Energy Materials

Due to their various applications, metal oxides are of high interest for fundamental research and commercial usage. Per applications as catalysts or electrochemical devices, the tailored design of metal oxides featuring a high specific surface area and additional functionalities is of the utmost importance for the performance of the resulting materials. We report a new method for preparing free-standing films consisting of hierarchically porous metal oxides (titanium and niobium based) by combining emulsion polymerization and shear-induced monodisperse particle self-assembly in the presence of sol–gel precursors. After thermal treatment, the resulting porous materials can be used as electrodes in Li-ion batteries. The titanium and niobium sol–gel precursors were partially immobilized to the surface of organic core–interlayer particles featuring hydroxyl groups to obtain hybrid organic–inorganic particles through the melt–shear organization process. Free-standing particle-based films, in analogy to elastomeric opal films and colloidal crystals, can be prepared in a convenient one-step preparation process. After thermal treatment, ordered pores are obtained, while the pristine metal oxide precursor shell can be converted to the (mixed) metal oxide matrix. Heat treatment under CO2 leads to mixed-TiNb oxide/carbon hybrid materials. The highly porous derivative structure enhances electrolyte permeation. When tested as Li-ion battery electrodes, it shows a specific capacity of 335 mAh·g–1 at a rate of 10 mA·g–1. After 1000 cycles at 250 mA·g–1, the electrodes still provided a specific capacity of 191 mAh·g–1.

Pillared Ti3C2Tz MXene with a large interlayer spacing (1.75 nm) is shown to be promising for high-power Li-ion batteries. Pillaring dramatically enhances the electrochemical performance, with superior capacities, rate capability, and cycling stability compared to the nonpillared material. In particular, at a high rate of 1 A g–1, the SiO2-pillared MXene has a capacity over 4.2 times that of the nonpillared material. For the first time, we apply in situ electrochemical dilatometry to study the volume changes within the MXenes during (de)lithiation. The pillared MXene has superior performance despite larger volume changes compared to the nonpillared material. These results give key fundamental insights into the behavior of Ti3C2Tz electrodes in organic Li electrolytes and demonstrate that MXene electrodes should be designed to maximize interlayer spacings and that MXenes can tolerate significant initial expansions. After 10 cycles, both MXenes show nearly reversible thickness changes after the charge–discharge process, explaining the stable long-term electrochemical performance.

Extraordinarily homogeneous, freestanding titania-loaded carbon spherogels can be obtained using Ti(acac)2(OiPr)2 in the polystyrene sphere templated resorcinol-formaldehyde gelation. Thereby, a distinct, crystalline titania layer is achieved inside every hollow sphere building unit. These hybrid carbon spherogels allow capitalizing on carbon's electrical conductivity and the lithium-ion intercalation capacity of titania.

Our investigations into molecular hydrogen (H2) confined in microporous carbons with different pore geometries at 77 K have provided detailed information on effects of pore shape on densification of confined H2 at pressures up to 15 MPa. We selected three materials: a disordered, phenolic resin-based activated carbon, a graphitic carbon with slit-shaped pores (titanium carbide-derived carbon), and single-walled carbon nanotubes, all with comparable pore sizes of <1 nm. We show via a combination of in situ inelastic neutron scattering studies, high-pressure H2 adsorption measurements, and molecular modelling that both slit-shaped and cylindrical pores with a diameter of ∼0.7 nm lead to significant H2 densification compared to bulk hydrogen under the same conditions, with only subtle differences in hydrogen packing (and hence density) due to geometric constraints. While pore geometry may play some part in influencing the diffusion kinetics and packing arrangement of hydrogen molecules in pores, pore size remains the critical factor determining hydrogen storage capacities. This confirmation of the effects of pore geometry and pore size on the confinement of molecules is essential in understanding and guiding the development and scale-up of porous adsorbents that are tailored for maximising H2 storage capacities, in particular for sustainable energy applications.

Electrochemical processes enable fast lithium extraction, for example, from brines, with high energy efficiency and stability. Lithium iron phosphate (LiFePO4) and manganese oxide (λ-MnO2) have usually been employed as the lithium gathering electrode material. Compared with λ-MnO2, LiFePO4 has a higher theoretical capacity and lower lithium insertion potential but suffers from low performance stability. Therefore, exploring the reason for capacity fading and putting forward an effective approach to address this issue is important. In this work, we studied the effect of additional present cations and dissolved oxygen on the stability of LiFePO4, using a rocking chair cell configuration to eliminate the effect of the other electrode. We found that adding Ca2+ to the solution and dissolved oxygen aggravate the capacity fading of LiFePO4, whereas Na+ and Mg2+ do not show an obvious influence on the stability of LiFePO4. By continuous nitrogen-flushing of the electrolyte and carbon coating of the electrode material, the stability of LiFePO4 was significantly enhanced. The lithium extraction capacity of LiFePO4/C is 21 mgLi gelectrode−1 with an energy consumption of 3.03 ± 0.5 W h molLi−1 and capacity retention of 82% in 10 cycles in 5 mM LiCl + 50 mM NaCl solution at a cell voltage range of −0.5 V to +0.5 V.

Abstract High-entropy materials (HEMs) with promising energy storage and conversion properties have recently attracted worldwide increasing research interest. Nevertheless, most research on the synthesis of HEMs focuses on a “trial and error” method without any guidance, which is very laborious and time-consuming. This review aims to provide an instructive approach to searching and developing new high-entropy energy materials in a much more efficient way. Toward materials design for future technologies, a fundamental understanding of the process/structure/property/performance linkage on an atomistic level will promote prescreening and selection of material candidates. With the help of computational material science, in which the fast development of computational capabilities that have a rapidly growing impact on new materials design, this fundamental understanding can be approached. Furthermore, high-throughput experimental methods, enabled by the advances in instrumentation and electronics, will accelerate the production of large quantities of results and stimulate the identification of the target products, adding knowledge in computational design. This review shows that combining computational preselection and verification by high-throughput can be an efficient approach to unveil the complexities of HEMs and design novel HEMs with enhanced properties for energy-related applications.

Quinone-containing materials have attracted significant attention for energy storage and electroswing carbon capture. Tailored redox-responsive core–shell particles are obtained in the present work via semicontinuous starved-feed emulsion polymerization and subsequent postmodification strategies with redox-responsive quinone moieties. The use of glycidyl methacrylate within the shell material offers the possibility of a ring-opening reaction with the redox-responsive 2-aminoanthraquinone (2-AAQ), which possesses a high affinity toward electrophilic carbon dioxide. The successful preparation of monodisperse particles, an essential prerequisite for colloidal self-assembly, was investigated by dynamic light scattering and transmission electron microscopy. The presence of reactive epoxy functionalities was achieved by the ring-opening reaction with the Preussmann reagent. Postsynthesis modification was investigated using X-ray photoelectron spectroscopy and cyclic voltammetry measurements. The redox-responsive core–shell particles were subjected to the melt-shear organization technique to prepare free-standing opal films featuring structural colors. The monodisperse 2-AAQ-containing particles were investigated for self-assembly inside conductive carbon felts, and their electrochemically mediated carbon capture capabilities were studied.

Abstract In this work, the preparation and fabrication of elastomeric opal films revealing reversible mechanochromic and pH-responsive features are reported. The core–interlayer–shell (CIS) particles are synthesized via stepwise emulsion polymerization leading to hard core (polystyrene), crosslinked interlayer (poly(methyl methacrylate-co-allyl methacrylate), and soft poly(ethyl acrylate-co-butyl acrylate-co-(2-hydroxyethyl) methacrylate) shell particles featuring a size of 294.9 ± 14.8 nm. This particle architecture enables the application of the melt-shear organization technique leading to elastomeric opal films with orange, respectively, green brilliant reflection colors dependent on the angle of view. Moreover, the hydroxyl moieties as part of the particle shell are advantageously used for subsequent thermally induced crosslinking reactions enabling the preparation of reversibly tunable mechanochromic structural colors based on Bragg's law of diffraction. Additionally, the CIS particles can be loaded upon extrusion or chemically by a postfunctionalization strategy with organic dyes implying pH-responsive features. This convenient protocol for preparing multi-responsive, reversibly stretch-tunable opal films is expected to enable a new material family for anti-counterfeiting applications based on external triggers.

Abstract The synthesis and characterization of polyferrocenylmethylene (PFM) starting from dilithium 2,2-bis(cyclopentadienide)propane and a Me2C[1]magnesocenophane is reported. Molecular weights of up to Mw = 11 700 g mol–1 featuring a dispersity, Ð, of 1.40 can be achieved. The material is studied by different methods comprising nuclear magnetic resonance (NMR) spectroscopy, matrix-assisted laser desorption/ionization time of flight (MALDI-ToF) mass spectrometry, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) measurements elucidating the molecular structure and thermal properties of these novel polymers. Moreover, cyclic voltammetry (CV) reveals quasi-reversible oxidation and reduction behavior and communication between the iron centers. Also, the crystal structure of a related cyclic hexamer is presented.

Subnanometer pores of carbon discriminate against ions based on their size. Capitalizing on such nuanced differences enables ion separation via charge/discharge cycling during ion electrosorption. Different ion uptake capacities in aqueous media with multiple, competing ions are also of high importance to understand capacitive deionization of surface water or industrial process water. In our experiments, we observed divalent cations sieving in pores smaller than 0.6 nm. By applying this phenomenon, a desalination cell with online concentration monitoring was used to study the ion-selectivity. We concluded that in pores below 0.6 nm, divalent Mg2+ and Ca2+ are entirely blocked, and the K+ over Na+ selectivity corresponds with their size ratio. Larger micropores show a preference for divalent cations with higher charge numbers. In both materials, a dynamic monovalent cation and divalent cation replacement dependent on the potential variation is observed.