Controlling the size and composition of nanosized Pt-Ni octahedra to optimize their catalytic activities toward the oxygen reduction reaction.

Electrocatalysts based on Pt-Ni alloys have received considerable interest in recent years owing to their remarkable activities toward the oxygen reduction reaction (ORR). Here, we report the synthesis of nanosized Pt-Ni octahedra with a range of controlled sizes and compositions in an effort to optimize their ORR activities. If we employed benzyl ether as a solvent for the synthesis, we could readily control the edge lengths of the Pt-Ni octahedra in the range of 6-12 nm and keep the Pt/Ni atomic ratio at around 2.4 by varying the amount of oleylamine added into the reaction system. If we adjusted the amount of Ni precursor, the atomic ratio of Pt to Ni in the Pt-Ni octahedra could be controlled in the range of 1.4-3.7 and their edge lengths were kept at 9 nm. For the catalysts with a Pt/Ni atomic ratio around 2.4, their specific ORR activities (per unit surface area) increased monotonically as the edge length increased from 6 to 12 nm. However, the mass activities (per unit mass of Pt) of these Pt-Ni octahedra showed a maximum value at an edge length of 9 nm. The specific and mass activities for the Pt-Ni octahedra with an edge length of 9 nm but different compositions both showed peak values at a Pt/Ni atomic ratio of 2.4.

The role of transition metals in the catalytic activity of Pt alloys: quantification of strain and ligand effects.

The oxygen reduction reaction (ORR) activity as a function of thickness of the Pt shell on Pt-Ni alloy nanoparticles was established. We demonstrated that the effects of transition metals could only extend to a very thin Pt shell, which was 0.9-1.0 nm for Pt3Ni.

Manipulating the oxygen reduction activity of platinum shells with shape-controlled palladium nanocrystal cores.

The oxygen reduction reaction (ORR) activities of Pt shells were found to have a strong dependence on the surface structure by depositing them on Pd nanocrystals with different shapes.

Core-shell catalysts consisting of nanoporous cores for oxygen reduction reaction.

A comprehensive experimental study was conducted on the dealloying of PdNi6 nanoparticles under various conditions. A two-stage dealloying protocol was developed to leach >95% of Ni while minimizing the dissolution of Pd. The final structure of the dealloyed particle was strongly dependent on the acid used and temperature. When H2SO4 and HNO3 solutions were used in the first stage of dealloying, solid and porous particles were generated, respectively. The porous particles have a 3-fold higher electrochemical surface area per Pd mass than the solid ones. The dealloyed PdNi6 nanoparticles were then used as a core material for the synthesis of core-shell catalysts. These catalysts were synthesized in gram-size batches and involved Pt displacement of an underpotentially deposited (UPD) Cu monolayer. The resulting materials were characterized by scanning transmission electron microscopy (STEM) and in situ X-ray diffraction (XRD). The oxygen reduction reaction (ORR) activity of the core-shell catalysts is 7-fold higher than the state-of-the-art Pt/C. The high activity was confirmed by a more than 40 mV improvement in fuel cell performance with a Pt loading of 0.1 mg cm(-2) by using the core-shell catalysts.

Synthesis and characterization of 9 nm Pt-Ni octahedra with a record high activity of 3.3 A/mg(Pt) for the oxygen reduction reaction.

Nanoscale Pt-Ni bimetallic octahedra with controlled sizes have been actively explored in recent years owning to their outstanding activity for the oxygen reduction reaction (ORR). Here we report the synthesis of uniform 9 nm Pt-Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni. Through the introduction of benzyl ether as a solvent, the coverage of both surfactants on the surface of resultant Pt-Ni octahedra was significantly reduced while the octahedral shape was still attained. By further removing the surfactants through acetic acid treatment, we observed a specific activity 51-fold higher than that of the state-of-the-art Pt/C catalyst for the ORR at 0.93 V, together with a record high mass activity of 3.3 A mgPt(-1) at 0.9 V (the highest mass activity reported in the literature was 1.45 A mgPt(-1)). Our analysis suggests that this great enhancement of ORR activity could be attributed to the presence of a clean, well-preserved (111) surface for the Pt-Ni octahedra.

Structural dependence of oxygen reduction reaction on palladium nanocrystals.

We have synthesized sub-10 nm Pd cubic and octahedral nanocrystals and then evaluated their activities towards oxygen reduction reaction (ORR). The ORR activity of Pd nanocubes was one order of magnitude higher than that of Pd octahedra, and comparable to that of the state-of-the-art Pt catalysts.