Oxygen reduction reaction activity on Pt{111} surface alloys

Area of Science:

• Materials Science
• Electrochemistry
• Surface Science

Background:

• Platinum-based materials are crucial for catalyzing reactions like the oxygen reduction reaction (ORR).
• Controlling the synthesis and surface structure of platinum-metal (PtM) overlayers is key to optimizing catalytic performance.
• Understanding the relationship between material phase and catalytic activity is essential for developing advanced electrocatalysts.

Purpose of the Study:

• To synthesize and characterize PtM (M=Fe, Co, Ni) overlayers on a Pt{111} surface.
• To investigate the influence of post-annealing cooling environments on the resulting PtM phase formation.
• To correlate the structural and chemical properties of different PtM phases with their activity for the oxygen reduction reaction (ORR).

Main Methods:

• Synthesis of PtM overlayers on Pt{111} via thermal annealing of M(Z+) cation-containing aqueous droplets.
• Controlled cooling of the annealed samples in either a nitrogen/water or hydrogen ambient.
• Characterization of the resulting PtM phases, distinguishing between metallic and mixed oxide/hydroxide structures.
• Electrochemical evaluation of the prepared PtM overlayers for oxygen reduction reaction (ORR) activity.

Main Results:

• Two distinct PtM phases were formed depending on the cooling environment: a mixed metal oxide/hydroxide shell over a metallic core, or an entirely metallic phase.
• The metallic PtM phase, obtained by cooling in hydrogen, exhibited flatter microcrystals compared to the oxide-rich phase.
• A positive shift in the onset of PtM oxide formation was observed, correlating with enhanced ORR activity.
• Increased availability of metallic platinum sites under ORR conditions was identified as the reason for improved activity.

Conclusions:

• The cooling environment critically dictates the phase and morphology of PtM overlayers synthesized on Pt{111}.
• The all-metallic PtM phase, favored by hydrogen cooling, demonstrates superior ORR activity compared to the oxide-rich phase.
• Optimizing PtM overlayer structure, particularly maximizing metallic platinum site availability, is crucial for enhancing electrocatalytic performance in reactions like ORR.