Researchers at the Joint Center for Artificial Photosynthesis (JCAP) report the development of the first complete, efficient, safe, integrated solar-driven system—an “artificial leaf”—for splitting water to produce hydrogen. JCAP is a US Department of Energy (DOE) Energy Innovation Hub established at Caltech and its partnering institutions in 2010.
The new system has three main components: two electrodes—one photoanode and one photocathode—and a membrane. The photoanode uses sunlight to oxidize water molecules, generating protons and electrons as well as oxygen gas. The photocathode recombines the protons and electrons to form hydrogen gas. A key part of the JCAP design is the plastic membrane, which keeps the oxygen and hydrogen gases separate. If the two gases are allowed to mix and are accidentally ignited, an explosion can occur; the membrane lets the hydrogen fuel be separately collected under pressure and safely pushed into a pipeline.
One possible existing approach to solar-driven hydrogen production involves connecting photovoltaic (PV) panels, modules or cells physically and electrically in series with an electrolyzer (E), the researchers noted. Peak system efficiencies of 12.6% and 24.6%, respectively, could be obtained by use of an electrolyzer in conjunction with a high-efficiency (21%) Si PV module or a high-efficiency (41%) III-V triple junction PV operated under optical concentration.
Such systems have been demonstrated at commercial, laboratory and research scales, the team said. However, at the commercial level, the high balance of systems cost and low capacity factor results in high levelized hydrogen costs relative to hydrogen produced by steam reforming or grid electrolysis using fossil or low-carbon electricity. Thus, a lower-cost “artificial leaf” technology could offer significant advantages as an alternative.
In a paper published in the RSC journal Energy & Environmental Science, the researchers reported that their photoelectrosynthetic cell (GaAs/GaInP2/TiO2/Ni photoanode connected to a Ni-Mo coated counterelectrode) exhibited a solar-to-hydrogen conversion efficiency (ηSTH) of 10.5% under 1 sun illumination, with stable performance for > 40 h of continuous operation at an efficiency of ηSTH >10%.
An intrinsically safe solar-hydrogen prototype system (1 cm2) built with the device exhibited a hydrogen production rate of 0.81 μL s-1 and a solar-to-hydrogen conversion efficiency of 8.6% under 1 sun illumination, with minimal product gas crossover while allowing for beneficial collection of separate streams of H2 and O2.
This result was a stretch project milestone for the entire five years of JCAP as a whole, and not only have we achieved this goal, we also achieved it on time and on budget. This new system shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more.
Our work shows that it is indeed possible to produce fuels from sunlight safely and efficiently in an integrated system with inexpensive components. Of course, we still have work to do to extend the lifetime of the system and to develop methods for cost-effectively manufacturing full systems, both of which are in progress.—Nate Lewis, Caltech George L. Argyros Professor and professor of chemistry, and the JCAP scientific director
Semiconductors such as silicon or gallium arsenide absorb light efficiently and are therefore used in solar panels. However, these materials also oxidize (or rust) on the surface when exposed to water, so cannot be used to directly generate fuel. A major advance that allowed the integrated system to be developed was previous work in Lewis’s laboratory, which showed that adding a nanometers-thick layer of titanium dioxide (TiO2) onto the electrodes could prevent them from corroding while still allowing light and electrons to pass through. The new complete solar fuel generation system uses such a 62.5-nanometer-thick TiO2 layer to prevent corrosion and improve the stability of a gallium arsenide–based photoelectrode.
Another key advance is the use of active, inexpensive catalysts for fuel production. The photoanode requires a catalyst to drive the essential water-splitting reaction. Rare and expensive metals such as platinum can serve as effective catalysts, but in its work the team discovered that it could create a much cheaper, active catalyst by adding a 2-nanometer-thick layer of nickel to the surface of the TiO2. This catalyst is among the most active known catalysts for splitting water molecules into oxygen, protons, and electrons and is a key to the high efficiency displayed by the device.
The photoanode was grown onto a photocathode, which also contains a highly active, inexpensive, nickel-molybdenum catalyst, to create a fully integrated single material that serves as a complete solar-driven water-splitting system.
A critical component that contributes to the efficiency and safety of the new system is the special plastic membrane that separates the gases and prevents the possibility of an explosion, while still allowing the ions to flow seamlessly to complete the electrical circuit in the cell. All of the components are stable under the same conditions and work together to produce a high-performance, fully integrated system.
|A fully integrated photoelectrochemical device performing unassisted solar water splitting for the production of hydrogen fuel. Credit: Erik Verlage and Chengxiang Xiang/Caltech|
Funding was provided by the Office of Science at the US Department of Energy, and the Gordon and Betty Moore Foundation.
Verlage, Erik and Hu, Shu and Liu, Rui and Jones, Ryan J. R. and Sun, Ke and Xiang, Chengxiang and Lewis, Nathan and Atwater, Harry A., Jr. (2015) “A Monolithically Integrated, Intrinsically Safe, 10% Efficient, Solar-Driven Water-Splitting System Based on Active, Stable Earth-Abundant Electrocatalysts in Conjunction with Tandem III-V Light Absorbers Protected by Amorphous TiO2 Films.” Energy and Environmental Science doi: 10.1039/C5EE01786F.