The idea of an “artificial leaf” sounds simple enough: Take a small, cheap, light-collecting device the size of a typical leaf, dunk it in a quart of water, and use solar energy to generate enough hydrogen gas for powering a small fuel cell. Scaled up, these solar-derived fuel cells would provide an energy storage solution that puts solar power on the same consistent, reliable footing as any fossil fuel.
Various researchers have gotten most of the way there only to get hung up on that little word, “cheap,” but a new study has revealed a pathway that could result in a cost-effective solution.
An Artificial Leaf On The Cheap
We’re calling this device an “artificial leaf” but to be more precise, it’s a photoelectrochemical (PEC) cell. In contrast to a photovoltaic cell, which converts light to electricity, a PEC cell collects photons and converts them to energetic electrons, which touch off a chemical reaction splitting water into hydrogen and oxygen gas.
One main hurdle has been the development of a catalyst that is efficient enough, and cheap enough, for such a device to hit a reasonable price point. Plenty of high-efficiency catalysts abound, relatively speaking, but they are far too expensive to make sense.
Relatedly, the most efficient semiconductors also tend to corrode when exposed to water, which kind of kills the whole deal right there.
The new study comes from the National Institute of Standards and Technology (NIST), which has proposed a silicon-based PEC, in which a layer of silicon dioxide insulates the semiconductor. The top layer of this “sandwich” is an array of tiny electrodes made of titanium and coated with platinum.
As described by NIST, the protective layer of silicon dioxide is thin enough to admit a sufficient amount of light to get the job done:
“…photons will travel through it to the semiconductor, and the photo-generated electrons will “tunnel” in the opposite direction to reach the electrodes, where the platinum catalyzes the reaction that produces hydrogen.”
NIST also notes that part of the savings would come in during the manufacturing process, which could be based on standard fabrication methods that are already common throughout the electronics industry.
The Tradeoff Between Cheap And Efficient
For its trouble, the NIST calculated an efficiency of 2.9 percent which does not sound like much, but keep in mind that we’re talking about PEC cells, not photovoltaic cells. At 2.9 percent, NIST’s results were 15 times more efficient than the next-best silicon based designs, and the team is already looking at new paths to improving on that.
The other thing to keep in mind is that solar conversion efficiency is not an end in itself, it is a means to affordability. In that regard, it’s helpful to look at the potential market for small scale PEC devices, as championed by Harvard researcher Daniel Nocera (formerly of MIT).
We’ve been following Nocera’s self-described artificial leaf for a couple of years now, and his silicon based PEC concept is aimed squarely at the potential fuel cell market in undeveloped communities.
Aside from quality of life improvements, the mass introduction of low cost PEC/fuel cell devices would help reduce dependency on wood, charcoal and other dirty fuels that are currently burned in primitive cookstoves in hundreds of millions of households, contributing a sizeable chunk of black carbon and greenhouse gas emissions to the climate change equation.
Another Obstacle For The Artificial Leaf
Aside from cost, there is a huge obstacle to mainstreaming the artificial leaf concept, especially in undeveloped communities, and that is the fresh water scarcity issue.
PEC cells need fresh, clean water to operate efficiently, partly because of the corrosion issue and partly because impurities build up a biofilm on the catalyst.
Nocera’s team recently solved part of the equation by coming up with a way to tweak the surface of the catalyst, roughing it up enough to prevent biofilm buildup.
Interestingly, the tweak also resulted in a self-repairing surface. The roughened surface is fragile and falls apart, but it also goes through a cycle of healing and reassembly.
That still leaves the issue of using non-potable water for fuel cells instead of irrigating crops, but with utility scale solar powered desalination technology in the foreseeable future, that problem could be at least partly solved by using seawater.
An Artificial Leaf, Really?
The first time we wrote about the artificial leaf concept, one of our commentors took issue with that description. Our thinking is that if it’s good enough for Daniel Nocera, it’s good enough for CleanTechnica, but if you really want to get a step closer to the real thing, take a look over at Oak Ridge National Laboratory, where researchers have been using spinach to generate hydrogen gas from water.
Well, not the whole spinach leaf. Spinach proteins, to be more precise.
A similar path is being pursued by a team at North Carolina State University, using a combination of chlorophyll and carbon nanotubes.
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