Prof. Dr. Volker Presser

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.

We pyrolyzed and activated novolac beads in one single step with ammonia at different temperatures (750–950 °C), which leads to a highly porous carbon with nitrogen-doping. The chemical and physical properties were characterized and correlated with the electrochemical performance as supercapacitor electrodes. The average pore size varied at 0.6–1.4 nm dependent on the synthesis temperatures. Three different electrolytes (aqueous, organic, and an ionic liquid) were tested. The specific capacitance in a symmetrical supercapacitor reached up to 173 F g−1 and was strongly dependent on the porosity of the electrode material and the kind of electrolyte. We found that the presence of nitrogen enhanced the electrochemical performance stability and led to a high specific energy of 50 Wh·kg−1 using an ionic liquid as electrolyte.

Electrodeposition is a simple and effective method for the synthesis of disordered hydrated vanadium pentoxide (V2O5[middle dot]nH2O). For the synthesis of energy storage electrodes with high power performance, electrodeposition of hydrated V2O5 inside carbon micropores is particularly attractive to synergize electric-double layer formation and lithium ion intercalation. Here, we demonstrate that hydrated V2O5 can be effectively electrodeposited in carbon micropores of activated carbon cloth. Our study indicates that carbon pores larger than 1 nm are essential for the effective decoration with hydrated V2O5. A thermal treatment after the electrodeposition is often used to enhance the crystal structure of hydrated V2O5. However, thermal annealing of the hydrated vanadium pentoxide decorated activated carbon cloth under an oxygen-rich environment at high temperature (>330 [degree]C) leads to a significant loss of pore volume, leading to a decreased electrochemical performance. At low annealing temperature (200 [degree]C), the vanadium pentoxide electrodeposited activated carbon cloth electrode exhibits a maximum specific capacity of 137 mA h g-1 with stable cycle performance over 1600 cycles at a rate of 4C.

Abstract Faradaic deionization is a promising new seawater desalination technology with low energy consumption. One drawback is the low water production rate as a result of the limited kinetics of the ion intercalation and insertion processes. We introduce the redox activities of iodide confined in carbon nanopores for electrochemical desalination. A fast desalination process was enabled by diffusionless redox kinetics governed by thin-layer electrochemistry. A cell was designed with an activated carbon cloth electrode in NaI aqueous solution, which was separated from the feedwater channel by a cation-exchange membrane. Coupled with an activated carbon counter electrode and an anion-exchange membrane, the half-cell in NaI with a cation-exchange membrane maintained performance even at a high current of 2.5 A g−1 (21 mA cm−2). The redox activities of iodide allowed a high desalination capacity of 69 mg g−1 (normalized by the mass of the working electrode) with stable performance over 120 cycles. Additionally, we provide a new analytical method for unique performance evaluation under single-pass flow conditions regarding the water production rate and energy consumption. Our cell concept provides flexible performance for low and high salinity and, thus, enables the desalination of brackish water or seawater. Particularly, we found a low energy consumption (1.63 Wh L−1) for seawater desalination and a high water production rate (25 L m−2 h−1) for brackish water.

Research on alternatives to replace conventional graphite anodes is needed to advance lithium-ion battery technology. In this work, an anatase nano-TiO2/carbon onion hybrid material (nano-TiO2-C) is introduced as a rapid and stable lithium storage anode material, synthesized by a simple synthetic route using tailored sol-gel chemistry. The nano-TiO2-C hybrid material provides highly reversible capacity (166 mA h g-1 at 0.02 A g-1), promising rate capability (61 mA h g-1 at 5 A g-1), and long-term cycle stability (capacity retention: 94% at 1 A g-1 for 1000 cycles). We demonstrate that hybridization of nano-TiO2 with carbon onions improves the high rate performance significantly.

A fundamental understanding of ion charge storage in nanoporous electrodes is essential to improve the performance of supercapacitors or devices for capacitive desalination. Here, we employ in situ X-ray transmission measurements on activated carbon supercapacitors to study ion concentration changes during electrochemical operation. Whereas counter-ion adsorption was found to dominate at small electrolyte salt concentrations and slow cycling speed, ion replacement prevails for high molar concentrations and/or fast cycling. Chronoamperometry measurements reveal two distinct time regimes of ion concentration changes. In the first regime the supercapacitor is charged, and counter- and co-ion concentration changes align with ion replacement and partially co-ion expulsion. In the second regime, the electrode charge remains constant, but the total ion concentration increases. We conclude that the initial fast charge neutralization in nanoporous supercapacitor electrodes leads to a non-equilibrium ion configuration. The subsequent, charge-neutral equilibration slowly increases the total ion concentration towards counter-ion adsorption.