New paper published in ChemSusChem in collaboration with the Kickelbick-Group at Saarland University. We have developed an exciting new class of inorganic-organic hybrid materials with redox-active components that have great potential for use in lithium-ion batteries (LIBs). The materials were prepared using an aqueous precipitation reaction of ammonium heptamolybdate (AHM) with para-phenylenediamine (PPD), and a low-energy continuous wet chemical synthesis process known as the microjet process. By varying the ratio of molybdate to organic ligand and pH, we were able to produce two different crystalline hybrid products with large surface areas in the submicrometer range and high purity and reproducibility on a large scale. The first product, [C6H10N2]2[Mo8O26] ⋅ 6 H2O, was obtained by using a ratio of para-phenylenediamine to ammonium heptamolybdate from 1 : 1 to 5 : 1. The second product, [C6H9N2]4[NH4]2[Mo7O24] ⋅ 3 H2O, was obtained by using higher PPD ratios from 9 : 1 to 30 : 1. Our electrochemical testing revealed that the second product showed exceptional battery performance, with a high capacity of 1084 mAh/g at 100 mA/g after 150 cycles. The product reached maximum capacity after an induction phase, which can be explained by a combination of a conversion reaction with lithium to Li2MoO4 and an additional in situ polymerization of PPD. We are excited about the potential of this hybrid material for use in LIB applications.
New paper published in Carbon. This is our latest work from our collaboration with our Austrian partners now being published in the January issue of Carbon. Combining the expertise in catalysis of the Eder group (TU Vienna) with the innovative carbon spherogel material developed by Michael Elsaesser from the Hüsing Group (Paris Lodron Universität Salzburg) makes up for an interesting system. Spherogels are hollow carbon spheres which, as shown by this work, can be conveniently loaded with electrocatalytically active species, such as titania. In our case, we studied the photocatalytic hydrogen evolution.
After a lot of work by our Ph.D. students Stefanie Arnold and Lei Wang, our invited article in the Wiley journal SMALL on the dual-use of seawater batteries for energy storage and water desalination. Seawater batteries are a unique type of device that capitalizes, in the most common design, on a ceramic separator, an electrocatalytic reaction on one electrode side, and reversible sodium-ion electrochemistry on the other side. Since simple seawater can be used as the aqueous electrolyte in the system, the system has become known as seawater batteries. In our opinion, this technology can do more than contribute toward beyond-lithium energy storage by also being used for desalination.
New paper published in ACS Omega. Batteries employing transition-metal sulfides enable high-charge storage capacities, but polysulfide shuttling and volume expansion cause structural disintegration and early capacity fading. The design of heterostructures combining metal sulfides and carbon with an optimized morphology can effectively address these issues. Our work introduces dopamine-coated copper Prussian blue (CuPB) analogue as a template to prepare nanostructured mixed copper–iron sulfide electrodes. The material was prepared by coprecipitation of CuPB with in situ dopamine polymerization, followed by thermal sulfidation. Dopamine controls the particle size and favors K-rich CuPB due to its polymerization mechanism. While the presence of the coating prevents particle agglomeration during thermal sulfidation, its thickness demonstrates a key effect on the electrochemical performance of the derived sulfides. After a two-step activation process during cycling, the C-coated KCuFeS2 electrodes showed capacities up to 800 mAh/g at 10 mA/g with nearly 100% capacity recovery after rate handling and a capacity of 380 mAh/g at 250 mA/g after 500 cycles.
New paper published in Electrochimica Acta on Ni-decorated AgAu alloy graphene/cobalt hydroxide electrodes for micro-supercapacitors to obtain high-performance micro-supercapacitors. A nanocomposite of graphene, cobalt hydroxide and nickel can was obtained from using gold-silver alloy lines. Using a two-step electrodeposition method, the scaly morphology is pre-deposited on a Ni film, followed by the interconnecting corrugated graphene/cobalt hydroxide composite nanomaterial. The resulting device, a graphene/cobalt hydroxide/Ni//activated carbon flexible micro-supercapacitor (MSC), was assembled by gel KOH-PVA electrolyte, graphene/cobalt hydroxide/Ni (positive electrode), and activated carbon (negative electrode). When testing, we obtained a volumetric energy of about 19 mWh/cm3 and the devices retained over 94% capacitance after 10,000 cycles. After 1,000 continuous bending/unbending cycles at a 180° bending angle with the frequency of 100 mHz, the capacitance retention of MSC is still maintained at 97% of the initial value.
new paper published in ACS Energy Letters on continuous electrochemical lithium-ion extraction. We used a redox electrolyte “engine” to drive the ion transfer (in our case: potassium ferricyanide). Employing a pair of ceramic lithium superionic conductor (LISICON) membranes meant that only Lithium ions were accessible to the redox electrolyte for charge compensation. And to complement the design, we used an anion exchange membrane to separate the inflow (e.g., seawater) from a recovery solution. By this way, we obtained an electrochemical system for the continuous extraction of Lithium ions. This sets this technology apart from earlier works (including our contributions) that relied on a cyclic operation to obtain ion separation. Yet, this is just one of many more steps towards seeing such technology toward application; future research must critically address cell design, optimization of the Li-membranes, and investigating the robustness and durability of continuous operation.
