**4. C-NCs-based heterogeneous catalyst for EH2ER**

The concept of using electric current to control various chemical reactions achieved much attention, since the time when humankind invented first power resources. Splitting of water to produce hydrogen and oxygen gas started much earlier, but now it has started at large scale in industrial process and seems to play a crucial role in combating the future energy crisis. The increasing demand for energy day by day and the shortage of fossil fuels have encouraged scientists to develop a renewable and clean source of energy.

Hydrogen is considered a clean source of energy because by-product of H2 combustion is H2O and the starting material to obtain H2 is water, therefore, an efficient and clean source to supplant the depleting fossil fuels [65–67].

Moreover, H2 produces highest energy on combustion of per unit mass relative to any other fuels, thus, leading to become a fuel of future. The H2 can be produced by electrochemical water splitting reaction which is an endothermic process with a potential of ΔE° = 1.23 V and ΔG° = 237.2 kJ mol−1. Water splitting generates both hydrogen and oxygen. For the process of electrochemical water splitting the reactions that occur at anode are called as oxygen evolution reaction (O2ER) and the reactions which occurs at cathode are called as H2 evolution reaction (H2ER) [68]. Electrocatalysis is actually an attempt to elucidate and predict observable phenomena like overall activity of the reactions that occur on the surface of electrode by the interactions of electrode/electrolyte interface. Development of an efficient electrocatalyst is important to minimize the energy losses during the electrocatalytic splitting of water to produce hydrogen and oxygen gas. **Figure 6** shows a diagrammatic representation of evolution of hydrogen (Depicted to the left-side of **Figure 6**) and oxygen (depicted to the right side of **Figure 6**) gas on the surface of glassy carbon electrode (GCE) after deposition of catalyst on its surface.

> 4H 4e 2H2 + − + → 2H O O 4H 4e 2 2 →+ ++ −

Net reaction : 2H O 2H O 2 22 → +

The reactions which are central to hydrogen energy are two types. These are hydrogen evolution (2H+ + 2e− → H2) and hydrogen oxidation (H2 → 2H+ + 2e− ) reactions. The research of oxidizing and evolving hydrogen was started in 1960 but it gained importance in 1970 and 1990 when the shortage of oil was realized [69]. The most success in this regard was achieved when precious metals like platinum (Pt) were used. Metal NPs on the surface of carbon also showed great success in H2ER and hydrogen oxidation reaction (H2OR).

In the world of EH2ER electrochemistry, recent merge of computational quantum chemistry and nanotechnology have shown great progress in explaining

#### **Figure 6.**

*Diagrammatic representation of formation of hydrogen during hydrogen evolution reaction and formation of oxygen during oxygen evolution reaction on the surface of glassy carbon electrode. Abbreviation: LSV - linear sweep voltammetry, WE - working electrode, RE - reference electrode, CE - counter electrode.*

**205**

**Figure 7.**

*GCE - glassy carbon electrode.*

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems…*

fundamentals and basics of EH2ER with much emphasize on its utility and storage [51, 70, 71]. Metallic Pt is considered as 'state of the art catalyst' and exhibits small Tafel slope values and extremely high exchange current density (j0) [72–74]. However, because of high cost and less availability of Pt, a sustainable, cost effective, and stable catalyst needs to be developed. So, there have been efforts to synthesize the EH2ER active catalysts from the transition metals that are abundant

EH2ER kinetics has a long history and have been explained in detail [69]. EH2ER

 +2e- → H2) is a process involving a series of electrochemical steps which takes place on the electrode surface and results in the evolution of hydrogen. There are two mechanisms in acidic and basic conditions accepted universally as shown in

( ) +− ∗ H M e M H acidic solution ++↔− (1)

( ) ∗ + M H H H acidic solution −+↔ <sup>2</sup> (3)

( ) ∗ − M H H O M OH H alkaline solution − + ↔+ + 2 2 (4)

( ) − ∗− H O M e M H OH alkaline solution <sup>2</sup> ++↔− + (2)

1.Electrochemical hydrogen adsorption (Volmer reaction) (Eq. (1), (2))

2.Electrochemical desorption (Heyrovsky reaction) (Eq. (3), (4))

*Mechanism of hydrogen evolution reaction on surface of glassy carbon electrode (GCE). Abbreviation:* 

*DOI: http://dx.doi.org/10.5772/intechopen.95338*

**4.1 Mechanistic overview of EH2ER**

**Figure 7 [**75]. These steps are:

This step is followed by.

in nature.

(2H<sup>+</sup>

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems… DOI: http://dx.doi.org/10.5772/intechopen.95338*

fundamentals and basics of EH2ER with much emphasize on its utility and storage [51, 70, 71]. Metallic Pt is considered as 'state of the art catalyst' and exhibits small Tafel slope values and extremely high exchange current density (j0) [72–74]. However, because of high cost and less availability of Pt, a sustainable, cost effective, and stable catalyst needs to be developed. So, there have been efforts to synthesize the EH2ER active catalysts from the transition metals that are abundant in nature.
