Kinetics of oxygen reduction reaction on supported Pt and several Pt alloy electrocatalysts (PtCo/C and PtFe/C) have been investigated in terms of the effect of alloying on the initiation and extent of surface oxide formation (water activation:  xH2O + Pt*(M) → (M)Pt−[OH] x + xH+ + xe-). For this, a systematic RRDE investigation has been conducted in trifluoromethane sulfonic acid (TFMSA) as a function of concentration (in the range 1 to 6 M) which corresponds to a change in mole ratio of water/acid from 50:1 in 1 M to 4:1 in 6 M TFMSA. This change in relative amount of water in the various concentrations can also be indirectly correlated to the relative humidity in an operating PEM fuel cell. The scope of this effort was (a) to confirm the shift and lowering of water activation on supported Pt alloy electrocatalysts relative to Pt at lower concentrations (1 M); (b) to compare the inherent activity for ORR on supported Pt and Pt alloy nanoparticles without the effect of oxide formation via activation of water, this was enabled at higher concentrations of TFMSA (6 M); (c) to relate the activation energy values at 1 M for Pt and Pt alloy electrocatalysts for further insight into the nature of the rate-determining step in the mechanism; and (d) to examine the relative formation of peroxides via a parallel pathway for Pt and Pt alloy electrocatalysts in 1 and 6 M TFMSA. Our results confirm that for fully hydrated systems akin to 1 M concentration the alloys shift the formation and extent of water activation on the Pt alloy surfaces; this has been correlated with in-situ XAS data (changes to Pt electronic states and short-range atomic order) as well as via direct EXAFS probe of the formation of oxygenated species above 0.75 V (typical potential for initiation of surface oxides on Pt). The lowering of oxide formation agrees well with the extent of enhancement of ORR activity. Activation energy determinations at 1 M concentration however revealed no difference between Pt and Pt alloys, indicating thereby that the rate-limiting step remains unchanged. At lower water activity (6 M) with negligible water activation (and hence surface oxides), the Pt surface was found to possess a higher activity for ORR as compared to the alloys. In addition, the determination of peroxide yield on the Pt surface showed that there was variation both in terms of alloy formation as well as the water activity at the interface. All these results have been discussed in the context of a PEM fuel cell operating in the low to medium temperature range (70−120 °C) and humidity variation (100 to 10%).

Neutron reflectometry measurements show that lamellar structures composed of thin alternating water-rich and Nafion-rich layers exist at the interface between SiO 2 and the hydrated Nafion film. Lamellae thickness and number of layers increase with humidity. Some lamellae remain in the film after dehydration. Multilayer lamellae are not observed for Nafion on Au or Pt surfaces. Instead, a thin partially hydrated single interfacial layer occurs and decreases in thickness to a few angstroms as humidity is reduced to zero. The absorption isotherm of the rest of the Nafion film is similar to that of bulk Nafion for all three surfaces investigated. The observed interfacial structures have implications for the performance, reliability, and improvements of fuel cell proton exchange membranes and membrane electrode assemblies.

An analysis of X-ray absorption spectroscopy ͑XAS͒ data ͓X-ray absorption near-edge structure ͑XANES͒ and extended X-ray absorption fine structure ͑EXAFS͔͒ at the Pt L 3 edge for Pt-M bimetallic materials ͑M = Co, Cr, Ni, Fe͒ and at the Co K edge for Pt-Co is reported for Pt-M/C electrodes in HClO 4 at different potentials. The XANES data are analyzed using the ⌬ method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode ͑RHE͒ representing a reference spectrum. These ⌬ data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt-M. This OH chemisorption, decreasing in the direction Pt Ͼ Pt-Ni Ͼ Pt-Co Ͼ Pt-Fe Ͼ Pt-Cr, is directly correlated with the previously reported fuel cell performance ͑electrocatalytic activities͒ of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt-Ni and Pt-Co clusters were more homogeneous with M atoms at the surface, while the Pt-Fe and Pt-Cr clusters had a "Pt skin." The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the presence of the Pt skin, or lateral interactions when M-OH groups exist on the surface.

A combined theoretical and experimental analysis of the electrode potential dependencies of activation energies is presented for the first step in oxygen reduction over platinum and platinum alloy catalysts in both polycrystalline and carbon supported form. Tafel data for several of the catalysts are used to predict potential-dependent activation energies for oxygen reduction over the 0.6-0.9 V range in strong and weak acid. Comparisons with the theoretical curve show good agreement above 0.8 V, suggesting a fairly constant preexponential factor. Arrhenius determinations of activation energies over the 0.7-0.9 V range yield little trend for weak acid, possibly because of the larger uncertainties in the Arrhenius fits, but the strong acid results have smaller uncertainties and for them the measured activation energies trend up with potential.

X-ray absorption spectroscopy (XAS) is utilized in situ to gain new insights into the electronic and chemical interactions of anions specifically adsorbed on Pt/C. A novel difference methodology was utilized, along with full-multiple scattering calculations using the FEFF8 code, to interpret the X-ray absorption near edge structure (XANES). Significant direct contact (“specific”) anion adsorption occurs in 1 M H2SO4 and 6 M TFMSA, while it does not in 1 M HClO4 and 1 M TFMSA. This specific anion adsorption significantly hinders O(H) chemisorption, particularly formation of subsurface O, causes the Pt nanoparticle to become more round, and weakens the Pt−Pt bonding at the surface. The specific anion adsorption becomes site-specific only after lateral interactions from other chemisorbed species such as OH on the surface force the anions to adsorb into specific sites. Alloying has a profound effect on the strength of the anion adsorption and whether site-specific or just specific adsorption occurs.