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Electrochemical surface enlargement of a niobium foil for electrolytic capacitor applications
Electrochemistry Communications 13 (2011) 298–301
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Electrochemical surface enlargement of a niobium foil for electrolytic capacitor applications Jeong Eun Yoo, Jinsub Choi ⁎ Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 13 December 2010 Received in revised form 7 January 2011 Accepted 13 January 2011 Available online 20 January 2011 Keywords: Metal oxides Electrolytic capacitor Niobium oxides Etching
a b s t r a c t Electrochemical etching for surface enlargement of niobium was investigated for electrolytic capacitor applications. We obtained a niobium capacitor with a capacitance of 271.4 μF/cm2, which is approximately four times larger than a conventional aluminum capacitor. The enhanced capacitance was attributed to surface enlargement by a factor of more than 40, which was prepared by the etching of niobium at 20 V in propanolic HF (2 wt.%) for 6 h. Electrochemical conditions and their corresponding capacitance are discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Although several types of capacitors have attracted great attention recently, the electrolytic capacitor is still considered an important component in an electric device because it provides a large capacitance per unit volume at a low cost [1–3]. More importantly, it can be operated in relatively high-current and low-frequency circuits [4]. Until recently, only a few materials have successfully been used in a commercialized capacitor. In the case of aluminum, although the dielectric constant of aluminum oxide is relatively low (ε ~7) compared to other dielectric materials such as BaTiO2, the surface enlargement by electrochemical etching greatly increases capacitance [5–11]. Capacitance, which is defined as the amount of electric charge on a conductor per unit of voltage between electrodes, increases with area and dielectric constant and decreases with separation. Electrochemical etching of Al in chloride solution is a well-known process that is used for making surface enlargements in Al by a factor of more than 100 [12]. A capacitor consisting of materials such as tantalum and niobium, which have relatively higher dielectric constants (εr ~ 25 for tantalum oxide and εr ~ 40 for niobium oxide), has been prepared by pressing metal oxide powders. It is more economical to make the capacitor based on metal oxide powders. In addition, there is no effective and cost-competitive electrochemical process that greatly increases the surface area [13,14].
Although electrochemical etching of niobium foils is rarely found in the literature, electrochemical anodization of niobium has recently been studied by several groups [15–20]. Because electrochemical etching is very similar to anodization, we anticipated that it would be possible to find appropriate conditions for electrochemical etching from anodization technologies. Based on experience with the anodization of niobium, we knew that the surface of niobium could be effectively etched in fluoride-based electrolytes [21]. In addition, additives and non-aqueous electrolytes can play a very important role in controlling the surface morphology [18,21,22]. Critical points differentiating etching and oxide formation strongly depend on potential and electrolytes. Aggressive electrolytes such as Cl− and F− ions and high potential typically lead to the etching of metal; however, there is not a strong understanding of the criteria to distinguish etching and oxide formation. Based on previous experiences, the electrochemical etching conditions for enlarging the surface area of niobium foils, which we anticipated could be used for electrolytic capacitor applications, are optimized in terms of electrolyte, voltage, and etching time. Because the electrochemical etching process is more beneficial than the method using metal oxide powders, we believe that the presented results suggest evidence for producing niobium-based electrolytic capacitors using electrochemical etching in aluminum-based electrolytic capacitors. 2. Experimental
⁎ Corresponding author. Tel.: +82 32 860 7476/82 32 860 7471; fax: +82 32 866 0587. E-mail address: [email protected] (J. Choi). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.01.009
Nb foils (99.9% purity) with a thickness of 0.25 mm were purchased from Goodfellow (England). Methanol (Aldrich), ethanol (Duksan, Korea), propanol (Aldrich), butanol (Aldrich) and HF
J.E. Yoo, J. Choi / Electrochemistry Communications 13 (2011) 298–301
