The 21st Century Solar Army 1 (2015)

Thank you, Barbara Grosz, for that very nice introduction. I don't know who invited me to give this talk because I'm not sure how the Society works. It's very mysterious, but whoever did, thank you very much. I've been coming to these wonderful meetings since I was elected in 2000. I just love them. I've made lots of great friends and now I have an opportunity (at last!) to speak to people my own age.Let's get off and running. There's lots of sunlight; we've got to figure out how to use it productively to make fuels and chemicals. We are going to stop using oil, gas, and coal, maybe not in this century but certainly in the next. Therefore, it's about time we started working on renewables. Fortunately, we have four great resources: sunlight, seawater, nitrogen from the air, and carbon dioxide. Starting with these resources, we will have to figure out how to make everything-not only fuel but also pharmaceuticals and polymers. Everything that we now get on the cheap from oil, gas, and coal, we will have to learn to make ourselves. So let's get started.I'm sure you're familiar with silicon solar cells, which people put on their roofs to generate electricity. We have had silicon solar cells with 10% conversion efficiencies for some time, and now we have 21%-efficient silicon solar cells but very few people are using them. We also have 28%-efficient gallium arsenide, but even with 10%-efficient silicon, we could provide electricity to much of our country. Nate Lewis and I picked out a part of the country to cover with silicon solar cells, a part where nobody lives (we think!), but if any of you in the audience live in that region, we will be happy to move you to Las Vegas. Just kidding! We must find better ways to power the planet.So, what are we doing? We are covering wasted spaces with solar cells. At Caltech, for example, we have several solar power stations. We put solar cells on the roofs of parking garages to generate electricity, which we sell to Los Angeles during the day and buy back at night. I can tell you that lots of universities are now converting to solar; for example, Arizona State has a big solar footprint, and Occidental College, which is close to Caltech, has a solar power station. Even the Pope is installing solar!The big challenge in the field is to store solar energy by making fuel, which can be made by splitting water into hydrogen and oxygen.2 Good solar water splitters will need to include components that absorb all the light that reaches the earth's surface filtered by the ozone layer. We started with molecular light absorbers to do this job but soon realized they're not going to make it in the long run, so now we use semiconductors with variable bandgaps to capture solar photons.Upon light excitation in a semiconductor, one electron and one hole are separated briefly. However, chemistry involves multiproton and multielectron processes, such as splitting water into hydrogen and oxygen. So, we have to couple the electrons and holes that are generated in semiconductors with catalysts to drive the desired chemical reactions. We will need to make integrated photoanodes and photocathodes to put into solar water splitting devices.3Andy Maverick and I laid out the territory for solar water splitting by molecular absorbers and catalysts many years ago.4 Since then, I've trained many brilliant students who like to show the old guy up; turns out two, Nate Lewis and Dan Nocera, are solar gurus. These guys wrote a big paper on solar fuels in the Proceedings of the National Academy of Sciences.5 Of course I responded by writing a one-page paper that summarizes all the important points they attempted to make.6 I'm just kidding! Give me a break; I love my former students.In the late '90s, John Turner's group at the National Renewable Energy Laboratory (NREL) made a big advance.7 He and his NREL coworkers showed us that artificial photosynthesis is not just a pipe dream: they constructed a solar-driven device that split water over 10 times more efficiently than can be done by green plants. …