Supercapacitors can store large amounts of energy very quickly before dispensing them again with just as much speed. This makes them an indispensable component of sustainable energy systems that have many fluctuating energy sources.
In order to effectively replace fossil fuels, we need energy storage devices. That can not only hold vast amounts of energy, but also dispense this energy again quickly when needed. After all, the increasing share of regenerative energy gives rise to fluctuations in the grid. That need to be counteracted within a matter of seconds to keep the mains power supply stable.
Supercapacitors as a hybrid solution
One particularly promising solution combines a battery and a capacitor. Hybrid supercapacitors like this have similar charging and discharging times to a typical capacitor. And can store roughly as much energy as conventional batteries. In addition, they can be charged much more rapidly and charged or discharged much more frequently. While a lithium-ion battery has a lifetime of just a few thousand cycles, a supercapacitor can reach around a million charging cycles. They are already commonplace in smartphones, laptops, electronic devices and electric vehicles. As the change of energy policy gathers steam, supercapacitors are also becoming an important element of sustainable energy systems. Especially those manufactured with environmentally friendly materials and processes.
The best of both worlds
An extremely powerful, sustainable, and inexpensive hybrid system like this is the development objective of the European research project HyFlow. In which eleven partners from Germany, Italy, Spain, the Czech Republic, Austria, Portugal, and Russia are collaborating under the direction of Landshut University of Applied Sciences. In this project, researchers are aiming to combine a high-performance vanadium redox flow battery with a supercapacitor. “A redox flow battery has a high storage capacity, yet it can only be charged and discharged slowly. A supercapacitor, on the other hand, boasts short charging times with a low energy density. This hybridisation should give rise to an energy storage concept that combines the advantages of both systems. High storage capacity and high performance,” explains Professor Karl-Heinz Pettinger, Scientific Head of the Energy Technology Centre at Landshut University of Applied Sciences, who is coordinating the project.
In this new design, the supercapacitor is intended to bolster the grid with a discharging capacity in the megawatt range. In the future, the planned storage system will be able to flexibly counterbalance the electricity and energy demand in the event of critical grid states. Such as peaks in load or generation.
Graphene brings high efficiency
One problem supercapacitors have faced so far is low energy density. While rechargeable lithium-ion batteries have an energy density of up to 265 watthours per kilogram, previous supercapacitors could only supply a tenth of that figure. Now, a team from the Technical University of Munich has developed a new kind of powerful yet sustainable graphene hybrid material for supercapacitors. It acts as the positive electrode in the energy storage device. The researchers combined this with a tried-and-tested, titanium and carbon-based negative electrode. In this configuration, the new energy storage device not only achieves an energy density of up to 73 watt-hours per kilogram. Which roughly corresponds to the energy density of a nickel-metal hydride battery. But also considerably surpasses the power densities of most other supercapacitors with its rating of 16 kilowatts per kilogram.
A system of carbon and salt water
One particularly sustainable variant of a hybrid supercapacitor is currently being investigated in more depth by researchers at Graz University of Technology. “The system we’ve been looking at so closely has nanoporous-carbon electrodes and an electrolyte comprising aqueous sodium iodide. In other words, salt water. These features make the system very environmentally friendly, cheap, incombustible, and easy to recycle,” explains Christian Prehal, the principal author of this study. He has recently transferred from the Institute for Chemistry and Technology of Materials at Graz University of Technology to ETH Zurich. Understanding the processes in these supercapacitors paves the way for hybrid supercapacitors. That boast a considerably higher energy density alongside very fast charging and discharging. They could be an especially attractive option when it comes to storing solar energy in private households. To name just one example.
Supercapacitors with low-priced polymer
The Advanced Technology Institute at the University of Surrey has developed a supercapacitor based on a material called polyaniline. This cheap polymer material stores a charge by capturing ions within the electrode. This happens through the exchange of electrons with the ion that “dopes” the material. Ash Stott, lead scientist on the project and a PhD student at the University of Surrey, explains: “Supercapacitors have already been proven to be one of the leading technologies for intermittent storage as well as high-power delivery. Our work has established a baseline for high-energy devices that also operate at high power, effectively widening the range of potential applications.”