The ORR Mechanism

Specific aspects examined here include insights and conclusions derived from the most recently performed density functional theory (DFT) calculations, which have been based on a comprehensive model of the electrochemical interface, and the strong disagreements (which seem to defy all recent theoretical efforts) that remain regarding proper interpretation of experimental ORR results and proper identihcation of the ORR mechanism in a PEFC cathode employing Pt catalysts. 

Elucidation of the actual values of key diagnostic parameters that have been used for a long time by ORR experimentahsts, including primarily the measured Tafel slope and the reaction order with regard to O2, and their predictive power regarding the ORR mechanism. 

To further substautiate these insights for the ORR mechanism, Wang and co-workers analyzed the four-electron ORR in acidic media using an intrinsic kinetic equation with the free energies of activation and adsorption as the kinetic parameters [Wang JX et al., 2007]. The kinetic model consisted of four essential elementary reactions ... 

The ORR mechanism is very complex and involves a number of different adsorbate intermediates. In addition to its complex reduction mechanism, the relatively high thermodynamic electrode potential of the ORR causes electrocatalytic surfaces... 

Next we analyze the activity of low-coordinated sites, which are a large percent of the active sites in nanoparticles. Although the ORR mechanism remains highly controversial, the OOH dis-... 

Analysis performed with the EQCM technique enabled to investigate the possible adsorption mode of oxygen on a catalyst and then to develop a hypothesis for the ORR mechanism. However, this method is less efficient for catalysts with multiple and complex oxide formation steps, like gold or platinum catalysts, since it is difficult to observe separated adsorption steps with the micro-balance. " ... 

However, due to a different experimental setup, the assignation of these species could not be confirmed from the infrared spectroscopy measurements carried out on a gold catalyst in alkaline medium, " which was found to present an exclusive series of oxygen reduction pathway. " The presence of these species as intermediates in the ORR mechanism was observed by Raman spectroscopy coupled with electrochemistiy in acidic medium on a gold electrode with bismuth adatoms. " Later, this mechanistic study of ORR by Raman spectroscopy was extended to a gold electrode in alkaline medium "" with an evidence for O2 formation as a reaction intermediate. H02 was also identified, but as a product of the ORR in alkaline medium. 

The ORR mechanism expressed by Reactions (4-V)—(4-X) may be too complicated to be quantitatively treated to obtain the reaction rate expression. In order to obtain the relationship between the current and the electrode potential, some reasonable assumptions may be made to simplify this mechanism. We may make two assumptions, one is to assume x = 0 so that the whole mechanism only goes through a four-electron pathway to produce water (this may be true for ORR catalyzed by Pt), and the other is to combine the fast Reactions (4-DQ and (4-X) together into one reaction. In this way, the ORR mechanism may be simplified as the following three reactions ... 

Generally, the ORR mechanism on carbon materials is complicated, involving several elementary steps. The first step was thought to involve adsorption and electron transfer to form superoxide (OJ). Following this, either further reduction or disproportionation of superoxide would happen. A step involving a transformation between two different forms of adsorbed superoxide was also proposed. In alkaline solutions... 

It is commonly recognized that the carhon surface chemistry plays a complex role in the ORR, which should be taken into account when considering the ORR mechanism. Some of the important surface features of carbon are shown in Figure 4.5. 

In order to further understand the ORR mechanism, another commonly used technique called rotating ring-disk electrode (RRDE) method has been demonstrated to be a powerful tool, particularly in detecting the intermediate such as peroxide produced during the ORR. 

In this chapter, both the fundamentals and applications of RRDE in studying electrochemical reaction kinetics such as the ORR mechanism will be presented in a detailed level. 

ORR is probably one of the most widely studied processes due to its important applications. In literature, the ORR mechanisms have been proposed and extensively studied using RRDE technique. Here, we take the RRDE measurements of ORR as examples to illustrate the RRDE data analysis methods. 

