- Email: [email protected]
Control of the electrode temperature for electrochemical studies: A new approach illustrated on porous anodizing of aluminium
Electrochemistry Communications 11 (2009) 2292–2295
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Control of the electrode temperature for electrochemical studies: A new approach illustrated on porous anodizing of aluminium Tim Aerts *,1, Iris De Graeve, Herman Terryn Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
a r t i c l e
i n f o
Article history: Received 7 September 2009 Received in revised form 9 October 2009 Accepted 9 October 2009 Available online 20 October 2009 Keywords: Temperature Electrode temperature Aluminium Anodizing Porous oxide
a b s t r a c t A new approach for studying the effect of temperature on electrochemical processes is presented in this paper. Using an in-house developed electrode holder, experiments are performed under conditions of applied and controlled electrode temperature. This new approach provides an improved temperature control during the experimental study and, additionally, allows distinguishing both the influences of the electrolyte and electrode temperatures. The advantages of the applied electrode temperature approach are illustrated by considering porous anodizing of aluminium. In a broad temperature range the electrochemical behaviour of the aluminium electrodes, recorded during the new and the conventional way of anodizing, are compared. Differences between the anodic potential evolutions in both approaches are observed, and are explained by a heat flux to the surroundings during the experiments at uncontrolled electrode temperature. These results illustrate the advantage of applying the electrode temperature. If the influence of temperature on a process is investigated by merely varying the electrolyte temperature, the electrode temperature is only indirectly influenced and can significantly differ from the electrolyte temperature. Therefore, when evaluating the influence of temperature on an electrochemical system the electrode temperature should be considered, and preferentially also controlled. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Whereas the influence of mass transfer on electrochemical systems is widely investigated and an abundant literature is available on this subject, only a limited number of studies deal with the influence of temperature and heat transfer on electrochemical processes. For many electrochemical processes the local effects of temperature at the electrode might be neglected and the temperature can be considered as a bulk parameter, influencing physical and electrokinetic data (such as pH, kinetics of reactions). However, in some cases the electrode temperature has a significant influence on the system, e.g. on processes which involve an important heat generation at the electrode, such as anodic oxide growth on aluminium. In general the influence of temperature on an electrochemical process is evaluated merely by considering the electrolyte temperature; the temperature of the electrode is generally not considered. This is for example the case for studies which can be considered as references describing the temperature dependency of anodizing of aluminium [1–8]. They acknowledge the important effect of temperature on the electrochemical behaviour of the aluminium anode * Corresponding author. Tel.: +32 2 629 35 35; fax: +32 2 629 32 00. E-mail address: [email protected] (T. Aerts). 1 ISE member. 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.10.013
and resulting oxide layer, though, do not explicitly consider the electrode temperature. It is assumed to equal the electrolyte temperature. More accurate information on the impact of the electrode temperature on an electrochemical system can be obtained by experiments in which this parameter is measured in situ. For anodizing of aluminium a very limited number of studies is available in which the global, or local aluminium temperature is monitored by in situ temperature measurements [9–14]. These studies demonstrate the significant influence of the electrode temperature on the anodic reaction. However, they still consider anodizing conditions in which the temperature of the aluminium is only affected indirectly and in a non-controlled way (i.e. by varying electrolyte convection). The recorded electrode temperatures are determined by thermal equilibrium under the prevailing conditions. This issue is overcome by the new approach introduced in this communication. Using an in-house developed electrode holder, electrochemical reactions can be investigated under conditions of applied and controlled electrode temperature, an approach which offers new possibilities for the study of the influence of temperature. In the present paper first the characteristics of the temperature controlling electrode holder are presented, followed by illustrations of the improved temperature control of the new approach. To this purpose the porous anodizing of aluminium is considered as a case study and experiments at applied electrode
