Prof. Dr. Volker Presser

Carbon nanostructures with a well-developed turbostratic sp2 structure and high porosity are synthesized at room temperature inside a planetary ball mill. The obtained carbons were analyzed in-depth by means of gas adsorption, wide-angle X-ray scattering (WAXS), Raman spectroscopy, and transmission electron microscopy (TEM). Our approach involves the solvent-free reaction between calcium carbide (CaC2) and hexachlorobenzene (C6Cl6) conducted under mechanochemical conditions. After certain mechanical activation time, the exothermic nature of the reaction (−492 kcal) provokes a combustion-like event that results in innocuous salt (CaCl2) and a carbonaceous material. Carbon with a high degree of structural order in the constituting graphene and the graphene stacks, possessing almost no internal surface, can be obtained after 5 min of milling time with a mass ratio CaC2/C6Cl6 of 0.9, while carbon exhibiting a surface area as high as 915 m2/g can be obtained after 2 h of milling time with a mass ratio CaC2/C6Cl6 of 5.1. WAXS results and TEM observations reveal a mixture of amorphous carbon and non-graphitic phases. Among the last one, spherical-shaped carbons and curved nanosized strips can be easily distinguished.

A sulfur-1,3-diisopropenylbenzene copolymer was synthesized by ring-opening radical polymerization and hybridized with carbon onions at different loading levels. The carbon onion mixing was assisted by shear in a two-roll mill to capitalize on the softened state of the copolymer. The sulfur copolymer and the hybrids were thoroughly characterized in structure and chemical composition, and finally tested by electrochemical benchmarking. An enhancement of specific capacity was observed over 140 cycles at higher content of carbon onions in the hybrid electrodes. The copolymer hybrids demonstrate a maximum initial specific capacity of 1150 mA h gsulfur-1 (850 mA h gelectrode-1) and a low decay of capacity to reach 790 mA h gsulfur-1 (585 mA h gelectrode-1) after 140 charge/discharge cycles. All carbon onion/sulfur copolymer hybrid electrodes yielded high chemical stability, stable electrochemical performance superior to conventional melt-infiltrated reference samples having similar sulfur and carbon onion content. The amount of carbon onions embedded in the sulfur copolymer has a strong influence on the specific capacity, as they effectively stabilize the sulfur copolymer and sterically hinder the recombination of sulfur species to the S8 configuration.

In this study, we explore carbon onions (diameter below 10 nm), for the first time, as a substrate material for lithium sulfur cathodes. We introduce several scalable synthesis routes to fabricate carbon onion-sulfur hybrids by adopting in situ and melt diffusion strategies with sulfur fractions up to 68 mass%. The conducting skeleton of agglomerated carbon onions proved to be responsible for keeping active sulfur always in close vicinity to the conducting matrix. Therefore, the hybrids are found to be efficient cathodes for Li-S batteries, yielding 97-98% Coulombic efficiency over 150 cycles with a slow fading of the specific capacity (ca. 660 mA h g-1 after 150 cycles) in long term cycle test and rate capability experiments.

Abstract Next generation electrochemical energy storage materials that enable a combination of high specific energy, specific power, and cycling stability can be obtained by a hybridization approach. This involves electrode materials that contain carbon and metal oxide phases linked on a nanoscopic level and combine characteristics of supercapacitors and batteries. The combination of the components requires careful design to create synergistic effects for an increased electrochemical performance. Improved understanding of the role of carbon as a substrate has advanced the power handling and cycling stability of hybrid materials significantly in recent years. This Concept outlines different design strategies for the design of hybrid electrode materials: (1) the deposition of metal oxides on readily existing carbon substrates and (2) co-synthesizing both carbon and metal oxide phase during the synthesis procedure. The implications of carbon properties on the hybrid material's structure and performance will be assessed and the impact of the hybrid electrode architecture will be analyzed. The advantages and disadvantages of all approaches are highlighted and strategies to overcome the latter will be proposed.

Merging of supercapacitors and batteries promises the creation of electrochemical energy storage devices that combine high specific energy, power, and cycling stability. For that purpose, lithium-ion capacitors (LICs) that store energy by lithiation reactions at the negative electrode and double-layer formation at the positive electrode are currently investigated. In this study, we explore the suitability of molybdenum oxide as a negative electrode material in LICs for the first time. Molybdenum oxide–carbon nanotube hybrid materials were synthesized via atomic layer deposition, and different crystal structures and morphologies were obtained by post-deposition annealing. These model materials are first structurally characterized and electrochemically evaluated in half-cells. Benchmarking in LIC full-cells revealed the influences of crystal structure, half-cell capacity, and rate handling on the actual device level performance metrics. The energy efficiency, specific energy, and power are mainly influenced by the overpotential and kinetics of the lithiation reaction during charging. Optimized LIC cells show a maximum specific energy of about 70 W·h·kg–1 and a high specific power of 4 kW·kg–1 at 34 W·h·kg–1. The longevity of the LIC cells is drastically increased without significantly reducing the energy by preventing a deep cell discharge, hindering the negative electrode from crossing its anodic potential limit.

