Russian Journal of Electrochemistry, Vol. 41, No. 9, 2005, pp. 964–970. Translated from Elektrokhimiya, Vol. 41, No. 9, 2005, pp. 1087–1093. Original Russian Text Copyright © 2005 by Gorodetskii, Neburchilov, Alyab’eva.
Titanium Anodes with Active Coatings Based on Iridium Oxides: An Attempt to Elucidate the Possibility of Enhancing Catalytic Activity and Corrosion Resistance of Anodes at the Expense of Variations in the Formation Regime of the Coatings V. V. Gorodetskii, V. A. Neburchilov, and V. I. Alyab’evaz State Scientific Center of the Russian Federation Karpov Research Institute of Physical Chemistry, ul. Vorontsovo Pole 10, Moscow, 105064 Russia Received October 15, 2004
Abstract—On the basis of the polarization, corrosion, and radiotracer measurements it is established that the optimum conditions for the deposition of active coatings consisting of IrO2 and IrO2 + TiO2 onto titanium anodes are the performing of the pyrolysis in air at T = 350°C for 15 min with a final anneal in the same environment at T = 450°C for 1 h. Removing the final anneal or reducing its temperature enhances the catalytic activity of the anodes but at the same time reduce their corrosion resistance. Raising the anneal temperature above 450°C makes no sense, as the catalytic activity of the anodes toward the chlorine evolution reaction substantially diminishes and the titanium support undergoes oxidation starting with 500°C. Key words: catalytic activity, corrosion resistance, selectivity, gamma-ray spectrometry, chromatography, pyrolysis, thermal treatment
INTRODUCTION In work [1], polarization measurements as well as chromatographic and radiotracer methods were used to investigate the corrosion and electrochemical behavior of anodes with an active coating (AC) of IrO2, 30 mol % IrO2 + 70 mol % TiO2 (OITA), and 30 mol % (IrO2 + RuO2) + 70 mol % TiO2 (OIRTA) in conditions that were similar to the conditions of a chlorine electrolysis (300 g l–1 NaCl, pH 2, T = 87°ë). It was established that the chlorine evolution rate on titanium anodes with a coating of IrO2 (with an iridium load of 6 g m–2), in contradistinction to ORTA and RuO2, was limited not by the diffusion stage of the removal of the reaction product, chlorine, away from the electrode, but by the stage of discharge of the chloride ions [1]. In so doing, an anodic polarization curve (PC) exhibited an extended Tafel portion with a slope of 40 mV, which shifted somewhat in the positive direction following an increase in the polarization duration. During anodic polarization of such an anode by constant current density i = 0.2 A cm–2, as was the case with ORTA and RuO2, there was observed a substantial decrease with time of the rate of dissolution of the noble metal out of the coating. The said decrease continued until a steady-state rate of the metal dissolution was established, specifically, q = 6 × 10–9 g cm–2 h–1 [1, 2]. This particular rate happened to be approximately z
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200 times as small as the dissolution rate (q = 1 × 10−6 g cm–2 h–1) of ruthenium out of RuO2 [3]. In so doing, the steady-state value of the concentration of oxygen in chlorine was equal to 0.3 vol %, which was 2 times smaller than on RuO2. Established was the existence of a Tafel relationship between the steadystate rate of dissolution of iridium out of IrO2 and the anode potential with a slope of 0.100 V, which points to an electrochemical mechanism of dissolution of IrO2. It was demonstrated that, in contradistinction to RuO2, the dissolution rate of IrO2 (at constant current density i = 0.2 A cm–2) is very weakly dependent on the solution acidity in the pH interval 0.8–6.5 [1, 2]. Upon going from anodes with a coating of IrO2 to OITA with 30 mol % IrO2 + 70 mol % TiO2, the steadystate dissolution rate of iridium out of the coating (at constant current density i = 0.2 A cm–2) was found to decrease to the quantity q = 3.2 × 10–9 g cm–2 h–1. This was presumably caused by that, in so doing, the partial concentration of iridium in the coating was decreasing, but simultaneously there was observed an increase in the electrode potential [1]. In order to prevent, as far as it was possible, such an increase in the potential of OITA, there was realized a replacement of a portion of IrO2 in the active coating of OITA by RuO2, which was considerably more active catalytically but less corrosion resistant. In so doing, the high electroconductance of the coating was retained at the expense of the maintaining of the overall concen-
