Interdisciplinary Research on the Topic Super-Capacitors
Graphene lends itself as a promising material not only for super-capacitors but also for battery technologies in general. Researchers from the Fraunhofer-Institute for Applied Solid State Physics (IAF), the Freiburger Materialforschungszentrum (FMF), the Institute for Macromolecular Chemistry (MAKRO), and the Department of Microsystems Engineering (IMTEK) are therefore currently interested in efficient super-capacitors with graphene. The interdisciplinary collaboration was made possible through the pilot project “G-ONET: Porous Graphene-Organic Networks for Super Capacitors” of the Sustainability Center Freiburg (LZN). The goal was to research functional, organic-chemically bound graphene materials as possible electrode materials in energy storage systems. Researchers from Fraunhofer IAF were occupied with the graphene production and characterization as well as the assessment of the produced materials. The researchers of the University of Freiburg were responsible for the examination of the linking of graphene flakes (MAKRO) and for the characterization of the graphene material in super-capacitors (IMTEK).
Graphene as an electrode material
Graphene is a one-dimensional form of carbon. Its characteristics make it a well-suited component for super-capacitor-electrodes. The material is not only thin but also boasts a high electrical conductivity. It can thereby be quickly charged and discharged. Beyond that, it has a large surface area which leads to a high energy density. “We gathered graphene by separating graphite layers,” explains Sarah Roscher from Fraunhofer IAF. In order to obtain graphene flakes of only a few atom-layer thickness, the graphite layers were separated using an electro-chemical process. As part of this process, a diamond-electrode, developed at Fraunhofer IAF, was utilized.
After the successful exfoliation, the quality of the graphene flakes was examined and three characteristics of the material were validated. First, the researchers could determine that the process used resulted in a yield of 70%. Furthermore, it was proven that the flakes demonstrated a low defect density and, as a last validation, the flakes retained their large lateral distension which indicates that the exfoliation method is relatively damage-free.
Super-capacitors as efficient energy Storages
A super-capacitor consists of two electrodes (positive and negative) which are electrically separated yet simultaneously connected by an electrolyte-permeable membrane. In order to serve as a suitable electrode material, the material, such as graphene, must display certain characteristics including a large surface area plus a suitable meso- and micro-porosity which can allow for a quick electrolyte transport and provides good access to the electrode surface area. Further factors include electrical contact between the particles in the electrode material and the environmental and sustainable components of the material.
Graphene cannot be singularly implemented as an electrode material, however, given its strong aggregative behavior, which can lead to a drastic reduction in storage capacity. Therefore, it is often combined with other components such as activated carbon or carbon nanotubes (CNTs), in order to inhibit the aggregation. Additionally, binders such as polytetrafluorethylene are added to the mixture. Within the boundaries of the G-ONET project, two strategies were carried out in order to maintain the porosity of the graphene electrodes. In the first strategy, the graphene flakes were to build a porose structure with help from the interconnection of electrically active Poly (3-hexylthiopene) (P3HT) chains. The second strategy included CNTs which were used to separate the graphene flakes. CNTs are commercially available but have a relatively small surface area. The electro-active polymer PEDOT was implemented in this project instead of polytetrafluorethylene, in order to ensure the cohesion of the composite. PEDOT, although it has great electro-activity, is not suitable as a solo electrode-material as it can only be slowly charged and discharged. However, it is a wonderful binding material. “We wanted to produce a CNT and graphene composite which also uses PEDOT as a binder,” explains Dr. Olena Yurchenko from FMF. In addition to the graphene manufactured at Fraunhofer IAF, the research team also examined commercially available graphene (Elicarb). CNTs and graphene can be disposed of in ordinary trash, given that they are both carbon-based. The Na2SO4 electrolyte and the PEDOT are also environmentally friendly. The researchers furthermore discovered, that the CNT-graphene-PEDOT mixture displayed a good adhesion to current wires and that a good connectivity between the graphene flakes within the composite existed.
Characterization of the electrochemical properties of graphene
The materials in the super-capacitor were analyzed based upon cyclovoltammetric examinations and galvanostatic charging/discharging, focused on the electro-chemical properties such as current density and storage capacity. In the former, the current path is depicted dependent on an applied voltage. In the latter, the super-capacitor is charged and discharged by a certain current density. “The electrodes made out of the composite had a significantly higher current density as compared to electrodes made exclusively of graphene,” explains Elmar Laubender, also from FMF. “That means, the composite has a higher storage capacity.” The research team also discovered that only insignificant Ohm-resistance occurs during the charging of the composite. However, the composite made of the Fraunhofer IAF-developed graphene showed a smaller capacity than the composite made of the Elicarb graphene. The reason behind this defect is the much higher quality of the commercially produced material, which originates from large, immaculate graphite flakes. However, the Fraunhofer-IAF-graphene-based composite showed great stability and thereby provides a long lifetime of the super-capacitors.
Super-capacitors with graphene: system of the future
The demand for this form of technology is rapidly growing which means, the research thereof also needs to continuously grow. A significant hurdle exists however as super-capacitors do not have the capacity to maintain constant voltage. That means, they are only suitable for specific applications and cannot replace all currently-used storage systems. However, they would be an acceptable replacement for batteries. In the area of electro-mobility for example, processes which require large amounts of energy for a short period of time, such as breaking or starting, could be supported by super-capacitors. Other similar processes with batteries could also be an implementation opportunity. Some driving systems of trains are already based upon this process. Super-capacitors can be more effectively and efficiently produced with the support of graphene as an electrode material. Thereby, these storage systems could play a vital role in a more sustainable future.