**5. Conclusions**

*Colloids - Types, Preparation and Applications*

with cost-effective nature.

Pd-Co catalyst, Ru-Co and Ru-Ni [101–104].

with either noble metal or transition metals, some examples are Pd-Au catalyst,

*(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 ©* 

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

**210**

**Figure 10.**

*2017, American Chemical Society.*

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