On Pt-based electrode surfaces

In a staged multi-scale approach, the energetics and reaction rates obtained from these calculations can be used to develop coarse-grained models for simulating kinetics and thermodynamics of complex multi-step reactions on electrodes (for example see [25, 26, 27, 28, 29, 30]). Varying levels of complexity can be simulated on electrodes to introduce defects on electrode surfaces, composition of alloy electrodes, distribution of alloy electrode surfaces, particulate electrodes, etc. Monte Carlo methods can also be coupled with continuum transport/reaction models to correctly describe surfaces effects and provide accurate boundary conditions (for e.g. see Ref. [31]). In what follows, we briefly describe density functional theory calculations and kinetic Monte Carlo simulations to understand CO electro oxidation on Pt-based electrodes. 

For details on the potential energy surface (PES) for CO oxidization, the reader is referred to Ref. [18]. In what follows, we describe the second stage of our Multi-scale approach. We show results of our Monte Carlo simulations, to elucidate the CO electrooxidation kinetics on Pt-based electrodes. 

This chapter discusses a staged multi-scale approach for understanding CO electrooxidation on Pt-based electrodes. In this approach, density functional theory (DFT) is used to obtain an atomistic view of reactions on Pt-based surfaces. Based on results from experiments and quantum chemistry calculations, a consistent coarse-grained lattice model is developed. Kinetic Monte Carlo (KMC) simulations are then used to study complex multi-step reaction kinetics on the electrode surfaces at much larger lengthscales and timescales compared to atomistic dimensions. These simulations are compared to experiments. We review KMC results on Pt and PtRu alloy surfaces. 

Methanol Fuel Cells The electrooxidation and dissociation of small organic molecules such as methanol on Pt-based electrodes is one of the most extensively studied systems in terms of both fundamental and applied research in surface (interfacial) electrochemistry (see also Chapter 3.5). It involves a very complex reaction occurring in many steps and various kinds of surface (interfacial) species, including reactants, intermediates, poisons, supporting electrolytes and even solvent... 

The dissociative adsorption of the oxygen molecule is unlikely and the first electron transfer to the oxygen molecule [Eq. (63)] is considered to be the rate determining step. The formation of H2O2 and various forms of platinum oxides on the surface reduces the theoretical potential obtainable by ORR. In fact, potential losses in PEMFC arise predominantly because of the sluggishness of the ORR. The structural sensitivity of ORR has been discussed over the Pt-based electrodes. Pt/C is commonly used for the cathode material, and Pt-based alloys, such as PtCr and PtNi, have also been investigated. 

In the equation 25 n is the number of electrons and F, k and C(02) are Faraday constant, rate constant and bulk O2 concentration, respectively. In addition, P and y are symmetry factors, while E is the electrode potential. The term 0ad relates to total surface coverage by OHads and adsorbed anions. The effect of surface oxides on the metal electrode surface was clearly demonstrated for series of Pt-based electrocatalysts [53, 54], leading to a general recipe for design of electrocatalysts with improved ORR activity. In specific, if oxide formation is hindered onset potential for ORR is shifted to higher anodic potentials. Underlying principles of this route have been set by combining electrochemical measurement... 

Previously, we have proposed that SFG intensity due to interfacial water at quartz/ water interfaces reflects the number of oriented water molecules within the electric double layer and, in turn, the double layer thickness based on the p H dependence of the SFG intensity [10] and a linear relation between the SFG intensity and (ionic strength) [12]. In the case of the Pt/electrolyte solution interface the drop in the potential profile in the vicinity ofelectrode become precipitous as the electrode becomes more highly charged. Thus, the ordered water layer in the vicinity of the electrode surface becomes thiimer as the electrode is more highly charged. Since the number of ordered water molecules becomes smaller, the SFG intensity should become weaker at potentials away from the pzc. This is contrary to the experimental result. 

Catalyst layer architecture As a consequence of the diminishing remrns from ever higher dispersion, the effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years primarily on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm. High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst... 

Besides these generalities, little is known about proton transfer towards an electrode surface. Based on classical molecular dynamics, it has been suggested that the ratedetermining step is the orientation of the HsO with one proton towards the surface [Pecina and Schmickler, 1998] this would be in line with proton transport in bulk water, where the proton transfer itself occurs without a barrier, once the participating molecules have a suitable orientation. This is also supported by a recent quantum chemical study of hydrogen evolution on a Pt(lll) surface [Skulason et al., 2007], in which the barrier for proton transfer to the surface was found to be lower than 0.15 eV. This extensive study used a highly idealized model for the solution—a bilayer of water with a few protons added—and it is not clear how this simplification affects the result. However, a fully quantum chemical model must necessarily limit the number of particles, and this study is probably among the best that one can do at present. 

In order to check the survival of methanol adsorbate to the transfer conditions, the following experiment was performed. After adsorption of methanol and solution exchange with base electrolyte, the Pt electrode was transferred to the UHV chamber over a period of ca. 10 min, then back to the cell where it was reimmersed into the pure supporting electrolyte. A voltammogram was run and compared with that of an usual flow cell experiment. The results, (see Fig. 2.5a,b), show that the transfer procedure is valid. The areas under the oxidation curve are the same. As in the case of adsorbed CO on Pt (see Fig. 1.4), the change in the double peak structure indicates that some surface re-distribution may occur. 

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