Prof. Dr. Volker Presser

Using electrodes with subnanometer pores and ionic liquid electrolytes can improve the charge storage capacity at the expense of the charging rate. The fundamental understanding of the charging dynamics of nanoporous electrodes can help to avoid compromising the power density. In this work, we performed molecular dynamics simulations to reveal the charging mechanism of subnanometer pores in ionic liquids. Different from the traditional view that a smaller pore results in slower charging, a non-monotonic relation is found between the charging rate and pore size, in which the charging process is accelerated in some subnanometer pores. Our analysis uncovers that the mechanism of the charging enhancement can be attributed to the transition of in-pore ion structure.

Electrodes based on alloying reactions for sodium-ion batteries (NIB) offer high specific capacity but require bespoken electrode material design to enable high performance stability. This work addresses that issue by systematically exploring the impact of carbon properties on antimony/carbon composite electrodes for NIBs. Since the Sb surface is covered by an insulating oxide layer, carbon additives are crucial for the percolation and electrochemical activity of Sb based anodes. Instead of using complex hybridization strategies, the ability of mechanical mixing to yield stable high-performance Sb/C sodium-ion battery (NIB) electrodes is shown. This is only possible by considering the physical, chemical, and structural features of the carbon phase. A comparison of carbon nanohorns, onion-like carbon, carbon black, and graphite as conductive additives is given in this work. The best performance is not triggered by the highest or lowest surface area, and not by highest or lowest heteroatom content, but by the best ability to homogenously distribute within the Sb matrix. The latter provides an optimum interaction between carbon and Sb and is best enabled by onion-like carbon. A remarkable rate performance is attained, electrode cracking caused by volume expansion is successfully prevented, and the homogeneity of the solid/electrolyte interphase is significantly improved as a result of it. With this composite electrode, a reversible capacity of 490 mA h g−1 at 0.1 A g−1 and even 300 mA g−1 at 8 A g−1 is obtained. Additionally, high stability with a capacity retention of 73% over 100 cycles is achieved at charge/discharge rates of 0.2 A g−1.

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine, and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descriptions of the electrochemical reaction mechanism to interconvert these species are elusive. Here we show that electrochemical oxidation of iodide in nanoporous carbons forms persistent solid iodine deposits. Confinement slows down dissolution into triiodide and pentaiodide, responsible for otherwise significant self-discharge via shuttling. The main tools for these insights are in situ Raman spectroscopy and in situ small and wide-angle X-ray scattering (in situ SAXS/WAXS). In situ Raman confirms the reversible formation of triiodide and pentaiodide. In situ SAXS/WAXS indicates remarkable amounts of solid iodine deposited in the carbon nanopores. Combined with stochastic modeling, in situ SAXS allows quantifying the solid iodine volume fraction and visualizing the iodine structure on 3D lattice models at the sub-nanometer scale. Based on the derived mechanism, we demonstrate strategies for improved iodine pore filling capacity and prevention of self-discharge, applicable to hybrid supercapacitors and batteries.

The freshwater scarcity and increasing energy demand are two challenging global issues. Herein, we propose a new route for desalination, self-sustained visible-light-driven electrochemical redox desalination. We propose a novel device architecture involving internal integration of a quasi-solid-state dye-sensitized solar cell and continuous redox-flow desalination units with a bifunctional platinized-graphite-paper electrode. Both the solar cell and redox-flow desalination units are integrated using the bifunctional electrode with one side facing the solar cell operating as a positive electrode and the other side facing the redox-flow desalination unit operating as a negative electrode. The solar cell contains a gel-based tri-iodide/iodide redox couple sandwiched between an N719 dye-modified photoanode and cathode. In contrast, the redox-flow desalination consists of re-circulating ferro/ferricyanide redox couple sandwiched between the anode and cathode with two salt streams located between these electrodes. The performances of bifunctional electrodes in both redox couples were thoroughly investigated by electrochemical characterization. The brackish feed can be continuously desalted to the freshwater level by utilizing visible light illumination. As a device, this architecture combines energy conversion and water desalination. This concept bypasses the need for electrical energy consumption for desalination, which provides a novel structural design using photodesalination to facilitate the development of self-sustained solar desalination technologies.

This study demonstrates the hybridization of Li4Ti5O12 (LTO) with different types of carbon onions synthesized from nanodiamonds. The carbon onions mixed with a Li4Ti5Ox precursor for sol–gel synthesis. These hybrid materials are tested as anodes for both lithium-ion battery (LIB) and sodium-ion battery (SIB). Electrochemical characterization for LIB application is carried out using 1 m LiPF6 in a 1:1 (by volume) ethylene carbonate and dimethyl carbonate as the electrolyte. For lithium-ion intercalation, LTO hybridized with carbon onions from the inert-gas route achieves an excellent electrochemical performance of 188 mAh g−1 at 10 mA g−1, which maintains 100 mAh g−1 at 1 A g−1 and has a cycling stability of 96% of initial capacity after 400 cycles, thereby outperforming both neat LTO and LTO with onions obtained via vacuum treatment. The performance of the best-performing hybrid material (LTO with carbon onions from argon annealing) in an SIB is tested, using 1 m NaClO4 in ethylene/dimethyl/fluoroethylene carbonate (19:19:2 by mass) as the electrolyte. A maximum capacity of 102 mAh g−1 for the SIB system is obtained, with a capacity retention of 96% after 500 cycles.