T. Aerts et al. / Electrochemistry Communications 11 (2009) 2292–2295
temperature are compared to corresponding experiments at uncontrolled electrode temperatures. Furthermore, a brief example of the possibility to independently vary the electrode and electrolyte temperatures, which enables to distinguish and compare the influences of both temperatures on the process, is presented. More details on these results, and on other findings on the influence of the electrode temperature during anodizing of aluminium obtained by this new approach, will be considered in future research papers. 2. Experimental setup A schematic cross-section of the temperature controlling electrode holder, with its different composing parts labeled, is presented in Fig. 1. The core of the holder is a thermo-electronic component (2) which acts a ‘heat pump’. Based on the Peltier effect it either transfers heat towards, or removes heat from the aluminium electrode, depending on its polarisation. Hence, ‘heated’, as well as ‘cooled’ anodes, i.e. with temperatures respectively higher or lower than the electrolyte, can be considered. On the electrode-side of the thermo-electronic component an aluminium bar with a diameter 60 mm and a height 50 mm is mounted (3). This part, which is encased in PVDF (7), acts as a ‘thermal capacitor’ and provides support for the electrode (5). As the Peltier element is only able to transfer heat from its one side to the other, the necessary ‘heat sink’ is provided by a flow-through heat-exchanger (1). The temperature of the electrode is controlled (±0.1 °C) by a PID controller unit in combination with PT-100 temperature probe, which is mounted in the aluminium bar at a depth of less than 5 mm from the electrode surface (4). The active electrode surface is limited to 10 mm by a PVDF cover (8), which is screwed onto the encased aluminium bar. A silicon spacer mat (6) is included to ensure a proper sealing of the non-exposed area on the electrode. The holder unit is fixed in a PMMA casing (10). During anodizing the entire holder is immersed into the electrolyte.
2293
3. Experimental conditions Anodizing experiments are performed in 50 l of 145 g=l H2 SO4 þ 5 g=l Al2 ðSO4 Þ3 18H2 O electrolyte, which is thermostatically controlled (±0.1 °C) by a Lauda RP845 thermostatically Intense electrolyte agitation was provided by a mechanical stirrer. Due to the high rotation speed of the stirrer the anode potential was no longer affected by a further increase in rotation speed. In the framework of this communication both the temperature of the electrolyte ðT H2 SO4 Þ and the temperature of the anode ðT Al Þ are varied in a wide range from 5 °C up to 65 °C. The influence of the electrode temperature is evaluated at the four evenly distributed temperatures 5, 25, 45 and 65 °C. Disk shaped AA1050 aluminium samples (99.5% Al sheet 0.3 mm) with a diameter of 55 mm were used, the diameter of the active anode surface limited to 10 mm. Prior to anodizing samples were alkaline etched in 60 g/l NaOH solution at 60 °C for 60 s, followed by a desmutting treatment in a 1:1 concentrated HNO3:H2O solution at room temperature for 90 s. The anodizing cell consists of a three electrode configuration, involving a large aluminium sheet as counter electrode and a Ag/ AgCl reference electrode. Anodizing is performed under galvano2 static conditions at a current density of 1 A=dm , the current being applied by a computer controlled Delta Elektronica SM 300–20 power source. A charge density of 512 C/dm2 is considered, corresponding to anodizing times of 512 s. The resulting potential evolutions are recorded with a National Instruments M-6220 DAQ card and using National Instruments VI-Logger software. The presented electrode potentials are corrected for the Ohmic drop between the anode and reference electrode. To check reproducibility at least three separate experiments are performed for each considered condition. During the entire anodizing treatment, i.e. from the onset of the anodic oxide growth, a constant electrode temperature is considered. This is ensured by the following experimental procedure, which is systematically applied. Before introducing the temperature controlling electrode holder into the electrolyte, the temperature of the mounted electrode is set to the desired value. Only after attaining a stable value of this electrode temperature the holder is immersed into the solution. Due to considered temperature differences between the electrode and the electrolyte (electrolyte temperatures, different from the electrode temperature, are evaluated) a minor deviation of the electrode temperature from its intended value can occur. Therefore, the holder is given the time to re-adjust the electrode temperature: from the moment of immersion on, a fixed timespan of 90 s is waited before applying the anodic current. Under all considered conditions this period is sufficient to guarantee a constant electrode temperature from the start of the experiments on.
