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Aluminum anode for aluminum–air battery – Part I: Influence of aluminum purity
Journal of Power Sources 277 (2015) 370e378
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Aluminum anode for aluminumeair battery e Part I: Influence of aluminum purity Young-Joo Cho, In-Jun Park, Hyeok-Jae Lee, Jung-Gu Kim* School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeonggi-Do 440-746, Republic of Korea
h i g h l i g h t s The performance of 2N5 Al is not as good as 4N Al due to the impurity layer. The impurity complex film is diminished with decreasing discharge voltage. 2N5 Al shows similar performance as 4N Al with decreasing discharge voltage. We find the way to optimize the performance of 2N5 Al as anode on Aleair battery.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2014 Received in revised form 5 December 2014 Accepted 9 December 2014 Available online 11 December 2014
2N5 commercial grade aluminum (99.5% purity) leads to the lower aluminumeair battery performances than 4N high pure grade aluminum (99.99% purity) due to impurities itself and formed impurity complex layer which contained Fe, Si, Cu and others. The impurity complex layer of 2N5 grade Al declines the battery voltage on standby status. It also depletes discharge current and battery efficiency at 1.0 V which is general operating voltage of aluminumeair battery. However, the impurity complex layer of 2N5 grade Al is dissolved with decreasing discharge voltage to 0.8 V. This phenomenon leads to improvement of discharge current density and battery efficiency by reducing self-corrosion reaction. This study demonstrates the possibility of use of 2N5 grade Al which is cheaper than 4N grade Al as the anode for aluminumeair battery. © 2014 Elsevier B.V. All rights reserved.
Keywords: Aluminum air battery Aluminum anode purity Impurity Battery efficiency Electrochemical impedance spectroscopy
1. Introduction Aluminum has been a very attractive source of metaleair battery for more than 50 years because of its inherent high theoretical energy density (8100 Wh kg1), lightweight (2.71 g cm3), negative standard potential (2.37 V vs. SHE), abundance (the third most abundant elements in the earth crust), environmental friendship, and recyclability [1e4]. However, aluminum has the protective oxide film which makes electron conducting hard in most aqueous conditions. This film results in high anodic dissolution overvoltage and makes unattractive for use as battery [5,6]. To overcome these problems, the electrolytes for aluminumeair battery system use saline, alkaline, and non-aqueous solutions. In the case of non-aqueous solutions such as organic solvents and
* Corresponding author. E-mail address: [email protected] (J.-G. Kim). http://dx.doi.org/10.1016/j.jpowsour.2014.12.026 0378-7753/© 2014 Elsevier B.V. All rights reserved.
ionic liquids, they have disadvantages in terms of high volatility and flammability, lower electrical conductivity, and high cost. Moreover, the stability of electrolyte becomes worse when oxygen dissolves in the electrolytes. For these reasons, alkaline or saline solution was commonly used in Aleair battery [4,7,8]. Among them, alkaline media displays good battery performance especially at high discharge current. However, aluminum suffers substantial corrosion in alkaline solution with the production of large amount of hydrogen gas covered on the surface of aluminum anode, induction of coulombic loss on discharge and stand-by [9,10]. In order to reduce the self-discharge of aluminum anode, it is considered to use high purity grade aluminum. The main reason for use high purity such as 4N (99.99% purity) or 5N (99.999% purity) grade aluminum is the absence of Fe impurities which presented in a raw cast aluminum and drastically increase the corrosion rate of aluminum. However, the production price of high grades, 10 to 20 times greater than the 2N5 commercial grade (99.5% purity), does
Y.-J. Cho et al. / Journal of Power Sources 277 (2015) 370e378
not satisfy the requirements for commercial application. Moreover, 4N or 5N grade aluminum anodes can reduce the overall thermodynamic efficiency of the battery due to the overpotential of both anodic and cathodic reactions when battery works. Until now, there were several researches on aluminum purity. However, it was inadequate because most of researches aimed at corrosion rate and hydrogen evolution by parasitic reaction not in terms of battery performance and influence of impurities [1,11]. For these reasons, the aim of this study is to investigate the battery performance and the behavior of impurities. In the present work, the battery performance of 2N5 grade Al and 4N grade Al have been studied in 4 M NaOH electrolyte. The potentiodynamic polarization test was carried out in the half cell assembly to analyze the anodic dissolution of aluminum. Besides, currentevoltage analysis and discharge efficiency were carried out in the full cell assembly to evaluate the aluminumeair battery performance. Furthermore, electrochemical impedance spectroscopy (EIS) and surface analysis were studied to investigate the mechanisms. 2. Experimental
Anode was consisted of aluminum plates cut from commercial 2N5 Al and high purity 4N grade Al. The detailed compositions of two aluminums were listed in Table 1. Cathode electrode was used the commercial gas diffusion electrode (GDE) including cobalt oxide as a catalyst (Meet Inc., Korea). For electrochemical tests, the surface of aluminum specimens was abraded with silicon carbide (SiC) papers down to 2000 grit. The aluminum specimen and gas diffusion electrode (GDE) were rinsed with ethanol, and dried with air. The electrolytes consisted of 4 M NaOH solutions at 25 C which corresponds to its peak electrolytic conductivity [10]. A saturated calomel electrode (SCE) was used as the reference electrode. To prevent the diffusion between OH ions and Cl ions in strong alkaline solution condition, the AMETEK G0194 polyethylene frit for use in highly alkaline solutions (pH > 10) was used as a reference tip. 2.2. Self-corrosion For self-corrosion evaluation, the specimens were F15 mm size controlled and immersed in 4 M NaOH for 60 min. The weight of the specimens before and after immersion was measured after cleaning the specimen. The corrosion rate is calculated by the following equation:
mm=yr
¼
dispersive spectroscopy (EDS) after immersion test. 2.3. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) Potentiodynamic polarization test was carried out to evaluate each Al oxidation reaction rate. This test was using a threeelectrode cell system and a VSP-300 (Biologic) potentiostat. The area of working electrode was controlled as 1.77 cm2. The potentiodynamic polarization test was swept at 0.166 mV s1 scanning rate from an initial potential of the open-circuit-potential (OCP) to a final potential of 1.2 V vs SCE. In order to evaluate the AC impedance change, three-electrode EIS test was performed by applying a sinusoidal wave to the working electrode with ±10 mV at frequencies ranging from 100 kHz to 100 mHz. As measuring the AC impedance change, the surface behavior of samples could be investigated as a function of time after immersion. In addition, the surface condition of sample could be analyzed by dynamic electrochemical impedance spectroscopy (DEIS) which measures the EIS under anodic polarized state. 2.4. Battery performance evaluation
2.1. Specimen and solution preparation
Corrosion rate
371
87:6W DAT
where W is the weight loss in milligram, D is the density in grams per cubic centimeter, A is the surface area in square centimeter, and T is the time in hour. To investigate morphology and impurity, specimens were examined using scanning electron microscopy (SEM) and energy
Table 1 Compositions of 4N grade Al and 2N5 grade Al. Materials
Fe
Cu
Si
Mn
Mg
Zn
Ti
Al
4N grade Al 2N5 grade Al
0.001 0.30
e 0.005
e 0.11
e 0.01
e 0.005
e 0.003
e 0.01
Rem. Rem.
