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Preparation and performance of lead foam grid for negative electrode of VRLA battery
Materials Chemistry and Physics 99 (2006) 431–436
Preparation and performance of lead foam grid for negative electrode of VRLA battery C.S. Dai ∗ , B. Zhang, D.L. Wang, T.F. Yi, X.G. Hu Department of Applied Chemistry, Harbin Institute of Technology, P.O. Box 411, Harbin 150001, PR China Received 14 June 2005; received in revised form 20 October 2005; accepted 11 November 2005
Abstract Lead (alloy) foam was prepared via electrodeposition by using copper foam as the substrate and adding element cerium to the electrodepositing solution under ultrasonic treatment. The performances and morphology of the lead foam were investigated by means of SEM and AFM. The results show that because of the addition of cerium and the application of ultrasound, the distribution thickness ratio (DTR) of the lead foam is decreased. The lead foam has a uniform three-dimensional reticulate structure with a specific surface area of about 5700 m2 m−3 , a porosity of about 88.1%, and an apparent resistivity of about 150 cm−1 . The results of the cyclic voltammetry indicate that the lead foam has a good stability when it is used as the negative electrode material of a lead acid battery. The battery testing revealed that, at 5, 2 h and high current discharge rates, there are improvements of 28.5%, 29.5% and 20.4% in the negative active material utilization efficiencies with the lead-foam VRLAB compared with the cast grid one and the mass specific capacities of the lead foam negative electrode are 36%, 44% and 33% higher than those of the cast grid one. © 2005 Elsevier B.V. All rights reserved. Keywords: Lead foam; Electrodeposition lead alloy; Negative electrode material
1. Introduction At present, exhaust emissions from fuel-driven automobile vehicles are a major cause of urban pollution and a source of the green house-gas CO2 , which accounts for 20% of the total. This problem has aroused increasing concern all over the world. Additionally, global oil storage capacity has declined gradually. Therefore, both low-emission vehicles (i.e., hybrid electric vehicles, HEVs) and zero-emission vehicles (i.e., electric vehicles, EVs) have thrived world wide and a new industry related to these two types of vehicles has appeared with a tremendous potential [1,2]. Although as a power supply, batteries cannot be considered as a match for oil, the advanced electric power sources are being developed, which are expected to have a high specific energy, a large specific power, a long cycle life, a wide operation temperature range, low cost, better safety and no pollution [3–5]. Due to the immaturity of the commercialization of fuel cells and the safety problems of Li-ion batteries, advanced lead-acid
∗
Corresponding author. Tel.: +86 451 86413751; fax: +86 451 86221048. E-mail address: [email protected] (C.S. Dai).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.11.014
batteries are still the most practical batteries for EVs and HEVs [5]. Usually, it is believed that the positive plate determines the performances of a lead-acid battery. However, because power batteries always work under a partial state of charge (PSoC), the negative plate has a great influence on a battery’s performances, which is reflected by a lower charging ability of a battery caused by the serious sulfation on the negative plat [6]. Recently, lead foam has been paid more attention to owing to its light weight large specific area and high porosity [7–10]. Cast reticulated lead foam, whose specific surface area is 14 cm2 cm−3 , has been reported to be applied as the current collector in an open cell. The utilization efficiency of the positive active material is as high as 50%, which is much higher than that of the traditional cast grids [7]. When the reticulated vitreous carbon is used as the substrate, lead foam prepared via electrodeposition can significantly increase the specific energy of a lead acid battery and the utilization efficiency of the active material [10]. In order to improve the charge/discharge performance of lead acid batteries, lead foam negative grids were produced by electrodepositing Pb alloy on a copper foam substrate which has good conductibility and a uniform three-dimensional reticulate structure.
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2. Experimental Lead foam negative grids were prepared on the copper foam substrate (30 PPI, apparent surface density is 770 ± 55 g m−2 ). The surface morphology was observed by using a Hitachi Japan S-4700 scanning electronic microscope. The lead foam was solidified by using epoxide resin, and polished before the cross-section observation. An atom force microscope (AFM, PicoPlus MOLECULAR IMAGING, USA) was used to study the detailed morphology of the sample. An inductively coupled plasma spectrometer (TJA USA IRIS Advantage) was applied to measure the content of Ce in the electrodeposited layer. The real surface area was tested by chronoamperometry and the porosity was measured by the weighing method. Cyclic voltammetry (CV) was performed by using a CHI model 630 electrochemical working station. The tested system was a three-electrode system. A pure lead plate with a large area was used as the counter electrode and Hg/Hg2 SO4 with a standard potential of 0.6210 V (versus SHE, 25 ◦ C) (the concentration of sulfuric acid was 4.8 mol L−1 ) served as the reference electrode. The electrolyte was 4.8 mol L−1 H2 SO4 and the potential applied to the working electrode was −0.6 to −1.2 V. The experimental temperature was 25 ± 1 ◦ C. The electrolyte was prepared by analytical reagents and distilled water. Cast grids and lead foam grids were used, respectively, as the current collector for a negative electrode, to assemble the valve regulated batteries with two positive electrodes and one negative electrode, whose capacities were limited by the negative electrodes. The charge and discharge tests were carried out by using a BTS 5 V/10 A battery testing system (made by Neware Technology Company, Shenzhen, China). The charge/discharge performances were evaluated according to the Mechanism Industry Standard of the People’s Republic of China (JB/T 10262-2001) for sealed lead acid batteries used for electric moped.
