Plasma Treatment As an Effective Way of Modifying Carbon-Based Electrocatalysts for an Enhanced Oxygen Reduction Reaction

Abstract

Oxygen reduction reaction (ORR) is pivotal in renewable energy technologies such as in fuel cells and metal-air batteries. Precious metal-free electrochemical ORR is a critical component in designing cost-effective electrochemical energy conversion devices. In this report we demonstrate an efficient surface modification of carbon nanotubes (CNT) through one-step oxygen plasma irradiation that induces doping and charge redistribution around the doped heteroatom oxygen that promotes ORR activity. The defect sites generated owing to oxygen dopant in CNTs were confirmed by Raman spectra, X-ray photoelectron spectroscopy (XPS) surface composition and CHNO elemental analysis. The O-doped CNTs were thoroughly characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) measurements. Our research shows that plasma-treated single-walled carbon nanotubes (SWCNT) are more effective ORR catalyst compared to multi-walled carbon nanotubes (MWCNT) due to the inherent structure of SWCNT that can access more defects and surface functional groups than MWCNT.In the following study, plasma modification approach was implemented in the synthesis of a highly active robust Metal-Nitrogen-Carbon (M-N-C) catalysts through NH3-plasma pretreatment on high surface area black pearl carbon. Pre-treated FeNC/NH3-C contains Fe3C and Fe-Nx as catalytically active sites. XPS confirms the presence of a high surface concentration of pyrrolic to pyridinic nitrogen and ferrous sites on FeNC/NH3-C that promotes low oxygen reduction overpotential (0.80 V vs RHE in 0.5 M H2SO4 and 0.96 V vs RHE in 0.1 M KOH). Hence plasma activates the surface and incorporates nitrogen which forms stable Fe-N coordination for accelerating oxygen reduction. Importantly, FeNC/NH3-C shows lower peroxide yield at 0.6V/RHE (<20% in 0.5 M H2SO4 and <2 % in 0.1 M KOH) that can reduce the detrimental effect of H2O2 on the catalysts support and exhibits high stability, seen from thousands of potential cycling stability tests of FeNC/NH3-C in acid solutions. Thereby this plasma-based new synthetic approach can promote more active and stable non-precious metal catalysts.