The University of Michigan in Ann Arbor has created a stable artificial photosynthesis device that harnesses sunlight to break apart fresh and salt water, generating hydrogen that can be used in fuel cells. The device could also be reconfigured to turn carbon dioxide back into fuel.
Hydrogen is the cleanest-burning fuel with water as its only emission. However, hydrogen production is not always environmentally friendly, as conventional methods require natural gas or electrical power. The new method of generating hydrogen uses only water and sunlight in a process called direct solar water splitting.
“If we can directly store solar energy as a chemical fuel, like what nature does with photosynthesis, we could solve a fundamental challenge of renewable energy,” says Zetian Mi, a professor of electrical and computer engineering at U-M who led the research while at McGill University in Montreal.
Faqrul Alam Chowdhury, a doctoral student in electrical and computer engineering at McGill, says the problem with solar cells is they cannot store electricity without batteries, which have a high overall cost and limited life.
The new device is made from the same materials as solar cells and other electronics, including silicon and gallium nitride, giving it an industry-ready design. It operates with just sunlight and seawater, paving the way for large-scale production of clean hydrogen fuel.
Mi and his team achieved more than 3 percent solar-to-hydrogen efficiency. Previous direct solar water splitters have achieved a little more than 1 percent stable solar-to-hydrogen efficiency. Other methods use costly, inefficient, or unstable materials such as titanium dioxide that also might involve adding highly acidic solutions to reach higher efficiencies.
“Although the 3 percent efficiency might seem low, when put in the context of the 40 years of research on this process, it’s actually a big breakthrough,” says Mi. “Natural photosynthesis, depending how you calculate it, has an efficiency of about 0.6 percent.”
To reach the higher efficiency, the researchers built a nano-sized cityscape of gallium nitride towers that generate an electric field. The gallium nitride turns light, or photons, into mobile electrons and positively charged vacancies called holes. These free charges split water molecules into hydrogen and oxygen.
Mi says 5 percent efficiency is the threshold for commercialization, but the researchers are aiming for 20 or 30 percent efficiency.
While the silicon backing of the chip does not contribute to its function, it could be doing more. The next step may be to use the silicon to help capture light and funnel charge carriers to the gallium nitride towers.
Mi conducts similar research to strip carbon dioxide of its oxygen to turn the resulting carbon into hydrocarbons, such as methanol and syngas. This research could remove carbon dioxide from the atmosphere like plants do.
The new device is documented in the study “A photochemical diode artificial photosynthesis system for unassisted high efficiency overall pure water splitting,” published in Nature Communications.