Research Progress and Perspectives

The increasingly apparent effects of global warming and resulting climate changes are forcing us to increase the use of renewable energy, such as solar energy. The global efforts in this directions aim to develop new energy conversion systems. The TiO2-based semiconductors are the promising candidates for the development of solar energy conversion systems with versatile applications. However, the research in development of high-performance systems for the modern day technology is mul­tidisciplinary. Therefore, the progress in the area of photocatalysis requires bringing

SOLAR HYDROGEN

Area of the solar panel required to cover all Australia’s energy needs

image654

FIGURE 8.45 Representation of area to be covered with a solar panel required for energy production to address Australia’s current energy needs. (Reprinted from Int J Hydrogen Energ, 30, T Bak, J Nowotny, M Rekas, CC Sorrell, Solar hydrogen. Environmentally safe fuel for the future, 521-544, Copyright 2005, with permission from Elsevier.)

together the concepts of several scientific disciplines, including solid-state science, surface chemistry, electrochemistry, and materials science.

As indicated in Chapter 2, only a small portion of the solar energy received by Earth would be sufficient to address our global energy needs. As shown in Figure 8.45, a square of approximately 40 x 40 km is sufficient to address the energy needs of Australia [129]. Such an area could be achieved if solar panels are installed on the roofs of all individual households in the sunshine country down under. In analogy, a square of approximately 161 x 161 km would cover the entire energy needs of the United States [130]. We are still far away from achieving these ambitious targets.

The increasingly apparent climate changes are forcing us to use renewable energy, such as solar energy, instead of fossil fuels. However, we need to develop less expensive materials able to efficiently harness solar energy. Awareness is growing that oxide semiconductors can be used to develop a new generation of solar energy conversion systems for the modern day technology. This book shows that the prop­erties of oxide semiconductors are profoundly influenced by lattice imperfections. Consequently, defect engineering may be applied to enhance their performance in providing the sustainable clean energy.

[1] , 1

MO ^ Mi + 2e’ + – O2 (1.19)

The related equilibrium constant is

K(Mi) = n2 [m* • ] p (O2 )1/2 (1.20)

The charge neutrality requires that

[2] The manufacturer of the High Temperature Seebeck Probe for simultaneous determination of both electrical conductivity and thermoelectric power at elevated temperatures and in the gas phase of controlled composition: Eco Materials & Equipment, 30 Fretus Ave, Woonona, NSW 2517, Australia.

[3] The high-temperature Kelvin probe was manufactured by EME (Eco Materials & Equipment Pty Ltd), 30 Fretus Ave, Woonona, NSW 2517, Australia.

[4] Kinetics controlled by the segregation-induced electric field

• Kinetics controlled by bulk diffusion

• Mixed kinetics

[5] Reduction of oxygen. Reduction of gaseous oxygen at the electrode of higher oxygen activity (cathode) leading to oxidation of the oxygen ion conductor on the cathode side.

• Transport of oxygen. The ionic transport of oxygen may be considered in terms of the following equivalent processes:

• Formation of doubly ionized lattice oxygen species that are transported from cathode to anode

• Formation of doubly ionized oxygen vacancies that are transported from anode to cathode

• Oxidation of oxygen. Oxidation of oxygen ions at the electrode of lower oxygen activity (anode) leading to the formation of gaseous oxygen, quasi­free electrons, and doubly ionized oxygen vacancies.

• Transport of electrons over the external circuit.

[6] Current Device. In this case the external circuit is connected through a resistor that allows the flow of current, which is related to the amount of oxygen passed through the cell. Such a cell may be used as a solid oxide

[7] Optical system. Its functions include collection of sunlight and its conver­sion into a parallel beam of photons that is directed onto the photoelectrode, resulting in the formation of a well-defined image of the sun.

• Photoelectrochemical cell. The electrochemical chain of the PEC equipped with one photoelectrode includes the following essential elements: (1) photoanode (n-type TiO2), (2) cathode, (3), aqueous electrolyte (forming

[8] Optical Reflection. The energy component related to optical energy losses, EOPT, is associated with the optical reflection. In consequence, only a frac­tion of photon energy is efficiently used for conversion.

• Formation of Heat Energy. The EHE component is related to the losses related to the conversion of light energy into heat energy.

• Recombination (EREC). Recombination-related energy losses may be reduced by the imposition of an electric field leading to charge separation. Such a field is formed at the electrode/electrolyte interface when the elec­trode is immersed in the electrolyte. This electric field may be tailored by surface and near-surface engineering, leading to the formation of concen­tration gradients and the related potential barriers.

• Electrical resistance (ER). These losses, which are related to charge trans­port, may be diminished by increasing the concentration of charge carriers with high mobility. This may be achieved by doping metal oxides with ions of controlled valency, leading to the formation of donors and acceptors. The

[9] Strongly Reduced Regime. The predominant defects in this regime are dou­bly ionized oxygen vacancies. Then the simplified charge neutrality condi­tion may be expressed as

[10] Oxidized Regime. Taking into account Equation 4.15, the concentration of electron holes in the oxidized regime is the following function of p(O2):

[11] Hossain 2006

FIGURE 4.6 Energy levels of points defects within band gap of TiO2, according to the data of Cronemeyer [11], He et al. [14], Ghosh et al. [12], and Hossain et al. [13]. (Reprinted with permission from T Bak, MK Nowotny, J Nowotny, Defect disorder of TiO2, J Phys Chem B, 110, 2006, 21560-21567. Copyright 2006 American Chemical Society.)

[12] Shift of the n-p transition points toward higher p(O2) values

• Reduction of the concentration of all positively charged defects (oxygen vacancies, titanium interstitials, and electron holes)

Updated: August 23, 2015 — 7:17 am