3. Results and discussion 3.1. Deciding of the solution for electrodeposition lead foam There have been some reports on the Pb electrodeposition in a pyrophosphate system and a sulfate system, but a fluoride boracic acid system is used more broadly in the commercial process. Therefore, a fluoride boracic acid system, as shown in Table 1, was chosen to study the electrodeposition of Pb on a copper foam substrate. In order to improve the uniformity of the lead foam, ultrasound was imposed during the electrodeposition of lead, which decreased the DTR of the lead foam. The copper foam substrate was 60 mm × 80 mm, to which a certain current density was applied. The electrodeposited efficiency was measured by the time when the copper foam substrate was completely covered by lead. The result shows that ultrasonic wave treating can significantly increase the electrodeposition efficiency (Fig. 1). It was found from the SEM image of the cross-section of lead foam obtained that the DTR of the lead foam is decreased from ca. 1.6
to 1.4. The mechanism of ultrasonic-catalysis is not related with the direct interaction between sound waves and molecules, but the liquid vacuum process. When air bubbles collapse, the local temperature will be above 5000 K and the local pressure will be up to 5 × 107 Pa. Meanwhile, the change rate of temperature is about 109 K s−1 and there are intense bow wave and 400 km h−1 current [11,12]. Under such a condition, some chemical reactions can occur, which is impossible under normal conditions. Ultrasonic wave makes the water molecules around Pb2+ in the surface liquid layer of the electrode to be rearranged, which results in the void electron energy level of Pb2+ rising to the Fermi energy level of the electrode and forming more Pb adsorbing atoms. Ultrasonic wave treating can increases the uniformity of the lead foam obtained via electrodeposition on copper foam. Due to the strong stirring of ultrasonic wave, the diffusion and the transfer of Pb2+ become easier, and the lead foam will be more uniform along the thickness direction. In the study of grid alloy used for lead acid batteries, it is found that the addition of rare earth metal Ce can improve the conductivity of the corrosion film of a positive electrode. To improve the performance lead foam, in this study Ce was added to the Pb electrodepositing solution, thereby, an Ce and Pb alloy was obtained. Because SO4 2− can interfere the lead foam electrodeposition, Ce was added without SO4 2− . The preparation of the Ce solution was as follows: 15.7 g of ceric sulfate was weighed and then dissolve in 5 mL of concentrated sulfuric acid (called solution A). Lead oxide (40 g) that was stoichiometrically excessive compared with the amount of SO4 2− in solution A was weighed and put into 100 g of fluoboric acid then the mixture was stirred till it was dissolved (called solution B). Solution A was poured into solution B and the sediment was filtered. A clear solution of cerium flouroborate and lead flouroborate, which contained no SO4 2− , was obtained. Then it fixed to 200 mL. The above solution (10–20 mL) was taken and added into the electrodepositing solution shown in Table 1 to obtain the cerium and lead electrodepositing solution. The result of the ICP experiment shows that the Ce content in the lead foam alloy produced by such a technique is in a range of 0.002–0.01 wt.%. By adding, the DTR of the lead foam can
Table 1 The composition of electrodeposition Pb solution and process parameters Pb2+ [added through Pb(BF4 )2 ] (g L−1 ) Dissociative fluoride boracic acid (HBF4 ) (g L−1 ) Boracic acid (H3 BO3 ) (g L−1 ) Peptone (g L−1 ) Anode Current density, j (A dm−2 ) Temperature, T (◦ C)
200 150 30 0.5 Pure lead 3 15–30
Fig. 1. Effect of ultrasonic wave on closed rate of lead foam electrodeposition.
