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

Unusually Huge Charge Storage Capacity of Mn₃O₄–Graphene Nanocomposite Achieved by Incorporation of Inorganic Nanosheets

Remarkable improvement in electrode performance of Mn₃O₄–graphene nanocomposites for lithium ion batteries can be obtained by incorporation of a small amount of exfoliated layered MnO₂ or RuO₂ nanosheets. The metal oxide nanosheet-incorporated Mn₃O₄–reduced graphene oxide (rGO) nanocomposites are synthesized via growth of Mn₃O₄ nanocrystals in the mesoporous networks of rGO and MnO₂/RuO₂ 2D nanosheets. Incorporation of metal oxide nanosheets is highly effective in optimizing porous composite structure and charge transport properties, resulting in a remarkable increase of discharge capacity of Mn₃O₄–rGO nanocomposite with significant improvement of cyclability and rate performance. The observed enormous discharge capacity of synthesized Mn₃O₄–rGO–MnO₂ nanocomposite (∼1600 mA·h·g^–1 for the 100th cycle) is the highest value among reported data for Mn₃O₄–rGO nanocomposite. Despite much lower electrical conductivity of MnO₂ than RuO₂, the MnO₂-incorporated nanocomposite at optimal composition (2.5 wt %) shows even larger discharge capacities with comparable rate characteristics compared with the RuO₂-incorporated homologue. This finding underscores that the electrode performance of the resulting nanosheet-incorporated nanocomposite is strongly dependent on its pore and composite structures rather than on the intrinsic electrical conductivity of the additive nanosheet. The present study clearly demonstrates that, regardless of electrical conductivity, incorporation of metal oxide 2D nanosheet is an effective way to efficiently optimize the electrode functionality of graphene-based nanocomposites

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

Bifunctional 2D Superlattice Electrocatalysts of Layered Double Hydroxide–Transition Metal Dichalcogenide Active for Overall Water Splitting

Bifunctional 2D superlattice electrocatalysts of alternating layered double hydroxide (LDH)–transition metal dichalcogenide (TMD) heterolayers were synthesized by interstratification of the exfoliated nanosheets. Density functional theory calculations predict an increased interfacial charge transfer between interstratified LDH and TMD nanosheets, which would lead to enhanced electrocatalytic activity. The electrostatically driven self-assembly of oppositely charged 2D building blocks, i.e., exfoliated Ni–Al-LDH/Ni–Fe-LDH and MoS₂ nanosheets, yields mesoporous heterolayered Ni–Al-LDH–MoS₂/Ni–Fe-LDH–MoS₂ superlattices. The synthesized superlattices show improved electrocatalytic activity with enhanced durability for oxygen and hydrogen evolution reactions and water splitting. The interstratification improves the chemical stability of LDH in acidic media, thus expanding its possible applications. The high electrocatalytic activity of the superlattices may be attributed to an enhanced affinity for OH^–/H⁺, improved electrical conduction and charge transfer, and the increase of active sites. This study indicates that the formation of superlattices via self-assembly of 2D nanosheets provides useful methodology to explore high-performance electrocatalysts with improved stability

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

Hybridization of a Metal–Organic Framework with a Two-Dimensional Metal Oxide Nanosheet: Optimization of Functionality and Stability

An effective way to improve the functionalities and stabilities of metal–organic frameworks (MOFs) is developed by employing exfoliated metal oxide 2D nanosheets as matrix for immobilization. Crystal growth of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals on the surface of layered titanate nanosheets yields intimately coupled nanohybrids of ZIF-8-layered titanate. The resulting nanohybrids show much greater surface areas and larger pore volumes than do the pristine ZIF-8, leading to the remarkable improvement of the CO₂ adsorption ability of MOF upon hybridization. Of prime importance is that the thermal- and hydrostabilities of ZIF-8 are significantly enhanced by a strong chemical interaction with the robust titanate nanosheet. A strong interfacial interaction between ZIF-8 and the layered titanate is verified by molecular mechanics simulations and spectroscopic analysis. The universal applicability of the present strategy for the coupling of MOFs and metal oxide nanosheets is substantiated by the stabilization of Ti-MOF-NH₂ via the immobilization on exfoliated V₂O₅ nanosheets. The present study underscores that hybridization with metal oxide 2D nanosheets provides an efficient and universal synthetic route to novel MOF-based hybrid materials with enhanced gas adsorptivity and stability

Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction

Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a “dual purpose” N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is <i>in situ</i> formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From <i>in situ</i> XANES, EDS, and first-principles calculations, we confirmed that an ordered fct-PtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells