Nanotubular Iridium–Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis

Here, we report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i>, where <i>x</i> is the relative Ir atomic content to the overall metal content. The formation of a variety of Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> (0 ≤ <i>x</i> ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO₂ and Co₃O₄, where Ir, Co, and O were homogeneously distributed throughout the entire nanostructures. The facile formation of Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> nanotubes was mainly ascribed to the inclination of Co₃O₄ to form the nanotubes during the calcination process, which could play a critical role in providing a template of tubular structure and facilitating the formation of IrO₂ by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i> nanotubes was characterized for oxygen evolution reaction (OER) with rotating disk electrode voltammetry in 1 M NaOH aqueous solution. Among diverse Ir_<i>x</i>Co_1–<i>x</i>O_<i>y</i>, Ir_0.46Co_0.54O_<i>y</i> nanotubes showed the best OER activity (the least-positive onset potential, greatest current density, and low Tafel slope), which was even better than that of commercial Ir/C. The Ir_0.46Co_0.54O_<i>y</i> nanotubes also exhibited a high stability in alkaline electrolyte. Expensive Ir mixed with cheap Co at an optimum ratio showed a greater OER catalytic activity than pure Ir oxide, one of the most efficient OER catalysts

Highly Efficient Silver–Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction

This paper reports the synthesis and characterization of silver–cobalt (AgCo) bimetallic composite nanotubes. Cobalt oxide (Co₃O₄) nanotubes were fabricated by electrospinning and subsequent calcination in air and then reduced to cobalt (Co) metal nanotubes via further calcination under a H₂/Ar atmosphere. As-prepared Co nanotubes were then employed as templates for the following galvanic replacement reaction (GRR) with silver (Ag) precursor (AgNO₃), which produced AgCo composite nanotubes. Various AgCo nanotubes were readily synthesized with applying different reaction times for the reduction of Co₃O₄ nanotubes and GRR. One hour reduction was sufficiently long to convert Co₃O₄ to Co metal, and 3 h GRR was enough to deposit Ag layer on Co nanotubes. The tube morphology and copresence of Ag and Co in AgCo composite nanotubes were confirmed with SEM, HRTEM, XPS, and XRD analyses. Electroactivity of as-prepared AgCo composite nanotubes was characterized for ORR with rotating disk electrode (RDE) voltammetry. Among differently synthesized AgCo composite nanotubes, the one synthesized via 1 h reduction and 3 h GRR showed the best ORR activity (the most positive onset potential, greatest limiting current density, and highest number of electrons transferred). Furthermore, the ORR performance of the optimized AgCo composite nanotubes was superior compared to pure Co nanotubes, pure Ag nanowires, and bare platinum (Pt). High ethanol tolerance of AgCo composite nanotubes was also compared with the commercial Pt/C and then verified its excellent resistance to ethanol contamination

Spongelike Nanoporous Pd and Pd/Au Structures: Facile Synthesis and Enhanced Electrocatalytic Activity

This paper reports the facile synthesis and characterization of spongelike nanoporous Pd (snPd) and Pd/Au (snPd/Au) prepared by a tailored galvanic replacement reaction (GRR). Initially, a large amount of Co particles as sacrificial templates was electrodeposited onto the glassy carbon surface using a cyclic voltammetric method. This is the key step to the subsequent fabrication of the snPd/Au (or snPd) architectures by a surface replacement reaction. Using Co films as sacrificial templates, snPd/Au catalysts were prepared through a two-step GRR technique. In the first step, the Pd metal precursor (at different concentrations), K₂PdCl₄, reacted spontaneously to the formed Co frames through the GRR, resulting in a snPd series. snPd/Au was then prepared via the second GRR between snPd (prepared with 27.5 mM Pd precursor) and Au precursor (10 mM HAuCl₄). The morphology and surface area of the prepared snPd series and snPd/Au were characterized using spectroscopic and electrochemical methods. Rotating disk electrode (RDE) experiments for oxygen reduction in 0.1 M NaOH showed that the snPd/Au has higher catalytic activity than snPd and the commercial Pd-20/C and Pt-20/C catalysts. Rotating ring-disk electrode (RRDE) experiments reconfirmed that four electrons were involved in the electrocatalytic reduction of oxygen at the snPd/Au. Furthermore, RDE voltammetry for the H₂O₂ oxidation/reduction was used to monitor the catalytic activity of snPd/Au. The amperometric <i>i</i>–<i>t</i> curves of the snPd/Au catalyst for a H₂O₂ electrochemical reaction revealed the possibility of applications as a H₂O₂ oxidation/reduction sensor with high sensitivity (0.98 mA mM^–1 cm^–2 (<i>r</i> = 0.9997) for H₂O₂ oxidation and −0.95 mA mM^–1 cm^–2 (<i>r</i> = 0.9997) for H₂O₂ reduction), low detection limit (1.0 μM), and a rapid response (<∼1.5 s)

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, Ir_<i>x</i>Ru_1–<i>x</i>O_<i>y</i> (<i>y</i> = 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 Ir_<i>x</i>Ru_1–<i>x</i>O_<i>y</i> 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 Ir_<i>x</i>Ru_1–<i>x</i>O_<i>y</i> nanofibers, the cathodically activated Ir_0.80Ru_0.20O_<i>y</i> shows the best HER activity and stability even compared with IrO_<i>y</i> and RuO_<i>y</i>, 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 Ir_0.80Ru_0.20 are close to those at Pt. This suggests that mixed metallic Ir and Ru are significant contributors to the improved HER activity of Ir_0.80Ru_0.20O_<i>y</i> 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

Hierarchically Driven IrO₂ Nanowire Electrocatalysts for Direct Sensing of Biomolecules

Applying nanoscale device fabrications toward biomolecules, ultra sensitive, selective, robust, and reliable chemical or biological microsensors have been one of the most fascinating research directions in our life science. Here we introduce hierarchically driven iridium dioxide (IrO₂) nanowires directly on a platinum (Pt) microwire, which allows a simple fabrication of the amperometric sensor and shows a favorable electronic property desired for sensing of hydrogen peroxide (H₂O₂) and dihydronicotinamide adenine dinucleotide (NADH) without the aid of enzymes. This rational engineering of a nanoscale architecture based on the direct formation of the hierarchical 1-dimensional (1-D) nanostructures on an electrode can offer a useful platform for high-performance electrochemical biosensors, enabling the efficient, ultrasensitive detection of biologically important molecules

Growth of Highly Single Crystalline IrO₂ Nanowires and Their Electrochemical Applications

We present the facile growth of highly single crystalline iridium dioxide (IrO₂) nanowires on SiO₂/Si and Au substrates via a simple vapor phase transport process under atmospheric pressure without any catalyst. Particularly, high-density needle-like IrO₂ nanowires were readily obtained on a single Au microwire, suggesting that the melted surface layer of Au might effectively enhance the nucleation of gaseous IrO₃ precursors at the growth temperature. In addition, all the electrochemical observations of the directly grown IrO₂ nanowires on a single Au microwire support favorable electron-transfer kinetics of [Fe­(CN₆)]^4–/3– as well as Ru­(NH₃)₆^3+/2+ at the highly oriented crystalline IrO₂ nanowire surface. Furthermore, stable pH response is shown, revealing potential for use as a miniaturized pH sensor, confirmed by the calibration curve exhibiting super-Nernstian behavior with a slope of 71.6 mV pH^–1

Highly Efficient Electrochemical Responses on Single Crystalline Ruthenium–Vanadium Mixed Metal Oxide Nanowires

Highly efficient single crystalline ruthenium–vanadium mixed metal oxide (Ru_1–<i>x</i>V_<i>x</i>O₂, 0 ≤ <i>x</i> ≤ 1) nanowires were prepared on a SiO₂ substrate and a commercial Au microelectrode for the first time through a vapor-phase transport process by adjusting the mixing ratios of RuO₂ and VO₂ precursors. Single crystalline Ru_1–<i>x</i>V_<i>x</i>O₂ nanowires show homogeneous solid-solution characteristics as well as the distinct feature of having remarkably narrow dimensional distributions. The electrochemical observations of a Ru_1–<i>x</i>V_<i>x</i>O₂ (<i>x</i> = 0.28 and 0.66)-decorated Au microelectrode using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrate favorable charge-transfer kinetics of [Fe­(CN)₆]^3–/4– and Ru­(NH₃)₆^3+/2+ couples compared to that of a bare Au microelectrode. The catalytic activity of Ru_1–<i>x</i>V_<i>x</i>O₂ for oxygen and H₂O₂ reduction at neutral pH increases as the fraction of vanadium increases within our experimental conditions, which might be useful in the area of biofuel cells and biosensors

Highly Branched RuO₂ Nanoneedles on Electrospun TiO₂ Nanofibers as an Efficient Electrocatalytic Platform

Highly single-crystalline ruthenium dioxide (RuO₂) nanoneedles were successfully grown on polycrystalline electrospun titanium dioxide (TiO₂) nanofibers for the first time by a combination of thermal annealing and electrospinning from RuO₂ and TiO₂ precursors. Single-crystalline RuO₂ nanoneedles with relatively small dimensions and a high density on electrospun TiO₂ nanofibers are the key feature. The general electrochemical activities of RuO₂ nanoneedles–TiO₂ nanofibers and Ru­(OH)₃-TiO₂ nanofibers toward the reduction of [Fe­(CN)₆]^3– were carefully examined by cyclic voltammetry carried out at various scan rates; the results indicated favorable charge-transfer kinetics of [Fe­(CN)₆]^3– reduction via a diffusion-controlled process. Additionally, a test of the analytical performance of the RuO₂ nanoneedles–TiO₂ nanofibers for the detection of a biologically important molecule, hydrogen peroxide (H₂O₂), indicated a high sensitivity (390.1 ± 14.9 μA mM^–1 cm^–2 for H₂O₂ oxidation and 53.8 ± 1.07 μA mM^–1 cm^–2 for the reduction), a low detection limit (1 μM), and a wide linear range (1–1000 μM), indicating H₂O₂ detection performance better than or comparable to that of other sensing systems