Researchers at Rice University are on to a relatively simple, low-cost way to pry hydrogen loose from water, using the sun as an energy source. The new system involves channeling high-energy “hot” electrons into a useful purpose before they get a chance to cool down. If the research progresses, that’s great news for the hydrogen fuel cell electric vehicle market, which has been growing in some niche sectors but stumbling over the cost barrier when it comes to passenger cars and buses.
Renewable Hydrogen From Water
For those of you new to the topic, the high energy density of hydrogen makes it ideal for fuel cell electric vehicles, but manufacturing hydrogen is an energy-intensive process that currently depends on fossil natural gas as a source.
The emergence of solar water-splitters could solve both of those problems together, by using renewable energy to split water into hydrogen and oxygen. Wind, hydropower, and tidal energy are also possible “clean” power sources for manufacturing hydrogen from water. Potable water resources aren’t necessarily compromised, as emerging technology works on less-than-clean water, including municipal wastewater.
The Rice University Solar Water-Splitter
The new Rice University hydrogen system resolves some of the problems besetting conventional water-splitting attempts.
The research team developed a three-layer material, which starts with a thin sheet of aluminum coated with a nanoscale, transparent layer of nickel oxide. The topmost layer is a smattering of ultra-tiny gold disks ranging from 10 to 30 nanometers in diameter.
The material can collect sunlight both directly and as a reflection from the aluminum layer. In either case, the gold nanoparticles convert light into high energy “hot” electrons (more on that later). Low-energy electron “holes” are attracted to the aluminum layer and the nickel oxide layer lets them pass through, while making the hot electrons stay behind on the gold discs.
So far, the researcher team has determined that the photocurrent generated by the new material is potentially sufficient for water-splitting, and is “on par” with more complex, costly systems.
The next step is to take direct measurements of the hydrogen and oxygen gases produced by the reaction.
Hot Electrons & Water-Splitting
Because they are very energetic, “hot” electrons can be very useful in driving chemical reactions. The problem is that they decay rapidly. To get a handle on just how rapidly, the Rice research team suggests that you consider this:
…most of the energy losses in today’s best photovoltaic solar panels are the result of hot electrons that cool within a few trillionths of a second and release their energy as wasted heat.
If you can grab hot electrons and put them to use before they cool down, the payoff is a huge improvement in solar conversion efficiency.
For a solution, the Rice team looked to the university’s previous work in plasmons. Plasmons refer to electrons that travel across metal surfaces like waves. As with hot electrons, plasmons have an extremely short lifespan, but the magic happens when you put the two together.
Hot electrons and their corresponding holes are caused by a plasmonic “jolt” of energy. The challenge is to keep the two states separated, so the hot electron can’t revert to its low energy state.
The conventional way to do this is by pushing the hot electrons over an energy barrier. It’s an inefficient approach but it is widely used because it is based on familiar technology. The Rice team came at the problem from the opposite angle:
We took an unconventional approach: Rather than driving off the hot electrons, we designed a system to carry away the electron holes. In effect, our setup acts like a sieve or a membrane. The holes can pass through, but the hot electrons cannot, so they are left available on the surface of the plasmonic nanoparticles.
It’s A Hydrogen World, Somewhere
When Tesla Motors cofounder Elon Musk famously quipped that fuel cell vehicles (FCEVs) are BS, at least one FCEV maker took him at his word, pointing out that you could potentially run a FCEV on hydrogen sourced from cow manure.
We’re not quite there yet — fossil natural gas is still the primary source of hydrogen for fuel cells for monetary reasons — but in the meantime, FCEVs are making inroads in a number of important niche markets, particularly logistics.
Things are also moving along on the research end. Last spring CleanTechnica was invited on a technology of Germany, which included a discussion of fuel cells with Professor Dr. Gunther Kolb, head of the Department of Decentralized and Mobile Energy Technology at Fraunhofer ICT-IMM. While showcasing one recent marketing fail, Dr. Kolb affirmed that fuel cell technology is “absolutely competitive” with battery technology for stationary storage.
Just a few weeks ago, CleanTechnica also visited Switzerland and got an up-close look at the country’s solar hydrogen and power-t0-gas research at École polytechnique fédérale de Lausanne, and the deployment of hydrogen in homes and vehicles at Empa, the Swiss federal Materials & Technology institute — where researchers made it clear that Switzerland is all over hydrogen as a long-term energy storage solution for winter, with is the “dry” season for the country’s massive hydropower systems.
As for the tension between battery and fuel cell EVs, during our tour, Empa showcased a battery-powered electric street sweeper that incorporates a fuel cell as a range extender, so stay tuned.
Image: via Rice University.