**2. One-dimensional nanostructure (1D)**

One-dimensional TMCs nanostructure is expected that it provides the 1D electron transfer pathways, promoting electrolyte penetration, and more reaction area [26–34]. However, the vertical 1D structure is rarely obtained because it is difficult to synthesize. Herein, we focus on that the 1D structure has been directly obtained without the template method, in **Figures 5** and **6**. Their corresponding efficiencies are listed in **Table 1**. In **Figure 5**, it shows horizontal 1D TMCs nanostructure SEM images of MoN nanorod, W18O49 nanowire, NiS nanorod, CoSe2 nanorod, Co0.85Se nanotubes, CoSe2/CoSeO3 nanorod, and Ni3S4 nanorod that were synthesized by Song et al., Zhou et al., Yang et al., Sun et al., Yuan et al., Huang et al., and Huang et al., respectively [27–33]. Song et al. reported that MoN nanorod morphology reveals enhancement of diffusion kinetics for the active electrochemical process, as shown in **Figure 5a** [27]. So that the MoN nanorod has higher *V*OC and *J*SC than MoN nanoparticle. Zhou synthesized W18O49 nanowire (**Figure 5b**), having oxygen vacancies within the range of WO2.625 to WO3, *via* the solvothermal method [28]. Their efficiency is 4.85% for Co ion electrolyte. Yang et al. obtained NiS nanorod (**Figure 5c**), which is α type, through chemical bath method [29]. It has *η* of 5.20%,

**Figure 5.**

*The SEM of horizontal 1D nanostructure with (a) MoN, (b) W18O49, (c) NiS, (d) CoSe2, (e) Co0.85Se, (f) CoSe2/CoSeO3, (g) Ni3S4 [27–33].*

**355**

than the Pt.

**Table 1.**

function for CE in DSSCs.

**3. Two-dimensional (2D)**

*Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts…*

**Materials** *η* **(%)** *V***OC (V)** *J***SC (mA cm−2) FF** *η***/***η***Pt Ref** CoS 7.67 0.71 16.31 0.66 1.00 [26] MoN 7.29 0.74 15.26 0.65 0.98 [27] W18O49 4.85 0.80 9.26 0.67 1.08 [28] NiS 5.20 0.68 11.42 0.67 0.83 [29] CoSe2 10.20 0.75 18.55 0.73 1.25 [30] Co0.85Se 5.34 0.71 14.51 0.52 0.71 [31] CoSe2/CoSeO3 7.54 0.82 14.32 0.64 0.95 [32] Ni3S4 7.31 0.75 15.53 0.63 0.93 [33] Co0.85Se 8.35 0.74 15.76 0.71 1.08 [34]

which is better than the nanoparticle NiS (4.20%). The reason is that the nanorod affords lower charge transfer resistance than the nanoparticle. Sun et al. acquired CoSe2 nanorod (**Figure 5d**), possessing a single orthorhombic crystal structure, by hydrothermal method [30]. The CoSe2 nanorod exits the excellent performance (10.20%), even better than the Pt. They remind that single CoSe2 nanorod has great electrocatalytic ability, lower charge resistance, and higher adsorption capacity for electrolyte. Yuan et al. prepared Co0.85Se nanotubes (**Figure 5e**) by a simple hydrothermal method [31]. It shows *η* of 5.34%, which lower than Pt, obviously. Huang et al. obtained CoSe2/CoSeO3 nanorod (**Figure 5f**) through a microemulsionassisted hydrothermal synthesis [32]. It reveals *η* of 7.54%, which is approach Pt performance. This result contributes to the 1D electron transfer pathways. Huang et al. synthesized the Ni3S4 nanorod (**Figure 5g**) *via* a one-pot colloidal synthesis [33]. And it has *η* of 7.31%, which is quite close Pt. As listed in **Table 1**, there have a few of the 1D TMCs nanostructures existing the better performance than the Pt. Most of them are vertical 1D TMCs nanostructures. The horizontal 1D TMCs nanostructures could not support the vertical electron transfer pathways and promote the electrolyte penetration. So most of them display lower performance

*A partial list of literature on the DSSCs with 1D TMCs nanostructure based CEs.*

The vertical 1D TMCs nanostructure is an ideal condition, as shown in **Figure 4**.

Geim and Grigorieva classified 2D materials into three groups [35]. First group, graphene type contains graphene, fluorographene, graphene oxide, hBN, *etc.*; second group, 2D chalcogenides (transition metal) type includes MoS2, NbS2, NbSe2, CoSe2, MoSe2, ZrSe2, GaSe, GaTe, InSe, Bi2Se3, Bi2Te3, *etc.*; final group, 2D oxides type involves TiO2, MnO2, V2O5, RuO2, perovskite-based materials (LaNb2O7, Ba4Ti3O12, Ca2Ta2TiO10 *etc.*), hydroxides (Ni(OH)2, Eu(OH)2, *etc.*), *etc*. Research of 2D TMCs

