EPFL/Technion team develops “champion” nanostructures for efficient solar water-splitting to produce hydrogen
|Hydrogen bubbles as they appear in a photoelectrochemical cell. © LPI / EPFL. Click to enlarge.|
Researchers from EPFL in Switzerland and Technion-Israel Institue of Technology have developed nanoparticle-based α-Fe2O3 (hematite) electrodes that achieve the highest photocurrent of any metal oxide photoanode for photoelectrochemical water-splitting under 100 mW cm−2 air mass, 1.5 global sunlight. A paper on their work is published in the journal Nature Materials.
With current methods, in which a conventional photovoltaic cell is coupled to an electrolyzer to produce hydrogen, the cost to produce hydrogen from water using the sun is around €15 per kilo at its cheapest, said research leader Dr. Michael Grätzel, Director of the Laboratory of Photonics and Interfaces (LPI) at EPFL and inventor of dye-sensitized photoelectrochemical cells. “We’re aiming at a €5 charge per kilo,” he said.
Batteries, fuel cells and solar-energy conversion devices have emerged as a class of important technologies that increasingly rely on electrodes derived from nanoparticles. These nanoparticle-based materials provide a unique challenge in assessing structure–property relationships because of the disordered arrangement of nanocrystals that results when nanoparticles collide and aggregate. The morphological evolution that follows aggregation further obscures the influence of particle size, shape and interfacial characteristics in defining the physical properties of these materials.
For the nanoparticle-based electrodes used in solar energy conversion, structural defects such as grain boundaries define pathways for charge transport by creating potential barriers and by promoting recombination. Owing to the complexity of these materials, within a single electrode there may exist a small proportion of champion nanostructures—by analogy with champion solar cells, these are nanostructures that provide the highest solar conversion efficiencies—that contribute most of the electrode’s photocurrent. Further improvement of device performance requires an analytical approach that identifies these champion nanostructures, quantitatively relating their microstructural features to their charge transport characteristics.—Warren et al.
The team, led by Dr. Grätzel and Prof. Avner Rothschild at Technion in Israel, developed an approach for correlating the spatial distribution of crystalline and current-carrying domains in entire nanoparticle aggregates. In correlating structure and charge transport with nanometer resolution across micrometer-scale distances, they identified the existence of these “champion” nanoparticle aggregates that are most responsible for the high photoelectrochemical activity of the electrodes.
Today we have just reached an important milestone on the path that will lead us forward to profitable industrial applications.—Michael Grätzel
The whole point of our approach is to use an exceptionally abundant, stable and cheap material: rust.—Scott C. Warren, first author, now at the University of North Carolina at Chapel Hill
At the end of last year, Kevin Sivula, one of the collaborators at the LPI laboratory, presented a prototype electrode based on the principle. Its efficiency was such that gas bubbles emerged as soon as it was under a light stimulus.
By using transmission electron microscopy (TEM) techniques, researchers were able to precisely characterize the movement of the electrons through the cauliflower-looking nanostructures forming the iron oxide particles, laid on electrodes during the manufacturing process.
By comparing several electrodes, whose manufacturing method is now mastered, scientists were able to identify the “champion” structure. A 10x10 cm prototype has been produced and its effectiveness is in line with expectations, the researchers said. The next step will be the development of the industrial process to large-scale manufacturing. A European funding and the Swiss federal government could provide support for this last part.
The long-term goal is to produce hydrogen in an environmentally friendly and especially competitive way.
Scott C. Warren, Kislon Voïtchovsky, Hen Dotan, Celine M. Leroy, Maurin Cornuz, Francesco Stellacci, Cécile Hébert, Avner Rothschild and Michael Grätzel (2013) Identifying champion nanostructures for solar water-splitting. Nature Materials doi: 10.1038/nmat3684