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Physics + MathPhysics & Math

Hydrogen’s Future as a Fuel May Hinge on the Fate of Four Electrons

ByTim De ChantNOVA NextNOVA Next

Hydrogen is often lauded as the clean fuel of the future, but it has a dirty secret—most hydrogen produced today comes from natural gas. While it’s cleaner than simply burning the natural gas, it’s certainly not a zero-emission fuel. If hydrogen is ever going to be truly clean, the best way to make it is by splitting plain water into hydrogen and oxygen.

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Few things on this planet do that better than plants. Exactly how they do it has remained a mystery, but in recent years, a flurry of breakthroughs has deepened our understanding of the process. Now, we’re tantalizingly close.

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citrus leaf
Plants use photosynthesis to split water into hydrogen and oxygen.

Two theories have emerged to describe the water-splitting process in plants, known as the Kok cycle. Central to the cycle is an enzyme known as the oxygen-evolving complex (OEC), which contains a core of manganese and calcium. The competing theories are at odds over how many electrons each manganese atom holds at each stage. Arguing over numbers of electrons may sound tedious, but determining the right answer could lead to an energy revolution.

Scientists have known for some time that the Kok cycle uses the OEC to split two molecules of H 2 O to produce one molecule of O

2 , four hydrogen ions (H + ), and four electrons. At each step along the way, photons of light interact with the OEC, changing the number of electrons the manganese atoms have and making them more reactive, which is required to break up the water molecules’ tight hydrogen-oxygen bonds.

Whether the manganese atoms gain or lose electrons has remained an unanswered question. In a quest to find the answer, Dimitrios Pantazis, a researcher at the Max Planck Institute for Chemical Energy Conversion in Germany, and his colleagues gathered data from published papers and entered it into a computer model that simulated a number of different scenarios, including ones where the manganese in OEC gained electrons others where it lost electrons. When the computers were finished crunching the numbers, they had their answer. Only by losing electrons, they say, can the manganese in the OEC successfully split water.

Not everyone agrees with these results, according to an article by Jason Woolford at Chemistry World, but Pantazis and his team’s findings give scientists a hypothesis to test. Subsequent experiments could reveal more secrets about the Kok cycle and OEC, including how we might use it to produce hydrogen on a commercial scale.

We’re still several steps from developing plant-inspired water-splitting devices, but we’re now a big step closer. Should scientists succeed in finding a way to create hydrogen from water and sunlight, we may finally have a clean source of potent and portable energy, one that could finally wean us from fossil fuels.

Photo credit: ZooFari/Wikimedia Commons (CC BY-SA)

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