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Fig. 2. AFM images of electrodeposited lead: (a) electrodeposited lead, (b) electrodeposited lead applying ultrasonic wave, (c) electrodeposited lead and cerium applying ultrasonic wave.
be decreased from 1.4 to 1.2 and the uniformity of the lead foam also can be improved. 3.2. AFM observation of electrodeposited lead foam Three samples were fabricated, respectively, by electrodepositing lead on copper plate, electrodepositing lead on copper plate under ultrasonic treatment, and electrodepositing leadcerium alloy under ultrasonic treatment. The three samples were polished by using the same emery paper and then were observed by using an AFM. The results are shown in Fig. 2. Fig. 3 shows the surface roughness along the diagonal direction of the three samples. It shows that ultrasonic treatment and the addition of cerium increase the surface roughness of the as-electrodeposited lead. The preparing process of the lead foam should include the following procedures: the electrodeposition of Pb–Ce alloy on copper foam [surface density was 770 ± 55 g m−2 ] under ultrasound treatment and the preparation of lead foam grids. The area density of the prepared lead foam grids was controlled at around 2200 ± 150 g m−2 .
Fig. 3. Surface roughness of diagonal direction of the samples in Fig. 2.
3.3. Three-dimensional reticular structure and surface morphology of the lead foam Similar to the copper foam, the lead foam also has a perfect three-dimensional reticular structure. All holes in lead foam are connected to each other. It has one more channel than the twodimensional cast grids. Therefore, it can increase the uniformity of an electrode. Fig. 4 shows the SEM image of the lead foam, in which the three-dimensional reticular structure of the lead foam is shown clearly.
Fig. 4. SEM image of lead foam made by electroplating (30 PPI).
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Table 2 Specific surface area of lead foam Foam (PPI) Specific surface area (m2 m−3 )
30 5700
3.4. Porosity, specific surface area and resistivity of lead foam High porosity is the unique characteristic of lead foam grids. Compared with other grids having the same volume, the containing capacity of the lead foam to active materials is the highest. The effective porosity of the lead foam is decided by the copper foam substrate, the porosity, the surface density and the thickness of the lead foam. The porosity of the lead foam was measured by the weighing method. And the result indicates that its porosity is about 88.1%. The specific surface area of lead foam is also an important parameter. It is beneficial to the connection of a substrate and an active material and the utilization efficiency of the active material can be increased when the specific surface area of lead foam is high. The specific surface area values of lead foam were measured by chronoamperometry and the results are shown in Table 2. The results show that the specific surface area of the lead foam is very high and is much higher than that of the lead foam (1800 m2 m−3 ) based on an RVC substrate. As a collector, the conductivity is a very important property while the conductivity of a metal foam greatly depends on its porosity. Langlois and Coeuret [13] found that the apparent resistivity of nickel foam is the function of the resistivity of nickel metal ρfoam =
4 ρNi 1−θ
(1) Fig. 6. Cyclic voltammograms of lead foam negative electrode cycled 100 times.
where ρfoam is the resistivity of nickel foam; ρNi the resistivity of nickel; θ the porosity of nickel foam. The effect of the production process on the resistivity of nickel foam was also studied. The function which is applicable to the calculation of the resistivity of nickel foam can also be used to calculate the resistivity of copper foam and lead foam. It is easily to be derived that ρfoam(Cu) = 163 ( cm−1 ), ρfoam(Pb) = 2070 ( cm−1 ). The electric resistance of the lead foam electrodeposited on copper foam can be regarded as the result of parallel connection of lead foam and copper foam that have the same length and area 1 1 1 = + ρfoam(Cu–Pb) ρfoam(Cu) ρfoam(Pb)
Fig. 5. Cyclic voltammograms of lead foam cycled 20 times.
(2)
According to (2), it can be obtained that ρfoam(Cu–Pb) = 151.1 ( cm−1 ). 3.5. Stability of lead foam in H2 SO4 The hydrogen evolution overpotential of lead is higher than that of copper. If copper in negative grids dissolves in the electrolyte while the battery is being discharged, the performance of the battery will be affected significantly. In order to investigate the stability of the lead foam in H2 SO4 , cyclic voltammograms
of the lead foam grids with different thicknesses of the electrodeposited Pb layer as well as those of the lead foam grids pasted with active materials were recorded. 3.5.1. Cyclic voltammetry of lead foam Fig. 5 shows the cyclic voltammogram of the lead foam in an electrolyte of 4.8 mol L−1 H2 SO4 . It can be
Table 3 Parameters of different electrodes Parameters
Lead foam electrode
Cast grid electrode
Grid mass before painting (g) Electrode mass after painting (g) Geometric area of grid (cm2 ) Real surface area of grid (cm2 ) α γ (g cm−2 )
9.0 41.0 31.5 546.84 0.26 0.05
13.6 45.6 31.5 31.14 0.34 0.84
α = Mcollector /(MNAM + Mcollector ), γ = MNAM /Scollector ; Mgrid is the grid mass; MNAM the mass of negative active material; Scollector the real surface area of grid.
