Understanding the Effects of Surface Chemistry and Microstructure on the Activity and Stability of Pt Electrocatalysts on Non-Carbon Supports

Abstract

In the first 2 1⁄2 years of this project, we have clearly shown that the electrocatalyst support has an impact on the activity and stability of Pt through at least two different mechanisms: 1) Electron transfer between the catalyst and support that shifted the binding energy of oxygen to more positive potentials, leaving a clean Pt surface for the ORR; and 2) Lattice matching, which led to preferential faceting of Pt (111), which has a very high ORR activity. In fact, preferential faceting and strong bonding between the catalyst and support led to a Pt/ITO catalyst with a specific activity of 750 μA/cmPt and mass activity of 621 mA/mgPt (both exceed 2015 DOE targets) with no measurable reduction in activity over 1000 accelerated degradation cycles between 0.0V and 1.4 V. vs. NHE. Pt/WC also showed improved activity compared to Pt/C, though surface oxidation and dissolution led to low catalyst life. However, we were also able to report, for the first time, a detailed mechanism for the electrochemical dissolution and instability of WC and WO3 in acid media. In addition, using templated carbon, we have found that pore sizes < 5 nm are not useful for ORR electrocatalysts. We have also found that having an amorphous support nanostructure significantly improves the Pt utilization. In addition, we found that doping carbonbased supports with surface N functional groups improves the stability of the carbon relative to commercial carbon and allows for very small Pt particle sizes (< 2 nm) even at very high Pt loading (50 wt%). This combination led to Pt/NOMC (OMC = ordered mesoporous carbon) catalysts with better activity and stability than commercial Pt/C (BASF). We also found during degradation testing of the N-OMC that it has very high double layer capacitance for its surface area Understanding the Effects of Surface Chemistry and Microstructure on the Activity and Stability of Pt Electrocatalysts on Non-Carbon Supports Mustain – University of Connecticut Hydrogen Fuel Cells 2 DOE Hydrogen and Fuel Cells Program 2013 Annual Merit Review and Peer Evaluation Meeting 4 times greater than that of Pt/C (156 ± 9 mA/mgPt) and far exceeded the 2015 DOE goal for Pt mass activity of 440 mA/mgPt. Characteristic voltammograms for Pt/ITO at various rotation rates are presented in Figure 1a. The higher specific activity of the Pt/ITO was found to be a result of synergistic effects between the surface Sn of ITO and the supported Pt NPs. Perhaps the most significant impact of the ITO was the preferential exposure of {111} facets on the Pt NPs on Pt/ITO compared to Pt NPs supported on carbon black, which typically contain mostly {100} facets. Interestingly, the Pt and ITO seemed to have similar cubic lattice spacing, which resulted in continuous lattice fringes from the ITO substrate to supported Pt particles, indicating that the Pt was grown epitaxially on the ITO surface The ORR activity on low-index crystallographic facets of Pt in a nonadsorbing electrolyte such as perchloric acid is known to increase on the order of Pt(100) ≪ Pt(111) < Pt(110), with a minor difference in catalytic activity between Pt(111) and Pt(110). This difference in ORR activity most likely arises from the structure-sensitive inhibiting effect of OHad species on Pt(hkl), which blocks the active site for O2 adsorption (156 F/g; 25.1 μF/cm) and is competitive with commercial carbons for supercapacitor applications. Most importantly, our initial work has validated our overall approach, allowed us to develop reliable experimental protocols, has used fundamental discoveries to make contributions to real devices and has led to interesting new questions that will be the subject of our work moving forward.