A super capacitor is an important component in modern automotives, especially for hybrid vehicles. Nowadays, there are three categories of materials used to make the electrode, carbon-based, conductive polymer and transition metal oxide. This paper is focused on one typical kind of transition metal oxide electrodes: manganese oxide/carbon nanotubes composite electrode, including its manufacturing process. Finally the advantages and disadvantages compared with other electrodes are evaluated.
For modern automotives, a mass of electrical components are connected together forming a complicated electrical system. As the main energy source for these electrical items, batteries in vehicles play an extremely important role in this system. Except for lighting the vehicle, the battery provides electrical energy in the starting procedure. At the starting instant of electric motor, its rotating speed is zero, as the result of which the starting current is quite high. For instance, the starting current of the battery (12V, 45Ah) which is used in a diesel engine, can reach up to 550A . Such a high current must damage to the battery in a certain degree although it has been well designed. To protect battery, parallel connecting a super capacitor to a battery is a feasible solution. Additionally, in this way, the size of internal combustion engine and the power of the battery can both decrease. Except for this case, super capacitors can be applied in other automotive industry fields. For electric or hybrid automotives, super capacitors can provide peak power for acceleration. Moreover, super capacitors can capture most energy from brake system, through which the energy consumption can be minimized . Therefore, super capacitors have attracted wide attention for their higher specific power than common batteries and higher energy than conventional dielectric capacitors because of their electrode materials .
To guarantee high performance, such as the long cycling life and large capacitance, the super capacitors` electrode materials must have predominant properties. Nowadays, there are about three main categories of materials used in super capacitors as electrodes: carbon-based materials, conductive polymers and transition metal oxides .
Amongst these three options, transition metal oxides seem to be the most appealing because of their high specific capacitance coupled with very low resistance resulting in a high specific power . In manufacture process, the products should be fabricated in nano-size to improve their specific capacitance. In addition, carbon nanotubes are also to be a popular component to form corresponding composites which show high performance in this application field. Among the transition metal oxides, manganese oxide is one of the most promising materials due to its abundance and high performance.
Transition Metal Oxide in Super Capacitor Electrodes.
The computing formula of the capacitance is (1)
Where: er is the relative static permittivity, e0 is the permittivity of free space, A is the area of overlap of the two electrode plates, and d is the distance of the two plates. According to equation (1), it is evident that capacitance is directly proportional to the area A, and it is inversely proportional to the interval distance d. To obtain a great capacitance, the area of overlap is required as large as possible. On the contrary, the distance between two plates should be designed as short as we can. For a particular capacitor, the areas of electrodes are limited. So the best way to improve its capacitance is that increasing the specific capacitance. This is also the development direction of super capacitor electrodes.
Seen from this point of view, transition metal oxide seems to be very appealing. One of representatives is RuO2. Satisfied the essential requirements of super capacitor electrode, RuO2 electrodes have high specific surface area, long cycle life and high conductivity. However, since noble metal, Ru, its cost becomes the biggest obstacle for commercial production. To make transition metal oxide electrodes commercialization, some substitute materials are suggested, such as manganese oxide, nickel oxide, and Fe3O4 , in which manganese oxide is the most representative one.
Manganese oxide dominates the transition metal oxide using in this field due to its low cost, high reserves, high theoretical specific capacitance and environmental compatibility. To improve the specific capacitance, CNTs usually are embedded into manganese oxide to synthesize composites. Since discovered in 1991 by lijima, carbon nanotubes (CNTs) have attracted a lot of interests in the materials researching field because of their superior properties . In addition to the outstanding mechanical properties, the high specific surface area and excellent electrical conductivity make CNTs have tremendous potential in the super capacitor application.
In term of the type of the composite materials, it is difficult to identify which component in this system is the matrix or the reinforcement, since it is quite different from the conventional composite types (ceramic matrix composites, metal matrix composites and polymer matrix composites). The matrix here should be carbon nanotubes reasonably. For the reinforcement, it is metallic oxides evidently. As nanocomposites, the reinforcement improves the electric performance.
