Abstract
The main drawback hindering the commercialization of direct alcohol fuel cells (DAFCs) is the sluggish reaction kinetics experienced on the electro-catalyst surface during alcohol electro-oxidation. It is of paramount importance to design cost-effective and highly efficient electro-catalysts for the commercialization of DAFCs. Palladium (Pd) based metal nanoparticles supported on carbon can alleviate the slow reaction kinetics experienced during alcohol electro-oxidation. The choice of carbon support greatly influences the metal nanoparticles' electrochemical properties during alcohol electro-oxidation in DAFCs. On this basis, this study synthesized carbon nano-onions (CNOs) and carbon nanotubes (CNTs) support materials with high surface area and suitable porosity using simple and inexpensive methods. This study focused on synthesizing Palladium-Niobium (Pd-Nb) nanoparticles supported by CNOs and CNTs to improve alcohol electro-oxidation in DAFCs.
CNOs were synthesized using the inexpensive soot-based approach from soot collected from a burning candle. CNTs were synthesized using the chemical synthesis method. The Pd/CNOs, Pd-Nb/CNOs, Pd/CNTs, and Pd-Nb/CNTs electro-catalysts were synthesized using the alcohol reduction method. Fourier-transform infrared spectroscopy (FTIR) analysis revealed the presence of oxygen moieties (O–H, C=C, and C=O) that anchored the metal nanoparticles on the surface of the CNOs and CNTs support. The High Resolution-Transmission Electron Microscopy (HRTEM) analysis confirmed the well-dispersed Pd-Nb metal nanoparticles on the surface of the CNOs and CNTs due to the anchoring ability of the oxygen moieties. The particle size of the Pd-Nb nanoparticles on the surface of CNOs and CNTs was 6.69 and 5.02 nm, respectively. The Raman spectroscopy of Pd-Nb/CNOs and Pd-Nb/CNTs displayed the highest ID/IG ratio of 1.61 and 1.86, respectively, confirming increased defects upon the introduction of Nb to the Pd/CNOs and Pd/CNTs structure. The electrochemical performance of the synthesized electro-catalysts was examined during isopropanol and ethanol oxidation. Pd-Nb/CNOs displayed the highest If/Ib ratio of 2.71 and 8.11 for isopropanol and ethanol oxidation, respectively. Pd-Nb/CNTs displayed the highest If/Ib ratio of 6.82 and 10.34 for isopropanol and ethanol oxidation, respectively, compared to the monometallic electro-catalysts. Chronoamperometry curves displayed low relative current loss of 55.3 % (isopropanol oxidation) and 36.5 % (ethanol oxidation) for Pd-Nb/CNOs and 53.8 % (isopropanol oxidation) and 39.6 % (ethanol oxidation) for Pd-Nb/CNTs. The results obtained in this study confirmed that the addition of Nb to Pd/CNOs and Pd/CNTs improved the catalytic activity and poisoning tolerance of the electro-catalysts. Both the bifunctional and electronic effects significantly increased the activity of the electro-catalysts upon adding Nb to the synthesized electro-catalysts.
Keywords: Carbon nano-onions, carbon nanotubes, electro-catalyst, alkaline medium, direct alcohol fuel cell.