**Abstract**

The serious threat that human beings face in near future will be shortage of fossil fuel reserves and abrupt changes in global climate. To prepare for these serious concerns, raised due to climate change and shortage of fuels, conversion of excessive atmospheric CO2 into valuable chemicals and fuels and production of hydrogen from water splitting is seen most promising solutions to combat the rising CO2 levels and energy crises. Amoung the various techniques that have been employed electrocatalytic conversion of CO2 into fuels and hydrogen production from water has gained tremendous interest. Hydrogen is a zero carbon-emitting fuel, can be an alternative to traditional fossil fuels. Therefore, researchers working in these areas are constantly trying to find new electrocatalysts that can be applied on a real scale to deal with environmental issues. Recently, colloidal nanocrystals (C-NCs)-based electrocatalysts have gained tremendous attention due to their superior catalytic selectivity/activity and durability compared to existing bulk electrodes. In this chapter, the authors discuss the colloidal synthesis of NCs and the effect of their physiochemical properties such as shape, size and chemical composition on the electrocatalytic performance and durability towards electrocatalytic H2 evolution reaction (EH2ER) and electrocatalytic CO2 reduction reactions (ECO2RR). The last portion of this chapter presents a brief perspective of the challenges ahead.

**Keywords:** colloidal nanocrystals, electrocatalysis, size control, shape control, CO2 reduction reactions, H2 evolution reaction

## **1. Introduction**

A clean environment and sustained energy resources are essential for future generations. With growing concerns for both dwindling traditional fossil fuels and global warming, there is an urgent need to develop renewable and environmentally benign alternatives to address these issues of mankind [1–3]. Currently, humans are mainly dependent on fossil fuels and thus extract carbon from the geosphere and put it into the atmosphere where it causes global warming. There are two methods that can be helpful in preventing and reducing carbon emissions. The first and

convenient way to stop carbon emissions is to move towards zero carbon-emitting resources. In view of this, hydrogen (H2) produced by photo/electrocatalytic water splitting has shown great potential to become the fuel of the future. The merits have been attributed to its high energy density and it produces only one by-product of water upon combustion [4]. Thus H2, which is a zero carbon-emitting fuel, can be a promising solution to the mitigation of climate change. The second method is to capture carbon from the atmosphere and then store it back into the geosphere. However, the geosphere sequestration of CO2 has no financial benefits. In contrast, chemical transformation of excess accumulated CO2 from air into valuable industrial products, such as fuels (methanol, ethanol), hydrocarbon (methane, ethylene) and chemicals (formic acid, acetic acid), is an effective way to solve both global warming and energy crises [5, 6]. Furthermore, it has economic significance from an industrial point of view. However, carbon-di-oxide reduction (CO2R) is a highly cumbersome and non-specific process. So far, several approaches, such as biochemical, chemical, thermal, photochemical and electrochemical catalysts have been explored to achieve aspirated activity and selectivity in this region [7–9]. Nonetheless, unlike other catalytic system electrocatalysts have gained tremendous attention due to its easy operation at ambient temperature and pressure. In addition, the selectivity of the product can be obtained by just adjusting reaction conditions, such as redox potential, electrode, and electrolyte, pH temperature and so on. The main advantage of using electrocatalysts is that they can be powered by renewable energy sources that emit zero carbon. For all these reasons, many research activities have shifted to the areas of EH2ER and ECO2RR (**Figure 1**) [10].

Over the years of time metal, metal oxide, metal sulphide have shown great promise in ECO2RR and EH2ER using electrocatalysis phenomenon. These electrocatalysts are being considered as a promising system that would be able to operate on a real scale without polluting environment. Whereas, there are still many limitations that are associated with electrocatalysts, such as high cost, poor product selectivity, high overpotential and low stability [11]. Colloidal Nanocrystal (C-NC) based electrocatalysts have become indispensable to overcome these limitations to certain extent owing to their larger surface to volume ratio, precise shape, long-term durability, and the plethora of configurations [12, 13]. These factors are important in influencing their efficiency, selectivity, and durability for EH2ER and ECO2RR. For example, variation in the size and/or shape causes alteration in reactivity at

#### **Figure 1.**

*Electrochemical CO2 conversion into fuel and H2 production by using colloidal nanocrystal-based electrocatalysts.*

