**1. Introduction**

Climate change and increasing energy demand have emphasized research on sustainable energy source [1, 2]. Day-by-day increase of human population and global requirements has compelled researchers to develop new renewable sustainable energy sources in replacement of hydrocarbon deposits [3]. Renewable sources such as solar power, wind, and water, storage of these energies for on-demand utilization, and transportation are the major challenges for researchers. To develop a clean and eco-friendly environment, splitting of water into hydrogen and oxygen is a tremendous way to produce sustainable energy. Hydrogen gas emerged as a green energy fuel due to its high-energy density and zero carbon dioxide (CO2) emission [4–6]. In this regard, electrocatalytic and photocatalytic H2 generation

from water has been considered as one of the most striking approaches [7–9]. In recent years, a substantial number of artificial photosynthesis have been developed, exploited solar power as electron and proton source to make a clean renewable fuel [10–14]. Light-induced splitting of water is a suitable process because the production of hydrogen is used as green fuel in future and even used for the synthesis of other chemicals [15–18].

Literature reports suggested more than 500 billion cubic meters (44.5 million tons) of hydrogen gas is produce yearly worldwide [19, 20]. In the current scenario, steam methane refining, coal gasification, and water electrolysis are the major way for hydrogen production. Nowadays 95% hydrogen gas is produced from steam methane reforming and coal gasification, however only 4% hydrogen from water electrolysis. Steam methane is a high-energy-intensive process maintained at high temperature with the formation of carbon dioxide and carbon monoxide: (i) CH4 + H2O = CO + 3H2 (ii) CO + H2O = CO2 + H2. Hence, it is not an eco-friendly method for hydrogen production. Water electrolysis is the most sustainable and clean approach for hydrogen production because its source is abundant. Since the most suitable way of light-driven energy conversion is water electrolysis, artificial photosynthesis (PS II) has been considered as primary goal to produce electron and proton [21, 22]. Water splitting is a redox reaction in which aqueous protons are reduced into H2 at cathode and water is oxidized to O2 at anode [23]. Both H2 (HER) and O2 (OER) reactions are rigorously coupled, which may lead to the formation of explosive H2/O2 mixtures due to gas crossover [24–26]. By far, only a few stable metal complexes as catalysts are achieved that can decompose water into H2 and O2 [27–31]. Water-splitting reactions are split into two half-reactions: water oxidation to O2 evolution and water reduction to H2 production:

$$\text{H}\_2\text{O} \rightarrow 2\text{H}\_2 + \text{O}\_2 \quad \text{E}^0 = \text{1.23V} \tag{1}$$

desorption pathway. Precious metal like Pt-based electrocatalysts is highly reactive for HER and is usually pursuing Volmer-Tafel mechanism. Lately few literatures

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal…*

Electrocatalytic water splitting is driven by passing the electric current through the water; conversion of electrical energy to chemical energy takes place at electrode through charge transfer process. During this process, water reacts at the anode form O2 and hydrogen (proton) produce at the cathode as we mentioned earlier. Suitable electrocatalysts can maximally reduce the overpotential which is highly desirable for driving a specific electrochemical reaction. However, the process of surface catalytic reactions in electrocatalysis is very similar to photocatalysis [38]. Photocatalytic is a simple water-splitting reaction in which H2 and O2 are produced from water by utilizing the energy of sunlight. **Figure 2(a)** shows the process of photocatalysis in which a metal catalyst contains chromophores that

[35–37] have been reported on Ni-based electrocatalysts which follows

*The inside mechanism of H2 evolution of electrocatalyst in acidic solution.*

*DOI: http://dx.doi.org/10.5772/intechopen.92854*

**1.1 Fundamental aspects of photocatalytic and electrocatalytic hydrogen**

*(a) Photocatalyst system for water splitting. (b) Molecular orbital diagram for d<sup>6</sup> metal complex*

Volmer-Heyrovsky path.

**Figure 1.**

**Figure 2.**

**99**

*chromophores.*

**production process**

$$\text{2H}\_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}\_2 \quad \text{E}^0 = \text{1.23 V vs. NHE} \tag{2}$$

$$4\text{H}^+ + 4\text{e}^- \rightarrow 2\text{H}\_2 \quad \text{E}^0 = \mathbf{0} \text{ V } \text{vs.}\text{ NHE} \tag{3}$$

The limitation of OER is that it takes place after the successive accumulation of four oxidized electrons and protons in Kok cycle (catalytic cycle of the water oxidation in PS II) that require much higher overpotential input than that of HER [32]. Thermodynamic potential is different for H+ /H2 (0 V vs. NHE) and OH- /O2 (1.23 V vs. NHE), and the overall solar energy conversion efficiency is only �15% in OER [33]. The hydrogen evolution reaction (HER, 2H<sup>+</sup> + 2e- = H2) is the cathodic reaction with the two-electron transfer in one catalytic intermediate and offers the potential to hydrogen production. However, hydrogen production technology requires proficient electrocatalysts and photocatalysts which support two key electrode reactions (OER and HER) at lower overpotentials.

