Hybrid energy storage devices combining carbon-nanotube/polyaniline supercapacitor with lead-acid battery assembled through a “directly-inserted” method

Abstract

In this work, we used a new method to assemble hybrid energy storage devices, combining electrochemical capacitor with lead-acid battery. The ultrathin, flexible and low-weight carbon-nanotube/polyaniline (CNT/PANI) composite films used as supercapacitor electrodes were “directly-inserted” into a lead-acid battery in series or parallel. This quite simple and effective method is promising for large-scale industrialization as it reduces production and process complexity. The assembled hybrid devices showed notably improved properties, including a 19% increase in specific capacity, a 21% increase in specific energy or a 6% increase in specific power when compared to the conventional independent lead-acid batteries, which can be attributed to the synergic effect of batteries and supercapacitors.

---

Introduction

With the approaching fossil energy source crisis and exacerbating environmental pollution, the use of clean energy, such as electrical energy, is urgently required. Electrical energy storage devices with high stability and excellent efficiency are required to collect the unstable electrical energy generated from wind or solar power systems, and to supply power for portable electronic equipment or electric vehicles. As the most common electrical energy storage devices, rechargeable batteries can supply high specific capacity and specific energy but their low specific power and short cycle-life limits their applications in instances such as high current charge–discharge.¹ Compared with rechargeable batteries, supercapacitors provide higher specific power and more outstanding cycle stability but their specific energy is much lower.^2–7 Therefore, the hybrids of rechargeable batteries and supercapacitors have been investigated to achieve electrical energy storage devices, which have both high specific energy and specific power.⁸

Some routes to hybrid rechargeable battery with supercapacitor have been attempted with conventional batteries, such as nickel metal hydride battery,⁹ lithium ion battery,^10–13 lithium sulfur battery,¹⁴ biofuel cell,¹⁵ etc., which show remarkable improvements in battery properties. Compared with the mentioned batteries above, lead-acid battery, a classic battery, with the merits of safety, reliability and mature manufacture technology, usually serves as an uninterruptible power supply (UPS) and power supply of various electrical equipment.¹⁶ Moreover, Pb is much cheaper, more abundant and more easily recycled source than most metals used in other batteries, which enables potentially wider applications of lead-acid battery such as supporting electrical energy for hybrid-electric vehicles (HEVs). However, the comparable low specific energy and power of lead-acid battery against other new-generation batteries has limited its uses in applications, which require higher power input. Therefore, further studies on the hybrids of supercapacitor and lead-acid battery in one cell have been done to improve the electrochemical performances of classic lead-acid batteries. The hybrid methods can be classified according to the following two types.

First, there have been studies on the combination of supercapacitor and lead-acid battery in new electrode configurations. As Lam et al.¹⁷ reported, PbO₂ was used as the common positive plate, and carbon-based supercapacitor electrode was connected internally with the Pb sponge in parallel to act as common negative plates. A hybrid energy-storage device, combining an asymmetric supercapacitor and lead-acid battery in one unit cell was achieved, which showed a higher discharging–charging power and longer cycle-life than the conventional lead-acid batteries. Yu et al.¹⁸ designed a hybrid supercapacitor with PbO₂ as the positive electrode and activated carbon as the negative electrode, which exhibited a larger specific capacitance, higher power and more stable cycle performance than the lead-acid battery.

Second, hybrids of supercapacitor and lead-acid battery have also been achieved by adding capacitive carbon materials to battery plates. When HEVs work under high rates and partial state-of-charge (HRPSoC) mode, PbSO₄ will be generated on the surface of Pb negative plates, which is also called sulfation, leading to the reduction of electrode effective surface-area, battery charging–discharging efficiency and cycle-life. Carbon materials such as activated carbons, carbon blacks and graphite have been added to the negative active material to form a capacitive carbon system combined with an electrochemical Pb system, which enhances the electroconductivity of the negative plates, and improves the charge–discharge characteristics of the batteries.^19–23 Shapira et al.²⁴ improved the cycle life of lead-acid batteries by adding properly oxidized carbon nanotubes (CNTs) to the positive active material of lead-acid batteries. Consisting of CNTs coated uniformly with Pb salts, a stable conductive grid was formed to enable the delivery of current to all active materials, which avoid the sulfation of lead-acid battery plates.

From abovementioned works, the electrode configurations of devices and the choices in adding capacitive carbon materials are identified as the two key points in the assembly of hybrid energy storage devices of combined supercapacitors with lead-acid batteries.

