Platinum nanoparticles with superacid-doped polyvinylpyrrolidone coated carbon nanotubes: electrocatalyst for oxygen reduction reaction in high-temperature proton exchange membrane fuel cell

Abstract

In order to improve the catalytic activity and durability of proton-exchange-membrane-fuel-cells (PEMFCs), Nafion-free Pt-based catalyst using the superacid-doped polymer coated multiwall carbon nanotubes (MWCNTs) was investigated. The modification and nano polymerization of MWCNTs were developed by polyvinylpyrrolidone (PVP). The following observations were made in the presence of polymer: better dispersion of MWCNTs, higher thermal stability of MWCNT/PVP than that of pristine MWCNT up to 450 °C as tested by thermal gravimetric analysis (TGA), homogeneous distribution of Pt without agglomeration as observed by transmission electron microscope (TEM), and not too much difference in Pt loading amount as analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Modification of the electrocatalyst was determined by Raman spectroscopy, X-ray diffraction (XRD) patterns and TEM. Proton conductivity of the electrocatalysts was carried out by employing the super proton conductor phosphotungstic acid (PWA). Presence of PWA in the MWCNT/PVP/Pt as well as electrostatic interaction between PVP chains and PWA particles confirmed based on X-ray photoelectron spectroscopy (XPS). We developed a novel cross-linking between MWCNTs using PWA/PVP composite. The individual MWCNTs were cross-linked with each other to form a network structure after addition of PWA which modulates the electronic network structure, considerably enhanced the electrocatalytic activity toward the oxygen reduction reaction (ORR) in acidic media with the most efficient four-electron transfer process. The results showed that by doping of PWA into MWCNT/PVP/Pt, utilization efficiency of the catalyst was significantly improved. The findings have huge implications for electrocatalysts and high-temperature proton-exchange-membrane (HTPEM) fuel cell systems.