10.1.1 Features and Objectives of Hydrogen Energy

Hydrogen is one of the most common elements in nature, and there is no depletion of hydrogen resources. Hydrogen has high calorific value; with its combustion product of water, it is emission free in a real sense and is also free from pollution. There are many ways to utilize hydrogen, for example to generate heat via burning or to generate power electrochemically. Hydrogen in storage and in transport can be gas, liquid, solid, or in compound form.

The key technology to achieve hydrogen economy is the development of an inexpensive and highly efficient hydrogen production technology that enables safe and efficient hydrogen storage technologies. Therefore, the priority is to develop new, highly efficient, and safe hydrogen storage materials and hydrogen storage technologies.

Vehicle hydrogen storage systems are designed in accordance with two standards issued, respectively, by the IEA (International Energy Agency) and DOE (US Department of Energy). Here, the IEA standard requires the mass hydrogen storage capacity to be more than 5% and requires the volume capacity to be more than 50 kg (H₂)/m³, whereas the DOE requires mass hydrogen storage capacity of more than 6.5% and volume capacity larger than 62 kg (H₂)/m³.

10.1.2 Comparison of Different Hydrogen Storage Methods

The characteristics of various hydrogen storage methods are described in Table 10.1. Obviously, solid-state hydrogen storage has many advantages.

10.1.3 Technology Status of Hydrogen Storage Materials

There are three kinds of hydrogen storage materials: metal hydrides, coordination hydrides, and nanomaterials.

Metal hydrides have the following hydrogen storage characteristics: reversible reactions, hydrogen atom storage structure, solid-state hydrogen storage, safety, reliability, and relatively high-volume density of hydrogen storage. At present, successful developments of metal hydrides as hydrogen storage materials are the RE La–Ni system, titanium–iron system, magnesium system, and titanium/zirconium system.

Hydrogen storage alloy in the RE La–Ni system has a representative, LaNi₅, which was first developed by the Philips Laboratories in the Netherlands. It is characterized by its easy activation, moderate and smooth equilibrium pressure, low equilibrium pressure in hydrogen absorption and desorption, good performance of anti-gas poisoning impurities, and is suitable for room temperature operation. By partial replacement of elements, MmNi_3.55Co_0.75Mn_0.47Al_0.3 (mixed rare earth, with major components of La, Ce, Pr, Nd) is widely used in nickel–hydrogen batteries.

The titanium–iron system has a representative, TiFe, which was first invented by the US-based Brookhaven National Laboratory. It features a low price, reversible hydrogen storage at room temperature, susceptibility to oxidation, difficult activation, and weak ability in anti-impurity gas poisoning; surface modification is required in the actual use of the alloy.

TiFe alloy is characterized by two hydride phases, namely β-phase (TiFeH_1.04) and γ-phase (TiFeH_1.95). The hydrogen absorption reactions of the TiFe alloy are:

$2.13{\mathrm{TiFeH}}_{0.10}+1/2{\mathrm{H}}_2\to 2.13{\mathrm{TiFeH}}_{1.04}$

$2.20{\mathrm{TiFeH}}_{1.04}+1/2{\mathrm{H}}_2\to 2.20{\mathrm{TiFeH}}_{1.95}$

The magnesium system has a representative, Mg₂Ni, which was first reported by the US Brookhaven National Laboratory. It is characterized by high hydrogen storage capacity, abundant resources, low cost, and high temperature of hydrogen release (250–300°C), but its hydrogen release dynamic performance is weak. It can go through mechanical alloying or can be added with TiFe and CaCu₅ for ball milling.

Titanium and zirconium materials are intermetallic compounds with the Laves phase structure. The atomic gap in these materials comes from the tetrahedral structure, with more spaces conducive to the adsorption of hydrogen atoms. Typical examples are TiMn_1.5H_2.5 and Ti_0.90Zr_0.1Mn_1.4V_0.2Cr_0.4. Characterized by good material activity, they are mainly used for hydrogen storage of hydrogen vehicles and the negative poles of batteries (Ovinic).

Coordination hydride hydrogen storage material is formed mainly by alkali metals (Li, Na, K) or alkaline earth metals (Mg, Ca) and group III elements (B, Al). It has high hydrogen storage capacity, and it is difficult to restore it to its hydride form (catalytic LiAlH₄ in TiCl₃ and TiCl₄ at 180°C, are able to obtain 5% of the reversible hydrogen storage capacity under a hydrogen pressure of 8 MPa).

The main properties of metal hydride ligands are indicated in Table 10.2.