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Table 4 Discharge performance of batteries with different negative electrodes Discharge rate
Discharge capacity (mAh)
Utilization efficiency (%)
Specific capacity of negative electrodes (mAh g−1 )
5h Foam Cast
2957.8 2383.6
46.48 36.17
71.15 52.27
2h Foam Cast
2684.2 2072.9
42.18 32.58
65.47 45.46
High current (3.0I2 ) Foam Cast
1818.4 1510.3
28.57 23.73
44.35 33.12
seen that the corresponding cyclic voltammogram has only one oxidation–reduction peak corresponding to Pb/PbSO4 but no peak belonging to Cu/Cu2+ after it was scanned many times. The conclusion can be drawn that there is no copper dissolved and the degree of hydrogen evolution does not increase. 3.5.2. Cyclic voltammetry of lead foam negative electrode Fig. 6 shows the cyclic voltammogram for the lead foam negative electrode scanned for 100 cycles. It shows that hydrogen evolution is not becoming more obvious after 100 cycles. However, the current of hydrogen evolution becomes smaller with the increase of scan times. Meanwhile, there is no Cu2+ in the electrolyte. It is reasonable to conclude that during the process of lead electrodeposition, if the surface density is limited to 2200 ± 150 g m−2 [the surface density of the copper foam is 770 ± 55 g m−2 ], the lead foam can be used as the negative electrode material for lead acid batteries when the batteries are not over-discharged. 3.6. Effect of the lead foam on parameters of electrode The specific surface area is an important parameter for the lead foam material. The bigger the specific surface area of lead foam and the better the connection between it and the active material, the higher the utilization efficiency of the active material. Usually a smaller value of α indicates a bigger amount of the active material and a higher capacity of the electrode, resulting in a greater γ value that can cause a decrease in the active material utilization efficiency. In contrast to it, a smaller γ value can lead to a higher energy output and active material utilization, which, however, will increase the corrosion rate of the positive grid [14]. The lead foam and cast grids were used, respectively, as the negative current collector. The parameters of the different electrodes were tested. The results are shown in Table 3. It indicates that compared with cast grids, the lead foam can cause a decrease in factor α and factor γ of the electrode, which will result in the improvement in the capacity of a lead-foam negative electrode and the utilization efficiency of the active material [14].
3.7. Discharge performance of batteries with different negative electrodes The discharge performances of batteries with different negative electrodes are shown in Table 4. At 5 and 2 h discharge rates and a high current discharge rate, there are improvements of 28.5%, 29.5% and 20.4% in the negative active material utilization efficiencies with the lead-foam VRLAB compared with the cast grid one and the mass specific capacities of the lead foam negative electrode are 36%, 44% and 33% higher than those of the cast grid one. As a result, the batteries are suitable for electric vehicles. 4. Conclusions (1) The lead foam production technology by electrodepositing in a fluoboric acid system was studied by using copper foam as the substrate and adding cerium under ultrasonic treatment. (2) The results of the performance test show that the lead foam has a uniform three-dimensional reticulate structure; the specific surface area is about 5700 m2 m−3 ; the porosity is about 88.1%; the apparent resistivity is about 150 cm−1 . (3) As a negative electrode material of a lead acid battery, lead foam has good stability. (4) At 5 and 2 h discharge rates and a high current discharge rate, there are improvements of 28.5%, 29.5% and 20.4% in the negative active material utilization efficiencies of the lead-foam VRLAB compared with the cast grid one and the mass specific capacities of the lead foam negative electrode are 36%, 44% and 33% higher than those of the cast grid one. References [1] [2] [3] [4]
P.T. Moseley, J. Power Sources 73 (1998) 122. P.T. Moseley, J. Power Sources 67 (1997) 115. Battlebury, R. David, J. Power Sources 80 (1999) 7. L.T. Lam, R.H. Newnham, H. Ozgun, F.A. Fleming, J. Power Sources 88 (2000) 92. [5] T.C. Dayton, D.B. Edwards, J. Power Sources 85 (2000) 137.
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[6] A. Cooper, P.T. Moseley, J. Power Sources 113 (2003) 200. [7] E. Gyenge, J. Jung, B. Mahato, J. Power Sources 113 (2003) 388. [8] E. Gyenge, J. Jung, S. Splinter, A. Snaper, J. Appl. Electrochem. 32 (2002) 287. [9] J.L. Arias, D.W. Boughn, J.J. Rowlette, Increasing the specific energy of deep-cycle VRLA batteries by improving material-utilization efficiencies [A], in: Proceedings of the Annual Battery Conference on Applications and Advances [C], 2001, pp. 213–217.