2.4.1. Full cell assembly For the Aleair full cell test, self-designed full cell assembly was used in these experiments as shown in Fig. 1. A piece of aluminum specimen was used as the anode electrode, a commercial gas diffusion electrode (GDE) including cobalt oxide was comprised as a cathode. Each area of anode and cathode electrodes exposed to the electrolyte was 1.77 cm2 under air condition. 2.4.2. Currentevoltage evaluation and constant-voltage discharge test Currentevoltage evaluation and constant-voltage discharge test were conducted for evaluating battery performance in the full cell assembly. Currentevoltage evaluation was carried out to evaluate the current discharge outputs of the full cell as decreasing cell voltages. The test was conducted to discharge voltage for decreasing at 0.166 mV s1 scanning rate from OCP to 0.6 V. In order to calculate the discharge efficiency of anode materials, the constant-voltage discharge test was carried out at 0.8 V and 1.0 V for 4 h. Discharge efficiency was calculated by the amount of generated charge and weight loss. 3. Results and discussion 3.1. Electrochemical characteristics 3.1.1. Open-circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) Fig. 2 and Table 2 show the open-circuit potential vs time curves and self-corrosion rate of 4N grade and 2N5 grade aluminum anodes in 4 M NaOH solution. Self-corrosion rate is obtained by weight loss measurement in 4 M NaOH solution for 60 min. It was found that the open-circuit potential and self-corrosion rate of 2N5 grade Al are higher than those of 4N grade Al. The difference in the open-circuit potential and corrosion rate for these aluminums can be explained on the basis of the impurities identified as Mg, Mn, Cu, Si and Fe. Among the impurities, commercial aluminums such as 2N5 grade Al contain Fe and Si as components [12]. These impurities are known as cathodic impurities which cause OCP to shift towards the noble direction. Especially it was reported that Fe had a detrimental effect upon corrosion characteristics in terms of the increase of corrosion rate
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Fig. 1. Schematic diagram of (a) A photograph of actual full cell for experiments, (b) Schematic full cell diagram of Aleair battery.
[7]. The impedance spectra were obtained in the form of Nyquist plots of data from the three-electrode half cell shown in Fig. 3. The spectra are plotted as a function of time immersed in 4 M NaOH solution. The Nyquist diagrams are consisted of high-frequencies and low-frequencies capacitive loops. According to K.C. Emregul et al., the behavior of the system was characterized by a highfrequencies capacitive loop related to the charge transfer process owing to the dissolution of aluminum and a second capacitive loop at low frequencies due to the film growth [13]. It can be simply expressed as an anhydrous film and a hydrous film suggested by Emergul et al. and the following general reactions proceed [14]: Anhydrous film as the alloy/film interface (anodic processes):
Fig. 2. Open-circuit potential vs time for 4N grade Al, 2N5 grade Al for half cell in 4 M NaOH.
2Al þ 3O2 solid state /Al2 O3 þ 6e
(1)
Al/Al3þ solid state þ 3e
(2)
Hydrous film as the film/electrolyte interface (chemical processes):
Y.-J. Cho et al. / Journal of Power Sources 277 (2015) 370e378 Table 2 Corrosion rates of 4N grade Al and 2N5 grade Al in 4 M NaOH solution. Materials
Weight loss (mg)
Corrosion rate (mm/yr)
4N grade Al 2N5 grade Al
25.4 31.5
465.59 577.40
373
high frequencies spectra are detected on the charge transfer owing to the dissolution of aluminum and the low frequencies spectra are detected on the adjustment of the surface film. In the case of 4N grade Al, the phase angle curve maximum and width at high frequencies are depressed as a function of time. This is due to the formation of pores of the anhydrous film by breaking the reaction of passive film which was covered by the oxide film of Al2O3 in atmosphere [15]. At the same time, the phase angle curve maximum and width at low frequencies is depressed at the initial stage and saturated as a function of time. It is related to the formation of aluminum hydroxide layer:
Al þ 3OH /AlðOHÞ3 þ 3e
(6)
AlðOHÞ3 þ OH /AlðOHÞ 4 þ ss
(7)
where “ss” represents the bare surface site. Since Al3þ ions are thermodynamically unstable, Al(OH)3 films are formed when aluminum dissolves in alkaline solution. After that, the hydroxide film forms soluble aluminate ions in the solution and regenerates a bare aluminum surface site [16]. For these reasons, the phase angle curve maximum and width at low frequencies is high initially as forming Al(OH)3 mainly. After that, this curve is depressed as a function of immersion time when aluminum hydroxide film dissolved and finally this curve is saturated.
