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Voltammetric and impedance study of the influence of the anode composition on the electrochemical ferrate(VI) production in molten NaOH
Electrochimica Acta 110 (2013) 581–586
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Voltammetric and impedance study of the influence of the anode composition on the electrochemical ferrate(VI) production in molten NaOH Lucia Hrnˇciariková, Miroslav Gál 1 , Kamil Kerekeˇs, Ján Híveˇs ∗,1 Department of Inorganic Technology, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 812 37 Bratislava, Slovakia
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Article history: Received 20 February 2013 Received in revised form 27 May 2013 Accepted 31 May 2013 Available online 19 June 2013 Keywords: Ferrate(VI) Impedance spectroscopy Hydroxide melt Anodic oxidation Electrochemical synthesis
a b s t r a c t Three typical anode materials: pure iron (Fe), silicon-rich steel (FeSi) and white cast iron (FeC) electrodes were used in the process of electrochemical ferrate(VI) synthesis in the molten sodium hydroxide. The voltammetric peak current densities corresponding to the first and second step of the anode dissolution in the case of FeC as well as FeSi electrode are higher compared to the pure iron electrode. After passivity region subsequently the transpassive iron dissolution, including ferrate(VI) formation together with an oxygen evolution occurs and the current shoulder is visible for all electrodes used. Measured electrochemical impedance spectra confirm the physical model of the polarized surface based on the concept of two macrohomogeneous surface layers. In all cases the resistance of both inner and outer layer decrease with increasing applied potential. With increasing temperature the resistance of inner and outer layer decreases. The capacity of inner and outer layer increases with increasing potential. This is in agreement with decrease of the resistances of both layers: layers are getting thinner or more disintegrated by oxygen evolution or strong anodic dissolution. The number of exchanged electrons calculated from a static polarization curve at the potentials in ferrate(VI) formation region is z = 3 for all electrode materials used. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Generally, Fe(II)/Fe(III) are the typical oxidation states in which iron can be usually found. However, other not very stable states 2−
have been also observed, e.g. ferrate(VI) [FeVI O4 ] [1]. The very first information about ferrates(VI) is dated back to 1702 [1]. In 1715 Stahl prepared K2 FeO4 by oxidation of iron filings in molten KNO3 [1]. After the dissolution of residues in alkaline solution and during the smelting of iron ore with potassium carbonate the violet coloration was observed. Fremy assumed that the violet color of this product was caused by the presence of FeVI OII4 [1]. Electrochemically ferrates(VI) were prepared for the first time by using anodic oxidation of iron in alkaline electrolyte by Poggendorf in 1841 [1,2]. At the beginning of the 20th century huge development of Ni–Fe alkaline batteries increased the interest in ferrates(VI). Haber and Pick [3,4] focused on the production of ferrates(VI) for electrochemical synthesis [2]. In the early fifties, Touˇska [5,6] and Helferich [7] began intensive research in the preparation of ferrates(VI) by
∗ Corresponding author. Tel.: +421 2 59 325 468; fax: +421 2 59 325 560. E-mail address: [email protected] (J. Híveˇs). 1 ISE member. 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.144
chemical synthesis (wet process) due to their excellent oxidation capability. One of the most promising application of ferrates(VI) is, nowadays, connected with, both drinking water and wastewater treatment. The main advantage of ferrates(VI) is the high oxidation potential that allows the decomposition of stable inorganic, organic and microbiological contaminants. Another reason for ferrates(VI) utilization for water treatment is a product of the ferrates(VI) reduction – ferric hydroxide [8]. Fe(OH)3 does not burden the environment, is non-toxic and is excellent coagulant and flocculant. In most cases, ferrates(VI) provide a complete degradation of the pollutant without harmless by-products [9]. Therefore, it can be nominated as a “green” environmental friendly oxidant [10]. In organic synthesis, ferrates(VI) are used for the selective oxidation of primary and secondary alcohols to aldehydes and ketones [11]. In the field of the corrosion protection, ferrates(VI) can be utilized for passivation of aluminium, zinc and iron products, or to dissolve resistant layer of deposits [12]. Three basic methods of ferrates(VI) preparation are reported in the literature [1]. The first one is called dry oxidation: iron or iron oxide is heated to a high temperature in the melt of alkali metal compounds (e.g. oxides, peroxides). The resulting product is corresponding ferrate of the alkali metal. The second one is wet
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oxidation: its principle is in the absorption of chlorine in solution of concentrated sodium hydroxide and subsequent reaction of the resulting hypochlorite with ferric ion to form ferrates(VI). The last one is the electrochemical anodic dissolution (oxidation) of iron or cast iron in the concentrated hydroxide solutions or melts until ferrates(VI) are prepared. There has been a gradual increase in attention to this method because it can provide solution for a majority of the problems associated with the previous ones [10]. In the case of the melt suitable temperature for ferrates(VI) synthesis is up to 200 ◦ C. In water solutions the typical temperature range is from 20 to 70 ◦ C [1]. Utilizing electrochemical method the high purity ferrates(VI) are prepared. However, the stability of the product in the presence of water remains the weak point of this approach. Even a small amount of water presented in the highly alkaline paste decomposes ferrate(VI) within hours. Sharma [10] suggested the utilization of organic solvents instead of usual ones. However, the lower electrical conductivity on the one side and higher toxicity on the other side disadvantage this method. An electrochemical treatment of iron in low-temperature melt binary hydroxide systems, e.g. KOH–LiOH, NaOH–KOH, CsOH–LiOH, KOH–RbOH has been proposed for ferrates(VI) preparation to minimize disadvantages of previous approaches [13]. The major advantage is usually the negligible presence of water: ferrates(VI) are prepared in dry form and therefore are stable. Another advantage of this technology is the fact that the reaction rate can be easily and continuously controlled by adjusting temperature in order to achieve the optimum anodic dissolution conditions. Híveˇs et al. [13] also reported the minimal thermal decomposition of ferrates(VI) during their electrochemical preparation. The most suitable eutectic mixture of NaOH and KOH (51.5% (n/n) of NaOH) was found in our previous studies [13–15]. This mixture is characterized by a relatively low eutectic melting point and high electrical conductivity [15]. In addition, the iron anode passivation is significantly reduced [1,14]. Ferrates(VI) formation can be detected easily by observing color change of the electrolyte. Titration, chemical precipitation, spectral and electrochemical methods can be used for the quantitative determination of ferrates(VI) concentration [11,16]. This work is focused on the preparation of ferrates(VI) by anodic oxidation of pure iron (Fe), silicon-rich steel (FeSi) and white cast iron (FeC) electrodes in the molten sodium hydroxide. All three electrodes were chosen in order to compare our results in molten hydroxide with previous reports by Mácová et al. [18–20] in strong alkaline aqueous solutions. The advantage of molten hydroxide to Mácová’s approach is almost water-free environment, higher temperature and, therefore, higher reaction kinetics and efficiency of the ferrate(VI) production. 2. Experimental 2.1. Chemicals Sodium hydroxides (Mikrochem Ltd., Pezinok, Slovakia) in p.a. grade with distilled water were used to prepare electrolytes. The purity of NaOH ranged from 72 to 82% (w/w), carbonate content of up to 0.6% (w/w) and water. Determination of NaOH purity was performed by acidimetric titration with HCl (Mikrochem Ltd., Pezinok, Slovakia, p.a.), using Methyl red indicator (Lachema Neratovice, Czech Republic, p.a.). Carbonates were determined gravimetrically, by precipitation with barium chloride (p.a., Lachema Neratovice, Czech Republic). 2.2. Apparatus and procedures An oil thermostat with calibrated sensor, stainless steel box and PTFE crucible with the sample was used for our experiments.