This work was the result of the collaboration of our Ph.D. students Lei Wang, Stefanie Arnold, Panyu Ren, and our former Postdoc (now group leader at Bavarian Center for Battery Technology (BayBatt)) Qingsong Wang, as well as our Chinese collaborators Jun Jin and Zahoyin Wen (Chinese Academy of Sciences).
New paper published in Materials Futures. Sodium-deficient, P2-type layered oxides are promising cathodes for sodium-ion batteries. Their open sodium cation transport pathways 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. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 V to 4.6 V. Advanced operando techniques and post-mortem analysis were used to thoroughly probe the underlying reaction mechanism. 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.
New paper published in Desalination on the ion selectivity of carbon nanopores. It is well known that electrolyte confinement inside carbon nanopores strongly affects ion electrosorption in capacitive deionization. A thorough understanding of the intricate pore size influence enables enhanced charge storage performance and desalination in addition to ion separation. In subnanometer pores, where the pore size is smaller than hydrated ion size, a dehydration energy barrier must be overcome before the ions can be electrosorbed into the pores. Ion sieving is observed when the dehydration energy is larger than the applied energy. However, when a high electrochemical potential is used, the ions can desolvate and enter the pores. Capitalizing on the difference in size and dehydration energy barriers, this work applies the subnanometer porous carbon material, and a high electrochemical ion selectivity for Cs+ and K+ over Na+, Li+, Mg2+, and Ca2+ is observed. This establishes a viable way for selective heavy metal removal by varying pore and solvated ion sizes. Our work also shows the transition from double-layer capacitance to diffusion-limited electrochemical features in narrow ultramicropores.
New paper published in Current Opinion in Green and Sustainable Chemistry on “Recent advances in wastewater treatment using semiconductor photocatalysts”. Can’t decide if you like water remediation or photocatalysis/semiconductors more? They both make a great match! Read about synergies and future possibilities in our latest review article in Current Opinion in Green and Sustainable Chemistry. The team of Prof. Xiao Su (Jaeyoung Hong & Ki-Hyun Cho) and I explore this interesting interfacial research – interfacial in double meaning: at the interface of fluid and solid, and at the interface of material science/electrochemistry and water research. It is exciting to explore semiconductors, for example, to target emerging contaminants, such as perfluorinated compounds.
New paper entitled “Spray-dried pneumococcal membrane vesicles are promising candidates for pulmonary immunization” published in the International Journal of Pharmaceutics. This collaborative work spearheaded by experts from the Helmholtz Institute for Pharmaceutical Research Saarland (HIPS) and Saarland University explores optimized vaccine microparticles with a mean particle size of 1–2 µm, corrugated surface, and nanocrystalline nature.
New paper published in Advanced Sustainable Systems on the ion selectivity of MXene electrodes during electrochemical operation. The Tortoise and the Hare is a classic Aesop fable that we heard growing up. We learned that a clever strategy could win over physical advantages in any match of unequal rivals. In the day and age of MXene, this fable returns when we explore MXenes as an electrode material for ion separation. The structure of MXenes impacts ion preference, as was shown before. For example, Amir Razmjou’s team wonderfully investigated the d-layer spacing’s effect on ion selectivity (10.1016/j.memsci.2021.119752). Our work now explores the ion exchange within the nanoconfined electrolyte space provided by MXene layers. We see that monovalent ions like potassium are initially preferred – only to be replaced over time by bivalent ions, like Magnesium. MXene behaves thereby like carbon nanopores, for which such ion exchange processes during continued charging were demonstrated before, among others, by the team of Maarten Biesheuvel (10.1016/j.jcis.2012.06.022). The combination of kinetic and intrinsic ion selectivity may enable novel applications within the energy/water research nexus. However, a higher ion selectivity will have to be enabled for industrial applications.
New paper published in the Journal of Materials Chemistry A. Our work entitled “Design of high-performance antimony/MXene hybrid electrodes for sodium-ion batteries” explores the synergy of the 2D nanomaterial MXene (conductive, nanotextured) and antimony (large sodium-ion storage capacity via alloying). This work is the latest outcome of our collaboration with the group and team of Riccardo Ruffo (Università degli Studi di Milano-Bicocca) and Stefano Marchionna (Ricerca sul Sistema Energetico). Special thanks to visiting Ph.D. student Antonio Gentile from Riccardo’s team!
