Here, we report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as IrCoO, where x is the relative Ir atomic content to the overall metal content. The formation of a variety of IrCoO (0 ≤ x ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that IrCoO polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO and CoO, where Ir, Co, and O were homogeneously distributed throughout the entire nanostructures. The facile formation of IrCoO nanotubes was mainly ascribed to the inclination of CoO 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 IrO by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained IrCoO nanotubes was characterized for oxygen evolution reaction (OER) with rotating disk electrode voltammetry in 1 M NaOH aqueous solution. Among diverse IrCoO, IrCoO 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 IrCoO 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.

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, IrRuO (y = 0 or 2) nanofibers with diverse compositions of Ir/IrO and RuO are synthesized by electrospinning and calcination procedures. Their HER activities are measured in 1.0 M NaOH. Interestingly, the HER activities of IrRuO 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 IrRuO nanofibers, the cathodically activated IrRuO shows the best HER activity and stability even compared with IrO and RuO, 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 IrRu are close to those at Pt. This suggests that mixed metallic Ir and Ru are significant contributors to the improved HER activity of IrRuO 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.

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Nanocomposites of gold (Au) and iridium (Ir) oxide with various compositions (denoted as Au x Ir 1−x O 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/IrO 2 . Different solvent composition in the electrospinning solution induced the formation of main frames with distinct nanostructures; nanoribbons (Au x Ir 1−x O y -70) with ethanol/DMF = 70:30; and nanofibers (Au x Ir 1−x O 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 x Ir 1−x O y . Oxygen evolution reaction (OER) activities of as-synthesized Au x Ir 1−x O y were investigated in 0.5 M H 2 SO 4 and compared to those of IrO y , Au, and commercial iridium (Ir/C, 20% Ir loading on Vulcan carbon). Among them, Au 0.10 Ir 0.90 O y -50 exhibited the best OER activity, even better than previously reported catalysts containing both Ir and Au. The high OER activity of Au 0.10 Ir 0.90 O y -50 was mainly attributed to the fiber frame structure and the optimal interfacial areas between Au and Ir/IrO 2 , which are electrophilic OER active sites. The stability of Au 0.10 Ir 0.90 O 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/IrO 2 surface improves the OER activity of Ir/IrO 2 .

RhCo alloy nanotubes were synthesized via the reduction of single-phase Co2RhO4 nanotubes. The reduction was conducted by thermal annealing of the Co2RhO4 nanotubes under hydrogen gas flow. The crystallinity of the prepared RhCo alloy nanotubes depended on the reduction temperature: amorphous phase (200 °C reduction) and the crystalline phase (300 °C reduction). The hydrogen evolution reaction (HER) on RhCo alloys was investigated with voltammetry in 1.0 M HClO4 solution. Amorphous RhCo alloys provided lower overpotential than the crystalline counterpart despite their similar morphology and composition. Of great interest, amorphous RhCo alloy nanotubes exhibited an outstanding HER electroactivity verified with a low overpotential at −10 mA cm–2 (−22 mV) and a small Tafel slope (−24.1 mV dec–1), outperforming commercial Pt, pure Rh metal, and the other previously reported Rh-based catalysts. This excellent HER activity of amorphous RhCo nanotubes was attributed to the amorphous structure having a large electrochemical surface area and maximized Rh–Co interfaces in the alloy facilitating HER. Active but expensive Rh alloyed with less active but cheap Co was successfully demonstrated as a potential cost-effective HER catalyst.