Bottling sunlight as hydrogen fuel

Home Technologist Online Bottling sunlight as hydrogen fuel

Scientists have converted solar energy into hydrogen with unrivalled efficiency using cheap, abundant materials, eliminating the need for rare metals. This new way of producing hydrogen fuel from sunlight and water is set to become a strong contender in the solar energy race towards a post-fossil fuel future.

Gas produced from perovskite solar cells combined with nickel and iron catalysts

One of the big challenges of harnessing the sun’s energy is to find an efficient way to store it – a must if solar energy is ever going to be used at the scale of fossil fuels. Renewable energy researchers have therefore worked long and hard to develop artificial photosynthesis systems to convert sunlight and water directly into hydrogen gas. This gas can either be burned as a fuel, releasing only water vapour, or piped through a fuel cell to generate electricity on demand – not only when the sun is shining.

“Once you have hydrogen, you [can] store it in a bottle and you can do with it whatever you want to, whenever you want it,” says Michael Grätzel from École Polytechnique Fédérale de Lausanne (EPFL) in a press release.

Grätzel heads the research laboratory where researchers from Switzerland, Singapore and Korea recently developed an exceptional device that converts 12.3 per cent of the energy from sunlight into hydrogen, as reported in the journal Science.

New record in solar water splitting using cheap, abundant materials

To do this, they combined a pair of solar cells, made with a mineral called perovskite as the absorber layer, and an electrolyser that split water molecules into hydrogen and oxygen.

The perovskite can be obtained in the laboratory from common materials, such as those used in conventional car batteries, eliminating the need for rare metals in the production of hydrogen fuel.

“Both the perovskite used in the cells and the nickel and iron catalysts making up the electrodes require resources that are abundant on Earth and that are also cheap,” says the lead author of the paper, Jingshan Luo, in the press release. “However, our electrodes work just as well as the expensive platinum-based models customarily used.”

The device’s 12.3 per cent solar-to-hydrogen is unrivalled by other systems composed entirely of Earth-abundant materials, as opposed to rare metals.

According to Thomas Hamann, a solar energy researcher from Michigan State University not involved in the research, very few other systems described so far produce over 10 per cent solar-to-hydrogen efficiencies. “There are no previous examples of over 10 per cent cent solar-to-hydrogen [conversion efficiency] from systems that have used only Earth-abundant elements and been driven by potentially cheap photovoltaic [systems],” he writes in a Science editorial.

Perovskite more efficient than silicon solar cells

The creators of the new device promise that its conversion efficiency will soon get even higher. The high efficiency values are possible thanks to a special characteristic of perovskite cells: their ability to generate an open circuit voltage greater than 1 V. In comparison, silicon solar cells stop at 0.7 V.

“A voltage of 1.7 V or more is required for water electrolysis to occur and to obtain exploitable gases,” Luo explains in the EPFL press release.

This means that three or more silicon cells need to be wired together to produce a voltage high enough to split water, whereas two perovskite cells are enough. “This is the first time we have been able to get hydrogen through electrolysis with only two cells,” Luo adds.

Three of four criteria met

In his editorial, Hamann states that Luo and colleagues have achieved three of the four criteria needed for a viable solar water splitting system. “It needs to be efficient, cost-effective, and scalable – thus composed of Earth-abundant materials – and stable enough to run for years. So far, nobody has been able to meet all four requirements,” he writes.

“One major drawback of this system is the instability of the perovskite photovoltaics, which results in a degradation of the photocurrent over a period of hours. The cause of the instability is not yet fully understood, and this issue clearly needs to be resolved if these perovskite photovoltaics are ever to be commercially viable.

“It will be exciting to see if perovskites will be the first to meet all four criteria to win the solar hydrogen race and beat out fossil fuels for our energy future.”

– by Lillian Sando

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