This has been adapted from a Berkeley Lab press release.
Three years ago, scientists at the University of Michigan discovered an artificial photosynthesis device made of silicon and gallium nitride (Si/GaN) that harnesses sunlight into carbon-free hydrogen for fuel cells with twice the efficiency and stability of some previous technologies.
Now, a team of Foundry users, working with staff, have uncovered a surprising, self-improving property in Si/GaN that contributes to the material’s highly efficient and stable performance in converting light and water into carbon-free hydrogen. Their findings, reported in the journal Nature Materials, could help radically accelerate the commercialization of artificial photosynthesis technologies and hydrogen fuel cells.
“Our discovery is a real game-changer,” said senior author Francesca Toma, a staff scientist in the Chemical Sciences Division at Berkeley Lab. Usually, materials in solar fuels systems degrade, become less stable and thus produce hydrogen less efficiently, she said. “But we discovered an unusual property in Si/GaN that somehow enables it to become more efficient and stable. I’ve never seen such stability.”
Previous artificial photosynthesis materials are either excellent light absorbers that lack durability; or they’re durable materials that lack light-absorption efficiency.
But silicon and gallium nitride are abundant and cheap materials that are widely used as semiconductors in everyday electronics such as LEDs (light-emitting diodes) and solar cells, said co-author Zetian Mi, a professor of electrical and computer engineering at the University of Michigan who invented Si/GaN artificial photosynthesis devices a decade ago.
The researchers carried out a photoconductive atomic force microscopy experiment at Toma’s lab to test how GaN photocathodes could efficiently convert absorbed photons into electrons, and then recruit those free electrons to split water into hydrogen, before the material started to degrade and become less stable and efficient.
They expected to see a steep decline in the material’s photon absorption efficiency and stability after just a few hours. To their astonishment, they observed a 2-3 orders of magnitude improvement in the material’s photocurrent coming from tiny facets along the “sidewall” of the GaN grain, Zeng said. Even more perplexing was that the material had increased its efficiency over time, even though the overall surface of the material didn’t change that much, Zeng said. “In other words, instead of getting worse, the material got better,” he said.
To gather more clues, the researchers used the scanning transmission electron microscopy (STEM) capabilities at the Foundry’s National Center for Electron Microscopy facility and angle-dependent X-ray photon spectroscopy (XPS) in the Imaging facility.
Those experiments revealed that a 1 nanometer layer mixed with gallium, nitrogen, and oxygen – or gallium oxynitride – had formed along some of the sidewalls. A chemical reaction had taken place, adding “active catalytic sites for hydrogen production reactions,” Toma said.
Looking ahead, Toma said that she and her team would like to test the Si/GaN photocathode in a water-splitting photoelectrochemical cell, and will experiment with similar materials to get a better understanding of how nitrides contribute to stability in artificial photosynthesis devices – which is something they never thought would be possible.
Read the full press release.
