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

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

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