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JCAP hybrid photocathode material shows promising performance in conversion of solar energy to hydrogen
9 March 2014
A new study by Berkeley Lab researchers at the Joint Center for Artificial Photosynthesis (JCAP) shows that nearly 90% of the electrons generated by a new hybrid photocathode material designed to store solar energy in hydrogen are being stored in the target hydrogen molecules (Faradaic efficiency).
Gary Moore, a chemist and principal investigator with Berkeley Lab’s Physical Biosciences Division, led an efficiency analysis study of the material he and his research group have developed for catalyzing the production of hydrogen fuel from sunlight. (Earlier post.) This material, a p-type (100) gallium phosphide (GaP) semiconductor functionalized with molecular hydrogen-producing cobaloxime catalysts via polymer grafting, has the potential to address one of the major challenges in the use of artificial photosynthesis to make renewable solar fuels.
The catalysts belong to the cobaloxime class of compounds that have shown recent promise in electrocatalysis and solar-to-fuels applications. Attachment of the complex to a semiconductor surface allows direct photoelectrochemical (PEC) measurements of performance.
Under simulated air mass 1.5 illumination, the catalyst-modified photocathode yields a 0.92 mA cm-2 current density when operating at the equilibrium potential for the hydrogen production half reaction. The open circuit photovoltage (VOC) is 0.72 V vs. a reversible hydrogen electrode (RHE) and the fill factor (FF) is 0.33 (a 258% increase compared to polymer-modified electrodes, without cobaloxime treatment).
The external quantum efficiency (EQE), measured under a reverse bias of +0.17 vs. RHE, shows a maximum of 67% under 310 nm illumination. Product analysis of the head-space gas yields a lower limit on the Faradaic efficiency of 88%. In addition, the near linear photoresponse of the current density upon increasing illumination indicates that photocarrier transport to the interface can limit performance.—Krawicz et al.
A paper describing this research is published in the RSC journal Physical Chemistry Chemical Physics.
Ultimately the renewable energy problem is really a storage problem. Given the intermittent availability of sunlight, we need a way of using the sun all night long. Storing solar energy in the chemical bonds of a fuel also provides the large power densities that are essential to modern transport systems. We’ve shown that our approach of coupling the absorption of visible light with the production of hydrogen in a single material puts photoexcited electrons where we need them to be, stored in chemical bonds.—Gary Moore
Bionic leaves that produce energy-dense fuels from nothing more than sunlight, water and atmosphere-warming carbon dioxide, with no byproducts other than oxygen, represent an ideal sustainable energy alternative to fossil fuels. However, realizing this artificial photosynthesis ideal will require a number of technological breakthroughs including high performance photocathodes that can catalyze fuel production from sunlight alone.
Last year, Moore and his research group at JCAP took an important step towards the photocathode goal with their gallium phosphide/cobaloxime hybrid. Gallium phosphide is an absorber of visible light, which enables it to produce significantly higher photocurrents than semiconductors that only absorb ultraviolet light. The cobaloxime catalyst is also Earth-abundant, meaning it is a relatively inexpensive replacement for the highly expensive precious metal catalysts, such as platinum, currently used in many solar-fuel generator prototypes.
The novelty of our approach is the use of molecular catalytic components interfaced with visible-light absorbing semiconductors. This creates opportunities to use discrete three-dimensional environments for directly photoactivating the multi-electron and multi-proton chemistry associated with the production of hydrogen and other fuels.—Gary Moore
The efficiency analysis performed by Moore and his colleagues also confirmed that the light-absorber component of their photocathode is a major bottleneck to obtaining higher current densities. Their results showed that of the total number of solar photons striking the hybrid-semiconductor surface, measured over the entire wavelength range of the solar spectrum (from 200 to 4,000 nanometers) only 1.5% gave rise to a photocurrent.
This tells us that the use of light absorbers with improved spectral coverage of the sun is a good start to achieving further performance gains, but it is likely we will also have to develop faster and more efficient catalysts as well as new attachment chemistries. Our modular assembly method provides a viable strategy to testing promising combinations of new materials.
Efficiency is not the only consideration that should go into evaluating materials for applications in solar-fuel generator technologies. Along with the durability and feasible scalability of components, the selectivity of photoactivating a targeted reaction is also critical. This is where molecular approaches offer significant opportunities, especially in catalyzing complex chemical transformations such as the reduction of carbon dioxide.—Gary Moore
JCAP, which has a northern branch in Berkeley and a southern branch on the campus of the California Institute of Technology (Caltech), was established in 2010 by the US Department of Energy (DOE) as an Energy Innovation Hub. Operated as a partnership between Caltech and Berkeley Lab, JCAP is the largest research program in the United States dedicated to developing an artificial solar-fuel technology. It is funded through the DOE Office of Science.
Alexandra Krawicz, Diana Cedeno and Gary F. Moore (2014) “Energetics and Efficiency Analysis of a Cobaloxime-Modified Semiconductor at Simulated Air Mass 1.5 Illumination,” Phys. Chem. Chem. Phys. doi: 10.1039/C4CP00495G
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