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Anodic dissolution of aluminum in KOH ethanol solutions
Electrochemistry Communications 6 (2004) 6–9 www.elsevier.com/locate/elecom
Anodic dissolution of aluminum in KOH ethanol solutions H.B. Shao a, J.M. Wang b
a,*
, X.Y. Wang a, J.Q. Zhang
a,b
, C.N. Cao
a,b
a Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Chinese State Key Laboratory for Corrosion and Protection, Shenyang 110015, PR China
Received 23 September 2003; received in revised form 6 October 2003; accepted 7 October 2003 Published online: 27 October 2003
Abstract The feasibility of employing aluminum in KOH ethanol solution as a battery anode system is discussed. Weight loss measurement showed that its corrosion rate is low enough to be used in sealed batteries. Through anodic polarization, EIS and galvanostatic discharge, it was found that Al can keep electrochemical activity in a wide potential range in this system, and its discharge mechanism is similar to that of Al in aqueous solution. The discharge potential increases rapidly after a period of time, which may be caused by the discharge products deposited on the electrode surface. Ó 2003 Published by Elsevier B.V. Keywords: Pure aluminum; KOH ethanol solution; Electrochemical performance
1. Introduction Aluminum is undoubtedly interesting material for the use of electrochemical energy providing. Its specific capacity is as high as 2.98 Ah/g due to its low atomic weight and three-electron transfer during oxidation. Despite its relatively low density, its capacity per unit volume 8.05 Ah/cm3 is much higher than that of the widely used anodic material zinc 5.88 Ah/cm3 . Much attention has been paid to the usage of aluminum as anodic material in batteries of saline [1,2], alkaline [3,4] and non-aqueous [5,6] system. However, generally, aluminum still cannot take the place of zinc, and only a few kinds of aluminum batteries have been industrially produced. The reason lies in the spontaneously formed passive film on the surface of aluminum. In aqueous and non-aqueous saline systems the passive film makes the potential of the Al anode much more positive than theoretical value, and as a result, cuts down the output voltage of the battery. Of course the surface film can be removed by strong acid or alkaline aqueous electrolytes. However, in such media aluminum undergoes serious *
Corresponding author. Tel.: +86-571-7951513/7951358; fax: +86571-7951895/7951358. E-mail address: [email protected] (J.M. Wang). 1388-2481/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/j.elecom.2003.10.007
self-discharge due to corrosion with the evolution of hydrogen. A successful system should keep the Al anodic active while reduce the corrosion rate to a low level. We now propose a new system employing KOH ethanol solution as electrolyte. In this medium, the passive film can be removed by the alkali, while the hydrogen evolution hardly occurs because the proton of ethanol is much less active than that of water.
2. Experimental details 2.1. Electrochemical measurements Electrochemical measurements were taken in a classical three-electrode glass cell. In order to reduce microgalvanic couple corrosion, aluminum of high purity is used. The working electrode was made of specpure aluminum rod (supplied by Johnson Matthey, purity no less than 99.9995%), 6 mm in diameter. The electrode surface was polished by 2000# waterproof abrasive paper, degreased in acetone, rinsed in deionized water and dried in air. The counter electrode was a platinum foil and the reference electrode was a Hg/HgO electrode. The measurement system was made up of an EG&G model 273A potentiostat and a model 5210 lock-in
H.B. Shao et al. / Electrochemistry Communications 6 (2004) 6–9
1 2
2
i (mA/cm )
amplifier, controlled by a microcomputer with certain software. The potentiodynamic polarization curves were measured at a scanning rate of 50 mV/s, and gavanostatic discharge was performed at various of current density. After discharging, the anode surface was observed with a Philips model XL30 ESEM. The electrochemical impedance spectra (EIS) were obtained in the frequency range of 120 kHz to 0.01 Hz and at a.c. signal amplitude of 10 mV. All the above electrochemical measurements were taken at 25 ± 1 °C, and all the solutions were prepared by KOH of A.R. grade, anhydrous ethanol (water content less than 0.3%) and/or deionized water.