4. Comparing ‘conventional’ and ‘applied electrode temperature’ approaches
Fig. 1. Schematic cross-section of the side-view of the temperature controlling electrode holder with the different parts labeled: (1) flow-through heat-exchanger, (2) thermo-electronic component, (3) aluminium bar, (4) inserted temperature probe, (5) electrode, (6) silicon spacer mat, (7) PVDF encasing of the aluminium bar, (8) screw cap, (9) electrical connection and (10) PMMA casing.
As indicated in the introduction, during the conventional way of anodizing the electrode temperature T Al is generally not known and is supposed to equal the electrolyte temperature. However, this might not always be correct. This is illustrated by comparing conventional anodizing experiments (i.e. without controlling the electrode temperature; referred to as ‘T Al free’ experiments) to experiments with applied electrode temperatures equal to the considered electrolyte temperature T H2 SO4 . Characteristic evolutions of the electrode potential U we [15,16], recorded during anodizing at corresponding temperatures for both approaches, are presented in Fig. 2. Clearly, differences in the potential evolutions between the uncontrolled and controlled elec-
2294
T. Aerts et al. / Electrochemistry Communications 11 (2009) 2292–2295
Fig. 2. Potential evolutions recorded at different temperatures during anodizing under conditions of controlled and uncontrolled (‘T Al free’) electrode temperatures.
trode temperature experiments are observed. At a low electrolyte temperature of 5 °C the experiments performed at uncontrolled electrode temperatures systematically yield lower electrode potentials than those at applied electrode temperatures equal to the electrolyte temperature. On the other hand, at an electrolyte temperature of 25 °C the conventional approach typically results
in higher anode potentials than the approach involving controlled electrode temperatures. This effect persists and even increases when the electrolyte temperature is further increased – cf. the potential evolutions at 45 and 65 °C. Even at equal electrolyte temperatures higher aluminium temperatures result in lower potentials [12,13]. The lower electrode
Fig. 3. Evolution of the stationary electrode potential U we;stat as a function of temperature. The full lines correspond to the evolution as a function of T Al at different, constant T H2 SO4 , the dashed lines to the evolution as a function of T H2 SO4 at different, constant T Al .
T. Aerts et al. / Electrochemistry Communications 11 (2009) 2292–2295
potentials recorded during the conventional approach at a low electrolyte temperature of 5 °C therefore indicate actual electrode temperatures which are higher than the applied electrode temperatures in the new approach. At higher electrolyte temperatures, on the other hand, the higher potentials observed under the uncontrolled approach correspond to actual electrode temperatures which are lower than the considered electrolyte temperatures. These effects are explained by considering the heat flux to the surroundings. At electrolyte temperatures lower than the ambient temperature in the lab (approximately 20 °C) an uptake of heat from the surroundings influences the actual electrode temperature, increasing it to a value higher than the considered low electrolyte temperature. When anodizing is performed at electrolyte temperatures higher than the ambient temperature a heat loss to the surroundings is expected, cooling the anode and reducing its temperature to a value lower than the electrolyte temperature. This heat loss is enhanced at higher electrolyte temperatures, increasing the difference between the actual electrode and the electrolyte temperature, which explains the increased potential difference between the conventional and the new approach at higher temperatures. These results illustrate the advantage of anodizing under applied electrode temperature. If the influence of temperature on the process is investigated by merely varying the electrolyte temperature, the electrode temperature is only indirectly influenced and can significantly differ from the electrolyte temperature. This effect, which is believed to be due to heat flux to the surroundings, increases with increasing difference between the considered anodizing temperature and the ambient temperature in the lab. 5. Illustration of the evaluation of influences of electrolyte and electrode temperatures Additional to an improved temperature control the temperature controlling electrode holder also enables varying the electrode temperature independently of the electrolyte temperature. Hence, it is possible to separately evaluate the influences of both the electrolyte and the electrode temperature. As an example this is illustrated by Fig. 3, which displays the evolutions of the stationary electrode potential U we;stat as a function of temperature. The full lines correspond to the evolution as a function of T Al at different, constant T H2 SO4 , whereas the dashed lines represent the evolution as a function of T H2 SO4 at different, constant T Al . For example, the larger decline in stationary potential upon increasing the electrode temperature at constant electrolyte temperature demonstrates the more important influence of T Al on this parameter than of T H2 SO4 . Moreover, a direct relation between U we;stat and the dimensions of the characteristic features of the porous film (e.g. the barrier layer and pore diameter) was observed in [3], this at different electrolyte temperatures. Although the latter study did not consider the electrode temperature, the general applicability of this relation has been demonstrated. Therefore, the presented larger variations in U we;stat as a function of T Al than as a function of T H2 SO4 indicate that also concerning the morphology of oxide layer the electrode tem-
2295
perature has more important influence. The precise impact of T Al on the electrochemical behaviour of the aluminium anodes and on the resulting oxide layer will elaborately be presented in a future paper. 6. Conclusions A new approach for the study of the influence of temperature on electrochemical processes is introduced. By performing experiments under conditions of controlled and applied electrode temperature an improved control over the temperature is obtained. This is illustrated by the presented results, recorded during porous anodizing of aluminium. The differences in potential evolutions, recorded during the new and the conventional approach, are believed to be due to heat fluxes to the surroundings under conditions of uncontrolled electrode temperature. This effect can significantly influence the actual electrode temperature during the conventional approach, resulting in important differences between the electrode and the electrolyte temperature. Therefore, when studying the influence of temperature on an electrochemical system the electrode temperature should be considered and preferentially also controlled. Furthermore, the new approach allows varying the electrolyte and electrode temperatures independently, which enables distinguishing the separate influences on the considered process. Acknowledgements The authors acknowledge the support from the Instituut voor de aanmoediging van innovatie door Wetenschap & Technologie in Vlaanderen (IWT, contract no. SBO 040092). Raf Claessens and Marnix Depauw are greatly acknowledged for the design and development of the temperature controlling electrode holder. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14]
[15] [16]
R.B. Mason, P.E. Fowle, J. Electrochem. Soc. 101 (1954) 53. M.S. Hunter, P. Fowle, J. Electrochem. Soc. 101 (1954) 514. J.P. O’Sullivan, G.C. Wood, Proc. R. Soc. London, Ser. A 317 (1970) 511. J.W. Diggle, T.C. Downie, C.W. Goulding, Electrochim. Acta 15 (1970) 1079. G.C. Wood, J.P. O’Sullivan, Electrochim. Acta 15 (1970) 1865. G. Bailey, G.C. Wood, Trans. Inst. Met. Finish. 52 (1974) 187. A. Despic, V.P. Parkhutik, in: J.O’.M. Bockris, Ralph E. White, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 18, Plenum Press, New York, 1989 (Chapter 6). G. Patermarakis, J. Chandrinos, K. Masavetas, J. Solid State Electrochem. 11 (2007) 1191. G.C. Tu, I.T. Chen, K. Shimizu, Keikinzoku 40 (1990) 382. L.E. Fratila-Apachitei, I. De Graeve, I. Apachitei, H. Terryn, J. Duszczyk, Surf. Coat. Technol. 200 (2006) 5343. L.E. Fratila-Apachitei, I. Apachitei, J. Duszczyk, J. Appl. Electrochem. 36 (2006) 481. I. De Graeve, H. Terryn, G. Thompson, J. Appl. Electrochem. 32 (2002) 73. I. De Graeve, H. Terryn, G. Thompson, J. Electrochem. Soc. 150 (2003) B156. A. Bosch, P. Boerstoel, T. Zuidwijk, A. Hovestad, A. Plomp, J. van de Heuvel, in: Proc. Second Int. Symp. on Aluminium Surface Science and Technology, Manchester, UK, 21–25 May, 2000, p. 65. V.P. Parkhutik, V.I. Shershul’skii, J. Phys. D.: Appl. Phys. 25 (1992) 1258. G.E. Thompson, Thin Solid Films 297 (1997) 192.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Control of the electrode temperature for electrochemical studies: A new approach illustrated on porous anodizing of aluminium Tim Aerts *,1, Iris De Graeve, Herman Terryn Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
a r t i c l e
i n f o
Article history: Received 7 September 2009 Received in revised form 9 October 2009 Accepted 9 October 2009 Available online 20 October 2009 Keywords: Temperature Electrode temperature Aluminium Anodizing Porous oxide