Fig. 1. Cyclic voltammograms of working electrodes, i.e. pure iron (solid line), FeSi (dashed line), and FeC (dotted line) at different temperatures in molten NaOH at scan rate of 400 mV s−1 ; temperatures are indicated on the graph; arrows indicate the potential sweep direction; insets represent zoomed part of CVs in order to make peaks better visible.
Reference connection of thermocouple was immersed in a Dewar flask with ice-water. Measuring connection of thermocouple was immersed into the melt at the same level as electrodes [17]. Electrochemical measurements were performed using AUTOLAB instrument PGSTAT 20 equipped with FRA2 module (ECO Chemie, The Netherlands). A three electrode electrochemical cell was used for all experiments. Working electrodes (WE) were made from: (A) pure iron (Fe) (99.95% (w/w) Fe, 0.005% (w/w) C, 0.0048% (w/w) Ni and 0.0003% (w/w) Mn), (B) silicon steel (FeSi) (96.1% (w/w) Fe, 3.17% (w/w) Si, 0.47% (w/w) Cu, 0, 23% (w/w) Mn, 0.03% (w/w) Ni), and (C) white cast iron (FeC) (96.354% (w/w) Fe, 3.17% (w/w) C in the form of Fe3 C, 0.44% (w/w) Mn and 0.036% (w/w) Ni). The geometric area of the working electrodes varied from 0.2 to 0.7 cm2 . The same material (Fe) served as the reference electrode (RE) in all cases. Counter electrode (CE) was made from mild steel (steel class 11). Measurements were carried out in a PTFE crucible containing the melt of 50 g (NaOH–NaOH·H2 O). The temperature was varied in the range 70–160 ◦ C. The lower limit is given by the temperature and composition of eutectic mixture NaOH–NaOH·H2 O which is 62.5 ◦ C and 74% (w/w) of NaOH in mixture. Cyclic voltammograms were recorded in the potential range from −0.3 to 1.8 V vs. RE. Impedance measurements were carried out in the same systems immediately after the measurement of polarization curves. The potential of the working electrode was gradually increased by 25 mV from 1.35 up to 1.75 V vs. RE. In this area, the formation of ferrates(VI) was expected. Frequency range used for the impedance measurements was from 10 Hz to 100 kHz. Perturbation signal had a sinusoidal shape with amplitude of 5 mV.
3. Results and discussion 3.1. Voltammetric analysis In Fig. 1 cyclic voltammograms of Fe, FeSi and FeC electrodes in the molten NaOH at various temperatures are shown. The anodic part of the potential window is limited by the decomposition of the melt and subsequent oxygen evolution while the hydrogen evolution limits cathodic one.
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All curves are characterized by the presence of several anodic and cathodic current peaks. On the anodic part of the cyclic voltammograms at 80 ◦ C two, well separated, current peaks A1 and A2 are observed. As stated previously [14] the current peak A1, which can be found at around 0.025 V vs. RE for all three electrodes, corresponds to Fe(II) formation according to equation: Fe + 2OH− → Fe(OH)2 + 2e−
(1)
Further anodic polarization of the electrode covered by Fe(OH)2 layer leads to the A2 peak formation (ca. 220 mV more positively than A1). The oxidation process can be described as follows [14]: Fe(OH)2 + OH− → Fe(OH)3 + e−
(2)
Fe(OH)2 + OH− → FeOOH + H2 O + e−
(3)
2Fe(OH)2 + 2OH− → Fe2 O3 + 3H2 O + 2e−
(4)
It is highly probable that Fe(OH)2 , Fe(OH)3 and Fe2 O3 compounds are in strong alkaline environment and at sufficient high electrode potentials further dissolved and form solu− ble species (Fe(OH)2 + 2OH− → FeO2− 2 + 2H2 O; Fe(OH)3 + OH → − − − FeO2 + 2H2 O and Fe2 O3 + 2OH → 2FeO2 + H2 O, respectively). The exact mechanism is not known and, unfortunately, it is not detectable in situ at the electrochemical conditions. However, it is hardly probable to expect complete stoichiometric oxidation of the original hydroxidic layer. In agreement with previous observations the formation of the layer composed of non-stochiometric species similar to the magnetite with better protecting properties is more probable. Due to its specific insulating properties and some kinetic limitation of such process current peak A2 is less visible than A1 one [14]. The current density for A1 and A2 peak in the case of FeC as well as FeSi electrode is higher compare to the pure iron electrode, but no so much as reported by Mácová et al. [1]. The structure of the pure Fe anode positively influences the ability of the Fe species to form corresponding oxo/hydroxy compounds. Only in the case of white cast iron (FeC) electrode another, small A2b, peak is observed around 250 mV positively to the peak A2. For all three electrodes, in the passivity region at about 850 mV vs. RE broad, not perfectly pronounced current peak A2c is observed. This peak together with A2b peak can be assigned to the restructuring, formation and/or transformation of an anodic solid surface layer. After passivity region subsequently the transpassive iron dissolution, including ferrate(VI) formation together with an oxygen evolution occurs and the current shoulder A3 is visible for all electrodes used. Only slight differences (max. 40 mV) in the A3 peak potentials are observed among electrodes. The most positive is situated E(A3) for pure iron and, on the other side, the most negative is E(A3) for FeC. Also, A3 peak for pure iron electrode is better developed than for the FeC one. Moreover, the A3 peak is hardly detectable in the case of FeC electrode for lower temperatures. In all cases the hysteresis in the course of voltammetric curve in the transpassive potential region was observed. It is probably caused, on one side, by the diminution of the inhibition of the electrode surface toward dissolution at high potentials and, on the other side, due to the massive oxygen evolution causing the mechanical disruption of the anode surface. The higher is the temperature of the system, the higher current densities are observed and A1, A2, and A2b peaks are slightly shifted to the more positive potentials indicating the higher energy demand for oxidation process. At 120 ◦ C A1 and A2 peaks are not well separated and at 160 ◦ C only one doubled peak A1 + A2 is observed. This is probably connected with reaction rate enhancement (formation of respective compounds containing Fe(II) and/or Fe(III) species and/or formation of intermediate products) in latter cases. Restructuralization A2c peak is shifted to the more negative
Fig. 2. The dependences of the anodic peak current densities (A3) on the square root of potential scan rate; 䊉 pure Fe electrode at 80 ◦ C; pure Fe electrode at 150 ◦ C; FeSi electrode at 80 ◦ C; FeSi electrode at 150 ◦ C; inset: 夽 FeC electrode at 140 ◦ C. Size of points represents an experimental error.
