Solar Fuel Production Just Needs a Change in Direction

Photoelectrodes that move electric charge diagonally split water better

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Electric charges can move more easily through cuprous oxide crystals diagonally than along their facets or edges.

Linfeng Pan

Solar and wind power are the fastest growing sources of electricity. However, they are both intermittent power sources—nighttime, cloudy days, and windless periods can all halt power generation. Scientists worldwide are investigating ways to harness solar power to produce storable solar fuels, like hydrogen, that can then be used when the sun doesn’t shine.

One way to produce solar fuels is with devices known as photoelectrodes, which can absorb light to split apart water molecules into hydrogen and oxygen. However, photoelectrodes are typically made using either expensive semiconductors and catalysts that are efficient at generating solar fuels, or abundant and inexpensive materials that are inefficient.

Now, researchers at the University of Cambridge, in England, have found that by guiding an electric charge diagonally through a particular crystal called cuprous oxide—instead of along its faces or edges—they can boost the performance of this cheap, abundant material.

The discovery is the latest advancement in a decade of progress researchers have made in boosting the performance of photoelectrodes made of cuprous oxide, which in addition to being cheap and plentiful is also nontoxic.

A key problem that currently limits cuprous oxide photoelectrodes is how the electric charges they generate upon exposure to light may recombine before they can accomplish useful work. Much remains poorly understood about this recombination in cuprous oxide, which limits attempts to improve its performance.

“We actually spent more time reproducing the synthesis process than in achieving the first good thin film.” —Linfeng Pan, University of Cambridge

One way to learn more about how electric charges move and recombine in cuprous oxide is to analyze their behavior in detail in single-crystal thin films of the material. However, making these kinds of thin films is normally a complex process involving high vacuum, high temperatures, and sophisticated instruments.

In the new study, researchers at the University of Cambridge grew high-quality single-crystal thin films of cuprous oxide at ambient pressure and room temperature. This greatly simplified the process of making them, which in turn made it easier to research them. In addition, by precisely controlling acidity and other factors in the growth chamber, the scientists could adjust the ways in which the atoms that make up the crystals were oriented in space.

The cuprous oxide crystals consist of cubes.The researchers discovered that when electrons move through these cubes diagonally, rather than along the faces or edges of the cubes, they can travel an order of magnitude farther.

Based on these findings, the scientists developed a simple, low-cost method to manufacture cuprous oxide photoelectrodes with a crystal orientation that favored diagonal charge mobility.

“To ensure the reproducibility of the method, we actually spent more time reproducing the synthesis process than in achieving the first good thin film,” says Linfeng Pan, a postdoctoral researcher at the University of Cambridge.

These devices showed a current density 75 percent greater than existing state-of-the-art photoelectrodes that were also made using relatively cheap metal oxides. That’s a “huge performance improvement,” Pan says.

In addition, cuprous oxide photoelectrodes favoring diagonal mobility operated without degrading while splitting water for more than 120 hours. In comparison, cuprous oxide photoelectrodes favoring other forms of mobility degraded after 7 to 30 hours.

The researchers’ next area of investigation is understanding why this diagonal mobility is better in terms of both current density and stability, Pan says. Eventually, this work boosting the performance of oxides may find use not just in solar fuels but also in photovoltaics, transistors, and detectors, the scientists note.

The scientists detailed their findings online 24 April in the journal Nature.

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