Nitrogen Doping Effects on Carbon Nanotubes and the Origin of the Enhanced Electrocatalytic Activity of Supported Pt for Proton-Exchange Membrane Fuel Cells

Abstract

Carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (CNx) were synthesized by the floating catalyst chemical vapor deposition (FCCVD) method. Pt nanoparticles were deposited onto the two catalyst supports with the ethylene glycol reduction method. Different from CNTs that contain straight hollow tubes, CNx contain a bamboolike structure with kinks along the tubes and more surface defects. X-ray photoelectron spectroscopy (XPS) confirms the nitrogen atoms in the graphite matrix and reveals the chemical natures of the doped nitrogen atoms. Carbon K-edge near-edge X-ray absorption fine structure (NEXAFS) and Raman characterizations reveal higher defectiveness in CNx than CNTs. The unique structure and surface property of CNx leads to a better dispersion of Pt nanoparticles on CNx than on CNTs, as revealed by TEM images. Pt supported on CNx (Pt/CNx) exhibited a higher electrochemical surface area (ECSA) and higher catalytic activity toward oxygen reduction reaction (ORR), in comparison to Pt supported on CNTs (Pt/CNTs). Better performance of Pt/CNx than Pt/CNTs has been confirmed by single-cell fuel cell tests.