Prof. Dr. Volker Presser

Due to their versatile properties and excellent electrical conductivity, MXenes have become attractive materials for alkali metal-ion batteries. However, as the capacity is limited to lower values due to the intercalation mechanism, these materials can hardly keep up in the ever-fast-growing community of battery research. Antimony has a promisingly high theoretical sodiation capacity characterized by an alloying reaction. The main drawback of this type of battery material is related to the high volume changes during cycling, often leading to electrode cracking and pulverization, resulting in poor electrochemical performance. A synergistic effect of combing antimony and MXene can be expected to obtain an optimized electrochemical system to overcome capacity fading of antimony while taking advantage of MXene charge storage ability. In this work, variation of the synthesis parameters and material design strategy have been dedicated to achieving the optimized antimony/MXene hybrid electrodes for high-performance sodium-ion batteries. The optimized performance does not align with the highest amount of antimony, the smallest nanoparticles, or the largest interlayer distance of MXene but with the most homogeneous distribution of antimony and MXene while both components remain electrochemically addressable. As a result, the electrode with 40 mass% MXene, not previously expanded, etched with 5 mass% HF and 60% antimony synthesized on the surfaces of MXene emerged as the best electrode. We obtained a high reversible capacity of 450 mA h g−1 at 0.1 A g−1 with a capacity retention of around 96% after 100 cycles with this hybrid material. Besides the successful cycling stability, this material also exhibits high rate capability with a capacity of 365 mA h g−1 at 4 A g−1. In situ XRD measurements and post mortem analysis were used to investigate the reaction mechanism.

In this work, a block copolymer (BCP) consisting of poly((butyl methacrylate-co-benzophenone methacrylate-co-methyl methacrylate)-block-(2-hydroxyethyl methacrylate)) (P(BMA-co-BPMA-co-MMA)-b-P(HEMA)) is prepared by a two-step atom-transfer radical polymerization (ATRP) procedure. BCP membranes are fabricated applying the self-assembly and nonsolvent induced phase separation (SNIPS) process from a ternary solvent mixture of tetrahydrofuran (THF), 1,4-dioxane, and dimethylformamide (DMF). The presence of a porous top layer of the integral asymmetric membrane featuring pores of about 30 nm is confirmed via scanning electron microscopy (SEM). UV-mediated cross-linking protocols for the nanoporous membrane are adjusted to maintain the open and isoporous top layer. The swelling capability of the noncross-linked and cross-linked BCP membranes is investigated in water, water/ethanol mixture (1:1), and pure ethanol using atomic force microscopy, proving a stabilizing effect of the UV cross-linking on the porous structures. Finally, the influence of the herein described cross-linking protocols on water-flux measurements for the obtained membranes is explored. As a result, an increased swelling resistance for all tested solvents is found, leading to an increased water flux compared to the pristine membrane. The herein established UV-mediated cross-linking protocol is expected to pave the way to a new generation of porous and stabilized membranes within the fields of separation technologies.

Semiconductor materials demonstrate promising potential for wastewater treatment due to their photocatalytic properties, which can be controlled through the design of the bandgap structure. The photogenerated electron and hole in semiconductor materials provide efficient oxidation/reduction performance for the degradation of pollutants, either directly or indirectly, through the generation of reactive species. Photocatalytic degradation has been utilized to treat contaminants ranging from dyes, chemical precursors, and pharmaceuticals, to diverse organic and inorganic waste. Over the past few years, advances in functional materials have achieved wider light absorption ranges and extended charge carrier lifetime through the doping of heteroatoms or the formation of heterojunctions. Despite these advances, innovative strategies are required to target emerging contaminants with environmental persistence, such as perfluorinated compounds, and improve the efficiency of these nanomaterials in real water matrices in the presence of multicomponent interfering ions. In this review, recent advances on the application of semiconductor catalysts for wastewater treatment and environmental remediation are reviewed, and new approaches that may overcome the current limitations are discussed.

Mixed-metal oxides (MMOx), oxides with more than one (transition) metal, provide versatile structural and electrochemical properties well exploited in energy conversion and electrochemical energy storage. The preparation of MMOx from single-source precursors benefits from homogeneous composition and uniform metal distribution. Herein, we describe layered mixed-metal carbides (MXenes) as templates to prepare MMOx. Through thermal oxidation of TiNb-based MXenes in CO2, mixtures of Ti and Nb oxides were produced. The Ti-to-Nb ratio in the MXene significantly affects the derived oxide composition but does not show a direct stoichiometric relation between them. At higher Ti ratios, oxide mixtures of TiO2 and titanium niobium oxide are obtained, while with Nb excess, only MMOx are produced. Multilayer MXenes retain carbon upon oxidation and produce TiNbOx/C hybrids, while delaminated MXenes lead to pure TiNbOx. When tested as Li-ion battery electrodes, the multilayer MXene-derived MMOx with Ti/Nb = 1:5 presented 226 mAh·g–1 at 10 mA·g–1 and 75% retention after 1000 cycles at 1 A·g–1.

