A compound that absorbs near-infrared light can be used to split hydrogen gas from water for future energy applications. The compound has three ruthenium atoms connected by an organic molecule that give it electronic characteristics not found in other related materials. The NIR photons allow its electrons to 'jump' into orbitals that are simply not present in those other materials. The subsequent energy exchange allows hydrogen to be split from water.
Hydrogen gas is a promising "green" fuel that could be used in fuel cells or other power sources for future low-emissions vehicles for instance. There are countless ways to split the hydrogen and oxygen atoms in water, which thus represents a useful source of this gas. Solar energy is perhaps the cleanest and most sustainable method. Now, researchers at Kyushu University, Japan, have overcome the problem of all that lost energy that lies below the visible spectrum. Their approach allows UV, visible, and NIR, to be harvested from sunlight by exploiting ruthenium chemistry. [Tsuji et al., Angew Chem Int Edn, (2017); DOI: 10.1002/anie.201708996]
The team points out that some metal-organic hybrid materials are good at trapping light, but these usually require highly energetic ultraviolet photons for the electrons to make the requisite leap. In conventional materials red, NIR, and even longer infrared are simply bounced back or pass through the material unused.
The Kyushu design is different: "We introduced new electron orbitals into the ruthenium atoms," explains team member Ken Sakai. "It's like adding rungs to a ladder - now the electrons in ruthenium don't have so far to jump, so they can use lower energies of light such as red and NIR. This nearly doubles the amount of sunlight photons we can harvest," he adds.
Their approach exploited a polypyridyl unit to hook together the three ruthenium ions to make their photosensitizer. This not only creates the extra "rungs" needed to allow red and NIR to be absorbed rather than reflected or lost as heat but it also makes the reaction more efficient due to spatial expansion of the light harvesting part of the molecule. The effect is to accelerate hydrogen production overall.
"It's taken decades of efforts worldwide, but we've finally managed to drive water reduction to evolve hydrogen gas using near infrared," Sakai explains. "We hope this is just the beginning -the more we understand the chemistry, the better we can design devices to make clean, hydrogen-based energy storage a commercial reality."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.