**4.1 Mechanistic overview of EH2ER**

*Colloids - Types, Preparation and Applications*

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

> 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

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

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

*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.*

+ 2e− → H2) and hydrogen oxidation (H2 → 2H+

 + 2e− )

**204**

**Figure 6.**

surface.

hydrogen evolution (2H+

H2ER and hydrogen oxidation reaction (H2OR).

EH2ER kinetics has a long history and have been explained in detail [69]. EH2ER (2H<sup>+</sup> +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 **Figure 7 [**75]. These steps are:

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

$$\text{CH}^+ + \text{M} + \text{e}^- \leftrightarrow \text{M} - \text{H}^\* \text{(acidic solution)}\tag{1}$$

$$\mathrm{H}\_{2}\mathrm{O} + \mathrm{M} + \mathrm{e}^{-} \leftrightarrow \mathrm{M} - \mathrm{H}^{\*} + \mathrm{OH}^{-} \text{(alkaline solution)}\tag{2}$$

This step is followed by.

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

$$\text{M} - \text{H}^\* + \text{H}^\* \leftrightarrow \text{H}\_2 \left( \text{acidic solution} \right) \tag{3}$$

$$\text{M} - \text{H}^\* + \text{H}\_2\text{O} \leftrightarrow \text{M} + \text{OH}^- + \text{H}\_2 \text{ (alkaline solution)}\tag{4}$$

#### **Figure 7.**

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

#### Or,

#### 3.Chemical desorption or combination reaction (Tafel reaction) (Eq. (5))

$$2\,\mathrm{M} - \mathrm{H}^\* \leftrightarrow 2\mathrm{M} + \mathrm{H}\_2 \left( \text{both acidic } \& \text{alkaline} \right) \tag{5}$$

In the above reactions, H\* indicates an adsorbed hydrogen atom that has been adsorbed chemically on the surface of electrode (M) at the active site. These reaction pathways are highly dependent on electronic and chemical properties of the electrode surface [76]. Tilak et al., explained that rate controlling steps (1, 2 and 3) is predicted by deducing Tafel slope values from EH2ER polarization curves [72]. The mechanism and rate determining step is studied by the Tafel slope. Tafel slope is an inherent and interesting property because it gives information about the potential difference required to increase or decrease the current density by 10-fold. Tafel slope is also useful to determine the effectiveness of a catalyst. In order to calculate the Tafel slope the linear portion of the Tafel plots is to be fitted in the Tafel equation (η = b log (j) + a, where η = overpotential, b = Tafel slope, and j = current density) [77, 78]. Theoretical facts about Tafel slope have been derived from Butler-Volmer equation and it is proved for three limited cases. First, if the discharge reaction proceeds very quickly and H2 is evolved by the rate determining combination reaction (Tafel step). The slope value is 29 mV dec-1 at 25°C (2.3RT/2F). Second, if the discharge reaction proceeds very quickly and H2 is evolved by the rate determining desorption reaction (Heyrovsky step). The slope value for this step is 40 mV dec-1 at 25°C (4.6RT/3F). Third, if the discharge reaction proceeds very slowly and then the rate determining step will be Volmer step irrespective of the fact whether H2 is evolved by the combination reaction or the desorption reaction. The Tafel slope is 116 mV dec-1 at 25°C (4.6RT/F). The detailed mechanism is shown in **Figure 7.** It is evident that reaction (1) represents chemical adsorption, whereas, reaction (2) and (3) exhibits H atoms desorption from the electrode surface, which are competing with each other. Sabatier and co-workers came with an idea (Sabatier principle) that a better catalyst should not only form a strong bond with absorbed H\* and facilitates the proton electron transfer process, but also it should be weak enough in facial bond breaking to assure quick release of H2 gas [79]. It is difficult to establish a quantitative relationship between energies of H\* intermediate and rate of electrochemical reaction owing to absence of directly measured surfaceintermediate bonding energy values [80]. However from the perspective of physical chemistry, both for H\* adsorption and H2 evolution on the catalyst surface can be determined from the change in free energy of H\* adsorption (∆GH\*) using EH2ER free energy diagram [81]. According to Sabatier principle, under the condition ∆GH\* = 0 will have maximum overall reaction rate (expressed in terms of EH2ER exchange current density, j0).

#### **4.2 Metal-based C-NCs for EH2ER**

An important correlation between ∆GH\* and j0 have been proposed in the form of "volcano curve" for a wide variety of electrode surfaces as illustrated in **Figure 8 [**81, 82]. Pt group of metals are the most efficient in the process and that's why are found at the top of the volcano curve, because they have small Tafel slope and quasi zero onset potential.

However, due to high cost of Pt, various research groups have been working on modifying Pt group metals such as engineering the NCs. Crystal plane (110) of Pt NCs has been proved good surface for EH2ER. Like Pt, Palladium (Pd) NCs have

**207**

**Figure 8.**

non-noble metals for the process.

