Composite material brings metal-air batteries a step closer

As a society, we are now heavily dependent on good battery technology. Indeed, as climate change starts to bite and hydrocarbon fuels become more expensive, the demand for better batteries is just going to increase. But the current best technology is simply not going to keep pace. Commercial Lithium ion batteries are approaching their theoretical maximum energy storage density, which is lower than that of gasoline by a factor of about 60-70. In the meantime, we want electric cars like the Tesla—but lighter, with longer range and faster recharging times.

One solution to some of these problems may be metal-air batteries. These batteries have maximum energy densities approaching that of gasoline. Better than that, they should be simpler to construct and could even be made from cheaper materials. In other words, when viewed through rose-tinted glasses, metal-air batteries are better in every way.

The problem is that no one knows how to make one that meets all of these criteria. A group of chemists from University of Waterloo in Canada may be heading in the right direction, though.

One of the big obstacles is getting the oxygen in the right form before it meets the metal. We want that metal to oxidize, but without a spectacular display of pyrotechnics. The way to control the reaction and use some of its energy is to find the right catalyst. A catalyst is a material that, simply put, gives reactions a helping hand.

A reaction is, at heart, just the transfer of electrons—in a battery we just make the electrons do some work along the way. At one electrode, electrons are given up when a metal is oxidized. These electrons travels out of the battery, get put up at a local capacitor, do some work, and return to a different electrode in the battery. Back in the battery, the electrons are used in a reaction that frees up oxygen to react with the metal at the other electrode.

In other words, we have two circuits. In one circuit, electrons flow and do work, while in the other, oxygen flows to generate and receive electrons. The big problem in this scheme is to get something like atomic (rather than molecular) oxygen around to react, and later on, convincing the oxygen to let go of the metal so you can recharge the battery.

We need a catalyst that will convince an oxygen molecule to split up to form some sort of radical (either a lone oxygen atom, or, more commonly, a reactive OH). Then, to allow recharging, we want a second catalyst that recombines oxygen atoms to make molecular oxygen. Traditionally, these catalysts are mixtures of things like palladium and platinum, along with other expensive metals. What is more, they tend to degrade with time, producing a battery that isn’t cheap, and doesn’t last for very long.

So, one of the big efforts in the field at the moment is the development of a catalyst that is stable and cheap. Various lines of earlier research have shown that metals like nickel and cobalt could make good catalysts for producing molecular oxygen, while carbon-based materials seem to make pretty good oxygen reducing catalysts. The trick, then, is to turn these into stable and efficient catalysts. This is where this latest bit of research comes in.

The researchers noted that a particular crystalline form of oxides, called perovskites, make a good support structure for the growth of carbon nanotubes—that is, if you grow nanotubes in their presence, the nanotubes grow all over the oxide material. They also noted that nitrogen doped nanotubes are, chemically, quite stable, making them a good candidate for the oxygen reducing catalyst. By using a dollop of nickel in the perovskite structure, instead of lanthanum oxide, they could create a lanthanum-nickel-oxide catalyst, one that would also support the growth of a carbon-based catalyst.