Pt Nanoparticle-Dispersed Graphene-Wrapped MWNT Composites As Oxygen Reduction Reaction Electrocatalyst in Proton Exchange Membrane Fuel Cell

Chemical and electrical synergies between graphite oxide and multiwalled carbon nanotube (MWNT) for processing graphene wrapped-MWNT hybrids has been realized by chemical vapor deposition without any chemical functionalization. Potential of the hybrid composites have been demonstrated by employing them as electrocatalyst supports in proton exchange membrane fuel cells. The defects present in the polyelectrolyte, which have been wrapped over highly dispersed MWNT, act as anchoring sites for the homogeneous deposition of platinum nanoparticles. Single-cell proton exchange membrane fuel cells show that the power density of the hybrid composite-based fuel cells is higher compared to the pure catalyst-support-based fuel cells, because of enhanced electrochemical reactivity and good surface area of the nanocomposites

Theoretical Insights into the Experimental Observation of Stable p‑Type Conductivity and Ferromagnetic Ordering in Vacuum-Hydrogenated TiO₂

Tuning of electrical and magnetic properties to achieve stable p-type conductivity and room temperature ferromagnetism in undoped TiO₂ is quite challenging. Here both are attained simultaneously through a facile method of vacuum-hydrogenation, wherein vacuum annealing as well as hydrogenation play crucial roles. The p-type conductivity in hydrogenated TiO₂ is investigated through the Hall measurement studies, which show considerable enhancement in Hall mobility and electrical conductivity. The high and low pressures of hydrogenation show strong and weak ferromagnetic ordering, respectively, whereas the pristine TiO₂ NPs manifest paramagnetic behavior. In order to understand the mechanism of these characteristic changes, density functional theory (DFT) calculations are performed. DFT calculations reveal that the smaller amount of hydrogenation leads to gap-states above valence band maximum (VBM) due to the effect of hydrogen atoms 1s orbitals and by the formation of ∼Ti–H and ∼O–H bonds. Further increase in the hydrogenation changes the ∼O–H bond to the ∼H₂O bond, and these H₂O molecules will be easily detached during the next vacuum annealing step. These processes will lead to the formation of excess oxygen vacancies and cause the localization of excess electrons on Ti atoms. This results in emergence of well pronounced midgap states in the forbidden bandgap. These midgap states are mostly contributed by the 3d orbitals of Ti atoms. DFT studies also disclose that the higher spin polarization for the high hydrogen concentration, which is reflected as the ferromagnetic ordering in the experimental results