Researchers from MIT and Sun Catalytix develop an artificial leaf for solar water splitting to produce hydrogen and oxygen
Researchers led by MIT professor Daniel Nocera have produced an “artificial leaf”—a solar water-splitting cell producing hydrogen and oxygen that operates in near-neutral pH conditions, both with and without connecting wires. (Earlier post.)
In a paper published in the journal Science, they report that the cells carry out the solar-driven water splitting reaction at direct solar-to-fuels efficiencies of 2.5% (wireless configuration) and 4.7% (wired configuration) when driven by a solar cell of 6.2% and 7.7% light-to-electricity efficiency, respectively, and when illuminated with 1 sun of AM 1.5 simulated sunlight. The cells consist of a triple junction, amorphous silicon photovoltaic interfaced to hydrogen and oxygen evolving catalysts made from an alloy of earth-abundant metals and a cobalt-borate catalyst, respectively.
By constructing a simple, stand-alone device composed of silicon-based light absorbers and earth-abundant catalysts, the results described herein provide a first step down a path aligned with the low-cost systems engineering and manufacturing that is required for inexpensive direct solar-to-fuels systems.—Reece et al.
Placed in a container of water and exposed to sunlight, the device quickly begins to generate oxygen from one side and hydrogen bubbles from the other. If placed in a container that has a barrier to separate the two sides, the two streams of bubbles can be collected and stored, and used later to deliver power: for example, by feeding them into a fuel cell that combines them once again into water while delivering an electric current.
Nocera, the Henry Dreyfus Professor of Energy and professor of chemistry at MIT, is the senior author of the paper, which was co-authored by his former student Steven Reece PhD ’07 (who now works at Sun Catalytix, a company started by Nocera to commercialize his solar-energy inventions), along with five other researchers from Sun Catalytix and MIT.
We show that water-splitting catalysts comprising earth- abundant materials can be integrated with amorphous silicon with minimal engineering to enable direct solar-to-fuels conversion based on water splitting. For the O2 evolving catalyst, we use a cobalt catalyst, Co-OEC, that self-assembles upon oxidation of Co2+, self-heals, and that can operate in buffered electrolyte with pure or natural water at room temperature. These attributes are similar to those of the OEC found in photosynthetic organisms.
The H2 evolving catalyst is a ternary alloy, NiMoZn. These catalysts have been interfaced directly with a commercial triple junction amorphous silicon (3jn-a-Si) solar cell (Xunlight Corp.) in wired and wireless configurations. For either, the cell uses stacked amorphous silicon and amorphous silicon-germanium alloy junctions deposited on a stainless steel substrate and coated with a 70 nm layer of Indium Tin Oxide (ITO). While the abundance of Ge may be a source of debate, the use of a silicon-based light absorber represents a major step towards a device composed of all earth-abundant materials for solar water splitting. Co-OEC is deposited directly onto the ITO layer (the illuminated side of the cell).
The NiMoZn alloy H2 catalyst was used in two configurations: (i) deposited on a Ni mesh substrate that is wired to the 3jn-a-Si solar cell and (ii) deposited directly on the opposing stainless steel surface of the 3jn-a-Si solar cell as a wireless device. The devices, which have not been optimized for performance may operate out of an open container of water containing borate electrolyte and with overall . The overall conversion efficiency of the wired cell indicates that a majority of the current from the solar cell can be converted directly to solar fuels and that a simply engineered functional artificial leaf comprising earth-abundant materials may be realized.—Reece et al.
The new device is not yet ready for commercial production, since systems to collect, store and use the gases remain to be developed. Ultimately, Nocera sees a future in which individual homes could be equipped with solar-collection systems based on this principle: Panels on the roof could use sunlight to produce hydrogen and oxygen that would be stored in tanks, and then fed to a fuel cell whenever electricity is needed.
Such systems, Nocera hopes, could be made simple and inexpensive enough so that they could be widely adopted throughout the world, including many areas that do not presently have access to reliable sources of electricity.
Professor James Barber, a biochemist from Imperial College London who was not involved in this research, says Nocera’s 2008 finding of the cobalt-based catalyst was a “major discovery,” and these latest findings “are equally as important, since now the water-splitting reaction is powered entirely by visible light using tightly coupled systems comparable with that used in natural photosynthesis. This is a major achievement, which is one more step toward developing cheap and robust technology to harvest solar energy as chemical fuel.”
There will be much work required to optimize the system, particularly in relation to the basic problem of efficiently using protons generated from the water-splitting reaction for hydrogen production. But there is no doubt that their achievement is a major breakthrough which will have a significant impact on the work of others dedicated to constructing light-driven catalytic systems to produce hydrogen and other solar fuels from water. This technology will advance side by side with new initiatives to improve and lower the cost of photovoltaics.—James Barber
Nocera’s ongoing research with the artificial leaf is directed toward driving costs lower and looking at ways of improving the system’s efficiency.
Steven Y. Reece, Jonathan A. Hamel, Kimberly Sung, Thomas D. Jarvi, Arthur J. Esswein, Joep J. H. Pijpers, and Daniel G. Nocera (2011) Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science DOI: 10.1126/science.1209816