CNTs were discovered in 1991 by Professor Iijima of Japan’s NEC Corporation, but research on the use of CNTs in hydrogen storage was made by the American scholar Dillon, who set a precedent in 1997 [2]. On coke containing 0.1–0.2 wt% single-wall carbon nanotubes (SWNTs), he used the temperature-programmed desorption method to measure the hydrogen adsorption capacity and then found that pure SWNTs have a hydrogen storage capacity of 5–10 wt% at room temperature and a hydrogen pressure of 300 Torr. He also pointed out that hydrogen at the adsorption sites with high temperature is physically adsorbed and the CNTs can have a hydrogen storage capacity 10 times that of activated carbon. The sample size was only 1 mg, and thus the results were obtained through extrapolation; therefore, these results are perhaps unconvincing. Nonetheless, the results are encouraging. As such, there has been much in-depth and extensive research conducted regarding CNTs with regard to their hydrogen storage properties [3].

CNT hydrogen storage can occur in a variety of ways. One is through adsorption of hydrogen storage of CNTs. Table 10.3 shows a comparison of the hydrogen storage capacity of a variety of CNTs in hydrogen absorption.

Another mode of hydrogen storage by CNTs is electrochemical hydrogen storage. Research in this area was first reported in 1999 by Nützenadel and associates. Multi-wall carbon nanotubes (MWNTs) after purification had a maximum discharge capacity of 1,157 mAh/g, which is equivalent to 4.1% of hydrogen storage capacity by weight. After 100 cycles of charge–discharge, 70% of its maximum capacity remained. The SWNTs had maximum discharge capacity of 503 mAh/g, which is equivalent to 1.84% of hydrogen storage capacity by weight. After 100 cycles of charge–discharge, 80% of its maximum capacity remained (Figures 10.1 and 10.2).

Hydrogen is the cleanest, most renewable energy and has gained great importance in developed countries over the past decade. China, in recent years, has also invested heavily in research and development in this field. Pilot operations of hydrogen-driven vehicles have been performed in some developed countries, and China is in the process of introducing hydrogen-driven transportation. One of the barriers to the commercialization of hydrogen vehicles is the high cost of hydrogen storage. Taking into account safety and cost, both liquid hydrogen and high-pressure hydrogen gas are not the best choices for the commercialization of hydrogen vehicles. Most hydrogen storage alloy products have a large self-weight, and their life expectancy is also a problem, whereas magnesium-based alloys with a lower self-weight are difficult to store/release hydrogen at room temperature. In addition, reversible hydrogen storage of hydride ligands still needs further development and research. Carbon absorption materials for hydrogen storage have been taken seriously, but their basic research is still not sufficient. Whether it can be put into practical use is still a question. At present, hydrogen storage with nanomaterials has the following problems. There remains a sharp difference in the measured hydrogen storage capacities around the world (from 0.01 up to 67 wt%) without accurate methods for determination. Also, the hydrogen storage mechanism is still unknown. Therefore, the promise of hydrogen energy storage has a long way to go.

10.2.1 Basic Concept

Fuel cells are one of the most important energy technologies in the twenty-first century and are a very promising alternative energy source. At present, the development of fuel cells used for vehicles and fuel cells for portable electronic devices has been classified as one of the top priorities in most developed countries. Fuel cells are a device in which an energy source can be directly converted; chemical energy in the fuel can combine with hydrogen and oxygen into water to produce electricity by using an electrochemical method rather than the burning mode. It is like installing a miniaturized generator in electronic products such as mobile phones, which require an endless fuel supply. In this sense, fuel cells differ from conventional secondary batteries. Therefore, fuel cells are actually power generators that are safe, clean, and characterized by long duration, long lifetime, portability, and cost-effectiveness. In addition, fuel cells can also be used in lighting devices, television sets, refrigerators and motors, and can also be widely used in military supplies. At present, the fuel cell is still in the development stage in the fields of materials and application. Any breakthrough in some key technologies will certainly result in a huge impact on the industry.

The basic principle of fuel cell power generation is based on a chemical reaction of hydrogen and oxygen that results in water, heat, and chemical energy, and the chemical energy can be directly converted into electricity. Specifically, the fuel cell is a “generator” using the reverse reaction of water electrolysis. It is composed of anode, cathode, and an electrolyte panel situated in the middle of the positive and negative electrodes. Initially, the electrolyte plate is formed by the use of electrolyte infiltration of the porous plate and is now being developed into a type that uses solids directly.

Under operating conditions, the negative terminal is supplied with fuel (hydrogen), while the positive terminal is supplied with oxidant (air). Hydrogen is broken down into H⁺ ions and e⁻ electrons at the negative terminal. Hydrogen ions will enter the electrolyte, while electrons move along the external circuit to the positive terminal. Electricity load is connected to the external circuit. In the cathode, the oxygen in the air and the hydrogen ions in the electrolyte will result in the absorption of electrons entering the cathode to form water. This is precisely the reverse reaction of the water electrolysis process. By taking advantage of this principle, fuel cells in operation can continuously provide electricity to an external power source, hence the name “generator.”

