Energie-Materialien

Stable crystalline aluminum doped zinc oxide (AZO) nanopowders were synthesized using hydrothermal treatment processing. Three different aluminum precursors have been used. The Al-precursors were found to affect the morphology of the obtained nanopowders. AZO nanoparticles based on zinc acetate and aluminum nitrate have been prepared with different Al/Zn molar ratios. XRD investigations revealed that all the obtained powders have single phase zincite structure with purity of about 99%. The effect of aluminum doping ratio in AZO nanoparticles (based on Al-nitrate precursor) on structure, phase composition, and particle size has been investigated. The incorporation of Al in ZnO was confirmed by UV-Vis spectroscopy revealing a blue shift due to Burstein-Moss effect.

Electrical energy storage (EES) is one of the most critical areas of technological research around the world. Storing and efficiently using electricity generated by intermittent sources and the transition of our transportation fleet to electric drive depend fundamentally on the development of EES systems with high energy and power densities. Supercapacitors are promising devices for highly efficient energy storage and power management, yet they still suffer from moderate energy densities compared to batteries. To establish a detailed understanding of the science and technology of carbon/carbon supercapacitors, this review discusses the basic principles of the electrical double-layer (EDL), especially regarding the correlation between ion size/ion solvation and the pore size of porous carbon electrodes. We summarize the key aspects of various carbon materials synthesized for use in supercapacitors. With the objective of improving the energy density, the last two sections are dedicated to strategies to increase the capacitance by either introducing pseudocapacitive materials or by using novel electrolytes that allow to increasing the cell voltage. In particular, advances in ionic liquids, but also in the field of organic electrolytes, are discussed and electrode mass balancing is expanded because of its importance to create higher performance asymmetric electrochemical capacitors.

Nuclear magnetic resonance (NMR) spectroscopy is increasingly being used to study the adsorption of molecules in porous carbons, a process which underpins applications ranging from electrochemical energy storage to water purification. Here we present density functional theory (DFT) calculations of the nucleus-independent chemical shift (NICS) near various sp2-hybridized carbon fragments to explore the structural factors that may affect the resonance frequencies observed for adsorbed species. The domain size of the delocalized electron system affects the calculated NICSs, with larger domains giving rise to larger chemical shieldings. In slit-pores, overlap of the ring current effects from the pore walls is shown to increase the chemical shielding. Finally, curvature in the carbon sheets is shown to have a significant effect on the NICS. The trends observed are consistent with existing NMR results as well as new spectra presented for an electrolyte adsorbed on carbide-derived carbons prepared at different temperatures.

Template-based chemical vapor deposition is an efficient one step process to synthesize carbon nanotubes (CNTs) for a wide range of applications. In this process, the choice of template dictates certain physical features of the CNT, such as length and outer diameter, while the process itself affects other features, such as tube wall thickness, carbon deposition rate and carbon morphology. Although it is generally understood that the process affects important CNT properties, little is known about how parameters affect synthesized CNTs. In this report, a systematic parametric study was conducted to determine how three key process parameters (deposition time, temperature, and gas flow rate) affect overall carbon mass deposition rate and CNT wall thickness and morphology. The findings show that process parameters can be independently utilized to produce CNTs with similar or differing cross-sectional dimensions and other useful features, each with distinct advantages.

The expansion of porous carbons used in electrochemical double layer capacitors during positive or negative charging has been investigated in several studies, however, the underlying mechanisms are not well understood yet. We therefore investigated dimensional changes of three different carbons in various electrolytes including ionic liquids e.g. [EMIM][BF4] with and without solvent and [EMIM][TFSI]. For all carbon/electrolyte combinations the expansion during positive charging was smaller than for negative charging. The expansion increased with decreasing average pore size of the carbon. Addition of solvents, such as acetonitrile or propylene carbonate, significantly increased the expansion for negative charging. These observations are discussed in the view of various existing models for charge induced strain of carbons.

This study investigates carbons onions (∼400 m2g-1)as a conductive additive for supercapacitor electrodes of activated carbon and compares their performance with carbon black with high or low internal surface area. We provide a study of the electrical conductivity and electrochemical behavior between 2.5 and 20 mass% addition of each of these three additives to activated carbon. Structural characterization shows that the density of the resulting film electrodes depends on the degree of agglomeration and the amount of additive. Additions of low surface area carbon black (∼80 m2g-1) enhances the power handling of carbon electrodes but significantly lowers the specific capacitance even when adding small amounts of carbon black. A much lower decrease in specific capacitance is observed for carbon onions and the best values are seen for carbon black with a high surface area (∼1390 m2·g-1). The overall performance benefits from the addition of any of the studied additives only at either high scan rates and/or electrolytes with high ion mobility. Normalization to the volume shows a severe decrease in volumetric capacitance and only at high current densities nearing 10 A g-1 we can see an improvement of the electrode capacitance.

Herein we show that heating 2D Ti3C2 in air resulted in TiO2 nanocrystals on thin sheets of disordered graphitic carbon structure that can handle extremely high cycling rates when tested as anodes in lithium ion batteries. Oxidation of 2D Ti3C2 in either CO2 or pressurized water also resulted in TiO2/C hybrid structure. Similarly, other hybrids can be produced, as we show here for Nb2O5/C from 2D Nb2C.

The electrochemical flow capacitor (EFC) has been recently introduced as a new concept for rapid and capacitive energy storage using flowable carbon-electrolyte suspensions. In our study, we investigate the EFC under static and constant flow condition. Unlike static carbon suspensions where poor particle-particle-contact and particle settling yield a highly resistive and time-dependent behavior, we show that flow operation of carbon suspensions reach high Coulombic efficiency and stable energy density performance. Our results also indicate that the specific capacitance per total mass of carbon electrodes in flow operation is comparable to conventional binder-bound carbon film electrodes.

Capacitive technologies, such as capacitive deionization and energy harvesting based on mixing energy ("capmix" and "CO2 energy"), are characterized by intermittent operation: phases of ion electrosorption from the water are followed by system regeneration. From a system application point of view, continuous operation has many advantages, to optimize performance, to simplify system operation, and ultimately to lower costs. In our study, we investigate as a step towards second generation capacitive technologies the potential of continuous operation of capacitive deionization and energy harvesting devices, enabled by carbon flow electrodes using a suspension based on conventional activated carbon powders. We show how the water residence time and mass loading of carbon in the suspension influence system performance. The efficiency and kinetics of the continuous salt removal process can be improved by optimizing device operation, without using less common or highly elaborate novel materials. We demonstrate, for the first time, continuous energy generation via capacitive mixing technology using differences in water salinity, and differences in gas phase CO2 concentration. Using a novel design of cylindrical ion exchange membranes serving as flow channels, we continuously extract energy from available concentration differences that otherwise would remain unused. These results may contribute to establishing a sustainable energy strategy when implementing energy extraction for sources such as CO2-emissions from power plants based on fossil fuels.

Extracting electric energy from small temperature differences is an emerging field in response to the transition toward sustainable energy generation. We introduce a novel concept for producing electricity from small temperature differences by the use of an assembly combining ion exchange membranes and porous carbon electrodes immersed in aqueous electrolytes. Via the temperature differences, we generate a thermal membrane potential that acts as a driving force for ion adsorption/desorption cycles within an electrostatic double layer, thus converting heat into electric work. We report for a temperature difference of 30 °C a maximal energy harvest of ~2 mJ/m2, normalized to the surface area of all the membranes.