The ubiquitous climate change and the attendant consequences in the forms of flash floods, earthquakes, tsunamis etc has further intensified efforts at seeking alternative sources of energy to the currently used fossil fuels that contribute so much to the deterioration of the environment and the menace of climate change. The alternative forms of energy, termed green energy are renewable sources of energy e.g. solar energy. Theses green sources of energy do not contribute to the deterioration of the environment. The industrial production of hydrogen gas H2 is dependent on heavy use of fossil fuel-generated electricity to power the process of electrolysis in use for the hydrogen gas production. In order to reduce dependence on fossil fuels for the generation of electricity for this process, alternative electricity generation using nanotechnology based on solar power, a green energy source, are being explored.
Use of Solar Cells in Electrolysis
In a paper, Singh (2010) explores the use of nanotechnology in the production of Hydrogen gas and identified nanotechnology-driven solar cells as veritable means of green energy generation for the electrolytic process involved in Hydrogen gas generation.
The process of electrolysis has been used for ages in generating Hydrogen gas with 99% purity using direct current to split water molecules into Hydrogen gas and Oxygen gas. The cathode and anode of the electrolyzer used depended on the electricity generated from the combustion of fossil fuels. The use of solar energy in the generation of hydrogen however is in tow forms – thermolysis and photo-electrolysis.
Cathode half reaction: 2H+(aq) + 2e- = H2(g)
Anode half reaction: 2H2O(liquid) = O2(g) + 4H+(aq) + 4e-
Balanced reaction: 2H2O(liquid) = 2H2(g) + O2(g)
The equations above show the cathodic reduction equation, the anodic oxidation equation and the overall equation of the process of electrolysis.
Thermolysis uses the energy from the sun to generate heat in materials that are used to generate the Hydrogen gas. In photo-electrolysis, the radiant heat from sunlight is employed directly in the generation of the Hydrogen gas. Before expounding the use of photo-electrolysis and thermolysis, an understanding of the use of solar cells and solar energy will suffice.
The sun being the source of the solar energy used is made up of very hot gases in form of a sphere of size 109 times bigger than the size of the earth. The estimated temperature of the surface of the sun is put at 5,726K which travels through the atmosphere before reaching the earth’s surface. Travelling through the atmosphere, solar radiation becomes reflected, scattered and absorbed by water vapour, clouds and other particles before reaching the surface of the earth. The average intensity of solar power reaching the earth is thus estimated to be 1353 W/m2.
Solar cells that generate electricity using solar power work based on the Photovoltaic (PV) effect. The PV effect describes a situation where a material generates electricity when light is shone upon it. The materials used in PV cells are semiconductor materials such as Silicon, Gallium Arsenide etc. The generation of electricity is attained when the energy from the incident light becomes sufficient enough to excite electrons from the valence band of the molecules of the semiconductor material to the conduction band. Generation of electricity from PV cells is easy and does not contribute to environmental degradation.
Solar Thermal Concentrators
Thermolysis involves the splitting of water molecules into Hydrogen gas and Oxygen gas by using PV cells that collect solar energy to attain very high temperatures. The collection of the thermal energy from the sun can be achieved by using concentrators. Different kinds of concentrators are used, but a simple one that will not require the mechanical tracking effort like in the parabolic dish concentrators is the use of a flat plate collector. In order to improve the yield of Hydrogen and the overall efficiency of the use of solar collectors, the use of nano tubes in the collectors is being explored (Singh, 7). The nano tubes are made up of a number of TiO2 in order to increase the movement of electrons.
Photo-electrochemical Cells (PEC)
The process of photolysis for the production of Hydrogen gas is made possible by the use of solar cells which are referred to as photo-electrochemical cells (PEC). The cells are irradiated with light around the visible region - solar energy. The cells then convert the visible light photons from the solar energy into electricity for the semiconductor anode and cathode immersed in an electrolyte (Singh, 6). The photolysis equations of the reactions at the photo anode are as shown
Photoanode: H2O(liquid) + 2holes+ = 2H+(aq) + 12O2(g)
Cathode: 2H+(aq) + 2e- = H2(g)
In the PEC cells, the semiconductor photo-anode absorbs the light photons themselves in order to facilitate the chemical reactions that take place in the electrolyte being used. Hydrogen gas is produced in the endothermic reaction that results in a net change in reactants. This process involved in the PEC is only slightly different from the normal electrolysis with the involvement of electrons and holes as charge carriers.
