DSA-type anode based on conductive porous p-silicon substrate
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Electrochemistry Communications 5 (2003) 365–368 www.elsevier.com/locate/elecom
DSA-type anode based on conductive porous p-silicon substrate M. Panizza a, L. Ouattara b, E. Baranova c, Ch. Comninellis a
c,*
Dipartimento di Ingegneria Chimica e di Processo, Universita di Genova, P.le J.F. Kennedy 1, 16129 Genova, Italy b Laboratoire de Chimie Physique, Universit e Cocody, 22 Bp 582 Abidjan 22, C^ ote, dÕIvoire c Swiss Federal Institute of Technology, SB-ISP-UGEC. CH-1015 Lausanne, Switzerland Received 28 February 2003; received in revised form 18 March 2003; accepted 18 March 2003
Abstract Porous silicon (PS) thin films have been prepared by electrochemical anodization of p-Si in HF–H2 O–EtOH solution and they have been used as substrate material for the preparation of iridium oxide based electrodes (PS/IrO2 ) using the thermal decomposition technique. The morphology and the electrochemical behaviour of the PS/IrO2 have been studied and the results have been compared with IrO2 electrodes deposited on a sandblasted p-silicon (p-Si/IrO2 ). SEM analyses have revealed that the PS/IrO2 electrodes are porous, rough and IrO2 appears to be deposited within some silicon pores, while the p-Si/IrO2 present a Ômud-crackedÕ surface. Cyclic voltammetries in 1 M HClO4 have shown that the PS/IrO2 presents higher surface area than p-Si/IrO2 . Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; DSAâ ; Iridium oxide; SEM; Cyclic voltammetry
1. Introduction The discovery of the dimensionally stable anodes (DSAÒ) has brought significant improvements in the chlor-alkaly industry and in many other applications such as water electrolysis, metal electrowinning, selective synthesis and destructive oxidation of organic contaminants. The DSAÒ electrodes consist of a pure or mixed metal oxide coatings deposited on a base metal using an appropriate precursor. Although many preparation techniques has been investigated, including, sol–gel technique [1–3], the Pechini method [4,5], laser calcinations [6] and induction heating [7,8]; thermal decomposition of the corresponding metal chloride salt is still preferred because such a method is simple and generates film with a surface morphology that are adequate for several applications [9–12]. The main problem of DSAÒ electrodes, in the case of long operation times in aggressive conditions, is the loss of activity, generally manifested by an increase in potential. One of the reasons for such an activity loss is the *
Corresponding author. Tel.: +41-21-693-3674; fax: +41-21-6933190. E-mail address: christos.comninellis@epfl.ch (C. Comninellis).
poor performance of the base metal used. Titanium, due to its excellent combination of mechanical properties, low density and corrosion resistance is the most commonly used substrate. However, a consequence of the thermal treatment or during anodic polarization is that the titanium is partially oxidised, forming a thin layer of TiOx at the Ti/electrocatalyst interface, which strongly reduces the service life of the electrodes [13]. Many other valve metals (zirconium, niobium and tantalum) have been suggested [13–15] and tantalum appeared to be the most suitable anode material owning to its corrosion resistance in aggressive media due to its protective oxide film [16]. However, its lower electrical resistivity, higher density and higher cost compared to titanium limit its industrial applications. In a previous paper [17], in order to overcome TiOx formation and the tantalum high cost, p-silicon (1– 3 mX cm) has been chosen as substrate material due to its high anodic stability and reasonable cost. DSA-type electrodes of IrO2 deposited on a p-silicon substrate (pSi/IrO2 ) by thermal decomposition, have shown a high activity for organic oxidation similar to noble metal and Ti/IrO2 electrodes. Moreover, the electrochemical stability of the p-Si/IrO2 has been evaluated by the accelerated test of life by anodically polarization at 5 A cm2
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00069-9
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in 30 wt% aqueous H2 SO4 solutions at 80 °C. The obtained values of the service life of the IrO2 deposited on silicon substrate was about one order magnitude higher than that deposited on titanium. The porous silicon (PS) thin films are of great interest for several applications in microelectronics, optoelectronics and micro-machining [18–21]. In fact, the porous silicon layer, usually prepared by anodising a Si wafer in hydrofluoric (HF) solution, has a number of interesting properties such as porous structure, high surface area (up to 600 m2 cm3 ), relatively low resistivity and good anodic stability [22,23]. These properties of PS suggest it to be a more suitable substrate than p-silicon previously reported for the production of the DSA-type electrodes with high surface area. In this paper we report preliminary data concerning the characterization of a DSAÒ-type electrode of IrO2 deposited on an anodised porous p-Si substrate (PS/ IrO2 ) by thermal decomposition technique. The electrochemical behaviour of PS/IrO2 electrodes has been compared with those of IrO2 deposited on silicon substrate (p-Si/IrO2 ).