Fig. 3. Impedance spectra presented in Nyquist plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH as a function of immersion time.
n Al2 O3 þ 2 x 3 OH þ 3H2 O/2 AlðOHÞx gel
(3)
n Al3þ ejected þ xOH / AlðOHÞx gel /AlðOHÞ3 þ x 3 OH
(4)
The cathodic hydrogen evolution is occurred at the same time:
2H2 O þ 2e /2Hads þ 2OH /H2 [
(5)
In the case of this study, the diameter of both capacitance loops of 4N grade and 2N5 grade aluminums are decreased as a function of immersion time and second capacitive loops were changed from unstable to stable circles. After overlapped the capacitance loop at about 60 min later, the diameter of 2N5 grade Al is smaller than that of 4N grade Al. It is evident that the corrosion rate of 2N5 grade Al is higher than that of 4N grade Al. Fig. 4 presents the EIS results in the form of Bode phase plots during 60 min of immersion at open-circuit potential (OCP). The
Fig. 4. Bode phase plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH as a function of immersion time.
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In the case of 2N5, a similar trend is observed that the phase angle curve maximum at high frequencies is depressed. However, the phase angle curve maximum and width at low frequencies is not only increased, but also the phase curve at high frequencies moves to medium frequencies region and combines with phase curve at low frequencies as a function of the immersion time. It is considered that the impurity film forms on the aluminum surface. The impurities are detached when aluminum dissolved and attached again spontaneously due to the higher electromotive force (EMF) potential of impurities such as Fe and Si. 3.1.2. Surface analysis The surface morphologies as a function of immersion time of 4N grade, 2N5 grade Al in 4 M NaOH solution are obtained by scanning electron microscopy. There are two distinct features of the 2N5 grade Al. First, impurity layer is formed on the 2N5 grade Al surface at the immersion. The surface morphologies of two Al electrodes after immersion of (a) 5 min, (b) 25 min, (c) 60 min are presented in Figs. 5 and 6. In the case of 4N grade Al, small pieces of inclusion such as Si, C, O, Fe can be detected on the 4N grade Al surface rarely. However, the impurity layer is formed on the surface as a function
Fig. 5. SEM images of 4N grade Al as a function of immersion time in 4 M NaOH and EDS diagram during 60 min immersion of (a) 5 min, (b) 25 min, (c) 60 min.
Fig. 6. SEM images of 2N5 grade Al as a function of immersion time in 4 M NaOH and EDS diagram during 60 min immersion of (a) 5 min, (b) 25 min, (c) 60 min.
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of immersion time (Fig. 6a / Fig. 6b / Fig. 6c) in the case of 2N5. This layer is consisted of impurities containing Fe, O, Al which are segregated in inclusions. The layer is formed by the reduced impurities which are dissolved on the aluminum surface. Second, the pore size of 2N5 grade Al is larger than that of 4N grade Al. In the research by K.C. Emregül et al., pore size is related to the resistance of hydrogen accumulation which is generated by aluminum reaction. Especially accumulation of hydrogen molecules on the surface occupies pores and results in resistance to current flow [13]. As can be seen in Fig. 7, the pore size of 4N grade Al is smaller than that of 2N5 grade Al. The accumulation of hydrogen molecules in these pores is likely to be a resistor at the anhydrous layer corresponding to high frequencies spectra of EIS data. This feature of difference is related to galvanic coupling reaction, when the relatively lowalloyed matrix is coupled to the highly segregated inclusions. In order to simply elucidate this reaction, inclusion as an effective cathode, will accelerate the anodic oxidation reaction and the film dissolution of aluminum matrix [14]. This procedure increases corrosion rates and assists expanding the pores. 3.1.3. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) Fig. 8 shows the anodic polarization curves of 4N grade Al and 2N5 grade Al in 4 M NaOH solution. The corrosion potential of 4N grade Al is more negative than that of 2N5 grade Al. The anodic current density of 4N grade Al is higher than that of 2N5 grade Al up to 1.32 VSCE, whereas the anodic current density of 4N grade Al is lower than 2N5 grade Al above the 1.32 VSCE. The reaction rate which is influenced by overvoltage is related to the reaction interface resistivity. Fig. 9 shows impedance spectra presented in Nyquist plots of 4N grade Al and 2N5 grade Al for the half cell in 4 M NaOH as a function of anodic polarization. The Nyquist diagrams with increasing degree of anodic polarization are consisted of highfrequencies and low-frequencies capacitive loops in common with the case of Fig. 3. In the case of 4N grade Al, the diameter of highfrequencies capacitive loop is decreased and the diameter of low-
375
Fig. 8. Anodic potentiodynamic polarization curves of 4N grade Al and 2N5 grade Al for half cell in 4 M NaOH.
frequencies capacitive loop is increased. On the other hand, both capacitance loop diameters of 2N5 grade Al are increased with anodic polarization. Fig. 10 presents the EIS results in the form of Bode phase plots during the anodic polarization at 1.5 VSCE, 1.4 VSCE and 1.3 VSCE. In the case of 4N grade Al, the phase angle curve maximum and width at high frequencies are depressed a little with increasing anodic polarization. This is due to the enlargement of pores in the anhydrous film and decrease of hydrogen evolution with increasing degree of polarization. At the same time, the phase angle curve maximum and width at low frequencies increase dominantly. This is due to the consumption of OH ions to produce Al(OH) 4 that lowers electrolyte conductivity allowing the hydrous film to thicken interrupting further dissolution. This overproduction of Al(OH)3 and Al(OH) 4 acts as a factor of resistance with an
Fig. 7. SEM images of 4N grade Al, 2N5 grade Al after 60 min of immersion in 4 M NaOH with surface cleaning: (a) 4N grade Al (50), (b) 4N grade Al (200), (c) 2N5 grade Al (50), (d) 2N5 grade Al (200).
