Oxygen reduction catalyzed by Au-TiO2 nanocomposites in alkaline media

Chemical Vapor Deposition-Grown Nickel-Encapsulated N-Doped Carbon Nanotubes as a Highly Active Oxygen Reduction Reaction Catalyst without Direct Metal-Nitrogen Coordination

Nickel-encapsulated nitrogen-doped carbon nanotubes (Ni-TiO₂-NCNTs) are synthesized via chemical vapor deposition by thermal decomposition of acetylene with acetonitrile vapor at 700 °C on the Ni-TiO₂ matrix. TiO₂ is used as a dispersant medium for Ni nanoparticles, which assists in higher CNT growth at high temperatures. A reference catalyst is made by following the similar procedure without acetonitrile vapor, which is called a Ni-TiO₂-CNT. Acid treatment of these two catalysts dissolved Ni on the surface of CNTs-NCNTs, producing catalysts with enhanced surface area and defects. The transmission electron microscopy-energy-dispersive X-ray spectra analysis of acid-treated version of the catalysts confirmed the presence of encapsulated Ni. Oxygen reduction reaction (ORR) activity of these catalysts was analyzed in 0.1 N KOH solution. Among these, the acid-treated Ni-TiO₂-NCNT exhibited highest ORR onset potential of 0.88 V versus reversible hydrogen electrode and a current density of 3.7 mA cm^-2 at 170 μg cm^-2 of catalyst loading. The stability of the acid-treated Ni-TiO₂-NCNT is proved by cyclic voltammetry and chronoamperometry measurements which are done for 800 cycles and 100 h, respectively. Primarily N doping of CNTs is the reason behind the improved ORR activity.

Towards High-Performance Electrocatalysts for Oxygen Reduction: Inducing Atomic-Level Reconstruction of Fe-Nx Site for Atomically Dispersed Fe/N-Doped Hierarchically Porous Carbon

A rational and effective strategy for the synthesis of a high-performance non-precious metal electrocatalyst for oxygen reduction reaction (ORR) was developed by inducing reconstruction of Fe-N_x site on pig-bone-derived nitrogen-doped hierarchically porous carbon. The resultant Fe/N-doped carbon electrocatalyst possessed abundant atomically dispersed non-planar Fe-N₄ ORR active sites, with absolute presence of active D1 (Fe^II -N₄ ) and D3 (N-Fe^II -N₂₊₂ ) sites, as well as large specific surface area and three-dimensional porous structure with hierarchical micro-/meso-/macro-pore distribution, which increased the utilization of active sites and promoted mass transport of ORR reactants. This resulted in a remarkably superior ORR activity with half-wave potential of 0.87 V (20 mV higher than Pt/C) and kinetic current density of 10.9 mA cm^-2 at 0.85 V (2.3-fold of Pt/C) in alkaline electrolyte. This methodology provides a route for atomic-level design of high-performance ORR electrocatalyst.

Mechanism of H adatoms improving the O2 reduction reaction on the Zn-modified anatase TiO2 (101) surface studied by first principles calculation

Surface H and subsurface Zn interstitials could facilitate O₂ adsorption and dissociation on the TiO₂ surface.

Enhanced Photoelectrocatalytic Reduction of Oxygen Using Au@TiO2 Plasmonic Film

Novel Au@TiO₂ plasmonic films were fabricated by individually placing Au nanoparticles into TiO₂ nanocavity arrays through a sputtering and dewetting process. These discrete Au nanoparticles in TiO₂ nanocavities showed strong visible-light absorption due to the plasmonic resonance. Photoelectrochemical studies demonstrated that the developed Au@TiO₂ plasmonic films exhibited significantly enhanced catalytic activities toward oxygen reduction reactions with an onset potential of 0.92 V (vs reversible hydrogen electrode), electron transfer number of 3.94, and limiting current density of 5.2 mA cm^-2. A superior ORR activity of 310 mA mg^-1 is achieved using low Au loading mass. The isolated Au nanoparticle size remarkably affected the catalytic activities of Au@TiO₂, and TiO₂ coated with 5 nm Au (Au₅@TiO₂) exhibited the best catalytic function to reduce oxygen. The plasmon-enhanced reductive activity is attributed to the surface plasmonic resonance of isolated Au nanoparticles in TiO₂ nanocavities and suppressed electron recombination. This work provides comprehensive understanding of a novel plasmonic system using isolated noble metals into nanostructured semiconductor films as a potential alternative catalyst for oxygen reduction reaction.

Porous Carbon-Supported Gold Nanoparticles for Oxygen Reduction Reaction: Effects of Nanoparticle Size

Porous carbon-supported gold nanoparticles of varied sizes were prepared using thiolate-capped molecular Au25, Au38, and Au144 nanoclusters as precursors. The organic capping ligands were removed by pyrolysis at controlled temperatures, resulting in good dispersion of gold nanoparticles within the porous carbons, although the nanoparticle sizes were somewhat larger than those of the respective nanocluster precursors. The resulting nanocomposites displayed apparent activity in the electroreduction of oxygen in alkaline solutions, which increased with decreasing nanoparticle dimensions. Among the series of samples tested, the nanocomposite prepared with Au25 nanoclusters displayed the best activity, as manifested by the positive onset potential at +0.95 V vs RHE, remarkable sustainable stability, and high numbers of electron transfer at (3.60-3.92) at potentials from +0.50 to +0.80 V. The performance is comparable to that of commercial 20 wt % Pt/C. The results demonstrated the unique feasibility of porous carbon-supported gold nanoparticles as high-efficiency ORR catalysts.