a b s t r a c t A new approach for studying the effect of temperature on electrochemical processes is presented in this paper. Using an in-house developed electrode holder, experiments are performed under conditions of applied and controlled electrode temperature. This new approach provides an improved temperature control during the experimental study and, additionally, allows distinguishing both the influences of the electrolyte and electrode temperatures. The advantages of the applied electrode temperature approach are illustrated by considering porous anodizing of aluminium. In a broad temperature range the electrochemical behaviour of the aluminium electrodes, recorded during the new and the conventional way of anodizing, are compared. Differences between the anodic potential evolutions in both approaches are observed, and are explained by a heat flux to the surroundings during the experiments at uncontrolled electrode temperature. These results illustrate the advantage of applying the electrode temperature. If the influence of temperature on a process is investigated by merely varying the electrolyte temperature, the electrode temperature is only indirectly influenced and can significantly differ from the electrolyte temperature. Therefore, when evaluating the influence of temperature on an electrochemical system the electrode temperature should be considered, and preferentially also controlled. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Whereas the influence of mass transfer on electrochemical systems is widely investigated and an abundant literature is available on this subject, only a limited number of studies deal with the influence of temperature and heat transfer on electrochemical processes. For many electrochemical processes the local effects of temperature at the electrode might be neglected and the temperature can be considered as a bulk parameter, influencing physical and electrokinetic data (such as pH, kinetics of reactions). However, in some cases the electrode temperature has a significant influence on the system, e.g. on processes which involve an important heat generation at the electrode, such as anodic oxide growth on aluminium. In general the influence of temperature on an electrochemical process is evaluated merely by considering the electrolyte temperature; the temperature of the electrode is generally not considered. This is for example the case for studies which can be considered as references describing the temperature dependency of anodizing of aluminium [1–8]. They acknowledge the important effect of temperature on the electrochemical behaviour of the aluminium anode * Corresponding author. Tel.: +32 2 629 35 35; fax: +32 2 629 32 00. E-mail address: [email protected] (T. Aerts). 1 ISE member. 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.10.013
and resulting oxide layer, though, do not explicitly consider the electrode temperature. It is assumed to equal the electrolyte temperature. More accurate information on the impact of the electrode temperature on an electrochemical system can be obtained by experiments in which this parameter is measured in situ. For anodizing of aluminium a very limited number of studies is available in which the global, or local aluminium temperature is monitored by in situ temperature measurements [9–14]. These studies demonstrate the significant influence of the electrode temperature on the anodic reaction. However, they still consider anodizing conditions in which the temperature of the aluminium is only affected indirectly and in a non-controlled way (i.e. by varying electrolyte convection). The recorded electrode temperatures are determined by thermal equilibrium under the prevailing conditions. This issue is overcome by the new approach introduced in this communication. Using an in-house developed electrode holder, electrochemical reactions can be investigated under conditions of applied and controlled electrode temperature, an approach which offers new possibilities for the study of the influence of temperature. In the present paper first the characteristics of the temperature controlling electrode holder are presented, followed by illustrations of the improved temperature control of the new approach. To this purpose the porous anodizing of aluminium is considered as a case study and experiments at applied electrode
T. Aerts et al. / Electrochemistry Communications 11 (2009) 2292–2295
