Over the past several decades our energy consumption needs have grown dramatically while our production levels have remained relatively constant. The United States has relied heavily on foreign imports to meet our fossil fuel needs. However with oil and gas prices heavily rising, it will soon become economically necessary to utilize alternative sources of energy.
Energy, in one form of another, is driving today's technically advanced, connected and highly industrial world. The United States is the largest energy consumer in terms of total use, using 1 quadrillion BTUs in 2005. This is three times the consumption by the United States in 1950. Energy is universally accepted as the core of an industrial society. Without the sufficient supply of the energy the stability of social, economic and political structure of the society is in threat. As the world supply of fossil energy sources decreases, the need for energy conversion, energy conservation, and renewable energy technologies, becomes critical.
Almost 85% of the fuel consumed in United States in the year 2007 was obtained from fossil fuels, with approximately 40% of the total energy coming from the petroleum sources, 23% coming from natural gas, 22% coming from coal, and 8% from the nuclear sources. The renewable energy sources provided around 6% of the total consumption of the energy. Due to the continuous increase in world population, the energy consumption of the world is increasing rapidly mainly because of the growing demand for energy in developing countries, as they attempt to improve their living standards. The following chart provides projected energy consumption of the world by the end of the year 2030.
The increase in demand for energy will be almost 50% in the next 20 years.
Sources of Renewable energy
Hydro power
Solar energy
Bio-Mass
Bio Fuels- Ethanol, DMF, Bio-diesel
Wind power
Tidal and wave energy
In these following plots we see the current and future renewable energy estimates.
Solar energy
The Earth receives 174 petawatts (PW) of incoming solar radiation at the upper atmosphere. Approximately 30% is reflected back to space while clouds, oceans and landmasses absorb the rest. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near ultraviolet. This energy is free and depends on the topology of the land. Major technologies like solar thermal systems, photovoltaic systems, and solar chemical systems, have been implemented to harness energy from the sun to accomplish routine tasks like lighting, water heating, cooking, heating, cooling and ventilation.
Most of these technologies are in the incubation stage, inefficient and costly. Photovoltaic systems are used to convert solar energy into useful electricity by incorporating semiconducting electrodes. According to the US Department of Energy's annual report in 2006, the electricity generation capacity of solar energy sources was 400 MW, or 0.39% of the total electricity production. Even though solar energy is abundant, free and can fulfill all our energy needs, the main problem is the cost of these devices and systems. NREL estimates the cost of solar energy to be 11 to 15 cents/kWh. The solution to this problem comes in the form of using solar energy to split water molecules and use the hydrogen, evolved in the process, as a fuel.
Hydrogen is the future of fuel. Many technologies are being developed in order to use hydrogen as a fuel instead of fossil fuel owing to the fact that the fossil fuel deposits are limited and are responsible for climate changes due to global warming. Due to the limited resources of fossil fuels the prices have been constantly increasing. Hydrogen as a source of energy is anticipated to compliment photovoltaic systems, for the development of sustainable energy systems.
We believe that hydrogen generation by using solar energy is the answer to the ever growing energy needs of the modern world.
Hydrogen as a fuel:
Hydrogen was first discovered as a flammable gas by British chemist Henry Cavendish in 1766. French scientist, Antoine Laurent in 1783, later rediscovered it as a constituent of the water molecule. Hydrogen is a highly reactive element, never exists in nature as a free element. It is present in naturally occurring substances such as water, hydrocarbons (natural gas, petroleum sources and coal), and biological materials (carbohydrates, fats and protein).
Key Aspects:
Hydrogen obtained from dissociating water molecules is a clean method and as water is abundant there would be no shortage of resources.
Combustion of hydrogen, when used as a fuel, results in the formation of steam and liquid water, so from an environmental standpoint it is perfectly safe, non toxic and renewable.
Hydrogen can be generated using various renewable energy sources such as solar, wind, hydroelectric, or hydrothermal energy.
It can be used in fuel cells, which can store large amounts of energy and convert chemical energy directly into electrical energy.
Due to its long-term advantages, development of hydrogen generation technologies using renewable energy sources is expected to get major support from governments and private energy producers.
Hydrogen also possesses a high calorific value when compared with other fuels as shown in the figure below.