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tration of noble metals, which was equal to 30 mol %, invariant [1]. It was demonstrated that such anodes as OIRTA with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2 do possess a high catalytic activity toward the chlorine evolution reaction as compared with IrO2 and OITA, though they are inferior in this respect to ORTA [1]. It was discovered that on OIRTA, as well as on IrO2 and OITA, there was observed a decrease in the anode potential in the course of the first hour of electrolysis, after which there was established its steady-state value [1]. As a result, at a standard value of the current density, specifically, i = 0.2 A cm–2, the steady-state potential of OIRTA was higher than the steady-state potential on ORTA by approximately 0.025 V, and the steadystate dissolution rate of iridium out of it was found to be equal to 1 × 10–9 g cm–2 h–1. The last value was approximately ten times as small as the dissolution rate of ruthenium out of ORTA, which was equal to 1.2 × 10−8 g cm–2 h–1 [3]. Under these conditions, the steadystate value of the oxygen concentration in chlorine amounted to 0.05% [1], which was smaller than that on ORTA by approximately 2.5 times [3]. As to the present work, its aim is the elucidation of the possibility of enhancing the catalytic activity and corrosion resistance of titanium anodes with an active coating of IrO2 and IrO2 + TiO2 at the expense of justified selection of conditions that would be optimum for the formation of their active coating.
EXPERIMENTAL All the anodes, with the exception of those mentioned specifically, were manufactured with the aid of the procedure that was described in detail in [4]. An active coating would be deposited in so doing on chemically polished titanium with a protective sublayer preliminarily formed on it, the sublayer in question consisting of metallic iridium, titanium, and their oxides [5]. The coating was produced by means of pyrolysis of corresponding salts in air at a temperature of 350°ë for the duration of 15 min. All the anodes had a constant iridium load in the coating, which was equal to 2.5 g m–2. The anodes differed from one another by the temperature of their final anneal and the composition of the gas phase in which this anneal was conducted. The catalytic activity of these anodes was determined by means of the recording of PC for the chlorine evolution in a solution of 300 g l–1 NaCl with an additive of HCl to pH 2 at a temperature of 87°ë on stationary vertically positioned anodes while constantly bubbling the electrolyte with argon. All the potentials are referred to a normal hydrogen electrode. The polarization curves are corrected for the ohmic drop of potential between the Luggin capillary and the electrode surface, found from curves of potential decay, which occurred after the opening of the polarizing circuit. A brief opening of the circuit was realized by means of a current interrupter, which had been designed and manufactured RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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in the State Scientific Center of the Russian Federation Karpov Research Institute of Physical Chemistry. The dissolution rate of iridium out of the coating was determined with the aid of a gamma-ray spectrometric method of measurements of low dissolution rates of metals, which had been developed in the above institute [6]. The measurements were taken in a cell of a flow-through type, which was described in detail in [3]. The radioisotope 192Ir with a half-value period equal to 79 days was formed in the coating when irradiating electrodes in a nuclear reactor by a neutron flux density equal to 3 × 1013 n cm–2 s–1 for the duration of 10 h. The dissolution rate of iridium was calculated on the basis of the radioactivity of the solution that flowed out of the cell in the course of a certain time interval. The recording of the activity of this solution was realized on a single-channel spectrometer of the type Spectrometer 20060 (German Democratic Republic) in a differential regime, in a specified energy interval. The geometry of the recording of gamma radiation was close to 2π. Performing electrolysis without interruption of polarization in the course of 1000 h and longer made it possible to reliably measure steady-state values of the dissolution rate of iridium. The anodes' selectivity was determined by means of measuring the concentration of oxygen in the gas phase with the aid of a gas chromatography procedure designed in [7]. The analysis of gases was conducted on a chromatograph of the type Tsvet 530 with use made of a molecular sieve of the type SA, on which there occurred effective absorption of chlorine and separation of peaks pertaining to light gases (oxygen, nitrogen). Bearing in mind the constancy of the ratio between peaks pertaining to nitrogen and oxygen in air, this made it possible to calculate, on the basis of the nitrogen peak, the concentration of oxygen in the gas phase, the oxygen in question penetrating into the electrolyzer because of the influx out of air. The difference between the overall concentration of oxygen in the gas phase and the concentration of oxygen penetrating into the gas phase out of air was used for the determination of the concentration of electrolytic oxygen in gaseous chlorine. All this made it possible to enhance the sensitivity of the gas chromatography method.
