Polimer elektrolit membranlı yakıt pilleri için elektrokatalizör olarak karbon aerojel destekli saf Pt ve Pt-Cu ikili alaşım nanoparçacıklarının süperkritik depozisyon yöntemi ile hazırlanması ve karakterizasyonu

Supercritical deposition method was used to synthesize carbon aerogel supported Pt, Pt-Cu and carbon black supported Pt-Cu nanoparticles. CAs with average pore diameters of 6, 8 and 19 nm (CA6, CA8 and CA19, respectively) were synthesized and impregnated with Pt(cod)me2 precursor using supercritical carbon dioxide followed by the thermal conversion at various temperatures between 200-1000˚C. All of the prepared CAs have high surface areas with very sharp pore size distributions. XRD and TEM results show increased Pt particle size with increasing conversion temperature with a homogenous distribution of nanoparticles on the CA supports. Cyclic voltammetry was used to determine the effect of CA pore properties on electrocatalytic activity. At a conversion temperature of 400 oC, the highest electrochemical surface area value were obtained for the CAs with higher average mesopore size (Pt/CA19). Furthermore, Pt/CA19 showed good mass activity whereas Pt/CA6 and Pt/CA8 had lower activity values towards ORR. The mass activity values for Pt/CA19 increased with increasing conversion temperature, except for the sample converted at 1000 ˚C which exhibited the lowest mass activity. The specific activity increased significantly with the conversion temperature up to 600 oC which gave a value six times that obtained at 200 oC. At 800 oC, the specific activity decreased slightly, probably due to a change in the CA structure at this elevated conversion temperature. The adsorption isotherms of Cu(tfa)2 on CA19 were measured in 35 oC and 10.7 MPa in scCO2 and fitted to Langmuir adsorption model. Both simultaneous and sequential SCD methods were applied for the preparation of preliminary Pt-Cu/CA19 samples, however, simultaneous SCD resulted low Cu loadings due to competitive adsorption of Pt(cod)me2 and Cu(tfa)2 on CA19. Sequential SCD was used to prepare Pt-Cu/CA19 samples. Decomposition of the precursors were carried out thermally under H2 flow, followed by the annealing under N2 at three different temperatures, 600 oC, 800 oC and 950 oC. The effects of deposition order, annealing temperature and metal composition on the average Pt-Cu particle size, dispersion, and particle morphology and electrocatalytic activity were investigated. Two sets of Pt-Cu/CA19 samples were prepared with different Pt:Cu mole ratios; first set was prepared with Pt:Cu mole ratio of 1:1 and the second set was prepared with Pt:Cu mole ratio of 1:3 using the single component adsorption isotherms of Pt(cod)me2 and Cu(tfa)2 in scCO2 and changing the deposition order. XRD and EDX mapping showed that all prepared samples formed disordered Pt-Cu alloy nanoparticles on CA19. The metal composition did not have a significant effect on the average particle size or dispersion of Pt-Cu nanoparticles. XRD and TEM images showed homogenous distribution of Pt-Cu nanoparticles on CA19 with narrow particle size distributions for all samples. All prepared samples have average Pt-Cu nanoparticles sizes between 1.8 nm and 4.5 nm. Furthermore, increasing annealing temperatures found to promote alloying process without excessive growth of Pt-Cu nanoparticles. Adsorption of first Cu(tfa)2 on CA19 resulted in low electrocatalytic activities for both 1:1 and 1:3 samples whereas the samples prepared by first adsorbing Pt(cod)me2 on CA19 had promising electrocatalytic activities. Samples with Pt:Cu mole ratios of 1:3 showed better catalytic activities than the ones with 1:1. Enhanced catalytic activity as compared to pure Pt/CA19 was obtained for Pt-Cu/CA19 sample (0.123 A/mgPt) annealed at 800 oC, where Pt(cod)me2 adsorbed firstly, with a Pt:Cu mole ratio of 1:3. The effect of support was also investigated by impregnating CA19 and Vulcan (carbon black) with Pt(cod)me2 and Cu(tfa)2 via in-situ sequential SCD method with Pt:Cu mole ratios of 1:3. The average particle size of Pt-Cu on CA19 was significantly lower (2.8 nm) than the average particle size on carbon black (4.9). Mole ratios obtained from XRD and XRF were quite close to each other when compared to other Pt-Cu/CA19 samples. Moreover, Pt-Cu/Vulcan sample resulted in higher electrocatalytic activity than the Pt-Cu/CA19 sample (0.09 A/mgPt and 0.04 A/mgPt).