Moreover discussion on the mechanism of HER, H+ adsorption on the hydrogen evolution catalyst surface is the first step, known as Volmer step, followed by Heyrovsky or Tafel steps shown in **Figure 1**. A suitable HER catalyst always binds H+ very fast and releases the product. Hence, electrochemical hydrogen evolution reaction (HER) facilitates for H2 production on large-scale.

Afterwards, H2 evolution may occur via two different reaction mechanisms depending on the action of catalyst [34]. Hydronium cation (H3O<sup>+</sup> ) is the proton source in acidic solution, and in alkaline condition H2O is the proton source. In Volmer-Tafel mechanism, two protons absorbed on the catalytic surface can combine to form H-H bond to yield H2. In Heyrovsky reaction route, a second electron and another proton from the solution are transferred to the catalyst surface which reacts with the absorbed H atom and generate H2. This is an electrochemical

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.92854*

#### **Figure 1.**

from water has been considered as one of the most striking approaches [7–9]. In recent years, a substantial number of artificial photosynthesis have been developed, exploited solar power as electron and proton source to make a clean renewable fuel [10–14]. Light-induced splitting of water is a suitable process because the production of hydrogen is used as green fuel in future and even used for the synthesis of

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

Literature reports suggested more than 500 billion cubic meters (44.5 million tons) of hydrogen gas is produce yearly worldwide [19, 20]. In the current scenario, steam methane refining, coal gasification, and water electrolysis are the major way for hydrogen production. Nowadays 95% hydrogen gas is produced from steam methane reforming and coal gasification, however only 4% hydrogen from water electrolysis. Steam methane is a high-energy-intensive process maintained at high temperature with the formation of carbon dioxide and carbon monoxide: (i) CH4 + H2O = CO + 3H2 (ii) CO + H2O = CO2 + H2. Hence, it is not an eco-friendly method for hydrogen production. Water electrolysis is the most sustainable and clean approach for hydrogen production because its source is abundant. Since the most suitable way of light-driven energy conversion is water electrolysis, artificial photosynthesis (PS II) has been considered as primary goal to produce electron and proton [21, 22]. Water splitting is a redox reaction in which aqueous protons are reduced into H2 at cathode and water is oxidized to O2 at anode [23]. Both H2 (HER) and O2 (OER) reactions are rigorously coupled, which may lead to the formation of explosive H2/O2 mixtures due to gas crossover [24–26]. By far, only a few stable metal complexes as catalysts are achieved that can decompose water into H2 and O2 [27–31]. Water-splitting reactions are split into two half-reactions: water oxidation

H2O ! 2H2 <sup>þ</sup> O2 E0 <sup>¼</sup> <sup>1</sup>*:*23V (1)

2H2O ! 4H<sup>þ</sup> <sup>þ</sup> 4e� <sup>þ</sup> O2 E0 <sup>¼</sup> <sup>1</sup>*:*23 V *vs:* NHE (2)

The limitation of OER is that it takes place after the successive accumulation of

(1.23 V vs. NHE), and the overall solar energy conversion efficiency is only �15% in OER [33]. The hydrogen evolution reaction (HER, 2H<sup>+</sup> + 2e- = H2) is the cathodic reaction with the two-electron transfer in one catalytic intermediate and offers the potential to hydrogen production. However, hydrogen production technology requires proficient electrocatalysts and photocatalysts which support two key

Moreover discussion on the mechanism of HER, H+ adsorption on the hydrogen

evolution catalyst surface is the first step, known as Volmer step, followed by Heyrovsky or Tafel steps shown in **Figure 1**. A suitable HER catalyst always binds H+ very fast and releases the product. Hence, electrochemical hydrogen evolution

Afterwards, H2 evolution may occur via two different reaction mechanisms

source in acidic solution, and in alkaline condition H2O is the proton source. In Volmer-Tafel mechanism, two protons absorbed on the catalytic surface can combine to form H-H bond to yield H2. In Heyrovsky reaction route, a second electron and another proton from the solution are transferred to the catalyst surface which reacts with the absorbed H atom and generate H2. This is an electrochemical

four oxidized electrons and protons in Kok cycle (catalytic cycle of the water oxidation in PS II) that require much higher overpotential input than that of HER

4H<sup>þ</sup> <sup>þ</sup> 4e� ! 2H2 E0 <sup>¼</sup> 0V *vs:* NHE (3)

/H2 (0 V vs. NHE) and OH-

/O2

) is the proton

to O2 evolution and water reduction to H2 production:

[32]. Thermodynamic potential is different for H+

electrode reactions (OER and HER) at lower overpotentials.

reaction (HER) facilitates for H2 production on large-scale.

**98**

depending on the action of catalyst [34]. Hydronium cation (H3O<sup>+</sup>

other chemicals [15–18].

*The inside mechanism of H2 evolution of electrocatalyst in acidic solution.*

desorption pathway. Precious metal like Pt-based electrocatalysts is highly reactive for HER and is usually pursuing Volmer-Tafel mechanism. Lately few literatures [35–37] have been reported on Ni-based electrocatalysts which follows Volmer-Heyrovsky path.