(Aldrich) were purchased as reagent-grade chemicals. Deionized water (DI water, N18 MΩ) was used to prepare an aqueous H3PO4 solution. The Nb foils were ultrasonicated in acetone for 5 min, washed with ethanol, and dried with a stream of N2 gas. The electrochemical etching of niobium for the surface enlargement was optimized in terms of etching time, voltage, and solvents. During the etching experiment, the temperature was maintained at 15 °C. Electrochemical etching was carried out at different etching times ranging from 1 h to 12 h, and the applied voltage was adjusted from 20 V to 50 V. Different concentrations of aqueous, methanolic, ethanolic, propanolic, and butanolic HF were used as an etchant. The cell was a twoelectrode system consisting of a Pt mesh acting as the counter electrode and the Nb foil with a size of 1 cm2 as the working electrode. During the electrochemical etching, the stirring rate of the electrolyte remained constant (~ 180 rpm). An LCR meter (ANDO AG 4303) was used to measure the capacitance. The hysteresis characteristic was observed at a fixed frequency of 120 Hz in 0.5 M ammonium adipate (1,4-butanedicarboxylic acid diammonium salt). The surface morphologies of the electrochemically etched niobium were characterized by a field emission scanning electron microscope (FE-SEM, 4300S), and the cross-sectional FE-SEM images were obtained using a focus ion beam (FIB, Helios NanoLab 600). The chemical composition of the etched niobium was characterized by X-ray photoelectron spectroscopy (XPS, VGESCALAB 220iXL spectrometer (Fisons)). When using XPS analysis, the residual pressure in the spectrometer ranged from 1.3 to 6.5 × 10−7 Pa. A monochromated Al anode (the energy of the Al Kα line was 1486.6 eV), powered at 10 keV and 20 mA, was used for X-ray production. 3. Results and discussion Fig. 1 shows FE-SEM images of a niobium foil, which was electrochemically etched in different electrolytes. Because electrochemical etching in an aqueous hydrofluoric acid solution is very aggressive and primarily occurs on the skin surface of the niobium [21], controllable etching in a depth direction for an extreme surface enlargement is obtained with great difficulty (not shown here). Thus,
299
organic solvents including methanol (Fig. 1(a)), ethanol (Fig. 1(b)), propanol (Fig. 1(c)), and butanol (Fig. 1(d)) were used. As shown in Fig. 1(a), the surface of the niobium foil etched in methanolic hydrofluoric acid contained many etched holes in the skin. Although its capacitance (Table 1) was relatively high compared to the nonetched foil, a higher capacitance in methanolic hydrofluoric acid was not obtained in this stage. Similarly, Fig. 1(b) displays a skin-etched surface, whereas Fig. 1(c) and (d) indicate the possibility of controlling the depth directional etching. From the perspective of surface enlargement, the depth directional etching and the skin etching used to increase the density of the etching pits should occur in an optimized fashion. Propanolic and butanolic etching appeared to satisfy both the depth directional and the skin etching (Fig. 1). The capacitance measurement (Table 1) showed that a higher capacitance was observed in propanolic etching. Thus, a propanolic electrolyte was selected and optimized in the following experiments. Fig. 2 shows the FE-SEM images of electrochemically etched niobium foils as a function of etching time, demonstrating that the pit density increased as etching time increased. Various diameters of pits ranging from a few hundred nanometers to tens of micrometers are observed in Fig. 2(a) and (b). The size of the microsized pits increased as the etching time increased. Interestingly, a large empty space existed underneath the microsized pits, implying that the structures had an inside volume larger than the mouth of the pits (Fig. 2(b)). The surface inside the empty space appeared to have many pits or holes. We believe that a long etching time leads to the complete removal of the skin surface above the empty space (Fig. 2(c)), exposing the surface inside the empty space to the electrolyte. This explanation was confirmed by cross-sectional views of the FE-SEM images shown in Fig. 2(d) and (e), which were taken using FIB techniques. Because the original thickness of the niobium foil was 250 μm, the thickness shown in the inset of Fig. 2(d) demonstrated that etching up to 6 h only created etched pits with an average thickness of 93 μm, without significant removal of the metal surface. However, the thickness dramatically decreased as the etching time increased. After 12 h of etching, the remaining niobium foil was approximately 150 μm (see inset of Fig. 2(e)) and contained a depth directional etched layer with a thickness of 2.27 μm. This result clearly demonstrated that there was a complete removal of the skin surface above the empty space. Thus, we can determine that the optimized etching for surface enlargement
Fig. 1. FE-SEM images of surface-enlarged niobium foils, which were prepared by electrochemical etching of a niobium foil with a thickness of 0.25 mm at 20 V and 15 °C for 1 h in (a) methanolic HF (1 wt.%), (b) ethanolic HF (1 wt.%), (c) propanolic HF (1 wt.%), and (d) butanolic HF (1 wt.%). Note that the insets show enlarged views of each picture.