Commercial processes for Hydrogen generation
Steam Reformation
This process converts natural gas (mainly methane and other hydrocarbons) into hydrogen and carbon monoxide by reacting it with steam, in the presence of a nickel catalyst. Methods used to produce hydrogen from natural gas are well researched, highly efficient, and developed and account for over 95% of all hydrogen produced in the U.S. and 48% globally. But this method is constrained by the cost of natural gas.
Bio-chemical process
Producing hydrogen using enzymes and organisms, in a photo-bioreactor is another method being widely studied. Algae produce hydrogen under certain conditions. Cyanobacteria break down water compounds into hydrogen and oxygen molecules through photosynthesis.
Gasification
Coal
Hydrogen is already being produced at an industrial scale by the gasification of coal. High temperature steam is used to breakdown coal into a gaseous form, from which hydrogen can be segregated. This process also deals with the same problem of limited fossil-fuel resources.
Biomass
Gasification of biomass is a thermal treatment for various cellulose based materials like wood, paper, agricultural residue, to produce a high proportion of gaseous products and small quantities of char (solid product) and ash. At relatively high temperatures (873-1,273 K), biomass undergoes thermal decomposition to produce gases like H2, CO, CO2, CH4, H2O, and other gaseous hydrocarbons. New technologies are being developed to intensify, reduce costs and increase the efficiency (oxygen intake) of these systems.
Photo Electrolysis of water
A lot of research articles indicate that the most promising method of hydrogen generation is based on photo- electrochemical water decomposition using solar energy. Honda et al. were the first to publish the use and impact of various materials on the performance of photo-electrochemical cells (PEC). Electrolysis of water produces high purity hydrogen, but considerable amount of energy is used in this process. This is a major roadblock as the 'required' energy is obtained from burning fossil fuels. Therefore devising a system, which uses low energy, is highly efficient and can produce large amounts of fuel in the form of hydrogen, is a necessity. This is the main idea of this research project.
Photoelectrochemical (PEC) generation of hydrogen from water
Photoelectrochemical water splitting is based on the conversion of solar energy into electricity by using a two-electrode system, immersed in an aqueous electrolyte. The anode is made of a semi conducting material that has the ability to absorb light. Geris in 1960 invented the Photoelectrochemical cell. The general setup of an electrochemical cell consists of a semiconducting photo anode, made of an n-type semi conductor and a cathode made of a metal (Pt). Metals are not suitable for light energy conversion as their exited electronic states have very small lifetimes and most of the energy is swiftly thrown out as heat. The semiconducting photo anode, synthesized by electrochemical method, is used to generate electron-hole pairs by capturing the energy from the solar spectrum. Water splitting is based on the conversion of light into electrical energy within the cell involving two electrodes in contact with aqueous electrolyte. The semiconductor photo electrode is exposed to light and able to absorb light .The electricity is then used for water splitting. This method ensures the cleanest way of producing hydrogen with no use of fossil fuel, utilizing sunlight and water. These PEC systems are simple, compact, low initial cost and high hydrogen generation efficiency. PEC has not yet given a high yield and opportunity to use in practical purposes. Performance of PEC is based on following terms:
1. Excitation of electron-hole pair in photo electrodes
2. Charge separation in semiconductor photo electrodes
3. Electrochemical oxidation and reduction processes with necessary charge transfer at anode and cathode solution interfaces
4. Generation of PEC voltage for water splitting
Present trends in research are focused to the development of high efficiency PEC cells and its commercialization.
Advantages and disadvantages of PEC cells
Advantages:
1. The PEC system is environmentally safe and simple.
2. They are regenerative and the composition of electrolyte solution remains unchanged.
3. Semiconductor-electrolyte junction is achieved quickly on contact with the solution, avoiding the requirement of doping and diffusion.
4. Numerous semiconductor materials and the redox electrolytes are available for research.
5. Utilization of electro catalysis helps to improve the kinetics of charge transfer process and thus improving the conversion of energy.
6. Requirement of anti reflection coating of semiconductors in PV devices is not necessary for PEC cells and thus reducing the cost.
Disadvantages:
1. Photo-corrosion of semiconductors by holes
2. Recombination of holes and semiconductors lead to less efficiency.
Reaction mechanism of PEC cell
Becquerel first observed the flow of current between two unsymmetrical metal electrodes after being illuminated by sunlight in 1839. This effect was called as Becquerel effect. This triggered the researchers to focus on photovoltaic field. Bell laboratories did a detailed study of Photoelectrochemical reactions in 1954 on Germanium. This research showed that varying the properties of Germanium could control electrochemical reactions. Honda et al in 1971 provided the electrochemical evidence for the mechanism of photosynthesis. Electrochemical system involving n-type TiO2 semiconductor electrodes mimicked a process similar to photosynthesis. The results established the nature's initial O2 evolution reaction in photosynthesis process.