In fact, the fuel cell had been invented before the internal combustion engine was invented in the nineteenth century. In 1839, William Grove made the first primitive model of the fuel cell, which was based on the different redox potentials of H₂ and O₂ to obtain an available electromotive force. However, there was a lack of environmental awareness at that time, and with a large reserve of petroleum being discovered and explored at the same time, people chose to use the low-cost method of burning oil to generate energy. As a result, the development of fuel cells was overlooked. On entering the 1980s, the oil crisis started to emerge. In this global oil shortage crisis, people began to attach importance to the development of novel energy sources. Currently, great progress has been made in the development of renewable and clean energies, including solar energy, nuclear energy, wind energy, and tidal energy. Fuel cells can supply sustainable and stable electricity, and thus are widely studied. Current energy mainly comes from nuclear power, oil, natural gas, coal, and other minerals. Power generation is based on the principle of combustion or reaction of minerals to produce large amounts of heat, which is then converted into mechanical energy, and then into electrical energy. The use of internal combustion engines to generate electricity usually results in low efficiency and polluting emissions. High-efficiency fuel cells can generate electricity in a clean way (~83%) and may ensure a low pollution emission, so it can replace the conventional power generation methods of using internal combustion engines. So far, many developed countries have invested in fuel cell research and development.

Hydrogen fuel cell technology has currently been fully developed abroad and many products have been introduced, such as electric vehicles and hybrid cars, as shown in Figure 10.3. However, proton-exchange membrane (PEM) materials applied in methanol fuel cells are still far from satisfactory. Early electric vehicles were mainly dependent on lead–acid batteries as the main power source. Because of the long charging time and heavy weight of these batteries, electric vehicles have always been unable to compete with conventional internal combustion engine-driven ones. The most striking difference between the typical internal combustion engines and fuel cells is the lack of endurance. In less than 90 min, the fuel cell vehicles may run out of fuel. To make things worse, it is difficult for them to find premises for refueling. The key to the operation of the PEM is the thin platinum catalyst (expensive); it is plated on both sides of the membrane, accounting for 40% of the cost of the batteries. The biggest problem in the design of fuel cell vehicles is how to make enough fuel available to meet the requirements of customers who drive a certain mileage. In general, a ride of approximately 640 km may take 5–7 kg of hydrogen, but the current prototype vehicles using fuel cells can only carry 2.5–3.5 kg. As a result, the design of fuel cell vehicles still has a long way to go.

10.2.2 Comparison of the Main Fuel Cells

There are many types of fuel cells. Depending on the electrolytes in use, fuel cells can be divided into the following categories: polymer electrolyte fuel cell (PEFC), solid polymer electrolyte fuel cell (SPEFC), proton-exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). A comparison between these categories is indicated in Table 10.4. Currently, the PEMFC is one of the hotspots in fuel cell research, because it is operable at low temperatures and used with solid-state electrolytes. In addition, its application can also be found in industries relying on mobile energy, such as the automobile industry and 3C electronics industry. The core components of PEMFCs include membrane electrode assembly (MEA), gas diffusion layer (hydrogen system), and conductive bipolar plates. Here, MEA is formed by the catalytic electrode and proton conduction membrane, as shown in Figure 10.4. At present, it is urgent to overcome the many flaws in the applications of PEMFCs, such as improving the battery life and lowering the price. PEMFC in the fuel in use can be divided into two categories, hydrogen fuel system and methanol fuel system. In general, the PEMFC is based on hydrogen as fuel, and the system with methanol as fuel is called the direct methanol fuel cell (DMFC). Regarding the global fuel manufacturing giants or famous car makers, such as Ballard, Toyota, Ford, and Honda, PEMCF has matured in device technology development. At present, various fuel cell manufacturing giants are committed to the goal of developing reduced manufacturing costs of PEMCF.

DMFCs are characterized by large units of energy density, easy assembly, small volume, and ample fuel supplements, so they are suitable for application in 3C electronics, such as portable computers or mobile phones. Thus, European, American, and Japanese manufacturers, such as Smart Fuel, Gore Associates, Toshiba, NEC, and Panasonic, are focusing on funding research and development to develop DMFCs. However, studies found that a DMFC, if working with the same accessories as hydrogen PEMFCs, may have very low power generation efficiency and short battery life. This results from the perfluorinated resin membrane more likely using a PEM in the hydrogen system. For example, Nafion, with a large swelling feature in methanol solutions, has poor barrier properties of methanol, resulting in the serious problem of the proliferation of methanol. Furthermore, it causes the platinum catalyst to be poisoned, which in turn leads to reduced life, or methanol may produce protons without being catalyzed by platinum, directly leading to a reaction with oxygen to reduce the efficiency of fuel use.

