Stanford chemical engineering professor Thomas Jaramillo is on a mission to save the planet, one artificial photosynthetic leaf at a time.
Speaking to a large audience on the lawn outside the Thursday evening, Jaramillo cited President Obama's 2011 State of the Union address, in which he called for America to get 80 percent of its power from “clean energy” sources by 2035. He cited converting sunlight and water into fuel as one of the ways to reach this goal.
Jaramillo acknowledges that the world is now producing and consuming 16 trillion watts of power. “That’s equivalent to every man, woman and child on the planet using two industrial strength hair dryers,” he said. Eighty percent of the power comes from fossil fuels with coal, natural gas and petroleum each contributing about one-third.
There are two major issues of concern with fossil fuels, however. One is global climate change due to releasing carbon dioxide into the atmosphere, he said, and the other is that “we’re energy hogs and we’re dependent on imported energy.”
However, Jaramillo said, “I love fossil fuels!” He listed their advantages. They’re easy to store and transport. They’re stable and have high energy density. He said a Honda Accord with 18 gallons of gas in its tank has the same energy as 100,000 fully charged iPhone batteries.
And even at $4 per gallon gasoline is as cheap as milk.
Nevertheless Jaramillo and others are working on developing new technologies for making fuels from sunlight. In July the Department of Energy awarded $122 million for five years to the Joint Center for Artificial Photosynthesis (JCAP). Stanford is one of the institutions participating in JCAP.
Jaramillo showed that existing technologies for converting sunlight to electricity have deficiencies. Photovoltaics are efficient but extremely expensive. Solar cells are at best three times more costly than electricity from fossil fuels.
“Technology exists for us to do anything we want but cost drives what we use,” Jaramillo said. He and his colleagues at Stanford have estimated the costs of plants based on “artificial leaves” that convert sunlight and water into hydrogen. Assuming 15 percent efficiency, a ten-year replacement cycle and material costs of $200 per square meter, the fuel cost turns out to be about $3 per kilogram of hydrogen.
What we need are materials that are non-toxic, stable in water, and inexpensive. Jaramillo’s team is looking at molybdenum disulfide, MoS2. This could be the photocathode that pulls the hydrogen out of the water. A mixed metal oxide that absorbs a different wavelength of light than that absorbed by the photocathode would act as the photoanode to pull out the oxygen.
Another approach is to find materials that break down carbon dioxide into methane.
Jaramillo concluded his talk by reiterating that we need to develop these technologies as soon as possible because they will take decades to implement and replace our existing fossil fuel systems.