A slight modification of this process using photo-catalysis employs the use of photo-electrodes that are exposed to light and Ultra Violet radiation. Instead of using solar cells to generate electrons and holes in the PEC, the photo-catalysts used as electrodes will reduce water directly to produce Hydrogen gas. Modifications of the photo-catalyst structure to produce nano crystalline coatings in order to improve the yield of Hydrogen gas has been explored by Alenzi et al. (2010).
The use of solar collectors and the use of photo-electrochemical cells have each been considered in the production of Hydrogen gas. These two methods have been proven to improve the yield of Hydrogen gas even using cheap sources of electricity obtained by harnessing solar energy (a renewable energy source) using nano technology. How about facilitating the industrial production of Hydrogen gas with a combination of these two processes? This has been explored and the process with the outcome as presented is termed the “Integrated Solar-Nano Hydrogen System”. This process is designed to boost the production of Hydrogen gas using these two above named techniques in a single system. Solar energy is harnessed simultaneously by the use of solar cells and the heat collectors (Singh, 8). The PECs absorb light photons to be used directly as electricity to reduce water while the heat obtained from the heat collectors may be used to also generate electricity or to increase the temperature of the setup. This combined method in a system has proven to increase the overall yield of Hydrogen gas production. The overall energy cost is economically cheaper since the maximum benefit is derived from the solar energy used for the two techniques simultaneously in one system.
The use of nanotechnology in the industrial production of Hydrogen gas so far have been considered in the use of the harnessed solar energy from the sun directly or converted to heat. The direct use of the irradiation of the sun is made possible in the use of a nano technology based apparatus – the PEC – which is made using PV cells from semiconductor materials. Likewise, the use of heat derived from the conversion of solar irradiation is also made possible with the use of semiconductor nano materials. These two points stress the importance of semiconductor materials in the production of Hydrogen gas from water splitting using a cheap, green and renewable source of energy from the sun. Modification of the structure of the semiconductors in order to alter its electronic properties to improve the efficiency of the processes and the yield of Hydrogen are considered. These modifications using metallic deposition on the surface of the semiconductor is meant to make the resulting nano composite crystalline.
Hydrogen Gas production with Photo-catalysts
In a review, Preethi and Kanmani (2013) examined how efficient different nano materials were in the production of Hydrogen gas using photo-catalysis. This review gives a broad idea and overview of the different nano materials used for the industrial production of Hydrogen gas by the splitting of Hydrogen sulfide gas and the splitting of water molecules. Hydrogen Sulfide gas is detrimental to the environment because of its corrosive nature in the form of sulfide, its contribution to acid rain and the unpleasant odour it presents. The use of H2S to produce Hydrogen gas is therefore preferred to the splitting of H2O. This is not only because it helps remove the environmentally unpleasant Hydrogen Sulfide gas but also because it is less costly and faster to achieve. However, despite this preference and desirability for the use of H2S in the production of Hydrogen gas, the near future depletion of the non-renewable energy sources (such as coal and fossil fuels) which are the major sources of H2S, will result in the non-availability of the gas for the production of Hydrogen gas (Preethi and Kanmani, 2). The implication of this is that the splitting of H2O for the production of Hydrogen gas will outlive the splitting of Hydrogen Sulfide gas.
Of all the different methods that have been employed to split H2S to produce Hydrogen gas which includes thermo-chemical methods, plasma-chemical methods, thermal methods, electrochemical methods and photochemical methods, photochemical methods have the greatest potential because it uses a renewable energy source - solar energy. The light from the sun is used to drive the production of Hydrogen from the splitting of Hydrogen Sulfide in a photo-catalytic reactor through a process called photo-catalysis.
