Energy Materials

The Junior Investigator Group “Energy Materials” investigates and develops advanced functional nanomaterials for electrochemical applications. In particular, we focus on sustainable and energy efficient applications to store, recover, and harvest energy and treat water. This multitude of applications is enabled via use of the electrical double layer which describes the phenomenon of highly efficient electrosorption of ions at electrically charged surfaces at the fluid-solid interface. We develop novel carbon and hybrid materials with customized chemical composition, tuned electrochemical properties, and optimized porosity. Such materials are derived from organic and inorganic precursors. A particular challenge is the optimization of the pore structure and pore size distribution as a function of the actual application and type of electrolyte.


Fig. 1 Schematic drawing of the ion electrosorption via the formation of an electrical double-layer.

ELECTROCHEMICAL ENERGY STORAGE (EES)

Electrochemical energy storage (EES) has the advantage of high efficiency and short charge/discharge cycles. In particular, this is true for supercapacitors which excel in high power density and longevity. Supercapacitors may be based on faradaic or non-faradaic reactions at the fluid-solid interface and we differentiate between pseudocapacitors and electrical double-layer capacitors (EDLC) accordingly. The latter enable energy storage solely based on non-chemical and highly reversible ion electrosorption (in contrast to chemical energy storage in common batteries). Our team develops reliable testing procedures and technologies to investigate the long term performance stability. We also employ in situ measurement techniques to gain new insights in energy storage and ion transport mechanisms. This way, we can cover the entire range from molecular synthesis of electrode materials and separators and testing of cells to employing in situ techniques. In addition, we investigate more environmentally friendly ways to synthesize and manufacture carbon composite electrodes.

Fig. 2 Scheme of non-faradaic and faradaic energy storage in supercapacitors.

CAPACITIVE DEIONIZATION (CDI)

The phenomenon of the electrical double-layer cannot only be used to sore energy, but also for energy efficient deionization of water (i.e., capacitive deionization = CDI). Just like for EDLCs, CDI capitalizes a reversible electrochemical mechanism, namely ion electrosorption. Within the electrical double-layer, ions are electrosorbed as a stream of feed water containing a salt flows by (or through the electrodes). This immobilization of ions causes the salt concentration of the feed stream to drop. In an ideal system, using Na-Cl as a model, we could obtain as much as one ion to be immobilized per one invested electron. This concept can also be adapted for other ion systems, such as nitrates, sulfates, or carbonates, which are becoming more and more important.


Fig. 3 Concept of capacitive deionization (CDI).

Flow electrodes are an emerging technology for scalable, low cost capacitive systems. They are based on a flowable suspension of electrolyte and porous carbon particles. This technology is not only suitable for scalable energy storage, but also for continuous desalination or energy harvesting from chemical concentrations (like sea water vs. river water). Besides difference in salt concentration, this technology can also tap into the chemical gradient energy from dissolved CO2 which enables to extract energy from the exhaust gases from industrial sites or power plants.


Fig. 4 Schematic drawing of the concept of flow electrodes. The example shown relates to the application of an electrochemical flow capacitor for energy storage.

FUNCTIONAL CARBON AND HYBRID NANOMATERIALS

Novel carbon and hybrid materials are an important aspect of our work. In particular, we focus on porous materials with a well-developed pore size distribution and high surface area. Precursor-derived carbons are an important group of materials which can be obtained from bio mass, polymers, or carbides. Polymers enable us to freely design complex shapes, such as thin films, beads, or fibers. Carbide-derived carbons (CDC) are particularly interesting because of the very high level of control over the pore size distribution. Such model carbons are ideal to investigate the electrochemical performance with in situ techniques, such as methods based on spectroscopy, diffraction, or dilatometry. Hybrid materials enable to capitalize the electrical conductivity of carbon and the redox activity of metal oxides, metal nitrides, or electroactive polymers. We also synthesize carbons with defined redox active functional surface groups. Redox active carbon hybrids enable us to significantly improve the energy density of electrochemical energy storage devices.


Fig. 5 Selection of carbon nanomaterials and hybrid materials for electrochemical applications: A) Mixture of activated carbon, carbon black, and polymer binder, B) Electrospun fibers with nanotexture, C) birnessite-carbon hybrid material, D) nanodiamond-derived carbon onions.