Production of Hydrogen without electricity from
Although a renewable energy source in conjunction with electrolysis would eliminate the dependence on fossil fuels, it still requires the production of electricity in the first place. The overall efficiencies of these processes are thereby reduced. Alternative methods without the need for electrical power include:
Photoelectrochemical (PEC) hydrogen
PEC systems use sunlight directly to generate sufficient energy to split water into hydrogen and oxygen. The advantage over conventional electrolysis using photovoltaic is the elimination of an electrical current network and the associated current transmission losses.
Biological Photolytic Hydrogen
Another way to directly tap solar energy for hydrogen production is to take advantage of certain microalgae and photosynthetic bacteria that sometimes use photosynthesis to make hydrogen instead of sugar and oxygen. However, the algal enzymes that trigger hydrogen production are inhibited by oxygen, so bioengineering of enzymes or a whole new organism would be required to make this process even remotely practical.
Conversion of Biomass and Wastes
Hydrogen can be produced via pyrolysis (thermochemical conversion) or anaerobic# digestion (fermentation) of biomass resources such as agricultural residues, wastes including plastics and waste grease; or biomass specifically grown for energy uses. Specific research areas include reforming of pyrolysis streams and development and testing of fluidizable catalysts.
Scientists are also working on “dark fermentation” reactions which do not do not require light energy at all. Here, a variety of bacteria ferment sugars and produce hydrogen using multi-enzyme systems. Sugars are relatively expensive substrates so engineering pretreatment technologies to convert lignocelluloses biomass into sugar-rich feedstock including hemicelluloses and cellulose that can be fermented directly to produce hydrogen, ethanol, and other high-value chemicals will be needed.
Solar Thermal Water Splitting
Water usually decomposes at temperatures of more than 2,500°C into hydrogen and oxygen. Researchers have demonstrated that highly concentrated sunlight can be used to generate these temperatures. However, catalysts based on metals or inorganic sulfur compounds can lower the heat needed to the more moderate range of 800 – 1,200°C. Such high-temperature, high-flux solar driven thermo-chemical processes offer a novel approach for the environmentally benign production of hydrogen, and is explored in more detail here:
All the above mentioned methods are still in experimental phases and capable of supplying only small amounts of hydrogen. It seems that many technical, economical, and even mental hurdles need to be overcome before widespread commercial scale production of clean hydrogen is possible.