Electrochemical Effects of Annealing Temperature on Anodic 304 Stainless Steels
Authors: Wang Yingge , Li Gang , Wang Kaiying , Chen Xuyuan 
Published: February 2020

Supercapacitors, as an energy storage device, have attracted growing interests [1]. Recently, iron oxides (a-Fe2O3 and Fe3O4) have been demonstrated to present comparable pseudocapacitive properties [2]. It was further observed that nanoporous oxide layers have been obtained on various SS substrates by electrochemical anodization process. They can provide more reaction sites due to high surface area, and acceptable electron conductivity due to multiple valence states of formed Fe3O4 for potential supercapacitor electrodes. Meanwhile, it has been envisaged that annealing at various temperatures may induce morphologgical modification and crystal-phase transition, which will strongly influence the supercapacitance behavior [3]. However, the effects of annealing temperature on supercapacitance behavior of anodic oxide films have not been well studied.
In the experiments, SUS 304-type SS foils were used as the anode for prorous formation. 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 as cathode. 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 200-600℃ in air for 3h, respectively.
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 h, respectively annealed at 200-600℃ in air. The self-organized pores exhibited a quasi-honeycomb structure with pore diameters ranging from 45 to 60 nm as shown in Fig. 1a. As shown in Fig. 1b, the thickness of 8.12 μm anodic layer was formed at 50 V for 1h and some cracks were further clearly observed across the anodic films by the residual stress produced during film growing process and further amplified during subsequent annealing process. Remarkably, the decrease of the pore size from the top to the bottom of the porous anodic film was clearly observed by SEM pictures shown in Fig. 1c, the pore size varies from 53±10nm (top) to 33±7nm (bottom). An anodic etching model will be applied to explain the phenonomenon. XRD spectra further reveals the annealed anodic films own crystalline structures, which are predominantly composed of three phases of hematite (Fe2O3), eskolaite (Cr2O3) and spinel (Fe3O4 and FeCr2O4), which was clearly increased with the increasing annealing temperature.
Fig. 2 (a) shows the C-V curves of the anodic layers formed at different annealing temperature. When tested as a binder-free electrode, all C-V curves exhibit a nearly symmetric CV plots, indicating good reversibility of faradaic redox reactions. Clearly, the anodic layer annealed at 400℃ exhibits the maximum specific area capacitance among the five samples, which can be explained by the increased amount of Fe2O3
and Fe3O4. Fig. 2 (b) shows the C-V curves of the anodic layers (400℃) recorded at the scan rates from 5 to 100 mV s-1. The enclosed area under the CV curve increases with the scan rate increases from 5 to 100 mV s-1. Similar results were observed in GCD plots as shown in Fig. 2 (c) and (d). The anodic layers exhibit the specific area capacitance ~ 54.5, 145.3, 80.6 and 18.3mF cm-2 at 1 mA cm-2, resepectively for the annealing temperature from 300-600℃. The optimum one exhibited the higher area capacitance ~ 145.3 mF cm-2 at 1 mA cm-2 and ~ 76 mF cm-2 at 4 mA cm-2, and the capacitance still retained 53.3% of its initial value after 1000 cycles. The Nyquist plots are presented in Fig. 2e to assess the resistance and capacitance of the formed anodic films. The lower resistance of anodic film annealed at 600℃ was attributed to the increased amount of high conductive Fe3O4.
In summary, nanostructured anodic films have been fabricated on 304 stainless steel via constant-potential anodization instead of galvanostatic anodization. The supercapacitance behavior of anodic film is strongly dependent on the annealing temperature. When tested as a binder-free electrode, the optimum anodic layers (400℃) exhibit the highest area capacitance ~ 145.3 mF cm-2 at 1 mA cm-2, and the capacitance still retained 53.3% of its initial value after 1000 cycles.

References

[1] B. Sarma, Y.R. Smith, A.L. Jurovitzki, R.S. Ray, S.K. Mohanty, M. Misra, Journal of Power Sources, 236 (2013) 103-111.
[2] K.Y. Xie, J. Li, Y.Q. Lai, W. Lu, Z.A. Zhang, Y.X. Liu, L.M. Zhou, H.T. Huang, Electrochemistry Communications, 13 (2011) 657-660.
[3] B. Sarma, A. L. Jurovitzki, Y. R. Smith, R. S. Ray, M. Misra, Journal of Power Sources, 272 (2014) 766-775.

Figure 1