In 1972 Honda et al gave the Photoelectrochemical reactions occurring during electrolysis of water. PEC involves several reactions occurring at semiconductor photo electrode/electrolyte surface, within photoeletrodes. They are as follows:
• "Intrinsic ionization of semiconducting material i.e. Photo anode due to light illumination, resulting the formation of quasi-free electrons and holes, generally known as charge carriers".
• "Water oxidation at the photo anode surface by holes"
• "Migration of H+ ions through the electrolyte, from the photo anode to the cathode and flow of electrons through external circuit, from anode to cathode".
• "Reduction of H+ ions by electrons at the cathode"
Intrinsic ionization of semiconductor due to light illumination can take place at the semiconductor electrode/electrolyte surface, if light with energy (hÏ…) equal or more than the band gap of semiconductor strikes the photo anode. This leads to the formation of electrons in conduction band and electron holes in valence band. The reaction can be represented as follows,
An electric field is required at the electrolyte/electrode interface to avoid the recombination of the charges. Electron hole due to light illumination helps in splitting of water molecules into hydrogen and gaseous Oxygen. The water splitting can be represented as follows,
This reaction takes place at the photo electrode i.e. anode and O2 is evolved at the interface. The electrons generated from the reaction are transferred to the external circuit. The electrons thus help in reducing H+ into gaseous hydrogen.
The overall water splitting reaction can be represented as follows,
H2O (liquid) →1/2 O2 (gas) + H2 (gas) (4) The reaction (4) is feasible if the energy of the photons captivated by the semiconductor photo-anode is equal to or larger than threshold energy Et = hυ = 1.23eV.
Factors to be considered while designing an energy efficient PEC system
Energy conversion efficiency should be greater than or equal to 10%. Energy conversion efficiency can be defined as ratio of amount of output chemical energy to the input chemical energy. Cost effective, must be analogous to other hydrogen generation technologies, robust and free from corrosion.
Material aspects for PEC system
The semiconductor photo electrodes should be able to perform the following fundamental functions.
• Requirement of optical function to harvest maximum solar energy
• Catalytic action for water splitting
The efficient photo electrodes consist of oxides and elements from II-VI, III-V compounds16. These semiconductor photo electrodes can either be oxide type or compound (non-oxide) semiconductors. The oxide semiconductors which are non- stoichiometric are of great importance to researchers to use the material as a photo electrode material. TiO2 is the mother of all oxides. The easiest way of obtaining the non-stoichiometric TiO2 i.e. TiO2-x, is to anneal under reducing atmosphere. The non- stoichiometric TiO2 decreases the electrical resistivity, which is one of the requirements for solar conversion efficiency. The other requirements for an efficient photo electrode can be listed as follows:
"Band gap"
"Flat band potential"
"Schottky barrier"
"Electrical resistance"
"Helmholtz potential"
"Corrosion resistance"
"Microstructure"
Kung et al have worked on variety of binary and ternary oxide anodes for photo assisted oxygen evolution reaction. The counter electrode consists of noble metals like platinum. As calculated from equation, a potential of 1.23eV is necessary to split the water molecules. Take into account of the ohmic and kinetic losses one needs to apply over- potential to drive a water splitting reaction. For a swift transport of charges at the semiconductor electrolyte interface, the valence and conduction bands should be far from potentials of H+/H2 and O2/H2 red-ox couple must fit inside the band gap (Eg) on the energy scale. Considering the above listed factors, a material with a band gap greater than 1.48 eV is suitable for water splitting reaction. Various compound semiconducting (inorganic) materials such as amorphous or crystalline silicon, gallium arsenide, cadmium telluride, and indium phosphide are known to harness solar light for converting into electricity. There is continuing interest in using these efficient semiconducting materials, as their band gaps are suitable for capturing a large volume of the solar energy, for conversion of solar energy to chemical energy. Cadmium sulfide is one of the most intriguing non-oxide semiconductors. The serious problems of non-oxide semiconductors are prone to photo corrosion in aqueous solution. Compared to non-oxide semiconductors oxide semiconductors are stable in aqueous solution. They possess suitable overlap of conduction and valence band edges of the semiconductor. They exhibit the rapid charge transfer form the semiconductor to the water20.
