**4.3 Non-nobel metal-based C-NCs for EH2ER**

To avoid using precious noble metals there have been a plethora of reports using non-noble metal-based C-NCs for EH2ER process. Some of them are transition Metal Oxides (TMOs), transition metal nitrides (TMNs), transition metal carbides (TMCs), transition metal borides (TMBs), transition metal phosphides (TMPs), transition metal dichalcogenides (TMDs). In this section the authors will discuss about the advancement in these types of NCs their advantage and disadvantages all. Although there are huge number of reports on transition metal-based compounds, but Mo and W display very good catalytic activity out of all of them. TMOs are easily available, stable, not harmful to the environment and obviously not precious like noble metals and thus is a good class of EH2ER catalysts. Out of all the TMOs, Mo (MoO2) and W (WO2) based EH2ER catalysts are best as they have high electrical conductivity as compared to other TMOs the credit goes to their monoclinic and distorted rutile crystal structure [93]. Compact MoO2 faces have a problem of aggregation and thus various research groups tried to bring changes in the catalyst by decreasing the size of structure, forming hybrid composites, doping the MoO2 or even introducing surface defects. One such example where Yu et al. encapsulated MoO2 with phosphorous-doped porous carbon embedded on rGO to form MoO2@PC-rGO which has a small tafel slope (41 mV dec-1 ) and less over potential (ƞ10 = 64 mV). Here electronic coupling between MoO2 and rGO along with the porous carbon layer helps in getting rid of the problem of aggregation and improves its efficiency as a EH2ER catalyst [94]. The other important metal oxide for EH2ER is WO2. Reports have revealed when WO2 have vacancies of oxygen, it provides large number of active sites and thus augmented the EH2ER process as compared to the compact WO2 counterpart. One such example where Shen et al. encapsulated WOx with Carbon on a Carbon support to form WOx@C/C which has EH2ER activity comparable to Pt metal with ultra-low overpotential (ƞ60 = 36 mV) and very small tafel slope (19 mV dec−1) which is because of the thick carbon shell helping in increase of charge transfer and also changes the Gibbs free energy values of H\* for different adsorption sites [95].

TMNs are also other class of noble metal-compound based NCs for EH2ER. The importance lies in the fact that they possess more contraction of the d bands and density of state near Fermi level because of interaction of negatively charged N atom which expands the lattice and increases it efficiency as a catalyst to make it comparable to noble metals like Pt, Pd [96]. As discussed earlier, whenever NCs are encapsulated with carbon material they tend to avoid aggregation and improve the efficiency as a catalyst. One such example where Shao et al. developed MoN

**209**

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

encapsulated with N-doped carbon material to form MoN@NC which exhibited very low overpotential (ƞ10 = 62 mV dec−1) and small tafel slope (54 mV) as depicted in **Figure 9 (c) [**97]. WN also possess same features as they exhibit less activity when used as it is but when encapsulated with carbon material its activity increases, one such example is using WN encapsulated with N doped graphene material (WNxNRPGC) which has high electrocatalytic activity because of the formed hetero-architecture [98]. Ni based TMNs have also been explored a lot as they pos-

*(a) Procedure for the synthesis of MoN-NC nano-octahedrons derived from Mo-based MOFs (b) polarization curves (iR compensated) in 0.5 M H2SO4 at a scan rate of 5 mV s−1 and (c) the corresponding Tafel plots of the MoN-NC nano-octahedrons, intermediate MoO2-C, bulk MoN, and 20% Pt/C catalysts. Reprinted with the* 

Whenever there is introduction of another element in the lattice of the metal synergistic effect comes into effect (intercalation of crystal planes, change in the metal–metal length to have strain and also formation of heteroatom bond to give ligand effect). This result in the change of electronic properties and morphology and hence in the electrocatalytic activity of the catalyst. To reduce loading of noble metals they are doped either with other comparatively cheap noble metal or sometimes with transition metal. The authors will discuss examples of both these types. Pt can be doped with Pd to form 1-D single crystalline material (thickness 3 nm) which increases the Pt utilization efficiency this was done by Liu et al. using solution phase method directed by surfactant [99]. Pt can also be doped with non-noble metals one such example is the use of Pt-Co alloy encapsulated on carbon material, thus possessing high activity as displayed by the small tafel slope of 20 mV dec−1. This catalyst requires very less loading of Pt (ca. 5 wt %) and has activity comparable to Pt/C catalyst [100]. Using the same strategy Pd and Ru can also be doped

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

sess high electrocatalytic activity.

