**2. Fundamentals of oxygen electrode reactions (ORR/OER)**

To provide good clarity of bifunctional electrocatalysis, first, the basics and fundamentals of oxygen electrode reactions and electrochemical parameters used in the evaluation of the performance of electrocatalysts are discussed in more detail. The intensive knowledge of the above-mentioned field is necessary to design a new nonprecious transition metal-based material and to understand its behavior toward ORR/OER.

#### **2.1 Mechanism of ORR**

ORR is an electrochemically sluggish reaction with four-electron transfer in a multiple-step process, where the reaction begins with the diffusion of oxygen (O2) toward the catalyst, adsorption of O2 molecule on the active sites of the catalyst, transfer of electrons to the adsorbed O2 molecules and passage through multiple steps. Finally, it is converted to products and desorbed from the active sites of the catalyst [14].

If we look at the ORR more deeply, there are two possible ways to adsorb O2 molecules on the electrode surface such as associative (bidentate, side-on, two oxygens atoms of O2 coordinates with the metal) O2 adsorption and dissociative (monodentate, end-on, one oxygen atom of O2 coordinates perpendicular to the metal) O2 adsorption as shown in **Figure 1**.

**Figure 1.**

*The schematic representation of the adsorption of O2 over the metal/oxide surface.*

The adsorption mode of oxygen had the foremost influence and is crucially significant for the number of electrons transferred (*n*) during the ORR process. The associative adsorption mode of oxygen leads to the direct four-electron transfer reduction path and the dissociative adsorption mode of oxygen results in the two-electron transfer reduction path through the peroxide intermediate formation. Typically, noble metals predominantly follow four-electron transfer reduction, and carbon-based materials follow two-electron transfer reduction. On the other hand, non-precious metal oxides can follow either indirect two- or direct four-electron transfer path or both, influenced by the nature of metal oxide and overpotential region. The higher overpotential region offers highly desirable direct four-electron transfer reduction, whereas, in the lower overpotential region, two-electron transfer reduction with peroxide formation occurs predominantly. Therefore, the exact number of electron transfers and the mechanism of ORR on the metal oxide surface still remain unclear. However, the widely accepted mechanism of ORR is fairly complicated and involves oxygen-containing intermediates such as O\*, OH\* and OOH\*. The mechanism of fourelectron transfer pathway of ORR on a catalytic metal surface can be described as follows:

$$\text{\textquotedblleft}^\* + \mathsf{O}\_2(\mathsf{g}) \to \mathsf{O}\_2\text{\textquotedblright}\tag{1}$$

$$\mathbf{1}/2\mathbf{O}\_2^\* \to \mathbf{O}^\* \tag{2}$$

$$\text{O}^\* + \text{H}^+(\text{aq}) + \text{e}^- \rightarrow \text{OH}^\* \tag{3}$$

$$\text{OH}^\* + \text{H}^+(\text{aq}) + \text{e}^- \rightarrow \text{H}\_2\text{O}^\* \tag{4}$$

$$\text{H}\_2\text{O}^\* \rightarrow \text{H}\_2\text{O} \tag{5}$$

Here, \* represents the active sites of the catalyst under investigation [13, 15, 16]. The two-electron transfer mechanism of ORR is as follows:

$$\text{O}\_2\text{ }^\* + \text{H}^+(\text{aq}) + \text{e}^- \rightarrow \text{HOO}^\* \tag{6}$$

$$\text{HOO}^\* + \text{H}^+ + \text{e}^- \rightarrow \text{H}\_2\text{O}\_2 \tag{7}$$

In some cases, OOH\* dissociation may be involved on the same catalytic surface as follows:

$$\text{HOO}^\* \rightarrow \text{O}^\* + \text{HO}^\* \tag{8}$$

*A Perspective on the Recent Amelioration of Co3O4 and MnO2 Bifunctional… DOI: http://dx.doi.org/10.5772/intechopen.109922*

All possible mechanisms of ORR are shortened here.

The heterogeneous electrocatalysis (ORR/OER) occurs at the catalyst–electrolyte– reactant triple point, where the binding energy of the reactive species (intermediates/ reactant) has been calculated to determine the catalytic performance of the electrocatalyst. The French chemist Paul Sabatier made a principle with respect to the binding energy of the reactive species and catalytic performance of the electrocatalyst in a heterogeneous system called the Sabatier principle (also called the volcano plot). It states that the interaction between the surface of the catalyst and the reactant should be optimum; That is, it should be neither too strong nor too weak [17]. Because too weak interactions result in the failure of making bonds between them, causing no further reaction, and too strong interactions lead to kinetically slow dissociation of the resultant intermediates, the catalytic surface is not accessible by the reactant for further reaction. In the case of ORR, three oxygen-containing species are involved as an intermediate such as O\*, OH\* and OOH\*. The binding energy of these reactive intermediates is the leading factor that decides the activity of the catalyst [18].

