-chemical etching of aluminum foils were observed. Tunnel clustering arose from the random distribution of high-density tunnels, greatly increasing the thickness of the branched layer; this occurred because the tunnel clustering increased the tunnel width, thereby facilitating electrolyte transport in vertical tunnels. In contrast, tunnel tapering blocked the branched tunnels generated at a certain depth. The thickened branched layer greatly reduced the foil thickness during tunnel widening, considerably reducing the surface area and the specific capacitance obtained for the etched aluminum foil.