New device advances commercial viability of solar fuels





Photoelectrochemical cell

A model of solar fueled device called a photoelectrochemical cell. A research team led by Francesca Toma, a scientist with the Liquid Sunlight Alliance in the Berkeley Laboratory’s Chemical Sciences Division, designed the model. Credit: Thor Swift/Berkeley Lab

The discovery significantly improves the stability of ethylene and hydrogen production via artificial photosynthesis.

A research team has developed a new artificial photosynthesis device component with remarkable stability and longevity because it selectively converts sunlight and carbon dioxide into two promising sources of renewable fuels – ethylene and hydrogen.

The researchers’ findings, which they recently reported in the journal natural energy, reveal how the device degrades with use, then demonstrate how to mitigate it. The authors also provide new insights into how electrons and charge carriers called “holes” contribute to the breakdown of artificial photosynthesis.

“By understanding how materials and devices transform during operation, we can design more sustainable approaches and thereby reduce waste,” said lead author Francesca Toma, a scientist in the Chemical Sciences Division of Liquid Sunlight Alliance (LiSA) Berkeley Lab.

For the current study, Toma and his team designed a model solar-powered device known as a copper (I) oxide or cuprous oxide (Cu) photoelectrochemical (PEC) cell.2O), a promising artificial photosynthesis material.

Cuprous oxide has long puzzled scientists because the material’s strength – its high reactivity to light – is also its weakness, as light causes the material to decompose within minutes of exposure. But despite its instability, cuprous oxide is one of the best candidate materials for artificial photosynthesis because it is relatively affordable and has characteristics suitable for absorbing visible light.

To better understand how to optimize working conditions for this promising material, Toma and his team took a closer look at the crystal structure of cuprous oxide before and after use.

Electron microscopy experiments at the Molecular Foundry have confirmed that cuprous oxide oxidizes or corrodes rapidly within minutes of exposure to light and water. In research on artificial photosynthesis, researchers have typically used water as an electrolyte in the reduction of carbon dioxide into renewable chemicals or fuels, such as ethylene and hydrogen – but water contains hydroxide ions. , resulting in instability.

But another experiment, this time using a technique called ambient pressure X-ray photoelectron spectroscopy (APXPS) at the Advanced Light Source, revealed an unexpected clue: cuprous oxide corrodes even faster in water containing hydroxide ions, which are negatively charged ions composed of an oxygen atom bonded to a hydrogen atom.

“We knew he was unstable – but we were surprised to learn how unstable he really is,” Toma said. “When we started this study, we wondered if the key to a better solar fuel device might not be in the material itself, but in the overall reaction environment, including the electrolyte.”

“This demonstrates that hydroxides contribute to corrosion. On the other hand, we felt that if you remove the source of corrosion, you remove the corrosion,” explained first author Guiji Liu, LiSA project scientist in the Chemical Sciences Division of Berkeley Lab.

Discover unexpected signs of corrosion

In electronic devices, electron-hole pairs split into electrons and holes to generate charge. But once separated, if the electrons and holes are not used to generate electricity, such as in a photovoltaic device that converts sunlight into electricity, or to perform a reaction in an artificial photosynthesis device, they can react with the material and degrade it.

In artificial photosynthesis, this recombination can corrode cuprous oxide if not properly controlled. Scientists have long assumed that electrons were solely responsible for the corrosion of cuprous oxide. But to Toma and Liu’s surprise, computer simulations performed at the National Energy Research Computing Science Center (NERSC) showed that the holes also play a role. “Before our study, most people assumed that light-induced degradation in cuprous oxide was primarily caused by electrons, not holes,” Liu said.

The simulations also hinted at a potential workaround to the inherent instability of cuprous oxide: a PEC of cuprous oxide coated with silver on top and gold/iron oxide below. This “Z pattern”, which is inspired by the transfer of electrons that takes place in natural photosynthesis, should create a “funnel” that sends holes from cuprous oxide to the gold/iron oxide “sink”. Additionally, the diversity of materials at the interface should stabilize the system by providing additional electrons to recombine with the holes in the cuprous oxide, Toma explained.

To validate their simulations, the researchers built a physical model of a Z-pattern artificial photosynthetic device at Toma’s LiSA laboratory at Berkeley Lab. To their delight, the device produced ethylene and hydrogen with unprecedented selectivity – and for more than 24 hours. “It’s an exciting result,” Toma said.

“We hope our work will encourage people to design strategies that accommodate the intrinsic characteristics of semiconductor materials in artificial photosynthesis devices,” Liu added.

The researchers plan to continue their work on the development of new solar-powered devices for the production of liquid fuels using their new approach. “Understanding how materials transform while functioning in an artificial photosynthetic device can enable preventative repair and prolonged activity,” Toma concluded.

Reference: “Investigation and mitigation of degradation mechanisms in Cu2O photoelectrodes for CO2 reduction to ethylene” by Guiji Liu, Fan Zheng, Junrui Li, Guosong Zeng, Yifan Ye, David M. Larson, Junko Yano, Ethan J. Crumlin, Joel W. Ager, Lin-wang Wang and Francesca M. Toma, November 8 2021, natural energy.
DOI: 10.1038/s41560-021-00927-1

Additional co-authors were Fan Zheng, Junrui Li, Guosong Zeng, Yifan Ye, David Larson, Junko Yano, Ethan Crumlin, Joel Ager, and Lin-wang Wang.

The Liquid Sunlight Alliance is a DOE Energy Innovation Hub. The Advanced Light Source, Molecular Foundry, and NERSC are Berkeley Laboratory user facilities.

This work was supported by the DOE Office of Science.





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