Manufacturing Process of Manganese Oxide/ CNTs Composite Super Capacitor Electrodes
Manufacturing process of manganese oxide
To manufacture manganese oxide, there are several alternatives. The first choice is called organic-aqueous interfacial method. This method has been confirmed very effective to produce porous MnO2. Just as the name implies, MnO2 were prepared at the interface between organic phase and aqueous phase. In the aqueous phase, there are two main components, potassium permanganate (KMnO4) and sodium dodecylsulfate (SDS). According to Xin-hui Yang `s report, the two phases system was kept for several minutes, then some products appeared at the interface. With the products increasing, the colour of aqueous phase turned lighter, since more KMnO4 transformed to MnO2. After the expected products had precipitated, the organic phase was removed by the pipette. Then the aqueous with MnO2 were separated with a centrifuge. Finally, MnO2 was dried at an ambient temperature approximate 70°C in vacuum for approximate 10h . The schema of how porous MnO2 is produced is illustrated in Figure 1 . As their experimental results, MnO2 is aggregated as spherical particles, the sizes of which are approximate 30 to 300 nm. Then, the highest specific capacitance could reach up to 260 F/g.
Figure 1.Schema of the formation of MnO2 at interface of aqueous and organic phases.
Manganese oxide films can also be manufactured by sol-gel process. Firstly, to obtain the manganese oxide precursor sols, manganese acetate (Mn(CH3COO3)·2H2O) was added into a citric acid containing n-propyl alcohol at room temperature, in which system the ratio of manganese acetate and citric acid was 1 to 2. The next step is to stir for about 12h. After that, ammonium hydroxide (NH3·H2O) was added in to adjust pH value to about 9. After that a graphite substrates, which was degreased, etched in 0.2 M H2SO4, and flushed by deionized water, was immerged into sol. Finally, sample was conducted drying process and heat treatment . As experimental results demonstrated, the specific capacitance of around 190 F/g was obtained, which increased with the heat treatment temperature within limits.
Manufacturing process of composite electrodes
There are several possible manufacture routes for fabricating the composites. One of the feasible methods is electrophoretic deposition (EPD). To produce manganese oxide/carbon nanotubes composite electrodes, manganese oxide nanofibres should be raw material, which were prepared by the aqueous-ethanol interfacial method. Electrophoretic deposition was conducted from aqueous suspension, in which there are 0-5 g/L manganese dioxide, 0.1-0.5 g/L sodium alginate and 0-0.5 g/L multi-walled carbon nanotubes. In this system, sodium alginate played a role as dispersant. In the process, the voltage between stainless steel foils and graphite substrates were controlled 10-50 V, and the distance between them is 15 mm . According to reports said, the films fabricated by this method exhibited good capacitive behaviour because of their fibrous porous microstructure and the micron-sized thickness (1 to 20 µm).
Another possible manufacture route utilized to synthesize manganese oxide/ carbon nanotubes composites is microwave-assisted method. For this method, manganese oxide does not need to prepare before synthesis. Instead, KMnO4 is the raw material, the amount of which has been predetermined (see Table 1) . As the first step, CNTs were refluxed in 10 wt% nitric acid for 30 min to remove the catalyst and other impurities. The definite purified CNTs were dispersed in 100 mL distilled water by ultrasonic vibration for 6 hours. Then, KMnO4 was added into the suspension. After about one hour`s stirring, the mixture was put into a microwave. Then the reaction went for 10 minutes around. After that, the products should be washed with distilled water for several times, and dried at 100°C for 12h .
Table 1.The amount of KMnO4 added into CNT suspension and percentage of MnO2 in the resulting CNT/MnO2 composites.