2. Experimental Siltronix silicon (1 0 0) p-type wafers of 0.01– 0:03 X cm resistivity doped with boron were used. PS was prepared by anodizing the wafer in 1:1 volume ratio of 40 wt% HF-aqueous and ethanol (EtOH) at an anodic current density of 10 mA cm2 for 1 h. Once anodization was performed, wafers were rinsed in deionised water. To ensure maximum adhesion of the coating, the silicon substrate for the p-Si/IrO2 electrode was sandblasted, degreased in iso-propanol with ultrasounds, washed with purified water and finally dried in air at 80 °C. The p-Si/IrO2 and PS/IrO2 electrodes were prepared by thermal decomposition technique [14] from a precursor solution of 0.086 M hexachloroiridic acid (H2 IrCl6 6H2 O) dissolved in iso-propanol. The precursor solution was applied on the pre-treated silicon substrate in two sequential coatings. After each coating, the sample was dried at 80 °C for 10 min for evaporating solvent and fired in an air oven at 400 °C for about 10 min, and the final layer was annealed at the same temperature for 1 h. The electrodes prepared in such a way presented an IrO2 catalyst loading of about 1:9 g m2 . The microstructural characterization of the coatings was performed by scanning electron microscopy (SEM) using a JEOL JMS-6300-F scanning electron microscope. The Si/IrO2 ohmic junction is due to that IrO2 has an electronic conductivity and behaves like a metal. Electrochemical measurements were performed in a conventional three-electrode cell using a computer-
controlled potentiostat EG&G Princeton Applied Research model 273. The counter electrode was a platinum wire and the reference electrode was a saturated sulphate electrode (SSE). The exposed apparent area of the electrodes was 1 cm2 . Cyclic voltammetries and linear polarizations were recorded in 1 M HClO4 solutions at 25 °C, with a scan rate of 100 and 10 mV s1 , respectively. All the experimental data were repeated two times and the results were well reproduced. SEM measurements were carried on porous silicon (PS), IrO2 coated porous silicon (PS/IrO2 ) and IrO2 coated sandblasted silicon (p-Si/IrO2 ) and electrochemical measurements were made on IrO2 coated porous silicon (PS/IrO2 ) and IrO2 coated sandblasted silicon (p-Si/IrO2 ).
3. Results and discussion Fig. 1 shows a scanning electron micrograph, respectively, of the freshly prepared PS layer, PS/IrO2 , pSi/IrO2 coatings and sandblasted silicon. The surface of the PS layer formed during anodization in hydrofluoric acid (Fig. 1(a)) is smooth and it presents regular distribution of pores ranging from 10 to 20 nm. The ‘‘apparent’’ porosity of PS was determined by the ratio of mass loss during the anodization to the total mass loss [24] ma mb papp ¼ ; ð1Þ ma mc where ma , mb and mc are the masses of silicon substrate before anodization in HF/EtOH, after anodization and after the removal of the porous layer in 1 M NaOH, during 2 min, respectively. The calculated porosity of the PS was about 80%. Fig. 1(b) shows that IrO2 layer deposited on PS was porous and rough, with an amorphous pattern of nucleation. Iridium oxide coating appears to be deposited within some silicon pores, however the coating is thin enough that some micropores can still be observed. On the contrary, the iridium oxide layer prepared on p-silicon (Fig. 1(d)) presents a typically Ômud-crackedÕ structure already observed on Ti substrate, consisting in a high number of islands separated by large fractures [25]. Fig. 2 shows a typical cyclic voltammograms recorded with a scan rate of 100 mV s1 on PS/IrO2 and p-Si/IrO2 electrodes in 1 M HClO4 . The voltammetric curves of both electrodes present two broad humps that have been attributed to the redox processes Ir(III)/Ir(IV) and Ir(IV)/Ir(VI) [25]. As pointed out by other authors [26], the electrochemical active surface area of IrO2 electrodes is proportional to the total anodic charge density (qa ) of the voltammogram between the hydrogen and oxygen evo-
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(a)
(b)
Fig. 2. Cyclic voltammograms recorded in 1 M HClO4 with a scan rate of 100 mV s1 , T ¼ 25 °C on (a) PS/IrO2 electrode; (b) p-Si/IrO2 electrode.
