Erscheinungsjahr 2023

In metal-air batteries, the integrated solid-state cathode is considered a promising design because it can solve the problem of high interfacial resistance of conventional solid-state cathodes. However, solid discharge products cannot be efficiently decomposed in an integrated solid-state cathode, resulting in batteries that are unable to operate for long periods of time. Herein, an integrated solid-state cathode (Gel-Tempo cathode) of sodium-air batteries (SABs) capable of promoting efficient decomposition of discharge product Na2O2 is designed. The Gel-Tempo cathode is synthesized by cationic-π interaction of redox mediator 2,2,6,6-tetramethyl-1-piperidinyloxy (Tempo) and ionic liquid with carbon nanotubes. The Gel-Tempo cathode serves multiple functions as a redox mediator, flame retardancy, and high stability to air. In quasi-solid-state SABs, the Gel-Tempo cathode reduces overpotential to 1.15 V and improves coulomb efficiency to 84.5% (at a limited discharge capacity of 3000 mAh g−1) compared to gel cathodes. Experiments and density functional theory calculations indicate that Tempo significantly reduces the Gibbs free energy in the decomposition reaction of Na2O2, and high Tempo content is more conducive to enhancing the decomposition kinetics of Na2O2 and hence resulting in an ultra-long cycle life (1746 h). This work is crucial to promote practical applications of SABs, providing guidelines for functionalization design of integrated solid-state cathodes for metal-air batteries.

This paper provides a fundamental understanding of “false friend” formation, i.e., hidden defects associated with lack of fusion, using an experimental setup that allowed an insight into the processing zone based on high-speed synchrotron X-ray imaging. The setup enabled the welding of a lap joint of AISI 304 high-alloy steel sheets (X5CrNi18-10/1.4301), with the ability to adjust different gap heights between top and bottom sheet (up to 0.20 mm) and to acquire high-speed X-ray images at 100 kHz simultaneously with the welding process. On this basis, a time-resolved description of the “false friend” formation can be provided by visualizing the interaction between keyhole and melt pool during laser welding and solidification processes within the gap area. The bridgeability of the gap was limited due to the gap height and insufficient melt supply leading to the solidification of the bridge. The distance between the solidified bridge and the keyhole increased with time, while the keyhole and melt pool dynamics initiated the formation of new melt bridges whose stability was defined by melt flow conditions, surface tension, and gap heights. The alternating formation and solidification of melt bridges resulted in entrapped areas of lacking fusion within the weld, i.e., “false friends.” Finally, based on the results of this study, a model concept is presented that concludes the main mechanisms of “false friend” formation.

Identifying an effective electrolyte is a primary challenge for hybrid ion capacitors, due to the intricacy of dual-ion storage. Here, this study demonstrates that the electrochemical behavior of graphite oxide in ether-solvent electrolyte outperforms those in ester-solvent electrolytes for the cathode of potassium-ion hybrid capacitor. The experimental and theoretical assessments verify that the anion and cation are isolated effectively in dimethyl ether, endowing a weaker interaction between cations and anions compared to that of ester-solvent electrolytes, which facilitates the dual-ion diffusion and thus enhances the electrochemical performance. This result provides a rational strategy to realize high-rate cations and anions storage on the carbon cathode. Furthermore, a new low-cost and high-performance capacitor prototype, modified graphite oxide (MGO) cathode versus pristine graphite (PG) in ether-solvent electrolyte (MGOǁDMEǁPG), is proposed. It exhibits a high energy density of 150 Wh kg^−1cathode at a high power density of 21443 W kg^−1cathode (calculation based on total mass: 60 Wh kg^−1 at 8577 W kg^−1).

The threshold for the onset of thermocapillary flow in a planar liquid layer heated from below is increased by a vertical magnetic field when the liquid is a good electric conductor. The magnetic damping effect is reduced when the induced eddy currents are blocked by insulating side walls. Neutral conditions for this specific Bénard-Marangoni stability problem with a vertical field and side walls are obtained numerically for three-dimensional perturbations assumed periodic in one horizontal direction. The domain is bounded by a free-slip wall at the bottom, a free surface at the top and two free-slip lateral walls in the other horizontal direction. Buoyancy forces and surface deformations are neglected and a constant heat flux is imposed on the free surface. Upon increasing the magnetic induction, the least stable modes become localized near the side walls and the convective threshold increases at a lower rate than for the least stable bulk mode.

With more than 50% of the world’s population living in urban areas, the smart city concept has been introduced as a solution to urbanization problems, with a focus on technological and social innovation. However, critics argue that the concept is more about marketing than actual benefits for citizens. Given the limitations of conventional and formalized e-participation and smart city procedures, we highlight the value of shared citizen knowledge and the potential of e-interaction in this context by analyzing city-related informal social media communication, following recent calls to embrace citizens’ opinions in the smart city framework. This work focuses on major German cities with more than 100,000 inhabitants. The authors identify nine categories of interest in citizens’ discussions. Unlike official channels, citizens tend to focus on social and societal issues. The results of this study can complement existing tools by including citizens’ perspectives in smart city decision-making processes.