To make the manufacture process more efficient, some more direct synthesis methods have been investigated. A method involved the thermally decomposing manganese nitrates is a very representative example. In this case, carbon nanotubes were produced by chemical vapour deposition (CVD) on graphite disk. Then CNTs/graphite electrodes were dipped into 30 wt% HNO3 for 4h. After that a predetermined amount of 50 wt% Mn (NO3)2 solution was added in. Finally, the composite electrode was heated at 250°C for 2 hours in an ambient atmosphere. During the heating process, Mn (NO3)2 would decomposing and formed manganese oxide. Through this way, the specific capacitance could get to 568 F/g .
The three production methods mentioned previously should be the most common ones at the present stage. Except for these, there are some other possible methods utilized, for instance, hydrothermal reduction of KMnO4 with CNTs which is another effective way similar to the last-mentioned one above .
Discussion of the Advantages and Disadvantages of MnO2/CNT Composites
The relative merits of the composite electrode can be identified from three aspects, compared with other transition metal oxides as candidates, conventional electrodes and other kinds of materials for super capacitor electrodes.
Compared with another most promising alternative, RuO2, the specific capacitance of this electrode is an inferior. For RuO2, its specific capacitance can reach up to 720 F/g, while the corresponding value is below 600F/g. However, the cost is much lower, because manganese is much more abundant than ruthenium (Ru) which is known as a noble metal.
The advantages compared with traditional electrodes are very evident. The manganese oxide/carbon nanotubes composite electrode has a much higher specific capacitance than traditional one, which profits from its particular materials and structure. According to statistics, the value is about 20-200 times larger than conventional one . This immeasurable benefit brings a broad application in modern automotives, especially for petrol-electric hybrid vehicles. With the aggravating of energy crisis, hybrid vehicles have been the potential direction of the development of automotive industry. As for disadvantages compared with conventional electrodes, high expense and relative complicated manufacturing process should be main points. However, drawbacks are negligible for their potential.
The comparison with other candidates of super capacitor electrodes should be taken into consideration. Carbon-based materials are the very first materials used in making super capacitors. These materials have their prominent merits, such as relatively low cost, non-toxicity and wide temperature and so on. These advantages should be the main reasons why they are chosen to be as the first generation super capacitor electrode. All the carbon-based materials have large specific surface area (SSA). For example, ordered meso/macroporous carbon monoliths synthesized by Zhao et al, has a high SSA (1585.72 m2/g) and a high specific capacity about 130 F/g, which is much higher than other commercial products . Different carbon-based materials have different manufacturing methods. A typical example for carbon nanotubes is reported. According to Honda group`s method, a vertically aligned MWCNTs sheet is prepared by a transfer procedure from a silicon substrate. On the substrate, catalyst grains are covered as the seedbed . The main limitation of carbon-based materials at the present stage is that the specific capacitance does not increase with the higher specific surface area, so the specific capacitance cannot reach the value excepted.
About conductive polymer electrodes, a number of researches have been reported. This category has its instinctive advantages, like low cost, high flexibility and simple manufacture process, while there is a prominent drawback that its cyclability is not good enough. At the present stage, a lot of researchers are focusing on the polymer/CNTs composites. For instance, polyaniline/MWCNTs, synthesized by an in situ chemical oxidative polymerization method, is a feasible choice as the polymeric electrode .
Thus, according to the statement above, each electrode material has its merits and drawbacks. Generally speaking, transition metal oxide ones have the edge over the other two, because their specific capacitance is higher than carbon-based electrodes, and their cycle life is longer than conductive polymer ones. Certainly, it is no denying the fact that like other transition metal oxide options, manganese minerals are limited. With the utilization on a large scale, the resource should be the barrier for this material in future.
In this paper, manganese oxide/carbon nanotubes composite as super capacitor electrode was discussed, including the possible manufacturing processes and relative merits compared with other object choices. In summary, MnO2/CNTs composite is a recommendable material in super capacitor electrode application area.