Hydrogen - A New Fuel

Hydrogen is the simplest molecule present in the universe. Production and use of hydrogen represent a potential alternative source of fuel. It can be easily collected, stored (as gas, liquid or hydrides of metals) and transported (by trucks, ships or trains). Hydrogen can be piped or transmitted over wires for a distance of over 10 km. After the use, hydrogen does not pollute the environment. For the production of hydrogen, water serves as a source of raw material. The bond between hydrogen and oxygen in water can be broken by providing necessary energy by heat, electricity or light photons as below:

	hy (photos)
H2O		½ O2 + H2
	Catalysts

Based on the types of energy used, the following categories of splitting water have been made :

1. Electrolysis: Electrical splitting
2. Thermolysis : Splitting of water by heat,
3. Thermochemical lysis : Splitting of water by both heat and chemical catalysts.
4. Photolysis : Splitting of water by light.

The first three approaches have been already in practice, whereas photolysis of water has much future prospect as far as seeking of alternative source of energy is concerned.

Solar energy available on earth surface constitutes an abundant and free energy source. It can be converted directly into heat, mechanical energy, electricity or fuel. Nowadays several processes have been envisaged for the conversion of optical energy to chemical energy. These processes arc given below:

(i) Photo-chemical Process : Hydrogen is produced by using photocatalysts such as compound salts and photosynthetic dyes.

(ii) Photo-electro-chemical (PEC) Process : In this process semiconductor photocatalysts (e.g. Sr. Ti O₃) are used for the production of hydrogen. Based on semi-conductor system PEC cells have been designed. PEC cells offer the unique advantages for converting optical energy to chemical energy. It is the most efficient chemical system designed so far.

(iii) Photobiological Process : This process involves the splitting of water by using natural or synthetic chlorophyll, algae and bacteria.

Photobiological Process of H₂ Production
Biophotolysis of water refers to break down of water and production of oxygen and hydrogen by biological process. Hydrogen production is brought about by the following means :

Hydrogenase and H₂ Production
In the early 1960s, production of hydrogen was demonstrated by using chloroplasts isolated from spinach (Spinacia oleracea) in the presence of artificial electron donors and bacterial extracts containing hydrogenase. Electron donors (organic compounds) transfer electrons to photosystem I of the chloroplast from where electrons are received by electron carriers (e.g. ferredoxin). Hydrogenase accepted electrons from the electron carrier as shown below. The organic compounds which acted as electron donor also served as a source of hydrogen (Sasson, 1984).

	e		e		e
Electron donor		Photo system (I)		Electron carrier (H+)		Hydrogenase		(H2)

Hall et al (1980) demonstrated the production of 50 micromolecule of H₂/mg chloroplast/h for about 6 hours from a mixture of chloroplast and bacterial (Clostridium) hydrogenase at pH 7 and temperature 25°C.

In the visible light, hydrogenase separates high energy electrons from ferredoxin and facilitates their transfer to H⁺; ultimately H₂ is evolved. Those plants which produce carbohydrates lack hydrogenase. This enzyme is restricted in a variety of bacteria, cyanobacteria and other green algae. Hydrogenase of these plants produces H₂ in the absence of CO₂.

Sensitivity of hydrogenase to O₂ varies with species. It is, however, unlikely to develop an efficient system for hydrogen production if considerable amount of O₂ is produced. Therefore, for the production of hydrogen from cyanobacteria addition of a suitable inhibitor which may stop photosystem II and produce free hydrogen has been suggested (Mukherjee et al., 1981). Research works are being done on the production of hydrogen from organic waste by using many anoxygenie phototrophic bacteria, water-using cyanobacteria or green algae grown in light condition (Klibanov, 1983). The algae which possess hydrogenase are given in Table 20.3.

Table 20.4. Algal species containing hydrogenase

Groups	Species
Blue-green algae (Cyanobacteria)	Anabeana azollae, A. cylinderica, Anacystis elongata, Nostoc muscorum, Spirulina platensis, Synechococcus elongatus
Green algae	Chlamydomonas moewusii, Chlorella fusca, C. homosphaera, C. kessleri, C. sorokiniana, Ulva lactuca
Brown algae	Ascophyllum nodosum
Red algae	Ceramium rubrum, Chondrus crispus, Corallina officinalis Porphyra sp, Porphyridium cruentum

There are many bacteria which contain hydrogenase as they possess nitrogenase for nitrogen fixation. Efficiency of nitrogen fixation in heterocystous cyanobacteria and root nodule bacteria (e.g. Rhizobium japonicum) is governed by the efficiency of hydrogenase (see Presence of hydrogenase).