potentials. The higher is the temperature the lower is the A2c peak. This feature is caused by the disrupted oxo–hydroxide surface film. In the cathodic direction a reduction peak of ferrate(VI) (peak C3) occurs at potentials of 0.42–0.90 V depending on the both temperature and electrode used. Compare to all faradaic anodic peaks, this one is badly pronounced. On the one hand, the higher is the temperature the lower is the C3 current peak for all electrodes. On the other hand, with increasing the temperature the reduction peaks of ferrates(VI) (peak C3) are shifted to the more positive potentials indicating less energy requirement for their reduction. This probably indicates that the collapse of compactness of anode passive layer occurred. It is important finding with respect to possible increase of anode efficiency of ferrate(VI) formation. In the case of peak C3 the highest current densities are observed for pure iron electrode and the lowest for FeC electrode. This means that the number of species undergoing reduction in this potential region is highest for pure iron electrode. Furthermore, for the pure Fe electrode C3 peak is observed at the most positive potentials and for FeC electrode at the most negative potentials for all temperatures. The similar behavior is found in the case of C2 and C1 peaks. Both are shifted to the more negative potentials with increasing temperature for all electrodes. Moreover, the most negative potentials are found in the case of FeC electrode. It seems that the structure of the anode plays an important role in ferrate(VI) production also in molten salts environment [1]. In Fig. 2 the dependences of the A3 peak current density on the square root of potential scan rate for both various materials and temperatures are shown. Since A3 peak for FeC electrode is hardly detectable at low temperatures, the only dependence at 140 ◦ C is plotted. It is known that an electrochemical reaction on an electrode surface can occur by two limiting mechanisms: the reaction is controlled by kinetics or the reaction is controlled by the diffusion of the electroactive species. When the kinetic of the reaction is fast enough, the phenomenon is controlled by the diffusion (mass transport) of the species that enters or leaves the electrode surface. This is indicated by the linear dependence of peak current vs. square root of scan rate. Since our experiments are carried out at elevated temperatures, formation of ferrates(VI) on the electrode surface is very fast and linear dependences of peak current on square root of polarization rate are found almost for all systems one can conclude
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Fig. 3. Nyquist plots of impedance spectra for (A) pure Fe, (B) FeSi, and (C) FeC electrodes at t = 160 ◦ C and at chosen potentials vs. RE: 1.45 V; 1.55 V; 1.65 V.
that that the electrode reaction kinetic is controlled by the mass transport under the semi-infinitive linear diffusion conditions. In the case of FeC at high temperature (see Fig. 2, stars) current density after reaching a certain maximum at about 400–500 mV s−1 decreases with increasing the polarization rate. It seems, that some chemical step due to the electrode structure become rate determining when scan rate reach ca. 400 mV s−1 (EC reaction mechanism). Our observation is in agreement with some previous results in hydroxide melts [13]. In the strong alkaline aqueous solutions a subsequent chemical reaction was also observed [18–20]. 3.2. Electrochemical impedance spectroscopy analysis In order to deeply characterize ferrate(VI) formation EIS was utilized. This technique is well suitable for surface layer structure and properties studies. An example of impedance spectra obtained for various electrodes at several chosen potentials is shown in Fig. 3. From all three figures A, B, and C two time constants with inductive passivation low frequency region can be easily recognize. The electrochemical behavior of the electrode in NaOH molten has been represented by EIS as an equivalent circuit (Fig. 4). The physical model based on the concept of two macrohomogeneous surface layers with theory of duplex sandwich assembly of passivating layers was taken into account in order to find the best equivalent circuit representation of our results. Two parallel R–Q (constant phase element) impedance elements characterizing individual layers (inner, outer) connected in series together with one resistant (RS ) and one inductance (L) element compose the simplest equivalent circuits that fit our experimental data with acceptably low
From Fig. 6 it can be seen that resistances of both inner and outer layer decrease with increasing applied potential. This means that either both layers are getting thinner with increasing potential or are disintegrated by strong anodic potential or by strong oxygen evolution. Resistance of outer layer is lower than resistance of inner one meaning that inner layer is little bit thicker or more compact. With increasing temperature the resistance of outer layer slightly and the resistance of inner layer significantly decreases. This means that the charge is at higher temperatures easily transferred; the layer is either thinner or less compact. At about 120 ◦ C the resistances are almost equal for individual sublayers. The capacity of inner and outer layer increases with increasing potential. This is in agreement with decrease of the resistances of both layers: layers are getting thinner or more disintegrated by oxygen evolution or strong anodic dissolution. Capacity of outer layer is lower than capacity of inner one. This means that outer layer is either thicker or more compact than inner one. Capacities of inner and outer layers increase with increasing temperature meaning that both layers are losing their thickness or compactness. At about 120 ◦ C individual capacities are approximately equal. Additional information can be gained from n parameter of constant phase element. In the case outer layer parameter n changes from almost unity (“ideal” behavior) to ca. 0.6 (non-ideal surface) with increasing potential. This means the quality of the outer layer is changing and the specific surface is increasing and is getting more porous. Bearing in mind the previous conclusions about Rout and Qout , one should think about the disintegration of the outer layer rather than reduction in its thickness. Values of nout are higher at
k
(Oi − Ei )/Ei , where Oi is the observed frequency for bin i 2 = i=1 and Ei is the expected frequency for bin i. Constant phase element (Q) was used instead of pure capacitance element (C) to underline the nonlinearity of the dissolution process. The accuracy of proposed equivalent circuit is documented on Bode plot (Fig. 5) where lines represent the results of the nonlinear regression analysis according to the model in Fig. 4. The optimized parameters of nonlinear analysis of the equivalent circuit for white cast iron (FeC) electrode at low and high temperatures in dependence on the electrode potential are shown in Fig. 6.
Fig. 4. Equivalent circuit representing the impedance of the system containing electrode and NaOH melt; RS , electrolyte resistance; L, outer inductance; Rin/out , inner/outer layer resistance; Qin/out , inner/outer layer constant phase element.
