Prof. Dr. Volker Presser

Electrochemical energy storage using nanoporous carbons and ionic liquids enables large cell voltages and is a promising way to increase the energy density of electrical double-layer capacitors. The structure of the double layer in solvent-free electrolytes is fundamentally different from other systems with organic or aqueous solvents. In our study, we investigate the physical behavior of nanoporous carbon electrodes in contact with ionic liquids with a multilength scale approach by combining electrochemical quartz-crystal microbalance and electrochemical dilatometry. Synergistic combination of both in situ methods allows one to correlate system properties on particle and electrode level. We find that the charging mechanism at low charge is characterized by the exchange of more smaller ions by fewer larger ions. At higher charges, the system is changing to preferred counterion adsorption, which is resulting in a strong increase in the electrode volume. The maximum linear strain for a bulk electrode is 2% in our study, which is quite high for a supercapacitor system.

We present a novel multichannel membrane flow-stream capacitive deionization (MC-MCDI) concept with two flow streams to control the environment around the electrodes and a middle channel for water desalination. The introduction of side channels to our new cell design allows operation in a highly saline environment, while the feed water stream in the middle channel (conventional CDI channel) is separated from the electrodes with anion- and cation-exchange membranes. At a high salinity gradient between side (1000 mm) and middle (5 mm) channels, MC-MCDI exhibited an unprecedented salt-adsorption capacity (SAC) of 56 mg g−1 in the middle channel with charge efficiency close to unity and low energy consumption. This excellent performance corresponds to a fourfold increase in desalination performance compared to the state-of-the-art in a conventional CDI cell. The enhancement originates from the enhanced specific capacitance in high-molar saline media in agreement with the Gouy–Chapman–Stern theory and from a double-ion desorption/adsorption process of MC-MCDI through voltage operation from −1.2 to +1.2 V.

Activated carbon cloth is a promising electrode material for capacitive deionization to accomplish energy efficient desalination of water. The most attractive feature is the combination of high porosity and the ability to shape binder-free electrodes by simple cutting. The macroporous inter-fiber space also assists facile flow of the aqueous medium. Our work presents a thorough benchmarking of activated carbon cloth materials with different pore structures which show different potentials at zero charge. The studied activated carbon cloth textiles possess a large microporosity with an average pore size of 0.7–1.3 nm and stable electrochemical performance in aqueous media with specific capacitance of up to 125 F/g. In aqueous 5 mM NaCl, the electrodes achieve up to 16 mg/g salt adsorption capacity with charge efficiency of 80% at cell voltage of 1.2 V. Further on, we investigated cell voltages between 0.6 V and 1.2 V and applied our predictive salt adsorption tool that is based on the pore structure to the entire voltage window range. Our work also shows that activated carbon cloth can even be operated without a current collector.

Dimensional changes in carbon-based supercapacitor electrodes were investigated using a combination of electrochemical dilatometry and in situ small-angle X-ray scattering. A novel hierarchical carbon material with ordered mesoporosity was synthesized, providing the unique possibility to track electrode expansion and shrinkage on the nanometer scale and the macroscopic scale simultaneously. Two carbons with similar mesopore structure but different amounts of micropores were investigated, employing two different aqueous electrolytes. The strain of the electrodes was always positive, but asymmetric with respect to positive and negative applied voltages. The asymmetry strongly increased with increasing microporosity, giving hints to the possible physical origin of electrosorption induced pore swelling.

Novolac is a low-cost carbon precursor which can be used to derive nanoporous carbon beads in sub-micrometer size. In this study, we introduce this material as a novel electrode material for capacitive deionization (CDI) with high performance stability and superior desalination rate. The polymer beads were synthesized employing a self-emulsifying system in an autoclave, pyrolyzed under argon, and activated with CO2, yielding a specific surface area of 1905 m2 g−1 with a high total pore volume of 1.26 cm3 g−1. After CO2 activation, the material shows a salt sorption capacity of ∼8 mg g−1, but the performance is highly influenced by functional groups, causing an inversion peak and fast performance decay. However, de-functionalization via hydrogen treatment is outlined as an effective strategy to improve the CDI performance. After hydrogen treatment of novolac-derived carbon beads, we obtained a salt sorption capacity of 11.5 mg g−1 with a charge efficiency of more than 80% and a performance stability of around 90% over more than 100 cycles. Particularly attractive for practical application is the very high average salt adsorption rate of 0.104 mg g−1 s−1, outperforming commercial activated carbons, which are commonly used for CDI, by at least a factor of two.

