"Since the 1970s, chemists have worked on storing solar energy in molecules that change state in response to light. These photoactive molecules could be the ideal solar fuel, as the right material should be transportable, affordable, and rechargeable. Unfortunately, scientists haven’t had much success. One of the best examples in recent years, tetracarbonly-diruthenium fulvalene, requires the use of ruthenium, which is rare and expensive.

Many cellulosic fuel producers are working with enzymes to break down tough, inedible plant parts, such as corncobs or switch grass, into simpler sugars that can be fermented to ethanol.