RESULTS AND DISCUSSION Shown was that anodes with a coating of IrO2, which was obtained by subjecting hydrogen hexachloroiridate(IV) hydrate to pyrolysis in air at a temperature of 350°ë, are amorphous and not dense (Fig. 1a). They possess the same hilly relief of surface as does chemically polished titanium (see Fig. 1 in [5]). Annealing such anodes in air at a temperature of 450°ë makes the coating denser and extended cracks appear in so doing on individual hills (Fig. 1b). Their emergence is probably caused by the break of the coating in the course of oxidation in the No. 9
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1.4
6 1
1.3
(a)
10 µm 3
1 –logi [A cm–2]
2
Fig. 2. Polarization curves for chlorine evolution in a 300 g l–1 NaCl solution of pH 2 at 87°C which was continuously bubbled with argon. The curves were obtained on the anodes with an active coating of IrO2 (load of iridium 2.5 g m–2), which were formed by means of pyrolysis in air at 350°C for the duration of 15 min as follows: (1) without subsequent final anneal; (2, 3, 4) with subsequent final anneal in air at temperatures of, respectively, 450, 500, and 550°C; (5) with subsequent final anneal in oxygen at 450°C; and (6) similar PC for RuO2 that passed final anneal in air at 450°C (load of ruthenium 1.25 g m–2). The final anneal of anodes 2 through 6 was conducted for the duration of one hour.
(b)
10 µm Fig. 1. Electron microscopy photographs taken in secondary electrons of the surface of anodes with an active coating of IrO2 obtained as follows: (a) by means of pyrolysis of hydrogen hexachloroiridate(IV) hydrate in air at 350°C for 15 min and (b) after final anneal of these anodes in air at 450°C for 1 h.
process of the anneal of the titanium support. This was later confirmed by the results of an x-ray diffraction analysis. Obtained were polarization curves for the chlorine evolution in a solution of 300 g l–1 NaCl at pH 2 and the temperature T = 87°ë on anodes with a coating of IrO2 without the final anneal and on anodes that passed the final anneal in air at temperatures of 450, 500, and 550°ë as well as in oxygen at a temperature of 450°ë (Fig. 2). It was established that the PC (curve 1), which was obtained on the anode that was not subjected to the final anneal, lies in so doing lower that all the others. The process of chlorine evolution on this anode is described by a Tafel plot with a slope of 0.034 V through the entire interval of currents studied in this work. In the course of the final anneal of such anodes at temperatures of 450, 500, and 550°ë there was observed an increase in the Tafel slope of PC from 0.034 V to 0.046 V (curves 2–4) with a simultaneous shift of the curves in the direction of more positive
potentials, which points to that the process of chlorine evolution slowed down. In so doing, in the region of high currents there took place a noticeable deviation from a Tafel plot in the direction of more positive potentials. Performing the final anneal of anodes with a coating of IrO2 not in air (curve 2) but in oxygen (curve 5) at the same temperature of 450°ë leads merely to an insignificant shift of PC in the direction of more positive potentials. In order to find out what defines the rate of the process of chlorine evolution on the anode that is most active out of all the anodes studied (curve 1), similar polarization measurements were conducted on an anode with a coating of RuO2 with the same load of the precious metal in its active coating expressed in g-atoms per m2 (ruthenium load 1.25 g m–2) (curve 6). The polarization curve that was obtained on this anode had a Tafel dependence with a slope of 0.036 V and practically coincided with curve 1. The reaction of chlorine evolution occurs on anodes of RuO2 at a high rate and is limited by the chlorine diffusion away from the electrode [8]. Consequently, the obtained coincidence of PC 1 and 6 points to that the reaction of chlorine evolution, when occurring on IrO2, is also limited in this case by the same stage. The characteristic feature of the behavior of anodes with a coating of IrO2 is a substantial displacement of
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polarization curves obtained on these anodes for the course of the first 100–150 h of electrolysis in the direction of more positive potentials, which points to a decrease in their catalytic activity and the passing of reaction in a kinetic regime. For example, for the anodes that passed the final anneal at temperatures of 450 and 500°C at a current density of 0.2 A cm–2, the potential of anodes increased, respectively, from 1.380 to 1.420 V and from 1.400 to 1.440 V. In order to find out what were the reasons for the decrease in the catalytic activity of IrO2 with increasing temperature of the final anneal of the anodes, we obtained voltammetric curves on these anodes in the potential region, which extended from 0.650 to 1.300 V, and from the area under the anodic branch of these curves determined values of the surface charge for the corresponding anodes. The result was that, upon going from the anode that passed no final anneal to the anodes that were scorched at temperatures of 400 and 450°C, the magnitude of the charge decreases, respectively, from 