Water splitting in photo-electrochemical cells to yield hydrogen is a promising way to sustainable fuels. A team of Swiss and U.S. scientists have now made major progress in developing highly efficient electrodes – made of an algal protein, thus mimicking a central step in natural photosynthesis.
Photosynthesis converts solar energy into storable fuel using nothing but water and carbon dioxide. Scientists have long tried to mimic the underlying natural processes and to optimize them for energy device applications such as photo-electrochemical cells (PEC), which use sunlight to electrochemically split water – and thus directly generate hydrogen, cutting short the more conventional approach using photovoltaic cells for the electrolysis of water.
Traditionally, PEC electrodes are made of semiconducting materials such as metal oxides, some of which are also known for their photocatalytic properties. Researchers at Empa’s Laboratory for High Performance Ceramics (LHPC) in Switzerland have been investigating nanoparticles of these materials, for instance titanium dioxide (TiO2), for the neutralization of organic pollutants in air and water. Collaborating with colleagues at the University of Basel and at Argonne National Laboratory, they have succeeded in making a nano-bio PEC electrode, consisting of iron oxide conjugated with a protein from blue-green algae (also known as cyanobacteria), which is twice as efficient in water splitting as iron oxide alone.
Iron oxide, in particular hematite (alpha-Fe2O3), is a promising electrode material for PEC because it is susceptible to visible wavelengths and thus uses sunlight more efficiently than photocatalysts like TiO2, which can only use the UV part of solar radiation. Hematite is also a low-cost and abundant material. The second ingredient is phycocyanin, a protein from blue-green algae.
After Empa researcher Debajeet K. Bora covalently cross-coupled phycocyanin to hematite nanoparticles that had been immobilized as a thin film, the conjugated hematite absorbed many more photons than without the algal protein. The induced photocurrent of the hybrid electrode was doubled compared to a typical iron oxide electrode.