With the known value of N and experimental results of disk current, 7d, and ring current, 7r, Eqns (6.41) and (6.52) are often used to calculate the apparent number of electrons transferred per O2 molecule, n, and the percentage of peroxide formation, % H2O2 to determine the ORR mechanism. 

In general, the ORR mechanism on carbon electrode surface in basic solution can be expressed as follows ... 

As mentioned above, the pretreatment of the carbon electrode surface can significantly affect its ORR activity, and even alter the ORR mechanism. Actually, the carbon surface can also be modified by some monolayer substances such as anthraquinones (AQ), and transition metal macrocycle complexes. Due to the conjugate structures of these molecules, they can irreversibly adsorb on the carbon surface to form monolayers that could serve as an electrocatalyst for ORR. This kind of electrode structure can be used to evaluate the electrocatalyst s ORR activity, particularly in acidic solution. 

As discussed above, the ORR mechanism catalyzed by the Co-N/C catalyst was a mixed process of 2- and 4-electron-transfer pathways, dominated by the 4-electron pathway. An ORR mechanism was proposed in this work as follows ... 

In Chapter 6, the importance of RRDE fundamentals and practical usage in ORR study is emphasized in terms of both the electron transfer process on electrode surface, diffusion-convection kinetics near the electrode, and the ORR mechanism, particularly the detection of intermediate such as peroxide. One of most important parameters of RRDE, the collection efficiency, is deeply described including its concept, theoretical expression, as well as experiment calibration. Its usage in evaluating the ORR kinetic parameters, the apparent electron transfer, and percentage of peroxide formation is also presented. In addition, the measurement procedure including RRDE preparation, current—potential curve recording, and the data analysis are also discussed in this chapter. 

Methanol (MeOH) crossover from the anode to the cathode in the direct methanol fuel cell (DMFC) is responsible for significant depolarization of the Pt cathode catalyst. Compared to Pt-based catalysts, NPMCs are poor oxidation catalysts, of methanol oxidation in particular, which makes them highly methanol-tolerant. As shown in Fig. 8.25, the ORR activity of a PANI-Fe-C catalyst in a sulfuric acid solution is virtually independent of the methanol content, up to 5.0 M in MeOH concentration. A significant performance loss is only observed in 17 M MeOH solution ( 1 1 water-to-methanol molar ratio), a solution that can no longer be considered aqueous. The changes to oxygen solubility and diffusivity, as well as to the double-layer dielectric environment, are all likely to impact the ORR mechanism and kinetics, which may not be associated with the electrochemical oxidation of methanol at the catalyst surface. Based on the ORR polarization plots recorded at... 

Due to its complexity, the ORR mechanisms on noble metal surfaces are not fully understood despite five decades extensive studies [8-10]. On Pt surfaces, it is generally accepted that the first electron transfer or oxygen adsorption together with an electron and proton transfer forming superoxide (Eq. 17.1) is the slow step determining the overall reaction rate of oxygen reduction ... 

Chapters 7-12 focus on the electrocatalysis of carbon-based non-precious metal catalysts. The unique properties and fuel cell applications of various carbon based catalysts are intensively discussed in these chapters. Chapter 7 summarizes the fundamental studies on the electrocatalytic properties of metallomacrocyclic and other non-macrocyclic complexes. Chapter 8 and 9 review the progress made in the past 5 years of pyrolyzed carbon-supported nitrogen-coordinated transition metal complexes. Chapter 10 gives a comprehensive discussion on the role of transitional metals in the ORR electrocatalysts in acidic medium. Chapter 11 introduces modeling tools such as density functional theory (DPT) and ah initio molecular dynamics (AIMD) simulation for chemical reaction studies. It also presents a theoretical point of view of the ORR mechanisms on Pt-based catalysts, non-Pt metal catalysts, and non-precious metal catalysts. Chapter 12 presents an overview on recent progresses in the development of carbon-based metal-free ORR electrocatalysts, as well as the correlation between catalyst structure and their activities. 

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