Exposed crystal facets directly affect the electrochemical/catalytic performance of MnO₂ materials during their applications in supercapacitors, rechargeable batteries, and fuel cells. Currently, the facet-controlled synthesis of MnO₂ is facing serious challenges due to the lack of an in-depth understanding of their surface evolution mechanisms. Here, combining aberration-corrected scanning transmission electron microscopy (STEM) and high-resolution TEM, we revealed a mutual energy-driven mechanism between beta-MnO₂ nanowires and microstructures that dominated the evolution of the lateral facets in both structures. The evolution of the lateral surfaces followed the elimination of the {100} facets and increased the occupancy of {110} facets with the increase in hydrothermal retention time. Both self-growth and oriented attachment along their {100} facets were observed as two different ways to reduce the surface energies of the beta-MnO₂ structures. High-density screw dislocations with the 1/2 <100> Burgers vector were generated consequently. The observed surface evolution phenomenon offers guidance for the facet-controlled growth of beta-MnO₂ materials with high performances for its application in metal-air batteries, fuel cells, supercapacitors, etc.

The discharge and charge mechanisms of rechargeable Li-O₂ batteries have been the subject of extensive investigation recently. However, they are not fully understood yet. Here we report a systematic study of the morphological transition of Li₂O₂ from a single crystalline structure to a toroid like particle during the discharge–charge cycle, with the help of a theoretical model to explain the evolution of the Li₂O₂ at different stages of this process. The model suggests that the transition starts in the first monolayer of Li₂O₂, and is subsequently followed by a transition from particle growth to film growth if the applied current exceeds the exchange current for the oxygen reduction reaction in a Li-O₂ cell. Furthermore, a sustainable mass transport of the diffusive active species (e.g., O₂ and Li⁺) and evolution of the underlying interfaces are critical to dictate desirable oxygen reduction (discharge) and evolution (charge) reactions in the porous carbon electrode of a Li-O₂ cell.

The present study explored a new method to improve the catalytic activity of non-precious metals, especially in electrochemical reactions. Highly ionized Fe plasma produced by arc discharge was uniformly deposited on a porous carbon substrate and formed atomic clusters on the carbon surface. The as-prepared FeO_x/C material was tested as a cathode material in a rechargeable Li–O₂ battery under different current rates. The results showed significant improvement in battery performance in terms of both cycle life and reaction rate. Furthermore, X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that the as-prepared cathode material stabilized the cathode and reduced side reactions and that the current rate was a critical factor in the nucleation of the discharge products.

Novel SnO_2–x/g-C₃N₄ heterojunction nanocomposites composed of reduced SnO_2–x nanoparticles and exfoliated g-C₃N₄ nanosheets were prepared by a convenient one-step pyrolysis method. The structural, morphological, and optical properties of the as-prepared nanocomposites were characterized in detail, indicating that the aggregation of g-C₃N₄ nanosheets was prevented by small, well-dispersed SnO_2–x nanoparticles. The ultraviolet–visible spectroscopy absorption bands of the nanocomposites were shifted to a longer wavelength region than those exhibited by pure SnO₂ or g-C₃N₄. The charge transfer and recombination processes occurring in the nanocomposites were investigated using linear scan voltammetry and electrochemical impedance spectroscopy. Under 30-W visible-light-emitting diode irradiation, the heterojunction containing 27.4 wt.% SnO_2–x exhibited the highest photocurrent density of 0.0468 mA·cm^–2, which is 33.43 and 5.64 times larger than that of pure SnO₂ and g-C₃N₄, respectively. The photocatalytic activity of the heterojunction material was investigated by degrading rhodamine B under irradiation from the same light source. Kinetic study revealed a promising degradation rate constant of 0.0226 min^-1 for the heterojunction containing 27.4 wt.% SnO_2–x, which is 32.28 and 5.79 times higher than that of pure SnO₂ and g-C₃N₄, respectively. The enhanced photoelectrochemical and photocatalytic performances of the nanocomposite may be due to its appropriate SnO_2–x content and the compact structure of the junction between the SnO_2–x nanoparticles and the g-C₃N₄ nanosheets, which inhibits the recombination of photogenerated electrons and holes.