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exhibits the higher cell current density than 2N5 grade Al until ca. 0.8 V, whereas the current density of 4N grade Al shows the lower cell current density than 2N5 above ca. 0.8 V. This phenomenon shows the similar result of anodic polarization characteristics as explained in section 3.1.3. Fig. 14 shows the results of discharge current at applied potentials of 1.0 V and 0.8 V for 4 h 4N grade Al discharges higher currents during 4 h in the case of discharge at 1.0 V although the discharge current density decreases steadily from 40 mA cm2 to 25 mA cm2. While the discharge current density of 2N5 grade Al is maintained at ca. 15 mA cm2. However, the current density of 4N grade Al and 2N5 grade Al was become similar at 0.8 V discharge voltage. From constant discharge test, the full cell discharge efficiencies can be defined as the following equation. The calculated values of the full cell discharge efficiency are presented in Table 3.
QWL ¼
3 96485 weight loss atomic weight
Efficiency ¼
Qreal 100 QWL
Fig. 9. Impedance spectra presented in Nyquist plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH as a function of anodic polarization.
increasing anodic reaction by anodic polarization [17,18]. On the other hand, the integrated shaped phase angle curve at intermediate frequencies is maintained until 1.4 V in the case of 2N5 grade Al. However, it is divided into a region of high-frequencies and low-frequencies in common with the case of 4N grade Al. The phase angle curve maximum and width at low frequencies is increased with increasing degree of anodic polarization. This is considered why the Fe complex layer is removed over the 1.3 V as shown in Fig. 11 which are photograph images of in situ for the half cell in 4 M NaOH as increasing degree of anodic polarization. In the case of 4N grade Al, there is no change on the surface as increasing degree of anodic polarization. On the other hand, black layer is formed on the surface of 2N5 grade Al at the open-circuit-potential and this layer maintained until 1.4 VSCE. However, the black layer is disappeared above 1.4 VSCE. For the detailed observation of surface condition above 1.4 VSCE, it is observed by BSE mode of SEM and EDS analysis in Fig. 12. This image is taken after anodic polarization at 1.3 VSCE in 4 M NaOH. In this case, there are inclusions which are distributed randomly and contained impurities such as Si, C, O, P, Fe, Ni. 3.2. Battery performance evaluation Fig. 13 shows the currentevoltage evaluation curves for the discharge currents as decreasing full cell voltages. The 4N grade Al
Fig. 10. Bode phase plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH with increasing degree of anodic polarization.
Y.-J. Cho et al. / Journal of Power Sources 277 (2015) 370e378
377
Fig. 11. Photograph images of in situ (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH with increasing anodic polarization potential.
Fig. 13. Currentevoltage evaluation curves for the full cell discharge currents with decreasing cell voltage.
Fig. 12. SEM image and EDS diagram of 2N5 grade Al after anodic polarization at 1.3 V.
Where Qreal denotes the amount of charge in the external circuit and QWL is the amount of charge corresponding to weight loss during the anodic dissolution. At 1.0 V discharge voltage, the discharge efficiency of 4N grade Al was 50.39% and that of 2N5 grade Al was 19.00%. This result indicates that inclusions and complex film reduce the efficiency due to the increasing selfcorrosion reaction as a parasitic corrosion reaction. However, at 0.8 V discharge voltage, discharge current densities of 4N and 2N5 grade Al became similar. The efficiency of 4N grade Al was 71.96% and 2N5 grade Al was 76.00%, respectively. This result was caused by the dissolution phenomenon of complex film of 2N5 grade Al at 1.3 VSCE. In addition, the lower impedance of 2N5 grade Al than 4N grade Al at 1.3 VSCE as shown in Fig. 9, led to the decrease of the
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dissolved with the aluminum in electrolyte and dissolved metallic ions are reduced on the aluminum surface spontaneously due to the difference of electromotive force (EMF) potential between the aluminum and dissolved impurities. The complex film containing Fe and Si impedes the ion-exchange on the aluminumeelectrolyte interface and reduces the stand-by battery voltage. It also acts as an effective cathodic site to accelerate the self-corrosion and decrease the discharge efficiency. Therefore, the battery performance of 2N5 grade Al is lower than 4N grade Al at stand-by and low-power discharge. However, at higher power discharge voltage, the complex film of 2N5 grade Al disappears and the discharge current density of 2N5 grade Al and 4N grade Al became similar value. Therefore, it is inferred that the cheaper 2N5 grade Al can be used as a promising anode instead of 4N grade Al at the high-power discharge condition for aluminumeair battery. Fig. 14. Discharge current density behavior of the full cell at applied potentials of 1.0 V and 0.8 V.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. NRF-2012R1A2A2A03046671).
