One of the main methods of producing hydrogen for fuel cells is to use artificial photosynthesis to split water into hydrogen and oxygen, but these devices still suffer from some efficiency issues. Now a new hybrid device may be able to recover some of the energy that would otherwise go to waste, by producing both hydrogen and electricity.

Great strides have been made recently in hydrogen production from artificial photosynthesis, improving the efficiency, lowering the cost, and developing clever systems to put it to work, like rigs that float on the ocean and harvest hydrogen from the water below.

But despite improvements, efficiency remains on ongoing problem. Many artificial photosynthesis devices can only make use of single-digit percentages of the sunlight that hits them, compared to regular photovoltaic systems that often reach 20 percent conversion efficiencies, and have been known to get as high as 45 percent. The researchers on the new study, from Berkeley Lab and the Joint Center for Artificial Photosynthesis (JCAP), blamed the non-silicon components of the water-splitting devices for bringing down the silicon's effectiveness.

"It's like always running a car in first gear," says Gideon Segev, lead author of the study. "This is energy that you could harvest, but because silicon isn't acting at its maximum power point, most of the excited electrons in the silicon have nowhere to go, so they lose their energy before they are utilized to do useful work."

The answer might be surprisingly simple – why not just let those electrons out? To do so, the researchers added a second electrical contact to the back of the silicon component in the device. That splits the current produced by the sunlight's energy, allowing some of the current to split the water into hydrogen and oxygen, and some to be captured as electricity. They dubbed the new device a hybrid photoelectrochemical and voltaic (HPEV) cell.

For reference, the researchers calculated that a conventional artificial photosynthesis device using silicon and bismuth vanadate would have an efficiency of 6.8 percent. By comparison, a HPEV cell made using these same components would convert an extra 13.4 percent of the solar energy into electricity. Along with the 6.8 percent that's going into producing hydrogen, the cell would have a combined efficiency of 20.2 percent.

The researchers first tested their HPEV design by running simulations, before building a prototype. Sure enough, the real-world device worked as hoped. The team plans to continue improving the device, as well as investigating other applications for it, including reducing CO2 emissions.

The study was published in the journal Nature Materials.

Source: Berkeley Lab

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