300
J.E. Yoo, J. Choi / Electrochemistry Communications 13 (2011) 298–301
Table 1 Capacitance of electrochemically etched niobium foils in terms of electrochemical conditions. Conditions (15°C)
Potential/V
Original Foil Acqueous 1 M H3PO4 Methanolic 1 wt.% HF Ethanolic 1 wt.% HF Propanolic 1 wt.% HF
20 20 20 20
Propanolic 2 wt.% HF
20
30 50 Propanolic 3 wt.% HF
20
Propanolic 4 wt.% HF Propanolic 5 wt.% HF Butanolic 1 wt.% HF
20 20 20
Time/h
Capacitance/μFcm−2
. 0.5 1 1 1 2 4 6 1 2 4 6 12 2 6 2 6 6 12 6 6 1
6.41 1.68 95.36 15.36 83.04 42.64 43.17 48.91 36.71 37.14 67.96 271.4 203.57 12 40.32 14 32.96 108.9 99.3 95.35 128.57 9.64
should be performed before the complete removal of the skin surface. The current–time transient during the etching is exhibited in Fig. 2(f). Current density linearly increased as a function of etching time, indicating that the surface area was enlarged. Current fluctuation was observed after 8 h, likely reflecting that the skin surface was seriously etched away. Thus, an optimized etching of 6 h was a good approximation. Fig. 3 displays the chemical composition of the etched niobium foil, suggesting that the outer layer consisted of Nb2O5 and contained a
small amount of F ions. The Nb 3d5/2 at 207.04 eV and the O 1 s at 530.12 eV corresponded to the binding energy of Nb2O5 [23–25]. The capacitance of the original foil, anodized foil, and electrochemically etched niobium foil were investigated. The original niobium metal showed a capacitance of 6.41 μF/cm2, corresponding to the natural oxide thickness of 5.5 nm, which is in agreement with previous results [26]. The natural oxide thickness was calculated using the equation C = ε0 εr A/d, where C is the capacitance, A is the area of overlap of the two plates, εr is the relative static permittivity of niobium oxide (εr ≈ 40), ε0 is the electric constant (ε0 ≈ 8.854 × 10− 12 F/m), and d is natural oxide thickness. It was expected that the anodization of niobium foil in 1 M H3PO4 would produce a thick barrier film, thus reducing the capacitance [21]. As mentioned in the introduction, the capacitance is proportional to the surface area; a higher capacitance corresponds to a larger surface area. As shown in Table 1, the optimized capacitance was achieved by electrochemical etching in propanolic HF (2 wt.%) for 6 h at 20 V. If the etching potential was higher than the optimized potential of 20 V, or the etching time was longer than the optimized time of 6 h, too much etching occurs. This results in the complete removal of the skin surface, leading to reduced total surface areas for capacitance. Because the aluminum electrolytic capacitor prepared by electrochemical etching is approximately 70 μF/cm2, the results obtained in niobium show almost a four-fold larger capacitance [27]. 4. Conclusions In the study presented, the surface enlargement of niobium by electrochemical etching was investigated for an electrolytic capacitance application in terms of etching solvents, time, and potential. When comparing different etching solvents containing hydrofluoric acid, etching in propanolic hydrofluoric acid showed not only depth-
Fig. 2. FE-SEM images of niobium prepared by electrochemical etching in propanolic HF (2 wt.%) at 20 V and 15 °C for (a) 1 h, (b) 6 h, and (c) 12 h. Note that the insets show enlarged views of the etched area. Fig. 2(d) and (e) are the cross-sectional images of Fig. 2(b) and (c), respectively. The insets show the thickness of the etched niobium foil. The original foil is 250 μm. The samples were prepared by FIB. Fig. 2(f) shows the current–time transient during etching in propanolic HF (2 wt.%) at 20 V and 15 °C for 12 h.
