http://www.scientificamerican.com/article.cfm?id=reinventing...

In photosynthesis, green leaves use the energy in sunlight to rearrange the chemical bonds of water and carbon dioxide, producing and storing fuel in the form of sugars. “We want to make something as close to a leaf as possible,” Lewis says, meaning devices that work as simply, albeit producing a different chemical output. The artificial leaf Lewis is designing requires two principal elements: a collector that converts solar energy (photons) into electrical energy (electrons) and an electrolyzer that uses the electron energy to split water into oxygen and hydrogen. A catalyst—a chemical or metal—is added to help achieve the splitting. Existing photovoltaic cells already create electricity from sunlight, and electrolyzers are used in various commercial processes, so the trick is marrying the two into cheap, efficient solar films.

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The problem is that commercial solar cells contain expensive silicon crystals. And electrolyzers are packed with the noble metal platinum, to date the best material for catalyzing the water-splitting reaction, but it costs $1,500 an ounce.

So the major hurdle for the electrolyzer component is platinum, because it is rare and expensive. But solar / photovoltaic-derived energy is the future. Ergo: investments in platinum could be very lucrative.