סמינר באלקטרוכימיה: Iron oxide photoanodes and tandem cells for solar energy conversion and storage.

Abstract:

Photovoltaics is on the rise, bringing about new challenges for affordable energy storage to buffer between the variable solar power production and demand. Likewise, there is a need for alternative fuels to replace fossil fuels. These challenges can be met, potentially, by fitting photovoltaic cells with photoelectrolysis cells that split water into hydrogen and oxygen. The hydrogen can be stored and converted to electricity and heat on demand. Alternatively, it may serve as feedstock for sustainable production of methanol or other liquid fuels for transportation by reaction with CO2, paving the road towards carbon-neutral synthetic fuels, so-called solar fuels. The first and foremost challenge toward this emerging technology is the development of chemically stable, efficient and affordable photoelectrodes for water photoelectrolysis.      

Photoelectrodes for solar powered water photoelectrolysis must employ a semiconductor material with exceptional stability against corrosion, as well as visible-light absorption. On top of that, it should also be abundant, inexpensive and non-toxic. Iron oxide (-Fe2O3) is one of few materials meeting these requirements, but its poor transport properties present challenges for efficient charge-carrier generation, separation, collection and injection. 

We explore an innovative solution to these challenges by means of resonant light trapping in ultrathin films designed as optical cavities. Interference between forward- and backward-propagating waves enhances the light absorption in quarter-wave or, in some cases, deeper subwavelength films, amplifying the intensity close to the surface wherein photogenerated minority charge carriers (holes) can reach the surface and oxidize water before recombination takes place. Combining this effect with photon retrapping using V-shaped structures provides efficient light harvesting in ultrathin films of high internal quantum efficiency, overcoming the intrinsic trade-off between light absorption and charge collection. A water photooxidation current density of 4 mA cm-2 was achieved using a V-shaped cell comprising 26-nm-thick Ti-doped -Fe2O3 films on back-reflector substrates coated with silver–gold alloy.* This sets a new record in water photoelectrolysis by abundant and stable photoanodes, paving the road towards potentially affordable large scale production of hydrogen using abundant and renewable sources: water and sunlight. The iron oxide photoanodes can be combined with photovoltaic cells to create tandem systems for solar energy conversion to electricity and storage in the form of hydrogen.