Nano-sandwich photocathode improves solar conversion of CO₂ into ethanol

Czech researchers at Charles University in Prague, led by Pavla Eliášová, have developed an advanced photocathode that helps convert the greenhouse gas carbon dioxide into pure ethanol using sunlight. The new technology solves a long-standing problem of material degradation in water, achieves record conversion efficiency and opens the way toward sustainable production of green fuels. The paper is published in the journal Advanced Energy Materials.

Converting carbon dioxide (CO₂) into useful fuels using solar energy represents the holy grail of modern green chemistry. The device enabling this conversion is called a solar reactor. Its key element is a photocathode: a material that absorbs light and enables the chemical reaction. Cuprous oxide has long appeared to be an ideal material because it is inexpensive, readily available and excels at capturing sunlight. In practice, however, it faced a fundamental obstacle: In an aqueous environment under light, it transforms and degrades extremely rapidly—much like iron undergoing swift corrosion—and loses its functionality within tens of minutes.

The team from the Chemistry Section of the Faculty of Science, CU, led by Eliášová, in collaboration with Tomáš Hrbek from the Faculty of Mathematics and Physics, CU, resolved this issue by creating a unique “nano-sandwich.” The scientists combined cuprous oxide with advanced two-dimensional materials called MXenes and, through precisely controlled heating, created an ultrathin layer of titanium dioxide on their surface. MXenes modified in this way can rapidly drain electric charge from the cuprous oxide, thereby slowing its decomposition. At the same time, they use the charge to convert carbon dioxide into liquid fuel.

“Surface-engineered MXene combined with cuprous oxide allowed us to unite the best of both materials. The copper base enables maximum utilization of light energy, while the conductive MXene framework ensures that the material remains active even after many hours of operation,” explains research team leader Eliášová from the Chemistry Section of the Faculty of Science, Charles University.

In laboratory tests, the new photocathode achieved a record solar-energy-to-ethanol conversion efficiency of 2.74%. A crucial improvement is the formation of only a single product. While other systems often produce a mixture of byproducts, the material designed by the research team produces pure ethanol as its sole liquid product, significantly simplifying its potential industrial application.

“This is a major step forward for us. At the laboratory scale, the system works very well and produces ethanol over many hours. In the next step, we want to further reinforce the stability of the entire photocathode and begin testing on a larger scale and under real daylight conditions,” adds physical chemist Eliášová.

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