Researchers from the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) in California are interfacing the semiconductor gallium phosphide with a cobaloxime catalyst to create an inexpensive photocathode for bionic leaves that produce energy-dense fuels, such as hydrogen gas, from sunlight, water and carbon dioxide. In their efficiency analysis study, nearly 90% of the photoexcited electrons are being stored in the target hydrogen molecules.
"Artificial photosynthesis has been described as a great scientific and moral challenge of our time," says Gary Moore, principal investigator in the Berkeley Lab’s Physical Biosciences Division.
Berkeley Lab’s novel photocathode consists of the semiconductor gallium phosphide and a molecular cobalt-containing hydrogen production catalyst. "In coupling the absorption of visible light with the production of hydrogen in one material, we can generate a fuel simply by illuminating the photocathode," Moore says. "Storing solar energy in the chemical bonds of a fuel provides the large power densities that are essential to modern transport systems."
Hydrogen gas provides greater densities (140 MJ/kg) than liquid fuels, but, according to Moore, there are concerns regarding its storage and distribution. "We showed a proof of principle. Hopefully by following our design logic and by using the appropriate light absorber/catalyst combination we can move beyond hydrogen generation," says Alexandra Krawicz, a postdoctoral researcher in Moore’s group.
Looking into the future, Moore says, "This research opens the possibility of using discrete three-dimensional (molecular) environments to directly photoactivate the multi-electron and multi-proton chemistry associated with fuels production." In the study, about 88% of the electrons generated by illuminating Berkeley Lab’s novel hybrid material were stored in molecules of hydrogen. "However, the light-absorber component of the photocathode is a major bottleneck to obtaining higher current densities," Krawicz advises. "Of the total number of solar photons striking the hybrid-semiconductor surface, measured over the entire wavelength range of the solar spectrum, only 1.5% give rise to a photocurrent." Adds Moore: "The most exciting part of our work is not necessarily the improved durability or efficiency of the photocathode following chemical modification, it’s the use of molecular components interfaced with visible-light absorbing semiconductors."
Moore, who agrees that the renewable energy problem is really a storage problem, deems light absorbers with improved spectral coverage of the sun "a good start," to optimising performance, but says researchers will likely have to develop faster, more efficient catalysts and new attachment chemistries, too. Efficiency, he says, should not be the only factor in evaluating a material’s suitability for solar fuel production. "Along with the durability and feasible scalability of components, the selectivity of photoactivating a targeted reaction is also critical." Molecular approaches offer significant opportunities here, especially in catalysing complex chemical transformations such as the reduction of carbon dioxide, according to the team.
Moore imagines a future where solar fuel technologies provides a "significant portion" of our energy, but says many components currently proposed for artificial photosynthesis are costly, nondurable or inefficient, and some are even based on highly caustic or toxic materials. "Clearly, a solution that is as or more polluting than the burning of fossil fuels is not practical." The best path forward, he argues, is "to focus on systems that operate under environmentally benign conditions using earth-abundant elements."
The Berkeley Lab’s findings are summarised in the paper "Energetics and efficiency analysis of a cobaloxime-modified semiconductor under simulated air mass 1.5 illumination," published in the journal Physical Chemistry Chemical Physics.
Written by Sandra Henderson, Research Editor Solar Novus Today