Simultaneous improvement of proton conductivity and chemical stability of Nafion membranes via embedment of surface-modified ceria nanoparticles in membrane surface

The lab-synthesized ceria (CeO₂) nanoparticles were surface-modified to provide proton conductivity. The dopamine sulfonated ceria (CeO₂-DS) nanoparticles were embedded into the thin surface layers of Nafion 212 membranes, resulting in the sandwiched structure. The structure, morphology and properties of the synthesized nanoparticles and membranes were analyzed using a variety of methods including TEM, FTIR, DLS, XRD, XPS, TGA, and SEM-EDS. The CeO₂-DS nanoparticles exhibited excellent OH• and OOH• radical scavenging effect for enhanced chemical stability, accompanied by a simultaneous improvement of proton conductivity of the membrane. The proton conductivity of the Nafion-CeO₂-DS8 membrane was 0.112 ∼ 0.199 S cm^-1 from room temperature to 80 °C, which was about 1.5-fold higher than that of pristine Nafion membrane. Nevertheless, the prepared sandwiched structure membrane demonstrated quite high electrical resistance due to the absence of electrically conductive ceria nanoparticles in the thick middle layer. Consequently, not only the durability but also the cell performance of the membrane was significantly enhanced, illustrating the maximum power density of 522 mW cm^-2, which was much higher than those of the pristine and single-layer composite membranes, 460 mW cm^-2 and 390 mW cm^-2, respectively.