[10] E. Gyenge, S. Splinter, J. Jung, et al., High specific surface area. Threedimensional reticulated electrodes for deep-cycle lead-acid batteries [A], in: Proceedings of the 17th Annual of Battery Conference on Application and Advanced [C], 2002, pp. 287–295. [11] E.B. Flint, K.S. Suslick, Science 253 (1991) 1397. [12] K.S. Suslick, Annu. Rev. Mater. Sci. 29 (1999) 295. [13] S. Langlois, F. Coeuret, J. Appl. Electrochem. 19 (1989) 43. [14] D. Pavlov, J. Power Sources 53 (1995) 9.
Preparation and performance of lead foam grid for negative electrode of VRLA battery C.S. Dai ∗ , B. Zhang, D.L. Wang, T.F. Yi, X.G. Hu Department of Applied Chemistry, Harbin Institute of Technology, P.O. Box 411, Harbin 150001, PR China Received 14 June 2005; received in revised form 20 October 2005; accepted 11 November 2005
Abstract Lead (alloy) foam was prepared via electrodeposition by using copper foam as the substrate and adding element cerium to the electrodepositing solution under ultrasonic treatment. The performances and morphology of the lead foam were investigated by means of SEM and AFM. The results show that because of the addition of cerium and the application of ultrasound, the distribution thickness ratio (DTR) of the lead foam is decreased. The lead foam has a uniform three-dimensional reticulate structure with a specific surface area of about 5700 m2 m−3 , a porosity of about 88.1%, and an apparent resistivity of about 150 cm−1 . The results of the cyclic voltammetry indicate that the lead foam has a good stability when it is used as the negative electrode material of a lead acid battery. The battery testing revealed that, at 5, 2 h and high current discharge rates, there are improvements of 28.5%, 29.5% and 20.4% in the negative active material utilization efficiencies with the lead-foam VRLAB compared with the cast grid one and the mass specific capacities of the lead foam negative electrode are 36%, 44% and 33% higher than those of the cast grid one. © 2005 Elsevier B.V. All rights reserved. Keywords: Lead foam; Electrodeposition lead alloy; Negative electrode material
1. Introduction At present, exhaust emissions from fuel-driven automobile vehicles are a major cause of urban pollution and a source of the green house-gas CO2 , which accounts for 20% of the total. This problem has aroused increasing concern all over the world. Additionally, global oil storage capacity has declined gradually. Therefore, both low-emission vehicles (i.e., hybrid electric vehicles, HEVs) and zero-emission vehicles (i.e., electric vehicles, EVs) have thrived world wide and a new industry related to these two types of vehicles has appeared with a tremendous potential [1,2]. Although as a power supply, batteries cannot be considered as a match for oil, the advanced electric power sources are being developed, which are expected to have a high specific energy, a large specific power, a long cycle life, a wide operation temperature range, low cost, better safety and no pollution [3–5]. Due to the immaturity of the commercialization of fuel cells and the safety problems of Li-ion batteries, advanced lead-acid
∗
Corresponding author. Tel.: +86 451 86413751; fax: +86 451 86221048. E-mail address: [email protected] (C.S. Dai).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.11.014
batteries are still the most practical batteries for EVs and HEVs [5]. Usually, it is believed that the positive plate determines the performances of a lead-acid battery. However, because power batteries always work under a partial state of charge (PSoC), the negative plate has a great influence on a battery’s performances, which is reflected by a lower charging ability of a battery caused by the serious sulfation on the negative plat [6]. Recently, lead foam has been paid more attention to owing to its light weight large specific area and high porosity [7–10]. Cast reticulated lead foam, whose specific surface area is 14 cm2 cm−3 , has been reported to be applied as the current collector in an open cell. The utilization efficiency of the positive active material is as high as 50%, which is much higher than that of the traditional cast grids [7]. When the reticulated vitreous carbon is used as the substrate, lead foam prepared via electrodeposition can significantly increase the specific energy of a lead acid battery and the utilization efficiency of the active material [10]. In order to improve the charge/discharge performance of lead acid batteries, lead foam negative grids were produced by electrodepositing Pb alloy on a copper foam substrate which has good conductibility and a uniform three-dimensional reticulate structure.