Fig. 5. Bode plot. FeSi electrode; t = 160 ◦ C; E = 1.55 V vs. RE. Points represent experimental data, curves nonlinear regression analysis according to the equivalent circuits in Fig. 4.
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Fig. 6. The fitted values of the equivalent circuit elements obtained for both inner and outer layer in dependence on the polarization potential of the FeC electrode at various temperatures: 80 ◦ C; 160 ◦ C.
low temperatures. This means that at low temperatures outer layer behaves more “ideally” than at higher temperatures. Contrary to previous observations, ninn is almost unchanged for all potential applied. Therefore, the conclusions made for Rinn and Qinn are connected to the reduction in thickness of the inner layer rather than its disintegration. Temperature has a low impact on ninn value. The inner layer is probably more stable than outer one. One can conclude that temperature has an impact on the thickness of the individual layers, but not on their compactness. This is in agreement with the theory concerning the mechanism of temperature influence on the surface behavior of the electrode consisting in a chemical interaction of the OH− anion with surface layer [18]. Parameters for other two electrodes show the same trend as in the case of FeC one (not shown). However, there are differences among them. Rinn is the highest for pure iron electrode and lowest for FeSi electrode. Qinn (FeC) are equal to Qinn (FeSi) and are higher than Qinn (Fe). Rout , Qout and nout are approximately similar for all three electrodes. This means that the structure of the electrode influence manly the inner layer of the anode, but not outer one. Utilizing, so called, static polarization curve method the impedance data were used to determine the number of electrons
involved in the electrode reaction of ferrate(VI) formation (peak A3). These curves were constructed as follows: DC current densities, jDC and DC potentials, EDC were recorded during impedance measurements and a static polarization curve ln(jDC ) vs. EDC was plotted. To obtain stationary conditions electrodes were polarized at chosen DC potential for 5 min prior to each experiment and then impedance spectra were recorded. The number of electrons was calculated from an exponent of the dependence of DC current density on DC potential jDC = e˛zFEDC /RT , where ˛ is a charge transfer coefficient, is an overpotential and R, T have an usual meaning, in the potential region where ferrates(VI) are formed. This analysis gave the average z = 3.08 ± 0.52 for the pure iron electrode, z = 2.74 ± 0.29 for FeSi electrode and z = 2.76 ± 0.19 for FeC electrode, where z is a number of electrons exchanged during the process corresponding to peak A3. Both findings, i.e. the number of exchanged electrons and the diffusion controlled reaction mechanism allow us to write following final electrochemical redox reaction of ferrate(VI) production: FeO2 − + 4OH− ↔ FeO4 2− + 2H2 O + 3e−
(5)
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4. Conclusions Three typical anode materials: pure iron (Fe), silicon-rich steel (FeSi) and white cast iron (FeC) electrodes were used in the process of electrochemical ferrate(VI) synthesis in the molten sodium hydroxide. Eutectic mixture NaOH–NaOH·H2 O was used in the temperature range 70–160 ◦ C. The cyclic voltammetric curves of the electrodes show characteristics similar for the all anode materials used. The peak current densities corresponding to the first and second step of the anode dissolution in the case of FeC as well as FeSi electrode are higher compare to the pure iron electrode. After passivity region subsequently the transpassive iron dissolution, including ferrate(VI) formation together with an oxygen evolution occurs and the current shoulder is visible for all electrodes used. Only slight differences (max. 40 mV) in the peak potentials are observed among electrodes. In all cases the hysteresis in the course of voltammetric curve in the transpassive potential region was observed. It is probably caused, on one side, by the diminution of the inhibition of the electrode surface toward dissolution at high potentials and, on the other side, due to the massive oxygen evolution causing the mechanical disruption of the anode surface. In order to deeply characterize ferrate(VI) formation EIS was utilized in the anode potential range of ferrate(VI) production. Measured impedance spectra confirm the physical model of the polarized surface based on the concept of two macrohomogeneous surface layers. In all cases the resistance of both inner and outer layer decrease with increasing applied potential. With increasing temperature the resistance of inner and outer layer decreases. The capacity of inner and outer layer increases with increasing potential. This is in agreement with decrease of the resistances of both layers: layers are getting thinner or more disintegrated by oxygen evolution or strong anodic dissolution. Additional information can be obtained from n parameter of constant phase element. In the case outer layer parameter n changes from almost unity (“ideal” behavior) to ca. 0.6 (non-ideal surface) with increasing potential. This means the quality of the outer layer is changing and the specific surface is increasing and is getting more porous. The number of exchanged electrons calculated from a static polarization curve at the potentials in ferrate(VI) formation region is z = 3 for all electrode materials used. Acknowledgements The authors gratefully acknowledge the financial support for this research by the Ministry of Education, Science, Research and Sport of the Slovak Republic within project VEGA 1/0985/12 and
by the Research & Development Operational Programme funded by the ERDF within project ITMS 26240220073. References [1] Z. Mácová, K. Bouzek, J. Hiveˇs, V.K. Sharma, R.J. Terryn, J.C. Baum, Research progress in the electrochemical synthesis of ferrate(VI), Electrochimica Acta 54 (2009) 2673. [2] J.C. Poggendorf, Ueber die Frage, ob es wirksane galvanische Ketten ohne primitive chemische Action gebe, und über die Bildung der Eisensäure auf galvanischem Wege, Poggendorf’s Annalen 54 (1841) 161. [3] F. Haber, On the soluble alkali salts of iron oxide and ferric acid, Zeitschrift für Elektrochemie 7 (1900) 215. [4] W. Pick, On the electrolytic formation of alkali ferrates, Zeitschrift für Elektrochemie 7 (1901) 713. [5] J. Touˇsek, Electrochemical preparation of sodium ferrate, Collection of Czechoslovak Chemical Communications 27 (1962) 914. [6] J. Touˇsek, The decomposition of sodium ferrate, Collection of Czechoslovak Chemical Communications 27 (1962) 908. [7] B. Helferich, K. Lang, Salts of ferric acid, Zeitschrift