10.2.3 Proton-Exchange Membrane

PEM used in fuel cells is required to have the following basic properties: first, high proton conductivity; second, good mechanical strength; third, excellent chemical resistance; and fourth, good durability.

When operating at low temperatures (<80°C), PEMFCs and DMFCs work with the Nafion-based PEM manufactured by DuPont Co. When the operating temperature is higher than 120°C (120–200°C), high-temperature hydrocarbon polymers more commonly used, as reported in the literature, include poly (phenoxy phosphazene) (POP), sulfonated naphthalic polyimide, polybenzimidazole (PBI), alkylsulfonated polybenzimidazole (PBI-AS), sulfonated poly(arylene ether ether ketone) (PEEK-SO₃H), sulfonated poly(arylenesulfone) (PSU-SO₃H), and so on.

The PEM for the DMFCs is essentially the same as that of the hydrogen PEMFC. The difference is that methanol as a fuel needs special consideration regarding methanol permeability issues. Methanol permeability can cause the overall decline of the fuel cell discharge performance and a shortened life expectancy. Thus, concerning the PEM for direct methanol, we must take into account the methanol permeability problem in the material selection and design.

There are currently many PEM producers. These producers and the materials used for PEM are listed in Table 10.5. As noted, in the hydrogen PEM fuel system, PEM must have good proton conductivity, mechanical strength, and durability. In 1960, DuPont developed Nafion, which was the first PEM broadly in line with these requirements. At present, some other companies also produce fluorine polymer proton-conducting membranes, but they are mostly modeled on the Nafion polymer structure, as shown in Figure 10.5. However, Nafion cannot be directly used in the DMFC system, because the perfluorinated PEM has extremely high penetration of methanol that may reduce the discharge life and performance of fuel cells. The high methanol permeability is rooted in a large channel formed by a large group of ions, and the formation of this large channel is associated with its molecular structure.

As indicated in Figure 10.5, perfluorinated PEM materials are usually designed by using carbon–fluorine resin as a polymer main chain to increase the membrane’s chemical stability, thermal stability, and mechanical strength. In addition, the side chain-induced sulfonic fluorocarbon short chain has a proton conduction function. Nafion has high proton conductivity because the short-chain fluorocarbon sulfonic acid group has good mobility and can form acid-based clusters, which, in the case of water absorption, will soften the main chain of fluorocarbon polymer and increase the proton conduction pathway. However, because of the increased proton pathway, in the methanol fuel cell system the methanol molecules travel through the large channel at an increased rate. A perfluorinated resin with high methanol permeability would cause a shortening of life expectancy as a whole, dramatically reducing the power generation efficiency and effectiveness. To increase the efficiency of methanol fuel cells in power generation and battery life, the current research and development trend in methanol fuel cells is to reduce the methanol poisoning of the platinum catalyst or the methanol permeability of the PEM. To reduce the methanol permeability of the PEM, the PEM must be improved. This can be achieved in two ways. One way is the original perfluorinated proton-exchange membrane. For example, Nafion is modified by way of adding solid acid or inorganic oxides to reduce the number of channels in the perfluorinated membranes for methanol permeability, or by the use of different polymers doped with other polymers to limit the swelling of Nafion. Alternatively, the inorganic porous material can be impregnated with Nafion to reduce the swelling of Nafion, or the Nafion PEM is added with a good methanol barrier membrane on both sides. Another method is to use nonfluoride, high-performance engineering plastics containing a benzene ring, such as polyamide, PES, PEEK, or polymers containing more benzene. These materials are inexpensive and characterized by good chemical stability, mechanical strength, and thermal stability. Many researchers are trying to use the nonfluorine polymer sulfonic acid treatment to obtain a sulfonation polymer that can be used in PEM. The generated sulfonic acid enables the sulfonic acid polymer to have proton conductivity, which is related to sulfonation. For the same polymer, the level of sulfonation is proportional to the level of proton conductivity. However, as sulfonation is increased, water absorption of materials will be increased as well. When sulfonation is greater than a specific value, the membrane immersed in water will absorb a large amount of water, causing the swelling of polymer or even dissolution. In the preparation of a PEM using sulfonic acid polymer, the requirement for proton conductivity to achieve the level of Nafion will lead to an increase of sulfonation. However, while enhancing the proton conductivity, membrane dissolution will occur subject to the increased moisture in the membrane. This makes the sulfonic membrane distinct from Nafion in terms of proton conductivity. Therefore, in the case of low moisture content, the postsulfonic acid polymer cannot have high proton conductivity. To improve this, research has shifted toward the direct synthesis of nonfluorinated sulfonic acid polymer systems. The results showed that, by way of direct synthesis of sulfonic acid-based polyimide, the resulting PEM can have conductivity slightly lower than Nafion. Nonetheless, the methanol permeability was less than one-tenth that of Nafion. In addition, sulfonic acid-based polyamide is also lower than Nafion in moisture content. It is critical for the proton conductive membrane to be able to maintain high proton conductivity at low moisture content. This is because when the DMFC anode has contact with air, proton conductive membranes with low moisture content may have a water evaporation rate lower than that of proton conductive membranes with high moisture content. Therefore, the water dissipation can be reduced to increase the usage time of methanol fuel.

