Advances in Producing Hydrogen by Mimicry of Photosynthesis
Image: Hydrogen spectrum, cosmiccandace
Last time we checked in on Thomas Mallouk's work applying biomimicry to generate hydrogen, he was reporting about 0.3 percent efficiency. According to his projections, the proof-of-concept device for producing hydrogen using the same trick applied in plants for photosynthesis could eventually reach efficiencies of 10-15%, beating nature's average of 1 to 3 percent. Mallouk is back at the annual meeting of the American Association for the Advancement of Science (AAAS). So what's the news on one promising option for generating large quantities of clean fuel for combustion engines or fuel cells?
The Hardest Way to Make Fuel
Calling his process "the hardest way to make fuel," Thomas Mallouk, Evan Pugh professor of materials chemistry and physics at Penn State, uses organic dyes to catch photons. In photosynthesis, light hits a color complex in the plant, where the energy from the light frees an electron. Both processes work a bit like those devices with five steel balls, where the leading ball is dropped, striking the other four balls. Instead of sending all four balls flying, three balls remain motionless, while the fifth ball flies off with all (OK, almost all) of the energy of the initial ball.
A photon, effectively a particle of light, hits the organic dye molecule (which we perceive as colored precisely because it reacts with light in interesting ways) and an electron flies off. If that electron can be made to strike a water molecule just right, the water will break into two parts: hydrogen and oxygen. Mallouk's system uses an iridium oxide catalyst to help the electron do its work.
The Most Expensive Way to Make Fuel
Electrodes in the system, which help to carry the electrons to where they can break the water, must be made of extremely stable components like titanium and platinum because the reactions take place in a very aggressive environment. Oxygen atoms freed from the water are powerful "oxidizing agents." Think of fire burning or your car rusting, but much faster. Currently, even using such high-quality components, the lifespan of Mallouk's system is measured in hours. In nature, the proteins and complexes exposed to oxygen also suffer, but nature constantly renews these parts of its machine to maintain efficiency. So until better substitutes are found, Mallouk's process is not only the hardest, but one of the most expensive.
Work to find more reasonably priced substitutes is ongoing at the Massachusetts Institute of Technology, Princeton, and Yale, among others. Investigations are targeting cobalt and nickel and manganese catalysts.
The Most Efficient Way to Make Fuel?
In theory, a photosynthesic process is much more efficient than catching light and turning it into electricity, for example with solar cells, before using that electricity to give water the huge energy jolt needed to force the hydrogen to leave the comfort of the water molecule. Using fossil fuels to split the water is even worse, ending up with less energy in the form of hydrogen than existed in the fuel in its original form. Benefits would be achieved only when centralized pollution controls are used, and the advantage of distributed use of the clean hydrogen fuel could be realized.
"Currently, we are getting only 2 to 3 percent yield of hydrogen," Mallouk told attendees at the AAAS meeting, according to the Penn State press release. "For systems like this to be useful, we will need to get closer to 100 percent," he added.
Options to improve the efficiency focus on understanding the details of each step in the process. One major loss of efficiency is due to the electrons recombining. Nature minimizes this loss by controlling the process speeds so that an electron is always in the right place at the right time to be used by oxygen-evolving complexes. After defining the kinetics (speeds of reactions), Mallouk can use computer modelling to test hypothesis about optimizing the process.
Other options to increase efficiency include using the full spectrum of light energy, instead of using only the blue light as is currently the case, and capturing not one, but two photons, from the incoming light.
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