für Anorganische und allgemeine Chemie 263 (1950) 169. [8] J.Q. Jiang, B. Lloyd, Progress in the development and use of ferrate(VI) salt as an oxidant and coagulant for water and wastewater treatment, Water Research 36 (2002) 1397. [9] S. Licht, L. Yang, B. Wang, Synthesis and analysis of Ag2 FeO4 Fe(VI) ferrate superiron cathodes, Electrochemistry Communications 7 (2005) 931. [10] V.K. Sharma, Potassium ferrate(VI): an environmentally friendly oxidant, Advances in Environmental Research 6 (2002) 143. [11] L. Delaude, P. Laszlo, A novel oxidizing reagent based on potassium ferrate(VI), Journal of Organic Chemistry 61 (1996) 6360. [12] S. Licht, X. Yu, Recent advances in Fe(VI) synthesis, in: Sharma (Ed.), ACS Symposium series 985, Ferrates, American Chemical Society, Washington, DC, 2008 (Chapter 1). [13] J. Híveˇs, M. Benová, K. Bouzek, V.K. Sharma, Electrochemical formation of ferrate(VI) in molten NaOH-KOH system, Electrochemistry Communications 8 (2006) 1737. [14] J. Híveˇs, M. Benová, K. Bouzek, J. Sitek, V.K. Sharma, The cyclic voltammetric study of ferrate(VI) formation in molten Na/K hydroxide mixture, Electrochimica Acta 54 (2008) 203. [15] M. Benová, J. Híveˇs, K. Bouzek, V.K. Sharma, Electrochemical ferrate(VI) synthesis: a molten salt approach, in: Sharma (Ed.), ACS Symposium series 985, Ferrates, American Chemical Society, Washington, DC, 2008 (Chapter 3). [16] S. Licht, V. Naschitz, L. Halperin, N. Halperin, L. Lin, J. Chen, S. Ghosh, B. Liu, Analysis of ferrate(VI) compounds and super-iron Fe(VI) battery cathodes: FTIR, ICP, titrimetric, XRD, UV/VIS, and electrochemical characterization, Journal of Power Sources 101 (2001) 167. [17] L. Hrnˇciariková, J. Híveˇs, K. Kerekeˇs, J. Kamenár, M. Gál, Electrochemical behaviour of Fe and Fe–Si electrodes in molten hydroxides mixture, Acta Chimica Slovaca 3 (2010) 74. [18] Z. Mácová, K. Bouzek, V.K. Sharma, The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part I: anodic dissolution kinetics of pure iron, Journal of Applied Electrochemistry 40 (2010) 1019. [19] Z. Mácová, K. Bouzek, The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part II: anodic dissolution kinetics of a steel anode rich in silicon, Journal of Applied Electrochemistry 41 (2011) 1125. [20] Z. Mácová, K. Bouzek, The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part III: anodic dissolution kinetics of a white cast iron anode rich in iron carbide, Journal of Applied Electrochemistry 42 (2012) 615.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Voltammetric and impedance study of the influence of the anode composition on the electrochemical ferrate(VI) production in molten NaOH Lucia Hrnˇciariková, Miroslav Gál 1 , Kamil Kerekeˇs, Ján Híveˇs ∗,1 Department of Inorganic Technology, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 812 37 Bratislava, Slovakia
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Article history: Received 20 February 2013 Received in revised form 27 May 2013 Accepted 31 May 2013 Available online 19 June 2013 Keywords: Ferrate(VI) Impedance spectroscopy Hydroxide melt Anodic oxidation Electrochemical synthesis
a b s t r a c t Three typical anode materials: pure iron (Fe), silicon-rich steel (FeSi) and white cast iron (FeC) electrodes were used in the process of electrochemical ferrate(VI) synthesis in the molten sodium hydroxide. The voltammetric peak current densities corresponding to the first and second step of the anode dissolution in the case of FeC as well as FeSi electrode are higher compared to the pure iron electrode. After passivity region subsequently the transpassive iron dissolution, including ferrate(VI) formation together with an oxygen evolution occurs and the current shoulder is visible for all electrodes used. Measured electrochemical impedance spectra confirm the physical model of the polarized surface based on the concept of two macrohomogeneous surface layers. In all cases the resistance of both inner and outer layer decrease with increasing applied potential. With increasing temperature the resistance of inner and outer layer decreases. The capacity of inner and outer layer increases with increasing potential. This is in agreement with decrease of the resistances of both layers: layers are getting thinner or more disintegrated by oxygen evolution or strong anodic dissolution. The number of exchanged electrons calculated from a static polarization curve at the potentials in ferrate(VI) formation region is z = 3 for all electrode materials used. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Generally, Fe(II)/Fe(III) are the typical oxidation states in which iron can be usually found. However, other not very stable states 2−
have been also observed, e.g. ferrate(VI) [FeVI O4 ] [1]. The very first information about ferrates(VI) is dated back to 1702 [1]. In 1715 Stahl prepared K2 FeO4 by oxidation of iron filings in molten KNO3 [1]. After the dissolution of residues in alkaline solution and during the smelting of iron ore with potassium carbonate the violet coloration was observed. Fremy assumed that the violet color of this product was caused by the presence of FeVI OII4 [1]. Electrochemically ferrates(VI) were prepared for the first time by using anodic oxidation of iron in alkaline electrolyte by Poggendorf in 1841 [1,2]. At the beginning of the 20th century huge development of Ni–Fe alkaline batteries increased the interest in ferrates(VI). Haber and Pick [3,4] focused on the production of ferrates(VI) for electrochemical synthesis [2]. In the early fifties, Touˇska [5,6] and Helferich [7] began intensive research in the preparation of ferrates(VI) by
∗ Corresponding author. Tel.: +421 2 59 325 468; fax: +421 2 59 325 560. E-mail address: [email protected] (J. Híveˇs). 1 ISE member. 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.144
chemical synthesis (wet process) due to their excellent oxidation capability. One of the most promising application of ferrates(VI) is, nowadays, connected with, both drinking water and wastewater treatment. The main advantage of ferrates(VI) is the high oxidation potential that allows the decomposition of stable inorganic, organic and microbiological contaminants. Another reason for ferrates(VI) utilization for water treatment is a product of the ferrates(VI) reduction – ferric hydroxide [8]. Fe(OH)3 does not burden the environment, is non-toxic and is excellent coagulant and flocculant. In most cases, ferrates(VI) provide a complete degradation of the pollutant without harmless by-products [9]. Therefore, it can be nominated as a “green” environmental friendly oxidant [10]. In organic synthesis, ferrates(VI) are used for the selective oxidation of primary and secondary alcohols to aldehydes and ketones [11]. In the field of the corrosion protection, ferrates(VI) can be utilized for passivation of aluminium, zinc and iron products, or to dissolve resistant layer of deposits [12]. Three basic methods of ferrates(VI) preparation are reported in the literature [1]. The first one is called dry oxidation: iron or iron oxide is heated to a high temperature in the melt of alkali metal compounds (e.g. oxides, peroxides). The resulting product is corresponding ferrate of the alkali metal. The second one is wet
