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Extremely Thin Absorbers for Solar Energy Conversion and Storage

Final Report Summary - ETASECS (Extremely Thin Absorbers for Solar Energy Conversion and Storage)

This ERC project investigated iron oxide (Fe2O3) photoanodes in photoelectrochemical (PEC) cell for solar water splitting. Fe2O3 is considered as one of the most promising candidates for this application because it combines exceptional stability in alkaline aqueous solutions, broadband absorption in the visible range and high catalytic activity for water (photo)oxidation. However, it is also known to suffer from fast charge carrier (electron – hole) recombination in the bulk that diminishes the photocurrent, and it requires external bias to suppress surface recombination and provide sufficient energy to the electrons to reduce water and produce hydrogen at the counter electrode (the cathode). Our strategy to combat the bulk recombination was to use ultrathin films with short transport length to the surface and to enhance their light absorption by employing a unique resonant light trapping method whereby the forward propagating radiation from the incoming sunlight resonates with the backward propagating radiation coming from the specular back reflector at the bottom of the iron oxide film.
Following this approach, the photoelectrochemical performance of ultrathin film Fe2O3 photoanodes was drastically improved by optimizing their deposition conditions, introducing heterogeneous doping profiles that enhance charge separation, enhancing the specular reflectance of the silver back reflectors, and using co-catalyst overlayers that suppress surface recombination. Our champion photoanodes comprised 14 nm thick Fe2O3 films and reached a photocurrent of 1.5 mA/cm2 at the reversible potential for water oxidation. This is a world record for such ultrathin films. The photocurrent can be further increased by ~50% by placing two photoanodes facing each other in a V-shaped cell where the back-reflect light bounces forth and back between them. We also developed semitransparent back reflectors that reflect short-wavelength photons back into the Fe2O3 layer while transmitting long-wavelength photons through the photoanode. The transmitted photons are utilized by a photovoltaic (PV) cell placed behind the photoanode. This enables integration of our photoanodes with commercial PV cells to construct PEC-PV tandem cells that can split water to produce both hydrogen (and oxygen) and electricity. For such devices to compete with water electrolysis powered by solar electricity, the photoanode performance should be improved much further that our current record to enable high photocurrent (> 8 mA/cm2) at low bias (< 1 V).
In addition to the progress made in improving the photoanode performance, we also developed advanced analytical methods to analyze different losses and their physical or (electro)chemical origin. A new loss mechanism was discovered that relates to futile light absorption that leads to localized electronic transitions without creating mobile charge carriers that contribute to the photocurrent. We estimate this loss diminishes the photocurrent by ~30-50% in hematite photoanodes.
Another important advancement made in this project is the development of a disruptive new approach to split water in separated hydrogen and oxygen cells. Unlike the conventional electrolytic water splitting architecture in which hydrogen and oxygen are produced together in the same cell, requiring a membrane to divide the cell into two compartments so as to prevent the formation of a flammable gas mixture, our approach allows physical separation of the hydrogen and oxygen cells over large distances with negligible loss in performance. Thereby, hydrogen can be produced in a central location while the PEC cells that are distributed in the solar field produce oxygen that can be released to the atmosphere without collection. This transformative approach solves the challenge of collecting hydrogen from thousands to millions of PEC cells in large solar plants for large scale hydrogen production.