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Organic solar cells can be manufactured as flexible and transparent modules which allows them to be combined with glass façades of buildings for example. (Photo: Christoph Brabec)

Abstract:
Nanotechnology offers many chances to benefit the environment and health. It can be applied to save raw materials and energy, develop enhanced solar cells and more efficient rechargeable batteries and replace harmful substances with eco-compatible solutions.

Going green with nanotechnology

Wurzburg, Germany | Posted on December 21st, 2016

"Nanotechnology is a seminal technology. The UMWELTnanoTECH project association has delivered excellent results. Even the smallest achievements can make a huge contribution to protecting the environment. We must treat the opportunities this future technology offers with responsibility; its eco-compatible use has top priority," said the Bavarian Minister of the Environment, Ulrike Scharf, in Erlangen on 23 November 2016 where the results were presented at the international congress "Next Generation Solar Energy Meets Nanotechnology". For three years, the Bavarian State Ministry for the Environment and Consumer Protection had financed the association consisting of ten individual projects with around three million euros.

Three Würzburg projects

Three of the ten projects were located in Würzburg. Professor Vladimir Dyakonov from the Department of Physics headed the project for environmentally compatible, highly efficient organic solar cells; he was also the spokesman of the "Organic Photovoltaics" section. Anke Krüger, Professor of Chemistry, was in charge of the project on ultra-fast electrical stores based on nano-diamond composites.

Responsibility for the third project rested with Professor Gerhard Sextl, Head of the Fraunhofer Institute for Silicate Research titled "Hybrid capacitors for smart grids and regenerative energy technologies". Sextl, who holds the Chair for Chemical Technology of Material Synthesis at the Julius-Maximilians-Universität (JMU) Würzburg, was also the spokesman of the "Energy storage" section.

Below are the three projects from Würzburg and their results.

Eco-friendly inks for organic solar cells

Organic solar cells have become quite efficient, converting about eleven percent of the solar energy received into electricity. What is more, they are relatively easy to manufacture using ink-jet printing processes where organic nanoparticles are deposited on non-elastic or flexible carrier materials with the help of solvents. This enables new applications in architecture, for example integrating solar cells in window façades or cladding concave surfaces.

There is, however, a catch to it: So far, most ink-jet printing processes have been based on toxic solvents such as dichlorobenzene. These substances are harmful for humans and the environment and require extensive and costly standards of safety.

The Professors Vladimir Dyakonov and Christoph Brabec (University of Erlangen-Nuremberg) have managed to use nanomaterials to develop ecologically compatible photovoltaic inks based on water or alcohol with equal efficiency. Moreover, the research team has developed new simulation processes: "They allow us to predict which combinations of solvents and materials are suitable for the eco-friendly production of organic solar cells," Dyakonov explains.

Nanodiamonds for ultra-fast electrical storage

In order to build highly efficient electric cars, more powerful energy stores are needed as the standard batteries still have some drawbacks, including low cycle stability and very limited power density. The first means that the battery capacity decreases following multiple charging and discharging cycles. The latter implies that only a fraction of the energy store is used during fast charging or discharging.

Supercapacitors play an important role as highly efficient energy stores besides batteries, because they outperform rechargeable batteries in terms of cycle stability and power density. Their energy density, however, is much lower compared with lithium-ion batteries. This is why supercapacitors need to be much bigger in size than batteries in order to deliver comparable amounts of energy.

Professor Anke Krüger has teamed up with Dr Gudrun Reichenauer from the Bavarian Center for Applied Energy Research (ZAE Bayern) to make progress in this regard. Their idea was to build the supercapacitors' electrodes not only of active charcoal, but to modify them using other carbon materials, namely nanodiamonds and carbon onions, which are small particles that have multiple layers like an onion.

Their approach is promising: By combining nanomaterials with suitable electrolytes, the performance parameters of the supercapacitors can be boosted. "Based on these findings, it is now possible to build application-oriented energy stores and test their applicability," Krüger further.

Increased storage capacity of hybrid capacitors

More efficient and faster energy stores were also the research focus of Professor Gerhard Sextl's project. His research team at the University of Würzburg managed to develop so-called hybrid capacitors further into highly efficient energy stores that can be manufactured in an environmentally compatible process.

Hybrid capacitors are a combination of supercapacitors based on electrochemical double-layer capacitors and charge storage in a battery. Firstly, they are capable of storing energy quickly by forming an electrochemical double layer as in a supercapacitor and also deliver the energy promptly when it is needed. Secondly, they hold more energy due to lithium ions embedded in an active battery material, analogously to lithium-ion batteries. By combining the two storage mechanisms, it is possible to implement systems with a high energy and power density at low costs.

The electrodes are the heart of the hybrid capacitors. They are coated with modified active materials: lithium iron phosphate and lithium titanate. This allows achieving storage capacities which are twice as high as those relying on conventional supercapacitor electrode materials.

"We have managed to develop a material that combines the advantages of both systems. This has brought us one step closer to implementing a new, fast and reliable storage concept," Sextl says. The activities at the university were supported by the Fraunhofer Institute for Silicate Research in Würzburg, one of the leading battery research centres in Germany.

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Contacts:
Dr. Esther Knemeyer Pereira

49-931-318-6002

Eco-friendly inks for organic solar cells
Prof. Dr. Vladimir Dyakonov
Department of Physics
University of Würzburg
T +49 931 31-83111

Nanodiamonds for ultra-fast electrical storage
Prof. Dr. Anke Krueger
Institute of Organic Chemistry
University of Würzburg
T +49 931 31-85334

Increased storage capacity of hybrid capacitors
Prof. Dr. Gerhard Sextl
Department for Chemical Technology of Material Synthesis
University of Würzburg
and Fraunhofer Institute for Silicate Research ISC
T +49 931 4100-100