Stabilization of cryptomelane α-MnO2 nanowires tunnels widths for enhanced electrochemical energy storage

Restricted and epitaxial growth of MnO2-x nano-flowers in/out carbon nanofibers for long-term cycling stability supercapacitor electrodes

Carbon nanofibers (CFs) have been widely applied as electrodes for energy storage devices owing to the features of increased contact area between electrodes and electrolyte, and shortened transmission route of electrons. However, the poor electrochemical activity and severe waste of space hinder their further application as supercapacitors electrodes. In this work, MnO2-x nanoflowers restricted and epitaxial growth in/out carbon nanofibers (MnO2/MnO@CF) were prepared as excellent electrode materials for supercapacitors. With the synergistic effect of uniquely designed structure and the introduction of MnO and MnO2 nanoflowers, the prepared interconnected MnO2/MnO@CF electrodes demonstrated satisfactory electrochemical performance. Furthermore, the MnO2/MnO@CF//activated carbon (AC) asymmetric supercapacitor offered an outstanding long-term cycle stability. Besides, kinetic analysis of MnO2/MnO@CF-90 was conducted and the diffusion-dominated storage mechanism was well-revealed. This concept of "internal and external simultaneous decoration" with different valence states of manganese oxides was proven to improve the electrochemical performance of carbon nanofibers, which could be generalized to the preparation and performance improvement of other fiber-based electrodes.

Accelerating the Low-Temperature Catalytic Oxidation of Acetone over Al-Substituted Mn-Al Oxides by Rate-Limiting Step Modulation

In order to enhance the catalytic activity and improve the stability of Mn-Al oxides in acetone oxidation, it is interesting to have found that modulating and accelerating the rate-limiting step by Al substitution rather than just mixing of Mn and Al is crucial for hydrocarbon efficient catalytic destruction. Here, a series of Mn-Al oxides with different Al substitution ratios were prepared by a scalable and facile hydrothermal-redox strategy. The reaction rate, selectivity, and stability of the representative α-MnO₂ catalyst in acetone oxidation can be remarkably promoted by simple replacing of the partial framework Mn with Al, which changes the rate-limiting step from acetic acid dissociation to ethanol decomposition accelerated by H₂O molecules. Among them, MnAl_0.5 displays the best catalytic performance with 90% of acetone converted at just 165 °C and a remarkable CO₂ yield. DFT results suggest that the p_y and p_x orbitals of the O element take part in the formation of the carbonyl group when the intermediate of removing H* from ethanol reacts with the hydroxyl group of H₂O. The d_xz orbital of Mn with p-electron of Al plays a vital role in the rate-limiting step. The work provides new insights into engineering catalysts for industrial VOC efficient and economical mineralization.

Cation intercalated one-dimensional manganese hydroxide nanorods and hierarchical mesoporous activated carbon nanosheets with ultrahigh capacitance retention asymmetric supercapacitors

We have reported the electrochemical performance of K⁺ ion doped Mn(OH)₄ and MnO₂ nanorods as a positive electrode and a highly porous activated carbon nanosheet (AC) made from Prosopis Juliflora as negative electrode asymmetric supercapacitor (ASC) with high rate capability and capacity retention. The cation K⁺ doped Mn(OH)₄ and MnO₂ nanorods with large tunnel sizes allow the electrolyte to penetrate through a well-defined pathway and hence benefits from the intercalation pseudocapacitance and surface redox reactions. As a result, they exhibit good electrochemical performance in neutral aqueous electrolytes. More specifically, the K⁺-Mn(OH)₄ nanorods exhibit higher capacitance values than K⁺-MnO₂ nanorods due to the homogenous distribution of 1D nanorods and optimum amount of OH bonds. The fabricated K⁺-Mn(OH)₄ symmetric electrochemical Pseudocapacitor shows very high energy density of 10.11 Wh/kg and high-power density of 51.04 W/kg over the range of 1.0 V in aqueous electrolyte. The energy density of AC||K⁺-Mn(OH)₄ ASC is improved significantly compared to those of symmetric supercapacitors. The fabricated ASC exhibits a wide working voltage window (1.6 V), high power (143.37 W/kg) and energy densities (41.38 Wh/kg) at 0.2 A g^-1, and excellent cycling behavior with 107.3% capacitance retention after 6000 cycles at 2 A g^-1 indicating the promising practical applications in electrochemical supercapacitors.