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

been showing great promise as it is in the same group and has almost same size and its lattice matches about 0.77% to Pt, good thing about Pd is that it is comparatively cheaper than Pt. Pd has one advantage that it can adsorb hydrogen from both electrolytes and the gas phase. Pd can be loaded on various supports to increase its activity as it alters surface area and electronic conductivity of the nanocrystals [84]. Huang et al. observed that Pd C-NCs deposited on carbon paper substrate has activity higher than Pt black electrode and it required very less catalyst loading that is 0.0106 mg cm−2 [85]. One thing to be noted about Pd is that it has higher activity in acidic medium than alkaline medium because of lower Pd-H binding energy and lower activation energy, acid (32.3 ± 0.7 kJ/mol) and base (38.9 ± 3.0 kJ/mol). Ruthenium (Ru) NCs are also being explored for EH2ER. As the Sabatier principle suggest that catalyst should not have much stronger binding to the hydrogen and should possess moderate binding capacity so desorption is easy, Ru-H follows this trend as it has ~65 kcal/mol energy for Ru-H bond and thus less activation barrier for desorption process [86]. Ru C-NCs are usually used with some support as they have durability problems because of aggregation. One such example where Joshi et al. used Ru C-NCs supported with Tungsten (W). DFT calculations suggest that Ru (0001) has high H2 binding to surface energy but using W support, it could reduce the H2 adsorption energy and changes the electronic environment thus making it similar to Pt (111) and increases its activity for EH2ER process [86]. Baek et al*.* revealed that Ru when deposited on graphene nanoplatelets (GnP) to form Ru@ GnP, its activity usually surpasses that of Pt/C in both acidic and alkaline medium. This happens because it is more stable, possess low Tafel slope (30 mV dec−1 in 0.5 M aq. H2SO4; and 28 mV dec−1 in 1.0 M aq. KOH) and also has comparatively low overpotential at 10 mA cm−2 (13 mV in 0.5 M aq. H2SO4; 22 mV in 1.0 M aq. KOH) [87]. Not only Pt, Pd, Ru, metals like Iridium (Ir) are also explored for the EH2ER process and also earth abundant metals are used but they are prone to corrosion in the presence of alkaline and acidic medium. Thus, the other way is using

*Volcano plot for log I0 values for HER as a function of M-H bond energy. Adapted from [83].*

Non-Noble metals follow this trend for the catalytic activity Nickel (Ni) > Molybdenum (Mo) > Cobalt (Co) > W > Iron (Fe) > Copper (Cu) which is calculated using the voltammetric techniques [88]. Ni shows a very good catalytic activity when using in the hybrid form of Ni/NiO/CoSe2 because this composite helps in less resistance to charge transfer. But this hybrid has poor

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

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

#### **Figure 8.**

*Colloids - Types, Preparation and Applications*

3.Chemical desorption or combination reaction (Tafel reaction) (Eq. (5))

In the above reactions, H\* indicates an adsorbed hydrogen atom that has been

An important correlation between ∆GH\* and j0 have been proposed in the form of "volcano curve" for a wide variety of electrode surfaces as illustrated in **Figure 8 [**81, 82]. Pt group of metals are the most efficient in the process and that's why are found at the top of the volcano curve, because they have small Tafel

However, due to high cost of Pt, various research groups have been working on modifying Pt group metals such as engineering the NCs. Crystal plane (110) of Pt NCs has been proved good surface for EH2ER. Like Pt, Palladium (Pd) NCs have

adsorbed chemically on the surface of electrode (M) at the active site. These reaction pathways are highly dependent on electronic and chemical properties of the electrode surface [76]. Tilak et al., explained that rate controlling steps (1, 2 and 3) is predicted by deducing Tafel slope values from EH2ER polarization curves [72]. The mechanism and rate determining step is studied by the Tafel slope. Tafel slope is an inherent and interesting property because it gives information about the potential difference required to increase or decrease the current density by 10-fold. Tafel slope is also useful to determine the effectiveness of a catalyst. In order to calculate the Tafel slope the linear portion of the Tafel plots is to be fitted in the Tafel equation (η = b log (j) + a, where η = overpotential, b = Tafel slope, and j = current density) [77, 78]. Theoretical facts about Tafel slope have been derived from Butler-Volmer equation and it is proved for three limited cases. First, if the discharge reaction proceeds very quickly and H2 is evolved by the rate determining combination reaction (Tafel step). The slope value is 29 mV dec-1 at 25°C (2.3RT/2F). Second, if the discharge reaction proceeds very quickly and H2 is evolved by the rate determining desorption reaction (Heyrovsky step). The slope value for this step is 40 mV dec-1 at 25°C (4.6RT/3F). Third, if the discharge reaction proceeds very slowly and then the rate determining step will be Volmer step irrespective of the fact whether H2 is evolved by the combination reaction or the desorption reaction. The Tafel slope is 116 mV dec-1 at 25°C (4.6RT/F). The detailed mechanism is shown in **Figure 7.** It is evident that reaction (1) represents chemical adsorption, whereas, reaction (2) and (3) exhibits H atoms desorption from the electrode surface, which are competing with each other. Sabatier and co-workers came with an idea (Sabatier principle) that a better catalyst should not only form a strong bond with absorbed H\* and facilitates the proton electron transfer process, but also it should be weak enough in facial bond breaking to assure quick release of H2 gas [79]. It is difficult to establish a quantitative relationship between energies of H\* intermediate and rate of electrochemical reaction owing to absence of directly measured surfaceintermediate bonding energy values [80]. However from the perspective of physical chemistry, both for H\* adsorption and H2 evolution on the catalyst surface can be determined from the change in free energy of H\* adsorption (∆GH\*) using EH2ER free energy diagram [81]. According to Sabatier principle, under the condition ∆GH\* = 0 will have maximum overall reaction rate (expressed in terms of EH2ER