Table 3 Discharge efficiencies of the full cell at different applied potentials. Discharge voltage
Materials Electric capacity from weight loss (C)
1.0 V
4N grade Al 2N5 grade Al 4N grade Al 2N5 grade Al
0.8 V
Acknowledgments
Electric capacity Efficiency during discharge (C) (%)
1477.26
752.31
50.93
1992.21
378.48
19.00
2212.14
1591.85
71.96
2152.13
1635.62
76.00
full cell resistance. These are the reason why the self-corrosion reaction was declined and the efficiency was raised on the 2N5 grade Al at 0.8 V discharge voltage. 4. Conclusion Two aluminum anodes have been compared to evaluate the effect of the impurity on the aluminumeair battery performance. In the case of 2N5 grade Al, it was identified as forming complex film which mainly contains Fe and Si as impurities by impedance spectra and surface analysis. The impurities of 2N5 grade Al are
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Aluminum anode for aluminumeair battery e Part I: Influence of aluminum purity Young-Joo Cho, In-Jun Park, Hyeok-Jae Lee, Jung-Gu Kim* School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeonggi-Do 440-746, Republic of Korea
h i g h l i g h t s The performance of 2N5 Al is not as good as 4N Al due to the impurity layer. The impurity complex film is diminished with decreasing discharge voltage. 2N5 Al shows similar performance as 4N Al with decreasing discharge voltage. We find the way to optimize the performance of 2N5 Al as anode on Aleair battery.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2014 Received in revised form 5 December 2014 Accepted 9 December 2014 Available online 11 December 2014
2N5 commercial grade aluminum (99.5% purity) leads to the lower aluminumeair battery performances than 4N high pure grade aluminum (99.99% purity) due to impurities itself and formed impurity complex layer which contained Fe, Si, Cu and others. The impurity complex layer of 2N5 grade Al declines the battery voltage on standby status. It also depletes discharge current and battery efficiency at 1.0 V which is general operating voltage of aluminumeair battery. However, the impurity complex layer of 2N5 grade Al is dissolved with decreasing discharge voltage to 0.8 V. This phenomenon leads to improvement of discharge current density and battery efficiency by reducing self-corrosion reaction. This study demonstrates the possibility of use of 2N5 grade Al which is cheaper than 4N grade Al as the anode for aluminumeair battery. © 2014 Elsevier B.V. All rights reserved.
Keywords: Aluminum air battery Aluminum anode purity Impurity Battery efficiency Electrochemical impedance spectroscopy
1. Introduction Aluminum has been a very attractive source of metaleair battery for more than 50 years because of its inherent high theoretical energy density (8100 Wh kg1), lightweight (2.71 g cm3), negative standard potential (2.37 V vs. SHE), abundance (the third most abundant elements in the earth crust), environmental friendship, and recyclability [1e4]. However, aluminum has the protective oxide film which makes electron conducting hard in most aqueous conditions. This film results in high anodic dissolution overvoltage and makes unattractive for use as battery [5,6]. To overcome these problems, the electrolytes for aluminumeair battery system use saline, alkaline, and non-aqueous solutions. In the case of non-aqueous solutions such as organic solvents and
* Corresponding author. E-mail address: [email protected] (J.-G. Kim). http://dx.doi.org/10.1016/j.jpowsour.2014.12.026 0378-7753/© 2014 Elsevier B.V. All rights reserved.
ionic liquids, they have disadvantages in terms of high volatility and flammability, lower electrical conductivity, and high cost. Moreover, the stability of electrolyte becomes worse when oxygen dissolves in the electrolytes. For these reasons, alkaline or saline solution was commonly used in Aleair battery [4,7,8]. Among them, alkaline media displays good battery performance especially at high discharge current. However, aluminum suffers substantial corrosion in alkaline solution with the production of large amount of hydrogen gas covered on the surface of aluminum anode, induction of coulombic loss on discharge and stand-by [9,10]. In order to reduce the self-discharge of aluminum anode, it is considered to use high purity grade aluminum. The main reason for use high purity such as 4N (99.99% purity) or 5N (99.999% purity) grade aluminum is the absence of Fe impurities which presented in a raw cast aluminum and drastically increase the corrosion rate of aluminum. However, the production price of high grades, 10 to 20 times greater than the 2N5 commercial grade (99.5% purity), does
Y.-J. Cho et al. / Journal of Power Sources 277 (2015) 370e378
not satisfy the requirements for commercial application. Moreover, 4N or 5N grade aluminum anodes can reduce the overall thermodynamic efficiency of the battery due to the overpotential of both anodic and cathodic reactions when battery works. Until now, there were several researches on aluminum purity. However, it was inadequate because most of researches aimed at corrosion rate and hydrogen evolution by parasitic reaction not in terms of battery performance and influence of impurities [1,11]. For these reasons, the aim of this study is to investigate the battery performance and the behavior of impurities. In the present work, the battery performance of 2N5 grade Al and 4N grade Al have been studied in 4 M NaOH electrolyte. The potentiodynamic polarization test was carried out in the half cell assembly to analyze the anodic dissolution of aluminum. Besides, currentevoltage analysis and discharge efficiency were carried out in the full cell assembly to evaluate the aluminumeair battery performance. Furthermore, electrochemical impedance spectroscopy (EIS) and surface analysis were studied to investigate the mechanisms. 2. Experimental
Anode was consisted of aluminum plates cut from commercial 2N5 Al and high purity 4N grade Al. The detailed compositions of two aluminums were listed in Table 1. Cathode electrode was used the commercial gas diffusion electrode (GDE) including cobalt oxide as a catalyst (Meet Inc., Korea). For electrochemical tests, the surface of aluminum specimens was abraded with silicon carbide (SiC) papers down to 2000 grit. The aluminum specimen and gas diffusion electrode (GDE) were rinsed with ethanol, and dried with air. The electrolytes consisted of 4 M NaOH solutions at 25 C which corresponds to its peak electrolytic conductivity [10]. A saturated calomel electrode (SCE) was used as the reference electrode. To prevent the diffusion between OH ions and Cl ions in strong alkaline solution condition, the AMETEK G0194 polyethylene frit for use in highly alkaline solutions (pH > 10) was used as a reference tip. 2.2. Self-corrosion For self-corrosion evaluation, the specimens were F15 mm size controlled and immersed in 4 M NaOH for 60 min. The weight of the specimens before and after immersion was measured after cleaning the specimen. The corrosion rate is calculated by the following equation:
mm=yr
¼
dispersive spectroscopy (EDS) after immersion test. 2.3. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) Potentiodynamic polarization test was carried out to evaluate each Al oxidation reaction rate. This test was using a threeelectrode cell system and a VSP-300 (Biologic) potentiostat. The area of working electrode was controlled as 1.77 cm2. The potentiodynamic polarization test was swept at 0.166 mV s1 scanning rate from an initial potential of the open-circuit-potential (OCP) to a final potential of 1.2 V vs SCE. In order to evaluate the AC impedance change, three-electrode EIS test was performed by applying a sinusoidal wave to the working electrode with ±10 mV at frequencies ranging from 100 kHz to 100 mHz. As measuring the AC impedance change, the surface behavior of samples could be investigated as a function of time after immersion. In addition, the surface condition of sample could be analyzed by dynamic electrochemical impedance spectroscopy (DEIS) which measures the EIS under anodic polarized state. 2.4. Battery performance evaluation
2.1. Specimen and solution preparation
Corrosion rate
371
87:6W DAT
where W is the weight loss in milligram, D is the density in grams per cubic centimeter, A is the surface area in square centimeter, and T is the time in hour. To investigate morphology and impurity, specimens were examined using scanning electron microscopy (SEM) and energy
Table 1 Compositions of 4N grade Al and 2N5 grade Al. Materials
Fe
Cu
Si
Mn
Mg
Zn
Ti
Al
4N grade Al 2N5 grade Al
0.001 0.30
e 0.005
e 0.11
e 0.01
e 0.005
e 0.003
e 0.01
Rem. Rem.