express only the electrochemically active sites and SEM gives information about the microscopic area. Both are related but generally are proportional. Furthermore, the ratio of the electrochemical charge for the PS/IrO2 and p-Si/IrO2 electrodes can be related to the ratio of the active sites of these electrodes. The oxygen evolution reaction on PS/IrO2 and p-Si/ IrO2 electrodes was studied by linear polarization in 1 M HClO4 with a scan rate of 10 mV s1 and the results obtained are presented in Fig. 3. This polarization shows that oxygen evolution commenced at low overpotential (1.4 V vs. SHE) with both electrodes, however PS/IrO2 seem to be more active for oxygen evolution. This difference between the electrodes is related to their effective electrochemical active surface area. Aforementioned results have shown that the effective area of the PS/IrO2 is higher than those of pSi/IrO2 , and consequently polarization curves should be normalized by the effective area. In order to achieve this normalization, the current have been divided by the voltammetric charge, which is proportional to the effective electrochemical active area, and
(a)
Fig. 1. SEM microphotographs of (a) PS freshly prepared by anodization in HF–H2 O–EtOH; (b) PS/IrO2 electrode; (c) sandblasted p-Si, (d) p-Si/IrO2 electrode.
lution region obtained by a dedicated software from integration of voltammograms shown in Fig. 2. The higher value of total charge density (78 mC cm2 ) for the PS/IrO2 compared to the one of p-Si/IrO2 (49 mC cm2 ) confirms the higher active surface of the PS/IrO2 already observed by SEM analysis. It is worthwhile to mention that the voltammetric charge
(b)
Fig. 3. Linear polarizations recorded in 1 M HClO4 with a scan rate of 10 mV s1 , T ¼ 25 °C on (a) PS/IrO2 electrode; (b) p-Si/IrO2 electrode.
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(b)
(a)
Acknowledgements The authors thank Brian Senior in the Material Institute in Swiss Federal Institute of Technology (EPFL) for making the SEM microphoto.
References
Fig. 4. Normalised linear polarizations and Tafel plots (inset) recorded in 1 M HClO4 with a scan rate of 10 mV s1 , T ¼ 25 °C on (a) PS/ IrO2 electrode; (b) p-Si/IrO2 electrode.
the results are shown in Fig. 4. The IrO2 electrodes deposited onto PS or p-Si showed almost the same activity for oxygen evolution, with ‘‘normalized’’ Tafel slopes measured at low and high overpotential region, of about 65 5 and 123 3 mV dec1 respectively, in agreement with literature data for Ti/IrO2 electrodes [27]. Furthermore, it was observed that the PS/IrO2 electrodes exhibited stable and reproducibility behaviour.
4. Conclusion In this paper, the use of porous silicon films as substrate material for DSA-type electrodes of IrO2 (PS/ IrO2 ) was investigated. PS substrates were formed by galvanostatic anodization in HF–H2 O–EtOH solution and they presented a regular distribution of nanometersize pores with a porosity of 80%. The morphology of the PS/IrO2 electrodes were completely different from that of p-Si/IrO2 . In fact PS/IrO2 electrodes were porous and rough without the typical Ômud-crackedÕ surface of the thermally prepared oxides on flat substrate. The higher value of the voltammetric, charge obtained during cyclic voltammetry, confirmed that, as a consequence of the porosity of the coating, the PS/IrO2 presented a higher electrochemical active surface area than the p-Si/IrO2 electrodes. These data suggest that PS is a suitable substrate material for the preparation of DSAÒ-type electrode with high electrochemical activity.
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