7
5
3
0 -1.5
-1.0
-0.5
0.0
2.2. Determination of corrosion rates
3 2
i (mA/cm )
Corrosion rate were determined by weight loss method. Specimens were made of aluminum wire of high purity (99.999%), 1 mm in diameter and 10 cm length, with a surface area of 3.16 cm2 . Specimens were degreased in acetone and weighed. After being kept completely immersed in test solutions at 25 ± 1 °C for certain time (480 h for KOH ethanol solution and 2 h in aqueous solution), the specimens were taken out, washed well with flowing water, rinsed in deionized water, dried and finally weighed. From the weight loss of specimens, the corrosion current density can be calculated.
Fig. 1. Anodic polarization curve of Al in: (1) 4 M, (2) 2 M and (3) 1 M KOH anhydrous ethanol solutions.
40
2 20
1 0 -1.5
3. Results and discussion As shown in Table 1, in a quite long period of time (480 h) the corrosion current of Al in KOH ethanol solution is very small. Even when the solution contains 10% water, the corrosion current is still less than 0.1% of that of Al in KOH aqueous solution, and almost equal to that of Zn in KOH aqueous solution, which makes it possible to use aluminum in sealed batteries, just as Zn in alkaline Zn/MnO2 battery. Anodic polarization was performed in order to investigate the electrochemical behavior of aluminum in the KOH anhydrous ethanol solutions, as shown in Fig. 1. It can be seen that aluminum can provide higher current density and more negative potential in the solution with higher KOH concentration. It appears that a Table 1 Corrosion currents of Al in various electrolytes System
Icorr (mA/cm2 )
Al in 4 M KOH ethanol solution Al in 2 M KOH ethanol solution Al in 4 M KOH ethanol solution + 5% H2 O Al in 4 M KOH ethanol solution + 10% H2 O Al in 4 M KOH aqueous solution Zn in 4 M KOH aqueous solution
0.0041 0.0102 0.0124 0.0986 15.7 0.0973
-1.0
-0.5
0.0
Fig. 2. Anodic polarization curve of Al in 4 M KOH ethanol solutions: (1) free of water, (2) containing 5% water and (3) containing 10% water.
current limit exists when the potential moves to positive direction, however, Al keeps active in the wide potential range where the measurement was performed. Added with small amount of water, the output current is greatly enhanced (Fig. 2), which may be caused by the improvement of mass transfer by decreasing the viscosity of the electrolyte. Another possible reason is that water promotes the electrochemical processes by participating the hydration of discharge products. After the anode had been immersed in test solution for 30 min to gain a stable potential, EIS was performed in order to reveal the mechanism of discharge process, shown in Fig. 3. This EIS is similar to the one of Al in KOH aqueous solutions [7–9], indicating that the mechanism is similar to the one of Al in KOH aqueous solutions. Electrochemical dissolution tends to occur in active sites of Al surface and form holes, therefore, the capacitive arc at the high frequency side may be caused by the relaxation process of the solution resistance in
H.B. Shao et al. / Electrochemistry Communications 6 (2004) 6–9
4
1.6k
120k
0.89
2 21k
0 67
-2 6
12
18
24
Fig. 3. EIS of pure Al in 4 M KOH ethanol solution containing 10% of water.
E (V vs. Hg/HgO)
2
-Z" (Ω /cm )
8
-1.00 15mA/cm -1.25
10mA/cm 5mA/cm
-1.50 0
5
10
2
2
2
15
20
Fig. 5. Galvanostatic discharge curves of pure Al in 4 M KOH ethanol solution containing 10% of water at various current density.
