Electrospun carbon nanofiber electrodes decorated with palladium metal nanoparticles : fabrication and characterization

Abstract

A new method was investigated to produce a novel oxygen reduction electrode comprised of carbon nanofibers for use in polymer electrolyte membrane (PEM) fuel cells and metal-air batteries. The process involved electrospinning a solution of polymer (polyacrylonitrile), noble metal salt (palladium (II) acetate), and organic solvent (n,n- dimethylformamide) to fabricate a porous, non-woven, free-standing nanofiber mesh. Through experimentation with multiple variables, the optimal electrospinning parameters were quantified. Post-process heating of the electrospun nanofibers included stabilization in air environment at 280⁰C for 2 hours, followed by carbonization in grade 5.0 argon environment to temperatures between 800 and 1100⁰ C for times varying between 1 minute to 1.5 hours. The carbonization step served the purpose of converting insulating polymer into conductive amorphous carbon and precipitating nanoparticles of palladium in a homogeneous distribution throughout the electrode. The electrode was characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD), transmission electron microscopy (TEM), microprobe station, and x-ray adsorption near edge structure (XANES). Electrochemical performance was characterized using cyclic voltammetry (CV), rotating disk electrode (RDE), and testing in a PEM fuel cell. It was demonstrated that palladium crystal size and particle size increased with heat treatment time and temperature. Lower concentrations of PAN in solution had the effect of thinner nanofibers (100-400nm diameters), which led to faster particle growth.

(cont.) Particle sizes were often distributed in a bimodal Gaussian distribution, centered around values on the order of lOnm and 100nm. In-situ TEM allowed for particle formation and growth to be investigated. Cross-sectional TEM showed that particle nucleation occurred within the fibers. Electrodes were spun as thin as 7 microns, and contained no significant amounts of graphite or palladium oxide. Electrochemical surface area was 7.17 m²/g catalyst, and the performance was comparable to commercially available E-TEK electrodes on a catalyst cost per power basis. It was shown platinum salts worked well in the process, allowing platinum electrodes to be fabricated.