Sulphur-Doped Ordered Mesoporous Carbon Hollow Spheres with High Catalytic Activity for the Oxygen Reduction Reaction and Exceptional Electrochemical Stability

Abstract

We reported the synthesis and characterization of the novel and less explored methanol-functionalized Sulphur-Doped Ordered Mesoporous Carbon Hollow Spheres (S-HWf) as metal-free electrocatalyst for the Oxygen Reduction Reaction (ORR) in alkaline media. The S-HWf was synthetized by the silica template-based method, using resorcinol and formaldehyde as carbon source. The doping was made in situ using 2-thiophenemethanol as sulphur source. The functionalization was carried out with methanol solution (0.5 mol L-1) with the aid of intermittent microwave heating. The catalytic activity of S-HWf was compared with that of non-doped Ordered Mesoporous Carbon Hollow Spheres (HW) synthetized by the same method. The morphology of S-HWf and HW analyzed in a Field Emission-Scanning Electron Microscope (FE-SEM) resulted in rough hollow spheres. The diameter and wall thickness were ~304 and ~90 nm for HW, while for S-HWf were ~248 and ~50 nm, respectively. The use of 2-thiophenemethanol as dopant, had an important effect in the morphology. The chemical surface composition determined by X-ray Photoelectron Spectroscopy (XPS) included the presence of C, O, and Si for HW. In addition to these elements, S was also detected at S-HWf (0.4 at. %). The textural properties were determined by N2 adsorption/desorption measurements. S-HWf and HW showed a surface area of 1007 and 505 m2 g-1, respectively, with micro-mesoporosity. Analysis of the electrocatalysts by X-Ray Diffraction (XRD) showed their amorphous structure with low-intensity crystalline phases. Meanwhile, from Raman spectroscopy S-HWf exhibited better ordering in its structure due to a lower value in ID/IG the intensity ratio compared to HW. The catalytic activity of the electrocatalysts for the ORR was evaluated by Rotating-Ring Disc Electrode (RRDE) technique. S-HWf showed a higher performance than HW, with an onset potential of 0.88 V half-wave potential 0.81 V and a current density (j) of -1.49 mA cm-2 at 0.8 V (all potentials vs. RHE). Moreover, its electron transfer number was n= 3.9 with a H2O2 production of ~3%. Furthermore, after accelerated degradation tests (3000 cycles, between 0.6 and 1.0 V), S-HWf showed an improvement in catalytic activity with onset potential of 0.88 V, half-wave potential of 0.82 V and j= -1.69 mA cm-2 at 0.8 V. Also, its n was 3.9 with a production of H2O2 of ~2%. In this study it was found that doping with sulphur and functionalizing with methanol have a positive effect on the catalytic activity for the ORR and the electrochemical stability of Ordered Mesoporous Carbon Hollow Spheres. The electrochemical behavior of the S-HWf electrocatalyst indicated that it is a potential candidate for Anion Exchange Membrane Fuel Cells (AEMFC) cathode applications.