**1. Introduction**

### **1.1 Dye-sensitized solar cells (DSSCs)**

Fossil fuel, as a limiting energy source, may be run out in the upcoming centuries. However, the consumption of energy increases every year [1, 2]. As a result, finding and developing renewable energy sources is an urgent problem. Due to the unlimitedness of renewable energy resources, they are candidates to be reliable replacement for sustainable usage in the future. Among them, the Sun has been considered as one of the most promising renewable energy sources. It provides about 120,000 terawatts to the earth, which equals thousand times of the current energy consumption rate. The solar cells can utilize the sunshine and transform to electricity [3–5]. Generally, solar cells can be classified to four generations: the first generation is silicon-based solar cells; the second generation is CIGS (CuInGaSe), CZTS (CuZnTiSe), and CdTe solar cells; the third generation is organic photovoltaics (OPVs) and dye-sensitized solar cells (DSSCs); and the fourth generation is

perovskite solar cells (PSCs). The first and second generations have been widely explored for decades, and they are the most common solar cells at present. However, they are fabricated through expensive, toxic, energy-intensive, high-temperature, and high-vacuum processes. Therefore, DSSCs are very competitive to the first and second generations of solar cells due to numerous advantages including easy fabrication, low cost (**Figure 1**) [3], and high performance at dim-light condition. Moreover, DSSCs can be used in indoor ambient applications [5–9].

A DSSC is composed of a photoanode, electrolyte, and counter electrode (CE), as shown in **Figure 2**. When a photoanode is excited by the sun or photon, it will release the electron to the external circuit. At the same time, the iodide/triiodide (I<sup>−</sup> /I3 − ) redox couple will relax the photoanode to its ground state. Then the CE will reduce the redox couple to regenerate the DSSC. Among them, the CE plays an important role to determine the DSSC performance [10]. At the CE/electrolyte interface, the electrochemical mechanism goes through I3 − decomposition (Eq. (1)) → adsorption (Eq. (2)) → catalytic reduction reaction (Eq. (3)) → desorption (Eq. (4)), and the overall reaction shows as Eq. (5) [11]. Among these reaction steps, Eq. (3) is found to be the slowest step, which means the rate-determining step to decide the DSSC performance.

$$\mathbf{I\_{3^{-}}} \leftrightarrow \mathbf{I\_{2}} + \mathbf{I^{-}} \tag{1}$$

$$\text{I}\_2 \star \text{2CE} \leftrightarrow \text{I} \text{ (CE)} \star \text{I} \text{ (CE)}\tag{2}$$

$$\text{I (CE)} \star \text{e}^- \leftrightarrow \text{I}^- \text{ (CE)}\tag{3}$$

$$\text{I}^- \text{ (CE)} \leftrightarrow \text{I}^- \star \text{CE} \tag{4}$$

$$\rm I\_{3^{-}} + 2e^{-} \leftrightarrow \rm 3I^{-} \tag{5}$$

**81**

**Figure 3.**

**Figure 2.**

*The structure and mechanism of DSSCs [2].*

*Structural Engineering on Pt-Free Electrocatalysts for Dye-Sensitized Solar Cells*

element, several types of materials, such as carbon materials [5, 12–28], conductive polymers [29–48], and transition metal compounds [37, 45, 49–79] have been extensively explored to elevate the cell efficiency (*η*) and decrease the cost of the CEs. To date, there have been a very limited number of non-Pt nanomaterials that

However, the specific structural designs of nanomaterials would largely increase the effective electrocatalytic surface area so as to provide better overall electrocatalytic ability than Pt. Moreover, the nanostructured electrocatalysts could have appropriate interfacial affinity, good electrochemical stability, or specific self-assembly natures; these properties may influence the DSSC performance as well. A typical nanostructured material can be defined if any dimension of the material is lower than 100 nm. The nanostructured material can be classified into three groups: zerodimensional (0D, e.g., nanoparticle, nanocube, etc.), one-dimensional (1D, e.g., nanorod, nanotube, nanoneedle, etc.), and two-dimensional (2D, e.g., nanosheet, nanopental, etc.) structures, as shown in **Figure 3**. Generally, 0D structure is

could have a comparable intrinsic heterogeneous rate constant to that of Pt.

