Single-Step Electrospun Ir/IrO2 Nanofibrous Structures Decorated with Au Nanoparticles for Highly Catalytic Oxygen Evolution Reaction.

Nanocomposites of gold (Au) and iridium (Ir) oxide with various compositions (denoted as Au xIr1- xO y, x = 0.05, 0.10, or 0.33, Au precursor molar ratio to Ir precursor) were synthesized via electrospinning and subsequent calcination method with two different solvent composition ratios of ethanol to N, N-dimethylformamide (DMF) in the electrospinning solution (ethanol/DMF = 70:30 or 50:50% v/v). Simple single-step electrospinning successfully fabricated a hierarchical nanostructure having Au nanoparticles formed on fibrous main frames of Ir/IrO2. Different solvent composition in the electrospinning solution induced the formation of main frames with distinct nanostructures; nanoribbons (Au xIr1- xO y-70) with ethanol/DMF = 70:30; and nanofibers (Au xIr1- xO y-50) with ethanol/DMF = 50:50. The pure Ir or Au counterparts (IrO y and Au) were also prepared by the same synthetic procedure as Au xIr1- xO y. Oxygen evolution reaction (OER) activities of as-synthesized Au xIr1- xO y were investigated in 0.5 M H2SO4 and compared to those of IrO y, Au, and commercial iridium (Ir/C, 20% Ir loading on Vulcan carbon). Among them, Au0.10Ir0.90O y-50 exhibited the best OER activity, even better than previously reported catalysts containing both Ir and Au. The high OER activity of Au0.10Ir0.90O y-50 was mainly attributed to the fiber frame structure and the optimal interfacial areas between Au and Ir/IrO2, which are electrophilic OER active sites. The stability of Au0.10Ir0.90O y-50 was also evaluated to be much higher than that of Ir/C during OER. The current study suggests that the presence of Au on the Ir/IrO2 surface improves the OER activity of Ir/IrO2.

Dielectric Breakdown and Post-Breakdown Dissolution of Si/SiO2 Cathodes in Acidic Aqueous Electrochemical Environment.

Understanding the conducting mechanisms of dielectric materials under various conditions is of increasing importance. Here, we report the dielectric breakdown (DB) and post-breakdown mechanism of Si/SiO2, a widely used semiconductor and dielectric, in an acidic aqueous electrochemical environment. Cathodic breakdown was found to generate conduction spots on the Si/SiO2 surface. Using scanning electrochemical microscopy (SECM), the size and number of conduction spots are confirmed to increase from nanometer to micrometer scale during the application of negative voltage. The morphologies of these conduction spots reveal locally recessed inverted-pyramidal structures with exposed Si{111} sidewalls. The pits generation preceded by DB is considered to occur via cathodic dissolution of Si and exfoliation of SiO2 that are induced by local pH increases due to the hydrogen evolution reaction (HER) at the conduction spots. The HER at the conduction spots is more sluggish due to strongly hydrogen-terminated Si{111} surfaces.

Fundamental Study of Facile and Stable Hydrogen Evolution Reaction at Electrospun Ir and Ru Mixed Oxide Nanofibers.

Electrochemical hydrogen evolution reaction (HER) has been an interesting research topic in terms of the increasing need of renewable and alternative energy conversion devices. In this article, IrxRu1-xOy (y = 0 or 2) nanofibers with diverse compositions of Ir/IrO2 and RuO2 are synthesized by electrospinning and calcination procedures. Their HER activities are measured in 1.0 M NaOH. Interestingly, the HER activities of IrxRu1-xOy nanofibers improve gradually during repetitive cathodic potential scans for HER, and then eventually reach the steady-state consistencies. This cathodic activation is attributed to the transformation of the nanofiber surface oxides to the metallic alloy. Among a series of IrxRu1-xOy nanofibers, the cathodically activated Ir0.80Ru0.20Oy shows the best HER activity and stability even compared with IrOy and RuOy, commercial Pt and commercial Ir (20 wt % each metal loading on Vulcan carbon), where a superior stability is possibly ascribed to the instant generation of active Ir and Ru metals on the catalyst surface upon HER. Density functional theory calculation results for hydrogen adsorption show that the energy and adsorbate-catalyst distance at metallic Ir0.80Ru0.20 are close to those at Pt. This suggests that mixed metallic Ir and Ru are significant contributors to the improved HER activity of Ir0.80Ru0.20Oy after the cathodic activation. The present findings clearly demonstrate that the mixed oxide of Ir and Ru is a very effective electrocatalytic system for HER.

Oxidation-state dependent electrocatalytic activity of iridium nanoparticles supported on graphene nanosheets.

Nanocomposites of iridium nanoparticles (Ir NPs), supported on graphene nanosheets, are synthesized and their electrocatalytic acitivities in the oxygen reduction reaction (ORR) are studied depending on their Ir oxidation state. Graphene functionalized with poly(vinyl pyrrolidone) (pRGO) is a suitable support for Ir NPs, producing well-monodispersed Ir NPs anchored strongly on the pRGO surface (Ir NP/pRGO) with a very high density. This was confirmed by scanning electron microscopy and transmission electron microscopy. The ORR activity of the Ir NP/pRGO nanocomposites in 0.5 M H2SO4 solution was observed to be dependent on the oxidation state of the immobilized Ir NPs. In fact, the nanocomposite composed of Ir(0) metal NPs, rather than Ir oxide (IrOx) NPs, exhibits higher ORR activity, such as more positive onset potential, higher and flatter limiting current density, a greater n value, and a sharper curve shape in the rotating disk electrode voltammetry experiments. Higher ORR activity of Ir is ascribed to the stronger adsorption of oxygen on the surface of Ir compared to IrOx. The practical stability of the Ir NP/pRGO composite was also confirmed under O2 saturated/acidic conditions.

Synthesis and electrocatalytic activity of highly porous hollow palladium nanoshells for oxygen reduction in alkaline solution.

A series of hollow Pd nanoshells are prepared by employing Co nanoparticles as sacrificial templates with different concentrations of a Pd precursor (1, 6, 12, 20, and 40 mM K2PdCl4), denoted hPd-X (X: concentration of K2PdCl4 in mM unit). The synthesized hPd series are tested as a cathodic electrocatalyst for oxygen reduction reaction (ORR) in alkaline solution. The morphology and surface area of the hPd catalysts are characterized using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), and cyclic voltammetry (CV). Rotating disk electrode (RDE) voltammetric studies show that the hPd-20 (prepared using 20 mM K2PdCl4) has the highest ORR activity among all the hPd series, while being comparable to commercial Pd and Pt catalysts (E-TEK). The more facilitated ORR at hPd-20 is presumably induced by the enhanced Pd surface area and efficiently high porosity of Pd nanoshells.