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Pt, PtCo, and PtNi electrocatalysts were prepared by mechanical alloying, and their activity for oxygen reduction reaction in 0.5 M KOH was investigated using cyclic voltammetry (CV) and rotating ring-disk electrode techniques. Electrocatalysts were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Physical characterization showed that all electrocatalysts are alloys in the form of agglomerated particles that consist of nanocrystals. To perform an electrocatalytic evaluation, polarization curves, Koutecky-Levich and Tafel plots were obtained to calculate the kinetic parameters. Additionally, the molar fraction of the generated HO and the number of transferred electrons were determined. PtCo alloy showed a better performance for oxygen reduction reaction with a higher exchange current density, a lower overpotential, as well as higher specific and mass activities due to a synergistic effect between Pt and Co atoms as a result of the alloying process. [ABSTRACT FROM AUTHOR]

The microstructures of Pt/C and PtM/C (M = Ni, Cu, or Ag) electrocatalysts were studied using X-ray diffraction and transmission electron microscopy (TEM). The electrochemically active surface areas of the prepared materials were estimated by cyclic voltammetry in 1 M HSO. The materials, with metal contents ranging from 30 to 35 wt.%, were synthesized by chemically reducing the metal precursors in water-ethylene glycol solutions. The actual composition of the bimetallic nanoparticles corresponds to a theoretical (1:1) composition for the PtAg/C catalysts, whereas in the PtNi/C and PtCu/C materials, a portion of the alloying component exists in an oxide form. Decreasing the average metallic crystallite sizes from 3.5 to 1.6 nm does not increase the electrochemically active surface area. This apparent contradiction is because a majority of the PtNi and PtCu nanoparticles consist of 2-4 disordered crystallites. In addition, a portion of the PtNi or PtCu nanoparticle surface is covered by nickel or copper oxides, respectively. PtAg nanoparticles, which have a smaller size relative to other bimetallic particles according to the TEM data, are characterized by an intense platinum surface segregation. The agglomeration processes are lowest for the PtAg nanoparticles. [ABSTRACT FROM AUTHOR]

We review our recent progress in the development of Pt-Ni bimetallic electrocatalysts with both high sustained activity and sustained stability for oxygen reduction reaction (ORR). This was achieved by an atomic understanding and rational control of the core-shell compositional patterns and size-related nanoporosity within the bimetallic nanoparticles formed during chemical and electrochemical pretreatment and electrocatalysis. In particular, we reveal how the size of the nanoparticle directly influences the nanoporosity formation and thereby the near surface composition, catalytic activity and stability. Our atomic insights provide a clearer picture on how bimetallic nanoparticles should be tailored for optimal ORR performance. [ABSTRACT FROM AUTHOR]

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O2-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H2O2-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, Ta2O5, Nb2O5, and ZrO2), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt–Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal–support interactions and interfacial structural changes affecting adsorption and activation of O2-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional. [ABSTRACT FROM AUTHOR]

Developments of high efficient materials for electrocatalyst are significant topics of numerous researches since a few decades. Recent global interests related with energy conversion and storage lead to the expansion of efforts to find cost-effective catalysts that can substitute conventional catalytic materials. Especially, in the field of fuel cell, novel materials for oxygen reduction reaction (ORR) have been noticed to overcome disadvantages of conventional platinum-based catalysts. Various approaching methods have been attempted to achieve low cost and high electrochemical activity comparable with Pt-based catalysts, including reducing Pt consumption by the formation of hybrid materials, Pt-based alloys, and not-Pt metal or carbon based materials. To enhance catalytic performance and stability, numerous methods such as structural modifications and complex formations with other functional materials are proposed, and they are basically based on well-defined and well-ordered catalytic active sites by exquisite control at nanoscale. In this review, we highlight the development of nano-structured catalytic materials for ORR based on recent findings, and discuss about an outlook for the direction of future researches. [ABSTRACT FROM AUTHOR]