Pair Distribution Function and Density Functional Theory Analyses of Hydrogen Trapping by γ-MnO2

Control of H-Related Defects in γ-MnO2 in a Hydrothermal Synthesis

Manganese dioxide is a good candidate for effective energy storage and conversion as it possesses rich electrochemistry. The compound also shows a wide polymorphism. The γ-variety, an intergrowth of β- and R-MnO2, has been extensively studied in several types of batteries (e.g., Zn/MnO2, Li-ion) and is a common electrode material for commercial batteries. It is well known that the insertion of protons thermodynamically stabilizes γ-MnO2 with respect to β-MnO2. Protons can enter the structure either by forming groups of 4 hydroxyls around a Mn4+ vacancy, called a Ruetschi defect, or by forming a hydroxyl group near a Mn3+ ion, called a Coleman defect. These defects differently affect the electrochemistry of manganese oxide, and tailoring their amount in the structure can be used to tune the material properties. Previous studies have addressed the proton insertion process, but the role of the synthesis pathway on the amount of defects created is not well understood. We here investigate how the parameters in a hydrothermal synthesis of γ-MnO2 nanoparticles influence the amount and type of H-related defects. Structural investigations are carried out using Pair Distribution Function analysis, X-ray absorption spectroscopy, thermogravimetric analysis, and inelastic neutron scattering. We demonstrate the possibility to control the amount and type of defects introduced during the synthesis. While the amount of Ruetschi defects increases with synthesis temperature, it decreases with extended synthesis time, along with the amount of Coleman defects. Moreover, we discuss the arrangement of the defects in the γ-MnO2 nanoparticles.

Characterisation of intergrowth in metal oxide materials using structure-mining: the case of γ-MnO2

Manganese dioxide compounds are widely used in electrochemical applications e.g. as electrode materials or photocatalysts. One of the most used polymorphs is γ-MnO₂ which is a disordered intergrowth of pyrolusite β-MnO₂ and ramsdellite R-MnO₂. The presence of intergrowth defects alters the material properties, however, they are difficult to characterise using standard X-ray diffraction due to anisotropic broadening of Bragg reflections. We here propose a characterisation method for intergrown structures by modelling of X-ray diffraction patterns and pair distribution functions (PDF) using γ-MnO₂ as an example. Firstly, we present a fast peak-fitting analysis approach, where features in experimental diffraction patterns and PDFs are matched to simulated patterns from intergrowth structures, allowing quick characterisation of defect densities. Secondly, we present a structure-mining-based analysis using simulated γ-MnO₂ superstructures which are compared to our experimental data to extract trends on defect densities with synthesis conditions. We applied the methodology to a series of γ-MnO₂ samples synthesised by a hydrothermal route. Our results show that with synthesis time, the intergrowth structure reorders from a R-like to a β-like structure, with the β-MnO₂ fraction ranging from ca. 27 to 82% in the samples investigated here. Further analysis of the structure-mining results using machine learning can enable extraction of more nanostructural information such as the distribution and size of intergrown domains in the structure. Using this analysis, we observe segregation of R- and β-MnO₂ domains in the manganese oxide nanoparticles. While R-MnO₂ domains keep a constant size of ca. 1–2 nm, the β-MnO₂ domains grow with synthesis time.

A methodology for characterisation of γ-MnO₂ intergrowths has been developed. By combining supercell modelling, structure-mining and machine learning, both qualitative and quantitative information on intergrowth domain distributions are extracted.

Adsorption of Greenhouse Decomposition Products on Ag2O-SnS2 and CuO-SnS2 Surfaces

In this paper, based on density functional theory, the adsorption mechanism and gas sensitivity of Ag₂O/CuO-modified SnS₂ were analyzed. The results were analyzed according to the adsorption energy, total density of states, partial density of states, and frontier molecular orbital theory. The results show that the adsorption of all gas molecules is exothermic. NH₃, Cl₂, and C₂H₂ gases are chemisorbed on the modified SnS₂ surfaces. After gas adsorption, the energy gap of the base changes by more than 10%, which fully shows that the conductivity changes greatly after gas adsorption, which can be reflected in the macroscopic resistance change. Ag₂O-SnS₂ is suitable as a gas sensor for NH₃ gas sensors in terms of moderate adsorption distance, large adsorption energy, charge transfer, and frontier molecular orbital theory, while CuO-SnS₂ is more suitable as a C₂H₂ gas sensor.

Pencil-Drawing Skin-Mountable Micro-Supercapacitors

In this study, integrated plaster-like micro-supercapacitors based on medical adhesive tapes are fabricated by a simple pencil drawing process combined with a mild solution deposition of MnO₂ . These solid micro-supercapacitors not only exhibit excellent stretchability, flexibility, and biocompatibility, but also possess outstanding electrochemical performances, such as exceptional rate capability and cycling stability. Hence they may act as skin-mountable and thin-film energy storage devices of high efficiency to power miniaturized and wearable electronic devices.