A partial list of literature on the DSSCs with 1D TMCs nanostructure based CEs.
Nowadays, the requirement of energy increases every year, however, the major energy resource is fossil fuel, a limiting source. Dye-sensitized solar cells (DSSCs) are a promising renewable energy source, which could be the major power supply for the future. Recently, the transition metal component has been demonstrated as potential material for counter electrode of platinum (Pt)-free DSSCs owing to their excellent electrocatalytic ability and their abundance on earth. Furthermore, the transition metal components exist different special nanostructures, which provide high surface area and various electron transport routs during electrocatalytic reaction. In this chapter, transition metal components with different nanostructures used for the application of electrocatalyst in DSSCs will be introduced; the performance of electrocatalyst between intrinsic heterogeneous rate constant and effective electrocatalytic surface area are also be clarified. Final, the advantages of the electrocatalyst with different dimensions (i.e., one to three dimension structures) used in DSSCs are also summarized in the conclusion.
- counter electrode
- dye-sensitized solar cells
- and transition metal components
In this century, the energy requisition and environment caring arrive at the highest point in history. The clean and economical renewable energy resource is urgently needed for us. Photovoltaics, named solar cells, tremendous progress has been achieved in efficiency (
Typically, DSSCs are consist of three sections, including photoanode, electrolyte, and counter electrode (CE), that respond to different functions, as shown in Figure 2 [1, 4, 5, 8, 9, 10]. The photoanode converts the photon into the electron by the dye. The electrolyte keeps the function of the photoanode by iodine ion. The CE catalyzes the redox reduction in the electrolyte, which is an obvious influence on the photovoltaic performance, long-term stability, and cost of the device. In other words, the CE is a crucial component of DSSCs.
The CE is classified into three components, that are electrocatalyst, transparent conducting oxide, and substrate, as shown in Figure 2. Among them, the electrocatalyst is the key factor to promise the function of CE [1, 7, 8, 9, 15, 16]. As shown in Figure 3, between electrolyte and CE, the reaction of reduction iodide/triiodide (I−/I3−) redox couple is that: The first stage, diffusion, triiodide diffuses from electrolyte bulk to near the CE for regenerating electrolyte. The second stage, decomposition, triiodide decomposes to iodide and iodine. The iodide is used to renew the dye and iodine will go to the next step. The third stage, adsorption, the CE adsorbs iodine near the CE. The fourth stage, electrocatalysis, electrocatalyst catalyzes reduction reaction, transferring iodine to iodide. The final stage, desorption, the CE desorbs iodide to complete regenerate the electrolyte. According to this mechanism, the electrocatalytic ability, it also represents the reaction rate in here, and the specific structure are the major affections for the reduction reaction.
The traditional electrocatalyst of DSSCs is Platinum (Pt), which has an outstanding electrocatalytic ability [10, 15, 16, 17, 18, 19, 20]. However, Pt, noble metal, is rare on earth that present expensive prices and difficult shapes the specific structure. Up to date, there are a few non-Pt nanomaterials that could have comparable electrocatalytic ability to that of Pt. There have two ways to raise the electrocatalytic reduction reaction. The intrinsic electrocatalytic ability of the electrocatalyst is directly related the electrocatalytic ability. In other words, the choice of material is very important. The other way is to design the nanostructure of the electrocatalyst for I3− reduction regarding with the charge transfer route and the surface area.