95 to 40 and to 14 mC cm–2 at the potential scan rate v = 0.020 V s–1 and from 51 to 20 and to 8 mC cm–2 at v = 0.200 V s–1. In accordance with [9–11], this decrease in the charge is probably caused by the decrease in so doing in the magnitude of the surface area of electrodes. The validity of this assumption was confirmed with the aid of measurements that were performed by means of a BET method [12]. It was demonstrated that the magnitude of the surface area of the anodes on the anodes with a coating of IrO2 that was obtained by pyrolysis at temperatures of 350, 450, and 550°C decreased from 26.5 to 6.5 and to 2.3 m2 g–1, respectively. These results point to that the decrease in the rate of the process of chlorine evolution, which was observed following an increase in the temperature of the final anneal, was caused by a decrease in the magnitude of the working surface area of the anodes. This leads to that the process of chlorine evolution on the IrO2 anodes with a thin active coating (with an iridium load of 2.5 g m–2), which were subjected to the final anneal at temperatures of 500 and 550°C, proceeds in a kinetic regime with the slope of PC ba = 0.046 V, which corresponds to the transfer coefficient of the anodic process β = 1.55. As we have already mentioned in the foregoing, on the anode with a thicker active coating of IrO2 (with an iridium load of 6 g m–2), an anodic PC for the reaction of chlorine evolution that was recorded in analogous conditions had the slope b‡ = 0.040 V, which corresponded to β = 1.8. It is most probable that this discrepancy between values of β, which is observed on anodes with different coating thickness, is caused by that the increase in the rate of chlorine evolution on the anode with a thicker coating with increasing polarization of the anode occurs not only as a consequence of an increase in so doing in the potential of the anode but also as a result of an increase in the depth of penetration of the reaction inside the porous coating [13]. This can take place if, in contradistinction to ORTA and RuO2, RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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–logqIr [g cm–2 h–1)] 6
7 1 8 2 3 9
100
300
500
τ, h
Fig. 3. Variation of the iridium dissolution rate with time on the anodes with an active coating of IrO2 in the course of their anodic polarization in a 300 g l–1 NaCl solution of pH 2 at 87°C and i = 0.2 A cm–2. The active coating was formed by means of pyrolysis in air at 350°C for the duration of 15 min as follows: (1) without subsequent final anneal and (2, 3) with final anneal in air for one hour at temperatures of, respectively, 400 and 450°C. The arrow marks an interruption of the anodic polarization.
the reaction of chlorine evolution on IrO2 proceeds with a true transfer coefficient of the overall process, which is equal not to 2 but to 1.5. The latter satisfactorily agrees with its magnitude that was obtained on the anode with a thin coating. It happened to be possible to obtain the true transfer coefficient on such an anode with a thin coating probably as a consequence of that the reaction of chlorine evolution on this anode occurs from the entire depth of the coating. The deviation from a Tafel dependence in the direction of higher potentials, which was observed on PC for chlorine evolution in the region of high currents, is most probably connected with the oxidation of the titanium support in the process of the final anneal, which leads to an increase in the resistance of a transition layer between titanium and the coating. It should be noted that the increase in the anodic potential with time, which was observed on IrO2 in conditions of the chlorine electrolysis, took place also in the case of polarization of these electrodes in conditions of evolution of oxygen on them. In the case of polarization of IrO2 in an NaClO4 solution of pH 1 at a temperature of 87°ë and i = 0.2 A cm–2, its potential increased within the first 18 h of the electrolytic process by almost 0.100 V, specifically, from 1.666 to 1.765 V. In so doing, the ohmic drop of potential between the electrode and siphon, which was found from the curves of the decay of potential, increased by a mere 0.030 V. Established was that, on the anodes with a coating of IrO2, which was obtained by pyrolysis in air at a temperature of 350°ë, as well as on analogous anodes that were then subjected to the final anneal at temperatures of 450 and 500°ë, in the case of their anodic polarization in conditions that were close to the conditions of a No. 9
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–logqIr [g cm–2 h–1)] 6
4 1.4
3
7 5 2 1 1
8
6
2 9
100
300
3 500
700
τ, h
1.3 3
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Fig. 4. The same curves as in Fig. 3, which were obtained for the anodes with a coating of IrO2, each layer of which was formed in accordance with a two-stage scheme of the final anneal, i.e. first in argon containing 1 vol % of oxygen at 350°C for one hour and then in air at 400°C for 15 min: (1) without final anneal and (2, 3) with final anneal in air for one hour at temperatures of, respectively, 400 and 450°C. The arrow marks an interruption of the anodic polarization.