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oxidation: its principle is in the absorption of chlorine in solution of concentrated sodium hydroxide and subsequent reaction of the resulting hypochlorite with ferric ion to form ferrates(VI). The last one is the electrochemical anodic dissolution (oxidation) of iron or cast iron in the concentrated hydroxide solutions or melts until ferrates(VI) are prepared. There has been a gradual increase in attention to this method because it can provide solution for a majority of the problems associated with the previous ones [10]. In the case of the melt suitable temperature for ferrates(VI) synthesis is up to 200 ◦ C. In water solutions the typical temperature range is from 20 to 70 ◦ C [1]. Utilizing electrochemical method the high purity ferrates(VI) are prepared. However, the stability of the product in the presence of water remains the weak point of this approach. Even a small amount of water presented in the highly alkaline paste decomposes ferrate(VI) within hours. Sharma [10] suggested the utilization of organic solvents instead of usual ones. However, the lower electrical conductivity on the one side and higher toxicity on the other side disadvantage this method. An electrochemical treatment of iron in low-temperature melt binary hydroxide systems, e.g. KOH–LiOH, NaOH–KOH, CsOH–LiOH, KOH–RbOH has been proposed for ferrates(VI) preparation to minimize disadvantages of previous approaches [13]. The major advantage is usually the negligible presence of water: ferrates(VI) are prepared in dry form and therefore are stable. Another advantage of this technology is the fact that the reaction rate can be easily and continuously controlled by adjusting temperature in order to achieve the optimum anodic dissolution conditions. Híveˇs et al. [13] also reported the minimal thermal decomposition of ferrates(VI) during their electrochemical preparation. The most suitable eutectic mixture of NaOH and KOH (51.5% (n/n) of NaOH) was found in our previous studies [13–15]. This mixture is characterized by a relatively low eutectic melting point and high electrical conductivity [15]. In addition, the iron anode passivation is significantly reduced [1,14]. Ferrates(VI) formation can be detected easily by observing color change of the electrolyte. Titration, chemical precipitation, spectral and electrochemical methods can be used for the quantitative determination of ferrates(VI) concentration [11,16]. This work is focused on the preparation of ferrates(VI) by anodic oxidation of pure iron (Fe), silicon-rich steel (FeSi) and white cast iron (FeC) electrodes in the molten sodium hydroxide. All three electrodes were chosen in order to compare our results in molten hydroxide with previous reports by Mácová et al. [18–20] in strong alkaline aqueous solutions. The advantage of molten hydroxide to Mácová’s approach is almost water-free environment, higher temperature and, therefore, higher reaction kinetics and efficiency of the ferrate(VI) production. 2. Experimental 2.1. Chemicals Sodium hydroxides (Mikrochem Ltd., Pezinok, Slovakia) in p.a. grade with distilled water were used to prepare electrolytes. The purity of NaOH ranged from 72 to 82% (w/w), carbonate content of up to 0.6% (w/w) and water. Determination of NaOH purity was performed by acidimetric titration with HCl (Mikrochem Ltd., Pezinok, Slovakia, p.a.), using Methyl red indicator (Lachema Neratovice, Czech Republic, p.a.). Carbonates were determined gravimetrically, by precipitation with barium chloride (p.a., Lachema Neratovice, Czech Republic). 2.2. Apparatus and procedures An oil thermostat with calibrated sensor, stainless steel box and PTFE crucible with the sample was used for our experiments.
Fig. 1. Cyclic voltammograms of working electrodes, i.e. pure iron (solid line), FeSi (dashed line), and FeC (dotted line) at different temperatures in molten NaOH at scan rate of 400 mV s−1 ; temperatures are indicated on the graph; arrows indicate the potential sweep direction; insets represent zoomed part of CVs in order to make peaks better visible.
Reference connection of thermocouple was immersed in a Dewar flask with ice-water. Measuring connection of thermocouple was immersed into the melt at the same level as electrodes [17]. Electrochemical measurements were performed using AUTOLAB instrument PGSTAT 20 equipped with FRA2 module (ECO Chemie, The Netherlands). A three electrode electrochemical cell was used for all experiments. Working electrodes (WE) were made from: (A) pure iron (Fe) (99.95% (w/w) Fe, 0.005% (w/w) C, 0.0048% (w/w) Ni and 0.0003% (w/w) Mn), (B) silicon steel (FeSi) (96.1% (w/w) Fe, 3.17% (w/w) Si, 0.47% (w/w) Cu, 0, 23% (w/w) Mn, 0.03% (w/w) Ni), and (C) white cast iron (FeC) (96.354% (w/w) Fe, 3.17% (w/w) C in the form of Fe3 C, 0.44% (w/w) Mn and 0.036% (w/w) Ni). The geometric area of the working electrodes varied from 0.2 to 0.7 cm2 . The same material (Fe) served as the reference electrode (RE) in all cases. Counter electrode (CE) was made from mild steel (steel class 11). Measurements were carried out in a PTFE crucible containing the melt of 50 g (NaOH–NaOH·H2 O). The temperature was varied in the range 70–160 ◦ C. The lower limit is given by the temperature and composition of eutectic mixture NaOH–NaOH·H2 O which is 62.5 ◦ C and 74% (w/w) of NaOH in mixture. Cyclic voltammograms were recorded in the potential range from −0.3 to 1.8 V vs. RE. Impedance measurements were carried out in the same systems immediately after the measurement of polarization curves. The potential of the working electrode was gradually increased by 25 mV from 1.35 up to 1.75 V vs. RE. In this area, the formation of ferrates(VI) was expected. Frequency range used for the impedance measurements was from 10 Hz to 100 kHz. Perturbation signal had a sinusoidal shape with amplitude of 5 mV.
3. Results and discussion 3.1. Voltammetric analysis In Fig. 1 cyclic voltammograms of Fe, FeSi and FeC electrodes in the molten NaOH at various temperatures are shown. The anodic part of the potential window is limited by the decomposition of the melt and subsequent oxygen evolution while the hydrogen evolution limits cathodic one.
