Electrodeposition of Void-Free Nanostructured Aluminum from AlCl3–Diglyme Electrolyte

Abstract

Since Al has a lower redox potential (–1.68 V vs. SHE), the use of aqueous baths fails to electrodeposit less noble Al. The development of AlCl3–based nonaqueous electrolytes attracts intense interest in the field of Al and its alloys electrodeposition. In our previous studies on AlCl3–glyme electrolytes, AlCl3–G2 has been successfully used for Al and Al–CNTs electrodeposition.The nature properties of AlCl3–G2 electrolyte will be influenced by amount of AlCl3. AlCl3–G2 electrolyte has been investigated via NMR, Raman, ionic conductivity, dynamic viscosity and cyclic voltammetry. AlCl3–G2 electrolyte contains ionic complex ([AlCl2(G2)2]+ and AlCl4 –) and neutral molecule (AlCl3·G2 and G2). Increasing the molar ratio (x) of AlCl3 from 0.1 to 0.2 and 0.4, the dynamic equilibrium towards the direction of forming ionic species. The AlCl3–G2 electrolyte with x = 0.4 has a lower conductivity (2.6 mS cm–1) than that of molar ratio x = 0.2 at 25oC, due to an increased viscosity (15.2 mPa s). However, it exhibits the best ionicity and can be used to electrodeposit void-free Al deposit with smooth appearance. As shown in Figure 1(a) and (b), the Al deposit from x = 0.4 electrolytes is accumulated by fine particles, giving rise to a compact and smooth surface with metallic luster. In contract, x = 0.2 electrolyte gives a flake-like Al deposit (Figure 1(e) and (f)), resulting in a rough surface with black appearance. In addition, adjacent Al flakes grow at a certain angle indicated by the yellow arrows shown in Figure 1(h), thus, the voids pointed by green circle are remained in the deposits during the continuous electrodeposition.The corrosion behaviour of Al deposits have been compared using EIS and cyclic polarization techniques. The corrosion behaviour shows a strong dependence on the crystalline size, orientation and compactness. Al deposit obtained from x = 0.4 electrolyte has a trait of nanosized crystalline (about 30 nm), <200> preferential crystal face and a compact microstructure. It accounts for an improved resistance to free corrosion and pitting corrosion in 3.5 wt% NaCl aqueous solution. By contrast, the preferential crystal faces are <111> and <220> for Al deposits obtained from x = 0.1 and 0.2, respectively. Besides, Al deposits from x = 0.1 and 0.2 have a much loose structure compared to that from x = 0.4, thereby leading to a more negative E corr and E pit. Figure 1