10.2.4 Nanofuel Cells

The nanofuel cell is one of the fuel cells based on the use of nanomaterials and nanotechnology. The small fuel cells (< 500 W) are currently undergoing research and development. It involves using 2-nm to 3-nm electrochemical catalysis and technology of organic–inorganic hybrid nanomaterials (e.g., PEM), leading to the development of integrated fuel cell technology. Low cost and high efficiency have become the most critical bottlenecks for its practical use.

Conventional fuel cells generally use hydrogen fuel. Compared with hydrogen, methanol as a liquid has high density and can be more safely stored, so the methanol fuel system has considerable potential for development. In addition, propane gas is currently applied in some of the nanofuel cells being developed. This material can be easily purchased. The US-based NanoDynamics, Inc. is now developing a 50-W SOFC. The prototype is only the size of a loaf of bread, with 20% of its components being manufactured with nanomaterials. As such, a battery can store 3,000 W·h of electricity using just 5 pounds of propane. Under the same conditions, the traditional SOFC can only store one-third to one-half the electricity of the present fuel cell. This 50-W SOFC with nanomaterials is a prototype of fuel cells that was originally designed for battlefield soldiers. Such a battery can supersede the traditional batteries weighing 35 pounds.

In addition to using nanomaterials, NanoDynamics also renovated the various parts of these battlefield batteries dedicated to the soldiers, including the membrane, electrode, and battery catalyst, to enable them to become more portable. At the same time, the storage capacity of the fuel cell is improved. This should be higher in the requirements for battery storage capacity.

Fuel cell energy comes from the chemical energy released from the generation of water in the reaction of hydrogen and oxygen. Platinum in the fuel cells is the catalyst in the catalytic reaction. However, subject to the side effects occurring in the vicinity of the cathode, platinum would gradually lose catalytic oxidation ability. This is one of the challenges in the practical application of fuel cells. Studies have shown that platinum electrodes for fuel cells, if plated on gold nanoparticles with a diameter of approximately 2–3 nm, will increase the platinum oxidation potential, making the metal less susceptible to oxidation. The platinum electrode can be protected from damage in this way. The studies also showed that the catalytic ability of platinum will not decline when the surface is blocked by gold nanoparticles, that is, platinum plated with nanogold still has catalytic reaction capabilities.

10.3.1 Status of Solar Cells

In 1954, Bell Labs in the United States produced the world’s first practical solar cell with efficiency of 4%, which was applied to the US satellite Vanguard-1 in 1958. Later, solar cells gradually entered into the ground application of electricity from space. A home photovoltaic system of 4 kW on the roof is able to meet the average family’s electricity needs, with the annual reduced CO₂ emissions equivalent to the annual emissions of a family car. Along with the continuous improvement in the material, structure, process, and other aspects, the present price of solar cells is less than 1% of that in the 1970s. It is expected that in 10 years, solar energy will be able to compete with thermal power in terms of cost of power generation in the United States, Japan, and Europe. At present, solar energy has an average annual growth rate of 35%, presenting the fastest growth in the energy technology industry.

10.3.2 Types of Solar Cell

Solar cells can be divided into the inorganic type or the organic type, according to the materials used. Inorganic materials commonly used in solar cells are semiconductor silicon (monocrystalline, polycrystalline, amorphous, complex, etc.) and compound semiconductors (GaAs, CuInSe₂, CdTe, InP, etc.). Silicon solar cells (monocrystalline silicon, polycrystalline silicon, amorphous silicon) have relatively higher photoelectric conversion efficiency, but the cost is also high and the preparation process is complex. In addition, the batteries made from multi-compound materials, such as inorganic salts, including gallium arsenide, cadmium sulfide, and copper indium selenium, contain cadmium and other toxic elements, as well as indium, selenium, and other rare elements, and therefore they are inferior in security and price.