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2. Experimental Lead foam negative grids were prepared on the copper foam substrate (30 PPI, apparent surface density is 770 ± 55 g m−2 ). The surface morphology was observed by using a Hitachi Japan S-4700 scanning electronic microscope. The lead foam was solidified by using epoxide resin, and polished before the cross-section observation. An atom force microscope (AFM, PicoPlus MOLECULAR IMAGING, USA) was used to study the detailed morphology of the sample. An inductively coupled plasma spectrometer (TJA USA IRIS Advantage) was applied to measure the content of Ce in the electrodeposited layer. The real surface area was tested by chronoamperometry and the porosity was measured by the weighing method. Cyclic voltammetry (CV) was performed by using a CHI model 630 electrochemical working station. The tested system was a three-electrode system. A pure lead plate with a large area was used as the counter electrode and Hg/Hg2 SO4 with a standard potential of 0.6210 V (versus SHE, 25 ◦ C) (the concentration of sulfuric acid was 4.8 mol L−1 ) served as the reference electrode. The electrolyte was 4.8 mol L−1 H2 SO4 and the potential applied to the working electrode was −0.6 to −1.2 V. The experimental temperature was 25 ± 1 ◦ C. The electrolyte was prepared by analytical reagents and distilled water. Cast grids and lead foam grids were used, respectively, as the current collector for a negative electrode, to assemble the valve regulated batteries with two positive electrodes and one negative electrode, whose capacities were limited by the negative electrodes. The charge and discharge tests were carried out by using a BTS 5 V/10 A battery testing system (made by Neware Technology Company, Shenzhen, China). The charge/discharge performances were evaluated according to the Mechanism Industry Standard of the People’s Republic of China (JB/T 10262-2001) for sealed lead acid batteries used for electric moped.
3. Results and discussion 3.1. Deciding of the solution for electrodeposition lead foam There have been some reports on the Pb electrodeposition in a pyrophosphate system and a sulfate system, but a fluoride boracic acid system is used more broadly in the commercial process. Therefore, a fluoride boracic acid system, as shown in Table 1, was chosen to study the electrodeposition of Pb on a copper foam substrate. In order to improve the uniformity of the lead foam, ultrasound was imposed during the electrodeposition of lead, which decreased the DTR of the lead foam. The copper foam substrate was 60 mm × 80 mm, to which a certain current density was applied. The electrodeposited efficiency was measured by the time when the copper foam substrate was completely covered by lead. The result shows that ultrasonic wave treating can significantly increase the electrodeposition efficiency (Fig. 1). It was found from the SEM image of the cross-section of lead foam obtained that the DTR of the lead foam is decreased from ca. 1.6
to 1.4. The mechanism of ultrasonic-catalysis is not related with the direct interaction between sound waves and molecules, but the liquid vacuum process. When air bubbles collapse, the local temperature will be above 5000 K and the local pressure will be up to 5 × 107 Pa. Meanwhile, the change rate of temperature is about 109 K s−1 and there are intense bow wave and 400 km h−1 current [11,12]. Under such a condition, some chemical reactions can occur, which is impossible under normal conditions. Ultrasonic wave makes the water molecules around Pb2+ in the surface liquid layer of the electrode to be rearranged, which results in the void electron energy level of Pb2+ rising to the Fermi energy level of the electrode and forming more Pb adsorbing atoms. Ultrasonic wave treating can increases the uniformity of the lead foam obtained via electrodeposition on copper foam. Due to the strong stirring of ultrasonic wave, the diffusion and the transfer of Pb2+ become easier, and the lead foam will be more uniform along the thickness direction. In the study of grid alloy used for lead acid batteries, it is found that the addition of rare earth metal Ce can improve the conductivity of the corrosion film of a positive electrode. To improve the performance lead foam, in this study Ce was added to the Pb electrodepositing solution, thereby, an Ce and Pb alloy was obtained. Because SO4 2− can interfere the lead foam electrodeposition, Ce was added without SO4 2− . The preparation of the Ce solution was as follows: 15.7 g of ceric sulfate was weighed and then dissolve in 5 mL of concentrated sulfuric acid (called solution A). Lead oxide (40 g) that was stoichiometrically excessive compared with the amount of SO4 2− in solution A was weighed and put into 100 g of fluoboric acid then the mixture was stirred till it was dissolved (called solution B). Solution A was poured into solution B and the sediment was filtered. A clear solution of cerium flouroborate and lead flouroborate, which contained no SO4 2− , was obtained. Then it fixed to 200 mL. The above solution (10–20 mL) was taken and added into the electrodepositing solution shown in Table 1 to obtain the cerium and lead electrodepositing solution. The result of the ICP experiment shows that the Ce content in the lead foam alloy produced by such a technique is in a range of 0.002–0.01 wt.%. By adding, the DTR of the lead foam can
Table 1 The composition of electrodeposition Pb solution and process parameters Pb2+ [added through Pb(BF4 )2 ] (g L−1 ) Dissociative fluoride boracic acid (HBF4 ) (g L−1 ) Boracic acid (H3 BO3 ) (g L−1 ) Peptone (g L−1 ) Anode Current density, j (A dm−2 ) Temperature, T (◦ C)
200 150 30 0.5 Pure lead 3 15–30
Fig. 1. Effect of ultrasonic wave on closed rate of lead foam electrodeposition.
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Fig. 2. AFM images of electrodeposited lead: (a) electrodeposited lead, (b) electrodeposited lead applying ultrasonic wave, (c) electrodeposited lead and cerium applying ultrasonic wave.
be decreased from 1.4 to 1.2 and the uniformity of the lead foam also can be improved. 3.2. AFM observation of electrodeposited lead foam Three samples were fabricated, respectively, by electrodepositing lead on copper plate, electrodepositing lead on copper plate under ultrasonic treatment, and electrodepositing leadcerium alloy under ultrasonic treatment. The three samples were polished by using the same emery paper and then were observed by using an AFM. The results are shown in Fig. 2. Fig. 3 shows the surface roughness along the diagonal direction of the three samples. It shows that ultrasonic treatment and the addition of cerium increase the surface roughness of the as-electrodeposited lead. The preparing process of the lead foam should include the following procedures: the electrodeposition of Pb–Ce alloy on copper foam [surface density was 770 ± 55 g m−2 ] under ultrasound treatment and the preparation of lead foam grids. The area density of the prepared lead foam grids was controlled at around 2200 ± 150 g m−2 .
Fig. 3. Surface roughness of diagonal direction of the samples in Fig. 2.
3.3. Three-dimensional reticular structure and surface morphology of the lead foam Similar to the copper foam, the lead foam also has a perfect three-dimensional reticular structure. All holes in lead foam are connected to each other. It has one more channel than the twodimensional cast grids. Therefore, it can increase the uniformity of an electrode. Fig. 4 shows the SEM image of the lead foam, in which the three-dimensional reticular structure of the lead foam is shown clearly.
Fig. 4. SEM image of lead foam made by electroplating (30 PPI).
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Table 2 Specific surface area of lead foam Foam (PPI) Specific surface area (m2 m−3 )
30 5700
3.4. Porosity, specific surface area and resistivity of lead foam High porosity is the unique characteristic of lead foam grids. Compared with other grids having the same volume, the containing capacity of the lead foam to active materials is the highest. The effective porosity of the lead foam is decided by the copper foam substrate, the porosity, the surface density and the thickness of the lead foam. The porosity of the lead foam was measured by the weighing method. And the result indicates that its porosity is about 88.1%. The specific surface area of lead foam is also an important parameter. It is beneficial to the connection of a substrate and an active material and the utilization efficiency of the active material can be increased when the specific surface area of lead foam is high. The specific surface area values of lead foam were measured by chronoamperometry and the results are shown in Table 2. The results show that the specific surface area of the lead foam is very high and is much higher than that of the lead foam (1800 m2 m−3 ) based on an RVC substrate. As a collector, the conductivity is a very important property while the conductivity of a metal foam greatly depends on its porosity. Langlois and Coeuret [13] found that the apparent resistivity of nickel foam is the function of the resistivity of nickel metal ρfoam =
4 ρNi 1−θ
(1) Fig. 6. Cyclic voltammograms of lead foam negative electrode cycled 100 times.
where ρfoam is the resistivity of nickel foam; ρNi the resistivity of nickel; θ the porosity of nickel foam. The effect of the production process on the resistivity of nickel foam was also studied. The function which is applicable to the calculation of the resistivity of nickel foam can also be used to calculate the resistivity of copper foam and lead foam. It is easily to be derived that ρfoam(Cu) = 163 ( cm−1 ), ρfoam(Pb) = 2070 ( cm−1 ). The electric resistance of the lead foam electrodeposited on copper foam can be regarded as the result of parallel connection of lead foam and copper foam that have the same length and area 1 1 1 = + ρfoam(Cu–Pb) ρfoam(Cu) ρfoam(Pb)
Fig. 5. Cyclic voltammograms of lead foam cycled 20 times.
(2)
According to (2), it can be obtained that ρfoam(Cu–Pb) = 151.1 ( cm−1 ). 3.5. Stability of lead foam in H2 SO4 The hydrogen evolution overpotential of lead is higher than that of copper. If copper in negative grids dissolves in the electrolyte while the battery is being discharged, the performance of the battery will be affected significantly. In order to investigate the stability of the lead foam in H2 SO4 , cyclic voltammograms
of the lead foam grids with different thicknesses of the electrodeposited Pb layer as well as those of the lead foam grids pasted with active materials were recorded. 3.5.1. Cyclic voltammetry of lead foam Fig. 5 shows the cyclic voltammogram of the lead foam in an electrolyte of 4.8 mol L−1 H2 SO4 . It can be
Table 3 Parameters of different electrodes Parameters
Lead foam electrode
Cast grid electrode
Grid mass before painting (g) Electrode mass after painting (g) Geometric area of grid (cm2 ) Real surface area of grid (cm2 ) α γ (g cm−2 )
9.0 41.0 31.5 546.84 0.26 0.05
13.6 45.6 31.5 31.14 0.34 0.84
α = Mcollector /(MNAM + Mcollector ), γ = MNAM /Scollector ; Mgrid is the grid mass; MNAM the mass of negative active material; Scollector the real surface area of grid.
