Melamine-driven solvation effect promotes oxygen reduction on platinum catalyst: machine learning-aided free energy calculations

The modification of Pt surfaces by organic compounds such as melamine is known to enhance the activity of the oxygen reduction reaction (ORR) and improve catalyst durability against potential cycling. Here, we present the mechanism of activity enhancement elucidated by first-principles (FP) free energy calculations using thermodynamic integration within finite-temperature molecular dynamics simulations. These calculations accelerate phase space sampling, including nanosecond-scale fluctuations of interfacial water, by employing machine-learned force fields. The errors introduced by the force fields are corrected via thermodynamic integration from the machine-learned potential to the FP potential, allowing us to obtain accurate FP free energies. The results reveal that melamine destabilizes the OH adsorbate, an intermediate in the ORR, thereby accelerating the OH removal step and pushing the catalytic activity to the top of the volcano plot. This behavior is similar to what is observed on Pt(1-x)Cux alloy surfaces evaluated using the same method. However, the mechanism of OH destabilization by melamine differs entirely from that on alloy surfaces. Unlike alloys, melamine does not shift the d-band center or shorten the metal-metal distance. Instead, melamine disrupts the hydrogen bonds between the OH adsorbate and the surrounding interfacial water. Structural and vibrational analyses have revealed that the solvation structure changes induced by melamine are manifested as alterations in the radial distribution function of water near the OH adsorbate and as a blue shift in the O-H stretching vibrations of interfacial water. These findings indicate that manipulating interfacial solvation with organic compounds could be a promising approach to enhance catalytic activity without compromising durability.