We explore different electrolytes containing lithium perchlorate (LiClO4) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salts in various solvents at different temperatures to associate the electrochemical stability of lithium titanium oxide (LTO)/lithium manganese oxide (LMO)-activated carbon (AC) composite Li-ion hybrid supercapacitors. By varying the solvents and operation temperature, we were able to detect a correlation between the electrochemical performance and the conductivity of each electrolyte. To test the electrochemical stability and longevity of the electrolytes in such a system, full-cells were analyzed via a voltage floating for 10 h at charged state (2.8 V) of the full-cell. Energy and power performance were the highest for acetonitrile (ACN) containing electrolytes due to their high lithium-ion conductivity. Additionally, the longevity of LiClO4 in 3-methoxypropionitrile (3-MPN) was superior to all tested electrolytes at ambient temperature with 97% capacity retention and energy efficiency of about 86% after 500 h of voltage floating. LiClO4 in a mixture of ACN and ethylene carbonate (EC) was the most stable electrolyte at high temperatures with 70% capacity retention after 500 h voltage floating at 60°C and 50% capacity retention after 250 h voltage floating. The longevity of LiTFSI in ACN and LiClO4 in 3-MPN electrolyte demonstrated a capacity retention of 97% and 92% after 500 h voltage floating, respectively, at low temperature (−10 °C).

Electrochemical processes enable a new generation of energy-efficient desalination technologies. While ion electrosorption via capacitive deionization is only suitable for brackish water with low molar strength, the use of Faradaic materials capable of reversible ion intercalation or conversion reactions allows energy-efficient removal of ions from seawater. However, the limited charge transfer/storage capacity of Faradaic materials indicates an upper limit for their desalination applications. Therefore, a new electrochemical concept must be explored to exceed the current state-of-the-art results and to push the desalination capacity beyond 100–200 mgNaCl/gelectrode. In this proof-of-concept work, we introduce the new concept of using metal–air battery technology for desalination. We do so by presenting performance data for zinc–air desalination (ZAD) in 600 mM NaCl. The ZAD cell provides a desalination capacity of 0.9–1.0 mgNaCl/cm2 (normalized to the membrane area; corresponding to 1300 mgNaCl/gZn) with a charge efficiency of 70% when charging/discharging the cell at 1 mA/cm2. The energy consumption of ZAD is 68–92 kJ/mol.

To meet the future demands for off-grid power, high-performance electrochemical energy storage based on earth-abundant materials is essential. Supercapacitors are attractive in this sense due to their sustainable carbon-based architecture, rapid charging/discharging, and long cycle-life in comparison to battery chemistries. However, hybridizing carbon electrodes with inorganic phases is intensively explored in supercapacitor research to mitigate their low energy content. Iron sulfides are attractive because they are non-toxic and composed of earth-abundant elements, but, despite their hydrophobic nature, they have only been studied in aqueous electrolytes, limiting the energy content due to the narrow voltage stability window of water. Here, exploiting a rapid growth method and a highly functionalized graphene support, we strongly immobilized greigite (Fe3S4) nanoparticles with an ultrasmall size which could not be attained in the absence of graphene. The respective supercapacitor cell was found significantly more electroactive in the ionic liquid electrolyte than in water, boosting the energy content. Furthermore, greigite has high conductivity and fast surface faradaic reactions due to the enzyme-mimicking triple redox state of its thiocubane basic structural unit. Thus, fully reversible and fast redox processes in the expanded voltage-window of the ionic liquid also endowed excellent rate capability, cycling stability, and power. The work demonstrates a pathway, not previously explored, whereby greigite/graphene hybrids can surpass in these aspects top-rated supercapacitor materials.

Two-dimensional, layered transition metal carbides (MXenes) are an intriguing class of intercalation-type electrodes for electrochemical applications. The ability for preferred counterion uptake qualifies MXenes as an attractive material for electrochemical desalination. Our work explores Ti3C2Tx-MXene paired with activated carbon in such a way that both electrodes operate in an optimized potential range. This is accomplished by electrode mass balancing and control over the cell voltage. Thereby, we enable effective remediation of saline media with low (brackish) and high (seawater-like) ionic strength by using 20 and 600 mM aqueous NaCl solutions. It is shown that MXene/activated-carbon asymmetric cell design capitalizes on the permselective behavior of MXene in sodium removal, which in turn forces carbon to mirror the same behavior in the removal of chloride ions. This has minimized the notorious co-ion desorption of carbon in highly saline media (600 mM NaCl) and boosted the charge efficiency from 4% in a symmetric activated-carbon/activated-carbon cell to 85% in a membrane-less asymmetric MXene/activated-carbon cell. Stable electrochemical performance for up to 100 cycles is demonstrated, yielding average desalination capacities of 8 and 12 mg/g, respectively, for membrane-less MXene/activated-carbon cells in NaCl solutions of 600 mM (seawater-level) and 20 mM (brackish-water-level). In the case of the 20 mM NaCl solutions, surprising charge efficiency values of over 100% have been obtained, which is attributed to the role of MXene interlayer surface charges.

Reversible electrochemical processes are a promising technology for energy-efficient water treatment. Electrochemical desalination is based on the compensation of electric charge by ionic species, through which the ions are immobilized and, thereby, removed from a feed-water stream flowing through a desalination cell. For decades, electrochemical desalination has focused on the use of carbon electrodes, but their salt-removal ability is limited by the mechanism of ion electrosorption at low molar concentrations and low charge-storage capacity. Recently, charge-transfer materials, often found in batteries, have demonstrated much larger charge-storage capacities and energy-efficient desalination at both low and high molar strengths. In this Review, we assess electrochemical-desalination mechanisms and materials, including ion electrosorption and charge-transfer processes, namely, ion binding with redox-active polymers, ion insertion, conversion reactions and redox-active electrolytes. Furthermore, we discuss performance metrics and cell architectures, which we decouple from the nature of the electrode material and the underlying mechanism to show the versatility of cell-design concepts. These charge-transfer processes enable a wealth of environmental applications, ranging from potable-water generation and industrial-water remediation to lithium recovery and heavy-metal-ion removal.