The comprehensive investigation of ternary PtIrCo-alloy nanoparticles supported on 3D carbon aerogel matrix toward Methanol Oxidation Reaction (MOR) and Oxygen Reduction Reaction (ORR) is focused on CO tolerance in MOR, methanol vs. oxygen selectivity in ORR, and the structural changes within the metal alloy nanoparticles caused by the heat treatment and the electrochemical conditioning. Amorphous trimetallic Pt59Ir34Co7-nanoparticles with mean size of 1.47 +/- 0.21 nm were formed within 3D CA matrix in supercritical CO2 (scCO(2)) from the Pt, Ir, and Co organometallic precursors. Depending on the heat-treatment temperature (600 degrees C or 900 degrees C), the amorphous organometallic phase of Pt, Ir and Co was transformed into a single-phase fcc PtIrCo-alloy nanoparticles with a mean particle size of 1.68 +/- 0.33 nm and 2.68 +/- 0.61 nm, respectively. The fcc lattice parameters of the PtIrCo-alloy (0.389 nm at 600 degrees C and 0.386 nm at 900 degrees C) were smaller than that of the Pt fcc lattice (0.392 nm) and the lattice parameters reduced proportionally to the heat-treatment temperatures. After 50 electrochemical cycles, PtIrCo/CA catalysts display improved MOR specific activity (similar to 2.5 X Pt/C), mass activity (similar to 3 X Pt/C) at 0.7 V vs. SHE, higher CO-tolerance, and better electrochemical stability than Pt/C, but less CO-tolerance and stability than PtRu/C. Regarding ORR, the PtIrCo/CA catalysts demonstrate enhanced ORR specific activity (similar to 5.5 X Pt/C) and mass activity (similar to 6.3 X Pt/C) at 0.7 V vs. SHE along with superior methanol-tolerance in comparison to Pt/C. The structural changes associated with PtIrCo/CA fcc-lattice parameter with respect to Pt-lattice have contributed to better MOR and ORR activities. Faster surface diffusion associated with weak adsorption of hydroxide species and higher selective adsorption of oxygen in comparison to methanol onto PtIrCo-alloy surface could have contributed to their better CO-tolerant MOR and methanol-resistant ORR activity compared to Pt-catalyst. (C) 2015 Elsevier Ltd. All rights reserved.