Platinum Nanostructure Tailoring for Fuel Cell Applications Using Levitated Water Droplets as Green Chemical Reactors

The role of Laplace pressure in the maximal weight of pendant drops

HYPOTHESIS

The value of the maximal weight of a pendant drop formed at the end of a syringe needle is lower than the intensity of the corresponding capillary force. The balance of the external forces applied to the maximal pendant drop must be completed by the overpressure generated by the piston of the syringe. Inside the drop, the Laplace pressure corresponds to this overpressure.

EXPERIMENTS

Pendant drops are made with three liquids and five different needle diameters. The influence of Laplace pressure on the maximal weight is experimentally highlighted by modulating the drop curvatures thanks to glass beads placed at the apex of the pendant drop. Their maximal weight and curvatures are measured by image analysis.

FINDINGS

Experiments confirm that the balance of external forces must be completed by the force acting on the syringe piston. The overpressure on the piston has an impact on the drops via the Laplace pressure. A master curve between the mean curvature and the maximal volume of the pendant drops is observed. This result allows to validate an expression of the maximal weight which integrates the Laplace pressure. This work contributes to a better understanding of the maximal pendant drop properties and beyond, of the capillary phenomenon.

Metal-to-Semimetal Transition in Platinum Nanotubes: Dependence on Thickness

The temperature dependence of the electrical conductivity of Pt nanotubes (NTs) with different thicknesses synthesized by a wetting method using an Al₂O₃ membrane was studied. Pt NTs exhibited circular pores with an average diameter of ∼200 nm. From XRD, the prepared Pt NTs displayed a cubic crystal structure. Pt metal was identified based on the binding energy peak at 71 eV via XPS analysis. Pt NTs with thicknesses of 5 and 12 nm behaved like a semimetal, whereas Pt NTs with thicknesses of 25 and 29 nm showed normal metallic electrical conduction characteristics. This metal-to-semimetal transition was induced as the thickness and grain sizes of the Pt NTs were decreased. The critical metal-to-semimetal transition temperature of Pt NTs with average tube wall thicknesses of ∼5 nm was measured at ∼37 °C. However, the critical temperature could not be measured for NTs with a thickness of 12 nm. It is assumed that the critical temperature would be far below 0 °C. This transition behavior resulted from both a discontinuity in the density of states due to the quantum confinement effect and the increased energy barrier for conduction of electrons accompanied by the increased density of grain boundaries. These results presented here signify a vital step in the direction of realizing high-performance nanoelectronic devices.

Polyelectrolyte-Induced Stereoassembly of Grain Boundary-Enriched Platinum Nanoworms on Ti3C2Tx MXene Nanosheets for Efficient Methanol Oxidation

Direct methanol fuel cells with high energy conversion efficiency and low hazard emissions have aroused great attention from both academic and industrial communities, but their large-scale commercial application has been blocked by high costs as well as short lifespan of the anode Pt catalysts. Here, we demonstrate a simple and scalable noncovalent strategy for the synthesis of quasi-one-dimensional (1D) Pt nanoworms grown on poly(diallyldimethyl-ammonium chloride) (PDDA)-functionalized Ti₃C₂T_x nanosheets as anode catalysts for methanol electrooxidation. Interestingly, the introduction of PDDA on Ti₃C₂T_x nanosheets can not only effectively adjust their surface charge property to strengthen the electrostatic interaction between metal and support but also induce the stereoassembly of worm-shaped Pt nanocrystals with abundant catalytically active grain boundaries, which enable the resulting hybrid to express high electrocatalytic activity, remarkable durability, and strong antipoisoning ability for methanol electrooxidation, which are better than those of the traditional Pt nanoparticle electrocatalysts loaded on carbon black, carbon nanotubes, reduced graphene oxide, and MXene matrixes. Theoretical simulations disclose that the more stable worm-shaped Pt configuration with an optimized electronic structure on the Ti₃C₂T_x surface possesses a weaker CO adsorption ability than that of the Pt nanoclusters, thereby providing a dramatically enhanced and sustainable electrocatalytic performance.

Conformal Solution Deposition of Pt-Pd Titania Nanocomposite Coatings for Light-Assisted Formic Acid Electro-Oxidation

Many nanofabrication processes require sophisticated equipment, elevated temperature, vacuum or specific atmospheric conditions, templates, and exotic chemicals, which severely hamper their implementation in real-world applications. In this study, we outline a fully wet-chemical procedure for equipping a 3D carbon felt (CF) substrate with a multifunctional, titania nanospike-supported Pt-Pd nanoparticle (Pt-Pd-TiO₂@CF) layer in a facile and scalable manner. The nanostructure, composition, chemical speciation, and formation of the material was meticulously investigated, evidencing the conformal coating of the substrate with a roughened layer of nanocrystalline rutile spikes by chemical bath deposition from Ti³⁺ solutions. The spikes are densely covered by bimetallic nanoparticles of 4.4 ± 1.1 nm in size, which were produced by autocatalytic Pt deposition onto Pd seeds introduced by Sn²⁺ ionic layer adsorption and reaction. The as-synthesized nanocomposite was applied to the (photo)electro-oxidation of formic acid (FA), exhibiting a superior performance compared to Pt-plated, Pd-seeded CF (Pt-Pd@CF) and commercial Pt-C, indicating the promoting electrocatalytic role of the TiO₂ support. Upon UV-Vis illumination, the performance of the Pt-Pd-TiO₂@CF electrode is remarkably increased (22-fold), generating a current density of 110 mA cm^-2, distinctly outperforming titania-free Pt-Pd@CF (5 mA cm^-2) and commercial Pt-C (6 mA cm^-2) reference catalysts. In addition, the Pt-Pd-TiO₂@CF showed a much better stability, characterized by a very high poisoning tolerance for in situ-generated CO intermediates, whose formation is hindered in the presence of TiO₂. This overall performance boost is attributed to a dual enhancement mechanism (∼30% electrocatalytic and ∼70% photoelectrocatalytic). The photogenerated electrons from the TiO₂ conduction band enrich the electron density of the Pt nanoparticles, promoting the generation of active oxygen species on their surfaces from adsorbed oxygen and water molecules, which facilitate the direct FA electro-oxidation into CO₂.