Structurally-Tuned Nitrogen-Doped Cerium Oxide Exhibits Exceptional Regenerative Free Radical Scavenging Activity in Polymer Electrolytes

Abstract

The regenerative free radical scavenging activity of cerium oxide (CeO2) nanoparticles was improved by tuning its microstructure via nitrogen doping (N-doping). Commercially available CeO2 (60 m2/g) and high-surface-area CeO2 (synthesized in-house; 220 m2/g) were doped with nitrogen by annealing in a nitrogen rich atmosphere. The evolution of CeO2 microstructure in Nitrogen doped (N-doped) commercial CeO2 (20 m2/g) and N-doped high-surface-area CeO2 (90 m2/g) was studied. XPS and Raman measurements revealed that N-doping of CeO2 enhanced Ce3+ surface concentration and concomitantly increased surface non-stoichiometry (surface oxygen vacancy concentration). XRD analysis of the doped samples confirmed that the nitrogen atom replaced the oxygen atom in the CeO2 lattice, yielding a lower electron density region around the N-doped sites. The high-surface-area CeO2 and its N-doped version had significantly larger lattice parameters than that of commercial CeO2 and its doped version. XAS studies revealed that the N-doped high-surface-area CeO2 had more Ce3+ active clusters than its counterparts, and effectively retained its active Ce3+ active clusters upon exposure to reactive oxygen species (ROS). To confirm, the regenerative ROS (hydroxyl radical) scavenging ability of N-doped CeO2 nanoparticles within the polymer electrolyte membrane (PEM) of an operating polymer electrolyte fuel cell (PEFC) was evaluated using in-situ fluorescence spectroscopy. The N-doped high-surface-area CeO2 showed at least 100 hours of efficient and quantitative ROS scavenging under harsh conditions - a 15-fold improvement over previous reports. This study unequivocally demonstrates that N-doping increases both the number of Ce3+ active clusters in the lattice and the Ce-O bond distance; these structural attributes enhance regenerative ROS scavenging activity of CeO2.