Efficient electrooxidation of ammonia using platinum supported on titania/sulfur-doped onion like carbon nanoparticles in direct ammonia fuel cells

This study reports on the synthesis of platinum supported on titania oxide nanoparticles decorated on sulfur doped onion-like carbon nanoparticles (S-OLCNs) as electrocatalysts for ammonia oxidation reactions in alkaline media. S-OLCNs (S = 1.6 %) were synthesized in a one-step flame pyrolysis method using thiophene as the starting material and titania nanoparticles were prepared using a microwave-assisted hydrolysis technique (5–10 % titania decoration). Pt/S-OLCN, Pt/TiO2 and Pt/TiO2/S-OLCN electrocatalysts (10 % Pt loading) were synthesized by the ethylene glycol assisted reduction method. Transmission electron microscopy studies confirmed the presence of well dispersed spherically shaped Pt NPs on the surface of the S-OLCN, TiO2 and TiO2/S-OLCN support materials (d = 1.93 ± 0.38 nm, 1.98 ± 0.79 nm, 2.13 ± 0.52 nm, respectively). XRD patterns confirmed the anatase phase for the titania. The Pt/S-OLCN, Pt/TiO2 and Pt/TiO2/S-OLCN and Pt/C electrocatalysts were investigated in the ammonia oxidation reaction (AOR). The existence of the well-dispersed TiO2 (and Pt) on the S-OLCNs gave rise to enhanced AOR catalytic performance when compared to Pt/S-OLCN, Pt/TiO2 and commercial 10 wt% Pt/C catalysts. Furthermore, the Pt/TiO2/S-OLCN electrocatalyst exhibited better long term stability and a faster charge transfer resistance than Pt/S-OLCN, Pt/TiO2 and commercial 10 wt% Pt/C electrocatalysts. These results indicate that the incorporation of TiO2 nanoparticles into Pt/S-OLCN is an effective strategy to improve the AOR activity of Pt by facilitating the electron transfer process during the catalytic reactions.