Bioproduction of Hydrogen with the Assistance of Electrochemical Technology

Abstract

The depletion of fossil fuel diverts us to the use of renewable resources as the supplement for fuel. Solar, hydroelectric power and microbial system are known to be abundant renewable resources for fuel production. Hydrogen has a energy yield of 122 kJ/g, which is 2.75 times greater than hydrocarbon fuels (Kapdan & Kargi, 2006). Hydrogen together with oxygen is the key element in the biological energy cycle on the earth. In all organic matter hydrogen atoms are bound to carbon, nitrogen, sulphur and other elements. Biological processes for the production of hydrogen, which are environment-friendly and less energy intensive, may be categorized into bio-photolysis, photo-fermentation and dark fermentation. Bio-photolysis occurs in organisms such as green algae or cyanobacteria, which carry out plant-type photosynthesis, using captured solar energy to split water. Non-sulphur purple photosynthetic bacteria undergo photo-fermentation to perform an anaerobic photosynthesis. By dark fermentation, a variety of different microbes anaerobically breaks down carbohydrate rich substrates to hydrogen and by-products (Das & Veziroglu, 2001; Hallenbeck & Benemann 2002). Gaseous hydrogen is produced as well as consumed by living microorganisms in the presence or absence of oxygen (under both oxic and anoxic conditions). The anoxic condition is observed during dark fermentation of microbes. Among the processes, dark fermentation presents a high rate of hydrogen production, using fermentative bacteria, such as Enterobacter species (Palazzi et al., 2000; Kumar & Das, 2000; Kumar & Das, 2001; Nakashimada et al., 2002; Kurokawa T Zhang et al., 2005; Shin et al., 2007), Clostridium species (Chin et al., 2003; Lee et al., 2004; Levin et al., 2006; Jo et al., 2008) and Escherichia coli (Yoshida et al., 2005). Hydrogen production through bacterial fermentation is currently limited to a maximum of 4 moles of hydrogen per mole of glucose, and under these conditions results in a fermentation end product (acetate; 2 mol/mol glucose) that bacteria were unable to further convert to hydrogen. Thermophiles produced up to 60–80% of the theoretical maximum, demonstrating that higher hydrogen yields can be reached by extremophiles than using mesophilic anaerobes (Chin et al., 2003). The oxidative pentose phosphate pathway as an alternative metabolic route exists for example in microalgae, which can produce stoichiometric amount of H2 from glucose. However, this pathway is usually not functional for energetic reasons (Lee et al., 2004). Several problems still remain for the commercial scale production of bio-hydrogen including low hydrogen yield. Alternatively the by-products are to be used by