Equations for the photo-catalysis process
H2S + OH- HS- + H2O
H2S + OH- S2- + H2O
Photocatalyst e-CB + h+VB
2 S2- + 2 h+VB S22-
2H S- + h+VB S22- + 2H+
2H+ + 2 e-CB H2
The net reaction is simply
H2S H2 + polysulfide
The process of photo-catalysis is entrenched in the promotion of an electron from the valence band (VB) to the conduction band (CB) when light of sufficient energy is absorbed by a semiconductor material. An electron jumps the energy band gap into the conduction band when it becomes excited as a result of irradiation with light of sufficient wavelength and leaves a hole in the valence band. The difference in the energy levels of the lowest level of the conduction band and the highest level of the valence band is the energy band gap of the semiconductor material. For the promotion of electrons from the valence band to the conduction band, the wavelength of the absorbed light must be less than or the same as the energy band gap of the semiconductor material. The promotion of an electron into the conduction band leaves a hole previously occupied by the electron in the valence band. Thus an electron-hole pair is generated by the photo-catalysis process. The generated electron-hole pair can participate in one of many pathways. The pathways useful for splitting H2S and H2O to produce Hydrogen gas are those involving the loss of electrons in the conduction band and loss of holes in the valence band to electrochemical processes. These pathways are influenced by controlling the semiconductor with surface modifications and by adding electrolytes and reactive redox (reduction - oxidation) species.
Different types of nano materials have been identified as useful in the nanotechnology production of Hydrogen gas. These materials include metal oxide semiconductor materials (such as TiO2, BaTi4O9 etc) and metal sulfide semiconductor materials (such as CdS, RuS2 etc).
Most of the metal oxide semiconductor materials can only use Ultra Violet light due to their wide energy band gaps and cannot make use of sunlight (visible light). Some of the metal sulfide semiconductors can however produce Hydrogen gas but only in the presence of an electron donor or hole scavenger. The electron donor and hole scavenger agents react with the holes that are photo-generated and prevent the photo-generated electron-hole pairs from recombining thus allowing the photo-generated electrons to react with the Hydrogen Sulfide gas for the production of hydrogen (Alenzi et al., 11769).
Most nano materials used as photo-catalysts to produce Hydrogen gas have been identified to give a higher yield in the production of Hydrogen as compared to micro materials (Preethi and Kanmani, 564). Those that do not give a maximum production are still better in their yield than micro materials. The technique used to prepare the photo-catalyst plays an important role in the yield of the nano material photo-catalyst. The technique affects the particle size of the catalyst and its crystallinity. The smaller the particle diameter of the catalyst, the more active is the catalyst and consequently, an increase in the yield of Hydrogen production. Improvement in the efficiency of the photo-catalyst is also achieved by doping the nano materials with transition elements.
The industrial production of Hydrogen using photo-catalysis is done in a reactor designed purposely for this process. Either of two phases namely liquid phase and gaseous phase can be used in the photo-catalytic Hydrogen production. The phase used determines the type of nano materials used for the production.
In the liquid phase, the reactor is designed such that there are two different tubes one for the elimination of Nitrogen gas, and the other for the collection of hydrogen gas. The different photo-catalysts that can be used under irradiation by visible light from a lamp include CuFeO2, CuLaO262, FeGaO3, CuCr2O4/TiO2 etc. These photo-catalysts are referred to as spinel photo-catalysts.
Another type of nano material used for the liquid phase photo-catalysis is the Cadmium Sulfide and other materials based on it. The Cadmium sulfide can be used alone or doped with TiO2 or ZnS in the production of Hydrogen gas from Hydrogen Sulfide dissolved in water or alkali solution. As noted earlier that the method of production of the photo catalyst influences the activity of the catalyst, the Cadmium Sulfide photo-catalysts are fabricated through solvo-thermal and sol-gel techniques to make it highly active for Hydrogen gas production from water containing Sulfide ions under irradiation with visible light. Cadmium Sulfide-based photo-catalysts produce similar results even in a system using Hydrogen Sulfide dissolved in NaOH solution. The semiconductor has the capability to absorb visible light but not in pure water because it undergoes anodic photodecomposition (Preethi and Kanmani, 566). That is why electron donors have to be added to the solution to prevent this.
Nano structured Zinc Oxide (ZnO) is also a very stable semiconductor material with photo-catalytic activity needed for photo-catalysis. Although due to the high band gap, photo excitation of Zinc Oxide to produce electron-hole pair needed for the production of Hydrogen gas from Hydrogen Sulfide gas needs Ultra Violet (UV) light (Preethi and Kanmani, 569). The use of visible light is however made possible when the Zinc Oxide is doped with Copper which has a low band gap and good crystallinity.