MXene-transition metal dichalcogenide (TMD) heterostructures are synthesized through a one-step heat treatment of Nb2C and Nb4C3. These MXenes are used without delamination or any pre-treatment. Heat treatments accomplish the sacrificial transformation of these MXenes into TMD (NbS2) at 700 and 900 °C under H2S. This work investigates, for the first time, the role of starting MXene phase in the derivative morphology. It is shown that while treatment of Nb2C at 700 °C leads to the formation of pillar-like structures on the parent MXene, Nb4C3 produces nano-mosaic layered NbS2. At 900 °C, both MXene phases, of the same transition metal, fully convert into nano-mosaic layered NbS2 preserving the parent MXene's layered morphology. When tested as electrodes for hydrogen evolution reaction, Nb4C3-derived hybrids show better performance than Nb2C derivatives. The Nb4C3-derived heterostructure exhibits a low overpotential of 198 mV at 10 mA cm−2 and a Tafel slope of 122 mV dec−1, with good cycling stability in an acidic electrolyte.

The capacitance of the electrochemical interface has traditionally been separated into two distinct types: non-Faradaic electric double-layer capacitance, which involves charge induction, and Faradaic pseudocapacitance, which involves charge transfer. However, the electrochemical interface in most energy technologies is not planar but involves porous and layered materials that offer varying degrees of electrolyte confinement. We suggest that understanding electrosorption under confinement in porous and layered materials requires a more nuanced view of the capacitive mechanism than that at a planar interface. In particular, we consider the crucial role of the electrolyte confinement in these systems to reconcile different viewpoints on electrochemical capacitance. We propose that there is a continuum between double-layer capacitance and Faradaic intercalation that is dependent on the specific confinement microenvironment. We also discuss open questions regarding electrochemical capacitance in porous and layered materials and how these lead to opportunities for future energy technologies.

Abstract Environmentally sustainable, low-cost, flexible, and lightweight energy storage technologies require advancement in materials design in order to obtain more efficient organic metal-ion batteries. Synthetically tailored organic molecules, which react reversibly with lithium, may address the need for cost-effective and eco-friendly anodes used for organic/lithium battery technologies. Among them, carboxylic group-bearing molecules act as high-energy content anodes. Although organic molecules offer rich chemistry, allowing a high content of carboxyl groups to be installed on aromatic rings, they suffer from low conductivity and leakage to the electrolytes, which restricts their actual capacity, the charging/discharging rate, and eventually their application potential. Here, a densely carboxylated but conducting graphene derivative (graphene acid (GA)) is designed to circumvent these critical limitations, enabling effective operation without compromising the mechanical or chemical stability of the electrode. Experiments including operando Raman measurements and theoretical calculations reveal the excellent charge transport, redox activity, and lithium intercalation properties of the GA anode at the single-layer level, outperforming all reported organic anodes, including commercial monolayer graphene and graphene nanoplatelets. The practical capacity and rate capability of 800 mAh g−1 at 0.05 A g−1 and 174 mAh g−1 at 2.0 A g−1 demonstrate the true potential of GA anodes in advanced lithium-ion batteries.

Energy-efficient technologies for the remediation of water and generation of drinking water is a key towards sustainable technologies. Electrochemical desalination technologies are promising alternatives towards established methods, such as reverse osmosis or nanofiltration. In the last few years, hydrogen-driven electrochemical water purification has emerged. This review article explores the concept of desalination fuel cells and capacitive-Faradaic fuel cells for ion separation.

P2-type layered oxides with the general Na-deficient composition Na x TMO2 (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na+ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V and metal dissolution at low potentials upon discharge results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na0.67(Mn0.55Ni0.21Co0.24)O2, Na0.67(Mn0.45Ni0.18Co0.24Ti0.1Mg0.03)O2 and Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)O2 with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications.

Lithium-ion batteries are the primary power source for electric vehicles and portable electronic devices, creating a massive demand to mine and extract lithium. So far, lithium extraction has focused on brine and geological deposits. Yet, access to the enormous amount of lithium (at low concentration) in the earth’s oceans and other aqueous media remains challenging. Electrodialysis with Li-selective ceramic membranes could effectively separate lithium from seawater but at a high energy cost. Reversible electrochemical processes, like redox flow batteries, can overcome the limitation of electrodialysis-based systems. Herein we propose a system combining Li-selective ceramic membranes and a simple redox flow electrolyte to accomplish continuous lithium recovery from seawater. The lithium-extraction redox flow battery (LE-RFB) extracts dissolved lithium with a purity of 93.5% from simulated seawater, corresponding to a high Li/Mg selectivity factor of about 500.000:1. Benefiting from a low operating voltage, 1 g of lithium is extracted with only 2.5 Wh of energy consumption.