Fig. 4. Circuit model of the EIS shown in Fig. 3.
holes (Rh in Fig. 4) in parallel with the double layer capacity in holes (Ch in Fig. 4). The circuit model of the EIS can be described as in Fig. 4, where the Faradic impedance Zf is caused by the electrochemical steps and the subsequent chemical step [7]. In parallel with the double layer capacity Cdl , Zf displays complex feature on the Nyquist plane – a high-frequency capacitive arc, a middle-frequency inductive arc and a low-frequency capacitive arc [7–9]. As our previous work described [8,9], the complex features of Zf can be well fitted with a circuit model including resistance, capacitor and inductance elements. And, based on its reaction mechanism, Zf can be described as the equation in [7]. That equation can well explain the complex features of Zf , on the other hand, it can well analyze the electrochemical reaction mechanism. It is clear that the electrochemical reaction rate was affected by the potential and the covering densities of reactants and intermediates. Galvanostatic discharge was performed at various current densities, shown in Fig. 5. It is interesting that the anode can provide much higher capacity at higher discharge current density, with more positive discharge potential. At 5 mA/cm2 the sample activates (E down), at 10 mA/cm2 discharge is stable for about 6000 s, but at 15 mA/cm2 the anode rapidly polarizes but then sustains steady dissolution for a much longer period than at lower currents. As the anodic dissolution is regarded as occurring at the active sites and tending to form microholes, we consider that the discharge behavior at different current density has some relationship with the active sites and the microholes on Al surface. Under anodic polarization, Al surface tends to form active sites, thus at 5 mA/cm2 the electrode actives (E down).
At higher current density, the microholes might tend to grow deeper, therefore the rapid polarization at 15 mA/ cm2 might be due to the increase of hole resistance. Note that at the end of discharge, the anode has not completely consumed. For instance, the anode provides 79.7 mAh/cm2 of capacity at current density of 15 mA/ cm2 (Fig. 5), and it can be calculated that only about 100 lm thickness of Al is consumed, assuming the electric efficiency is 100%. We consider that the rapid increase of potential could result from the deposition of discharge products on the electrode surface. Fig. 6 shows the SEM image of the electrode surface after discharge. It can be seen that its surface has been almost completely covered by discharge products. Therefore, higher discharge capacity that the anode provide at higher current density might be due to that products generated at larger discharge current are easy to leave the surface or have less effect on mass transfer of the electrolyte. In order to make the Al anode completely consumed to provide electricity, it might be practical to employ Al foil or powder as anodic material. For instance, if Al can discharge actively for 100 lm in certain condition, its powder of 200 lm diameter can be completely used.
Fig. 6. SEM image of the surface of Al anode after discharge.
H.B. Shao et al. / Electrochemistry Communications 6 (2004) 6–9
Another possible method is to add additives (for example, surfactants) into the electrolyte in order to make the discharge products less tight.
9
and National Natural Science Foundation of China (50071054). The authors also gratefully acknowledge the financial support of the Chinese State Key Laboratory for Corrosion and Protection.
4. Conclusion 1. The corrosion rate of Al in KOH ethanol solution is low enough to be used in sealed batteries. 2. Al can keep active in a wide potential range in KOH ethanol solution, and its discharge mechanism is similar to that of Al in aqueous solution. 3. The discharge potential increases rapidly after a period of time, which may be caused by the discharge products depositing on the electrode surface. Acknowledgements This work was supported by Special Funds of the China Major State Basic Research Projects (G19990650)
References [1] J.J. Stokes Jr., Electrochem. Technol. 6 (1968) 36. [2] G. Burri, W. Luedi, O. Haas, J. Electrochem. Soc. 136 (1989) 2167. [3] S. Licht, C. Marsh, US Patent no. 5,549,991, August 27 1996. [4] S. Licht, J. Electrochem. Soc. 144 (1997) L134. [5] G.F. Reynolds, C.J. Dymek, J. Power Sources 15 (1985) 109. [6] J. Dymek, G.F. Reynolds, J.S. Wilkes, J. Power Sources 17 (1987) 134. [7] H.B. Shao, J.M. Wang, Z. Zhang, J.Q. Zhang, C.N. Cao, J. Electroanal. Chem. 549 (2003) 145. [8] H.B. Shao, J.M. Wang, Z. Zhang, J.Q. Zhang, C.N. Cao, Corrosion 57 (2001) 577. [9] H.B. Shao, J.M. Wang, Z. Zhang, J.Q. Zhang, C.N. Cao, Mater. Chem. Phys. 77 (2002) 305.
