Imagine a future astronaut, hazmat worker, or deep-sea diver, needing to breathe air in a hazardous or even liquid environment. With a few grains of a new material from a Danish lab, they could breathe safely without oxygen tanks or pumps!
Oxygen gas (O2) is among the most reactive chemicals we encounter regularly — left to its own devices, it corrodes metal and makes food rancid. Add a spark and fuel, and you have fire. So it was no surprise when researchers at the University of Southern Denmark reported that they had created crystals that react with oxygen. What is remarkable about these crystals is that they react reversibly with O2. When these crystals are exposed to heat or low pressure, the oxygen is released.
The crystals are considered cobalt salts because they consist of positively charged cobalt ions and a variety of negatively charged counter-ions. The structural difference between these cobalt complexes and common table salt (positively charged sodium and negatively charged chloride) is that, in these cobalt salts, the ions are crystallized with a molecule made of carbon, nitrogen and oxygen, which provides a framework to hold the cobalt ions the right distance apart to react with oxygen.
Picture the fuel cells of a future vehicle, fed by oxygen stored in these crystals. The crystals can function as a battery, “charging” by absorbing oxygen and releasing it when needed by the fuel cells. They can store remarkable amounts of oxygen — one bucket-full could absorb all the oxygen in a room. This is because the reversible chemisorbsion process allows oxygen atoms to sit much closer together in the crystal structure than they do in air. The cobalt ions are positively charged, and attract the electrons in the O2 molecule in an interaction called coordination. Each kind of metal can coordinate with a set number of “ligand” atoms — in cobalt’s case, the number is six. Five of these come from the ring framework, and oxygen supplies the sixth. The molecular framework holds pairs of cobalt ions just the right distance apart so that one can interact with each oxygen atom, binding it tightly. Such efficient filtering of gas might, if adapted to CO2 or other pollutants, clean up our air with staggering efficiency.
This is a reversible reaction, which means the oxygen can be removed by heat or low pressure, but it also means that individual cobalt pairs can pass their oxygen molecule on to pairs deeper in the crystal. This way, even though only the molecules on the surface can pull oxygen straight from the air, the molecules deeper in the crystal can fill up quickly as well, filling every slot in less than a second. The Danish team can control this speed, though. Some negative counter-ions, such as NO3-, coordinate with cobalt in a way that competes with O2, slowing the chemisorbsion process. If layers of different crystals are stacked, they can direct oxygen through the crystal. Our diver could then wear a mask — perhaps as light as a snorkeling mask — allowing him to take oxygen directly from the water. Our astronaut or hazmat worker, too, could find oxygen in any atmosphere, however unpleasant. Mankind’s habitable range may have just expanded immensely, thanks to the work of one remarkable crystal.