J.E. Yoo, J. Choi / Electrochemistry Communications 13 (2011) 298–301
540
535
530
525
Binding Energy/eV
520 220
215
210
205
301
200
Binding Energy/eV
Fig. 3. XPS data displaying the chemical composition of the niobium foil prepared by electrochemical etching in propanolic HF (2 wt.%) at 20 V and 15 °C for 1 h.
directional etched pits but also a large density of etched pits. The optimized conditions for surface enlargement were obtained when the skin surface was not completely removed. In this structure, narrow mouth pits and a large empty space underneath the skin surface were assumed. If the skin surface was completely removed as the etching time increased or the applied potential increased, the capacitance dramatically decreased because of reduction of the surface area. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Education, Science and Technology (2010-0011197). References [1] M. Jayalakshmi, K. Balasubramanian, Int. J. Electrochem. Sci. 3 (2008) 1196. [2] K. Watanabe, M. Sakairi, H. Takahashi, S. Hirai, S. Yamaguchi, J. Electroanal. Chem. 473 (1999) 250. [3] S.S. Park, B.T. Lee, J. Electroceram. 13 (2004) 111. [4] A. Nashino, J. Power Sources 60 (1996) 137. [5] R.S. Alwitt, J. Electrochem. Soc. 131 (1984) 13.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
X. Du, Y. Zu, Thin Solid Films 516 (2008) 8436. W.M. Moore, C.T. Chen, G.A. Shim, Corros. Sci. 40 (1984) 644. J. Kang, Y. Shin, Y. Tak, Electrochim. Acta 51 (2005) 1012. D.G.W. Goad, H. Uchi, J. Appl. Electrochem. 30 (2000) 285. R.G. Xiao, K.P. Yan, Corros. Sci. 50 (2008) 3256. N. Osawa, K. Fukuoka, Corros. Sci. 42 (2000) 585. J. Flis, L. Kowalczyk, J. Appl. Electrochem. 25 (1995) 501. N.F. Jackson, J.C. Hendy, Electrocomponent Sci. Technol. 1 (1974) 27. Y. Pozdeev, Qual. Reliab. Eng. Int 14 (1998) 79. J. Choi, J.H. Lim, J. Lee, K.J. Kim, Nanotechnology 18 (2007) 055603. R.L. Karlinsey, Electrochem. Commun. 7 (2005) 1190. J. Zhao, X. Wang, R. Xu, Y. Mi, Y. Li, Electrochem. Solid-State Lett. 10 (2007) C31. J.E. Yoo, J. Choi, Electrochim. Acta 55 (2010) 5142. S. Ono, K. Kuramochi, H. Asoh, Corros. Sci. 51 (2009) 1513. I. Sieber, H. Hildebrand, A. Friedrich, P. Schmuki, Electrochem. Commun. 7 (2005) 97. J. Choi, J.H. Lim, S.C. Lee, J.H. Chang, K.J. Kim, M.A. Cho, Electrochim. Acta 51 (2006) 5502. H. Habazaki, Y. Oikawa, K. Fushimi, Y. Aoki, K. Shimizu, P. Skeldon, G.E. Thompson, Electrochim. Acta 54 (2009) 946. J. Choi, J.H. Lim, J. Lee, K.J. Kim, Nanotechnology 18 (2007) 055603. S.A. O'Neill, I.P. Parkin, R.J.H. Clark, A. Mills, N. Elliott, J. Mater. Chem. 13 (2003) 2952. N. Magnumen, L. Quinones, D.C. Dufner, D.L. Cocke, E.A. Schweikert, B.K. Patnaik, C.V. Barros Leite, B.G. Baptista, Chem. Mater. 1 (1989) 220. I. Arsova, L. Arsov, N. Hebestreit, A. Anders, W. Plieth, J. Solid State Electrochem. 11 (2007) 209214. Z. Hou, J. Zeng, J. Chen, S. Liao, Mater. Chem. Phys. 123 (2010) 625.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Electrochemical surface enlargement of a niobium foil for electrolytic capacitor applications Jeong Eun Yoo, Jinsub Choi ⁎ Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 13 December 2010 Received in revised form 7 January 2011 Accepted 13 January 2011 Available online 20 January 2011 Keywords: Metal oxides Electrolytic capacitor Niobium oxides Etching
a b s t r a c t Electrochemical etching for surface enlargement of niobium was investigated for electrolytic capacitor applications. We obtained a niobium capacitor with a capacitance of 271.4 μF/cm2, which is approximately four times larger than a conventional aluminum capacitor. The enhanced capacitance was attributed to surface enlargement by a factor of more than 40, which was prepared by the etching of niobium at 20 V in propanolic HF (2 wt.