Anodic dissolution of aluminum in KOH ethanol solutions H.B. Shao a, J.M. Wang b
a,*
, X.Y. Wang a, J.Q. Zhang
a,b
, C.N. Cao
a,b
a Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Chinese State Key Laboratory for Corrosion and Protection, Shenyang 110015, PR China
Received 23 September 2003; received in revised form 6 October 2003; accepted 7 October 2003 Published online: 27 October 2003
Abstract The feasibility of employing aluminum in KOH ethanol solution as a battery anode system is discussed. Weight loss measurement showed that its corrosion rate is low enough to be used in sealed batteries. Through anodic polarization, EIS and galvanostatic discharge, it was found that Al can keep electrochemical activity in a wide potential range in this system, and its discharge mechanism is similar to that of Al in aqueous solution. The discharge potential increases rapidly after a period of time, which may be caused by the discharge products deposited on the electrode surface. Ó 2003 Published by Elsevier B.V. Keywords: Pure aluminum; KOH ethanol solution; Electrochemical performance
1. Introduction Aluminum is undoubtedly interesting material for the use of electrochemical energy providing. Its specific capacity is as high as 2.98 Ah/g due to its low atomic weight and three-electron transfer during oxidation. Despite its relatively low density, its capacity per unit volume 8.05 Ah/cm3 is much higher than that of the widely used anodic material zinc 5.88 Ah/cm3 . Much attention has been paid to the usage of aluminum as anodic material in batteries of saline [1,2], alkaline [3,4] and non-aqueous [5,6] system. However, generally, aluminum still cannot take the place of zinc, and only a few kinds of aluminum batteries have been industrially produced. The reason lies in the spontaneously formed passive film on the surface of aluminum. In aqueous and non-aqueous saline systems the passive film makes the potential of the Al anode much more positive than theoretical value, and as a result, cuts down the output voltage of the battery. Of course the surface film can be removed by strong acid or alkaline aqueous electrolytes. However, in such media aluminum undergoes serious *
Corresponding author. Tel.: +86-571-7951513/7951358; fax: +86571-7951895/7951358. E-mail address: [email protected] (J.M. Wang). 1388-2481/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/j.elecom.2003.10.007
self-discharge due to corrosion with the evolution of hydrogen. A successful system should keep the Al anodic active while reduce the corrosion rate to a low level. We now propose a new system employing KOH ethanol solution as electrolyte. In this medium, the passive film can be removed by the alkali, while the hydrogen evolution hardly occurs because the proton of ethanol is much less active than that of water.
2. Experimental details 2.1. Electrochemical measurements Electrochemical measurements were taken in a classical three-electrode glass cell. In order to reduce microgalvanic couple corrosion, aluminum of high purity is used. The working electrode was made of specpure aluminum rod (supplied by Johnson Matthey, purity no less than 99.9995%), 6 mm in diameter. The electrode surface was polished by 2000# waterproof abrasive paper, degreased in acetone, rinsed in deionized water and dried in air. The counter electrode was a platinum foil and the reference electrode was a Hg/HgO electrode. The measurement system was made up of an EG&G model 273A potentiostat and a model 5210 lock-in
H.B. Shao et al. / Electrochemistry Communications 6 (2004) 6–9
1 2
2
i (mA/cm )
amplifier, controlled by a microcomputer with certain software. The potentiodynamic polarization curves were measured at a scanning rate of 50 mV/s, and gavanostatic discharge was performed at various of current density. After discharging, the anode surface was observed with a Philips model XL30 ESEM. The electrochemical impedance spectra (EIS) were obtained in the frequency range of 120 kHz to 0.01 Hz and at a.c. signal amplitude of 10 mV. All the above electrochemical measurements were taken at 25 ± 1 °C, and all the solutions were prepared by KOH of A.R. grade, anhydrous ethanol (water content less than 0.3%) and/or deionized water.