Fig. 5. Polarization curves for chlorine evolution (in the same conditions as those pointed out in the caption to Fig. 2) on OITA that were formed by means of pyrolysis in air at 350°C for 15 min: (1) without final anneal; (2–6) with final anneal for one hour; (2, 3, 4) in air at temperatures of, respectively, 400, 450, and 500°C; (5) in oxygen at 450°C; and (6) in argon at 450°C.
chlorine electrolysis, the rate of dissolution of iridium out of the coating decreased with time (Fig. 3). In so doing, if the steady-state rate of the iridium dissolution in the absence of the stage of final anneal amounted to a quantity that was equal to 3.1 × 10–8 g cm–2 h–1, then, after the final anneal of the anodes at temperatures of 450 and 500°ë, it decreased by almost an order of magnitude and had values that were equal to, respectively, 3.6 × 10–9 g cm–2 h–1 (curve 2) and 4.0 × 10–9 g cm–2 h–1 (curve 3). As in the case of ORTA and RuO2, an interruption in the polarization led to a subsequent increase in the rate of dissolution of the precious metal; moreover, the former rate of dissolution of iridium was reached only in 50–70 h after the current was switched on. A weaker effect of the final anneal on the stability of a coating of IrO2 was observed on the anodes whose every layer was formed in accordance with the two-stage scheme (the first stage was an anneal in Ar + 1 vol % O2 at a temperature of 350°ë for 1 h and the second stage, an anneal in air at a temperature of 400°ë for 15 min) (Fig. 4). Upon going from the anode that was obtained without a final anneal (Fig. 4, curve 1) to the anodes that were scorched at temperatures of 450 and 500°ë (Fig. 4; curves 2, 3), the steady-state rate of dissolution of iridium diminished, respectively, from 1.2 × 10−8 g cm–2 h–1 to 7.3 × 10–9 g cm–2 h–1 and then to 4.3 × 10–9 g cm–2 h–1. It should be noted that these electrodes were conspicuous by a high stability of their potentials with time, which for electrodes 1, 2, and 3 (Fig. 2) altered in the intervals, respectively, 1.400–1.410, 1.388–1.407, and 1.404–1.414 V for the duration of 500 h of electrolysis. The effect of the temperature and the composition of the gas phase of the final anneal on the catalytic activity
and corrosion resistance of anodes was also investigated on OITA with 30 mol % IrO2 + 70 mol % TiO2. Polarization curves for the reaction of chlorine evolution on OITA, which were prepared by pyrolysis in air at a temperature of 350°ë for the duration of one hour without a final anneal and with a final anneal in air at temperatures of 400, 450, and 500°ë, as well as in argon and oxygen at a temperature of 450°ë, were obtained (Fig. 5). It was established that, at low currents, the reaction of chlorine evolution on all these anodes, with the exception of the anode that was scorched in air at a temperature of 500°ë (curve 4), proceeded at close rates, and in PC there was observed an extended Tafel portion with a somewhat different slope. The latter led to a discernible difference in the rate of chlorine evolution at a constant potential in the region of higher currents. Besides, at current densities in excess of 0.05 A cm–2, all the PC exhibited deviation from a Tafel dependence in the direction of a steeper slope. On OITA that was not subjected to a final anneal, the slope of the Tafel portion in PC was equal to 0.022 V, and the reaction of chlorine evolution occurred at a maximum rate. Following an increase in the temperature of the roasting of OITA from 400 to 450°C, the Tafel slope of PC would increase from 0.028 to 0.041 V. After a further increase in the temperature of the roasting of OITA to 500°C, the increase in the Tafel slope of PC to 0.046 V was accompanied by a shift of the PC in the positive direction by more than 0.02 V, which points to a substantial slowing down in so doing of the electrode process. Judging from the magnitude of the slope of curves 3 and 4 obtained on these anodes, the chlorine evolution reaction is limited predominantly by the electrochemical stage of discharge of chloride ions.
