Highly Dispersive Gold Nanoparticles on Carbon Black for Oxygen and Carbon Dioxide Reduction

Highly dispersive Au nanoparticles on carbon black (Au NPs/CB) were synthesized in situ with co-present two different reducing agents of NaBH₄ at various concentrations and citrate at a constant concentration of 3 mM. The average diameters of Au NPs on carbon support were in the range from 5.8 (±2.4) to 2.0 (±0.4) nm, with 50 particles quantified. Electrocatalytic activities of as-prepared Au NPs/CB were explored for oxygen reduction reaction (ORR) in basic solution with rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) voltammetry. In results, the Au NPs/CB synthesized with 0.2 mM NaBH₄ (2.0 nm of Au NPs diameter) represented the highest ORR catalytic activity with electron transfer number of 3.9 and mass activity of 0.25 mA cm⁻² μg⁻¹as well as a perfect resistance to methanol contamination. Especially, the half-wave potential of ORR curve which related to the kinetics of oxygen reduction was more positive compared with previously reported Au-based ORR catalysts. In addition, the Au NPs/CB prepared with 0.2 mM NaBH₄ was also examined as a CO₂ reduction catalyst in KHCO₃ with KCl solution with scanning electrochemical microscopy (SECM). CO₂ was reduced to CO selectively without hydrogen evolution at Au NPs/CB substrate electrode, which was directly monitored with an electrochemical CO microsensor as a tip electrode in SECM. In addition, we have identified the products of CO₂ reduction through gas chromatography (GC)-mass spectrometry (MS), flame ionization detector (FID), and thermal conductivity detector (TCD).