7
5
3
0 -1.5
-1.0
-0.5
0.0
2.2. Determination of corrosion rates
3 2
i (mA/cm )
Corrosion rate were determined by weight loss method. Specimens were made of aluminum wire of high purity (99.999%), 1 mm in diameter and 10 cm length, with a surface area of 3.16 cm2 . Specimens were degreased in acetone and weighed. After being kept completely immersed in test solutions at 25 ± 1 °C for certain time (480 h for KOH ethanol solution and 2 h in aqueous solution), the specimens were taken out, washed well with flowing water, rinsed in deionized water, dried and finally weighed. From the weight loss of specimens, the corrosion current density can be calculated.
Fig. 1. Anodic polarization curve of Al in: (1) 4 M, (2) 2 M and (3) 1 M KOH anhydrous ethanol solutions.
40
2 20
1 0 -1.5
3. Results and discussion As shown in Table 1, in a quite long period of time (480 h) the corrosion current of Al in KOH ethanol solution is very small. Even when the solution contains 10% water, the corrosion current is still less than 0.1% of that of Al in KOH aqueous solution, and almost equal to that of Zn in KOH aqueous solution, which makes it possible to use aluminum in sealed batteries, just as Zn in alkaline Zn/MnO2 battery. Anodic polarization was performed in order to investigate the electrochemical behavior of aluminum in the KOH anhydrous ethanol solutions, as shown in Fig. 1. It can be seen that aluminum can provide higher current density and more negative potential in the solution with higher KOH concentration. It appears that a Table 1 Corrosion currents of Al in various electrolytes System
Icorr (mA/cm2 )
Al in 4 M KOH ethanol solution Al in 2 M KOH ethanol solution Al in 4 M KOH ethanol solution + 5% H2 O Al in 4 M KOH ethanol solution + 10% H2 O Al in 4 M KOH aqueous solution Zn in 4 M KOH aqueous solution
0.0041 0.0102 0.0124 0.0986 15.7 0.0973
-1.0
-0.5
0.0
Fig. 2. Anodic polarization curve of Al in 4 M KOH ethanol solutions: (1) free of water, (2) containing 5% water and (3) containing 10% water.
current limit exists when the potential moves to positive direction, however, Al keeps active in the wide potential range where the measurement was performed. Added with small amount of water, the output current is greatly enhanced (Fig. 2), which may be caused by the improvement of mass transfer by decreasing the viscosity of the electrolyte. Another possible reason is that water promotes the electrochemical processes by participating the hydration of discharge products. After the anode had been immersed in test solution for 30 min to gain a stable potential, EIS was performed in order to reveal the mechanism of discharge process, shown in Fig. 3. This EIS is similar to the one of Al in KOH aqueous solutions [7–9], indicating that the mechanism is similar to the one of Al in KOH aqueous solutions. Electrochemical dissolution tends to occur in active sites of Al surface and form holes, therefore, the capacitive arc at the high frequency side may be caused by the relaxation process of the solution resistance in
H.B. Shao et al. / Electrochemistry Communications 6 (2004) 6–9
4
1.6k
120k
0.89
2 21k
0 67
-2 6
12
18
24
Fig. 3. EIS of pure Al in 4 M KOH ethanol solution containing 10% of water.
E (V vs. Hg/HgO)
2
-Z" (Ω /cm )
8
-1.00 15mA/cm -1.25
10mA/cm 5mA/cm
-1.50 0
5
10
2
2
2
15
20
Fig. 5. Galvanostatic discharge curves of pure Al in 4 M KOH ethanol solution containing 10% of water at various current density.