A hybrid membrane pseudocapacitive deionization (MPDI) system consisting of a hydrated vanadium pentoxide (hV2O5)-decorated multi-walled carbon nanotube (MWCNT) electrode and one activated carbon electrode enables sodium ions to be removed by pseudocapacitive intercalation with the MWCNT–hV2O5 electrode and chloride ion to be removed by non-faradaic electrosorption of the porous carbon electrode. The MWCNT–hV2O5 electrode was synthesized by electrochemical deposition of hydrated vanadium pentoxide on the MWCNT paper. The stable electrochemical operating window for the MWCNT–hV2O5 electrode was between −0.5 V and +0.4 V versus Ag/AgCl, which provided a specific capacity of 44 mAh g−1 (corresponding with 244 F g−1) in aqueous 1 m NaCl. The desalination performance of the MPDI system was investigated in aqueous 200 mm NaCl (brackish water) and 600 mm NaCl (seawater) solutions. With the aid of an anion and a cation exchange membrane, the MPDI hybrid cell was operated from −0.4 to +0.8 V cell voltage without crossing the reduction and oxidation potential limit of both electrodes. For the 600 mm NaCl solution, the NaCl salt adsorption capacity of the cell was 23.6±2.2 mg g−1, which is equivalent to 35.7±3.3 mg g−1 normalized to the mass of the MWCNT–hV2O5 electrode. Additionally, we propose a normalization method for the electrode material with faradaic reactions based on sodium uptake capacities.

A key challenge for present-day electric energy storage systems, such as supercapacitors and batteries, is to meet the world's growing demand for high performances, low cost, and environmental-friendliness. Here, we introduce a hybrid energy storage system combining zinc iodide (ZnI2) as redox electrolyte with a nanoporous activated carbon fiber (ACF) cathode and a zinc disk anode. We found that the nanopores (<1 nm) of ACF lead to a strong adsorption behavior of iodide and triiodide. Hence, this system exhibits low self-discharge rates without applying an ion exchange membrane. The high power performance (20.0 kW kg-1) originates from the enhanced redox kinetics of the iodide system as evidenced by electrochemical analysis. Considering the high specific energy (226 W h kg-1), the ACF/Zn ZnI2 battery represents an alternative for lead acid, Ni-Zn, and Ni-Cd batteries, while providing a supercapacitor-like power performance in the range of seconds to minutes charging times.

In recent decades, redox-active electrolytes have been applied in stationary energy storage systems, benefitting from Faradaic reactions of the electrolyte instead of the electrode material. One of the challenging tasks is to balance the redox activities between the negative and positive electrode. As a possible solution, a mixed electrolyte with vanadyl and tin sulfate was previously suggested; however, a low power performance is a great challenge to be overcome. Here, we found that the origin of the poor power performance in the mixture electrolyte system (vanadium complex and tin solution) is the reduction of the pore volume at the positive electrode via irreversible tin dioxide formation. To prevent the latter, we introduce a hybrid energy storage system exhibiting both battery-like and supercapacitor-like features via asymmetric redox electrolytes at the microporous activated carbon electrodes; SnF2 solution as anolyte and VOSO4 as catholyte. By employing an anion exchange membrane, the irreversible SnO2 formation at the positive electrode is effectively suppressed; thus, an asymmetric 1 M SnF2|3 M VOSO4 system provides a high maximum specific power (3.8 kW kg-1 or 1.5 kW L-1), while still exhibiting a high maximum specific energy up to 58.4 W h kg-1 (23.4 W h L-1) and a high cycling stability over 6500 cycles.

A solvent-free synthesis of hierarchical porous carbons is conducted by a facile and fast mechanochemical reaction in a ball mill. By means of a mechanochemical ball-milling approach, we obtained titanium(IV) citrate-based polymers, which have been processed via high temperature chlorine treatment to hierarchical porous carbons with a high specific surface area of up to 1814 m2 g−1 and well-defined pore structures. The carbons are applied as electrode materials in electric double-layer capacitors showing high specific capacitances with 98 F g−1 in organic and 138 F g−1 in an ionic liquid electrolyte as well as good rate capabilities, maintaining 87% of the initial capacitance with 1 M TEA-BF4 in acetonitrile (ACN) and 81% at 10 A g−1 in EMIM-BF4.

A new carbon model derived from in situ small-angle X-ray scattering (SAXS) enables a quantitative description of the voltage-dependent arrangement and transport of ions within the nanopores of carbon-based electric double-layer capacitors. In the first step, ex situ SAXS data for nanoporous carbon-based electrodes are used to generate a three-dimensional real-space model of the nanopore structure using the concept of Gaussian random fields. This pore model is used to derive important pore size characteristics, which are cross-validated against the corresponding values from gas sorption analysis. In the second step, simulated in situ SAXS patterns are generated after filling the model pore structure with an aqueous electrolyte and rearranging the ions via a Monte Carlo simulation for different applied electrical potentials. These simulated SAXS patterns are compared with in situ SAXS patterns recorded during voltage cycling. Experiments with different cyclic voltammetry scan rates revealed a systematic time lag between ion transport processes and the applied voltage signal. Global transport into and out of nanopores was found to be faster than the accommodation of the local equilibrium arrangement in favor of sites with a high degree of confinement.