**2.3 Transition metal composites**

Transition metal composites (TMC) possess high potential to replace Pt CE in DSSCs because of the similar electronic structures between TMCs and Pt. Metal compounds, including carbides, nitrides, chalcogenides, oxides, phosphides, and so on, have been applied as an electrocatalyst in DSSCs to replace expensive Pt. It is still a challenge to replace Pt with TMCs due to the relatively low conductivity of TMCs. Accordingly, various TMC structures, including nanoparticle, hollow sphere, nanorod array, nanowall, hierarchical nanorod, etc., are investigated to improve the performance of TMC-based CEs, as shown in **Figure 8**. The corresponding *η*s of DSSCs with various structures are listed in **Table 3**.

α-NiS has a sphere-like morphology with a diameter of 50–80 nm, as shown in **Figure 8(a)** [78]. The other NiS (β-NiS) has a nanorod 2–5 μm in length and 1000 nm in diameter. The DSSC of α-NiS CE has a better *η* (5.20%) than the β-NiS (*η* of 4.20%). The particle size of α-NiS is much smaller than nanorods of β-NiS. The smaller the size of a particle, the larger specific surface area it possesses. With the increase of specific surface area of α-NiS, the conversion efficiency reaches a higher value. The nanoparticle of CoSe2/CoSeO3 (CoSe2/CoSeO3-NP) has a diameter of 50–60 nm, as shown in **Figure 8(b)** [74]. And CoSe2/CoSeO3-NP has a larger reaction area than the nanorod and nanocube of CoSe2/CoSeO3, confirmed by the electrochemical double-layer capacitance, which is positively related to the

#### **Figure 8.**

*The structures of transition metal materials including (a and b) nanoparticle [78], (c) double-shelled ball-in-ball hollow sphere [70, 74], (d) hollow spherical particle [71], (e) acicular nanorod array [49], (f–h) nanorod [53, 54, 75], (i and j) nanosheet [55, 66], (k and l) nanowall [64, 72], and (m) hierarchical nanosphere with nanorod [77].*

**87**

0.740 V, a JSC of 15.26 mA cm<sup>−</sup><sup>2</sup>

the Pt CE.

**Table 3.**

*DSSCs is N719.*

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

**Materials** *η* **(%)** *η* **of Pt (%) Structure Ref** NiS 5.20 6.30 Nanoparticle [78] CoSe2/CoSeO3 9.27 7.91 Nanoparticle [74]

NiCo0.2@C 9.30 8.04 Hollow spherical particle [71] CoS 7.67 7.70 Acicular nanorod array [49] MoN 7.29 7.42 Nanorod [54] CoSe2 10.20 8.17 Nanorod [53] Ni3S4-Pt2Fe1 8.79 7.83 Nanorod [75] NbSe2 7.73 7.01 Nanosheet [55] WSe2 7.48 7.91 Nanosheet [66] CoSe2 8.92 8.25 Nanoclimbing wall [64] CuxZnySnzS 7.44 7.21 Nanowall [72] TiO1.1Se0.9 9.47 7.75 Nanosphere and nanorod [77]

sphere

[70]

NiCo2S4 9.49 8.30 Double-shelled ball-in-ball hollow

reaction area. The DSSC of CoSe2/CoSeO3-NP CE has an *η* of 9.27%, which is better than those of the nanorod and nanocube of CoSe2/CoSeO3 and is higher than that of the Pt CE (7.91%). The double-shelled hollow sphere (BHSs) structure exists in NiCo2S4 BHSs with the separation of hollow and solid parts. In **Figure 8(c)** [70], the diameter of the inner shell is 300 nm and that of the outer shell is approximately 550 nm. The thickness of the outer thin shell is 10–30 nm, which is quite less than that of the inner shell. The DSSC of NiCo2S4 BHSs CE exhibits an *η* of 9.49%, which is higher than that of the Pt CE (*η* of 8.30%). From a broken NiCo0.2@C microsphere shown in **Figure 8(d)** [71], the well-defined hollow structure with a shell thickness of around 200 nm can be observed. Meaningfully, the hollow spherical space can greatly shorten the diffusion paths within the electrode and serves as a robust reservoir for ions. The NiCo0.2@C exhibits an *η* of 9.30%, which is higher than that of the Pt CE (*η* of 8.04%). Most of the nanoparticles, double-shelled ballin-ball hollow sphere, and hollow spherical particle structures have better *η*s than

*A partial list of literature studies on the DSSCs with conductive polymer material-based CEs. The dye of* 