Materials commonly used in organic solar cells are organic semiconductors (zinc phthalocyanine, polyaniline, poly p-phenylene vinylene, etc.). There are also other photochemical solar cells using nano-TiO₂. Solar cells made from functional polymer materials are presently still in the early stage of research and development and are characterized by low conversion efficiency and short service life.

10.3.3 Dye-Sensitized Nanocrystalline Solar Cells [6–8]

DSSCs are developed mainly on the basis of mimicking photosynthesis. It is a new type of solar cell, with its main advantages being abundance of raw materials, low cost, relatively simple technology, and more advantages in large-scale industrial production. Furthermore, all raw materials and production processes are nontoxic and nonpolluting, and some of the materials can be fully recovered. This is of great significance to the protection of the human environment. Dye-sensitized nanocrystalline solar cells may have a cost that is merely one-fifth to one-tenth that of silicon solar cells and a service life of up to 15–20 years. Thus, DSSCs represent a promising clean solar energy conversion device. Research on DSSCs will help alleviate the energy crisis in the world today with significant practical potential.

10.3.3.1 The History of Dye-Sensitized Nanocrystalline Solar Cells

In 1991, O’Regan and Gratzel [9] at Ecole Polytechnique Fédérale de Lausanne, Switzerland, reported a new type of highly efficient solar cell with dye-sensitized nanocrystalline TiO₂ films as light anode, symbolizing the emergence of the world’s first nanosolar cell. DSSCs can achieve photoelectric conversion efficiency more than 7% at a lower cost, providing a new way to use solar power.

In 1997, the DSSC’s photoelectric conversion efficiency reached 10–11%, short-circuit current reached up to 18 mA/cm², and open circuit voltage reached 720 mV.

In 1998, the use of solid organic hole transport materials to supersede the solid-state DSSCs with liquid electrolytes was successful. Its monochromatic photoelectric conversion efficiency of up to 33% attracted worldwide attention.

At present, the photoelectric conversion efficiency of DSSCs can be maintained at 10%, life expectancy is 15–20 years, and its manufacturing cost is only one-fifth to one-tenth that of silicon solar cells [10].

DSSCs comprise an organic–inorganic composite system. The advantage of DSSCs is that they can be made into products of transparency, so they have a wide range of applications. They can be used in a variety of lighting conditions and have high light utilization efficiency. In addition, they are not sensitive to light and shadows and can work in a wide temperature range (Figure 10.6).

10.3.3.2 Cell Structure

Dye-sensitized nanocrystalline solar cells (DSSCs; also known as Grätzel-type photoelectrochemical solar cells) mainly include glass substrates coated with a transparent conductive film, the dye-sensitized semiconductor materials, such as electrode pair and electrolyte several parts (Figure 10.7). In Figure 10.7, the anode in the electrode pair is dye-sensitized semiconductor film, such as TiO₂ film (5–20 μm, 1–4 mg/cm²); the cathode is platinum-plated conductive glass and the electrolyte is ${\mathrm{I}}_3^{-}/{\mathrm{I}}^{-}$.

Titanium dioxide thin-films electrolyte Pt mirror

10.3.3.3 Working Principle

The working principle of DSSCs is shown in Figure 10.8. Semiconductor TiO₂ has a band gap of 3.1 eV, so absorption occurs in the ultraviolet region. After surface adsorption to the dye sensitization, the good response of dye to visible light can help to expand its absorption into the visible light region. DSSCs are actually a heterojunction photoelectric conversion device. The sunlight on the cell will be absorbed by electrons in dye molecules, which can then jump to the excited state. Excited state electrons will be quickly injected into the TiO₂ conduction band via TiO₂ film, quickly reaching the contact surface between the membrane and conductive glass. It will be collected in the conductive substrate and flow toward the electrode through the external circuit.

The principle in Figure 10.8 can be expressed by the following reaction equation, with the same symbols as shown in the figure:

$\begin{array}{l}\mathrm{S}+h\nu \to {\mathrm{S}}^{*}\hfill \\ \mathrm{S}+h\nu \to {\mathrm{S}}^{*}\hfill \\ {\mathrm{S}}^{*}\to {\mathrm{S}}^{+}+{\mathrm{e}}^{-}\to {\text{CB}(\text{TiO}}_2)\hfill \\ {\mathrm{S}}^{*}\to {\mathrm{S}}^{+}+{\mathrm{e}}^{-}\to {\text{CB}(\text{TiO}}_2)\hfill \\ {\mathrm{S}}^{+}+{\mathrm{A}}^{-}\to \mathrm{S}+\mathrm{A}\hfill \\ {\mathrm{S}}^{+}+{\mathrm{A}}^{-}\to \mathrm{S}+\mathrm{A}\hfill \\ \mathrm{A}+{\mathrm{e}}^{-}(\mathrm{CE})\to {\mathrm{A}}^{-}\hfill \\ \mathrm{A}+{\mathrm{e}}^{-}(\mathrm{CE})\to {\mathrm{A}}^{-}\hfill \end{array}$

10.3.3.4 Parameters for Performance Evaluation

The performance indicators of DSSCs are mainly the incident photon-to-current conversion efficiency (IPCE) and the total conversion efficiency (ratio of output power and input power) ${\eta }_{\mathrm{total}}$.