**Figure 9.**

**4.4 Metal-alloy-based C-NCs for EH2ER**

*permission from [97]. Copyright © 2017, American Chemical Society.*

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

**Figure 9.**

*Colloids - Types, Preparation and Applications*

**4.3 Non-nobel metal-based C-NCs for EH2ER**

MoO2@PC-rGO which has a small tafel slope (41 mV dec-1

discover in this field.

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

To avoid using precious noble metals there have been a plethora of reports using

) and less over potential

non-noble metal-based C-NCs for EH2ER process. Some of them are transition Metal Oxides (TMOs), transition metal nitrides (TMNs), transition metal carbides (TMCs), transition metal borides (TMBs), transition metal phosphides (TMPs), transition metal dichalcogenides (TMDs). In this section the authors will discuss about the advancement in these types of NCs their advantage and disadvantages all. Although there are huge number of reports on transition metal-based compounds, but Mo and W display very good catalytic activity out of all of them. TMOs are easily available, stable, not harmful to the environment and obviously not precious like noble metals and thus is a good class of EH2ER catalysts. Out of all the TMOs, Mo (MoO2) and W (WO2) based EH2ER catalysts are best as they have high electrical conductivity as compared to other TMOs the credit goes to their monoclinic and distorted rutile crystal structure [93]. Compact MoO2 faces have a problem of aggregation and thus various research groups tried to bring changes in the catalyst by decreasing the size of structure, forming hybrid composites, doping the MoO2 or even introducing surface defects. One such example where Yu et al. encapsulated MoO2 with phosphorous-doped porous carbon embedded on rGO to form

(ƞ10 = 64 mV). Here electronic coupling between MoO2 and rGO along with the porous carbon layer helps in getting rid of the problem of aggregation and improves its efficiency as a EH2ER catalyst [94]. The other important metal oxide for EH2ER is WO2. Reports have revealed when WO2 have vacancies of oxygen, it provides large number of active sites and thus augmented the EH2ER process as compared to the compact WO2 counterpart. One such example where Shen et al. encapsulated WOx with Carbon on a Carbon support to form WOx@C/C which has EH2ER activity comparable to Pt metal with ultra-low overpotential (ƞ60 = 36 mV) and very small tafel slope (19 mV dec−1) which is because of the thick carbon shell helping in increase of charge transfer and also changes the Gibbs free energy values of H\* for

TMNs are also other class of noble metal-compound based NCs for EH2ER. The importance lies in the fact that they possess more contraction of the d bands and density of state near Fermi level because of interaction of negatively charged N atom which expands the lattice and increases it efficiency as a catalyst to make it comparable to noble metals like Pt, Pd [96]. As discussed earlier, whenever NCs are encapsulated with carbon material they tend to avoid aggregation and improve the efficiency as a catalyst. One such example where Shao et al. developed MoN

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different adsorption sites [95].

*(a) Procedure for the synthesis of MoN-NC nano-octahedrons derived from Mo-based MOFs (b) polarization curves (iR compensated) in 0.5 M H2SO4 at a scan rate of 5 mV s−1 and (c) the corresponding Tafel plots of the MoN-NC nano-octahedrons, intermediate MoO2-C, bulk MoN, and 20% Pt/C catalysts. Reprinted with the permission from [97]. Copyright © 2017, American Chemical Society.*

encapsulated with N-doped carbon material to form MoN@NC which exhibited very low overpotential (ƞ10 = 62 mV dec−1) and small tafel slope (54 mV) as depicted in **Figure 9 (c) [**97]. WN also possess same features as they exhibit less activity when used as it is but when encapsulated with carbon material its activity increases, one such example is using WN encapsulated with N doped graphene material (WNxNRPGC) which has high electrocatalytic activity because of the formed hetero-architecture [98]. Ni based TMNs have also been explored a lot as they possess high electrocatalytic activity.