%) for 6 h. Electrochemical conditions and their corresponding capacitance are discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Although several types of capacitors have attracted great attention recently, the electrolytic capacitor is still considered an important component in an electric device because it provides a large capacitance per unit volume at a low cost [1–3]. More importantly, it can be operated in relatively high-current and low-frequency circuits [4]. Until recently, only a few materials have successfully been used in a commercialized capacitor. In the case of aluminum, although the dielectric constant of aluminum oxide is relatively low (ε ~7) compared to other dielectric materials such as BaTiO2, the surface enlargement by electrochemical etching greatly increases capacitance [5–11]. Capacitance, which is defined as the amount of electric charge on a conductor per unit of voltage between electrodes, increases with area and dielectric constant and decreases with separation. Electrochemical etching of Al in chloride solution is a well-known process that is used for making surface enlargements in Al by a factor of more than 100 [12]. A capacitor consisting of materials such as tantalum and niobium, which have relatively higher dielectric constants (εr ~ 25 for tantalum oxide and εr ~ 40 for niobium oxide), has been prepared by pressing metal oxide powders. It is more economical to make the capacitor based on metal oxide powders. In addition, there is no effective and cost-competitive electrochemical process that greatly increases the surface area [13,14].
Although electrochemical etching of niobium foils is rarely found in the literature, electrochemical anodization of niobium has recently been studied by several groups [15–20]. Because electrochemical etching is very similar to anodization, we anticipated that it would be possible to find appropriate conditions for electrochemical etching from anodization technologies. Based on experience with the anodization of niobium, we knew that the surface of niobium could be effectively etched in fluoride-based electrolytes [21]. In addition, additives and non-aqueous electrolytes can play a very important role in controlling the surface morphology [18,21,22]. Critical points differentiating etching and oxide formation strongly depend on potential and electrolytes. Aggressive electrolytes such as Cl− and F− ions and high potential typically lead to the etching of metal; however, there is not a strong understanding of the criteria to distinguish etching and oxide formation. Based on previous experiences, the electrochemical etching conditions for enlarging the surface area of niobium foils, which we anticipated could be used for electrolytic capacitor applications, are optimized in terms of electrolyte, voltage, and etching time. Because the electrochemical etching process is more beneficial than the method using metal oxide powders, we believe that the presented results suggest evidence for producing niobium-based electrolytic capacitors using electrochemical etching in aluminum-based electrolytic capacitors. 2. Experimental
⁎ Corresponding author. Tel.: +82 32 860 7476/82 32 860 7471; fax: +82 32 866 0587. E-mail address: [email protected] (J. Choi). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.01.009
Nb foils (99.9% purity) with a thickness of 0.25 mm were purchased from Goodfellow (England). Methanol (Aldrich), ethanol (Duksan, Korea), propanol (Aldrich), butanol (Aldrich) and HF
J.E. Yoo, J. Choi / Electrochemistry Communications 13 (2011) 298–301
(Aldrich) were purchased as reagent-grade chemicals. Deionized water (DI water, N18 MΩ) was used to prepare an aqueous H3PO4 solution. The Nb foils were ultrasonicated in acetone for 5 min, washed with ethanol, and dried with a stream of N2 gas. The electrochemical etching of niobium for the surface enlargement was optimized in terms of etching time, voltage, and solvents. During the etching experiment, the temperature was maintained at 15 °C. Electrochemical etching was carried out at different etching times ranging from 1 h to 12 h, and the applied voltage was adjusted from 20 V to 50 V. Different concentrations of aqueous, methanolic, ethanolic, propanolic, and butanolic HF were used as an etchant. The cell was a twoelectrode system consisting of a Pt mesh acting as the counter electrode and the Nb foil with a size of 1 cm2 as the working electrode. During the electrochemical etching, the stirring rate of the electrolyte remained constant (~ 180 rpm). An LCR meter (ANDO AG 4303) was used to measure the capacitance. The hysteresis characteristic was observed at a