Solar Closing in on "Practical" Hydrogen Production

Israeli and Italian scientists have developed a renewable energy technology that converts solar energy to hydrogen fuel - and it's reportedly at the threshold of practical" viability.

The new solar tech would offer a sustainable way to turn water and sunlight into storable energy for fuel cells, whether that stored power feeds into the electrical grid or goes to fuel-cell powered trucks, trains, cars, ships, planes or industrial processes.

Think of this research as a sort of artificial photosynthesis, said Lilac Amirav, associate professor of chemistry at the Technion - Israel Institute of Technology in Haifa. (If it could be scaled up, the technology could eventually be the basis of solar factories" in which arrays of solar collectors split water into stores of hydrogen fuel--as well as, for reasons discussed below, one or more other industrial chemicals.)

We [start with] a semiconductor that's very similar to what we have in solar panels," says Amirav. But rather than taking the photovoltaic route of using sunlight to liberate a current of electrons, the reaction they're studying harnesses sunlight to efficiently and cost-effectively peel off hydrogen from water molecules.

The big hurdle to date has been that hydrogen and oxygen just as readily recombine once they're split apart-that is, unless a catalyst can be introduced to the reaction that shunts water's two component elements away from one another.

Enter the rod-shaped nanoparticles Amirav and co-researchers have developed. The wand-like rods (50-60 nanometers long and just 4.5 nm in diameter) are all tipped with platinum spheres 2-3 nm in diameter, like nano-size marbles fastened onto the ends of drinking straws.

Since 2010, when the team first began publishing papers about such specially tuned nanorods, they've been tweaking the design to maximize its ability to extract as much hydrogen and excess energy as possible from "solar-to-chemical energy conversion."

Which brings us back to those "other" industrial chemicals. Because creating molecular hydrogen out of water also yields oxygen, they realized they had to figure out what to do with that byproduct. When you're thinking about artificial photosynthesis, you care about hydrogen-because hydrogen's a fuel," says Amirav. Oxygen is not such an interesting product. But that is the bottleneck of the process."

There's no getting around the fact that oxygen liberated from split water molecules carries energy away from the reaction, too. So, unless it's harnessed, it ultimately represents just wasted solar energy-which means lost efficiency in the overall reaction.

So, the researchers added another reaction to the process. Not only does their platinum-tipped nanorod catalyst use solar energy to turn water into hydrogen, it also uses the liberated oxygen to convert the organic molecule benzylamine into the industrial chemical benzaldehyde (commonly used in dyes, flavoring extracts, and perfumes).

All told, the nanorods convert 4.2 percent of the energy of incoming sunlight into chemical bonds. Considering the energy in the hydrogen fuel alone, they convert 3.6 percent of sunlight energy into stored fuel.

These might seem like minuscule figures. But 3.6 percent is still considerably better than the 1-2 percent range that previous technologies had achieved. And according to the U.S. Department of Energy, 5-10 percent efficiency is all that's needed to reach what the researchers call the practical feasibility threshold" for solar hydrogen generation.

Between February and August of this year, Amirav and her colleagues published about the above innovations in the journals NanoEnergy and Chemistry Europe. They also recently presented their research at the fall virtual meeting of the American Chemical Society.

In their presentation, which hinted at future directions for their work, they teased further efficiency improvements courtesy of new new work with AI data mining experts.

We are looking for alternative organic transformations," says Amirav. This way, she and her collaborators hope, their solar factories can produce hydrogen fuel plus an array of other useful industrial byproducts. In the future, their artificial photosynthesis process could yield low-emission energy, plus some beneficial chemical extracts as a practical" and "feasible" side-effect.