Carbon Nickel System Supercapacitor

At present, many carbon-nickel system supercapacitors are used. Here, the energy storage principle of this new type of supercapacitor is introduced in detail.

Two identical carbon electrodes are connected in series in the carbon-nickel system electric double-layer supercapacitor, and the capacity of the entire capacitor can only get half of the capacitance on one electrode. Selecting a metal oxide to replace a carbon electrode can make the voltage of one electrode change without polarization or a very small degree of polarization on the other plate, which can not only improve the capacitance, but also make more full use of the Faraday quasi-capacitance The effect stores energy. This composite system is called an “electric double layer-quasi-capacitor,” or hybrid supercapacitor. For metal electrode materials to replace carbon electrodes, it is required to have good reversibility.

As mentioned earlier, RuO, with its high conductivity, low decay performance, and good reversibility, is a good choice. However, the reserves of Ru are scarce and the material cost is too high, so that this capacitor cannot be applied to the civilian field on a large scale. On the basis of the accumulated research results of nickel-cadmium batteries and nickel-hydrogen batteries for many years, metal nickel oxide electrode materials have been introduced into the research of supercapacitors. Carbon-nickel supercapacitor is such a hybrid supercapacitor, which combines the energy storage principles of electric double layer capacitors (DLC) and quasi-capacitors (Pseudo-Capacitor), thus showing advantages in specific power and specific energy indicators. The structure of carbon-nickel system supercapacitor can be represented by the following formula:

Carbon-nickel system supercapacitor charging process

During the charging process, an oxidation reaction occurs on the positive nickel oxide electrode, which is similar to the alkaline nickel-metal hydride battery. The reaction formula is as follows:

The nickel electrode of the capacitor cannot be fully polarized, and a large amount of oxygen will be released when the charging is completed 70% to 80%, and the amount of precipitation increases with the increase of temperature. This is due to the non-uniform charge distribution in the nickel oxide electrode, and the overcharge of the boundary part causes the decomposition of nickel oxide. When the voltage on the nickel oxide positive electrode reaches 0.45~0.49V, the voltage at the electrode boundary has reached 0.51~0.6V, which will cause oxygen atoms to be generated in the electrolyte. Increasing both the charging time and the charging current density leads to the generation and release of more oxygen atoms into the electrolyte, which will release oxygen under certain conditions.

On the negative electrode, the carbon electrode still stores energy through the electric double layer effect. When the voltage on the negative plate reaches -0.3~-0.4V, in the electric double layer structure, K+ mainly accumulates on the phase surface in contact with the electrolyte. As the voltage on the negative plate gradually increases, when it reaches -0.75~-0.85V, the H+ voltage begins to appear in the electric double layer structure. When the voltage is low, the H+ in the electric double layer structure will be adsorbed by the carbon electrode, and continue to increase the voltage on the negative electrode. The voltage will cause the precipitation of H2.

Discharge process of carbon-nickel system supercapacitor

During the discharge process, NiOOH on the positive electrode was converted back to nickel oxide, and the electric double layer structure on the negative electrode gradually weakened until it disappeared.
The nickel oxide electrode shows good reversibility during the whole charge-discharge process.
Through the above analysis, three voltage working ranges of carbon-nickel system supercapacitors can be found.
There is no additional reaction area. The electromotive force EMF of the capacitor in this area is
EMF=ф+-ф-=0.49-(-0.4)=0.89V
There are some additional reactions on the electrode and gas is generated, but it can be adsorbed by the active surface of the electrode. The electromotive force of the capacitor in this region
ЕМF=ф+-ф-=0.55—(-0.85)=1.4V
③The working area of gas evolution
ЕМF=ф+-ф-=0.65—(—1.05)=1.7V

These calculations show that the maximum voltage of this carbon-nickel system supercapacitor is between 0.8 and 1.7V. By using high-purity electrolyte, controlling the thickness of the electrode plate and improving the process level of sintering and curing, the supercapacitor monomer can work stably without gas evolution under the condition of the highest voltage of 1.6V. The structure of this carbon-nickel system supercapacitor. The current research focuses on further increasing the specific surface area of the activated carbon electrode, increasing the specific capacity, and introducing organic electrolytes into this architecture to further increase the monomer voltage, thereby improving the specific energy index of the capacitor.