fixed frequency of 120 Hz in 0.5 M ammonium adipate (1,4-butanedicarboxylic acid diammonium salt). The surface morphologies of the electrochemically etched niobium were characterized by a field emission scanning electron microscope (FE-SEM, 4300S), and the cross-sectional FE-SEM images were obtained using a focus ion beam (FIB, Helios NanoLab 600). The chemical composition of the etched niobium was characterized by X-ray photoelectron spectroscopy (XPS, VGESCALAB 220iXL spectrometer (Fisons)). When using XPS analysis, the residual pressure in the spectrometer ranged from 1.3 to 6.5 × 10−7 Pa. A monochromated Al anode (the energy of the Al Kα line was 1486.6 eV), powered at 10 keV and 20 mA, was used for X-ray production. 3. Results and discussion Fig. 1 shows FE-SEM images of a niobium foil, which was electrochemically etched in different electrolytes. Because electrochemical etching in an aqueous hydrofluoric acid solution is very aggressive and primarily occurs on the skin surface of the niobium [21], controllable etching in a depth direction for an extreme surface enlargement is obtained with great difficulty (not shown here). Thus,
299
organic solvents including methanol (Fig. 1(a)), ethanol (Fig. 1(b)), propanol (Fig. 1(c)), and butanol (Fig. 1(d)) were used. As shown in Fig. 1(a), the surface of the niobium foil etched in methanolic hydrofluoric acid contained many etched holes in the skin. Although its capacitance (Table 1) was relatively high compared to the nonetched foil, a higher capacitance in methanolic hydrofluoric acid was not obtained in this stage. Similarly, Fig. 1(b) displays a skin-etched surface, whereas Fig. 1(c) and (d) indicate the possibility of controlling the depth directional etching. From the perspective of surface enlargement, the depth directional etching and the skin etching used to increase the density of the etching pits should occur in an optimized fashion. Propanolic and butanolic etching appeared to satisfy both the depth directional and the skin etching (Fig. 1). The capacitance measurement (Table 1) showed that a higher capacitance was observed in propanolic etching. Thus, a propanolic electrolyte was selected and optimized in the following experiments. Fig. 2 shows the FE-SEM images of electrochemically etched niobium foils as a function of etching time, demonstrating that the pit density increased as etching time increased. Various diameters of pits ranging from a few hundred nanometers to tens of micrometers are observed in Fig. 2(a) and (b). The size of the microsized pits increased as the etching time increased. Interestingly, a large empty space existed underneath the microsized pits, implying that the structures had an inside volume larger than the mouth of the pits (Fig. 2(b)). The surface inside the empty space appeared to have many pits or holes. We believe that a long etching time leads to the complete removal of the skin surface above the empty space (Fig. 2(c)), exposing the surface inside the empty space to the electrolyte. This explanation was confirmed by cross-sectional views of the FE-SEM images shown in Fig. 2(d) and (e), which were taken using FIB techniques. Because the original thickness of the niobium foil was 250 μm, the thickness shown in the inset of Fig. 2(d) demonstrated that etching up to 6 h only created etched pits with an average thickness of 93 μm, without significant removal of the metal surface. However, the thickness dramatically decreased as the etching time increased. After 12 h of etching, the remaining niobium foil was approximately 150 μm (see inset of Fig. 2(e)) and contained a depth directional etched layer with a thickness of 2.27 μm. This result clearly demonstrated that there was a complete removal of the skin surface above the empty space. Thus, we can determine that the optimized etching for surface enlargement
Fig. 1. FE-SEM images of surface-enlarged niobium foils, which were prepared by electrochemical etching of a niobium foil with a thickness of 0.25 mm at 20 V and 15 °C for 1 h in (a) methanolic HF (1 wt.%), (b) ethanolic HF (1 wt.%), (c) propanolic HF (1 wt.%), and (d) butanolic HF (1 wt.%). Note that the insets show enlarged views of each picture.
