Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts for Dye-Sensitized Solar Cells

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.


Introduction
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 (η), reproducibility, and stability [1][2][3].It has been considered as one of the most promising renewable energy sources.Photovoltaics are classified to three generations, as shown in Figure 1 [1,[4][5][6][7].The first-generation solar cells, named silicon-based solar cells or the traditional solar cells, made up of crystalline silicon.These solar cells demonstrate high efficiency and significant demand in the market, but the production cost of crystalline silicon materials limited the large-scale industrial applications.The second-generation is cadmium telluride (CdTe)/cadmium indium gallium diselenide (CIGS) based solar cells.The solar cells could be produced with large-scale and well efficiency (14-22%).The first and second generations are the most widely solar cells at present.However, they are scarcity, the toxicity of materials, high-temperature, and high-vacuum processes that restrict further applications.Dye-sensitized solar cells (DSSCs), classed thirdgeneration solar cells, have gained attention and be regarded as prospective solar cells for the photovoltaic technologies in recent years as potential cost-effective alternatives to the first and second generations solar cells [8][9][10][11].Furthermore, the DSSCs have outstanding performance in an indoor, dim light environment [12][13][14].
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, 7-9, 15, 16].As shown in Figure 3, between electrolyte and CE, the reaction of reduction iodide/ triiodide (I − /I 3 − ) 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 I 3 − reduction regarding with the charge transfer route and the surface area.
Transition metal compounds (TMCs) possess d-electron filling in e g 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, e.g.nanoparticle, nanocube, etc.), one-dimensional (1D, e.g.nanorod, nanotube, nanoneedle, etc.), two-dimensional (2D, e.g.nanosheet, nanopental etc.) and hierarchical nanostructures, as shown in Figure 4.In view of 1D, 2D, and hierarchical nanostructures have  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 DSSCs performance.

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  [27].So that the MoN nanorod has higher V OC and J SC than MoN nanoparticle.Zhou synthesized W 18 O 49 nanowire (Figure 5b), having oxygen vacancies within the range of WO 2.625 to WO 3 , 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%,    5d), possessing a single orthorhombic crystal structure, by hydrothermal method [30].The CoSe 2 nanorod exits the excellent performance (10.20%), even better than the Pt.They remind that single CoSe 2 nanorod has great electrocatalytic ability, lower charge resistance, and higher adsorption capacity for electrolyte.Yuan et al. prepared Co 0.85 Se nanotubes (Figure 5e) by a simple hydrothermal method [31].It shows η of 5.34%, which lower than Pt, obviously.Huang et al. obtained CoSe 2 /CoSeO 3 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 Ni 3 S 4 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  [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.
Besides that, the 2D TMCs nanostructure has advantages including enhancing the diffusion of electrolyte, vertical electron channel, and special optical property.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    8c).The CoNi 2 S 4 exhibits η of 8.86%, which attributes to a larger active surface area with favorable charge transport.The pseudo-vertical 2D nanostructure has obviously improvement of electrocatalytic ability than the normal 2D nanostructure.It is not only providing vertical transport pathways and active surface area but also contributes to reflection photon.Those properties make 2D TMCs nanostructure have the potential to alternative Pt as an electrocatalyst.

