**2.1 Carbon materials**

Carbon materials, composed of carbon atoms having *sp*<sup>2</sup> -hybridization (e.g., graphite, graphene, graphene oxide, and carbon nanotube) or *sp*<sup>3</sup> -hybridization (e.g., carbon black and activated carbon), have been widely investigated as the CEs due to their material specialties such as low cost, high electrical conductivity, high thermal stability, and good corrosion resistance [3, 4, 80–84]. The carbon materials with *sp*<sup>2</sup> -hybridization normally exhibit a 1D or 2D structure, and those with *sp*<sup>3</sup> hybridization have a 0D structure. However, carbon materials often exhibit serious aggregation that reduces the material conductivity and limits the electrochemical surface area. Here we include several studies which provided practical strategies to overcome this problem and reach comparable/better cell efficiencies than the Pt-incorporated DSSCs, as listed in **Table 1**.

For example, Fan et al. used a small 0D porous carbon nanoball (diameter = 20 ± 3 nm) to assemble a large 0D hollow nanoball (diameter = 100 ± 10 nm), as shown in **Figure 4(a)** [12]. Tseng et al. introduced a one-step synthetic method to make tens of 2D nitrogen-doped graphene with a thickness of ~3.5 nm stacking together to form a building block as a 0D hollow nanoball, as shown in **Figure 4(b)** [22]. Fan et al. used a small 0D porous carbon nanoball (diameter = 20 ± 3 nm) to assemble a large 0D hollow nanoball (diameter = 100 ± 10 nm), as shown in **Figure 4(b)** [12]. The hollow nanoballs consisting of nitrogen-doped graphene and porous carbon gave their DSSCs *η*s of 7.53 and 8.67%, respectively, which were comparable to the Pt-based cells. Besides, nitrogen-doped graphene (**Figure 4(c)**, 7.07%) [13] and the wrinkled 2D graphene (**Figure 4(d)**, 7.80%) [14] nanosheets were used to form a honeycomb-like structure having extra surface area and vertically aligned subnanosheets as the additional electron transfer routes.


**83**

**Figure 5.**

I

**Figure 4.**

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

The combination of few kinds of carbon materials was reported to form a hierarchical structure, which could not only create a high surface area but also a directional electron transfer pathway. Dong et al. made 1D few-walled carbon nanotubes (CNTs, tens of microns long) vertically fuse onto the 2D graphene nanosheet (<1 nm thick), as shown in **Figure 4(e)** [15]. The red seven-membered rings at the neck seamlessly fuse the tubular CNTs to the planar graphene without obvious CNT aggregation (**Figure 5(a)**). Even though the CNTs only covered few parts of the electrode surface, they still benefited the electrolyte wetting and electron transfer rate within the counter electrode, leading to a better *η* (8.2%) than the Pt-based cell (6.4%). Ma et al. reported a similar hierarchical structure, where the single-walled carbon nanotube (SWCNT) was located on a flat N-doped graphene (N-doped GN) nanosheet by the z-axis direction (**Figure 4(f )**) [16]. As a result, composite SWCNT@N-doped GN reached higher *η* (8.31%) than Pt (7.56%). Yeh et al. prepared a hybrid heterostructure of multiwalled carbon nanotube (MWCNT, diameter = 15 nm) and reduced graphene oxide (rGO, thickness = 25 nm) nanosheet, where the 2D rGO nanosheet-like shell was wrapped around the 1D CNT core (**Figure 4(g)**) [27]. In the MWCNT@rGO composite material, the tubular MWCNTs functioned as the 1D heterogeneous electron transfer pathways, which provided sufficient electrons to the electrochemical reaction; at the same time, the 2D rGO nanosheet supplied multiple edges as the active sites to reduce I3

*The structures of carbon materials including (a and b) hollow nanoball [12, 22], (c) nanosheet [13],* 

*(d) honeycomb [14], and (e–g) nanotube with nanosheet [15, 16, 27].*

<sup>−</sup> (**Figure 5(b)**). The hybrid heterostructure of MWCNT@rGO was found to avoid

*The scheme of hierarchical structures of (a and b) nanotube with nanosheet [15, 27].*

<sup>−</sup> to

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

### **Table 1.**

*A partial list of literature on the DSSCs with carbon material-based CEs.*

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

**Figure 4.**

*Nanostructures*

included.

with *sp*<sup>2</sup>

**2.1 Carbon materials**

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




[15]

[16]

**2. Nanostructure materials of counter electrode**

Pt-incorporated DSSCs, as listed in **Table 1**.

nanosheets as the additional electron transfer routes.