temperature are compared to corresponding experiments at uncontrolled electrode temperatures. Furthermore, a brief example of the possibility to independently vary the electrode and electrolyte temperatures, which enables to distinguish and compare the influences of both temperatures on the process, is presented. More details on these results, and on other findings on the influence of the electrode temperature during anodizing of aluminium obtained by this new approach, will be considered in future research papers. 2. Experimental setup A schematic cross-section of the temperature controlling electrode holder, with its different composing parts labeled, is presented in Fig. 1. The core of the holder is a thermo-electronic component (2) which acts a ‘heat pump’. Based on the Peltier effect it either transfers heat towards, or removes heat from the aluminium electrode, depending on its polarisation. Hence, ‘heated’, as well as ‘cooled’ anodes, i.e. with temperatures respectively higher or lower than the electrolyte, can be considered. On the electrode-side of the thermo-electronic component an aluminium bar with a diameter 60 mm and a height 50 mm is mounted (3). This part, which is encased in PVDF (7), acts as a ‘thermal capacitor’ and provides support for the electrode (5). As the Peltier element is only able to transfer heat from its one side to the other, the necessary ‘heat sink’ is provided by a flow-through heat-exchanger (1). The temperature of the electrode is controlled (±0.1 °C) by a PID controller unit in combination with PT-100 temperature probe, which is mounted in the aluminium bar at a depth of less than 5 mm from the electrode surface (4). The active electrode surface is limited to 10 mm by a PVDF cover (8), which is screwed onto the encased aluminium bar. A silicon spacer mat (6) is included to ensure a proper sealing of the non-exposed area on the electrode. The holder unit is fixed in a PMMA casing (10). During anodizing the entire holder is immersed into the electrolyte.
2293
3. Experimental conditions Anodizing experiments are performed in 50 l of 145 g=l H2 SO4 þ 5 g=l Al2 ðSO4 Þ3 18H2 O electrolyte, which is thermostatically controlled (±0.1 °C) by a Lauda RP845 thermostatically Intense electrolyte agitation was provided by a mechanical stirrer. Due to the high rotation speed of the stirrer the anode potential was no longer affected by a further increase in rotation speed. In the framework of this communication both the temperature of the electrolyte ðT H2 SO4 Þ and the temperature of the anode ðT Al Þ are varied in a wide range from 5 °C up to 65 °C. The influence of the electrode temperature is evaluated at the four evenly distributed temperatures 5, 25, 45 and 65 °C. Disk shaped AA1050 aluminium samples (99.5% Al sheet 0.3 mm) with a diameter of 55 mm were used, the diameter of the active anode surface limited to 10 mm. Prior to anodizing samples were alkaline etched in 60 g/l NaOH solution at 60 °C for 60 s, followed by a desmutting treatment in a 1:1 concentrated HNO3:H2O solution at room temperature for 90 s. The anodizing cell consists of a three electrode configuration, involving a large aluminium sheet as counter electrode and a Ag/ AgCl reference electrode. Anodizing is performed under galvano2 static conditions at a current density of 1 A=dm , the current being applied by a computer controlled Delta Elektronica SM 300–20 power source. A charge density of 512 C/dm2 is considered, corresponding to anodizing times of 512 s. The resulting potential evolutions are recorded with a National Instruments M-6220 DAQ card and using National Instruments VI-Logger software. The presented electrode potentials are corrected for the Ohmic drop between the anode and reference electrode. To check reproducibility at least three separate experiments are performed for each considered condition. During the entire anodizing treatment, i.e. from the onset of the anodic oxide growth, a constant electrode temperature is considered. This is ensured by the following experimental procedure, which is systematically applied. Before introducing the temperature controlling electrode holder into the electrolyte, the temperature of the mounted electrode is set to the desired value. Only after attaining a stable value of this electrode temperature the holder is immersed into the solution. Due to considered temperature differences between the electrode and the electrolyte (electrolyte temperatures, different from the electrode temperature, are evaluated) a minor deviation of the electrode temperature from its intended value can occur. Therefore, the holder is given the time to re-adjust the electrode temperature: from the moment of immersion on, a fixed timespan of 90 s is waited before applying the anodic current. Under all considered conditions this period is sufficient to guarantee a constant electrode temperature from the start of the experiments on.
