**3. Efforts on the in-built possession of the catalyst materials to enhance the bifunctional activity**

After the detailed discussion of the fundamental electrochemical parameters, it is now essential to highlight the significance of in-built possessions of the materials, which decide the properties of the catalyst materials. As stated earlier, a reputable candidate for electrochemical reaction must be recognized with certain properties such as intrinsic activity, density of active sites, conductivity, surface area, wettability (hydrophilicity) and stability. But all these properties are decided by their appropriate association with in-built possessions like phase, morphology, particle size, defects and *A Perspective on the Recent Amelioration of Co3O4 and MnO2 Bifunctional… DOI: http://dx.doi.org/10.5772/intechopen.109922*

strain. A slight change in these in-built possessions leads to the huge impact on the properties of catalyst materials. In addition, practically, there is no catalyst material with an adequate quantity of all these mentioned properties. To resolve this inadequateness, many efforts have been devoted on catalyst materials. A catalyst with optimum quantity of all the aforementioned properties can be attained only by a systematic fine-tuning of its in-built possessions such as phase, morphology, crystal facets, particle size, defects and strain. Therefore, to understand the linear thread connection between the in-built possessions, inherent properties and outperforming activity of the electrocatalyst, all these possessions have been discussed in more detail in this section.

#### **3.1 Phase engineering**

The smallest repeating domain containing identical (in both distance and space) arrangement of atoms is called a crystal lattice. The extent of a single or a group of crystal lattice can be defined as a phase, a key factor in deciding the properties of the materials. Each phase has its own property and the variation in phases causes dramatic changes in the properties especially in the intrinsic activity, density of active sites, conductivity, wettability and stability, thus having more impact on the bifunctional activity. Controlling materials with desirable phases is an art associated with synthetic methodology. Hence, it is recommended to choose a suitable methodology to prepare the materials with desirable phase selectivity, so that the extent of the activity can be attained [24]. Therefore, it has been concluded that the phase of the material is the primary in-built possession deciding almost all inherent properties, and hence, by choosing the appropriate methodology, the selectively particular phase can be attained.

#### **3.2 Morphology engineering**

An inherent property, surface area is often morphology dependent since different morphologies have different surface areas even for identical phases [25]. The electrochemical reaction takes place only at the interface, where surface atoms are in contact with electrolytes rather than interior atoms. Although all surface atoms are in contact with the electrolyte, only their certain portions are capable of catalyzing bifunctional reactions called active sites. The morphology with the exposure of the maximum number of active sites is the most opted for better bifunctional catalysis.

#### **3.3 Defect engineering**

Defect engineering is another way to tune the properties of the catalyst material for enhancing the catalytic activity; due to this reason, it is intentionally introduced into the lattice of the materials although it drags the stability of the catalyst materials. The introduction of defects on the lattice can tailor the intrinsic property, especially the electronic configuration, conductivity, chemical reactivity and stability by adjusting electron distribution on the lattice. Defects also lead to the formation of unsaturated atoms on the surface of the catalyst, which are capable of intensively catalyzing the reaction and hence are highly desirable to promote the oxygen electrode reactions [26]. Defects can exist in many forms, namely, structure distortion, oxygen vacancy, cation vacancy, anion vacancy and lattice defects. All the forms of defects lead to the enhancement of the catalytic activity somehow, mainly impacting the

conductivity and density of active sites of the catalyst materials [27]. Caution must be taken while improving the activity of the catalyst by defect engineering since a larger extent of defect density leads to a fall in stability due to the increase in the number of dangling bonds in the material. And it is recommended to adopt an effective and feasible way to create defects, rather than following tedious processes and harsh conditions so as to retain the other properties of the catalysts.