#### **2.2 Reaction mechanism of OER**

Water oxidation reaction or OER is the reversible reaction of ORR, which occurs at a higher positive overpotential than ORR; to attain the reversibility of each step involved, the ORR should be reversed. Moreover, it is the core electrochemical reaction in fuel cells, MABs and water splitting with their complementary reaction such as ORR and/or HER. It is a multi-step electron transfer reaction; each step requires some energy to overcome the energy barrier, drag the overall kinetics of OER and necessitate a large overpotential [19].

The proposed mechanism of OER on the metal oxide surface is as follows:

$$\rm{^{\cdot}} + \rm{H\_2O} \rightarrow \rm{^{\cdot}OH} + \rm{H} + \rm{+e} \tag{9}$$

$$\text{C}^\*\text{OH} \rightarrow \text{^\*O} + \text{H} + + \text{e-or} \\ \text{2}^\*\text{OH} \rightarrow \text{^\*O} + \text{^\*} + \text{H}\_2\text{O} \tag{10}$$

$$\text{O} \ 2^{\ast} \text{O} \to 2^{\ast} + \text{O}\_{2} \text{or}^{\ast} \text{O} + \text{H}\_{2}\text{O} \to ^{\ast} + \text{O}\_{2} + 2\text{H}^{+} + 2\text{e}^{-} \tag{11}$$

Although OER is relatively complex, it is considerably facile on the metal oxide surface rather than on the bare metal surface, since bare metals are more prone to oxidation at higher positive overpotentials. Moreover, the capability to exist in multiple oxidation states of metal oxides makes it a highly desirable candidate for OER, where the interaction between the metal and oxygen intermediates initiates the bond formation between them by change in the oxidation state of the metal. Therefore, metal oxides are the best choice of catalyst for OER. However, each metal oxide has a different OER mechanism on its surface; even oxides with identical element compositions due to the difference in the surface property of materials originated from the

preparation method. The volcano plots relating the required overpotential to attain 1 mA current vs ΔGO - ΔGOH for OER have been constructed by the study of OER catalysis on a wide variety of metal oxide surfaces [20].

#### **2.3 Bifunctional (ORR/OER) electrocatalysis**

A single catalyst employment for the catalysis of two electrochemical reactions (both a reaction and its complementary reaction) is called bifunctional electrocatalysis. The catalyst used in the bifunctional electrocatalysis with the capability to catalyze two electrochemical reactions is termed as bifunctional electrocatalyst. The bifunctional electrocatalyst is primarily used to facilitate the electron transfer from the electrode to the reactant, which promotes oxygen electrode reactions (ORR/OER). A single catalyst employment to catalyze both OER/ORR, that is, a bifunctional catalyst is highly preferable particularly for energy storage and conversion devices, rather than employing two different catalysts separately. Either the catalyst itself or the catalyst decorated on the electrode surface is mostly used as a bifunctional electrocatalyst. The principal role of the electrocatalyst is to adsorb the reactant on its surface and develop the adsorbed reactant/intermediate, thereby promoting the kinetics of charge transfers from the electrode to the reactant. The equilibrium potential of the oxygen electrode reaction (ORR and OER) is 1.23 V versus reversible hydrogen electrode (RHE) [21]. However, the complicated mechanisms and sluggish kinetics of these reactions demand high overpotentials. In order to facilitate the oxygen electrode reaction, the bifunctional electrocatalyst must adhere to certain intrinsic structural features, that is, high inherent bifunctional catalytic activity; enlarged surface area; high electrical conductivity and favorable morphology with exposure of maximum active sites; a high electrochemical surface area; high chemical, electrochemical and mechanical stability; and good contact of electrochemical (electrode electrolyte) interfaces [22]. Although a material with aforementioned properties can perform as good bifunctional electrocatalyst, it has to be evaluated with certain electrochemical parameters for its comparison with other catalysts.

For a fair evaluation of the performance of electrocatalysts, all the electrochemical parameters must be taken into account such as the onset/overpotential, exchange current density, Tafel slope, turnover frequency (TOF), potential gap, number of electron transfers, amount of peroxide formation and electrochemical surface area [23]. Each and every parameter is very crucial and can provide much more insightful information about the catalyst material related to the mechanism of electrochemical reaction. For this reason, the detailed introduction of all the fundamental electrochemical parameters has been discussed elaborately in the following sections.
