Researchers have developed a model photoelectrochemical (PEC) cell with remarkable stability and longevity as it selectively converts sunlight and carbon dioxide into two promising sources of renewable fuels—ethylene and hydrogen.
“By understanding how materials and devices transform under operation, we can design approaches that are more durable and thus reduce waste,” said senior author Francesca Toma, a staff scientist in the Liquid Sunlight Alliance (LiSA) and Berkeley Lab’s Chemical Sciences Division.
The model PEC cell incorporated a photocathode of cuprous oxide (Cu2O), a promising artificial-photosynthesis material. Cuprous oxide has long puzzled scientists, because its strength—high reactivity to light—is also its weakness, as light causes the material to break down.
To better understand why, the team took a closer look at cuprous oxide’s crystal structure before and after use. Electron microscopy at the Molecular Foundry confirmed that cuprous oxide quickly oxidizes or corrodes within minutes of exposure to light and water. At the Advanced Light Source, ambient-pressure x-ray photoelectron spectroscopy (APXPS) at Beamline 9.3.1 revealed an unexpected clue: hydroxide ions in water cause the cuprous oxide to corrode faster. Computer simulations at the National Energy Research Scientific Computing Center (NERSC) showed that holes (and not just electrons, as previously thought) play a part in the corrosion of cuprous oxide.
The simulations also hinted at a potential workaround: a cuprous oxide PEC was coated with silver on top and gold/iron oxide underneath. This “Z scheme,” inspired by the Z-shaped energy diagram of electron transfers that takes place in natural photosynthesis, would funnel holes away from the cuprous oxide to the gold/iron oxide.
To validate their simulations, the researchers designed a physical model of a Z-scheme artificial-photosynthesis device, which produced ethylene and hydrogen with unprecedented selectivity—and for more than 24 hours.
The researchers plan to continue their work on developing new solar fuel devices for liquid fuels production by using their new approach. “Understanding how materials transform while they are functioning in an artificial-photosynthesis device can enable preventive repair and prolonged activity,” said Toma.
G. Liu, F. Zheng, J. Li, G. Zeng, Y. Ye, D.M. Larson, J. Yano, E.J. Crumlin, J.W. Ager, L.-W. Wang, and F.M. Toma, “Investigation and mitigation of degradation mechanisms in Cu2O photoelectrodes for CO2 reduction to ethylene,” Nat. Energy 6, 1124 (2021), doi:10.1038/s41560-021-00927-1.
Adapted from the Berkeley Lab news release, “New Device Advances Commercial Viability of Solar Fuels.”