Flexible film supercapacitors based on CNT, graphene or other carbon nanoporous materials have recently attracted research interest because of their potential applications in wearable electronics.^25–30 The superaligned CNTs fabricated in our laboratory have the merits of high aspect ratio, large special surface-area and excellent electric conductivity,^31,32 which have been used as flexible film supercapacitor electrodes with high specific capacitance.^33–35 Polyaniline (PANI), a conventional conducting polymer, shows a larger pseudo capacitance than the electrical double-layer capacitance of CNTs but has a worse cycle-life and mechanical properties. By coating PANI on freestanding super-aligned CNT networks, CNT/PANI composite films have been fabricated in our laboratory as supercapacitor electrodes, which are ultrathin, extremely light-weight, highly flexible and with enhanced electrochemical properties.^36,37 This composite film is a good choice for supercapacitor electrodes in hybridizing with the lead-acid batteries.

In this work, we tried to assemble hybrid energy storage devices combining CNT/PANI composite film supercapacitor with lead-acid battery using a much easier method, in which the supercapacitor electrodes were “directly-inserted” into the lead-acid battery, which have never been reported previously. These new hybrid devices showed a higher specific capacity, specific energy and specific power than conventional independent lead-acid battery.

Experimental

Fabrication of CNT/PANI composite film electrodes

Superaligned CNTs used in this work were fabricated by chemical vapor deposition (CVD) on silicon wafers with iron as the catalyst and acetylene as the precursor. A uniform suspension of CNTs was obtained by ultrasonication (800 W, 10 min) of superaligned CNTs in ethanol. After filtration of CNTs suspension through a microporous membrane under vacuum, CNT networks were formed and dried at 80 °C for 12 h in a vacuum oven to be peeled off from the microporous membrane. The as-prepared CNT networks were immersed in 40 mL aqueous solution, containing 0.04 mol HCL and 0.002 mol aniline monomers (purity ≥99.5%) for 10 min of complete infiltration. As an oxidant for polymerization, 40 mL precooled aqueous solution containing 0.002 mol ammonium persulfate, was dropped slowly into the abovementioned solution. The mixed solution was cooled to 0 °C for 24 h for complete reaction, resulting in an uniform PANI coating on the CNT networks. The obtained CNT/PANI composite film was picked out from the reacted solution, cleaned with deionized water, acetone and ethanol and dried at 80 °C in a vacuum oven for 12 h.

Fabrication of Pb negative plates and PbO₂ positive plates

We disassembled a commercial valve regulated lead-acid (VRLA) battery (6V 1.3Ah, Jiangsu Jiuhua Energy Technology Co., Ltd., China), and directly used the negative and positive plates to assemble new devices.

Assembling of batteries

The CNT/PANI composite film electrodes and the disassembled battery plates were cut into rectangles of the size of 1.2 cm × 1.4 cm. Then, we sealed the structures constructed by the electrodes and plates with a plastic film, and used glass fiber cross wall (also called AGM) as the separator and 5 mol L⁻¹ H₂SO₄ aqueous solutions as the electrolyte.

Characterization

The microstructures of the CNT/PANI composite films were characterized by scanning electron microscopy (SEM) (Sirion 200, resolution 1.0 nm, FEI, USA). The electrochemical performances were studied by Galvanostatic charging–discharging experiments (Land Battery Testing System, China).

Results and discussion

First, we fabricated CNT/PANI composite film by an in situ chemical solution method reported previously in our laboratory. As Fig. 1a–d demonstrate, the composite film as thin as a piece of printing paper (about 100 μm) can be easily folded and bended. Combining flexibility and thinness, it shows great potential in matching up with the battery plates of various shapes. Fig. 1e shows the scanning electron microscopy (SEM) image of the CNT/PANI composite film. CNTs are connected with each other to form nanoporous networks with pore size of about 100 nanometers where PANI is coated uniformly on the CNTs.

Fig. 1 Photographs of CNT/PANI film in (a) flat, (b) folded and (c) bended state, respectively. (d) The CNT/PANI composite film shown with measurement by a micrometer caliper. (e) SEM image of the microstructures of CNT/PANI composite film.

