POSITION PAPER ON HYDROGEN ECONOMY
b.
Photo fermentation
Another photosynthetic pathway is photo fermentation that utilises light energy to convert organic acids such as acetate, lactate butyrate to H2 and catalytic CO2 under anaerobic anoxic conditions. The microorganism that utilises production of Hand activity, and suppression of nitrogenases 2, including low this pathway is the purple nonsulfur (PNS) photosynthetic bacteria, including Rhodobacter species. expression by NH4 and lower photochemical ability. Genetic manipulation of the PNS The PNS perform the fermentation using nitrogenases when in the absence of ammonium. Although there is no presence of O2, the bacteria could overcome the disadvantages. nitrogenases showed drawbacks in the production of H2, including low catalytic activity, suppression of nitrogenases expression by NH4 and lower photochemical ability. Genetic manipulation of the PNS bacteria could overcome the c. Dark fermentation disadvantages. c. Dark Facultative andfermentation obligate anaerobic bacteria are mainly used for generating H2 through dark fermentation pathway. Unlike photofermentation and photolysis, dark fermentation is carried in thebacteria dark atareroom temperature and often gives H2 Facultative and obligate out anaerobic mainly used for generating H2 through thehigher dark fermentation pathway. production rate: 4 and 2and moles of molecular H2 for is acetate andin butyrate Unlike photofermentation photolysis, dark fermentation carried out the dark atpathways room temperature and often respectively from mol of glucose degraded. The primaryH2drawback thebutyrate low yield of gives a higher H2 1production rate: 4 and 2 moles of molecular for acetateisand pathways, respectively, H2 from on substrates due degraded. to formation of various by-products. To ofovercome this issue, 1 mol of glucose The primary drawback is the low yield H2 on substrates due to the formation of culture conditions, including thethis C/Nissue, andculture C/P ratios, carbon sources, pH,and temperature, various by-products. To overcome conditions, including the C/N C/P ratios, carbon sources, pH, andtemperature, effect of metal ionsofonmetal enzyme are being are studied. and effect ions on enzymes, being studied. Microbial ElectrolysisCell Cell d. d.Microbial Electrolysis Microbial electrolysis cell (MEC) is a modified Microbial fuel cell (MFC). Certain types of bacteria on the anode (called
Microbial electrolysis cell (MEC) is a modified Microbial fuel cell (MFC). Certain types exoelectrogens) oxidise organic material to CO2 and proton, and release electrons to the anode. In MEC, electrons are of bacteria on the anode (called exoelectrogens) oxidize organic material to CO and used to reduce protons and produce H2. Figure 42 shows Hydrogen production from microbial2electrolysis cells. proton, and release electrons to the anode. In MEC, electrons are used to reduce proton and produced H2. Figure 40 shows Hydrogen production from microbial electrolysis cell.
Figure 42: Hydrogen Production from Microbial Electrolysis Cell
Figure 40: Hydrogen Production from Microbial Electrolysis Cell
However, this reaction does not occur spontaneously due to the thermodynamic reason. In practice, 400 to 1,000 mV is usually applied to the circuit to overcome energy losses due to electrode over-potentials and internal resistance. Platinum cathode and Nafion membranes are commonly used to catalyze H2 evolution and
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