Although TMCs present good electrocatalytic ability, the electrons may be insufficient at active sites. The rod structure is claimed to provide the specific electron transfer. It can supply sufficient electrons to keep consistent electrocatalytic reaction. From **Figure 8(e)**, it can be observed that CoS has 1D acicular nanorod arrays with the relatively rough surface of the nanorods (noted CoS ANRAs-24h) [49]. It is vertical to the FTO substrate and has a height of about 7 μm. The DSSC with CoS ANRAs-24h CE shows an *η* of 7.67%, which is virtually the same as the sputtered Pt-CE (*η* of 7.70%). The MoN nanorod (NR) on the Ti substrate has a one-dimensional structure with a diameter of 40–100 nm and a length of 0.5–2 mm, as shown in **Figure 8(f )** [54]. The electrode structure is expected to trigger positive effects on the electrochemical processes occurring in the electrode films. The MoN NR-Ti CE shows comparable performance to that using a Pt-FTO glass electrode with a VOC of

CoSe2 has nanorods 50–800 nm in length and 20–150 nm in width, as shown in **Figure 8(g)**, and possesses a lattice spacing of 3.71 ± 0.01 Å, corresponding to the

, a FF of 0.65, and an *η* of 7.29%. The single-crystal

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


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

#### **Table 3.**

*Nanostructures*

**2.3 Transition metal composites**

Transition metal composites (TMC) possess high potential to replace Pt CE in DSSCs because of the similar electronic structures between TMCs and Pt. Metal compounds, including carbides, nitrides, chalcogenides, oxides, phosphides, and so on, have been applied as an electrocatalyst in DSSCs to replace expensive Pt. It is still a challenge to replace Pt with TMCs due to the relatively low conductivity of TMCs. Accordingly, various TMC structures, including nanoparticle, hollow sphere, nanorod array, nanowall, hierarchical nanorod, etc., are investigated to improve the performance of TMC-based CEs, as shown in **Figure 8**. The corre-

α-NiS has a sphere-like morphology with a diameter of 50–80 nm, as shown in **Figure 8(a)** [78]. The other NiS (β-NiS) has a nanorod 2–5 μm in length and 1000 nm in diameter. The DSSC of α-NiS CE has a better *η* (5.20%) than the β-NiS (*η* of 4.20%). The particle size of α-NiS is much smaller than nanorods of β-NiS. The smaller the size of a particle, the larger specific surface area it possesses. With the increase of specific surface area of α-NiS, the conversion efficiency reaches a higher value. The nanoparticle of CoSe2/CoSeO3 (CoSe2/CoSeO3-NP) has a diameter of 50–60 nm, as shown in **Figure 8(b)** [74]. And CoSe2/CoSeO3-NP has a larger reaction area than the nanorod and nanocube of CoSe2/CoSeO3, confirmed by the electrochemical double-layer capacitance, which is positively related to the

*The structures of transition metal materials including (a and b) nanoparticle [78], (c) double-shelled ball-in-ball hollow sphere [70, 74], (d) hollow spherical particle [71], (e) acicular nanorod array [49], (f–h) nanorod [53, 54, 75], (i and j) nanosheet [55, 66], (k and l) nanowall [64, 72], and (m) hierarchical* 

sponding *η*s of DSSCs with various structures are listed in **Table 3**.

**86**

**Figure 8.**

*nanosphere with nanorod [77].*

*A partial list of literature studies on the DSSCs with conductive polymer material-based CEs. The dye of DSSCs is N719.*

reaction area. The DSSC of CoSe2/CoSeO3-NP CE has an *η* of 9.27%, which is better than those of the nanorod and nanocube of CoSe2/CoSeO3 and is higher than that of the Pt CE (7.91%). The double-shelled hollow sphere (BHSs) structure exists in NiCo2S4 BHSs with the separation of hollow and solid parts. In **Figure 8(c)** [70], the diameter of the inner shell is 300 nm and that of the outer shell is approximately 550 nm. The thickness of the outer thin shell is 10–30 nm, which is quite less than that of the inner shell. The DSSC of NiCo2S4 BHSs CE exhibits an *η* of 9.49%, which is higher than that of the Pt CE (*η* of 8.30%). From a broken NiCo0.2@C microsphere shown in **Figure 8(d)** [71], the well-defined hollow structure with a shell thickness of around 200 nm can be observed. Meaningfully, the hollow spherical space can greatly shorten the diffusion paths within the electrode and serves as a robust reservoir for ions. The NiCo0.2@C exhibits an *η* of 9.30%, which is higher than that of the Pt CE (*η* of 8.04%). Most of the nanoparticles, double-shelled ballin-ball hollow sphere, and hollow spherical particle structures have better *η*s than the Pt CE.

