Semiconductor nanowires capture diffuse sunlight to split water and store energy as hydrogen

A U of A engineering researcher is using sunlight and semiconductor catalysts to produce hydrogen by splitting apart water molecules into their constituent elements.

“The process to form the semiconductor, called thermal condensation polymerization, uses cheap and Earth-abundant materials, and could eventually lead to a more efficient, economical path to clean energy than existing solar technologies,” says project lead Karthik Shankar of the Department of Electrical and Computer Engineering, an expert in the field of photocatalysis.

In a collaboration between the U of A and the Technical University of Munich, results of the research were published in the Journal of the American Chemical Society.

Shankar uses carbon nitride derived from urea—a widely available chemical used in fertilizers—to absorb sunlight which excites electrons to a higher energy level. Each electron, and the “hole” left in its absence, produce two quasi-particles in quantum terms called an “electron hole pair.”

Left to themselves, the quasi-particles will recombine to restore equilibrium. When in contact with a titanium dioxide catalyst, carbon nitride forms a junction between dissimilar semiconductors, a so-called semiconductor heterojunction, preventing them from recombining.

The titanium dioxide—another abundant and cheap material—binds to the carbon nitride electrons, which react with protons to create hydrogen. The hole binds to hydroxyl ions in water, generating oxygen.

A technology already exists for the first step in the process, producing the electron pair through photovoltaics coupled with electrolysis, says Shankar, but the two-part process is more expensive and less efficient.

“You use a solar panel to generate electricity, and then use that electricity to do what’s called dry water splitting through electrolysis,” he says. “That involves a lot of energy loss, whereas using sunlight directly to generate hydrogen is far more efficient.”

Shankar’s technology solves two problems inherent in solar cells. First, the cells only operate intermittently depending on the presence of sunlight, and are far less efficient when the sunlight is indirect. Shankar’s carbon nitride surface, designed with vertically oriented nanowires to capture diffuse light from any angle, can function even on cloudy days.

Energy produced by a solar cell also has to be stored, and advances in battery technology have been relatively sluggish. Hydrogen fuel acts as an efficient store. “In compressed form, it is dense and portable—it can be used whenever you need it,” says Shankar.

Solar-panel technology is also highly reliant on silicon, he adds, with silicon photovoltaics making up 85% of the global market.

“The process used to make photovoltaic panels has an extremely large environmental footprint,” says Shankar, adding that silicon has to be heated to 2,000 or 3,000 degrees Celsius, producing high CO₂, NO_x and SO₂ emissions.

And while silicon is one of the most abundant elements in Earth’s crust, says Shankar, 75% of it is produced in China and Russia—”not exactly best friends of the West”—for commercial use.

In addition to being inexpensive and abundant, carbon nitride has an astonishing chemical resilience, mechanical flexibility and thermal stability, withstanding temperatures of a few hundred degrees Celsius and resistant to a large number of acids and bases.

Melamine, a widely available chemical used in the cement industry and to make plastic dinnerware and laminates, can also be used in the process instead of carbon nitride. Shankar’s lab has also produced hydrogen from methanol—not as clean as using water but useful in some contexts.

If all goes well, large-scale commercialization could become a reality in three to five years, he says.