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–logqIr [g cm–2 h–1] 6
7 1 2
8 3 9 (a)
200
10 µm
400
600
τ, h
Fig. 7. Variation of the iridium dissolution rate with time out of OITA in the course of their anodic polarization in a 300 g l–1 NaCl solution of pH 2 at 87°C and i = 0.2 A cm–2. The active coating was formed by means of pyrolysis in air at 350°C for the duration of 15 min as follows: (1) without subsequent final anneal and (2, 3) with final anneal in air for one hour at temperatures of, respectively, 400 and 450°C. The arrow marks an interruption of the anodic polarization.
(b)
(c)
10 µm
10 µm
Fig. 6. Electron microscopy photographs in secondary electrons of the surface of OITA, which passed a final anneal for one hour at 450°C in (a) air, (b) oxygen, and (c) argon.
It proved to be far more difficult to find out the reason for the anomalously small slope (smaller than 2.3 RT/nF) of PC curves 1, 2, and 6. Judging from the position of these curves as compared with a similar curve that was obtained on RuO2 (which was deposited on chemically polished titanium, curve 6 in Fig. 2), the reaction of chlorine evolution on the said OITA, as in RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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the case of RuO2, is also limited by the removal of chlorine away from the surface of anodes. In so doing, the anomalously small slope of PC on OITA is explained probably by that the removal of chlorine away from the surface of the anode is realized in this case not only by means of diffusion but also as a result of evolution of bubbles of gas, which leads to an intensive agitation of solution in the vicinity of the surface of the anode [14–17].1 The catalytic activity of OITA noticeably depends on the composition of the gas phase, in which the final anneal of anodes is conducted. For example, at the same temperature of 450°C of the final anneal, OITA that was roasted in oxygen (Fig. 5, curve 5) possesses a higher catalytic activity (i.e. PC lie lower) than OITA that was roasted in air. It is most probable that in air, as opposed to oxygen, there always is a certain amount of water vapor, whose presence hinders crystallization process. Indeed, as follows from electron microscopy measurements performed in the case of the final anneal in oxygen (Fig. 6), crystallites of the coating happen to be better pronounced and at the boundaries between large crystallites there occurs segregation of impurities. The decrease in the catalytic activity of OITA, which was observed to occur upon increasing the temperature of the final anneal (Fig. 5, curves 1–4), is caused probably by that, as is the case with IrO2, there occurs in so doing a decrease in the magnitude of surface as a result of the intensification of processes of crystallization and the densification of coatings. 1 This
portion of PC must not be confused with a portion of small polarizability (PSP), which was discovered for the first time in [14] and construed theoretically in [18]. In PSP PC must have a slope that is close to a horizontal one [19].
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Obtained were curves of variations with time in the rate of dissolution of iridium out of coating OITA in the case of their polarization in conditions that were close to the conditions of a chlorine electrolysis (Fig. 7). It was demonstrated that the steady-state rate of corrosion 2.9 × 10–8 g cm–2 h–1 on OITA that was not subjected to a final anneal (Fig. 7, curve 1) happens to be discernibly higher than on OITA that were subjected to the final anneal at temperatures of 400 and 450°C (curves 2, 3), respectively, 1.8 × 10–8 and 3.5 × 10–9 g cm–2 h–1. In so doing, it is essential that, same as on IrO2, a steady-state rate of corrosion was attained faster on the anode that passed the final anneal at a higher temperature. Thus, as a result of performed investigations it was established that the conditions that are optimum for the formation of an active coating of IrO2 and IrO2 + TiO2 on titanium anodes are the performance of pyrolysis of relevant salts (deposited on the surface in the form of a covering solution) in air at a temperature of 350°ë with a subsequent final anneal of the entire coating (after the deposition of the required number of its layers) in air at a temperature of 450°ë. Such a choice of the pyrolysis temperature provides for a sufficiently complete decomposition of relevant salts [4] and a high catalytic activity of the coatings. The performance of the final anneal at a temperature of 450°ë permits the rising of the corrosion resistance of coatings studied in this work by almost an order of magnitude with a certain simultaneous drop of their catalytic activity toward the reaction of evolution of chlorine.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13. 14. 15.
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RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41
No. 9
2005