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

these features will be discussed in detail as this chapter unfolds.

different locations (edges/corners/faces) of the C-NC due to changes in a specific atomic-arrangement of active centers and crystal surface energy. The impact of

The purpose of this chapter is to elaborate on recent research developments and challenges in the field of heterogeneous C-NCs-based electrocatalysts for ECO2RR and EH2ER. In the first part of this chapter, colloidal synthesis of nanocrystals will be discussed. The second part of this chapter will address the structural aspects, such as size, shape, and composition, are important in tuning the catalytic efficiency, selectivity, and durability of NC-based catalysts for ECO2RR. Here a brief introduction of effect of ligand functionalization and effect of MOF/NCs hybrid system on ECO2RR activity will be also discussed. In the third part of this chapter, the role of C-NCs-based catalysts on EH2ER, its activity and stability is given. Moreover, the detailed mechanism of EH2ER is also discussed in this part. Finally, Authors have given extractive commentary that sheds light on the future perspec-

The C-NC is an inorganic material with a size of 1–100 nm and surface covering of protecting capping agents like polymer and surfactants molecules. Generally, the inorganic part exhibits characteristic features, such as optical, electrical, magnetic, and catalytic, that can be tuned by changing their physicochemical parameters, while surface capping guarantees the stabilization of these structures and paves the way for synthesizing more complex structures [14, 15]. The physical parameters like morphology and chemical composition of C-NCs can be easily adapted by varying their reaction parameters like monomer concentration forming inorganic core of NCs and judicious choice of capping substances for surface covering. Over the last two-three decades, researchers have gained good control over synthesis of highquality and cost-effective NCs with uniform morphology and chemical constituents using colloidal synthesis [16–19]. The C-NCS approach has not only enhanced efficiency, selectivity of NCs, but also improved their service life. So far, researchers have found many commercial applications of C-NCs in various fields ranging from life sciences to the material world. One of the striking applications of C-NCs is in the field of biological imaging of cells, where quantum dots are used owing to their excellent fluorescent properties and also they do not photo-chemically bleach out like organic dyes [20]. Recently, quantum dots are being used commercially in LED displays also known as *QLED*-displays [21]. In addition, C-NCs-based photo/ electro-catalysts for ECO2RR and EH2ER are being developed to solve the energy crisis and global warming. However, their uses at economical scale in this area is still facing challenges. Deep insights of C-NCs synthesis and the effect of C-NCs physiochemical parameters on their electrocalytic properties need to be investigated for

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

tive of ECO2RR and EH2ER in conclusion.

their successful applications at the economical level.

In general, C-NCs can be synthesized in both water and organic solvents. However, synthesizing a broad spectrum of NCs requires different reaction conditions that are much more feasible to achieve in organic solvents compared to water that is mainly used in the synthesis of noble metal C-NCs [14]. Therefore, in this section, authors will focus on organic phase C-NCs synthesis. Generally, C-NCs synthesis requires three major elements: 1) precursor molecules or building blocks forming inorganic core of NCs, 2) capping agents, and 3) organic solvents. Capping agents sometime act as solvent. The process of nanocrystal formation starts with transformation of precursor molecules into unstable and reactive species or monomers that usually occurs at quite high temperature. Thereafter, these monomers

**2. Colloidal synthesis of NCs**

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

different locations (edges/corners/faces) of the C-NC due to changes in a specific atomic-arrangement of active centers and crystal surface energy. The impact of these features will be discussed in detail as this chapter unfolds.

The purpose of this chapter is to elaborate on recent research developments and challenges in the field of heterogeneous C-NCs-based electrocatalysts for ECO2RR and EH2ER. In the first part of this chapter, colloidal synthesis of nanocrystals will be discussed. The second part of this chapter will address the structural aspects, such as size, shape, and composition, are important in tuning the catalytic efficiency, selectivity, and durability of NC-based catalysts for ECO2RR. Here a brief introduction of effect of ligand functionalization and effect of MOF/NCs hybrid system on ECO2RR activity will be also discussed. In the third part of this chapter, the role of C-NCs-based catalysts on EH2ER, its activity and stability is given. Moreover, the detailed mechanism of EH2ER is also discussed in this part. Finally, Authors have given extractive commentary that sheds light on the future perspective of ECO2RR and EH2ER in conclusion.