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All curves are characterized by the presence of several anodic and cathodic current peaks. On the anodic part of the cyclic voltammograms at 80 ◦ C two, well separated, current peaks A1 and A2 are observed. As stated previously [14] the current peak A1, which can be found at around 0.025 V vs. RE for all three electrodes, corresponds to Fe(II) formation according to equation: Fe + 2OH− → Fe(OH)2 + 2e−
(1)
Further anodic polarization of the electrode covered by Fe(OH)2 layer leads to the A2 peak formation (ca. 220 mV more positively than A1). The oxidation process can be described as follows [14]: Fe(OH)2 + OH− → Fe(OH)3 + e−
(2)
Fe(OH)2 + OH− → FeOOH + H2 O + e−
(3)
2Fe(OH)2 + 2OH− → Fe2 O3 + 3H2 O + 2e−
(4)
It is highly probable that Fe(OH)2 , Fe(OH)3 and Fe2 O3 compounds are in strong alkaline environment and at sufficient high electrode potentials further dissolved and form solu− ble species (Fe(OH)2 + 2OH− → FeO2− 2 + 2H2 O; Fe(OH)3 + OH → − − − FeO2 + 2H2 O and Fe2 O3 + 2OH → 2FeO2 + H2 O, respectively). The exact mechanism is not known and, unfortunately, it is not detectable in situ at the electrochemical conditions. However, it is hardly probable to expect complete stoichiometric oxidation of the original hydroxidic layer. In agreement with previous observations the formation of the layer composed of non-stochiometric species similar to the magnetite with better protecting properties is more probable. Due to its specific insulating properties and some kinetic limitation of such process current peak A2 is less visible than A1 one [14]. The current density for A1 and A2 peak in the case of FeC as well as FeSi electrode is higher compare to the pure iron electrode, but no so much as reported by Mácová et al. [1]. The structure of the pure Fe anode positively influences the ability of the Fe species to form corresponding oxo/hydroxy compounds. Only in the case of white cast iron (FeC) electrode another, small A2b, peak is observed around 250 mV positively to the peak A2. For all three electrodes, in the passivity region at about 850 mV vs. RE broad, not perfectly pronounced current peak A2c is observed. This peak together with A2b peak can be assigned to the restructuring, formation and/or transformation of an anodic solid surface layer. After passivity region subsequently the transpassive iron dissolution, including ferrate(VI) formation together with an oxygen evolution occurs and the current shoulder A3 is visible for all electrodes used. Only slight differences (max. 40 mV) in the A3 peak potentials are observed among electrodes. The most positive is situated E(A3) for pure iron and, on the other side, the most negative is E(A3) for FeC. Also, A3 peak for pure iron electrode is better developed than for the FeC one. Moreover, the A3 peak is hardly detectable in the case of FeC electrode for lower temperatures. In all cases the hysteresis in the course of voltammetric curve in the transpassive potential region was observed. It is probably caused, on one side, by the diminution of the inhibition of the electrode surface toward dissolution at high potentials and, on the other side, due to the massive oxygen evolution causing the mechanical disruption of the anode surface. The higher is the temperature of the system, the higher current densities are observed and A1, A2, and A2b peaks are slightly shifted to the more positive potentials indicating the higher energy demand for oxidation process. At 120 ◦ C A1 and A2 peaks are not well separated and at 160 ◦ C only one doubled peak A1 + A2 is observed. This is probably connected with reaction rate enhancement (formation of respective compounds containing Fe(II) and/or Fe(III) species and/or formation of intermediate products) in latter cases. Restructuralization A2c peak is shifted to the more negative
Fig. 2. The dependences of the anodic peak current densities (A3) on the square root of potential scan rate; 䊉 pure Fe electrode at 80 ◦ C; pure Fe electrode at 150 ◦ C; FeSi electrode at 80 ◦ C; FeSi electrode at 150 ◦ C; inset: 夽 FeC electrode at 140 ◦ C. Size of points represents an experimental error.
potentials. The higher is the temperature the lower is the A2c peak. This feature is caused by the disrupted oxo–hydroxide surface film. In the cathodic direction a reduction peak of ferrate(VI) (peak C3) occurs at potentials of 0.42–0.90 V depending on the both temperature and electrode used. Compare to all faradaic anodic peaks, this one is badly pronounced. On the one hand, the higher is the temperature the lower is the C3 current peak for all electrodes. On the other hand, with increasing the temperature the reduction peaks of ferrates(VI) (peak C3) are shifted to the more positive potentials indicating less energy requirement for their reduction. This probably indicates that the collapse of compactness of anode passive layer occurred. It is important finding with respect to possible increase of anode efficiency of ferrate(VI) formation. In the case of peak C3 the highest current densities are observed for pure iron electrode and the lowest for FeC electrode. This means that the number of species undergoing reduction in this potential region is highest for pure iron electrode. Furthermore, for the pure Fe electrode C3 peak is observed at the most positive potentials and for FeC electrode at the most negative potentials for all temperatures. The similar behavior is found in the case of C2 and C1 peaks. Both are shifted to the more negative potentials with increasing temperature for all electrodes. Moreover, the most negative potentials are found in the case of FeC electrode. It seems that the structure of the anode plays an important role in ferrate(VI) production also in molten salts environment [1]. In Fig. 2 the dependences of the A3 peak current density on the square root of potential scan rate for both various materials and temperatures are shown. Since A3 peak for FeC electrode is hardly detectable at low temperatures, the only dependence at 140 ◦ C is plotted. It is known that an electrochemical reaction on an electrode surface can occur by two limiting mechanisms: the reaction is controlled by kinetics or the reaction is controlled by the diffusion of the electroactive species. When the kinetic of the reaction is fast enough, the phenomenon is controlled by the diffusion (mass transport) of the species that enters or leaves the electrode surface. This is indicated by the linear dependence of peak current vs. square root of scan rate. Since our experiments are carried out at elevated temperatures, formation of ferrates(VI) on the electrode surface is very fast and linear dependences of peak current on square root of polarization rate are found almost for all systems one can conclude
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Fig. 3. Nyquist plots of impedance spectra for (A) pure Fe, (B) FeSi, and (C) FeC electrodes at t = 160 ◦ C and at chosen potentials vs. RE: 1.45 V; 1.55 V; 1.65 V.
