Prof. Dr. Volker Presser

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.

Mechanically reversible compressible resorcinol–formaldehyde (RF) aerogels can be converted into mechanically reversible compressible carbon aerogels (CA) by carbonization in an inert atmosphere. By incorporation of polystyrene spheres into the RF gels as a sacrificial template, it is possible to create macropores with controlled size within the carbon framework during carbonization. The resulting templated carbon aerogel shows enhanced mechanical flexibility during compression compared to pristine samples. In addition, the presence of hierarchical porosity provides a porous architecture attractive for energy storage applications, such as supercapacitors.

Ion intercalation materials are emerging as a highly attractive class of electrodes for efficient energy water desalination. Most materials and concepts so far have focused on the removal of cations (especially sodium). Anion intercalation, however, remains poorly investigated in water desalination. We present a study on the capability of Mo1.33C-MXene for removing cations and anions and demonstrate the desalination performance in brackish water and seawater concentrations. Mo1.33C-MXene was prepared via acid treatment of the transition metal carbide MAX phase (Mo2/3Sc1/3)2AlC. Binder-free electrodes were obtained by entangling MXene with carbon nanotubes and tested without the use of any ion exchange membrane at low (5 mM NaCl) and high (600 mM NaCl) salt concentrations. Such electrodes showed a promising desalination performance of 15 mg/g in 600 mM NaCl with high charge efficiency up to 95%. By employing chemical online monitoring of the effluent stream, we separated the cation and anion intercalation capacity of the electrode material.

Aqueous electrolytes can be used for electrical double-layer capacitors, pseudocapacitors, and intercalation-type batteries. These technologies may employ different electrode materials, most importantly high-surface-area nanoporous carbon, two-dimensional materials, and metal oxides. All of these materials also find more and more applications in electrochemical desalination devices. During the electrochemical operation of such electrode materials, charge storage and ion immobilization are accomplished by non-Faradaic ion electrosorption, Faradaic ion intercalation at specific crystallographic sites, or ion insertion between layers of two-dimensional materials. These processes may or may not be associated with a (partial) loss of the aqueous solvation shell around the ions. Our work showcases the electrochemical quartz crystal microbalance as an excellent tool for quantifying the change in effective solvation. We chose sodium as an important cation for energy storage materials (sodium-based aqueous electrolytes) and electrochemical desalination (saline media). Our data show that a major amount of water uptake occurs during ion electrosorption in nanoporous carbon, while battery-like ion insertion between layers of titanium disulfide is associated with an 80% loss of the initially present solvation molecules. Sodiation of MXene is accomplished by a loss of 90% of the number of solvent molecules, but nanoconfined water in-between the MXene layers may compensate for this large degree of desolvation. In the case of sodium manganese oxide, we were able to demonstrate the full loss of the solvation shell.

Abstract The selective removal of ions by an electrochemical process is a promising approach to enable various water-treatment applications such as water softening or heavy-metal removal. Ion intercalation materials have been investigated for their intrinsic ability to prefer one specific ion over others, showing a preference for (small) monovalent ions over multivalent species. In this work, we present a fundamentally different approach: tunable ion selectivity not by modifying the electrode material, but by changing the operational voltage. We used titanium disulfide, which shows distinctly different potentials for the intercalation of different cations and formed binder-free composite electrodes with carbon nanotubes. Capitalizing on this potential difference, we demonstrated controllable cation selectivity by online monitoring the effluent stream during electrochemical operation by inductively coupled plasma optical emission spectrometry of aqueous 50 mm CsCl and MgCl2. We obtained a molar selectivity of Mg2+ over Cs+ of 31 (strong Mg preference) in the potential range between −396 mV and −220 mV versus Ag/AgCl. By adjusting the operational potential window from −219 mV to +26 mV versus Ag/AgCl, Cs+ was preferred over Mg2+ by 1.7 times (Cs preference).

In the present study, nickel matrix composites reinforced with a fine distribution of nanodiamonds (6.5 vol%) as reinforcement phase are annealed in vacuum at different temperatures ranging from 750 °C to 1300 °C. This is carried out to evaluate the in-situ transformation of nanodiamonds to carbon onions within a previously densified composite. The resulting materials are thoroughly analyzed by complementary analytical methods, including Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The proposed in-situ transformation method presents two main benefits. On one hand, since the particle distribution of a nanodiamond-reinforced composite is significantly more homogenous than in case of the carbon onions, it is expected that the transformed particles will preserve the initial distribution features of nanodiamonds. On the other hand, the proposed process allows for the tuning of the sp3/sp2 carbon ratio by applying a single straightforward post-processing step.