Supercapacitive properties of nanoporous oxide layer formed on 304 type stainless steel
Introduction
The electrochemical capacitors (ECs) are developed to obtain high power and energy density [1]. The ECs are divided into two categories on the basis of electrochemical storage such as electric double layer capacitors (EDLCs) and pseudocapacitors. The energy storage in the EDLCs and pseudocapacitors rely on non-faradic and faradic reactions at the electrode and electrolyte interface, respectively [2]. The EDLCs type includes carbon based materials such as carbon aerogel [3] and graphene [4] which undergo degradation for longer exposure to an electrolyte. The transition metal oxides such as MnO2 [5], NiO [6], Co3O4 [7] and Fe3O4 [8], [9] show pseudocapacitive nature. These metal oxides undergo several redox transitions because of their multiple valence states. Chromium oxide (Cr2O3) have +2 and +3 oxidation states. In literature, less attention has been paid to evaluate (Cr2O3) electrode for supercapacitor application. Xu et al. [8] studied supercapacitive performance of Cr2O3 electrode and reported specific capacitance (Cs) of 203 F g−1. Various methods such as hydrothermal [10], pulsed laser deposition [11], and chemical vapor deposition [12] have been used for synthesis of Cr2O3 electrode. The literature survey reveals that the nanostructured Cr2O3 surface is formed by the electropolishing on 304 stainless steel (SS) substrate [13]. Kure et al. [13] formed the nanoporous layer of Cr2O3 on 304 SS substrate using anodization. The oxides of Ni and Fe are present along with oxide of Cr in the oxide layer of SS [14]. Apart from these, chemical oxidation method proved to be one of the easily adoptable and facile synthesis method to form oxide layer on the surface of SS substrate, using the elements (Cr, Ni and Fe) present in the SS substrate itself [15].
In the chemical oxidation method, when the SS substrate is immersed in a mixture of hot solution of 5 M H2SO4 and 2.5 M CrO3 for different time intervals, the thickness of oxide layer is varied and the different colored layers are formed due to the interference [16]. The nanoporous layers have many potential applications, since these can be used as intermediate layers for polymer coatings with improved adhesion, an insulating layer for electronic device applications and a corrosion-protection layer after pore filling [17]. These colored layers are mostly used for the decorative applications [18].
In this work, the porous oxide layer is formed on 304 type SS using simple chemical oxidation method. The oxide layer over SS is characterized by X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), contact angle and energy-dispersive X-ray spectroscopy (EDS) techniques. The supercapacitive properties are studied using the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques.
Section snippets
Experimental procedure
The SS substrate of 304 type containing Cr (18.5%), Ni (8.8%), Fe (71%) and Mn (1.7%) was used. The well cleaned SS substrate was immersed in the mixed solution of 5 M H2SO4 (5 CC) and 2.5 M CrO3 (5 CC) kept at constant 353 K. Purple1 colored oxide layer was obtained after 20 min of dipping is shown in Fig. 1(A). The color of oxide layer depends on the various parameters such as concentration of H+
Characterization techniques
The surface morphologies and elemental composition analyses of as oxidized and annealed oxidized layers were visualized using FE-SEM and EDS technique (JEOL-JSM 6360). The three dimensional (3D) morphology was obtained using AFM model INNOVA 1B3BE. Contact angle measurement was carried out by Rame-Hart USA equipment with CCD camera. The phase composition analysis of annealed oxide layer was carried out using XPS study. The electrochemical properties were studied using the cyclic voltammetry
Results and discussion
The FE-SEM images of bare stainless steel, as-oxidized and annealed oxide layer at magnification of 25,000× are shown in Fig. 2(A)–(C) respectively and inset of figure shows contact angle images. Fig. 2(A) shows surface morphology of bare stainless steel substrate having contact angle of 18°. Fig. 2(B) shows that after oxidation, homogenous, compact, and dense oxide layer is formed and due to the compact nature of oxide particles, the contact angle also changed to 40°. After annealing at 773 K,
Supercapacitive study
The supercapacitive study of as-oxidized and annealed oxide layers is performed using three electrode system consisting of platinum as a counter electrode, saturated calomel electrode (SCE) as a reference electrode and oxide layer as a working electrode. The measurements are carried out in non-aqueous LiClO4-PC electrolyte within the potential range of 0.0 to −1.3 V/SCE. The inorganic solvents such as KOH [9] and LiPF6 [29] are used as the electrolytes for anodized stainless steel electrode.
Conclusions
The oxide layer is formed on 304 type stainless steel using the simple chemical oxidation method. The FE-SEM study reveals nanoporous morphology for annealed oxide layer. From the XPS study, formation of a mixture of Cr and Fe oxides and enriched Cr2O3 layer are confirmed. Annealed oxide layer shows electrochemical stability about 40% over the 1000 cycles. The interfacial capacitance of 75 mF cm−2 is observed. Thus, chemically oxidized annealed layer containing 304 SS electrode may be the good
Acknowledgments
This work was supported by the Human Resources Development program (No. 20124010203180) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Trade, Industry and Energy and supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A2A2A01006856).
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