*The scheme of zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) structure.*

*DOI: http://dx.doi.org/10.5772/intechopen.85307*

There are two ways to enhance the electrocatalytic reduction reaction. One is to increase the heterogeneous rate constant, relating to the intrinsic electrocatalytic ability of the electrocatalyst. The other is to engineer the structure of the electrocatalyst for I3 <sup>−</sup> reduction with regard to the charge transfer route and the surface area. To replace a traditional platinum (Pt) electrocatalyst, where Pt is a rare and expensive

**Figure 1.**

*Efficiency and cost projection for first- (I), second- (II), and third-generation (III) photovoltaic technology [3].*

*Structural Engineering on Pt-Free Electrocatalysts for Dye-Sensitized Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.85307*

**Figure 2.** *The structure and mechanism of DSSCs [2].*

*Nanostructures*

mechanism goes through I3

perovskite solar cells (PSCs). The first and second generations have been widely explored for decades, and they are the most common solar cells at present. However, they are fabricated through expensive, toxic, energy-intensive, high-temperature, and high-vacuum processes. Therefore, DSSCs are very competitive to the first and second generations of solar cells due to numerous advantages including easy fabrication, low cost (**Figure 1**) [3], and high performance at dim-light condition.

A DSSC is composed of a photoanode, electrolyte, and counter electrode (CE), as shown in **Figure 2**. When a photoanode is excited by the sun or photon, it will release

couple will relax the photoanode to its ground state. Then the CE will reduce the redox couple to regenerate the DSSC. Among them, the CE plays an important role to determine the DSSC performance [10]. At the CE/electrolyte interface, the electrochemical

<sup>−</sup> ↔ I2 + I

I2 + 2CE ↔ I (CE) + I (CE) (2)

<sup>−</sup> (CE) ↔ I

<sup>−</sup> + 2e− ↔ 3I<sup>−</sup>

There are two ways to enhance the electrocatalytic reduction reaction. One is to increase the heterogeneous rate constant, relating to the intrinsic electrocatalytic ability of the electrocatalyst. The other is to engineer the structure of the electrocata-

replace a traditional platinum (Pt) electrocatalyst, where Pt is a rare and expensive

*Efficiency and cost projection for first- (I), second- (II), and third-generation (III) photovoltaic technology [3].*

<sup>−</sup> reduction with regard to the charge transfer route and the surface area. To

lytic reduction reaction (Eq. (3)) → desorption (Eq. (4)), and the overall reaction shows as Eq. (5) [11]. Among these reaction steps, Eq. (3) is found to be the slowest step, which means the rate-determining step to decide the DSSC performance.

decomposition (Eq. (1)) → adsorption (Eq. (2)) → cata-

<sup>−</sup> (1)

<sup>−</sup> (CE) (3)

<sup>−</sup> + CE (4)

(5)

/I3 − ) redox

Moreover, DSSCs can be used in indoor ambient applications [5–9].

−

I3

I

I3

I (CE) + e− ↔ I

the electron to the external circuit. At the same time, the iodide/triiodide (I<sup>−</sup>

**80**

**Figure 1.**

lyst for I3

element, several types of materials, such as carbon materials [5, 12–28], conductive polymers [29–48], and transition metal compounds [37, 45, 49–79] have been extensively explored to elevate the cell efficiency (*η*) and decrease the cost of the CEs.

To date, there have been a very limited number of non-Pt nanomaterials that could have a comparable intrinsic heterogeneous rate constant to that of Pt. However, the specific structural designs of nanomaterials would largely increase the effective electrocatalytic surface area so as to provide better overall electrocatalytic ability than Pt. Moreover, the nanostructured electrocatalysts could have appropriate interfacial affinity, good electrochemical stability, or specific self-assembly natures; these properties may influence the DSSC performance as well. A typical nanostructured material can be defined if any dimension of the material is lower than 100 nm. The nanostructured material can be classified into three groups: zerodimensional (0D, e.g., nanoparticle, nanocube, etc.), one-dimensional (1D, e.g., nanorod, nanotube, nanoneedle, etc.), and two-dimensional (2D, e.g., nanosheet, nanopental, etc.) structures, as shown in **Figure 3**. Generally, 0D structure is

expected to supply the high electrochemical surface area, and 1D/2D structures are claimed to have directional electron transfer pathways. In this chapter, different strategies of designing nanostructured carbon materials, conductive polymers, and transition metal compounds to increase their active surface area/charge transfer route will be systematically discussed. The corresponding DSSC performance is also included.
