Pt Oxide Formation and Reduction

Obtaining the solution for these coupled functions represents a self-consistency problem, the subject of research in an emerging scientific field offirst-principles electrochemistry. The main objective is to employ ab initio methods for the modeling of 

FIG U RE 3.17 The self-consistency problem in Pt electrocatalysis. The metal phase potential determines oxidation state and charging properties at the catalyst surface. These properties in turn determine the local reaction conditions at the Hehnholtz or reaction plane. At this point, structural design and transport properties of the catalyst layer come into play (as illustrated for conventional and ultrathin catalyst layers). Newly developed methods in the emerging field of first-principles electrochemistry attempt to find self-consistent solutions for this conpled problem. 

A potentiodynamic kinetic model of Pt oxide formation that is consistent with CV data over a wide range of scan rates and for different catalyst surface structures is an important step in understanding Pt oxide formation and reduction. Evaluation of the model against a range of electrochemical and spectroscopic data and comparison with theoretical calculations of reaction pathways and energetics could help furnishing details of reaction mechanisms. 

Upon increasing the electrode potential in the anodic direction, the first step of the reaction sequence considered in Rinaldo et al. (2014) involves adsorption of hydroxide from interfacial water on two distinct adsorption sites A and B  

FIGURE 3.18 A comparison of the proposed model of surface oxide formation and reduction at Pt(lll) in nonadsorbing acidic electrolyte with CV data of Gomez-Marin et al. (2013). The scan rate varies by four orders of magnitude from (a) to (f), as indicated in the plots. (Reprinted from Electrocatalysis, Mechanistic principles of platinum oxide formation and reduction, 2014, 1-11, Rinaldo et al. Copyright (2014) Springer. With permission.) 

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Features in Figure 3.16 that correspond to Pt oxide formation and reduction are labeled and explained in the pictorial legend. It should be noted for the illustrated CV that the surface processes do not involve any supplied reactants. They correspond to the transformation of interfacial water molecules into surface species and vice versa. A CV shows the amount of electronic charge withdrawn from the metal surface in anodic scan direction (oxidative current, j > 0) or transported to it in cathodic scan direction (reductive current, < 0). The electrical charge flux generated or consumed at the interface is the result of double layer charging and faradaic processes. An interesting aspect of cyclic voltammetry is that the surface is never in a steady state. The potential is continuously ramped up and down, at a constant scan rate, Vs, and between precisely controlled upper and lower potential bounds. 

Note that a normalization to the saturated oxide coverages is used here, in contrast to the section Mod-eling Pt Oxide Formation and Reduction where normalization was done to the number of available adsorption sites. 

At the second sweep, the oxidation current increased again above 1.05 V. This increase is probably due to the active siurface Pt-OH. At the reverse sweep, the current increase was seen at the potential range where the Pt-OH is reduced as usually seen on polycrystalline platinum. It is because Pt(lll) was rearranged by the Pt-OH formation and reduction and has become a more polycrystalline-like surface. 

Figure 3.16 Cyclic voKammograms for the formation and reduction of surface oxide species on Pt in 0.5 M HjSO, from 0.06 V vs. RHE to various reversal potentials in the anodic sweep. Scan rate was O.IOVs . From Angerstein-Ko/lowska et til. (1973).

Figure 2—17 shows the voltammogram for PtClOO). At the hydrogen region, there are two peaks, one at 250 mV, the other at 350 mV. The oxidation of platinum starts at 850 mV. The peak at 350 mV disappeared after the electrode experienced the formation and reduction of Pt-OH. 

Tilak, B.V., Conway, B.E., and Angerstein-Kozlowska, H. (1973) The real condition of oxidized Pt electrodes, Part III. Kinetic theory of formation and reduction of surface oxides. J. Electroanal. Chem., 48, 1-23. 

Results discussed in this section reveal important trends in the stability of Pt nanoparticles. They identify the surface tension as a valid descriptor of nanoparticle stability. The surface tension must play an important role in the kinetic modeling of nanoparticle dissolution (Rinaldo et al., 2010, 2012). However, the main kinetic mechanisms that contribute to Pt nanoparticle dissolution proceed via formation and reduction of surface oxide intermediates at Pt. This well-founded observation suggests that stability studies, reported here for bare Pt nanoparticles evaluated in vacuo, should be expanded to Pt nanoparticles of varying surface oxidation state as well as conditions that mimic electrochemical conditions that the fuel cell catalyst is exposed to. 

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