The oxygen reduction reaction (ORR) on platinum catalysts is essential in fuel cells. Quantitative predictions of the relative ORR activity in experiments, in the range of 1 to 50 times, have remained challenging due to incomplete mechanistic understanding and small differences in associated activation energies (&lt;2.3 kcal/mol). We elucidated the mechanism of adsorption of molecular oxygen, which severs as the first step of ORR, on Pt (100), (111), and (110) surfaces, Pt nanowires and Pt nanoplates in high accuracy (0.1 kcal/mol) using molecular dynamics (MD) simulation, followed by local density functional theory calculations. By employing realistic electrolyte conditions and newly developed models for molecular oxygen and Pt, the oxygen adsorption-desorption dynamics can be monitored, which has been inaccessible in previous quantum mechanical and experimental studies. The metal-electrolyte-gas interfaces are described using the Interface force field (IFF) which exceeds the accuracy of prior models by a factor of ten, enabling the quantification of inhibition by superficial water layers and co-electrolytes, site-specific oxygen binding, and molecular orientations. We find that the oxygen affinity to differently structured Pt surfaces, e.g., in the form of free energy curves or contact time, directly correlates with the ORR activity measured in experiments. When the site-blocking electrolytes is absent such as in perchloric acid solution, the affinity of oxygen molecules to Pt surfaces increases in the order Pt(100)&lt;Pt(111)&lt;Pt(110), and even further for regular Pt nanowires, Pt nanoplates, and jagged Pt nanowires, which matches the trend of higher ORR activity observed in experiments. The site-blocking effects from sulfuric acid and phosphoric acid solutions are also observed in the simulation which is in consistent with experiments. We expect the relative ORR activities to be regulated by the availability of oxygen on different surfaces, which is modulated by the solvent and competing electrolytes, while the slowest step exhibits a constant rate regardless of the of the Pt(hkl) surface environment and electrolyte. The MD methods facilitate quantitative ORR activity predictions for any Pt nanostructures, understanding of electrolyte interfaces in other catalysts, and more powerful combinations of MD and DFT methods.

Direct correlation of oxygen adsorption on platinum-electrolyte interfaces with the activity in the oxygen reduction reaction

ACS Fall 2021

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