Berkeley researchers’ nanowire “artificial forest” for photosynthetic hydrogen production consists of silicon “trunks” and titanium oxide “leaves.” Credit: Lawrence Berkeley National Laboratory.

Hydrogen is considered by many to be the Holy Grail of clean energy, but a major hurdle to moving toward the hydrogen economy is producing enough of the gas to make a significant impact on the world’s energy needs. Scientists have taken up that challenge by exploring hydrogen production via photovoltaic electrolysis, from biomass, and, most intriguing, using photosynthetic electrolysis.

CTT has already reported on use of ceramic (mixed-metal oxide) catalysts for more efficient hydrogen production using PV electrolysis. Now, ceramic materials are at the center of advances toward more efficient production of hydrogen from biomass and by photosynthetic electrolysis.

Biomass-derived hydrogen is produced by reformation of organic sources such as methanol. The resulting hydrogen-rich gas mixture can be used in fuel cells, but a byproduct of the approach is carbon monoxide, which is not only toxic but also quickly damages fuel cell catalyst materials.

At Duke University, engineers working on a new method to produce clean hydrogen from biomass hit on a new catalytic approach that combines nanoparticle gold with iron oxide to reduce production of carbon monoxide to near zero. The improved catalyst resulted in hydrogen production with less than 20 ppm CO at 80°C, a lower temperature than previous methods.

The researchers ran the reaction for more than 200 hours and found no reduction in effectiveness for the gold/iron oxide catalyst. They are currently working to understand the reasons for the material’s catalytic efficiency. Full results of the research are reported the  Journal of Catalysis (subscription required).

Like PV electrolysis, photosynthetic electrolysis generates molecular hydrogen by breaking down water into its constituent components. The advantage: the photosynthetic process skips the electrical power input needed to provide the energy for PV electrolysis. Instead, it directly uses the power of the sun in a process analogous to the one green plants use to convert sunlight and carbon dioxide into glucose and oxygen.

“Artificial leaf” is the popular term for such a system but, according to scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory, an “artificial forest” may be the key to success.

Schematic shows TiO2 nanowires (blue) grown on silicon nanowires. Insets: photoexcited electron-hole pairs generate hydrogen with the help of co-catalysts (yellow and gray dots).

Schematic shows TiO2 nanowires (blue) grown on silicon nanowires. Insets: photoexcited electron-hole pairs generate hydrogen with the help of cocatalysts (yellow and gray dots). Credit: Lawrence Berkeley National Laboratory.

The LBNL team developed what they say is the first fully integrated nanosystem for artificial photosynthesis, which consists of two semiconductor light absorbers, an interfacial layer for charge transport, and spatially separated cocatalysts. The team synthesized tree like nanowire structures consisting of silicon “trunks” and titanium oxide “branches.” The silicon serves as a hydrogen-generating photocathode while the titanium oxide acts as an oxygen-generating photoanode. The two materials absorb different regions of the solar spectrum, and the system’s tree like architecture is said to maximize performance by suppressing sunlight reflection and providing more surface area for fuel-producing reactions.

Under simulated sunlight, the system achieved solar-to-fuel conversion efficiency of 0.12 percent—comparable to some natural photosynthetic efficiencies, but too low for commercial use. However, the scientists say, the system’s modular design allows easy integration of new individual components that can improve performance.

More information on the research is available here.

The paper is “Novel nano-scale Au/alpha-Fe2O3 catalyst for the preferential oxidation of CO in biofuel reformate gas,” Titilayo Shodiya, et. al, Journal of Catalysis (DOI 10.1016/j.cat.2012.12.027)