Photoelectrochemical water splitting for solar energy conversion and storage

Photoelectrochemical (PEC) water splitting is a promising route for solar energy conversion to hydrogen. It produces clean hydrogen that can be used for refueling fuel cell electric vehicles or serve as a feedstock for the production of drop-in liquid fuels by CO2 hydrogenation or ammonia via the Haber–Bosch process. The greatest challenges towards PEC solar water splitting technology lay in the selection and optimization of stable photocatalytic materials for water photo-oxidation, and the design of scalable PEC devices that produce hydrogen at a competitive cost. Iron oxide (a-Fe2O3, hematite) is one of few materials meeting the basic selection criteria for stable photoanodes, but its poor charge transport properties and fast recombination present challenges for efficient charge separation and collection. We explore innovative solutions to these challenges using ultrathin (20-30 nm) films on specular back reflectors. This optical design traps the light in otherwise nearly translucent ultrathin films, amplifying the intensity close to the surface wherein photogenerated charge carriers can reach the surface and split water before recombination takes place.1 This is the enabling key towards the development of high-efficiency epilayers whose properties can be tailored by material design at the atomic scale.2 Our recent efforts to uncover the design rules of these photoanodes will be presented. On the other end of the spectrum we explore innovative device architectures and operation schemes for scalable and competitive PEC solar water splitting technology. These include power and optical management schemes for optimizing the hydrogen and power outputs of PEC – PV tandem cells,3 and separating the hydrogen production from the oxygen production for safe operation and on-site hydrogen production.4 References: 1. Dotan, Nature Materials 12, 158-164 (2013). 2. Grave, The Journal of Physical Chemistry C 120, 28961–28970 (2016). 3. Rothschild, ACS Energy Letters 2, (2017) 45-51. 4. Landman, Nature Materials (2017, DOI: 10.1038/nmat4876)

Iron oxide thin film photoelectrodes for water splitting

Reliable utilization of solar power on a large scale requires affordable energy storage technology, cheaper than batteries, in order to synchronize the variable power production with the changing demand. Likewise, there is a need for renewable fuels to replace fossil fuels. These challenges can be achieved, potentially, by splitting water into hydrogen and oxygen, H2O à H2 + ½O2, using solar power to drive this endergonic reaction uphill. The hydrogen can be stored and converted to electricity and heat on demand. Alternatively, it may serve as feedstock for the production of liquid fuels for transportation by reaction with CO2, paving the road to carbon-neutral synthetic fuels, so-called solar fuels. The first and foremost challenge toward this ambitious goal is the development of chemically stable, efficient and affordable photoelectrodes for water splitting. Photoelectrodes for solar-powered water splitting 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 (a-Fe2O3, hematite) is one of few materials meeting these criteria, but its poor transport properties and ultrafast charge carrier recombination present a challenge for efficient charge carrier generation, separation and collection. We explore an innovative solution to this challenge using ultrathin (20-30 nm) quarter-wave films on back reflector substrates. This simple optical cavity design effectively traps the light in otherwise nearly translucent films, amplifying the intensity close to the surface wherein photogenerated charge carriers can reach the surface and split water before recombination takes place. This is the enabling key towards the development of high efficiency photoelectrodes that could potentially lead to affordable solar energy storage and solar fuel production. The research leading to these results has received funding from the European Research Council under the European Union\u27s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [617516]

Heteroepitaxial hematite photoanodes as a model system for solar water splitting

Heteroepitaxial multilayer Pt(111)/Fe2O3(0001) films were deposited on sapphire c-plane (0001) substrates by RF magnetron sputtering and pulsed laser deposition, respectively. The films were highly crystalline, displaying an in-plane mosaic spread of less than 1° and a homogenous surface morphology with roughness of ∼3 Å. Ellipsometry and UV-vis spectroscopy measurements were shown to be in excellent agreement with modelling, demonstrating that the optics of the system including absorption in the hematite layer are well described. For polycrystalline hematite photoanodes deposited on platinum, full characterization of the system is hampered by the inability to make measurements in alkaline electrolyte containing hydrogen peroxide (H2O2) due to spontaneous decomposition of H2O2 by the exposed platinum. The pin-hole free high quality of the heteroepitaxial films is demonstrated by the ability to make stable and reproducible measurements in H2O2 containing electrolyte allowing for accurate extraction of charge separation and injection efficiency. The combination of excellent crystalline quality in addition to the well characterized optics and electrochemical properties of the heteroepitaxial hematite photoanodes demonstrate that Al2O3(0001)/Pt(111)/Fe2O3(0001) is a powerful model system for systematic investigation into solar water splitting photoanodes