#### **4.4 Metal-alloy-based C-NCs for EH2ER**

Whenever there is introduction of another element in the lattice of the metal synergistic effect comes into effect (intercalation of crystal planes, change in the metal–metal length to have strain and also formation of heteroatom bond to give ligand effect). This result in the change of electronic properties and morphology and hence in the electrocatalytic activity of the catalyst. To reduce loading of noble metals they are doped either with other comparatively cheap noble metal or sometimes with transition metal. The authors will discuss examples of both these types. Pt can be doped with Pd to form 1-D single crystalline material (thickness 3 nm) which increases the Pt utilization efficiency this was done by Liu et al. using solution phase method directed by surfactant [99]. Pt can also be doped with non-noble metals one such example is the use of Pt-Co alloy encapsulated on carbon material, thus possessing high activity as displayed by the small tafel slope of 20 mV dec−1. This catalyst requires very less loading of Pt (ca. 5 wt %) and has activity comparable to Pt/C catalyst [100]. Using the same strategy Pd and Ru can also be doped

with either noble metal or transition metals, some examples are Pd-Au catalyst, Pd-Co catalyst, Ru-Co and Ru-Ni [101–104].

Transition metal can be alloyed with transition metal itself and thus it can be a very good alternative for noble metal electrocatalysts because of the cost-effective nature of them. These alloys could either be binary alloys or ternary alloys, the authors will discuss examples from both binary and ternary alloys. Ni can form alloy with various other transition metal, but Ni-Mo binary alloy is considered best for the EH2ER. Zhang et al. worked on the synthesis of MoNi4 supported over the MoO2 cuboids over the Ni foam. This catalyst has the activity similar to Pt/C with zero onset potential and ƞ10 = 15 mV and very low tafel slope of 30 mV dec−1 [105]. Ni binary alloys face the problem of corrosion which can be overcome by using the carbon support along with the Ni based alloy. Co also form binary alloys with Fe using N-doped carbon-based support. Also, Co can be alloyed with Mo forming good electrochemical catalyst for EH2ER such as Co3Mo having an overpotential of ƞ10 = 68 mV and tafel slope of 61 mV dec−1 [106]. Ternary alloys in the recent times have gained popularity for EH2ER electrochemical catalysts as electronic and morphological features of catalyst can be tuned by variation in compositions of various metals and thus it can act as the good promising substitute for the noble-metal-based EH2ER catalyst. One such example is the use of small amount of Pt (4.6%) to the Fe-Co binary alloy to form the PtCoFe@CN electrocatalyst which demonstrated activity similar to that of commercial 20% Pt/C having an overpotential of ƞ10 = 45 mV as depicted in **Figure 10(b) [**107]. Research is still going on to prepare the ternary alloys without using the noble metals in it and that will really be the landmark in this field as it will be a catalyst with cost-effective nature.

#### **Figure 10.**

*(a) Polarization curves (b) Tafel plots of samples (c) polarization curves of PtCoFe@CN 1st and 10000th cycles (d) Amperometric i-t curves of PtCoFe@CN. Reprinted with the permission from [107]. Copyright © 2017, American Chemical Society.*

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

This chapter discuses a thorough study of recent acheivements by C-NCs-based

RN, AP (Abhay Prasad), AP\*(Ashish Parihar) designed the study, reviewed the literature, wrote and edited the manuscript. MSA and RS provided the inputs while editing the manuscript. The complete review article was edited and finalized by

elelctroctalysts for ECO2RR and EH2ER. In this chapter, the authors have tried to summarize role of C-NCs in ECO2RR and EH2ER and the scope of these two important electrocatalytic reaction in combating the energy crises for human kind in introduction part. The examples that are discussed in this chapter were taken from recent reports in literature. The first part of this chapter sheds light on colloidal synthesis of nanocystals. The second part of this chapter emphasizes on effect of shape, size and composition in determining the catalytic activity, selectivity and stability of C-NCs for ECO2RR. Here the effect of ligand functionalization and MOF/NCs hybrid system on ECO2RR activity is also illustrated. The third part of this chapter addreses the role of C-NCs-based electrocatalysts on EH2ER, its activity and stability. In this part, indepth study about mechanism of EH2ER is also discussed. Although a lot has been done in ECO2RR but ECO2RR still face a big challenge in selectivity, similarly in EH2ER a lot of research has been dedicated to find a substitute of state-of-art Pt/C catalyst which is very much expensive. However, researchers have been sucessful in discovering the costeffective electrocatalysts

which are as good as Pt/C but they are still facing the stability issues.

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

**5. Conclusions**

**Author Contributions**

RN, AP, and AP\*.

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