Solar energy is the perfect energy source, except for one thing: you can’t store it, at least, you couldn’t until now. Five years ago Caltech established the Joint Center for Artificial Photosynthesis (JCAP) along with its partnering institution the U.S. Department of Energy’s Energy Innovation Hub with one goal: creating a cost-effective system that, like plants, uses only sunlight, water, and carbon dioxide to create storable energy. Over the past five years, JCAP researchers at have made major advances toward this goal, and yesterday they reported that they have, in fact, created the first complete, efficient, safe, integrated solar-driven system for splitting water to create hydrogen fuel.
Nate Lewis, a Caltech professor of chemistry, and the JCAP scientific director explains, ”This result was a stretch project milestone for the entire five years of JCAP as a whole, and not only have we achieved this goal, we also achieved it on time and on budget.”
Harry Atwater, director of JCAP and Howard Hughes Professor of Applied Physics and Materials Science says, "This accomplishment drew on the knowledge, insights and capabilities of JCAP, which illustrates what can be achieved in a Hub-scale effort by an integrated team. The device … grew out of a multi-year, large-scale effort to define the design and materials components needed for an integrated solar fuels generator.”
The new system, referred to as the artificial leaf, has the same components as a hydrolysis system you may have seen in your high school chemistry class: two electrodes: an anode and a cathode, and a membrane. But in the solar powered artificial leaf system, the anode is a photoanode and the cathode is a photocathode. The photoanode uses sunlight to oxidize water molecules. This generates protons, electrons, and oxygen gas. The photocathode recombines the protons and electrons to form hydrogen gas. A key innovation in the JCAP design is the plastic membrane that keeps the oxygen and hydrogen gases separate. This is crucial, because if the two gases were allowed to mix they would form a highly volatile and highly flammable compound that could easily be accidentally ignited and cause an explosion. The artificial leaf’s membrane allows the hydrogen fuel be separately collected under pressure and safely pushed into a pipeline.
Another of the JCAP team’s important innovations is their choice of material to use in conjunction with their semiconductors. Silicon and gallium arsenide absorb light efficiently and are therefore commonly used in solar panels. But, these materials also oxidize (or rust) when they are exposed to water, meaning they can not be used to directly generate fuel in a hydrolysis system. The team’s research found that adding a nanometers-thick layer of titanium dioxide (TiO2) to the outsides of the electrodes could prevent them from corroding but still allow light and electrons to pass through.