There is one more way of classifying the semiconductor photo electrodes based on the charge carriers. Semiconductor photo electrodes can be classified as n-type and p-type semiconductors. In an n-type semiconductor the majority charge carriers are electrons. Upon illumination of light in n-type semiconductors the electrochemical oxidation and reduction processes occur, O2 and H2 are evolved at photocathode and metal cathode respectively. Similarly in p-type the photocathode reduces H+ ion into H2 upon illumination and O2 is evolved at metal anode.
Drawbacks of p-type semiconductor
Threshold intensity is required to lower the Fermi energy level, so that O2 evolution can occur. Band bending is large and large Surface recombination occurs.
Nanotechnology, nanostructured materials and their properties
Nanotechnology is one the promising emerging technologies. It is a forerunner for a key technology in this 21st century that will contribute towards the sustainability of mankind's development. The concept of sustainable development needs a production and consumption outline to be shaped for the current generation in such a way so as to meet the demands of future generation without affecting the social and political life. The limited availability of natural resources like clean water, fossil fuels etc, highlights the importance of the efficiency of their use, recycling and substituting the non-renewable energy sources by renewable energy sources. Nanotechnology has wealth of applications that has been already proposed. Nanotechnology promises for many scientists and engineers less material and energy consumption and less waste and pollution from environment. Nanotechnology is expected to enable new technological methods that reduce the environmental footprints of existing industrial technologies in the developed countries or to allow developing countries to use the nanotechnology for some of their demanding needs. Science and technology and nanotechnology together might not provide the magic bullets to solve all sustainability problems. But nanoscience and nanotechnology may be critical enabling component of sustainable development when they are wisely used.
Nanostructured materials
Nanostructured materials have structure less than 100nm in at least one dimension. Nanoparticles and colloids are three dimensions in size. Nanotubes and nanorods are two-dimensioned structures.
Applications of nanostructures
Catalysts are the among the first industrial nanotechnologies applications. Nanoparticles and nanostructured materials offer new ways of designing and controlling catalytic functions, including the provision of enhanced activity and selectivity for target reactions. The number of atoms increases as the surface area increases and more electrons are available for catalytic reaction.
The most important and flexible energy technology is the direct conversion of sunlight to electrical energy by the use of nanostructured photovoltaic devices. Existing photovoltaic devices are limited by low conversion efficiency and high cost. But by the use of nanostructured quantum dots have a potential for cost effectiveness. They absorb the light in broader wavelength of the spectrum. Hetero-structured absorber layers will increase the efficiency of the cell, which leaves us an option of using lesser quality materials and thus reducing the cost. Photoexitation of the electrons in photoactive semiconductors mainly occurs at the surface. Therefore nanomaterials are more reactive than bulk materials in catalytic processes. Nanostructured materials also find application in generation of hydrogen by the photocatalytic conversion of oxygen and hydrogen by using nanostructured catalyst or nanoscale additives. Other application of nanotechnology includes the use of nanomembranes, zeolites and nonoporous polymers for water purification and desalination.
Properties of nanostructure materials
In nanomaterials, the quantum confinement can be explained by dispersed spread of allowed energy states. Density of state becomes discreet as the size of the particle decreases from bulk to the nanoscale. The charge transport becomes easier under applied field perpendicular to the quantized path due to the separation of electrons along the applied filed in the minibands.
Photo electrochemical behavior of the nanoparticles differs from their bulk properties. While a semiconductor is in contact with an electrolyte, an electrical double layer is formed at the interface. Due to potential gradient at the interface, the valence and conduction band bend occurs. This is called as band bending. This is a energy loss process, as extra energy is required to keep the photogenerated holes and electrons away from each other. This band bending depends on particle size. Smaller the size, small will be the band deformation. In nanoparticles, band bending will be less; no internal field acts on holes and electrons. Therefore as soon as the holes and electrons are generated, they are scavenged at the semiconductor/electrolyte interface28 29. TiO2 nanotubes are considered as one-dimensional structure due to wall thickness of around 20 to 90 nm and diameter of about 100nm. Electrons in nanoparticles penetrate by hopping mechanism, but in ordered arrays of one-dimensional nanotubular TiO2, electrons flows directly under the applied electric field radially in one direction due to quantum confinement. The ordered array of nanotubular structures on a conducting substrate is better due to ease in transport of charge from the surface towards the conducting material. The quantum confinement also helps in minimizing charge recombination.
