Hey, whatever happened to the artificial leaf? It was all the rage a few years ago, when scientists figured out that you could dunk a solar device in water, and out would come hydrogen gas — aka green hydrogen. But then, electrolysis kind of took over and the artificial leaf concept seemed to wither on the vine. And now all of a sudden it’s back. What is going on??
Electrolysis Vs. Photoelectrochemical Reaction For Green Hydrogen
For those of you new to the topic, hydrogen is a zero emission fuel, but right now the primary source of H2 is fossil gas, but also right now there is a lot of activity going on in the field of electrolysis, in which hydrogen gas pops out of plain water when you apply an electrical current.
So much the better if you get that electricity from solar panels or wind turbines. Et voilà, green hydrogen.
That sounds simple enough. The main hurdle is the cost of the electrolysis equipment, which used to be sky high but now those costs have been sinking rapidly, and electrolyzers are already coming into play commercially.
Meanwhile, the artificial leaf concept has been swirling around in the laboratory, along with its “solar fuel” cousins the supersonic leaf and the bionic leaf.
The basic idea is to skip the electricity middleperson and spark a reaction in water directly, through solar energy. If you’re thinking of a photoelectrochemical reaction, run right out and buy yourself a cigar.
The beauty of this direct solar-to-hydrogen is simplicity. A photoelectrochemical device requires no other inputs aside from solar energy, which means the technology could be dirt cheap, eventually.
Green Hydrogen & Perovskite Solar Cells
The artificial leaf concept has a lot of catching up to do to beat electrolysis on cost, which is the aim. However, based on a pair of new developments this spring, it looks like photoelectrochemistry is making a run for it and perovskites could be the winning ticket.
For those of you new to the perovskite topic, that refers to a super-cheap but fairly efficient crystalline material that only burst onto the solar cell field a few years ago but has already captured hearts and minds of thousands of solar researchers around the world.
Where were we? Oh right, those two developments, which involve perovskites. First up is the Rice University Brown School of Engineering, which dropped this nugget into the media waters back on May 4:
“…the lab of Rice materials scientist Jun Lou integrates catalytic electrodes and perovskite solar cells that, when triggered by sunlight, produce electricity. The current flows to the catalysts that turn water into hydrogen and oxygen, with a sunlight-to-hydrogen efficiency as high as 6.7%.”
That figure of 6.7% sounds pretty impressive considering the state of the research, but check out last week’s news from a research team based at the Australian National University:
“Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p+nn+ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination.”
Next Steps For Global Decarbonization, STAT
That’s nothing to sneeze at, but check out what happened when they tweaked their device just a little bit:
“Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting.”
Just goes to show you can’t look back nowadays because somebody might be gaining on you. The figure of 17.6% is significant because 20% efficiency is the goal for cost parity with electrolysis. That’s what they’re saying, anyways. If you can find a source for that 20% goal, drop us a note in the comment thread.
Meanwhile, keep an eye on the UK’s “Gigastack” green H2 project for an idea of how quickly commercial electrolysis technology can scale up.
Anyways, all of this is important because, although fans of hydrogen fuel cell passenger cars may have a long wait ahead of them, it looks like a consensus is forming around the use of green hydrogen to decarbonize heavy-duty sectors globally including long haul trucking and marine transportation, as well as industries like steel making.
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Image: via Rice University, “A schematic and electron microscope cross-section show the structure of an integrated, solar-powered catalyst to split water into hydrogen fuel and oxygen. The module developed at Rice University can be immersed into water directly to produce fuel when exposed to sunlight,” by Jia Liang.
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