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 Table 2.
A partial list of literature on the DSSCs with 2D TMCs nanostructure based CEs.
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 Ni 3 Se 4 with sea urchins-like structure, TiO 1.1 Se 0.9 with nanospheres and 1D nanorods, NiCo 0.2 with hollow structure and nanoclusters, NiCo  [49][50][51][52][53][54][55][56].And their efficiency parameters are listed in Table 3. Lee et al. synthesized the Ni 3 Se 4 sea urchins-like structure (Figure 10a) through one-step and low temperature hydrothermal process [49].It reveals η of 8.31%, which attribute to the high active electrocatalytic surface area.Li et al. obtained TiO 1.1 Se 0.9 with nanospheres and 1D nanorods (Figure 10b) via a simple dip-coating process and rapid thermal annealing (RTA) process [50].The TiO 1.1 Se 0.9 exhibits η of 9.47%, which is better than the Pt.The result is established that the nanospheres can work as electro-catalytic active sites, and the nanorods can function not only as electro-catalytic active sites but also as fast electron transport channels, as shown in Figure 11a.Jiang et al. synthesized NiCo 0.2 hollow structure and nanoclusters, having uniform spherical particles with an average diameter of about 2 μm and shell thickness of around 200 nm (Figure 10c), via a thermal method [51].It shows η of 9.30% and displays that the novel spherical structures can efficiently promote the transfer of electrons from the conductive carbon frameworks to metal nanoparticles, thus resulting in high electrocatalytic activity for the reduction.Jiang et al. acquired NiCo 2 S 4 ball-in-ball structure (Figure 10d) by a thermal method [52].Its efficiency is 9.49%, which is attributed to the rougher surface, higher surface area, and high diffusion coefficient for redox.Su et al. obtained NiS@MoS 2 feather duster-like hierarchical structure, which has η of 8.58% [53].They propose that feather duster-like hierarchical structure array can support the fast electron transfer and electrolyte diffusion channels, moreover, it also can render abundant active catalytic sites and large electron injection efficiency from CE to the electrolyte.Huang et al. gained CoSe 2 /CoSeO 3 hierarchical urchin-like structure (Figure 10g), the nanoparticle-composed sphere is the central core with a diameter of about 50 nm surrounded by several hexagonal prisms, through a one-step hydrothermal method [54].The CoSe 2 /CoSeO 3 reveals η of 9.29% and is mention that the  10f) via a facile self-templated method [55].The CuO/Co 3 O 4 has η of 8.34% and an excellent electronic transmission channel and more adsorption sites for the redox couple, which greatly enhances the subsequent redox process.Huang et al. acquired CoS 2 /NC@Co-WS 2 with yolk-shell structure (Figure 10g) [56].By virtue of larger surface area and more effective active sites, the CoS 2 /NC@Co-WS 2 (η of 9.21%) has better performance than the Pt.
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.

Conclusion
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    [50,54].

Figure 1 .
Figure 1.The scheme of three generation photovoltaic solar cells.

Figure 2 .
Figure 2. The scheme of dye-sensitized solar cells and counter electrode (cathode).

Figure 4 .
Kung et al. and Jin et al. directly synthesized pseudo-vertical 1D nanostructure array with CoS and Co 0.85 Se, respectively, as shown in Figure 6

Figure 7 ,
Ibrahem et al., Huang et al., and Mohammadnezhad et al. applied the horizontal 2D nanostructure with NbSe 2 , MoSe 2 , and Cu 2 ZnSnS x Se 4-x in CE for DSSCs [41-43].Ibrahem et al. reported that the NbSe 2 nanosheet (Figure 7a) has the best performance among nanosheet, nanorod, and nanoparticle [41].They mention that nanosheet could provide high surface area and coverage.And the NbSe 2 nanosheet existed η of 7.73%, which is better than the Pt CE.The result indicates that NbSe 2 nanosheet substitutes to the noble metal Pt in DSSCs.Following the idea, MoSe 2 and Cu 2 ZnSnS x Se 4-x nanosheet show the η of 7.58% and 5.73%, respectively.However, both of their values of η are lower than the Pt.To increase the performance of 2D TMCs nanostructure, the pseudo-vertical 2D nanostructure was synthesized and provide the vertical electron channel.The pseudo-vertical 2D nanostructure with MoS 2 , CoSe 2 , MoS 2 , Cu x Zn y Sn z S, and CoNi 2 S 4 were obtained by Antonelou et al., Chiu et al., Raj et al., Chiu et al., and Patil et al., respectively [44-48].Antonelou et al. obtained the MoS2 nanosheet with η of 8.40%, which has thicknesses down to the 1-2 nm scale.Chiu et al. acquired the nanoclimbing-wall-like CoSe 2 (Figure 8a) through an electrodeposition process, by using bathes with different pH values.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 MoS 2 nanosheet (Figure 8b), which has η of 7.50%, through chemical vapor deposition (CVD).The reflectivity of MoS 2 nanosheet is

Table 3 .
A partial list of literature on the DSSCs with hierarchical TMCs nanostructure based CEs.