Recent developments in solar water-splitting photocatalysis

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Driving force for cheap solar fuels The world’s population is increasing and is expected to continue increasing until at least mid-century. All of these new inhabitants will require water, food, shelter, transportation, and consumer products, all of which need energy to produce and/or deliver. The population increase, along with the rapid growth of many developing economies, indicates that the demand for energy may more than double by mid-century. Even though they are a finite resource, fossil fuels would be capable of meeting this demand, albeit at an increasing cost as the easy-to-extract sources are depleted. However, the increasing concentration of CO2 in the atmosphere, as a result of the combustion of these carbon-based fuels, is predicted to result in unacceptable changes in the Earth’s climate. In addition, pollution such as sulfur dioxide emissions and oil spills also point to the need for carbon-free alternative energy sources. Nuclear power and wind are both commercially viable carbon-free technologies that can contribute to the present and future energy mix. However, finding a viable way to tap solar energy, the most abundant and universally available source of renewable energy, is urgently needed.1 One problem with solar energy is that it is quite diffuse, requiring large areas of solar collectors to harvest significant amounts of power. Therefore, cost reductions in the production of thin-film solar cells are needed to produce solar electricity at a cost that can compete with conventional power sources if the costs of carbon

emissions and other pollution are included. The other “Achilles’ heel” of solar energy is that it is difficult to store as fuel for nighttime and transportation uses. Since fuels account for more than 70% of current energy consumption, there is an urgent need to convert solar energy directly to fuels, ideally liquid fuels that are easy to transport and store. To try and quickly develop systems to directly convert solar energy to fuels, the Department of Energy recently awarded a five-year $125 million research grant to establish a “Solar Hub” to improve what are now only ideas or inefficient laboratory demonstrations into a viable solar fuels technology operating at 10% efficiency or higher.

Solar-based water splitting methods Plants can convert and store solar energy as complex molecules, such as carbohydrates and other biomass; however, photosynthesis is not very efficient, and the stored energy in plants and the large amounts of arable land needed to grow them compete with other land uses such as food production. Storing solar energy by splitting water into hydrogen and oxygen has long been considered a promising idea; in fact, in 1874, Jules Verne, recognizing the finite supply of coal and the possibilities of hydrogen derived from water electrolysis, made the comment that “water will be the coal of the future” (Jules Verne, The Mysterious Island (available at http://www.literatureweb.net/verne/mysteriousisland, 1874). However, as of yet,

Frank E. Osterloh, University of California,

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