300
J.E. Yoo, J. Choi / Electrochemistry Communications 13 (2011) 298–301
Table 1 Capacitance of electrochemically etched niobium foils in terms of electrochemical conditions. Conditions (15°C)
Potential/V
Original Foil Acqueous 1 M H3PO4 Methanolic 1 wt.% HF Ethanolic 1 wt.% HF Propanolic 1 wt.% HF
20 20 20 20
Propanolic 2 wt.% HF
20
30 50 Propanolic 3 wt.% HF
20
Propanolic 4 wt.% HF Propanolic 5 wt.% HF Butanolic 1 wt.% HF
20 20 20
Time/h
Capacitance/μFcm−2
. 0.5 1 1 1 2 4 6 1 2 4 6 12 2 6 2 6 6 12 6 6 1
6.41 1.68 95.36 15.36 83.04 42.64 43.17 48.91 36.71 37.14 67.96 271.4 203.57 12 40.32 14 32.96 108.9 99.3 95.35 128.57 9.64
should be performed before the complete removal of the skin surface. The current–time transient during the etching is exhibited in Fig. 2(f). Current density linearly increased as a function of etching time, indicating that the surface area was enlarged. Current fluctuation was observed after 8 h, likely reflecting that the skin surface was seriously etched away. Thus, an optimized etching of 6 h was a good approximation. Fig. 3 displays the chemical composition of the etched niobium foil, suggesting that the outer layer consisted of Nb2O5 and contained a
small amount of F ions. The Nb 3d5/2 at 207.04 eV and the O 1 s at 530.12 eV corresponded to the binding energy of Nb2O5 [23–25]. The capacitance of the original foil, anodized foil, and electrochemically etched niobium foil were investigated. The original niobium metal showed a capacitance of 6.41 μF/cm2, corresponding to the natural oxide thickness of 5.5 nm, which is in agreement with previous results [26]. The natural oxide thickness was calculated using the equation C = ε0 εr A/d, where C is the capacitance, A is the area of overlap of the two plates, εr is the relative static permittivity of niobium oxide (εr ≈ 40), ε0 is the electric constant (ε0 ≈ 8.854 × 10− 12 F/m), and d is natural oxide thickness. It was expected that the anodization of niobium foil in 1 M H3PO4 would produce a thick barrier film, thus reducing the capacitance [21]. As mentioned in the introduction, the capacitance is proportional to the surface area; a higher capacitance corresponds to a larger surface area. As shown in Table 1, the optimized capacitance was achieved by electrochemical etching in propanolic HF (2 wt.%) for 6 h at 20 V. If the etching potential was higher than the optimized potential of 20 V, or the etching time was longer than the optimized time of 6 h, too much etching occurs. This results in the complete removal of the skin surface, leading to reduced total surface areas for capacitance. Because the aluminum electrolytic capacitor prepared by electrochemical etching is approximately 70 μF/cm2, the results obtained in niobium show almost a four-fold larger capacitance [27]. 4. Conclusions In the study presented, the surface enlargement of niobium by electrochemical etching was investigated for an electrolytic capacitance application in terms of etching solvents, time, and potential. When comparing different etching solvents containing hydrofluoric acid, etching in propanolic hydrofluoric acid showed not only depth-
Fig. 2. FE-SEM images of niobium prepared by electrochemical etching in propanolic HF (2 wt.%) at 20 V and 15 °C for (a) 1 h, (b) 6 h, and (c) 12 h. Note that the insets show enlarged views of the etched area. Fig. 2(d) and (e) are the cross-sectional images of Fig. 2(b) and (c), respectively. The insets show the thickness of the etched niobium foil. The original foil is 250 μm. The samples were prepared by FIB. Fig. 2(f) shows the current–time transient during etching in propanolic HF (2 wt.%) at 20 V and 15 °C for 12 h.