Oxygen reduction catalyzed by gold nanoclusters supported on carbon nanosheets

Nanocomposites based on p-mercaptobenzoic acid-functionalized gold nanoclusters, Au102(p-MBA)44, and porous carbon nanosheets have been fabricated and employed as highly efficient electrocatalysts for oxygen reduction reaction (ORR). Au102(p-MBA)44 clusters were synthesized via a wet chemical approach, and loaded onto carbon nanosheets. Pyrolysis at elevated temperatures led to effective removal of the thiolate ligands and the formation of uniform nanoparticles supported on the carbon scaffolds. The nanocomposite structures were characterized by using a wide range of experimental techniques such as transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, UV-visible absorption spectroscopy, thermogravimetric analysis and BET nitrogen adsorption/desorption. Electrochemical studies showed that the composites demonstrated apparent ORR activity in alkaline media, and the sample with a 30% Au mass loading was identified as the best catalyst among the series, with a performance comparable to that of commercial Pt/C, but superior to those of Au102 nanoclusters and carbon nanosheets alone, within the context of onset potential, kinetic current density, and durability. The results suggest an effective approach to the preparation of high-performance ORR catalysts based on gold nanoclusters supported on carbon nanosheets.

Gold nanoparticles supported on layered TiO2–RGO hybrid as an enhanced and recyclable catalyst for microwave-assisted hydration reaction

A novel composite (Au–SO₄²⁻/TiO₂–RGO) is synthesized and serves as an enhanced catalyst for alkyne hydration.

In situ preparation of multi-wall carbon nanotubes/Au composites for oxygen electroreduction

Multi-wall carbon nanotubes (CNTs)/Au nanocomposites have been prepared by the in situ reduction approach for oxygen reduction reaction.

Biomass-derived nitrogen self-doped porous carbon as effective metal-free catalysts for oxygen reduction reaction

Biomass-derived nitrogen self-doped porous carbon was synthesized by a facile procedure based on simple pyrolysis of water hyacinth (eichhornia crassipes) at controlled temperatures (600-800 °C) with ZnCl2 as an activation reagent. The obtained porous carbon exhibited a BET surface area up to 950.6 m(2) g(-1), and various forms of nitrogen (pyridinic, pyrrolic and graphitic) were found to be incorporated into the carbon molecular skeleton. Electrochemical measurements showed that the nitrogen self-doped carbons possessed a high electrocatalytic activity for ORR in alkaline media that was highly comparable to that of commercial 20% Pt/C catalysts. Experimentally, the best performance was identified with the sample prepared at 700 °C, with the onset potential at ca. +0.98 V vs. RHE, that possessed the highest concentrations of pyridinic and graphitic nitrogens among the series. Moreover, the porous carbon catalysts showed excellent long-term stability and much enhanced methanol tolerance, as compared to commercial Pt/C. The performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon catalysts for ORR. The results suggested a promising route based on economical and sustainable biomass towards the development and engineering of value-added carbon materials as effective metal-free cathode catalysts for alkaline fuel cells.

Defective TiO2-supported Cu nanoparticles as efficient and stable electrocatalysts for oxygen reduction in alkaline media

Nanocomposites based on TiO2-supported copper nanoparticles were prepared by a hydrothermal method where copper nanoparticles with or without the passivation of 1-decyne were chemically grown onto TiO2 nanocolloid surfaces (and hence denoted as CuHC10/TiO2 and Cu/TiO2, respectively). Transmission electron microscopy measurements showed that the size of the hybrid nanoparticles was 5-15 nm in diameter with clearly defined lattice fringes for anatase TiO2(101) and Cu(111). The formation of anatase TiO2 nanoparticles was also observed by X-ray diffraction measurements. FTIR measurements confirmed successful attachment of alkyne ligands onto the surface of the copper nanoparticles via Cu-C≡ interfacial bonds in CuHC10/TiO2. XPS measurements suggested the formation of CuO in both samples with a higher concentration in Cu/TiO2, and interestingly Ti(3+) species were found in CuHC10/TiO2 but were absent in Cu/TiO2 or TiO2 nanoparticles. Electrochemical studies demonstrated that both Cu/TiO2 and CuHC10/TiO2 exhibited a markedly improved electrocatalytic performance in the oxygen reduction reaction, as compared to TiO2 nanocolloids alone, in the context of the onset potential, the number of electrons transferred and the kinetic current density. Importantly, among the series, CuHC10/TiO2 exhibited the best ORR activity with a high current density, an almost four-electron reduction pathway and long-term stability after 4000 cycles at high potentials, which may be ascribed to the defective TiO2 structures in combination with surface ligand engineering.

Ordered mesoporous carbons-supported gold nanoparticles as highly efficient electrocatalysts for oxygen reduction reaction

Mesoporous carbons-supported gold nanoparticles exhibit apparent electrocatalytic activity towards oxygen reduction reaction, due to their intimate interactions that facilitate fast electron transfer and rapid mass transport.