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Table 4 Discharge performance of batteries with different negative electrodes Discharge rate
Discharge capacity (mAh)
Utilization efficiency (%)
Specific capacity of negative electrodes (mAh g−1 )
5h Foam Cast
2957.8 2383.6
46.48 36.17
71.15 52.27
2h Foam Cast
2684.2 2072.9
42.18 32.58
65.47 45.46
High current (3.0I2 ) Foam Cast
1818.4 1510.3
28.57 23.73
44.35 33.12
seen that the corresponding cyclic voltammogram has only one oxidation–reduction peak corresponding to Pb/PbSO4 but no peak belonging to Cu/Cu2+ after it was scanned many times. The conclusion can be drawn that there is no copper dissolved and the degree of hydrogen evolution does not increase. 3.5.2. Cyclic voltammetry of lead foam negative electrode Fig. 6 shows the cyclic voltammogram for the lead foam negative electrode scanned for 100 cycles. It shows that hydrogen evolution is not becoming more obvious after 100 cycles. However, the current of hydrogen evolution becomes smaller with the increase of scan times. Meanwhile, there is no Cu2+ in the electrolyte. It is reasonable to conclude that during the process of lead electrodeposition, if the surface density is limited to 2200 ± 150 g m−2 [the surface density of the copper foam is 770 ± 55 g m−2 ], the lead foam can be used as the negative electrode material for lead acid batteries when the batteries are not over-discharged. 3.6. Effect of the lead foam on parameters of electrode The specific surface area is an important parameter for the lead foam material. The bigger the specific surface area of lead foam and the better the connection between it and the active material, the higher the utilization efficiency of the active material. Usually a smaller value of α indicates a bigger amount of the active material and a higher capacity of the electrode, resulting in a greater γ value that can cause a decrease in the active material utilization efficiency. In contrast to it, a smaller γ value can lead to a higher energy output and active material utilization, which, however, will increase the corrosion rate of the positive grid [14]. The lead foam and cast grids were used, respectively, as the negative current collector. The parameters of the different electrodes were tested. The results are shown in Table 3. It indicates that compared with cast grids, the lead foam can cause a decrease in factor α and factor γ of the electrode, which will result in the improvement in the capacity of a lead-foam negative electrode and the utilization efficiency of the active material [14].
3.7. Discharge performance of batteries with different negative electrodes The discharge performances of batteries with different negative electrodes are shown in Table 4. At 5 and 2 h discharge rates and a high current discharge rate, there are improvements of 28.5%, 29.5% and 20.4% in the negative active material utilization efficiencies with the lead-foam VRLAB compared with the cast grid one and the mass specific capacities of the lead foam negative electrode are 36%, 44% and 33% higher than those of the cast grid one. As a result, the batteries are suitable for electric vehicles. 4. Conclusions (1) The lead foam production technology by electrodepositing in a fluoboric acid system was studied by using copper foam as the substrate and adding cerium under ultrasonic treatment. (2) The results of the performance test show that the lead foam has a uniform three-dimensional reticulate structure; the specific surface area is about 5700 m2 m−3 ; the porosity is about 88.1%; the apparent resistivity is about 150 cm−1 . (3) As a negative electrode material of a lead acid battery, lead foam has good stability. (4) At 5 and 2 h discharge rates and a high current discharge rate, there are improvements of 28.5%, 29.5% and 20.4% in the negative active material utilization efficiencies of the lead-foam VRLAB compared with the cast grid one and the mass specific capacities of the lead foam negative electrode are 36%, 44% and 33% higher than those of the cast grid one. References [1] [2] [3] [4]
P.T. Moseley, J. Power Sources 73 (1998) 122. P.T. Moseley, J. Power Sources 67 (1997) 115. Battlebury, R. David, J. Power Sources 80 (1999) 7. L.T. Lam, R.H. Newnham, H. Ozgun, F.A. Fleming, J. Power Sources 88 (2000) 92. [5] T.C. Dayton, D.B. Edwards, J. Power Sources 85 (2000) 137.
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