Generate Hydrogen from Sun

Hydrogen is an ideal, clean, carbon-free carrier of energy that produces only water vapor as a waste product and has potential applications in automobiles, airplanes, and also in home-heating.

The projected mean power generation for the years 2050 and 2100 has been estimated to be 28 and 46 terawatt (TW), respectively. By far, sunlight provides the largest of all carbon-neutral energy sources. In fact, 14 TW of solar energy falls on the earth every hour. Thus, more energy from sunlight strikes the earth in 1 h than all the energy consumed on the planet in a year.

The most successful technologies taking advantage of this resource are photovoltaics (PVs; solar electricity), a $10 billion industry that is currently growing at a rate of 35–40% each year. Continued growth of the PV sector at a rate of ~25% would increase the production level from 1.7 GW in 2005 to 380 GW in 2030, and thus would satisfy a significant fraction of the world energy demand. Moreover, among the renewable sources, PVs have the highest potential to reduce the costs compared to biomass, geothermal, wind, and solar thermal.

Photoelectrolysis of water and especially the use of semiconductor–electrolyte interfaces illuminated with sunlight for the production of hydrogen from water and other suitable solvents have been reviewed in several excellent publications. At present, only about 5% of the commercial hydrogen production is primarily via water electrolysis, whereas the other 95% is mainly derived from fossil fuels. This does not represent inhouse consumption of hydrogen such as oil refi neries and ammonia plants where the bulk of hydrogen is consumed.

There are two principal methods for the electrolysis of water to generate hydrogen viz. active, that is, photoassisted and passive or dark and nonphotoassisted. The active method consists of utilization of photogenerated charge carriers in the electrolysis of water and other products. In the passive method, the electrolysis is carried out in the dark at low (80°C) temperatures using an alkaline electrolyte such as NaOH or proton exchange membrane cells involving polymeric sulfonic acids, at intermediate (200–500°C) temperatures for which suitable electrolytes are still being sought, and at high (>800°C) temperatures using oxide electrolytes such as yttria-stabilized zirconia.