4. Comparing ‘conventional’ and ‘applied electrode temperature’ approaches
Fig. 1. Schematic cross-section of the side-view of the temperature controlling electrode holder with the different parts labeled: (1) flow-through heat-exchanger, (2) thermo-electronic component, (3) aluminium bar, (4) inserted temperature probe, (5) electrode, (6) silicon spacer mat, (7) PVDF encasing of the aluminium bar, (8) screw cap, (9) electrical connection and (10) PMMA casing.
As indicated in the introduction, during the conventional way of anodizing the electrode temperature T Al is generally not known and is supposed to equal the electrolyte temperature. However, this might not always be correct. This is illustrated by comparing conventional anodizing experiments (i.e. without controlling the electrode temperature; referred to as ‘T Al free’ experiments) to experiments with applied electrode temperatures equal to the considered electrolyte temperature T H2 SO4 . Characteristic evolutions of the electrode potential U we [15,16], recorded during anodizing at corresponding temperatures for both approaches, are presented in Fig. 2. Clearly, differences in the potential evolutions between the uncontrolled and controlled elec-
2294
T. Aerts et al. / Electrochemistry Communications 11 (2009) 2292–2295
Fig. 2. Potential evolutions recorded at different temperatures during anodizing under conditions of controlled and uncontrolled (‘T Al free’) electrode temperatures.
trode temperature experiments are observed. At a low electrolyte temperature of 5 °C the experiments performed at uncontrolled electrode temperatures systematically yield lower electrode potentials than those at applied electrode temperatures equal to the electrolyte temperature. On the other hand, at an electrolyte temperature of 25 °C the conventional approach typically results
in higher anode potentials than the approach involving controlled electrode temperatures. This effect persists and even increases when the electrolyte temperature is further increased – cf. the potential evolutions at 45 and 65 °C. Even at equal electrolyte temperatures higher aluminium temperatures result in lower potentials [12,13]. The lower electrode
Fig. 3. Evolution of the stationary electrode potential U we;stat as a function of temperature. The full lines correspond to the evolution as a function of T Al at different, constant T H2 SO4 , the dashed lines to the evolution as a function of T H2 SO4 at different, constant T Al .
T. Aerts et al. / Electrochemistry Communications 11 (2009) 2292–2295
potentials recorded during the conventional approach at a low electrolyte temperature of 5 °C therefore indicate actual electrode temperatures which are higher than the applied electrode temperatures in the new approach. At higher electrolyte temperatures, on the other hand, the higher potentials observed under the uncontrolled approach correspond to actual electrode temperatures which are lower than the considered electrolyte temperatures. These effects are explained by considering the heat flux to the surroundings. At electrolyte temperatures lower than the ambient temperature in the lab (approximately 20 °C) an uptake of heat from the surroundings influences the actual electrode temperature, increasing it to a value higher than the considered low electrolyte temperature. When anodizing is performed at electrolyte temperatures higher than the ambient temperature a heat loss to the surroundings is expected, cooling the anode and reducing its temperature to a value lower than the electrolyte temperature. This heat loss is enhanced at higher electrolyte temperatures, increasing the difference between the actual electrode and the electrolyte temperature, which explains the increased potential difference between the conventional and the new approach at higher temperatures. These results illustrate the advantage of anodizing under applied electrode temperature. If the influence of temperature on the process is investigated by merely varying the electrolyte temperature, the electrode temperature is only indirectly influenced and can significantly differ from the electrolyte temperature. This effect, which is believed to be due to heat flux to the surroundings, increases with increasing difference between the considered anodizing temperature and the ambient temperature in the lab. 5. Illustration of the evaluation of influences of electrolyte and electrode temperatures Additional to an improved