Physiochemical Investigation of Shape-Designed MnO₂ Nanostructures and Their Influence on Oxygen Reduction Reaction Activity in Alkaline Solution

Abstract

In this work, five types of MnO 2 nanostructres (nanowires, nanotubes, nanoparticles, nanorods, and nanoflowers) were synthesized with a fine control over their α-crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined toward oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS), and electrochemical properties (Tafel, Koutecky−Levich plots and % of H 2 O 2 produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO 2 coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nanoshapes and follow an order nanowire > nanorod > nanotube > nanoparticle > nanoflower. α-MnO 2 nanowires possess enhanced electrocatalytic activity compared to other shapes, even though the nanotubes possess a much higher BET surface area. In the ORR studies, α-MnO 2 nanowires displayed Tafel slope of 65 mV/decade, n-value of 3.5 and 3.6% of hydrogen peroxide production. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened Mn− O bonds along with a Mn−Mn distance of 2.824 Å, which provides an optimum requirement for the adsorbed oxygen in a bridge mode favoring the direct 4 electron reduction. In accordance with the first principles based density functional theory (DFT), the enhancement in ORR activity is due to the less activation energy needed for the reaction by the (211) surface than all other surfaces.

2D and 3D Silica‐Template‐Derived MnO2 Electrocatalysts towards Enhanced Oxygen Evolution and Oxygen Reduction Activity

“…The strong absorption at 3410 cm −1 attributed to the stretching vibration of O−H which may arises from adsorbed water. Similarly, the strong broad band at 570 to 635 cm −1 in Raman spectra was correlated to the nature of the tunnel species and it belongs to A g spectroscopic species that originate from the breathing vibration of MnO 6 octahedra within a tetragonal framework . The N 2 ‐sorption studies (Figure S3) were showed a type IV isotherm with H1 hysteresis loop which further confirms the porous nature and ordered arrangement in the materials.…”

“…In the recent years, many studies have been focused on the development of bifunctional electrodes based on non‐precious materials like metal oxides, carbon supported materials and hybrid materials, by adapting different structural modifications . Among the oxides materials, manganese oxide create a center of attention more due to the merits of natural abundance, low toxicity, structural diversity and multi valance nature . It has the potential to impact in various fields like catalysis,, supercapacitors,, and metal‐air batteries, etc.…”

Facile synthesis of MnO2/rGO/Ni composite foam with excellent pseudocapacitive behavior for supercapacitors

“…In addition to this, the Raman peak at 565.5 cm À1 can be attributed to the symmetric stretching vibration (MneO) of MnO 2 [20,21], indicating the existence of MnO 2 in the composite foam, too. Furthermore, there presents a new Raman peak at 1100 cm À1 , which may be attributed to the interaction between rGO and MnO 2.…”

“…So, the development of MnO 2 /rGO nanocomposite materials could draw on each other's merits and raise the level together for the both electrode materials. Currently, many strategies have been developed for the synthesis of MnO 2 /rGO nanocomposites, such as hydrothermal route [16], solution phase approach [17,18], solidestate reaction approach [19], chemical precipitation approach [20,21], and microwave heating approach [22]. However, products generated by the most methods were often in powder composite type.…”

Tunable Performance of Manganese Oxide Nanostructures as MRI Contrast Agents

“…Samples S0 , S0.2 , S0.02 and S0.002 previously identified as MnO 2 , were further confirmed to present a αMnO 2 structure by Raman scattering. αMnO 2 presents two (2) vibrational diagnostic bands in the region of 570 and 640 cm −1 corresponding to A g spectroscopic species originating from breathing vibrations of MnO 6 octahedra within a tetragonal hollandite‐type structure . These two bands are present in all the aforementioned samples.…”