In this new electrolyzer, an improved ion-conducting membrane (yellow film, right) enables hydrogen generation from water without expensive catalysts.
Yu Seung Kim/Los Alamos National Laboratory
By Robert F. Service
Running the world on renewable energy is simple, in principle: Harvest solar and wind energy, and use any extra to power devices called electrolyzers that split water into oxygen (O2) and hydrogen gas. Hydrogen (H2) can serve as a fuel; it is also a staple of the chemical industry. The trouble is that current electrolyzers are costly, requiring either expensive catalysts or pricey metal housings. Now, researchers report combining the best of both approaches to make a version that needs only cheap materials.
“I consider this a great breakthrough,” says Hui Xu, a chemical engineer at Giner Inc., an electrochemistry company. Xu says he and his colleagues presented similar results at a Department of Energy meeting last year, but have not yet published them. Their work and another team’s new device, described this week in Nature Energy, could bolster the global embrace of renewable energy if the new electrolyzers prove to be cheap and stable during many years of operation. “We are on the cusp of getting that done,” says Yushan Yan, a chemical engineer at the University of Delaware, Newark, who is working on similar technology. A handful of small companies, including one he founded, have formed to commercialize it.
Scientists have known how to split water into H2 and O2 for more than 200 years: Put two metal electrodes in a jar of water, apply an electrical voltage between them, and H2 and O2 will bubble up at separate electrodes. Because a mix of the gases can explode, today’s most common setups separate the anode and cathode with a thick, porous plastic sheet. They also use metal catalysts—most often inexpensive ones such as nickel and iron—to speed the reactions.
To make the water able to better conduct ions that move through the devices, today’s most common electrolyzers add high levels of potassium hydroxide (KOH) to the water. At the cathode, or negative electrode, water molecules split into H+ and OH– ions. The H+ ions combine with electrons from the cathode to make H2. The OH– ions diffuse through the membrane to the anode, or positive electrode, where they react to generate O2 and water.
But KOH is highly caustic, so engineers have to build their devices out of expensive inert metals such as titanium, says Yu Seung Kim, a chemist at Los Alamos National Laboratory. That drawback prompted researchers in the 1960s to develop a version of the technology known as a proton-exchange membrane (PEM) electrolyzer, in which the dividing membrane is designed to selectively allow H+ ions through. A PEM cell’s catalysts aren’t on the electrodes themselves, but are tethered to opposite sides of the membrane. In this setup, catalysts on the anode side split water molecules into H+ and OH– ions, with the latter instantly reacting at the catalysts to form O2 molecules. The H+ ions then migrate through the plastic membrane to the cathode side, where catalysts tethered to the membrane turn the H+ ions into H2.
Because OH– ions don’t migrate through PEM cells, there’s no need for highly alkaline conditions. The devices also typically produce hydrogen at five times the rate of the alkaline version. But these membrane cells have their own downsides: They still need some expensive corrosion-resistant metals to withstand acidic conditions produced by the proton-conducting membrane. They also require catalysts made from platinum and iridium. Those metals are expensive and rare. For example, the global production of iridium is only 7 tons. “There is simply not enough [precious metals] for large-scale hydrogen production,” Xu says.
Now, Kim and his colleagues at Los Alamos, along with researchers at Washington State University, say they’ve combined the best of both approaches. Their new device creates a highly alkaline environment to encourage water splitting. But it does so with the PEM approach of tethering catalysts to opposite faces of an ion-conducting membrane. As with the KOH setup, catalysts on the cathode side split water molecules into H+ and OH– ions. The former converts to H2, and the latter travels through the membrane, known as an anion exchange membrane (AEM). It is designed to create a highly alkaline local environment that speeds the travel of OH– ions to the anode side, where tethered catalysts prompt them to react to make O2.
The upshot is that alkaline conditions near the membrane allow the electrolyzer to rely on cheap and abundant nickel-, iron-, and molybdenum-based catalysts to split water. Yet, because the alkalinity is localized, the electrolyzer can be built from stainless steel. The new device generates hydrogen about three times faster than conventional alkaline devices, though still more slowly than commercial PEM electrolyzers, Kim and his colleagues report. “The combination of the older alkaline technology and membrane PEM technology is the path forward,” Xu says.
The new setup needs to prove its durability. Initial indications suggest the membrane begins to break down after only about 10 hours of operation. Kim says the main problem is likely that the polymer membrane readily absorbs water. Over time, this may cause the catalyst particles to come unglued and drift away. The team hopes that adding fluorine to the membrane will repel the water. With that and other fixes, Kim hopes, AEM electrolyzers could join solar cells and windmills as a key technology for a carbon-free world.
Source: Science Mag