2.4.1. Full cell assembly For the Aleair full cell test, self-designed full cell assembly was used in these experiments as shown in Fig. 1. A piece of aluminum specimen was used as the anode electrode, a commercial gas diffusion electrode (GDE) including cobalt oxide was comprised as a cathode. Each area of anode and cathode electrodes exposed to the electrolyte was 1.77 cm2 under air condition. 2.4.2. Currentevoltage evaluation and constant-voltage discharge test Currentevoltage evaluation and constant-voltage discharge test were conducted for evaluating battery performance in the full cell assembly. Currentevoltage evaluation was carried out to evaluate the current discharge outputs of the full cell as decreasing cell voltages. The test was conducted to discharge voltage for decreasing at 0.166 mV s1 scanning rate from OCP to 0.6 V. In order to calculate the discharge efficiency of anode materials, the constant-voltage discharge test was carried out at 0.8 V and 1.0 V for 4 h. Discharge efficiency was calculated by the amount of generated charge and weight loss. 3. Results and discussion 3.1. Electrochemical characteristics 3.1.1. Open-circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) Fig. 2 and Table 2 show the open-circuit potential vs time curves and self-corrosion rate of 4N grade and 2N5 grade aluminum anodes in 4 M NaOH solution. Self-corrosion rate is obtained by weight loss measurement in 4 M NaOH solution for 60 min. It was found that the open-circuit potential and self-corrosion rate of 2N5 grade Al are higher than those of 4N grade Al. The difference in the open-circuit potential and corrosion rate for these aluminums can be explained on the basis of the impurities identified as Mg, Mn, Cu, Si and Fe. Among the impurities, commercial aluminums such as 2N5 grade Al contain Fe and Si as components [12]. These impurities are known as cathodic impurities which cause OCP to shift towards the noble direction. Especially it was reported that Fe had a detrimental effect upon corrosion characteristics in terms of the increase of corrosion rate
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Fig. 1. Schematic diagram of (a) A photograph of actual full cell for experiments, (b) Schematic full cell diagram of Aleair battery.
[7]. The impedance spectra were obtained in the form of Nyquist plots of data from the three-electrode half cell shown in Fig. 3. The spectra are plotted as a function of time immersed in 4 M NaOH solution. The Nyquist diagrams are consisted of high-frequencies and low-frequencies capacitive loops. According to K.C. Emregul et al., the behavior of the system was characterized by a highfrequencies capacitive loop related to the charge transfer process owing to the dissolution of aluminum and a second capacitive loop at low frequencies due to the film growth [13]. It can be simply expressed as an anhydrous film and a hydrous film suggested by Emergul et al. and the following general reactions proceed [14]: Anhydrous film as the alloy/film interface (anodic processes):
Fig. 2. Open-circuit potential vs time for 4N grade Al, 2N5 grade Al for half cell in 4 M NaOH.
2Al þ 3O2 solid state /Al2 O3 þ 6e
(1)
Al/Al3þ solid state þ 3e
(2)
Hydrous film as the film/electrolyte interface (chemical processes):
Y.-J. Cho et al. / Journal of Power Sources 277 (2015) 370e378 Table 2 Corrosion rates of 4N grade Al and 2N5 grade Al in 4 M NaOH solution. Materials
Weight loss (mg)
Corrosion rate (mm/yr)
4N grade Al 2N5 grade Al
25.4 31.5
465.59 577.40
373
high frequencies spectra are detected on the charge transfer owing to the dissolution of aluminum and the low frequencies spectra are detected on the adjustment of the surface film. In the case of 4N grade Al, the phase angle curve maximum and width at high frequencies are depressed as a function of time. This is due to the formation of pores of the anhydrous film by breaking the reaction of passive film which was covered by the oxide film of Al2O3 in atmosphere [15]. At the same time, the phase angle curve maximum and width at low frequencies is depressed at the initial stage and saturated as a function of time. It is related to the formation of aluminum hydroxide layer:
Al þ 3OH /AlðOHÞ3 þ 3e
(6)
AlðOHÞ3 þ OH /AlðOHÞ 4 þ ss
(7)
where “ss” represents the bare surface site. Since Al3þ ions are thermodynamically unstable, Al(OH)3 films are formed when aluminum dissolves in alkaline solution. After that, the hydroxide film forms soluble aluminate ions in the solution and regenerates a bare aluminum surface site [16]. For these reasons, the phase angle curve maximum and width at low frequencies is high initially as forming Al(OH)3 mainly. After that, this curve is depressed as a function of immersion time when aluminum hydroxide film dissolved and finally this curve is saturated.