The nanotubular structures have better light absorption because of scattering of light inside the nanotubes. The light absorption can also be amplified by increasing the length of nanotubes without disturbing the properties of charge transport. The nanotubular structure also has high geometrical area, which increases with increase in length of nanotubes. The straight nanotubular arrays formed in the structure makes the transport of gas easier, without bubbles getting confined in the structure like mesoporous materials with twisted structure.
Synthesis of one-dimensional TiO2 nanotubular structures
There are numerous reports available, which demonstrate capability of synthesizing the one-dimensional titanium nanotubes. Various processes are used for development of these nanotubes. Template directed growth using porous, hydrothermal process, and anodization process. Among these processes synthesis by anodization route is easier due to ease in electrochemical set up and chemicals handling as well control over growth morphology.
Zwilling et al described the formation of nanoporous surface of titania films by electrochemical anodization. The detailed study of nanotubes formation was studied and reported by Grimes et al and Raja et al. The nanotubes synthesized by electrochemical anodization process are found to be highly ordered and uniform compared to other methods, their exclusive symmetric cylindrical arrangement and other properties related with nanotubular TiO2 are reflected to have many uses such as anode material in lithium battery, hydrogen sensors and photo catalysis.
Mechanistic model for growth of TiO2 nanotubes
Numerous reports have proposed the mechanisms for the growth of TiO2 nanotube formation. The arrays of nanopores and nanotubes formed by single step anodization can be subdivided into two process namely (a) Initiation and (b) growth There is no universally accepted mechanism for the nucleation of the arrays. Most researchers argue that initiation occurs at randomly distributed sites, such as unspecified defects, at the oxide/electrolyte interface and that organization occurs during the growth of the pores. According to Yasuda et al the larger volume of oxidizable metal associated with deep pores creates the greater acidification of the pore electrolyte, which leads to faster oxide dissolution and pore penetration into titanium. Self-organization is the result of synergism between oxidation and oxide dissolution. Another theory proposed by Raja et al suggests that planar instability induced by strain energy due to compressive stresses from electrostriction, electrostatic forces and volume expansion leads to formation of nanopores. Another mechanism for the pore formation put forward by Devine et al pores initiate as a consequence of compositional and potential gradients in the oxide/electrolyte interface and capillary effects, which lead to the breakdown of the planar interface into an initially disorganized array of nanopores. The same authors in another work report that pore organization starts as a consequence of a particular pore diameter/spacing propagating at a rate faster than all others. The faster growing pore diameter is a function of the oxide/electrolyte interfacial energy and the gradients of potential and composition in the oxide/electrolyte interface. The growth mechanism as proposed by Raja et al can be summarized as follows,
Barrier layer formation during first few seconds of anodization route. The barrier layer act as inactive oxide film on the surface. Enlargement of the barrier layer by and subsequent micro fissuring, normally refereed as formation of easy path.
"Nucleation of secondary oxide films and pore nucleation through easy path".
"Growth of secondary oxide and pores on entire surface." "Separation of pores and formation of individual self-ordered nanotubes".
"One creates the cation vacancies"
Under the influence of applied field, vacancies move toward the oxide/metal interface
"If these vacancies are not healed, they form voids"
"When the dissolution occurs at faster rate the density of vacancies is very high."
"The radial transport of these vacancies across the pore walls causes the buildup of repulsive forces and pore separation which subsequently grows as a nanotube"
"In the anodization process, the anodic current recorded is primarily due to the dissolution of Ti (III) and Ti (IV) species"
"This observation is evident by the continuous mass decrease observed in the nano-gravometric analysis of anodization process."
"The comparison of theoretical and practical rate of mol mass change during the anodization process, conclude that a small percentage of the anodic current is contributed by the oxidation of Ti"
"The oxide films synthesized by electrochemical anodization process consists of number of point defects and surface states created by the source of point defect model, it is proposed that that the dissolution of Ti surface hydroxyl groups, absorbed oxide molecules, coordinately unsaturated sites and low valance metal ions trapped in side the thick oxide layer".
"These inherent defects give TiO2 nanotubes unexpected properties such as high absorption of oxygen or decreased resistance when exposed to the hydrogen".