Although TMCs present good electrocatalytic ability, the electrons may be insufficient at active sites. The rod structure is claimed to provide the specific electron transfer. It can supply sufficient electrons to keep consistent electrocatalytic reaction. From **Figure 8(e)**, it can be observed that CoS has 1D acicular nanorod arrays with the relatively rough surface of the nanorods (noted CoS ANRAs-24h) [49]. It is vertical to the FTO substrate and has a height of about 7 μm. The DSSC with CoS ANRAs-24h CE shows an *η* of 7.67%, which is virtually the same as the sputtered Pt-CE (*η* of 7.70%). The MoN nanorod (NR) on the Ti substrate has a one-dimensional structure with a diameter of 40–100 nm and a length of 0.5–2 mm, as shown in **Figure 8(f )** [54]. The electrode structure is expected to trigger positive effects on the electrochemical processes occurring in the electrode films. The MoN NR-Ti CE shows comparable performance to that using a Pt-FTO glass electrode with a VOC of 0.740 V, a JSC of 15.26 mA cm<sup>−</sup><sup>2</sup> , a FF of 0.65, and an *η* of 7.29%. The single-crystal CoSe2 has nanorods 50–800 nm in length and 20–150 nm in width, as shown in **Figure 8(g)**, and possesses a lattice spacing of 3.71 ± 0.01 Å, corresponding to the

(110) planes of orthorhombic CoSe2 [53]. Impressively, the single-crystal CoSe2 CE produces an *η* of 10.20% with a VOC of 0.753 V, a JSC of 18.55 mA/cm<sup>−</sup><sup>2</sup> , and a FF of 0.73, which is better than the Pt CE (8.17%). The Ni3S4-PtFe heteronanorods are highly monodispersed with an average length of ∼34.0 nm and an average diameter of 9.0 nm, as shown in **Figure 8(h)** [75]. The DSSCs using Ni3S4-PtFe produce an *η* of 8.79%, which is higher than that of the Pt CE (7.83%). The 1-D structure is obviously promoting the electrocatalytic ability of TMCs and most of the 1-D structures for TMCs exhibit a better *η* than Pt CE. It can be claimed that the 1-D structures of TMCs could replace the Pt CE.

The 2D structure of TMCs also has a specific electron pathway and it could be vertical to the substrate to offer sufficient electrons on active sites. Moreover, the hierarchical structure has both the advantages of a large reaction area and vertical electron pathway. For example, the direction of the fractured NbSe2 sheet shows a structure with the [001] crystallographic orientation and revealed a very thick (>100 mm), disordered network arrangement of 2D sheets, as shown in **Figure 8(i)**; in comparison, the ground materials were very thin, separated nanosheets [55]. The NbSe2 sheet CE has an *η* of 7.73%, which reveals the potential to replace Pt CE (7.01%). The WSe2 is composed of several interlaced nanosheets with an average thickness of approximately 15 nm and a width between 60 and 100 nm, as shown in **Figure 8(j)** [66]. The WSe2 CE shows good electrical conductivity, subsequent energy band calculation results, and large reaction area that exhibits an *η* of 7.48%.

Vertically-aligned structures of electrocatalysts were reported to facilitate faster charge transport from the substrate through the electrocatalysts to the electrolyte [64, 72, 77], as shown in **Figure 9**. This structure is expected to have better electrocatalytic ability. The nanowall and the hierarchical nanorod are used with TMCs. The CoSe2 nanoclimbing wall (CoSe2/C-NCW) reveals arrays of vertically-aligned nanowalls with sharp edges, as shown in **Figure 8(k)** [64]. In addition, the nanowalls are covered with dot-matrix-like projections; these projections are expected to provide a large surface area to the film. On account of direct electron transfer and large surface area, the CoSe2/C-NCW film, on the whole, could be a better electrocatalyst for the reduction of I3 <sup>−</sup> to I<sup>−</sup>, as shown in **Figure 9(a)**. The cell with CoSe2/C-NCW CE reaches the highest efficiency of 8.92%, with a VOC of 0.73 V, a JSC of 18.03 mA cm<sup>−</sup><sup>2</sup> , and an FF of 0.67; this efficiency is even higher than that of the cell with Pt (8.25%). The CZTS nanowall electrodes (NWD) on Mo substrate show nanowalls with a width of ~500 nm, a thickness of nearly 15 nm, and a height of ~1.5 μm, which were adequately aligned in a densely packed array, which was nearly perpendicular to the surface of the Mo substrate, as shown in **Figure 8(l)** [72]. In this case, CZTS-NWD demonstrates a concept of "nano-geogrid"-reinforced CZTS nanowall electrode by synthesizing a thin layer of a porous CZTS nanostructure mimicking a geogrid on a substrate and then fabricating a CZTS nanowall on top of the nanostructure, as shown in **Figure 9(b)**. The *η* of the NWD device is 7.44%, which is comparable to the Pt device (7.21%). **Figure 8(m)** shows the film of

#### **Figure 9.**

*The scheme of vertical aligned structures of (a and b) nanowall and (c) nanosphere with nanorod [64, 77].*

**89**

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

They exhibit great *ηs*, which are better than that of the Pt CE.