Fig. 3. Impedance spectra presented in Nyquist plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH as a function of immersion time.
n Al2 O3 þ 2 x 3 OH þ 3H2 O/2 AlðOHÞx gel
(3)
n Al3þ ejected þ xOH / AlðOHÞx gel /AlðOHÞ3 þ x 3 OH
(4)
The cathodic hydrogen evolution is occurred at the same time:
2H2 O þ 2e /2Hads þ 2OH /H2 [
(5)
In the case of this study, the diameter of both capacitance loops of 4N grade and 2N5 grade aluminums are decreased as a function of immersion time and second capacitive loops were changed from unstable to stable circles. After overlapped the capacitance loop at about 60 min later, the diameter of 2N5 grade Al is smaller than that of 4N grade Al. It is evident that the corrosion rate of 2N5 grade Al is higher than that of 4N grade Al. Fig. 4 presents the EIS results in the form of Bode phase plots during 60 min of immersion at open-circuit potential (OCP). The
Fig. 4. Bode phase plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH as a function of immersion time.
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In the case of 2N5, a similar trend is observed that the phase angle curve maximum at high frequencies is depressed. However, the phase angle curve maximum and width at low frequencies is not only increased, but also the phase curve at high frequencies moves to medium frequencies region and combines with phase curve at low frequencies as a function of the immersion time. It is considered that the impurity film forms on the aluminum surface. The impurities are detached when aluminum dissolved and attached again spontaneously due to the higher electromotive force (EMF) potential of impurities such as Fe and Si. 3.1.2. Surface analysis The surface morphologies as a function of immersion time of 4N grade, 2N5 grade Al in 4 M NaOH solution are obtained by scanning electron microscopy. There are two distinct features of the 2N5 grade Al. First, impurity layer is formed on the 2N5 grade Al surface at the immersion. The surface morphologies of two Al electrodes after immersion of (a) 5 min, (b) 25 min, (c) 60 min are presented in Figs. 5 and 6. In the case of 4N grade Al, small pieces of inclusion such as Si, C, O, Fe can be detected on the 4N grade Al surface rarely. However, the impurity layer is formed on the surface as a function
Fig. 5. SEM images of 4N grade Al as a function of immersion time in 4 M NaOH and EDS diagram during 60 min immersion of (a) 5 min, (b) 25 min, (c) 60 min.
Fig. 6. SEM images of 2N5 grade Al as a function of immersion time in 4 M NaOH and EDS diagram during 60 min immersion of (a) 5 min, (b) 25 min, (c) 60 min.
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of immersion time (Fig. 6a / Fig. 6b / Fig. 6c) in the case of 2N5. This layer is consisted of impurities containing Fe, O, Al which are segregated in inclusions. The layer is formed by the reduced impurities which are dissolved on the aluminum surface. Second, the pore size of 2N5 grade Al is larger than that of 4N grade Al. In the research by K.C. Emregül et al., pore size is related to the resistance of hydrogen accumulation which is generated by aluminum reaction. Especially accumulation of hydrogen molecules on the surface occupies pores and results in resistance to current flow [13]. As can be seen in Fig. 7, the pore size of 4N grade Al is smaller than that of 2N5 grade Al. The accumulation of hydrogen molecules in these pores is likely to be a resistor at the anhydrous layer corresponding to high frequencies spectra of EIS data. This feature of difference is related to galvanic coupling reaction, when the relatively lowalloyed matrix is coupled to the highly segregated inclusions. In order to simply elucidate this reaction, inclusion as an effective cathode, will accelerate the anodic oxidation reaction and the film dissolution of aluminum matrix [14]. This procedure increases corrosion rates and assists expanding the pores. 3.1.3. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) Fig. 8 shows the anodic polarization curves of 4N grade Al and 2N5 grade Al in 4 M NaOH solution. The corrosion potential of 4N grade Al is more negative than that of 2N5 grade Al. The anodic current density of 4N grade Al is higher than that of 2N5 grade Al up to 1.32 VSCE, whereas the anodic current density of 4N grade Al is lower than 2N5 grade Al above the 1.32 VSCE. The reaction rate which is influenced by overvoltage is related to the reaction interface resistivity. Fig. 9 shows impedance spectra presented in Nyquist plots of 4N grade Al and 2N5 grade Al for the half cell in 4 M NaOH as a function of anodic polarization. The Nyquist diagrams with increasing degree of anodic polarization are consisted of highfrequencies and low-frequencies capacitive loops in common with the case of Fig. 3. In the case of 4N grade Al, the diameter of highfrequencies capacitive loop is decreased and the diameter of low-
375
Fig. 8. Anodic potentiodynamic polarization curves of 4N grade Al and 2N5 grade Al for half cell in 4 M NaOH.
frequencies capacitive loop is increased. On the other hand, both capacitance loop diameters of 2N5 grade Al are increased with anodic polarization. Fig. 10 presents the EIS results in the form of Bode phase plots during the anodic polarization at 1.5 VSCE, 1.4 VSCE and 1.3 VSCE. In the case of 4N grade Al, the phase angle curve maximum and width at high frequencies are depressed a little with increasing anodic polarization. This is due to the enlargement of pores in the anhydrous film and decrease of hydrogen evolution with increasing degree of polarization. At the same time, the phase angle curve maximum and width at low frequencies increase dominantly. This is due to the consumption of OH ions to produce Al(OH) 4 that lowers electrolyte conductivity allowing the hydrous film to thicken interrupting further dissolution. This overproduction of Al(OH)3 and Al(OH) 4 acts as a factor of resistance with an
Fig. 7. SEM images of 4N grade Al, 2N5 grade Al after 60 min of immersion in 4 M NaOH with surface cleaning: (a) 4N grade Al (50), (b) 4N grade Al (200), (c) 2N5 grade Al (50), (d) 2N5 grade Al (200).
