Origin of Pt Site Poisoning by Impurities for Oxygen Reduction Reaction Catalysis: Tailored Intrinsic Activity of Pt Sites

Meeting the long-term durability target still poses a significant challenge for the large-scale commercialization of proton exchange membrane fuel cells (PEMFCs) for electric vehicle application. The air impurities (e.g., SO2, NOx, and halogen ions) are regarded as one of the major factors that significantly suppresses the output performance of fuel cells. Herein, we revealed the origin of halogen ions poisoning on Pt sites of the state-of-the-art Pt/C catalyst for oxygen reduction reaction (ORR) catalysis through adsorption kinetic investigation and density functional theory (DFT) calculations. Results show that the origin of halogen ions poisoning on Pt sites not only arises from the decrease of Pt reactive sites for ORR catalysis through covering the Pt surface but also depends on the tailored Pt intrinsic activity by modifying the Pt electronic structure that relies on the bonding form between the Pt and impurity. The poisoning kinetics of halogen ions follows the decrease trend of I– > Br– > Cl– > F– with poisoning rates of 1.33, 0.36, 0.20, and 0.19 h–1, respectively. Both Br– and I– mainly bond with Pt in the form of covalent bonding that leads to occupation of the Pt 5d orbital and thus impedes the interaction with the 2p orbital of oxygen, leading to the decrease of ORR activity, while both Pt–F and Pt–Cl are the primary ionic bonding that makes the Pt 5d orbital electrons decrease and thus promotes the formation of the Pt–O bond. The ORR rate-determining step is changed from the *OH hydrogenation step (*OH → H2O) for normal ORR catalysis without impurities to the O2 hydrogenation step (O2 → *OOH) for Pt sites poisoned by Cl–, Br–, and I–. This work provides an insight into the poisoning mechanism of impurities on Pt sites and offers informative guidance for the development of robust antipoisoning catalysts for PEMFC application.