Second, we disassembled a commercial VRLA to obtain the positive and negative plate. As Fig. 2a illustrates, the positive plate (thickness = 3.17 mm) and negative plate (thickness = 2.49 mm) in this battery are constructed by a current collector prepared of a thick grid of lead alloys with calcium, which are coated with positive and negative paste materials of lead oxide and lead, respectively. Compared with these battery plates, the fabricated CNT/PANI composite films are ultrathin and low-weight.

Fig. 2 (a) Photographs of the positive plate (left) and negative plate (right) disassembled from a commercial VRLA. The schematic structures of the three hybrid energy storage devices: (b) lead-acid battery + supercapacitor in series inside; (c) lead-acid battery + supercapacitor in series outside; (d) lead-acid battery + supercapacitor in parallel.

Finally, we directly inserted the CNT/PANI film electrodes into the cell of the disassembled VRLA in the following three ways:

1. Positive-plate//CNT/PANI//separator//CNT/PANI//negative-plate, also called lead-acid battery + supercapacitor in series inside;

2. CNT/PANI//positive-plate//separator//negative-plate//CNT/PANI, also called lead-acid battery + supercapacitor in series outside;

3. Positive-plate//separator//negative-plate in parallel with CNT/PANI//separator//CNT/PANI, also called lead-acid battery + supercapacitor in parallel.

Fig. 2b–d demonstrate the structures mentioned above.

For comparison, we also manufactured an independent symmetric supercapacitor with CNT/PANI as electrodes and an independent lead-acid battery in the following ways:

1. CNT/PANI//separator//CNT/PANI;

2. Positive-plate//separator//negative-plate.

All the abovementioned devices were assembled under the same conditions, including supercapacitor electrodes and lead-acid battery plates with the same mass and size, the same electrolyte, separator, current collector and packaging method.

To compare the electrochemical performances of the independent lead-acid battery and three hybrid energy storage devices at high rate charge–discharge state, galvanostatic charging-discharging experiments were performed according to the following steps:

1. Charged at the same current of 60 mA (the rate of about 1.2 C) for 1 h from the initial window potential of 1.6 V;

2. Discharged at the same current of 60 mA to the potential of 1.6 V.

As the charge–discharge curve in Fig. 3 displays all the devices have the same charge capacity of 60 mA h, while the lead-acid battery + supercapacitor in series outside displays a longer discharging time compared with the other two types of hybrids and independent lead-acid battery, which indicates that this device has the largest discharge capacity among all the tested devices.

Fig. 3 The galvanostatic charging–discharging curves of the independent lead-acid battery, three types of hybrid energy storage devices tested according to the following steps: (1) charge at the same current of 60 mA for 1 h from the initial potential of 1.6 V; (2) discharge at the same current of 60 mA to the potential of 1.6 V.

To study the high rate charge–discharge performance of the independent supercapacitor, a galvanostatic charging–discharging experiment was carried out at the current of 60 mA in the potential window of 0–0.8 V (this potential window is adaptable to the pseudo capacitance of PANI). Fig. 4 shows the charge–discharge curve of the supercapacitor. The discharging time (24 s) of the supercapacitor is much shorter than the lead-acid battery (2736 seconds), demonstrating that the supercapacitor has an enhanced ability for rapid charge and discharge.

Fig. 4 The constant current charging–discharging curves of the independent supercapacitor at a current of 60 mA in the potential window of 0–0.8 V.

We analysed the results obtained from the abovementioned experiments in order to study some characterizations of the three hybrid devices, the lead-acid batteries and supercapacitors such as the mass of all electrodes in one cell (M), specific capacity (C_s), specific energy (E_s) and specific power (P_s) in the discharge process. The specific capacity is calculated according to

C_s = I × Δt/M (1)

where I is the discharging current and Δt is the discharging time. The specific energy is calculated according to

E_s = ∫U × Idt/M (2)

where U is the discharging voltage. The specific power is calculated according to

P_s = E_s/Δt (3)

As Table 1 demonstrates, the independent supercapacitor shows a higher specific capacity (23.8 mA h g⁻¹) of 2.8 times than the lead-acid battery (8.4 mA h g⁻¹) and an extra higher specific power (1095.0 W kg⁻¹) of 52.9 times than the lead-acid battery (20.7 W kg⁻¹); however, its specific energy (7.2 W h kg⁻¹) is much lower than the lead-acid battery (15.9 W h kg⁻¹).

