Modification of Nafion membrane with biofunctional SiO2 nanofiber for proton exchange membrane fuel cells
Abstract Proton currents are an integral part of the most important energy converting structures in biology. We prepared a new type of bioinspired Nafion (Bio Nafion) membrane composited of biofunctional SiO2 (Bio SiO2) nanofiber and Nafion matrix. SiO2 nanofibers were prepared by electrospinning silica sol prepared from tetraethyl orthosilicate. Meanwhile, Bio SiO2 nanofibers were synthesized by immobilizing amino acids (cysteine, serine, lysine, and glycine) on SiO2 nanofibers, which acted as efficient proton conducting pathways that involved numerous H+ transport sites. In our study, the SiO2 nanofibers biofunctionalized with cysteine were further oxidized, and the composite membranes were designated as Nafion Cys, Nafion Lys, Nafion Ser, and Nafion Gly, respectively. We then investigated the different polar groups SO3H, OH, and NH2) of the amino acids that contributed to membrane properties of thermal stability, water uptake (WU) dimensional stability, proton conductivity, and methanol permeability. Nafion Cys exhibited the highest proton conductivity of 0.2424 S/cm (80 degC) Nafion Gly showed the lowest proton conductivity and WU because glycine contains the least number of hydrophilic groups among the amino acids. Overall, the introduction of Bio SiO2 nanofiber to composite membranes significantly improved proton conductivity, dimensional stability, and methanol permeability.