St Petersburg University physicists increase the efficiency of supercapacitors using carbon nanotubes and transition metal oxides
A team of scientists working at St Petersburg University and Omsk Scientific Centre of the Siberian Branch of the Russian Academy of Sciences has created a composite material based on multi-walled carbon nanotubes and manganese oxide with rhenium additive. The developed material will improve the energy efficiency of supercapacitors used in alternative energy.
Carbon nanotubes are a promising material, which is a carbon cylindrical structure created from graphene (carbon lattice). Nanotubes are highly durable and dense, while their thickness is less than a human hair. We can say that these are "materials of the future" because, when small volumes of such matter (only 1% to 3% of the total volume) are added to a certain medium, nanotubes can significantly improve the characteristics of this medium. Thus, experiments were conducted to add nanotubes to: road surfaces; auto tyres; Li-ion batteries; and even paper. As a result, the substance became stronger and more efficient.
The research findings are published in Applied Sciences, a Swiss scientific journal.
Nanotubes can be either single-walled or multi-walled. The single-walled ones have a one-dimensional structure, while the multi-walled ones consist of several concentrically bound carbon nanotubes. They can be only a few micrometres long with a diameter of less than 100 nanometres which is almost imperceptible to the eye (there are 10 million nanometres in one centimetre). By comparison, the diameter of a human hair is about 50 micrometres. Multi-walled nanotubes conduct electricity better, and their surface is chemically inert, i.e. it does not allow any reactions to start. All of it suggests that multi-walled nanotubes are the most beneficial substance to be used in the production of supercapacitors, Li-ion batteries, and other elements.
The team of scientists used the infrastructure of the St Petersburg University Research Park, namely the equipment of: the Centre for Physical Methods of Surface Investigation; the Centre for X-Ray Diffraction Studies; and the Interdisciplinary Resource Centre for Nanotechnology.
St Petersburg University scientists have developed new ways to increase the efficiency of supercapacitors by using a combination of multi-walled nanotubes and transition metal oxides. One of the approaches is to increase the surface area that ensures the energy efficiency of the electrode. Usually, various types of carbon with a high specific surface area are used as the basis for electrodes of industrial supercapacitors. These types of carbon include soot, activated carbon, carbon black, graphene, carbon nanotubes, and other substances.
As of today, to improve the energy efficiency and stability of supercapacitors, scientists have been developing hybrid materials that accumulate energy both due to the electrical double layer and reversible electrochemical processes occurring on the electrode surface in the presence of transition metal oxides, for example. These include oxides of: cobalt; vanadium; ruthenium; and other metals. According to Petr Korusenko, a research associate at St Petersburg University and one of the contributors to this R&D, manganese oxides are now a promising option as transition metals. They have a high specific capacitance, low toxicity and low production cost.
We offer a composite based on multi-walled carbon nanotubes and manganese oxide with rhenium additive. Rhenium is a heavy metal. The resulting composite had a high capacitance, i.e. the accumulated charge per one mass unit. This is one of the main characteristics of such materials.
Petr Korusenko, a research associate at St Petersburg University and one of the contributors
’The more charge a composite can accumulate in a short period of time and give it away, the greater its efficiency,’ said Petr Korusenko, a research associate in the Department of Solid State Electronics, St Petersburg University.
During the experiment, the scientists deposited layers of manganese oxide on the surface of the nanotubes. Then, they carried out thermal treatments to crystallise and form nanoparticles. That made it possible to more than double the specific capacitance, but this indicator quickly decreased. It became possible to improve the electrochemical properties by selecting the optimal treatment temperature for the composite and the subsequent addition of rhenium oxide, which, like manganese, has several oxidation states. As experiments showed, rhenium oxide was fixed mainly near manganese nanoparticles and made it possible to increase the proportion of electrochemically active manganese oxide MnO2 by additional oxidation of MnOх.
Due to that, the scientists were able to make the material more stable during charge/discharge cycle tests. Such a high result is due to the synergistic effect achieved by combining the properties of manganese and rhenium oxides, as well as carbon nanotubes. On the one hand, it leads to an increase in the contribution of reversible electrochemical processes to the specific capacitance. On the other hand, it leads to a noticeable increase in the contribution of the electrical double layer during charge accumulation.
The findings of St Petersburg University scientists will significantly improve the efficiency of pulse power sources that generate a large amount of energy in a short time. Today, supercapacitors are used in alternative energy, transportation systems, energy storage in households and other branches of science and technology. Increasing their energy efficiency is important for many areas, since the main task of supercapacitors is generation of a powerful energy pulse.