The incident monochromatic light photoelectric conversion efficiency (IPCE) is defined as

$\mathrm{IPCE}=\frac{(1.25\times {10}^3)\cdot J_{\mathrm{p}}}{\lambda \cdot {\Phi }_{\mathrm{p}}}$ (10.1)

(10.1)

Here, $J_{\mathrm{p}}$ is photocurrent density in ${\mathrm{\mu }A/\text{cm}}^2$, $\lambda$ is the optical wavelength in nm, and ${\Phi }_{\mathrm{p}}$ is the flux by ${\mathrm{W}/\mathrm{m}}^2$.

Alternatively, IPCE can be expressed as:

$\mathrm{IPCE}=\mathrm{LHE}(\lambda ){\varphi }_{\mathrm{inj}}{\eta }_{\mathrm{c}}$ (10.2)

(10.2)

Here, $\mathrm{LHE}(\lambda )$ is the light absorption efficiency, $\mathrm{LHE}(\lambda )=1-{10}^{-\Gamma \delta (\lambda )}$; Γ is the number of moles of dye covering the surface of membrane per unit of square centimeter, δ(λ) is cross-sectional area of dye absorption, ${\varphi }_{\mathrm{inj}}$ is the efficiency of electron injection, ${\varphi }_{\mathrm{inj}}=k_{\mathrm{inj}}/({\tau }^{-1}+k_{\mathrm{inj}})$, k_inj indicates rate constant of electron injection, τ is the lifetime of the excited state, and ${\eta }_{\mathrm{c}}$ is the efficiency of the electrode in charge collection.

There are three main factors that may affect the photoelectric conversion efficiency of the cells: (1) lighting efficiency, mainly dependent on the optical absorption of an organic photosensitive dye; (2) electronic injection, mainly dependent on the energy level matching between the organic photosensitive materials and nanocrystalline semiconductor materials; and (3) collection efficiency, mainly determined by the proliferation of electronic properties in the film.

The total conversion efficiency (ratio of output power and input power) is calculated as follows:

${\eta }_{\mathrm{global}}=i_{\mathrm{ph}}{V}_{\mathrm{oc}}(\mathit{ff})/I_{\mathrm{s}}$ (10.3)

(10.3)

Here, i_ph is short-circuit photocurrent or, in other words, the current at short circuit (i.e., resistance is zero, only to connect the electrode pair and ammeter), which is the largest photocurrent that can be generated by photovoltaic cells. V_oc is open circuit photovoltage, which is the voltage at open circuit (infinite resistance, only to connect the reference electrode and voltmeter). It is the maximum voltage that can be generated by photovoltaic cells. Theoretically, the open circuit voltage V_oc is equal to the Fermi energy level of the semiconductor TiO₂ in light minus the Nernst reversible redox potential of electron pairs in electrolyte; ff is fill factor. It is defined as the ratio between the product of current (I_opt) and voltage (V_opt) against the product of the battery short-circuit current and open circuit voltage when the battery has a maximum output power (P_opt):

$\mathit{ff}=\frac{P_{\mathrm{opt}}}{I_{\mathrm{sc}}\cdot V_{\mathrm{oc}}}=\frac{I_{\mathrm{opt}}\cdot V_{\mathrm{opt}}}{I_{\mathrm{sc}}\cdot V_{\mathrm{oc}}},$ (10.4)

(10.4)

I_s is the incident light intensity.

10.3.3.5 Research Progress

There has been progress in the research on dye-sensitized nanocrystalline solar cells in the aspects of sensitizer, nanosemiconductor materials, and electrolytes.

10.3.3.5.1 Sensitizer

Dye-sensitized nanocrystalline solar cells require the sensitizer to be:

1. able to absorb sunlight as much as possible;

2. closely adsorbed on the surface of the nanocrystalline network electrode;

3. able to match the corresponding nanocrystalline energy band;

4. long enough in the excited state;

5. stable in the long term.

Dye-sensitized nanocrystalline solar cell sensitizers are mainly divided into the following four series: bipyridyl metal complex series, phthalocyanine series, porphyrin series, and pure organic dye series.

