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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.

Tris­(4-isocyanatophenyl)­methane (TIPM) and N,N′-dimethylformamide react at room temperature with no externally added catalyst to yield polyisocyanurate (PIR) gels. The obtained PIR gels were converted to N- and S-doped porous carbon monoliths by thermal treatment at 1000 °C with elemental sulfur under inert conditions. The PIR linkage acts as precursor for carbon and nitrogen, and %S doping was varied by changing the concentrations of elemental sulfur during pyrolysis. The optimized concentration of sulfur (5.6%) into the carbon matrix displayed excellent oxygen reduction activity with direct four-electron transfer relative to its pristine counterparts by (1) introducing micro- and mesopores in addition to the already existing macropores by etching the carbon surface (confirmed by N2 sorption isotherms and microscopic images) with the increase in the external surface area providing more active centers and efficient diffusion of electrolyte ions, (2) providing more – C–S–C– active species than oxidized sulfur species (confirmed by XPS and FT-IR) with more oxygen adsorption sites, and (3) filling the micropores of the carbon as a monolayer, affording increased electronic conductivity to the amorphous carbon. This simple and facile method of incorporating N- and S- together into the porous carbon matrix can be considered as an alternate for nonprecious metal catalysts for oxygen reduction reaction.

Manganese oxides (MnO 2 ) with nanowire morphology materials are promising candidates for improving oxygen evolution and oxygen reduction reaction (OER/ORR) performance. In this work, we developed transition metal cation doping strategy into the α-MnO 2 tunnel structure to tune the Mn oxidation states and control the uniform nanowire morphology, crystalline structure to investigate the effect of doping over bifunctional activity. The single Ni 2 + cation doping in α-MnO 2 with various loading concentrations resulted in 8NiÀ MnO 2 exhibiting remarkable OER and ORR activity owing to their excessive concentration of Mn 3 + and Mn 4 + octahedral sites respectively. Further, Co 2 + cation doping in 8NiÀ MnO 2 leads to an enhanced synergistic effect that significantly improves the fraction of Mn 3 + quantity which is confirmed by average oxidation state. For electrochemical OER performance, 8CoÀ 8NiÀ MnO 2 exhibits a potential of 1.77 V, Tafel slope value of 68 mV dec À 1 and lower charge transfer resistance and it is active in ORR with more positive onset potential of 0.90 V, half-wave potential of 0.80 V, better current density (4.7 mA cm À 2 ) and a four-electron pathway. Moreover, bifunctional activity (ΔE = E OER @10 mA cm À 2 -ORR@E 1/2 ) of 8CoÀ 8NiÀ MnO 2 demonstrated 0.97 V, indicates an excellent activity in alkaline electrolyte solution.