A partial list of literature on the DSSCs with carbon material-based CEs.
In recent decades, plenty of nanomaterials have been investigated as electrocatalysts for the replacement of the expensive platinum (Pt) counter electrode in dye-sensitized solar cells (DSSCs). The key function of the electrocatalyst is to reduce tri-iodide ions to iodide ions at the electrolyte/counter electrode interface. The performance of the electrocatalyst is usually determined by two key factors, i.e., the intrinsic heterogeneous rate constant and the effective electrocatalytic surface area of the electrocatalyst. The intrinsic heterogeneous rate constant of the electrocatalyst varies by different types of materials, which can be roughly divided into five groups: non-Pt metals, carbons, conducting polymers, transition metal compounds, and their composites. The effective electrocatalytic surface area is determined by the nanostructure of the electrocatalyst. In this chapter, the nanostructural design and engineering on different types of Pt-free electrocatalysts will be systematically introduced. Also, the relationship between various nanostructures of electrocatalysts and the pertinent physical/electrochemical properties will be discussed.
- counter electrode
- dye-sensitized solar cells
1.1 Dye-sensitized solar cells (DSSCs)
Fossil fuel, as a limiting energy source, may be run out in the upcoming centuries. However, the consumption of energy increases every year [1, 2]. As a result, finding and developing renewable energy sources is an urgent problem. Due to the unlimitedness of renewable energy resources, they are candidates to be reliable replacement for sustainable usage in the future. Among them, the Sun has been considered as one of the most promising renewable energy sources. It provides about 120,000 terawatts to the earth, which equals thousand times of the current energy consumption rate. The solar cells can utilize the sunshine and transform to electricity [3, 4, 5]. Generally, solar cells can be classified to four generations: the first generation is silicon-based solar cells; the second generation is CIGS (CuInGaSe), CZTS (CuZnTiSe), and CdTe solar cells; the third generation is organic photovoltaics (OPVs) and dye-sensitized solar cells (DSSCs); and the fourth generation is perovskite solar cells (PSCs). The first and second generations have been widely explored for decades, and they are the most common solar cells at present. However, they are fabricated through expensive, toxic, energy-intensive, high-temperature, and high-vacuum processes. Therefore, DSSCs are very competitive to the first and second generations of solar cells due to numerous advantages including easy fabrication, low cost (Figure 1) , and high performance at dim-light condition. Moreover, DSSCs can be used in indoor ambient applications [5, 6, 7, 8, 9].
A DSSC is composed of a photoanode, electrolyte, and counter electrode (CE), as shown in Figure 2. When a photoanode is excited by the sun or photon, it will release the electron to the external circuit. At the same time, the iodide/triiodide (I−/I3−) redox couple will relax the photoanode to its ground state. Then the CE will reduce the redox couple to regenerate the DSSC. Among them, the CE plays an important role to determine the DSSC performance . At the CE/electrolyte interface, the electrochemical mechanism goes through I3− decomposition (Eq. (1)) → adsorption (Eq. (2)) → catalytic reduction reaction (Eq. (3)) → desorption (Eq. (4)), and the overall reaction shows as Eq. (5) . Among these reaction steps, Eq. (3) is found to be the slowest step, which means the rate-determining step to decide the DSSC performance.
There are two ways to enhance the electrocatalytic reduction reaction. One is to increase the heterogeneous rate constant, relating to the intrinsic electrocatalytic ability of the electrocatalyst. The other is to engineer the structure of the electrocatalyst for I3− reduction with regard to the charge transfer route and the surface area. To replace a traditional platinum (Pt) electrocatalyst, where Pt is a rare and expensive element, several types of materials, such as carbon materials [5, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28], conductive polymers [29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48], and transition metal compounds [37, 45, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79] have been extensively explored to elevate the cell efficiency (
To date, there have been a very limited number of non-Pt nanomaterials that could have a comparable intrinsic heterogeneous rate constant to that of Pt. However, the specific structural designs of nanomaterials would largely increase the effective electrocatalytic surface area so as to provide better overall electrocatalytic ability than Pt. Moreover, the nanostructured electrocatalysts could have appropriate interfacial affinity, good electrochemical stability, or specific self-assembly natures; these properties may influence the DSSC performance as well. A typical nanostructured material can be defined if any dimension of the material is lower than 100 nm. The nanostructured material can be classified into three groups: zero-dimensional (0D, e.g., nanoparticle, nanocube, etc.), one-dimensional (1D, e.g., nanorod, nanotube, nanoneedle, etc.), and two-dimensional (2D, e.g., nanosheet, nanopental, etc.) structures, as shown in Figure 3. Generally, 0D structure is 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 included.