At present, bipyridyl metal complex is still the best and the most widely used dye sensitizer. Bipyridyl metal complex series have typical representatives N3 and black dye. Figure 10.9 indicates the formulae of N3 and black dye.

Figure 10.10 is a comparison of N3 and black dye regarding the incident monochromatic IPCE.

The pure organic dye family may be either semi-cyanine dye or coumarin derivatives. Their performance is not discussed here.

10.3.3.5.2 Nanosemiconductor materials

Oxides of metal sulfides, metal selenium compounds, perovskite, and titanium, tin, zinc, tungsten, zirconium, hafnium, strontium, iron, and cerium can all be used as semiconductor materials in DSSCs. For example, the optical and chemical properties of Nb₂O₅ DSSCs were reported in 1999 [11], and those of the nanocrystalline In₂O₃ thin-film electrode were reported in 2000 [12].

The use of nanoscale semiconductor materials such as TiO₂, ZnO, and SnO₂ as photovoltaic solar cells is extremely popular in research worldwide; the nontoxic nano-TiO₂ of light stability is the most commonly used material for study of photovoltaic solar energy conversion cells.

As mentioned, the two major crises humankind is facing in the twenty-first century are energy and pollution. TiO₂ will play a significant role in resolving these two issues. In Chapter 9, for example, we introduced the photocatalytic application of TiO₂. Here, we mainly focus on the application of solar cells.

As indicated in Table 10.6, TiO₂ is a high-refractive index semiconductor with a high dielectric constant that is conducive to the absorption of UV light; therefore, it can be used as micro-electromechanical material, photocatalyst, and in micro-light decomposition, as well as anti-fog and self-cleaning coatings and for light-grating. TiO₂ nanoporous membrane has the advantages of having high porosity and a large surface area. When applied in DSSCs, it can absorb more dye molecules. In addition, the mutual reflections between the grains within the films are able to enhance the absorption of sunlight. Thus, in addition to ensuring high photoelectric conversion quantum efficiency, dye-sensitized nanocrystalline TiO₂ semiconductor electrodes can further guarantee the high efficiency of light capture. The nanometer TiO₂ electrode is the key to the performance of solar cells, which is directly related to the efficiency of solar cells.

Table 10.6

TiO₂ Features

Physical Properties	Rutile	Anatase
Molecular weight (g/mol)	79.866	79.866
Density (cm3/g)	4.25	3.89
Lattice constant a (nm)	0.4594	0.3785
Lattice constant c (nm)	0.2962	0.9514
Crystal structure	Square	Square
Number of formula units in the cell	2	4
Band gap (eV) (corresponding to UV light wavelength, nm)	3.0 ~410	3.2 ~385
Hardness (Mohs)	6–7	5.5–6
Refractive index
Air	2.7	2.55
Water	2.1	1.9
Oil	1.85	1.7
Dielectric constant (powder state)	114	48
Specific heat (kJ/°C kg)	0.7	0.7
Melting point(°C)	1,855	Transformed into rutile

Methods of preparation of nano-TiO₂ thin-film electrode materials mainly include the sol–gel method, hydrothermal reaction, sputtering, alkoxide solution, sputtering deposition, plasma spraying, and screen-printing methods.

10.3.3.5.3 Electrolyte

Liquid electrolyte is found to have some obvious shortcomings. It tends to lead to dye-sensitized desorption. It is a volatile solvent in reaction with sensitizing dyes and leads to dye degradation. It has a complex sealing process, with long-term placement caused by electrolyte leakage. Sealant reaction with the electrolyte still exists in cell, electrode, with light corrosion, and can easily become dye-sensitized in desorption. It has a very slow carrier mobility rate and instability in high-intensity illumination. It also has the presence of other oxidation–reduction reactions. At present, researchers have prepared all solid-state nanosolar cells using solid-state organic hole transport materials to replace the liquid electrolyte, allowing for gratifying progress. For example, in 1996, Masamitsu and colleagues prepared complete solid-state solar cells using solid polymer electrolyte and obtained high ionic conductivity of the electrolyte using a special preparation method. A continuous photocurrent was achieved with 0.49% of the photoelectric conversion efficiency. In 1998, Grätzel and associates used 2,2′, 7,7′-4 (N, N-2 pairs of p-methoxyphenyl-amino)-9,9′-spiro-2-fluorene (OMeTAD, shown in Figure 10.11) as a hole transport material. The result was a cell with 0.74% of the photoelectric conversion efficiency and color efficiency as high as 33%. This has sparked worldwide attention and enabled nanosolar cells to make a huge step forward in the solid state [13].