*A partial list of literature on the DSSCs with carbon material-based CEs.*

Carbon materials, composed of carbon atoms having *sp*<sup>2</sup>

(e.g., graphite, graphene, graphene oxide, and carbon nanotube) or *sp*<sup>3</sup>

(e.g., carbon black and activated carbon), have been widely investigated as the CEs due to their material specialties such as low cost, high electrical conductivity, high thermal stability, and good corrosion resistance [3, 4, 80–84]. The carbon materials

hybridization have a 0D structure. However, carbon materials often exhibit serious aggregation that reduces the material conductivity and limits the electrochemical surface area. Here we include several studies which provided practical strategies to overcome this problem and reach comparable/better cell efficiencies than the


For example, Fan et al. used a small 0D porous carbon nanoball (diameter = 20 ± 3 nm) to assemble a large 0D hollow nanoball (diameter = 100 ± 10 nm), as shown in **Figure 4(a)** [12]. Tseng et al. introduced a one-step synthetic method to make tens of 2D nitrogen-doped graphene with a thickness of ~3.5 nm stacking together to form a building block as a 0D hollow nanoball, as shown in **Figure 4(b)** [22]. Fan et al. used a small 0D porous carbon nanoball (diameter = 20 ± 3 nm) to assemble a large 0D hollow nanoball (diameter = 100 ± 10 nm), as shown in **Figure 4(b)** [12]. The hollow nanoballs consisting of nitrogen-doped graphene and porous carbon gave their DSSCs *η*s of 7.53 and 8.67%, respectively, which were comparable to the Pt-based cells. Besides, nitrogen-doped graphene (**Figure 4(c)**, 7.07%) [13] and the wrinkled 2D graphene (**Figure 4(d)**, 7.80%) [14] nanosheets were used to form a honeycomb-like structure having extra surface area and vertically aligned sub-

**Materials** *η* **(%)** *η* **of Pt (%) Structure Ref** Porous carbon 8.67 9.34 Hollow nanoball [12] Nitrogen-doped graphene 7.53 7.70 Hollow nanoball [22] Nitrogen-doped graphene 7.07 7.44 Honeycomb [13] Graphene 7.80 8.00 Honeycomb [14]

8.2 6.4 Nanotube vertically fused onto the

6.91 7.26 Nanotube embedded in nanosheet [27]

8.31 7.56 Nanotube intertwined with

nanosheet

nanosheet

**82**

**Table 1.**

Carbon nanotubes and

Carbon nanotube and N-doped graphene

Carbon nanotube and graphene oxide

graphene

*The structures of carbon materials including (a and b) hollow nanoball [12, 22], (c) nanosheet [13], (d) honeycomb [14], and (e–g) nanotube with nanosheet [15, 16, 27].*

The combination of few kinds of carbon materials was reported to form a hierarchical structure, which could not only create a high surface area but also a directional electron transfer pathway. Dong et al. made 1D few-walled carbon nanotubes (CNTs, tens of microns long) vertically fuse onto the 2D graphene nanosheet (<1 nm thick), as shown in **Figure 4(e)** [15]. The red seven-membered rings at the neck seamlessly fuse the tubular CNTs to the planar graphene without obvious CNT aggregation (**Figure 5(a)**). Even though the CNTs only covered few parts of the electrode surface, they still benefited the electrolyte wetting and electron transfer rate within the counter electrode, leading to a better *η* (8.2%) than the Pt-based cell (6.4%). Ma et al. reported a similar hierarchical structure, where the single-walled carbon nanotube (SWCNT) was located on a flat N-doped graphene (N-doped GN) nanosheet by the z-axis direction (**Figure 4(f )**) [16]. As a result, composite SWCNT@N-doped GN reached higher *η* (8.31%) than Pt (7.56%). Yeh et al. prepared a hybrid heterostructure of multiwalled carbon nanotube (MWCNT, diameter = 15 nm) and reduced graphene oxide (rGO, thickness = 25 nm) nanosheet, where the 2D rGO nanosheet-like shell was wrapped around the 1D CNT core (**Figure 4(g)**) [27]. In the MWCNT@rGO composite material, the tubular MWCNTs functioned as the 1D heterogeneous electron transfer pathways, which provided sufficient electrons to the electrochemical reaction; at the same time, the 2D rGO nanosheet supplied multiple edges as the active sites to reduce I3 <sup>−</sup> to I <sup>−</sup> (**Figure 5(b)**). The hybrid heterostructure of MWCNT@rGO was found to avoid

**Figure 5.** *The scheme of hierarchical structures of (a and b) nanotube with nanosheet [15, 27].*

the aggregations among the MWCNTs or among the rGOs. Thus, the MWCNT@ rGO rendered its DSSC an *η* of 6.91%, which is close to the Pt-based cell (7.26%).