Fig. 4. Circuit model of the EIS shown in Fig. 3.
holes (Rh in Fig. 4) in parallel with the double layer capacity in holes (Ch in Fig. 4). The circuit model of the EIS can be described as in Fig. 4, where the Faradic impedance Zf is caused by the electrochemical steps and the subsequent chemical step [7]. In parallel with the double layer capacity Cdl , Zf displays complex feature on the Nyquist plane – a high-frequency capacitive arc, a middle-frequency inductive arc and a low-frequency capacitive arc [7–9]. As our previous work described [8,9], the complex features of Zf can be well fitted with a circuit model including resistance, capacitor and inductance elements. And, based on its reaction mechanism, Zf can be described as the equation in [7]. That equation can well explain the complex features of Zf , on the other hand, it can well analyze the electrochemical reaction mechanism. It is clear that the electrochemical reaction rate was affected by the potential and the covering densities of reactants and intermediates. Galvanostatic discharge was performed at various current densities, shown in Fig. 5. It is interesting that the anode can provide much higher capacity at higher discharge current density, with more positive discharge potential. At 5 mA/cm2 the sample activates (E down), at 10 mA/cm2 discharge is stable for about 6000 s, but at 15 mA/cm2 the anode rapidly polarizes but then sustains steady dissolution for a much longer period than at lower currents. As the anodic dissolution is regarded as occurring at the active sites and tending to form microholes, we consider that the discharge behavior at different current density has some relationship with the active sites and the microholes on Al surface. Under anodic polarization, Al surface tends to form active sites, thus at 5 mA/cm2 the electrode actives (E down).
At higher current density, the microholes might tend to grow deeper, therefore the rapid polarization at 15 mA/ cm2 might be due to the increase of hole resistance. Note that at the end of discharge, the anode has not completely consumed. For instance, the anode provides 79.7 mAh/cm2 of capacity at current density of 15 mA/ cm2 (Fig. 5), and it can be calculated that only about 100 lm thickness of Al is consumed, assuming the electric efficiency is 100%. We consider that the rapid increase of potential could result from the deposition of discharge products on the electrode surface. Fig. 6 shows the SEM image of the electrode surface after discharge. It can be seen that its surface has been almost completely covered by discharge products. Therefore, higher discharge capacity that the anode provide at higher current density might be due to that products generated at larger discharge current are easy to leave the surface or have less effect on mass transfer of the electrolyte. In order to make the Al anode completely consumed to provide electricity, it might be practical to employ Al foil or powder as anodic material. For instance, if Al can discharge actively for 100 lm in certain condition, its powder of 200 lm diameter can be completely used.
Fig. 6. SEM image of the surface of Al anode after discharge.
H.B. Shao et al. / Electrochemistry Communications 6 (2004) 6–9
Another possible method is to add additives (for example, surfactants) into the electrolyte in order to make the discharge products less tight.
9
and National Natural Science Foundation of China (50071054). The authors also gratefully acknowledge the financial support of the Chinese State Key Laboratory for Corrosion and Protection.
4. Conclusion 1. The corrosion rate of Al in KOH ethanol solution is low enough to be used in sealed batteries. 2. Al can keep active in a wide potential range in KOH ethanol solution, and its discharge mechanism is similar to that of Al in aqueous solution. 3. The discharge potential increases rapidly after a period of time, which may be caused by the discharge products depositing on the electrode surface. Acknowledgements This work was supported by Special Funds of the China Major State Basic Research Projects (G19990650)
References [1] J.J. Stokes Jr., Electrochem. Technol. 6 (1968) 36. [2] G. Burri, W. Luedi, O. Haas, J. Electrochem. Soc. 136 (1989) 2167. [3] S. Licht, C. Marsh, US Patent no. 5,549,991, August 27 1996. [4] S. Licht, J. Electrochem. Soc. 144 (1997) L134. [5] G.F. Reynolds, C.J. Dymek, J. Power Sources 15 (1985) 109. [6] J. Dymek, G.F. Reynolds, J.S. Wilkes, J. Power Sources 17 (1987) 134. [7] H.B. Shao, J.M. Wang, Z. Zhang, J.Q. Zhang, C.N. Cao, J. Electroanal. Chem. 549 (2003) 145. [8] H.B. Shao, J.M. Wang, Z. Zhang, J.Q. Zhang, C.N. Cao, Corrosion 57 (2001) 577. [9] H.B. Shao, J.M. Wang, Z. Zhang, J.Q. Zhang, C.N. Cao, Mater. Chem. Phys. 77 (2002) 305.