Moreover, N₂ fixation in root nodule bacteria does not operate efficiency and about half the electrons are used for H₂ production. In USA, it has been estimated that the soybean plantations leak about 30 billion m³ H₂/annum which has an energy equal to about 300 billion square feet of _o natural gas (Brill, 1977). From this system production of an adequate amount of H₂ can be done by allowing the ATP to produce H₂ under conditions of low partial pressure of N₂, low pH and high temperature.

In the 7th International Symposium on Biotechnology, held in New Delhi (from February 19-25,1984), Dr. A. Mitsui of the University of Miami (U.S.A.) emphasized the production of H₂ by using blue-green algae (e.g. Synechococcus). This alga fixes atmospheric nitrogen under aerobic conditions and also produces H₂ and O₂ by splitting water in light conditions. Mitsui’s group is currently working on sulphur-tolerant photosynthetic bacteria for growth and H₂ production.

Halobacteria
Recently, a group of microorganism, the purple bacteria, has been discovered which is another potential source of hydrogen. Two species of Halobacterium, e.g. H. halobium and H. curtirubrum, are known. Halobacteria are rod-shaped and physiologically a unique bacteria, as they are highly halophilic (salt loving). They differ from other bacteria in respect of cell-wall and energy producing mechanisms. They require the high concentrations of salt (e.g. 3-4 M sodium chloride) and low amount of oxygen. Therefore, they can grow in saline water i.e. saline lakes and sea. In salt saturated (> 3M NaCl) and oxygen poor environment they utilize sunlight to produce ATP and preserve their structure. When salt concentration reaches below 3 M the rod shaped cells become irregular or spherical. Below 1 M NaCl concentration both cell membrane and cell disintegrate. It is not known how NaCl helps in growth and integrity of the cells. No salt, like sodium chloride has been found to support growth of the cells.

Bacteriorhodopsin. In saline condition the bacteria survive only by incorporating a purple pigment to cell membrane in sunlight. This pigment is commonly known as bacteriorhodopsin (bR). It is linked with retinal in 1:1 ratio. Retinal is the aldehyde form of vitamin A which after reduction yields vitamin A, the retinol (alcohol form). The pigment is constituted by protein (75 per cent) and lipids (25 per cent) and occupies about 50 per cent area of the total cell surface. The pigment occurs on cell membrane in separate patches of 3 molecules. Each patch contains about 1,20,000 molecules of the pigment. A total of 40,000 patches are found on each purple membrane. The three molecules work synergistically (Stoeckenius, 1976). The bR molecules function normally even after destroying the bacterium only if the pigment is not disorganized.

Presence of light plays a major role in the synthesis of the pigment. Halobacterium synthesizes a purple membrane in the presence of light and red membrane in the absence of light.

W. Stoeckenius and coworkers of the University of California (U.S.A.) have shown that in the presence of light, Halobacterium survives even in anaerobic condition, but in the same condition, in dark it dies rapidly. Bacteriorhodopsin exists in two forms according to light and shade conditions. In dark it exists as bR 560 and in light as bR 570. The two forms are inter-conversible, which undergo fastly a series of photoreaction cycle when they are illuminated.

Bacteriorhodopsin acts as a light driven proton pump and the changes occur in about 10 milli second. During photo-reaction cycle bR molecules take up the protons on the inner surface of membrane and release them on outer surface. Proton concentration gradient develops on cell membrane and electric potential is generated in the same way on inner and outer cell membrane. Consequently light energy is converted into electrochemical gradient (Stoeckenius, 1976).

Each bR molecule pumps about 200 H⁺ per second under light condition. Thus, the number of protons pumped per second per sheat is 24 x 10⁶ (1,20,000 x 200 = 24 x 10⁶). It has also been estimated that each molecule of the unit patch of the bR pigment is capable of generating about 200 mV photopotential if the rate of proton pump per second is 200.

Biocells
Prentis (1981) has found that membrane fragments of some of strains of, halobium contain 1,00,000 bR molecules and a 10 liter bacterial culture would yield 0.5 gram of purple membrane. Now, it has become possible to sandwich the purple membrane and lipid between two platinum electrode. In this device the photovoltages are generated as a result of photochemical conversion of purple pigment after illumination. This voltage can be measured. This device is termed as biocell (Hwang et al, 1978). The device of preparing (biocells can be improved by replacing the eleptrodes (e.g. Ag-AgCl electrode or others). Therefore, due to increasing energy demand utilization of solar energy by designing biocells offers a great potential-. However, much work is needed to make it cheap and popular.