Photo Credit:
Prof. Lilac Amirav, Technion-Israel Institue of Technology

Researchers Philip Kalisman, Yifat Nakibli, and Lilac Amirav of the Technion-Israel Institute of Technology in Haifa, Israel, have recently published a paper based on their research on reaching perfect efficiency in splitting water using sunlight as the only source of power. The main application of splitting water into its components of oxygen and hydrogen is that the hydrogen can then be used to deliver energy to fuel cells for powering vehicles and electronic devices.

Their paper, Perfect Photon-to-Hydrogen Conversion Efficiency, appeared in a recent issue of Nano Letters. They introduced their findings, writing, “We report a record 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water-splitting reduction half-reaction. … The implications of such record efficiency for the prospects of realizing practical over all water splitting and solar-to-fuel energy conversion are discussed.”

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“I strongly believe that the search for clean and renewable energy sources is crucial,” Amirav told Phys.org, adding, “With the looming energy crisis on one hand, and environmental aspects, mainly global warming, on the other, I think this is our duty to try to amend the problem for the next generation.”

The results, according to Phys.org, blow out of the water (so to speak) the previous record of 60% efficiency for hydrogen production with visible light (meaning the two-step process of splitting water), and consequently future research should focus on the other step (oxidation) in order to achieve practical overall water splitting.

Under visible light illumination, the nanoscale photocatalysts perform the water-splitting reduction half-reaction with 100% efficiency. Credit: Lilac Amirav, Technion-Israel Institue of Technology

To get a water molecule to split up, the full reaction takes two H2O molecules, and proceeds with two separate half-reactions. In the oxidation half-reaction, four hydrogen atoms are produced and an O2 molecule (which is discarded). Then, in the reduction half-reaction, the four hydrogen atoms are paired up into two H2 molecules by adding electrons, which produces the useful form of hydrogen: the H2 gas. Hydrogen gas is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.

The Technion researchers showed in their new study that it is possible to get the reduction half-reaction with perfect efficiency, using specially designed nanorods that are placed in a water-based solution under visible light. The light provides the energy required to stimulate the reaction, with the nanorods acting as photocatalysts by absorbing the photons and releasing the electrons for the reaction.

The 100% efficiency in the photon-to-hydrogen conversion means that all the photons that reach the photocatalyst have generated an electron, and then every two electrons have produce one H2 molecule. This 100% outcome means the half-reaction produces some 100 H2 molecules per second (or one every 10 milliseconds) on each nanorod, with a typical sample containing some 600 trillion nanorods.

“Our work shows that it is possible to obtain a perfect 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water splitting reduction half-reaction,” Amirav told Phys.org. “These results shatter the previous benchmarks for all systems, and leave little to no room for improvement for this particular half-reaction. With a stable system and a turnover frequency of 360,000 moles of hydrogen per hour per mole of catalyst, the potential here is real.”

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