Preparation and thermal treatment influence on Pt-decorated electrospun carbon nanofiber electrocatalysts

Abstract

Crystalline platinum nanoparticles supported on carbon nanofibers were synthesized for use as an electrocatalyst for polymer electrolyte membrane fuel cells. The nanofibers were prepared by a method of electrospinning from polymer solution with subsequent pyrolysis. Pt nanoneedles supported on polyacrylonitrile pyrolyzed electrospun nanofibers were synthesized by chemical reduction of H₂[PtCl₆] in aqueous solution. The synthesized electrocatalysts were investigated using scanning, high resolution transmission and scanning transmission electron microscopies, EDX analysis and electron diffraction. The shape and the size of the electrocatalyst crystal Pt nanoparticles were controled and found to depend on the method of H₂[PtCl₆] reduction type and on conditions of subsequent thermal treatment. Soft Pt reduction by formic acid followed by 100 °C thermal treatment produced needle-shape Pt nanoparticles with a needle length up to 25 nm and diameter up to 5 nm. Thermal treatment of these nanoparticles at 500 °C resulted in partial sintering of the Pt needles. When formic acid was added after 24 h from the beginning of platinization, Pt reduction resulted in small-size spherical Pt nanoparticle of less than 10 nm in diameter. Reduction of H₂[PtCl₆], preadsorbed on electrospun nanofibers in formic acid with further treatment in H₂ flow at 500 °C, resulted in intensive sintering of platinum particles, with formation of conglomerates of 50 nm in size, however, individual particles still retain a size of less than 10 nm. Electrochemically active surface area (ECSA) of Pt/C catalyst was measured by electrochemical hydrogen adsorption/desorption measurements in 0.5 M H₂SO₄. ECSA of needle-shape Pt nanoparticles was 25 m² g⁻¹. It increased up to 31 m² g⁻¹ after thermal treatment at 500 °C, likely, due to amorphous structures removal from carbon nanofibers and retaining of Pt nanoneedle morphology. ECSA of small-size spherical Pt nanoparticles was 26 m² g⁻¹. Further thermal treatment at 500 °C in vacuum decreased ECSA down to 20 m² g⁻¹ due to Pt sintering and Pt active sites deactivation. The thermal treatment of small-size spherical Pt nanoparticles in H₂ flow at 500 °C produced agglomerates of Pt nanoparticles with ECSA of 14 m² g⁻¹.