New paper published in Nature Energy entitled “Continuous transition from double-layer to Faradaic charge storage in confined electrolytes”. Our paper explores the fascinating world from ion electrosorption transitioning towards Faradaic processes when electrolytes are nanoconfined. This work was a collaboration with several groups, espcially the team of Veronica Augustyn (NC State), Yury Gogotsi (Drexel), Patrice Simone (Toulouse) and more.
New paper published in Chemical Engineering Journal. The work with the title “Electro-assisted removal of polar and ionic organic compounds from water using activated carbon felt” was done in collaboration with the Department of Environmental Engineering at the UFZ site in Leipzig.
New paper published in Macromolecular Rapid Communications. The paper is entitled “Nanoporous block copolymer membranes with enhanced solvent resistance via UV-mediated cross-linking strategies” and was done in collaboration with the Gallei Group at Saarland University and partners at University of Freiburg and TU Darmstadt. The work is featured on the front cover of the journal.
New paper published in Advanced Energy Materials. Our work used densely carboxylated but conducting
graphene derivative (graphene acid) to enable effective operation without compromising the mechanical or chemical stability of the electrode. In our studies, we found a maximum performance of 800 mAh/g at a rate of 0.05 A/g and 174 mAh/g at 2.0 A/g. This Czech-German work was done in strong collaboration especially with the team of Aristides Bakandritsos from the Technical University of Ostrava.
New review paper published in Electrochemistry Communications. Energy-efficient technologies for the remediation of water and the generation of drinking water are essential to sustainable technologies. However, we cannot have sustainable energy technology without sustainable water remediation (and vice versa). Among many possible applications, large-scale seawater desalination is a much-needed step towards large-scale hydrogen generation via power-to-gas. However, this can only be considered sustainable when done effectively and energy-efficiently. 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 joint Israeli-German review article explores the concept of desalination fuel cells and capacitive-Faradaic fuel cells for ion separation. This work was done in collaboration with the research teams of Matthew Suss and of Yuri Gendel (both at Technion, Israel).
New paper published in Cell Press Physical Science on the use of sub-nanometer pores for capacitive deionization to enable membrane-free seawater desalination. Big pores are mighty powerful when it comes to capacitive deionization (CDI). CDI is highly appreciated as a potentially energy-efficient desalination technology, rendering saline water into desalinated (potable/processable) water. However, once we move from saline media with low salt concentrations (like in brackish water regimes) towards higher salt concentrations (as you find in ordinary seawater), CDI become less attractive: the desalination capacity and charge efficiency (think of it as salt removal per invested charge) drop drastically. This issue is linked to the limited permselectivity of carbon pores commonly found in CDI electrodes. Put simply: the invested charge is not only used for adding “extra “ions into the pore (thereby: lowering the feedwater ion concentration) but also to eject ions that are already inside the pore (which basically increases the ion concentration in the effluent stream). We can address this issue by implementing an ion exchange membrane (adding costs and a more complex design) or using charge-transfer materials (giving rise to desalination batteries). But is there a way to keep low-cost, nanoporous carbon and still enable direct, membrane-free seawater desalination? The answer is a resounding YES. In our 2020 paper in Sustainable Energy & Fuels, we showed already the proof of concept of using quasi-ionophobic, and thereby permselective, carbon pores. Now, our work extends the scope and demonstrates this effect’s intricate pore size dependency. The key is a subtle play of pore size and hydrated ion diameter, which allows the pore to only uptake (extra) ions once an electric potential is applied. This work was a great collaboration with the team of Guang Feng at HUST, China, and Christian Prehal at ETH Zürich that puts together simulation and experimental work.
New paper published in Desalination with the title “Particle size distribution influence on capacitive deionization: Insights for electrode preparation”. Our work explores the particle size dispersity of commercially available activated carbon. No activated carbon powder is “perfect”, that is, every powder contains (a bit) larger and smaller particles. Size separation allows to capitalize on “one powder – several sizes” aspect. Comparing mixed-sized, small-size, and large-size activated carbon classes (of the same activated carbon powder), our work shows that large particles suffer from ion transport limitation, but so do electrodes composed of (well-packed) small particles. The best performance was found to be in the middle: a hierarchic mixture of larger and smaller activated carbon particles.
New paper published in ACS Applied Materials & Interfaces. This work in collaboration with the teams of Markus Gallei and Guido Kickelbick (both at Saarland University) explores shear-induced co-assembly as a step towards creating unique (ordered) materials. The latter can be conveniently converted to metal oxide / carbon hybrids via thermal annealing. For example, titanium niobium oxide / carbon obtained this way profided 335 mAh/g at 10 mA/g and a capacity retention of 84% after 1000 cycles at 250 mA/g.