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exhibits the higher cell current density than 2N5 grade Al until ca. 0.8 V, whereas the current density of 4N grade Al shows the lower cell current density than 2N5 above ca. 0.8 V. This phenomenon shows the similar result of anodic polarization characteristics as explained in section 3.1.3. Fig. 14 shows the results of discharge current at applied potentials of 1.0 V and 0.8 V for 4 h 4N grade Al discharges higher currents during 4 h in the case of discharge at 1.0 V although the discharge current density decreases steadily from 40 mA cm2 to 25 mA cm2. While the discharge current density of 2N5 grade Al is maintained at ca. 15 mA cm2. However, the current density of 4N grade Al and 2N5 grade Al was become similar at 0.8 V discharge voltage. From constant discharge test, the full cell discharge efficiencies can be defined as the following equation. The calculated values of the full cell discharge efficiency are presented in Table 3.
QWL ¼
3 96485 weight loss atomic weight
Efficiency ¼
Qreal 100 QWL
Fig. 9. Impedance spectra presented in Nyquist plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH as a function of anodic polarization.
increasing anodic reaction by anodic polarization [17,18]. On the other hand, the integrated shaped phase angle curve at intermediate frequencies is maintained until 1.4 V in the case of 2N5 grade Al. However, it is divided into a region of high-frequencies and low-frequencies in common with the case of 4N grade Al. The phase angle curve maximum and width at low frequencies is increased with increasing degree of anodic polarization. This is considered why the Fe complex layer is removed over the 1.3 V as shown in Fig. 11 which are photograph images of in situ for the half cell in 4 M NaOH as increasing degree of anodic polarization. In the case of 4N grade Al, there is no change on the surface as increasing degree of anodic polarization. On the other hand, black layer is formed on the surface of 2N5 grade Al at the open-circuit-potential and this layer maintained until 1.4 VSCE. However, the black layer is disappeared above 1.4 VSCE. For the detailed observation of surface condition above 1.4 VSCE, it is observed by BSE mode of SEM and EDS analysis in Fig. 12. This image is taken after anodic polarization at 1.3 VSCE in 4 M NaOH. In this case, there are inclusions which are distributed randomly and contained impurities such as Si, C, O, P, Fe, Ni. 3.2. Battery performance evaluation Fig. 13 shows the currentevoltage evaluation curves for the discharge currents as decreasing full cell voltages. The 4N grade Al
Fig. 10. Bode phase plots of (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH with increasing degree of anodic polarization.
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377
Fig. 11. Photograph images of in situ (a) 4N grade Al, (b) 2N5 grade Al for the half cell in 4 M NaOH with increasing anodic polarization potential.
Fig. 13. Currentevoltage evaluation curves for the full cell discharge currents with decreasing cell voltage.
Fig. 12. SEM image and EDS diagram of 2N5 grade Al after anodic polarization at 1.3 V.
Where Qreal denotes the amount of charge in the external circuit and QWL is the amount of charge corresponding to weight loss during the anodic dissolution. At 1.0 V discharge voltage, the discharge efficiency of 4N grade Al was 50.39% and that of 2N5 grade Al was 19.00%. This result indicates that inclusions and complex film reduce the efficiency due to the increasing selfcorrosion reaction as a parasitic corrosion reaction. However, at 0.8 V discharge voltage, discharge current densities of 4N and 2N5 grade Al became similar. The efficiency of 4N grade Al was 71.96% and 2N5 grade Al was 76.00%, respectively. This result was caused by the dissolution phenomenon of complex film of 2N5 grade Al at 1.3 VSCE. In addition, the lower impedance of 2N5 grade Al than 4N grade Al at 1.3 VSCE as shown in Fig. 9, led to the decrease of the
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dissolved with the aluminum in electrolyte and dissolved metallic ions are reduced on the aluminum surface spontaneously due to the difference of electromotive force (EMF) potential between the aluminum and dissolved impurities. The complex film containing Fe and Si impedes the ion-exchange on the aluminumeelectrolyte interface and reduces the stand-by battery voltage. It also acts as an effective cathodic site to accelerate the self-corrosion and decrease the discharge efficiency. Therefore, the battery performance of 2N5 grade Al is lower than 4N grade Al at stand-by and low-power discharge. However, at higher power discharge voltage, the complex film of 2N5 grade Al disappears and the discharge current density of 2N5 grade Al and 4N grade Al became similar value. Therefore, it is inferred that the cheaper 2N5 grade Al can be used as a promising anode instead of 4N grade Al at the high-power discharge condition for aluminumeair battery. Fig. 14. Discharge current density behavior of the full cell at applied potentials of 1.0 V and 0.8 V.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. NRF-2012R1A2A2A03046671).
Table 3 Discharge efficiencies of the full cell at different applied potentials. Discharge voltage
Materials Electric capacity from weight loss (C)
1.0 V
4N grade Al 2N5 grade Al 4N grade Al 2N5 grade Al
0.8 V
Acknowledgments
Electric capacity Efficiency during discharge (C) (%)
1477.26
752.31
50.93
1992.21
378.48
19.00
2212.14
1591.85
71.96
2152.13
1635.62
76.00
full cell resistance. These are the reason why the self-corrosion reaction was declined and the efficiency was raised on the 2N5 grade Al at 0.8 V discharge voltage. 4. Conclusion Two aluminum anodes have been compared to evaluate the effect of the impurity on the aluminumeair battery performance. In the case of 2N5 grade Al, it was identified as forming complex film which mainly contains Fe and Si as impurities by impedance spectra and surface analysis. The impurities of 2N5 grade Al are
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