Capacitive deionization (CDI) is a promising desalination process, but conventional static electrode CDI operates by sequentially cycling through charging and discharging to produce fresh and concentrated water, respectively. However, an effective continuous operation is desirable for optimized system operation. Here, we report a semi-continuous desalination process with a novel modified CDI cell architecture using a multi-channel flow stream and ion exchange membranes (MC-MCDI). This MC-MCDI consists of two channels including side and middle channels with a pair of cation and anion ion exchange membranes where the feed streams can be separately distributed without mixing. The MC-MCDI design allows semi-continuous production of clean water since the separated middle and side channels are alternately desalinated and regenerated: one channel is being desalinated while the other channel is regenerated. Therefore, the cell can produce clean water during both charging and discharging, enabling semi-continuous operation. In addition, with the benefit from similar cell configuration with membrane CDI, the MC-MCDI design exhibits a high salt adsorption capacity (SAC) of 22 ± 2 mg/g and charge efficiency of 90 ± 2% at middle and side channels during charging and discharging with reverse voltage operation (cell voltage of + 1.2 V vs. − 1.2 V).

Capacitive deionization (CDI) is a promising desalination technology based on ion electrosorption. The desalination capacity of CDI using carbon electrodes has remained limited due to a low operating cell voltage of around 1.2 V originating from the electrochemical stability window of water. Here, we report a novel multi-channel membrane CDI system that allows extension of the cell voltage by immersing one carbon electrode in an organic electrolyte and the other in an aqueous electrolyte. The resulting membrane CDI system using an aqueous/organic bi-electrolyte consists of two side-channels for supporting electrolytes with activated carbon electrodes and middle-channel for the feedwater flow. The middle-channel is separated by ion exchange membranes from the side-channels allowing highly concentrated water and organic supporting electrolytes (1 M NaCl in water and 1 M NaClO4 in propylene carbonate), respectively. Using an organic electrolyte for negative electrode (Na+ adsorption), the stable operating cell voltage was increased to 2.4 V. At the operating cell voltage of 2.4 V, the system provided an excellent desalination capacity of 63.5 ± 4 mg/g with charge efficiency of 95%.

Abstract Ordered mesoporous carbon materials, prepared from co‐assembly of a block copolymer and a commercial resol, were investigated as a sulfur host for LiS‐battery cathodes. We studied two activation methods for such carbons, namely annealing in ammonia (NH3) and carbon dioxide (CO2). We found that both activation environments drastically increased the specific surface area and establish a micro‐ and mesoporous pore structure. Treatment with NH3 also introduced nitrogen groups, which increased the initial specific capacity. The non‐activated carbon yielded carbon/sulfur cathodes with an initial capacity of ∼900 mAh/gsulfur (150 mAh/gsulfur after 100 cycles). The initial capacity was increased to 1300 mAh/gsulfur for the NH3 activated sample but with poor cycling stability. Enhanced performance stability was found for the CO2 treated sample with an initial capacity of 1100 mAh/gsulfur (700 mAh/gsulfur after 100 cycles).

Silicon oxycarbides are promising anode materials for lithium-ion batteries. In this study, we used the continuous MicroJet reactor technique to produce organically modified silica (ORMOSIL) spheres which were pyrolyzed to obtain silicon oxycarbides. The continuous technique allows the production of large quantities with a constant quality. Different alkoxysilanes were used to produce the silicon oxycarbides with different compositions. Thereby, the amounts of silicon–carbon bonds, as well as the free carbon content, were modified. Electrochemical testing was carried out in 1 M LiPF6 in ethylene carbonate/dimethyl carbonate. A mixture of vinyl- and phenyltrimethoxysilane was identified as the best anode material with a stable performance due to the increased carbon content. The first-cycle delithiation capacity of the most stable material was 922 mA h/g, and the capacity retention after 100 cycles was 83% (767 mA h/g).

Abstract We investigated the influence of nitrogen groups on the electrochemical performance of carbide-derived carbons by comparing materials with a similar pore structure with and without nitrogen-doping. These materials were tested in a half-cell and full-cell supercapacitor setup with a conventional organic electrolyte (1 M tetraethylammonium tetrafluoroborate in acetonitrile) and an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate). Varying the nitrogen content in the range of 1–7 mass % had no systematic influence on the energy storage capacity but a stronger impact on the rate handling ability. The highest specific capacitance in a half-cell supercapacitor at a negative potential was 215 F/g in EMIM-BF4. Using the best-performing carbide-derived carbon with and without nitrogen-doping (i. e., by applying a synthesis temperature of 800 °C), the full-cell performance was 174 F/g, which results in a high specific energy of 61 Wh/kg in EMIM-BF4. For the same materials, the corresponding specific energy was about 30 Wh/kg when using the organic electrolyte.