By Peter Weiss
Here’s a recipe for a cleaner, healthier planet: Take some water, add solar energy, extract hydrogen, and use it to power fuel cells for running cars and other machines. Then, collect their water emissions and start the procedure again.
One look at the list of ingredients in today’s fuel cells, however, shows that this ideal isn’t yet being followed. Because processes that use sunlight to extract hydrogen remain costly and inefficient, fossil fuels still supply the hydrogen in most fuel cells.
Hoping to break the fossil fuel habit, a team of Israeli, German, and Japanese scientists has created a device that boosts the efficiency of solar-powered hydrogen extraction by 50 percent.
The group placed a photovoltaic cell on top of two flat, finger-long electrodes. The combination “is very efficient in converting solar energy [into an electric current] but also provides nearly the ideal voltage for splitting water” into hydrogen and oxygen, says team leader Stuart Licht of the Technion in Haifa, Israel. A water molecule splits, or undergoes electrolysis, at only 1.23 volts.
Licht and his colleagues describe their device in the Sept. 28 Journal of Physical Chemistry B. The gadget converts sunlight to an electrolysis current with 18.3 percent efficiency. In turn, the current creates hydrogen gas as it passes through acidic water.
The device is “showing the pathway towards higher efficiencies for direct solar-to-hydrogen production,” comments John A. Turner of the National Renewable Energy Laboratory (NREL) in Golden, Colo. The newly achieved efficiency may already be high enough for commercial hydrogen generators to be feasible. “That still needs to be figured out,” Turner says.
In 1998, he and Oscar Khaselev, then also of NREL, demonstrated a novel apparatus for solar-to-hydrogen conversion (SN: 4/18/98, p. 246). To achieve unprecedented efficiency, the device used multiple layers of semiconductor materials. The researchers arranged the layers to form two active regions, or junctions, that would absorb solar photons that dislodge electrons. Some of the less energetic photons weren’t captured in the first junction but passed to the second, where they generated more current.
The design gained an energy advantage by combining solar electricity and water splitting into one unit. Their cell’s 12.4 percent efficiency—nearly twice that of any previous solar-to-hydrogen device—has held as the record until now.
Licht and his colleagues have improved upon that pioneering effort in several crucial ways. In one sense, the NREL device was all wet: It had to be completely immersed in water to operate. That feature forced the researchers to select semiconductors that wouldn’t break down in solution.
By keeping their stack of semiconductor layers high and dry, Licht and his group were free to optimize them for both converting sunlight to electricity and water splitting. Their design permits a low electrolysis current, which also reduces energy waste.
Licht and his coworkers say that besides besting the solar-to-hydrogen conversion record, their work opens the way to efficiencies not considered possible before. Using measured photoelectric efficiencies of seven semiconductor combinations not yet tested in hydrogen generation, they predict maximum solar-to-hydrogen conversion efficiencies of up to 31 percent.
Thermodynamics theory says the maximum could range above 40 percent for a two-junction converter, but no one has previously predicted better than 24 percent performance for practical devices, Turner says. Experimentally achieving the new prediction “would be an accomplishment indeed!” he adds.