Kung et al. and Jin et al. directly synthesized pseudo-vertical 1D nanostructure array with CoS and Co0.85Se, respectively, as shown in **Figure 6** [26, 34]. This structure sufficiently acts the 1D TMCs nanostructure advantages, including favorable for fast diffusion of redox species within the CE film, 1D direction electron channel, enhance electrolyte penetration, and more reaction area. Both of them exhibit higher value of *η* than Pt. In other words, they could straightly replace the Pt

*DOI: http://dx.doi.org/10.5772/intechopen.94021*

**Figure 6.**

*The pseudo-vertical 1D nanostructure with (a) and (b) CoS and (c) and (d) Co0.85Se [26, 34].*


*Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts… DOI: http://dx.doi.org/10.5772/intechopen.94021*

**Table 1.**

*Solar Cells - Theory, Materials and Recent Advances*

**2. One-dimensional nanostructure (1D)**

DSSCs performance.

complex structure. In this chapter, we will systematically discuss their plural strategies (including high electrochemical surface area, directional electron transferring pathways, decrease diffusion control, *etc.*) to promote the electrocatalytic ability for

One-dimensional TMCs nanostructure is expected that it provides the 1D electron transfer pathways, promoting electrolyte penetration, and more reaction area [26–34]. However, the vertical 1D structure is rarely obtained because it is difficult to synthesize. Herein, we focus on that the 1D structure has been directly obtained without the template method, in **Figures 5** and **6**. Their corresponding efficiencies are listed in **Table 1**. In **Figure 5**, it shows horizontal 1D TMCs nanostructure SEM images of MoN nanorod, W18O49 nanowire, NiS nanorod, CoSe2 nanorod, Co0.85Se nanotubes, CoSe2/CoSeO3 nanorod, and Ni3S4 nanorod that were synthesized by Song et al., Zhou et al., Yang et al., Sun et al., Yuan et al., Huang et al., and Huang et al., respectively [27–33]. Song et al. reported that MoN nanorod morphology reveals enhancement of diffusion kinetics for the active electrochemical process, as shown in **Figure 5a** [27]. So that the MoN nanorod has higher *V*OC and *J*SC than MoN nanoparticle. Zhou synthesized W18O49 nanowire (**Figure 5b**), having oxygen vacancies within the range of WO2.625 to WO3, *via* the solvothermal method [28]. Their efficiency is 4.85% for Co ion electrolyte. Yang et al. obtained NiS nanorod (**Figure 5c**), which is α type, through chemical bath method [29]. It has *η* of 5.20%,

*The SEM of horizontal 1D nanostructure with (a) MoN, (b) W18O49, (c) NiS, (d) CoSe2, (e) Co0.85Se, (f)* 

*The pseudo-vertical 1D nanostructure with (a) and (b) CoS and (c) and (d) Co0.85Se [26, 34].*

**354**

**Figure 6.**

**Figure 5.**

*CoSe2/CoSeO3, (g) Ni3S4 [27–33].*

*A partial list of literature on the DSSCs with 1D TMCs nanostructure based CEs.*

which is better than the nanoparticle NiS (4.20%). The reason is that the nanorod affords lower charge transfer resistance than the nanoparticle. Sun et al. acquired CoSe2 nanorod (**Figure 5d**), possessing a single orthorhombic crystal structure, by hydrothermal method [30]. The CoSe2 nanorod exits the excellent performance (10.20%), even better than the Pt. They remind that single CoSe2 nanorod has great electrocatalytic ability, lower charge resistance, and higher adsorption capacity for electrolyte. Yuan et al. prepared Co0.85Se nanotubes (**Figure 5e**) by a simple hydrothermal method [31]. It shows *η* of 5.34%, which lower than Pt, obviously. Huang et al. obtained CoSe2/CoSeO3 nanorod (**Figure 5f**) through a microemulsionassisted hydrothermal synthesis [32]. It reveals *η* of 7.54%, which is approach Pt performance. This result contributes to the 1D electron transfer pathways. Huang et al. synthesized the Ni3S4 nanorod (**Figure 5g**) *via* a one-pot colloidal synthesis [33]. And it has *η* of 7.31%, which is quite close Pt. As listed in **Table 1**, there have a few of the 1D TMCs nanostructures existing the better performance than the Pt.

Most of them are vertical 1D TMCs nanostructures. The horizontal 1D TMCs nanostructures could not support the vertical electron transfer pathways and promote the electrolyte penetration. So most of them display lower performance than the Pt.

The vertical 1D TMCs nanostructure is an ideal condition, as shown in **Figure 4**. Kung et al. and Jin et al. directly synthesized pseudo-vertical 1D nanostructure array with CoS and Co0.85Se, respectively, as shown in **Figure 6** [26, 34]. This structure sufficiently acts the 1D TMCs nanostructure advantages, including favorable for fast diffusion of redox species within the CE film, 1D direction electron channel, enhance electrolyte penetration, and more reaction area. Both of them exhibit higher value of *η* than Pt. In other words, they could straightly replace the Pt function for CE in DSSCs.
