To reveal a possibility of developing catalytic activity of Pt/carbon ORR catalyst using “anchor effect” of Pt atoms, which are embed in graphite structure, Pt species were directly embed/deposited on glassy carbon (GC) disks via arc plasma deposition (APD) method. APD is a type of physical vapor deposition methods in which the metal target was vaporized to plasma under pulsed arc-discharge, followed by fabricating nano-particles (or thin films) on the substrate. The particle size, morphology, concentration and the binding affinity for substrate material of the deposited metals are varied by the operating factors of e.g. arc voltage V, capacitance of capacitor C, triggered frequency F and shot number N. Hence in this study, the characters and the ORR performance of the obtained Pt/GC samples were investigated focusing on the difference of “discharge energy” for generating Pt plasma during the APD process.
2. Experimental
Commercial GC disks (φ 4 mm, T 4 mm) were placed into the ultrahigh-vacuumed APD chamber, and the Pt plasma was irradiated under the APD conditions of V=300 V, C=50-300 μF (discharge energy=1/2CV 2=2.25-13.5 J) and F=1 Hz. The values of N were optimized within 30-300 to adjust the amount of Pt on the GC disks irradiated under the different discharge energies. The obtained Pt/GC samples were denoted by GC-50, 100, 200 and 300 using the C value; the amount of Pt on the samples was estimated (using ICP-MS) to be 1.03, 0.955, 1.43 and 0.955 μg cm-2 for GC-50, 100, 200 and 300, respectively. As a comparison, GC disks casted with “catalyst ink” of the commercial catalyst (TEC10E50E, TKK) dispersed in iso-propanol aq. solution was also prepared. The obtained TEC10E50E/GC sample was denoted by TEC, on which the amount of Pt was 18 μg cm-2. The characterization of the samples was conducted with XRD, Raman, SEM-EDX, TEM, and the ORR performance was investigated with CV and LSV using RDE under 0.1 M HClO4 aq. solution at 30ºC.
3. Results and Discussion
The comparison of the Raman spectra of the obtained Pt/GC samples revealed that the IG/ID ratio decreased with the increase of discharge energy; the values were 0.81 for GC-50, 0.77 for -100, 0.74 for -200, 0.71 for -300 while the value of GC disk before the plasma-irradiation was 0.88. The results suggested that the Pt species broke the C-C bond and embed in graphite structure during the APD process. Based on the CV and LSV curves, the specific activity (Is) for ORR against the "electrochemically effective surface area" (ECA) was estimated and the values were compered in terms of the IG/ID ratio (Fig. 1); the Is of the Pt/GC samples were increased with the increase of IG/ID ratio (increased damage of graphite structure). The highest value of 260 μA cm-2-ECA for GC-300 was as 1.2 times as 210 μA cm-2-ECA of the typical Is for TEC. The mass activity (Im) was also estimated for GC-300 and TEC; the values were 302 mA g-1-Pt and 88.6 mA g-1-Pt, respectively, suggesting that the strong interaction between the carbon and “embed Pt” on the Pt/GC samples activated the ORR catalytic performance.