Table 1 Characterizations of the devices

Device	M (g)	Cs (mA h g^−1)	Es (W h kg^−1)	Ps (W kg^−1)
Supercapacitor	0.017	23.8	7.2	1095.0
Lead-acid battery	5.500	8.4	15.9	20.7
Lead-acid battery + supercapacitor in series inside	5.517	7.1	12.9	19.6
Lead-acid battery + supercapacitor in series outside	5.517	10.0	19.3	20.9
Lead-acid battery + supercapacitor in parallel	5.517	9.1	18.5	22.0

---

The lead-acid battery + supercapacitor in series outside shows a 19% improvement in specific capacity (10.0 mA h g⁻¹) over the lead-acid battery (8.4 mA h g⁻¹), a specific energy (19.3 W h kg⁻¹) with a 21% improvement over the lead-acid battery (15.9 W h kg⁻¹), and a similar specific power (20.9 W kg⁻¹) to the lead-acid battery (20.7 W kg⁻¹). The lead-acid battery + supercapacitor in parallel shows an 8% improvement in specific capacity (9.1 mA h g⁻¹) over the lead-acid battery (8.4 mA h g⁻¹), a specific energy (18.5 W h kg⁻¹) with a 16% improvement over the lead-acid battery (15.9 W h kg⁻¹), and a specific power (22.0 W kg⁻¹) with a 6% improvement over the lead-acid battery (20.7 W kg⁻¹). However, the mass of all the electrodes of the hybrid device (5.517 g) is only 0.3% higher than the lead-acid battery (5.500 g), indicating that the addition of low-weight supercapacitive electrodes can markedly enhance the electrochemical performance of the lead-acid battery. The lead-acid batteries and the hybrid energy storage devices here were manually assembled in the laboratory. Therefore, their electrochemical properties can be further optimized by industry standards in the production of batteries. However, these factors do not affect the relative comparison among the assembled devices. The lead-acid battery + supercapacitor in series inside show the worst performance among all the hybrid devices and the independent lead-acid battery.

The enhanced electrochemical performances of the hybrid energy storage devices: (1) lead-acid battery + supercapacitor in series outside; and (2) lead-acid battery + supercapacitor in parallel can be attributed to the synergistic effect of the lead-acid battery and supercapacitor. The hybrid approach improves the overall utilization of all the electrode materials. In the high current discharging process of the hybrid device, the supercapacitor shares a large percent of the whole current because of its higher power ability than the lead-acid battery such that the lead-acid battery discharged in a lower current rather than the whole current. Because of electrode polarization in high current discharge, the discharge capacity of the same battery will decrease with an increase in discharge current. Because the lead-acid battery in the hybrid device is discharged in a lower current than the independent lead-acid battery, the former can supply a higher capacity than the latter. Meanwhile, the mass of all the electrodes of hybrid device is only a little higher than the independent lead-acid battery. Therefore, the hybrid device shows a higher specific capacity than the independent lead-acid battery. The discharge specific energy can be calculated according to

E_s = U_p × C_s (4)

where U_p is the discharge voltage platform. It is shown in Fig. 3 that the hybrid device combined in series outside and in parallel has a higher discharge voltage platform than the independent lead-acid battery. Hence, the specific energy of the hybrid device is also higher than the independent lead-acid battery. Nevertheless, the specific mechanism of the enhanced electrochemical performances for these new hybrids of supercapacitor and lead-acid battery requires further investigation.

Conclusions

In summary, we used a quite simple and effective method to assemble new hybrid energy storage devices, which combined supercapacitor with lead-acid battery in one cell. The ultrathin, flexible and low-weight CNT/PANI composite film supercapacitor electrodes are “directly-inserted” between lead-acid battery plates, which have never been reported previously. Compared to the independent lead-acid battery, the hybrid device combined in series outside shows a 19% increase in specific capacity and a 21% increase in specific energy, and the hybrid device combined in parallel shows a 6% increase in specific power. The improvement of electrochemical properties of hybrid devices can be attributed to the synergic effect of lead-acid battery and supercapacitor.

Acknowledgements

This work was supported by National Basic Research Program of China (2012CB932301) and the Natural Science Foundation of China (51173098).