( ) <sup>∗</sup> 2 M H 2M H both acidic & alkaline −↔ + <sup>2</sup> (5)

Or,

**206**

exchange current density, j0).

**4.2 Metal-based C-NCs for EH2ER**

slope and quasi zero onset potential.

*Volcano plot for log I0 values for HER as a function of M-H bond energy. Adapted from [83].*

been showing great promise as it is in the same group and has almost same size and its lattice matches about 0.77% to Pt, good thing about Pd is that it is comparatively cheaper than Pt. Pd has one advantage that it can adsorb hydrogen from both electrolytes and the gas phase. Pd can be loaded on various supports to increase its activity as it alters surface area and electronic conductivity of the nanocrystals [84]. Huang et al. observed that Pd C-NCs deposited on carbon paper substrate has activity higher than Pt black electrode and it required very less catalyst loading that is 0.0106 mg cm−2 [85]. One thing to be noted about Pd is that it has higher activity in acidic medium than alkaline medium because of lower Pd-H binding energy and lower activation energy, acid (32.3 ± 0.7 kJ/mol) and base (38.9 ± 3.0 kJ/mol). Ruthenium (Ru) NCs are also being explored for EH2ER. As the Sabatier principle suggest that catalyst should not have much stronger binding to the hydrogen and should possess moderate binding capacity so desorption is easy, Ru-H follows this trend as it has ~65 kcal/mol energy for Ru-H bond and thus less activation barrier for desorption process [86]. Ru C-NCs are usually used with some support as they have durability problems because of aggregation. One such example where Joshi et al. used Ru C-NCs supported with Tungsten (W). DFT calculations suggest that Ru (0001) has high H2 binding to surface energy but using W support, it could reduce the H2 adsorption energy and changes the electronic environment thus making it similar to Pt (111) and increases its activity for EH2ER process [86]. Baek et al*.* revealed that Ru when deposited on graphene nanoplatelets (GnP) to form Ru@ GnP, its activity usually surpasses that of Pt/C in both acidic and alkaline medium. This happens because it is more stable, possess low Tafel slope (30 mV dec−1 in 0.5 M aq. H2SO4; and 28 mV dec−1 in 1.0 M aq. KOH) and also has comparatively low overpotential at 10 mA cm−2 (13 mV in 0.5 M aq. H2SO4; 22 mV in 1.0 M aq. KOH) [87]. Not only Pt, Pd, Ru, metals like Iridium (Ir) are also explored for the EH2ER process and also earth abundant metals are used but they are prone to corrosion in the presence of alkaline and acidic medium. Thus, the other way is using non-noble metals for the process.

Non-Noble metals follow this trend for the catalytic activity Nickel (Ni) > Molybdenum (Mo) > Cobalt (Co) > W > Iron (Fe) > Copper (Cu) which is calculated using the voltammetric techniques [88]. Ni shows a very good catalytic activity when using in the hybrid form of Ni/NiO/CoSe2 because this composite helps in less resistance to charge transfer. But this hybrid has poor

stability as Ni does not work well in acidic medium [89]. Recent studies by Qiu et al. found that when Ni is used with graphene it forms Ni-C bonds which increases the stability as well as the activity of the catalyst and is the best one proved for the process using Ni [90]. Co when embedded with Nitrogen rich CNTs forms a very good catalyst that catalyzed at all pH ranges. The reason for this good activity at all pH ranges is the N-doped content and the structural defect caused by the caused by pyrolysis of the Co-NRCNTs at higher temperatures [91]. These Nitrogen rich Co based catalyst can also be dispersed over the nanofibers for improving the catalytic activity. These particular catalysts also showed good stability for various potential cycles of process [92]. Along with Co and Ni Other Non-Noble metals are also used for the EH2ER process including Fe, W, Mo but these all face the problem of their stability and there is still a lot to discover in this field.