that that the electrode reaction kinetic is controlled by the mass transport under the semi-infinitive linear diffusion conditions. In the case of FeC at high temperature (see Fig. 2, stars) current density after reaching a certain maximum at about 400–500 mV s−1 decreases with increasing the polarization rate. It seems, that some chemical step due to the electrode structure become rate determining when scan rate reach ca. 400 mV s−1 (EC reaction mechanism). Our observation is in agreement with some previous results in hydroxide melts [13]. In the strong alkaline aqueous solutions a subsequent chemical reaction was also observed [18–20]. 3.2. Electrochemical impedance spectroscopy analysis In order to deeply characterize ferrate(VI) formation EIS was utilized. This technique is well suitable for surface layer structure and properties studies. An example of impedance spectra obtained for various electrodes at several chosen potentials is shown in Fig. 3. From all three figures A, B, and C two time constants with inductive passivation low frequency region can be easily recognize. The electrochemical behavior of the electrode in NaOH molten has been represented by EIS as an equivalent circuit (Fig. 4). The physical model based on the concept of two macrohomogeneous surface layers with theory of duplex sandwich assembly of passivating layers was taken into account in order to find the best equivalent circuit representation of our results. Two parallel R–Q (constant phase element) impedance elements characterizing individual layers (inner, outer) connected in series together with one resistant (RS ) and one inductance (L) element compose the simplest equivalent circuits that fit our experimental data with acceptably low
From Fig. 6 it can be seen that resistances of both inner and outer layer decrease with increasing applied potential. This means that either both layers are getting thinner with increasing potential or are disintegrated by strong anodic potential or by strong oxygen evolution. Resistance of outer layer is lower than resistance of inner one meaning that inner layer is little bit thicker or more compact. With increasing temperature the resistance of outer layer slightly and the resistance of inner layer significantly decreases. This means that the charge is at higher temperatures easily transferred; the layer is either thinner or less compact. At about 120 ◦ C the resistances are almost equal for individual sublayers. The capacity of inner and outer layer increases with increasing potential. This is in agreement with decrease of the resistances of both layers: layers are getting thinner or more disintegrated by oxygen evolution or strong anodic dissolution. Capacity of outer layer is lower than capacity of inner one. This means that outer layer is either thicker or more compact than inner one. Capacities of inner and outer layers increase with increasing temperature meaning that both layers are losing their thickness or compactness. At about 120 ◦ C individual capacities are approximately equal. Additional information can be gained from n parameter of constant phase element. In the case outer layer parameter n changes from almost unity (“ideal” behavior) to ca. 0.6 (non-ideal surface) with increasing potential. This means the quality of the outer layer is changing and the specific surface is increasing and is getting more porous. Bearing in mind the previous conclusions about Rout and Qout , one should think about the disintegration of the outer layer rather than reduction in its thickness. Values of nout are higher at
k
(Oi − Ei )/Ei , where Oi is the observed frequency for bin i 2 = i=1 and Ei is the expected frequency for bin i. Constant phase element (Q) was used instead of pure capacitance element (C) to underline the nonlinearity of the dissolution process. The accuracy of proposed equivalent circuit is documented on Bode plot (Fig. 5) where lines represent the results of the nonlinear regression analysis according to the model in Fig. 4. The optimized parameters of nonlinear analysis of the equivalent circuit for white cast iron (FeC) electrode at low and high temperatures in dependence on the electrode potential are shown in Fig. 6.
Fig. 4. Equivalent circuit representing the impedance of the system containing electrode and NaOH melt; RS , electrolyte resistance; L, outer inductance; Rin/out , inner/outer layer resistance; Qin/out , inner/outer layer constant phase element.
Fig. 5. Bode plot. FeSi electrode; t = 160 ◦ C; E = 1.55 V vs. RE. Points represent experimental data, curves nonlinear regression analysis according to the equivalent circuits in Fig. 4.
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Fig. 6. The fitted values of the equivalent circuit elements obtained for both inner and outer layer in dependence on the polarization potential of the FeC electrode at various temperatures: 80 ◦ C; 160 ◦ C.
low temperatures. This means that at low temperatures outer layer behaves more “ideally” than at higher temperatures. Contrary to previous observations, ninn is almost unchanged for all potential applied. Therefore, the conclusions made for Rinn and Qinn are connected to the reduction in thickness of the inner layer rather than its disintegration. Temperature has a low impact on ninn value. The inner layer is probably more stable than outer one. One can conclude that temperature has an impact on the thickness of the individual layers, but not on their compactness. This is in agreement with the theory concerning the mechanism of temperature influence on the surface behavior of the electrode consisting in a chemical interaction of the OH− anion with surface layer [18]. Parameters for other two electrodes show the same trend as in the case of FeC one (not shown). However, there are differences among them. Rinn is the highest for pure iron electrode and lowest for FeSi electrode. Qinn (FeC) are equal to Qinn (FeSi) and are higher than Qinn (Fe). Rout , Qout and nout are approximately similar for all three electrodes. This means that the structure of the electrode influence manly the inner layer of the anode, but not outer one. Utilizing, so called, static polarization curve method the impedance data were used to determine the number of electrons
involved in the electrode reaction of ferrate(VI) formation (peak A3). These curves were constructed as follows: DC current densities, jDC and DC potentials, EDC were recorded during impedance measurements and a static polarization curve ln(jDC ) vs. EDC was plotted. To obtain stationary conditions electrodes were polarized at chosen DC potential for 5 min prior to each experiment and then impedance spectra were recorded. The number of electrons was calculated from an exponent of the dependence of DC current density on DC potential jDC = e˛zFEDC /RT , where ˛ is a charge transfer coefficient, is an overpotential and R, T have an usual meaning, in the potential region where ferrates(VI) are formed. This analysis gave the average z = 3.08 ± 0.52 for the pure iron electrode, z = 2.74 ± 0.29 for FeSi electrode and z = 2.76 ± 0.19 for FeC electrode, where z is a number of electrons exchanged during the process corresponding to peak A3. Both findings, i.e. the number of exchanged electrons and the diffusion controlled reaction mechanism allow us to write following final electrochemical redox reaction of ferrate(VI) production: FeO2 − + 4OH− ↔ FeO4 2− + 2H2 O + 3e−
(5)
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4. Conclusions Three typical anode materials: pure iron (Fe), silicon-rich steel (FeSi) and white cast iron (FeC) electrodes were used in the process of electrochemical ferrate(VI) synthesis in the molten sodium hydroxide. Eutectic mixture NaOH–NaOH·H2 O was used in the temperature range 70–160 ◦ C. The cyclic voltammetric curves of the electrodes show characteristics similar for the all anode materials used. The peak current densities corresponding to the first and second step of the anode dissolution in the case of FeC as well as FeSi electrode are higher compare to the pure iron electrode. After passivity region subsequently the transpassive iron dissolution, including ferrate(VI) formation together with an oxygen evolution occurs and the current shoulder is visible for all electrodes used. Only slight differences (max. 40 mV) in the peak potentials are observed among electrodes. In all cases the hysteresis in the course of voltammetric curve in the transpassive potential region was observed. It is probably caused, on one side, by the diminution of the inhibition of the electrode surface toward dissolution at high potentials and, on the other side, due to the massive oxygen evolution causing the mechanical disruption of the anode surface. In order to deeply characterize ferrate(VI) formation EIS was utilized in the anode potential range of ferrate(VI) production. Measured impedance spectra confirm the physical model of the polarized surface based on the concept of two macrohomogeneous surface layers. In all cases the resistance of both inner and outer layer decrease with increasing applied potential. With increasing temperature the resistance of inner and outer layer decreases. The capacity of inner and outer layer increases with increasing potential. This is in agreement with decrease of the resistances of both layers: layers are getting thinner or more disintegrated by oxygen evolution or strong anodic dissolution. Additional information can be obtained from n parameter of constant phase element. In the case outer layer parameter n changes from almost unity (“ideal” behavior) to ca. 0.6 (non-ideal surface) with increasing potential. This means the quality of the outer layer is changing and the specific surface is increasing and is getting more porous. The number of exchanged electrons calculated from a static polarization curve at the potentials in ferrate(VI) formation region is z = 3 for all electrode materials used. Acknowledgements The authors gratefully acknowledge the financial support for this research by the Ministry of Education, Science, Research and Sport of the Slovak Republic within project VEGA 1/0985/12 and
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