Colloidal semiconductor nano crystals are also used as photo catalysts by controlling the electrical and optical properties of the materials (Preethi and Kanmani, 569). These materials use hole scavengers to remove holes from the nanostructure and prevents the recombination of the photo-generated electron-hole pair and stabilizing the nano crystals against photo oxidation thereby facilitating the production of Hydrogen gas. Generally, the use of hole scavengers in the process is considered a disadvantage since fresh hole scavengers need to be continuously supplied in order to maintain the photo-catalytic activity. This however is a plus economically and environmentally. This is because the Sulfides used as hole scavengers are by-products of fossil fuels and so are cheap and lessen the burden of pollution of such by-products on the environment (Preethi and Kanmani, 562).
Fabrication of Photo-catalyst Nano composite films
(Alenzi et al., 2010) also proposed a high efficiency, high throughput method of using photo-catalyst films for producing Hydrogen gas from water splitting using solar energy.
In harnessing the energy from the sun, semiconductors have been used in the form of nano films to produce photovoltaic solar cells. In this research effort, the synthesis and use of nano films is being extended to hydrogen production as a photo-catalyst through the splitting of water by electrochemical means.
The photo-catalyst used is the anatase Ag/TiO2 nano composite film. The production of this film is described for completeness. The precursor solution used was composed of Titanium (IV) isopropoxide, HCl and isopropyl alcohol. Polished pyrex wafers cleaned in piranha solution for purification was used to hold the precursor solution and heat it to a temperature of about 5000C for 5 hours. The pyrex wafers are transparent in the near Ultra Violet range. After the anatase crystal of TiO2 is formed, Silver nitrate was used to reduce silver from the nitrate solution using a UV light source. The characterization of the nano composite Ag/TiO2 films was done with the aid of Scanning Electron Microscopy and X-ray photoelectron spectrometry produced the x-ray diffraction patterns.
The photo-electrochemical production of Hydrogen gas with the anatase Ag/TiO2 thin film sample was done in a pyrex flask containing a mixture of water and methanol in the volume ratio 1:1. Since the pyrex flask is transparent to light in the UV range, UV light was used to irradiate the pyrex flask.
hv e- + hole+
4 hole+ + 2H2O O2 + 4H+
2H+ + 2e- H2
Overall reaction: 4hv + 2H2O O2 + 2H2
A consideration and analysis of the results obtained in the course of the experiment will give further insight into the properties of the anatase Ag/TiO2 thin film that made it useful as a nano material in the photo-electrochemical production of Hydrogen gas.
The anatase TiO2 film had a low energy bandgap as a result of the higher absorbance wavelength of the material. The crystal structure of the TiO2 film as revealed by the X-ray Diffraction patterns characterization was a consequence of the presence of silver on the surface of the film. It has been confirmed that the crystalline form of TiO2 has increased photo-catalytic activity compared with the amorphous TiO2 (Preethi and Kanmani, 569). To confirm the effect of the Silver deposited on the surface of the anatase TiO2, a control experiment carried out without the use of TiO2 film without depositing Silver on the surface. The outcome clearly showed a tremendous reduction in the rate of production of Hydrogen gas (Alenzi et al., 11774).
The implication of this is that the crystalline structure of the photo-catalyst contributes a great deal to increasing the rate of Hydrogen production. This crystalline structure is achieved with a novel nano technology technique of deposition of Silver on the surface of the TiO2 film. This modified the electronic structure of the thin film to increase photo-activity.
Alenzi, Naser, Liao, Wei-Ssu, Cremer, Paul S., Sanchez-Torres, Viviana, Wood, Thomas K., Ehlig-Economides, Christine and Cheng, Zhengdong. "Photoelectrochemical Hydrogen Production from Water / Methanol Decomposition using Ag/TiO2 Nanocomposite Thin Films". International Journal of Hydrogen Energy, 35 . Elsevier (2010): 1768 - 11775
Mahinder Singh, B.S. "Nanotechnology in Solar Hydrogen Production", Springer-Verlag (2010) El. Print.
Preethi, V. and Kanmani, S. "Photocatalytic Hydrogen Production". Materials Science in Semiconductor Processing. Elsevier (2013): 561 - 575