Notes and references

1. T. Reddy, Linden's Handbook of Batteries, McGraw-Hill Companies, Inc., New York, USA, 4th edn, 2010.
2. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845.
3. H. Sun, L. Cao and L. Lu, Energy Environ. Sci., 2012, 5, 6206.
4. C. Guan, X. Xia, N. Meng, Z. Zeng, X. Cao, C. Soci, H. Zhang and H. J. Fan, Energy Environ. Sci., 2012, 5, 9085.
5. L. Estevez, R. Dua, N. Bhandari, A. Ramanujapuram, P. Wang and E. P. Giannelis, Energy Environ. Sci., 2013, 6, 1785.
6. C. Peng, D. Hu and G. Z. Chen, Energy Environ. Sci., 2014, 7, 1018.
7. C. Meng, J. Maeng, S. W. M. John and P. P. Irazoqui, Adv. Energy Mater., 2014, 4 DOI:10.1002/aenm.201301269.
8. D. Cericola and R. Kötz, Electrochim. Acta, 2012, 72, 1.
9. P. A. Nelson and J. R. Owen, J. Electrochem. Soc., 2003, 150, A1313.
10. Y. Wang and Y. Xia, Electrochem. Commun., 2005, 7, 1138.
11. Q. Wang, Z. H. Wen and J. H. Li, Adv. Funct. Mater., 2006, 16, 2141.
12. A. Yuan and Q. Zhang, Electrochem. Commun., 2006, 8, 1173.
13. F. Zhang, T. Zhang, X. Yang, L. Zhang, K. Leng, Y. Huang and Y. Chen, Energy Environ. Sci., 2013, 6, 1623.
14. G. Li, G. Li, S. Ye and X. Gao, Adv. Energy Mater., 2012, 2, 1238.
15. C. Agnès, M. Holzinger, A. Le Goff, B. Reuillard, K. Elouarzaki, S. Tingry and S. Cosnier, Energy Environ. Sci., 2014, 7, 1884.
16. B. Culpin and D. A. J. Rand, J. Power Sources, 1991, 36, 415.
17. L. T. Lam and R. Louey, J. Power Sources, 2006, 158, 1140.
18. N. Yu and L. Gao, Electrochem. Commun., 2009, 11, 220.
19. P. T. Moseley, J. Power Sources, 2009, 191, 134.
20. M. Fernández, J. Valenciano, F. Trinidad and N. Muñoz, J. Power Sources, 2010, 195, 4458.
21. D. Pavlov and P. Nikolov, J. Power Sources, 2013, 242, 380.
22. N. Fan, X. Li, H. Li, C. Sun, D. Kong and Y. Qian, J. Power Sources, 2013, 223, 114.
23. M. Saravanan, M. Ganesan and S. Ambalavanan, J. Power Sources, 2014, 251, 20.
24. R. Shapira, G. D. Nessim, T. Zimrin and D. Aurbach, Energy Environ. Sci., 2013, 6, 587.
25. Z. Weng, Y. Su, D. Wang, F. Li, J. Du and H. Cheng, Adv. Energy Mater., 2011, 1, 917.
26. G. Xiong, C. Meng, R. G. Reifenberger, P. P. Irazoqui and T. S. Fisher, Adv. Energy Mater., 2014, 4 DOI:10.1002/aenm.201300515.
27. X. Yang, C. Cheng, Y. Wang, L. Qiu and D. Li, Science, 2013, 341, 534.
28. C. Meng, O. Gall and P. Irazoqui, Biomed. Microdevices, 2013, 1.
29. Z. Niu, H. Dong, B. Zhu, J. Li, H. H. Hng, W. Zhou, X. Chen and S. Xie, Adv. Mater., 2013, 25, 1058.
30. L. Chen, Z. Huang, H. Liang, W. Yao, Z. Yu and S. Yu, Energy Environ. Sci., 2013, 6, 3331.
31. K. Jiang, Q. Li and S. Fan, Nature, 2002, 419, 801.
32. K. Jiang, J. Wang, Q. Li, L. Liu, C. Liu and S. Fan, Adv. Mater., 2011, 23, 1154.
33. H. Zhang, C. Feng, Y. Zhai, K. Jiang, Q. Li and S. Fan, Adv. Mater., 2009, 21, 2299.
34. R. Zhou, C. Meng, F. Zhu, Q. Li, C. Liu, S. Fan and K. Jiang, Nanotechnology, 2010, 21, 345701.
35. Y. Yin, C. Liu and S. Fan, J. Phys. Chem. C, 2012, 116, 26185.
36. C. Meng, C. Liu and S. Fan, Electrochem. Commun., 2009, 11, 186.
37. C. Meng, C. Liu, L. Chen, C. Hu and S. Fan, Nano Lett., 2010, 10, 4025.

---