temperature control the temperature controlling electrode holder also enables varying the electrode temperature independently of the electrolyte temperature. Hence, it is possible to separately evaluate the influences of both the electrolyte and the electrode temperature. As an example this is illustrated by Fig. 3, which displays the evolutions of the stationary electrode potential U we;stat as a function of temperature. The full lines correspond to the evolution as a function of T Al at different, constant T H2 SO4 , whereas the dashed lines represent the evolution as a function of T H2 SO4 at different, constant T Al . For example, the larger decline in stationary potential upon increasing the electrode temperature at constant electrolyte temperature demonstrates the more important influence of T Al on this parameter than of T H2 SO4 . Moreover, a direct relation between U we;stat and the dimensions of the characteristic features of the porous film (e.g. the barrier layer and pore diameter) was observed in [3], this at different electrolyte temperatures. Although the latter study did not consider the electrode temperature, the general applicability of this relation has been demonstrated. Therefore, the presented larger variations in U we;stat as a function of T Al than as a function of T H2 SO4 indicate that also concerning the morphology of oxide layer the electrode tem-
2295
perature has more important influence. The precise impact of T Al on the electrochemical behaviour of the aluminium anodes and on the resulting oxide layer will elaborately be presented in a future paper. 6. Conclusions A new approach for the study of the influence of temperature on electrochemical processes is introduced. By performing experiments under conditions of controlled and applied electrode temperature an improved control over the temperature is obtained. This is illustrated by the presented results, recorded during porous anodizing of aluminium. The differences in potential evolutions, recorded during the new and the conventional approach, are believed to be due to heat fluxes to the surroundings under conditions of uncontrolled electrode temperature. This effect can significantly influence the actual electrode temperature during the conventional approach, resulting in important differences between the electrode and the electrolyte temperature. Therefore, when studying the influence of temperature on an electrochemical system the electrode temperature should be considered and preferentially also controlled. Furthermore, the new approach allows varying the electrolyte and electrode temperatures independently, which enables distinguishing the separate influences on the considered process. Acknowledgements The authors acknowledge the support from the Instituut voor de aanmoediging van innovatie door Wetenschap & Technologie in Vlaanderen (IWT, contract no. SBO 040092). Raf Claessens and Marnix Depauw are greatly acknowledged for the design and development of the temperature controlling electrode holder. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14]
[15] [16]
R.B. Mason, P.E. Fowle, J. Electrochem. Soc. 101 (1954) 53. M.S. Hunter, P. Fowle, J. Electrochem. Soc. 101 (1954) 514. J.P. O’Sullivan, G.C. Wood, Proc. R. Soc. London, Ser. A 317 (1970) 511. J.W. Diggle, T.C. Downie, C.W. Goulding, Electrochim. Acta 15 (1970) 1079. G.C. Wood, J.P. O’Sullivan, Electrochim. Acta 15 (1970) 1865. G. Bailey, G.C. Wood, Trans. Inst. Met. Finish. 52 (1974) 187. A. Despic, V.P. Parkhutik, in: J.O’.M. Bockris, Ralph E. White, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 18, Plenum Press, New York, 1989 (Chapter 6). G. Patermarakis, J. Chandrinos, K. Masavetas, J. Solid State Electrochem. 11 (2007) 1191. G.C. Tu, I.T. Chen, K. Shimizu, Keikinzoku 40 (1990) 382. L.E. Fratila-Apachitei, I. De Graeve, I. Apachitei, H. Terryn, J. Duszczyk, Surf. Coat. Technol. 200 (2006) 5343. L.E. Fratila-Apachitei, I. Apachitei, J. Duszczyk, J. Appl. Electrochem. 36 (2006) 481. I. De Graeve, H. Terryn, G. Thompson, J. Appl. Electrochem. 32 (2002) 73. I. De Graeve, H. Terryn, G. Thompson, J. Electrochem. Soc. 150 (2003) B156. A. Bosch, P. Boerstoel, T. Zuidwijk, A. Hovestad, A. Plomp, J. van de Heuvel, in: Proc. Second Int. Symp. on Aluminium Surface Science and Technology, Manchester, UK, 21–25 May, 2000, p. 65. V.P. Parkhutik, V.I. Shershul’skii, J. Phys. D.: Appl. Phys. 25 (1992) 1258. G.E. Thompson, Thin Solid Films 297 (1997) 192.