TiO1.1Se0.9 wrapping around a carbon fiber, and many nanospheres and nanorods of TiO1.1Se0.9 are grown on the TiO1.1Se0.9 under-layer [77]. The TiO1.1Se0.9 nanorods are perpendicular to the surface of the carbon fiber; TiO1.1Se0.9 nanorods, therefore, are expected to facilitate a fast and 1D electron transport from the CC substrate to the

show the hierarchical electron transfer route electrode, where the carbon fiber in the CC serves for transporting the main stream of electrons through its 1D direction, and the deposited electrocatalysts provide secondary channels of electrons for executing

shows the best performance and gives the best *η* of 9.47%. Furthermore, the DSSC is used for further comparison with those cells with the Pt/CC (7.75%). In conclusion, the TMCs have good electrocatalytic ability and possess vertically aligned structures.

The counter electrode is a paramount part of DSSCS and has a significant influence on both the photovoltaic performance and the device cost of DSSCs. As a counter

electrolyte regeneration, as well as good stability. The DSSC devices employing CEs of different materials including carbon materials, conductive polymers, and transition metal composites have been summarized and discussed. One key point is that the CE performance can be optimized by combining special nanostructures into CE films to promote the industrialization of Pt-free CE catalysts. The nanostructure can briefly be classified into 0D, 1D, and 2D, which have different properties. The different materials with various nanostructures can overcome the problem of the material. The carbon materials have numerous advantages including low cost, plasticity, simple fabrication procedures, high electrical conductivity, high thermal stability, and good corrosion resistance. The *η* of carbon materials has been improved by the hierarchal structures (nanotube with nanosheet and nanotube with nanoribbon) and most of the carbon materials with hierarchal structure CE have a better value of *η* than the traditional Pt CE. However, most of the performances of the DSSCs with carbon material CEs are still slightly lower than those DSSCs with Pt CEs. This mostly results from various resistances associated with the structurally complex carbon electrodes, such as bulk resistance through the comparatively thick carbon CE, contact resistance to the TCO substrate, the diffusion resistance in the pores of the CE, etc. The conductive polymer materials possess outstanding electron conductivity, good adhesion, and easy fabrication. According to the literature above, it can be concluded that the 1D structure conductive polymer material-based CE can provide better *η* than the particles, nanosphere, and nanosheet structures. Although conductive polymer materials have larger reaction area and specific electron pathway, most of the conductive polymer material-based CEs still have a lower *η* than the Pt-based CEs. Only a few examples show better performance than Pt CE. It means that the conductive polymer materials need a hybrid with other electrocatalysts to

The TMCs exhibit great electrocatalytic ability, easy preparation, and modification. However, the poor conductivity needs to be solved in order to replace Pt CE. By synthesizing nanostructures, including nanoparticle, double-shelled ballin-ball hollow sphere, hollow spherical particle, acicular nanorod array, nanorod, nanosheet, nanoclimbing wall, hierarchical nanorod, etc., TMCs reveal better performances than the Pt CE. It can be said that the TMCs with nanostructure suc-

electrode, it must possess high conductivity and good catalytic activity toward

<sup>−</sup>, as shown in **Figure 9(c)**. The DSSC with the TiO1.1Se0.9–3/CC

<sup>−</sup> reduction occurs The correctional images

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

interface with the electrolyte, where I3

**3. Summary and future prospects**

obtain better electrocatalytic ability.

cessfully replace Pt CE.

the reduction of I3

TiO1.1Se0.9 wrapping around a carbon fiber, and many nanospheres and nanorods of TiO1.1Se0.9 are grown on the TiO1.1Se0.9 under-layer [77]. The TiO1.1Se0.9 nanorods are perpendicular to the surface of the carbon fiber; TiO1.1Se0.9 nanorods, therefore, are expected to facilitate a fast and 1D electron transport from the CC substrate to the interface with the electrolyte, where I3 <sup>−</sup> reduction occurs The correctional images show the hierarchical electron transfer route electrode, where the carbon fiber in the CC serves for transporting the main stream of electrons through its 1D direction, and the deposited electrocatalysts provide secondary channels of electrons for executing the reduction of I3 <sup>−</sup>, as shown in **Figure 9(c)**. The DSSC with the TiO1.1Se0.9–3/CC shows the best performance and gives the best *η* of 9.47%. Furthermore, the DSSC is used for further comparison with those cells with the Pt/CC (7.75%). In conclusion, the TMCs have good electrocatalytic ability and possess vertically aligned structures. They exhibit great *ηs*, which are better than that of the Pt CE.
