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. 

Can an ORR mechanism at Pt metal in an acid electrolyte with the Reaction (1.2) as the first and rate-limiting step be defended in light of the recently reported apparent Tafel slope and reaction order for ORR in the PEFC cathode ... 

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 ... 

Note that for metal nanoparticles supported on porous carbon materials, it is even more difficult to establish the mechanism of the ORR. Indeed, for the above-described thin layer or porous RRDE (Section 15.3), H2O2 has very little chance to escape from the CL and be detected at the ring. H2O2 can readsorb either on Pt particles or on the carbon support, and undergo chemical decomposition or further electrochemical reduction, while diffusing out of the CL. This implies great difficulties in establishing the detailed ORR mechanism on nanometer-sized metal nanoparticles. 

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... 

The exact ORR mechanism is still a topic of much debate. Two representative mechanisms, namely, a dissociative mechanism (reactions 11, 15, and 16) and an associative mechanism (reactions 12 to 16), are illustrated here ... 

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-... 

Hence, the study of the different adsorption modes of oxygen is an interesting way to bring a deeper understanding of ORR mechanism on a catalyst. The Electrochemieal Quartz Crystal Miero-balance (EQCM) is a partieularly convenient analytieal method to study adsorption - desorption of species on a eatalyst smfaee. 

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. 

Let us consider the oxygen reduction reaction (ORR) that occurs in the cathode of the polymer electrolyte membrane fuel cell (PEMFC), in an acidic environment. Although a variety of ORR mechanisms have been proposed, the four-electron pathway is primarily used to characterize the behavior of this reaction at a platinum electrode or a glassy carbon electrode coated with a platinum-based catalyst. The overall reaction is given by... 

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. 

Proposed ORR Mechanisms and Their Analysis Based on the RRDE Data 216... 

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. 

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