2. Nanostructure materials of counter electrode
2.1 Carbon materials
Carbon materials, composed of carbon atoms having
|Porous carbon||8.67||9.34||Hollow nanoball|||
|Nitrogen-doped graphene||7.53||7.70||Hollow nanoball|||
|Carbon nanotubes and graphene||8.2||6.4||Nanotube vertically fused onto the nanosheet|||
|Carbon nanotube and N-doped graphene||8.31||7.56||Nanotube intertwined with nanosheet|||
|Carbon nanotube and graphene oxide||6.91||7.26||Nanotube embedded in nanosheet|||
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) . 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) . 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) . The hollow nanoballs consisting of nitrogen-doped graphene and porous carbon gave their DSSCs
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) . 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
2.2 Conductive polymer materials
Since 2000, the conductive polymer material has been discovered by Shirakawa, MacDiarmid, and Heeger . Conductive polymers have attracted much attention as DSSC CEs owing to their excellent conductivity, good adhesion to the substrate, easy fabrication, light-weight, and good accessibility in terms of roll-to-roll processing. Common conductive polymers include poly(3,4-ethylenedioxythiophene) (PEDOT) [30, 35, 36, 40, 41, 42, 43], poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) [30, 43, 48], poly(hydroxymethyl 3,4-ethylenedioxythiophene) (PEDOT-MeOH) , poly(3,4-propylenedioxythiophene) (PProDOT) [85, 86], poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) [85, 86, 87], poly(2,2-dimethyl-3,4-propylenedioxythiophene) (PProDOT-Me2) , polythiophene (PT) , sulfonated poly(thiophene-3-[2-(2-methoxyethoxy) ethoxy]-2,5-diyl) (s-PT) , polyaniline (PANI) , and polypyrrole (PPy) [31, 38, 39, 44, 48], and their molecular structures are shown in Figure 6. However, conductive polymers often showed a flat or a mesoporous structure, meaning their lacks of the directional electron transfer pathways. Because of the synthetic difficulties, very few conductive polymers can form a 0D/1D/2D structure, as listed in Table 2.
The hierarchical nanosphere with PPy (denoted PPy-HNS) has the hierarchical nanospherical structure with an average diameter of 100–200 nm, as shown in Figure 7(a) . The PPy-HNS has the following photovoltaic parameters: a VOC of 0.70 V, a JSC of 16.49 mA cm−2, a FF of 0.58, and an
The flexible PPy membrane is composed of nanotubes that are about 50 nm in diameter, as shown in Figure 7(c) . The paper-like PPy membranes exhibit
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
|NiCo2S4||9.49||8.30||Double-shelled ball-in-ball hollow sphere|||
|NiCo0.2@C||9.30||8.04||Hollow spherical particle|||
|CoS||7.67||7.70||Acicular nanorod array|||
|TiO1.1Se0.9||9.47||7.75||Nanosphere and nanorod|||
α-NiS has a sphere-like morphology with a diameter of 50–80 nm, as shown in Figure 8(a) . 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
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) . It is vertical to the FTO substrate and has a height of about 7 μm. The DSSC with CoS ANRAs-24h CE shows an
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  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 . The NbSe2 sheet CE has an
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) . 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− to I−, 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−2, 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) . 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
3. Summary and future prospects
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 electrode, it must possess high conductivity and good catalytic activity toward 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
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
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 ball-in-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 successfully replace Pt CE.
Moreover, changing DSSC electrolyte toward the Cu, Co, Fe, etc. redox couples is another important research topic. Furthermore, the dim light condition application is another prospect of DSSCs. Among them, the matching material of CE is a key point to promise the
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan. Professor Chuan-Pei Lee especially thanks the financial support of MOST, under grant numbers 107-2113-M-845-001-MY3.