Prof. Dr. Volker Presser

As population demands for freshwater increase, existing natural freshwater resources face significant strains. Currently, over 2.5 billion people live in localities that are subject to severe water scarcity at least 1 month of the year.1 Scarcity affects all types of localities, such as urban, rural, coastal areas, landlocked areas, and off-grid locations. Increasingly, active water purification technologies are being used to boost and secure freshwater supplies. A widely used desalination technology is seawater reverse osmosis (SWRO), in which pumps pressurize the feedwater to well above its osmotic pressure to pump water molecules through a membrane largely impermeable to salt ions (posmotic ∼ 25 bar).2 City-scale SWRO plants are operational in several countries, delivering on the order of 106 m3 of treated water per day (<0.1% of the total global daily water consumption). However, as the need for water purification increases and the requirements for each locality becomes more diverse, SWRO plants alone cannot meet the growing demand for a technological solution. Barriers toward increased penetration of SWRO include the enormous investment required to develop such plants, poor downscalability of the technology, the geographical limitation to coastal areas and near urban environments, and high energy requirements (typically about 4 kWh/m3).2

Vanadium oxide nanostructures are constantly being researched and developed for cathodes in lithium- and sodium-ion batteries. To improve the internal resistance and the discharge capacity, this study explores the synthesis and characterization of continuous one-dimensional hybrid nanostructures. Starting from a sol–gel synthesis, followed by electrospinning and controlled thermal treatment, we obtained hybrid fibers consisting of metal oxide crystals (orthorhombic V2O5 and monoclinic VO2) engulfed in conductive carbon. For use as Li-ion battery cathode, a higher amount of carbon yields a more stable performance and an improved capacity. Monoclinic VO2/C fibers present a specific capacity of 269 mAh·gVOx–1 and maintain 66% of the initial capacity at a rate of 0.5 A·g–1. Orthorhombic V2O5/C presents a higher specific capacity of 316 mAh·gVOx–1, but a more limited lithium diffusion, leading to a less favorable rate handling. Tested as cathodes for Na-ion batteries, we confirmed the importance of a conductive carbon network and nanostructures for improved electrochemical performance. Orthorhombic V2O5/C hybrid fibers presented very low specific capacity while monoclinic VO2/C fibers presented an improved specific capacity and rate performance with a capacity of 126 mAh·gVOx–1.

Free-standing, binder-free, titanium–niobium oxide/carbon hybrid nanofibers are prepared for Li-ion battery applications. A one-pot synthesis offers a significant reduction of processing steps and avoids the use of environmentally unfriendly binder materials, making the approach highly sustainable. Tetragonal Nb2O5/C and monoclinic Ti2Nb10O29/C hybrid nanofibers synthesized at 1000 °C displayed the highest electrochemical performance, with capacity values of 243 and 267 mAh g−1, respectively, normalized to the electrode mass. At 5 A g−1, the Nb2O5/C and Ti2Nb10O29/C hybrid fibers maintained 78 % and 53 % of the initial capacity, respectively. The higher rate performance and stability of tetragonal Nb2O5 compared to that of monoclinic Ti2Nb10O29 is related to the low energy barriers for Li+ transport in its crystal structure, with no phase transformation. The improved rate performance resulted from the excellent charge propagation in the continuous nanofiber network.

Continuous fiber mats are attractive electrodes for lithium-ion batteries, because they allow operation at high charge/discharge rates in addition to being free of polymer binders and conductive additives. In this work, we synthesize and characterize continuous Sn/SiOC fibers (diameter ca. 0.95 [small mu ]m), as a Li-ion battery anode. Our synthesis employs electrospinning of a low-cost silicone resin, using tin acetate in a dual role both as a polymer crosslinker and as a tin precursor (6-22 mass%). The hybrid electrodes present very high initial reversible capacities (840-994 mA h g-1) at 35 mA g-1, and retain 280-310 mA h g-1 at 350 mA g-1. After 100 cycles at 70 mA g-1, the hybrid fibers maintained 400-509 mA h g-1. Adding low amounts of Sn is beneficial not just for the crosslinking of the polymer precursor, but also to decrease the presence of electrochemically inactive silicon carbide domains within the SiOC fibers. Also, the metallic tin clusters contribute to a higher Li+ insertion in the first cycles. However, high amounts of Sn decrease the electrochemical performance stability. In SiOC fibers synthesized at high temperatures (1200 [degree]C), the Cfree phase has a significant influence on the stability of the system, by compensating for the volume expansion from the alloying systems (Sn and SiO2), and improving the conductivity of the hybrid system. Therefore, a high amount of carbon and a high graphitization degree are crucial for a high conductivity and a stable electrochemical performance.

Selecting the most suitable force field is a key to meaningful molecular dynamics (MD) simulation. To select the appropriate gold force field to model the Au(111)/ionic liquid interface, a systematic comparison of four different widely used force fields of gold and a typical carbon force field has been studied by MD simulations with constant potential method. We calculated the ion adsorption behavior and differential capacitance of interfaces between the gold electrode and ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([PYR][TFSI]) in comparison with the experimental results and showed the effects on the solid-liquid interfaces from the van der Waals interaction, image force effect and cumulative ions. Based on the comparison between the results of simulations and experiments, we recommend two types of force fields to properly model the Au(111)/ionic liquid interfaces.