Transition metal compounds (TMCs) possess d-electron filling in eg orbitals, which promote excellent electrocatalytic performance in partially filled condition [4, 19, 21, 22, 23, 24]. So, they are interested to replace Pt. But most of TMCs still show poorer electrocatalytic ability than Pt. To overcome the challenge, TMCs are synthesized with various nanostructure, which is an important factor for increasing electrocatalytic ability [20, 21, 22, 25]. A nanostructure is defined if any dimension of the structure is lower than 100 nm, the structure is the nanostructure. Basically, nanostructure divides into four groups: zero-dimensional (0D,
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, 27, 28, 29, 30, 31, 32, 33, 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, 28, 29, 30, 31, 32, 33]. Song et al. reported that MoN nanorod morphology reveals enhancement of diffusion kinetics for the active electrochemical process, as shown in Figure 5a . So that the MoN nanorod has higher
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
3. Two-dimensional (2D)
Geim and Grigorieva classified 2D materials into three groups . First group, graphene type contains graphene, fluorographene, graphene oxide, hBN,
In this section, the partial works of literature are chosen depending on the electrocatalytic performance and structure. Their corresponding SEM images and efficiency parameters are shown in Figures 7 and 8, and Table 2, respectively. In Figure 7, Ibrahem et al., Huang et al., and Mohammadnezhad et al. applied the horizontal 2D nanostructure with NbSe2, MoSe2, and Cu2ZnSnSxSe4-x in CE for DSSCs [41, 42, 43]. Ibrahem et al. reported that the NbSe2 nanosheet (Figure 7a) has the best performance among nanosheet, nanorod, and nanoparticle . They mention that nanosheet could provide high surface area and coverage. And the NbSe2 nanosheet existed
Its performance is 8.92%. They mentioned that vertical nanowall provides conducting charge for electrocatalytic reduction, as shown in Figure 9a. Raj et al. synthesized reflectivity of MoS2 nanosheet (Figure 8b), which has
4. Hierarchical nanostructure
Basically, 0D nanostructure possesses a high reaction area; 1D and 2D nanostructure offers directional electron pathways and enhance electrolyte penetration. But they have their own weakness. For example, 0D nanostructure is easy aggregation and has larger heterogeneous resistance; 1D and 2D nanostructure have lower reaction area. A hierarchical nanostructure consists of the nanostructure with multidimensional subunits (0D, 1D, and 2D). It merges various subunits, so it has multidimensional nanostructure advantages, including high reaction area, benefit electron transfer, avoiding aggregation, enhance electrolyte diffusion, and offer directional electron pathways.
Herein, we list partial literature with hierarchical TMCs nanostructure. Figure 10 shows SEM of Ni3Se4 with sea urchins-like structure, TiO1.1Se0.9 with nanospheres and 1D nanorods, NiCo0.2 with hollow structure and nanoclusters, NiCo2S4 with ball-in-ball structure, NiS@MoS2 with feather duster-like hierarchical structure, CoSe2/CoSeO3 with hierarchical urchin-like structure, CuO/Co3O4 with core-shell structure and CoS2/NC@Co-WS2 with yolk-shell structure by Lee et al., Li et al., Jiang et al., Jiang et al., Su et al., Huang et al., Liao et al., and Huang et al., respectively [49, 50, 51, 52, 53, 54, 55, 56]. And their efficiency parameters are listed in Table 3. Lee et al. synthesized the Ni3Se4 sea urchins-like structure (Figure 10a) through one-step and low temperature hydrothermal process . It reveals
In this section, it can be found that the hierarchical TMCs nanostructure has better performance than the Pt in CE. In other words, they can efficiently raise the TMCs performance, so the hierarchical TMCs nanostructure could replace Pt directly.
The electrocatalytic ability of catalysts is usually determined by below two points: one is the intrinsic electrocatalytic activity, and another is the nanostructure. The nanostructure of TMCs can briefly be classified into 0D, 1D, 2D, and hierarchical nanostructures; those have different properties and could obviously affect the electrocatalytic ability. Herein, the partial reports about DSSCs with the electrocatalysts having 1D, 2D, or hierarchical nanostructures are selected for introduction and discussion. 1D nanostructure possesses several advantages, including the 1D electron transfer pathways, promoting electrolyte penetration, avoiding stack problem, and high reaction area. However, not all the electrocatalysts with 1D nanostructure show better performance than the Pt in DSSC application. Some of them lied down on substrate; so, the advantage on vertical electron transport rout is not given. Furthermore, as the stacking problem comes out, it will lose surface are for reaction. 2D nanostructures possess the active site on edges or defects, and their 2D structure could provide the benefits below, such as directional electron and diffusion channels; these properties boost their DSSC performances obviously. However, the stacking problem and poor activity on basal plane of 2D materials also retarding their practical performance in DSSCs. Hierarchical nanostructure incorporates the profits of subunits, so it displays high reaction area, benefit electron transport rout, avoiding aggregation, enhanced electrolyte diffusion,
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under grant numbers 107-2113-M-845-001-MY3.