267596 Influence of Chemistry and Structure of Nitrogen Doped Mesoporous Carbon Support On the ORR Activity of Platinum for Fuel Cell Applications
It is well known that the bonding between Pt and graphitic carbon surface is weak . This weak interaction has important implications on the mass activity, dispersion and stability of Pt nanoparticles supported on these materials. One way that all three can be concomitantly improved is by supporting the Pt electrocatalyst on a support material that interacts more strongly. This can be accomplished on carbon materials by introducing an adatom to the surface that disrupts the π-stabilized sp2 bonding, such as nitrogen . However, the physicochemical properties of nitrogen doped carbon are multivariate and complex. Here, we investigate the effect of both the nitrogen content and the level of disorder in the support microstructure on the catalyst properties and stability of supported Pt.
Nitrogen-doped mesoporous carbons were prepared using a hard templating synthesis procedure. The template used was SBA-15, which has a hexagonally ordered mesoporous structure .One of the unique properties of SBA-15 is that the carbon casted on it is its exact inverse replica. This allows for rational control of the pore structure of the carbon. Here, carbons with different pore structure were made by modifying the template with various secondary sol-gel treatments. Pt was deposited on such carbons using a modified polyol process. The physical structure and surface chemistry of the carbons were characterized using BET, XPS, XRD, Raman and TEM techniques. The activity of the supported Pt toward the oxygen reduction reaction (ORR) was analyzed by thin-film RDE method and stabilities were investigated by AC polarography accelerated degradation testing and cyclic voltammetry.
The SBA-15 was prepared by self-assembly of triblock copolymer PluronicŪ P123 (E20PO70EO20, BASF) and tetraethyl orthosilicate (TEOS) catalyzed by 2 M HCl . As synthesized SBA-15 was modified further with 0, 4 and 8 mL of TEOS in 2 M HCl which resulted in SBA-15 with decreasing pore diameter, evidenced by the pore size distribution plot from nitrogen adsorption isotherm. Then, pyrrole was vacuum infiltrated into the templates, polymerized to polypyrrole, and carbonized at high temperature for 3 h in N2 atmosphere. Finally, the template was removed in hot 10 M KOH. Pt was deposited on these carbon supports with hexachloroplatinic acid as a Pt precursor and ethylene glycol as the stabilizing and reducing reagent. The ORR activities of Pt on these carbons support were investigated in O2 saturated 0.1 M HClO4 at 1600 RPM and 25°C.
In Figure 1, the templates are denoted as SBA for as prepared and SBA-3.5/4 and SBA-3.5/8 for SBA-15 modified with 4 and 8 mL of TEOS respectively. The resulting nitrogen doped mesoporous carbon supports are referenced as CPPy-3.5/X and are shown below their respective templates.
Preliminary results suggest that we have prepared nitrogen doped mesoporous carbons with varying degree of structural order. The ORR polarization curves for Pt deposited on CPPy, CPPy-3.5/4 and CPPy-3.5/8 are shown in Figure 2. A slight enhancement in ORR activity of Pt on CPPy-3.5/8 can be observed. Further characterization and ORR activity of Pt on such carbons will be presented and discussed.
Figure 1. TEM images of nitrogen doped mesoporous carbons and their respective templates. The 3.5 g of original template with ordered mesopores (SBA) was modified with 4 and 8 mL of tetraethyl orthosilicate (TEOS) to give SBA-3.5/4 and SBA-3.5/8. The carbons synthesized on these templates are labeled CPPy, CPPy-3.5/4 and CPPy-3.5/8.
Figure 2. ORR polarization curves of Pt supported on nitrogen doped mesoporous carbon. The curves were measured in 0.1 M HClO4 at 1600 RPM with a scan rate of 5 mV/s.
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