J.E. Yoo, J. Choi / Electrochemistry Communications 13 (2011) 298–301
540
535
530
525
Binding Energy/eV
520 220
215
210
205
301
200
Binding Energy/eV
Fig. 3. XPS data displaying the chemical composition of the niobium foil prepared by electrochemical etching in propanolic HF (2 wt.%) at 20 V and 15 °C for 1 h.
directional etched pits but also a large density of etched pits. The optimized conditions for surface enlargement were obtained when the skin surface was not completely removed. In this structure, narrow mouth pits and a large empty space underneath the skin surface were assumed. If the skin surface was completely removed as the etching time increased or the applied potential increased, the capacitance dramatically decreased because of reduction of the surface area. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Education, Science and Technology (2010-0011197). References [1] M. Jayalakshmi, K. Balasubramanian, Int. J. Electrochem. Sci. 3 (2008) 1196. [2] K. Watanabe, M. Sakairi, H. Takahashi, S. Hirai, S. Yamaguchi, J. Electroanal. Chem. 473 (1999) 250. [3] S.S. Park, B.T. Lee, J. Electroceram. 13 (2004) 111. [4] A. Nashino, J. Power Sources 60 (1996) 137. [5] R.S. Alwitt, J. Electrochem. Soc. 131 (1984) 13.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
X. Du, Y. Zu, Thin Solid Films 516 (2008) 8436. W.M. Moore, C.T. Chen, G.A. Shim, Corros. Sci. 40 (1984) 644. J. Kang, Y. Shin, Y. Tak, Electrochim. Acta 51 (2005) 1012. D.G.W. Goad, H. Uchi, J. Appl. Electrochem. 30 (2000) 285. R.G. Xiao, K.P. Yan, Corros. Sci. 50 (2008) 3256. N. Osawa, K. Fukuoka, Corros. Sci. 42 (2000) 585. J. Flis, L. Kowalczyk, J. Appl. Electrochem. 25 (1995) 501. N.F. Jackson, J.C. Hendy, Electrocomponent Sci. Technol. 1 (1974) 27. Y. Pozdeev, Qual. Reliab. Eng. Int 14 (1998) 79. J. Choi, J.H. Lim, J. Lee, K.J. Kim, Nanotechnology 18 (2007) 055603. R.L. Karlinsey, Electrochem. Commun. 7 (2005) 1190. J. Zhao, X. Wang, R. Xu, Y. Mi, Y. Li, Electrochem. Solid-State Lett. 10 (2007) C31. J.E. Yoo, J. Choi, Electrochim. Acta 55 (2010) 5142. S. Ono, K. Kuramochi, H. Asoh, Corros. Sci. 51 (2009) 1513. I. Sieber, H. Hildebrand, A. Friedrich, P. Schmuki, Electrochem. Commun. 7 (2005) 97. J. Choi, J.H. Lim, S.C. Lee, J.H. Chang, K.J. Kim, M.A. Cho, Electrochim. Acta 51 (2006) 5502. H. Habazaki, Y. Oikawa, K. Fushimi, Y. Aoki, K. Shimizu, P. Skeldon, G.E. Thompson, Electrochim. Acta 54 (2009) 946. J. Choi, J.H. Lim, J. Lee, K.J. Kim, Nanotechnology 18 (2007) 055603. S.A. O'Neill, I.P. Parkin, R.J.H. Clark, A. Mills, N. Elliott, J. Mater. Chem. 13 (2003) 2952. N. Magnumen, L. Quinones, D.C. Dufner, D.L. Cocke, E.A. Schweikert, B.K. Patnaik, C.V. Barros Leite, B.G. Baptista, Chem. Mater. 1 (1989) 220. I. Arsova, L. Arsov, N. Hebestreit, A. Anders, W. Plieth, J. Solid State Electrochem. 11 (2007) 209214. Z. Hou, J. Zeng, J. Chen, S. Liao, Mater. Chem. Phys. 123 (2010) 625.