Abstract In recent years, numerous studies have explored ways to overcome the low intrinsic electrical conductivity of lithium titanate (Li4Ti5O12, LTO) for energy storage with lithium-ion batteries. These approaches almost exclusively considered element doping and elaborate LTO-carbon nanocomposites, whereas simple adjustment of the defect concentration remains largely unexplored. In our study, we tune the Ti3+/Ti4+ concentration of a commercial LTO nanopowder through oxygen vacancy formation during thermal annealing in hydrogen atmosphere. We investigate the impact of the treatment on material properties like energy band structure, electrical conductivity, crystallinity, phase distribution, surface chemistry, and particle morphology, and correlate these parameters to the electrochemical performance. At optimum treatment conditions, the intrinsic electrical conductivity can be greatly improved, while circumventing LTO phase transformations or amorphization. This enables the reduction of the carbon concentration to 5 mass%, while yielding a high electrode capacity of about 70 mAh/g (82 mAh/g based on active mass) at ultrahigh C-rates of 100C. When combined with an activated carbon/lithium manganese oxide composite cathode, an excellent energy and power performance of 70 Wh/kg and 47 kW/kg were obtained (82 Wh/kg and 55 kW/kg based on active mass), while maintaining 83 % of its energy ratings after 5000 cycles at 10C (78 % after 15000 cycles at 100C).

Synthesis of high surface area carbon materials with hierarchical pore structure is reported. Combined salt templating with ZnCl2 and hard templating with SBA-15 is used to produce ordered mesoporous and microporous hard–salt-templated carbons (OM-HSTCs) from simple sucrose as carbon precursor. OM-HSTCs achieve specific surface areas of more than 2600 m2 g−1 and total pore volumes up to 2.2 cm3 g−1. In comparison to purely hard-templated ordered mesoporous carbons, the additional salt template leads to high micropore volume and provides control over the size/distribution of micro- and mesopores and over the carbon microstructure. This method combines carbonization and the formation of well-defined micropores in one step and is more versatile in terms of resulting pore structure than previously reported routes toward ordered mesoporous/microporous carbons. When applied as electrode materials in electric double-layer capacitors with 1 m tetraethylammonium tetrafluoroborate in acetonitrile organic electrolyte, OM-HSTCs combine high gravimetric capacitance (133 F g−1 at 0.1 A g−1) resulting from high micropore volume with high capacitance retention under high-power conditions (126 F g−1 at 40 A g−1), exceeding the purely microporous or purely ordered mesoporous reference materials.

Abstract Lignin-derived carbon is introduced as a promising electrode material for water desalination by using capacitive deionization (CDI). Lignin is a low-cost precursor that is obtained from the cellulose and ethanol industries, and we used carbonization and subsequent KOH activation to obtain highly porous carbon. CDI cells with a pair of lignin-derived carbon electrodes presented an initially high salt adsorption capacity but rapidly lost their beneficial desalination performance. To capitalize on the high porosity of lignin-derived carbon and to stabilize the CDI performance, we then used asymmetric electrode configurations. By using electrodes of the same material but with different thicknesses, the desalination performance was stabilized through reduction of the potential at the positive electrode. To enhance the desalination capacity further, we used cell configurations with different materials for the positive and negative electrodes. The best performance was achieved by a cell with lignin-derived carbon as a negative electrode and commercial activated carbon as a positive electrode. Thereby, a maximum desalination capacity of 18.5 mg g−1 was obtained with charge efficiency over 80 % and excellent performance retention over 100 cycles. The improvements were related to the difference in the potential of zero charge between the electrodes. Our work shows that an asymmetric cell configuration is a powerful tool to adapt otherwise inappropriate CDI electrode materials.

The MicroJet reactor technique is an excellent continuous method to produce spherical and homogeneous organically modified silica (ORMOSIL) particles in a large scale (10-15 g min-1). We applied this method to manufacture polyorganosilsesquioxanes with different ratios of phenyl and vinyl functional groups, which were later pyrolyzed to obtain silicon oxycarbides. Such polymer-derived ceramic (PDC) materials are highly suited as precursor for carbide-derived carbon (CDC) synthesis. Chlorine etching of PDC at high temperatures removed silicon and oxygen, yielding the formation of nanoporous carbon. Pure poly(phenyl-silsesquioxane) spheres lost their shape during the thermal process by undergoing further condensation reactions. Yet, the spherical shape was conserved during thermal processing after adding vinyl functionalities. The ratio of vinyl and phenyl functionalities controlled the pore structure and the total CDC yield, enabling an increase from 2 mass% to 22 mass%. The total pore volume varied between 1.3-2.1 cm3 g-1 and the specific surface area between 2014-2114 m2 g-1. The high surface area and large pore volume makes these materials attractive for high power supercapacitor electrodes. The specific capacitance of the best sample at low rates in 1 M tetraethylammonium tetrafluoroborate in acetonitrile was 116 F g-1 (at 5 mA g-1) and still 80 F g-1 at very high rates (at 100 A g-1).

Novolac-derived nanoporous carbon beads were used as conductive matrix for lithium-sulfur battery cathodes. We employed a facile self-emulsifying synthesis to obtain sub-micrometer novolac-derived carbon beads with nanopores. After pyrolysis, the carbon beads showed already a specific surface area of 640 m2 g−1 which was increased to 2080 m2 g−1 after physical activation. The non-activated and the activated carbon beads represent nanoporous carbon with a medium and a high surface area, respectively. This allows us to assess the influence of the porosity on the electrochemical performance of lithium-sulfur battery cathodes. The carbon/sulfur hybrids were obtained from two different approaches of sulfur infiltration: melt-infusion of sulfur (annealing) and in situ formation of sulfur from sodium thiosulfate. The best performance (∼880 mAh gsulfur−1 at low charge rate; 5th cycle) and high performance stability (&gt;600 mAh gsulfur−1 after 100 cycles) were found for the activated carbon beads when using melt infusion of sulfur.