Fabrication and Supercapacitive Properties of Thick Nanostructured Anodic Films on 304 Stainless Steel

Abstract

Supercapacitors, also known as electrochemical capacitors, are new energy storage components between traditional capacitors and batteries, attracting a great deal of attention in recent years [1]. However, the main problem facing supercapacitors electrode materials is the low energy density. For both EDLCs and pseudocapacitors, electrodes with large surface area, good electrical conductivity and high-mass loading active materials are extremely desired to enhance the capacitance and further storage energy density [2]. Recently, porous oxide films formed on stainless steel (SS) foils attracted attention as promising nanoporous active material due to theoretically high specific capacitance, however, the nanoporous oxide films on SS foils by galvanostatic anodization typically suffer from low capacitance due to the limited layer thickness [3-4], which greatly restricts the mass of active materials thus large number charge storage sites. Therefore, ultra-thick nanostructured anodic films are highly desired to develop high-performance supercapacitors. In the experiments, the SUS 304-type SS foils containing C (0.07 wt%), Cr (17.01wt%), Ni (8.02%) and Fe balance was used as the anodizing substrate. Prior to anodization, all samples were ultrasonically cleaned in acetone and distilled water respectively for 15 minutes followed by air drying. Then, the anodization of SS foils was carried out in ethylene glycol containing 0.1 M distilled water and 0.1 M NH4F using a two-electrode cell. The SS foils worked as anode and graphite plate (2 mm in thickness) as cathode. Both were immersed in as-made electrolyte (10 mm in depth) parallel with each other. The distance between the two electrodes was fixed at 3.5 cm. The potential of 50 V was employed for 1, 2 and 3 h, respectivel. During the anodization process, the long-term anodization was kept by slightly decreasing electrolyte temperature to rescrict the temperature rise. The as-fabricated samples were soaked in 99.9% ethanol for 1 h followed by annealing at 500℃ in air for 3h. Fig.1 presents the SEM images of self-organized porous anodic films formed on the surface of SS foils by constant potential anodization of 50 V for 1, 2 and 3 h, respectively. The formed self-organized pores all exhibited a quasi-honeycomb structure with pore diameters mainly ranging from 45 to 60 nm as shown in Fig. 1a. The thickness of 8.9, 16.8, 26.9 μm anodic layer was respectively formed at 50 V for 1, 2, 3 h as shown in Fig. 1b-d. The achieved maximum thickness is over 7 times higher than the reported value obtained from galvanostatic anodization [5]. However, some cracks were observed across the thick anodic films, which were possibly formed by the residual stress produced during film growing process and further amplified during subsequent annealing process. Fig. 2 (a), (b) and (c) show the C-V curves of the anodic layers formed at 50 V for 1, 2 and 3 h at different scan rates, respectively. Clearly, the C-V curves exhibit a nearly symmetric CV plots when tested as a binder-free electrode, indicating good reversibility of faradaic redox reactions. The anodic layers exhibit the specific area capacitance ~ 81, 157 and 215 mF cm-2 at the current density of 1 mA cm-2. Furthermore, the thickest one surprisingly exhibited the higher area capacitance ~ 215 mF cm-2 at 1 mA cm-2 and ~ 128 mF cm-2 at 4 mA cm-2, and the capacitance still retained 63.9% of its initial value after 1000 cycles. The obtained highest capacitance is 3 times higher than the reported value obtained from galvanostatic anodization [5]. In summary, ultra-thick nanostructured anodic films have been fabricated on 304 stainless steel via long-term constant-potential anodization instead of galvanostatic anodization. The long-term anodization was kept by slightly decreasing electrolyte temperature to rescrict the temperature rise during anodization process. When tested as a binder-free electrode, the formed anodic layers exhibit the highest area capacitance ~ 215 mF cm-2 at 1 mA cm-2, and the capacitance still retained 63.9% of its initial value after 1000 cycles.