Hydrogen and oxygen gas generated by water-splitting solar-powered electrodes (Alain Herzog, EPFL)
26 September 2014. Engineers at Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland designed a solar energy system made of inexpensive and abundant materials that efficiently splits water into hydrogen and oxygen for producing electricity. The team from the lab of EPFL’s Michael Grätzel, with colleagues from Singapore and Korea, published its findings in today’s issue of the journal Science (paid subscription required).
Producing energy from the sun is a longstanding goal of scientists, business people, and policy makers eager for alternatives to fossil fuels that create greenhouse gases as well as economic dependence on hostile or unstable parts of the world. Developing energy systems that harness the sun, however, are hampered by continuing challenges of cost, efficiency, scalability, and storage for times when sunlight isn’t available.
The researchers led by postdoctoral researcher Jingshan Luo devised a system combining solar cells made of the perovskite, a calcium-titanium oxide mineral, with electrodes made of inexpensive metals. Perovskite is attracting increasing scientific and industrial interest as an alternative to silicon in photovoltaic solar cells, with new companies being formed to commercialize advances in the technology.
The system designed by Luo and colleagues simulates the photosynthesis process in plants, converting energy from the sun into a chemical fuel, in this case hydrogen. The solar cells built from perovskite produce an electric current, with the system using that current to split water into its hydrogen and oxygen components. The hydrogen produced by the system can then be captured and stored for fuel cells that generate electric power on demand, with water their only byproduct.
Most water-splitting systems up to now require platinum as a catalyst in electrodes to generate an electrochemical reaction that releases hydrogen and oxygen. Platinum, however, is expensive and Grätzel’s lab is working on finding alternatives as catalysts in energy systems. For this system, the EPFL team tested electrodes made of various materials, but found nickel-iron hydroxides deposited in foam layers got the best results, comparable to platinum. The foam configuration provided more surface area for the catalyst than a smooth surface.
The researchers wired two perovskite solar cells hooked to the nickel-iron electrodes, in series and in close proximity, in an aqueous alkaline solution. The team needed 2 solar cells to generate enough current, about 10 milliamperes per square centimeter, to power the system, although 3 or more silicon-based solar cells are usually needed for that output, say the authors. In lab tests, the team found the system produces hydrogen gas with a solar-to-hydrogen efficiency of 12.3 percent.
The system still has some drawbacks, say the researchers. The perovskite solar cells degrade after a few hours, and greater efficiencies are probably achievable with a more integrated system. Because perovskite solar cells are rapidly developing greater capacity and performance, the authors believe greater energy output and stability are feasible.
Luo tells more about the EPFL system in the following video.
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