Direct four-electron pathway, oxygen reduction

The electrocatalytic reduction of oxygen according to the overall equations following a direct four-electron pathway (Eqs. 9-6 and 9-7) ... 

The reduction of oxygen is itself a complex electrochemical process. As described in Section 9.2, O2 reduction is considered to proceed along two parallel pathways, the direct four-electron pathway and the peroxide pathway (Eq. 9-9). Both pathways... 

In spite of the considerable effort expended in trying to unravel the fundamental aspects of the O2 electroreduction reaction, many details about the mechanism are not fully understood. The electrochemical reduction of oxygen is a multielectron reaction that occurs via two main pathways one involving the transfer of two electrons to give peroxide, and the so-called direct four-electron pathway to give water. The latter involves the rupture of the 0-0 bond. The nature of the electrode strongly influences the preferred pathway. Most electrode materials catalyze the reaction via two electrons to give peroxide Peroxide pathway in acid... 

Studies have revealed that the type of interaction between catalyst and oxygen dictates the oxygen reduction reaction pathway and end products. The direct four-electron pathway requires dissociation of oxygen prior to the transfer of the first electron [25]. Research has demonstrated that both side-on and bridge interactions weaken the 0-0 bond to such an extent that a break is inevitable. Therefore, four-electron reduction will follow and the final product will be water. On the other hand, catalysts that cannot effectively stretch the 0-0 bond will only end up with the two-electron reduction product H2O2, which makes the catalyst unstable and degrades the fiiel cell system [31]. 

Figure 2.4 The possible reaction mechanistic pathways for oxygen reduction. The direct four-electron pathway k ) is desired as it is more efficient and does not generate the potentially damaging peroxide intermediate. Reproduced from Ref. [2].

Nowadays, it has been demonstrated that the reaction is indeed structure sensitive with a multielectron transfer process that involves several steps and the possible existence of several adsorption intermediates [93-96]. The main advantage that we have with the new procedures with respect to cleanliness is that we have well-ordered surfaces to study a complex mechanism such as the oxygen electroreduction reaction [96-99]. In aqueous solutions, the four-electron oxygen reduction appears to occur by two overall pathways a direct four-electron reduction and a peroxide pathway. The latter pathway involves hydrogen peroxide as an intermediate and can undergo either further reduction or decomposition in acid solutions to yield water as the final product. This type of generic model of a reaction has been extensively studied since the early 1960s by different authors [100-108]. 

The oxygen reduction reaction is covered in considerable detail in a review text by Kinoshita [10]. The charge-transfer reaction itself is quite complicated, and controversy still exists around the details of which of the many possible charge-transfer mechanisms determine electrode performance. The two generalized pathways that are considered are the direct four-electron reaction ... 

The oxygen reduction reaction can proceed by two pathways in aqueous electrolytes. The first one, the so-called direct pathway, involves releasing four electrons per oxygen molecule to yield HgO. The indirect pathway involves releasing two electrons to yield hydrogen peroxide (HgOg) that in successive steps can produce water. Two further pathways, which are combinations of the above, can be envisaged. The series pathway implies sequential two or... 

Some studies have shown that certain modification procedures can be used to transform two-electron reduction metalloN4-macrocyclic complexes into hybrid materials with the capability to reduce oxygen to water, either via the direct four-electron transfer pathway or in the series two-electron transfer pathway. Carbon nanomaterials, carbon nanotubes in particular [58-65], have been reported to significantly increase the catalytic oxygen reduction current, with a substantial reduction of the overpotential for ORR reported in some cases, as shown by the examples in Table 7.4. 

The oxygen reduction reaction (ORR) at the PEM fuel cell cathode is a multielectron, multistep reaction with a sluggish kinetics thus, a catalyst is generally required to accelerate the reaction. At present, platinum (Pt)-based catalysts are the most practical catalysts for the ORR in PEM fuel cells. The mechanism of the Pt-catalyzed ORR has been an active research area for about the past 40 years [21-24]. Yet, despite numerous studies, the detailed mechanism remains elusive. Figure 6.2 illustrates the simplified mechanism [24]. On Pt, the oxygen reduction reaction can proceed along several pathways for example, a "direct" four-electron reduchon to water, a two-electron pathway to hydrogen peroxide, and a "series" pathway with two- and four-electron reduction to water. 

The oxygen reduction reaction (ORR) is the primary electrochemical reaction occurring at the cathode of a PEMFC, and is central to this promising technology for efficient and clean energy generation. The ORR is a multi-electron reaction that follows the direct four-electron mechanism on platinum-based electro-catalysts. It appears to occur in two pathways in acid electrolytes (Adzic and Lima, 2009) ... 

The key step in the reduction of oxygen at a catalytic surfece is the breaking of the 0—0 bond that requires four coupled proton and electron transfers, opening up the possibility of many side reactions and products (see Figure 2.4) [6]. The complexity of the ORR and its numerous potential side products means that it is still relatively poorly understood, although the consensus is that it proceeds either via a direct four-electron reduction pathway or via a peroxide intermediate in a 2 + 2 serial four-electron pathway [16-18]. 

Oxygen reduction reaction (ORR) can occur following two different pathways. " The direct pathway involves four exchanged electrons and leads to the direct formation of water ... 

---