By splitting a water molecule into two atoms of hydrogen and one of oxygen, scientists can use the Sun’s energy to make a clean fuel. Splitting a water molecule requires a metal catalyst to get the reaction started. Recently, much scientific attention has focused on cobalt, a relatively abundant and inexpensive catalyst that — in the right circumstances — can serve as an escort to an electronic connection between hydrogens and oxygens.
Cobalt oxygen-evolving catalysts are the active components in technologies like artificial leaves and other materials in which light can be harvested to drive the synthesis of solar fuels. The overall water-splitting reaction has two halves. The first half, called water oxidation, requires the transfer of four protons and four electrons, and eventually results in the formation of an oxygen-oxygen bond. For this process, the oxygens need a temporary “partner,” which is the cobalt catalyst.
The reason this process isn’t yet well understood is that the transfers and the formation of the bond happen in an instant — the entire process takes less than a billionth of a second. To understand the nuances of the bonding action, the researchers performed X-ray absorption spectroscopy measurements.
In their analysis, they focused on a particularly unique chemical twist. At the beginning of the process, a bridge of two oxygen atoms connects two cobalt ions. Each of the cobalt ions, in turn, is connected to its own water molecule. At this point, the process is stable. When a cobalt ion adds an additional positive charge, it temporarily increases a characteristic number called an oxidation state. In the case of cobalt, the oxidation state changes, just for an instant, from three to four.
When two cobalt ions with an oxidation state of four come into contact, the process begins in earnest. The charge transfers cause the hydrogen atoms of the water molecules to dissociate from their oxygen bonds, leaving the cobalt atoms bonded just to oxygen ions. The key moment follows immediately afterwards, when the cobalt centers each receive an extra electron from the newly exposed oxygen atoms. When this happens, a bond is formed between the two oxygens, creating a molecular intermediate stage called a peroxide, which can be rapidly oxidized to release a dioxygen molecule. The electrons obtained from water during this process can be used to make solar fuels.
Researchers were able to directly measure cobalt oxidation states, and then used theory to calculate a quantity known as exchange coupling, a quantum mechanical value that identifies the relationship between the spins of the electrons that are shuttled between the oxygen and cobalt atoms. These electron spins are in opposite directions — they are anti-ferromagnetically coupled. Anti-ferromagnetism plays an important role in the formation of the oxygen-oxygen bond since it provides a way to simultaneously transfer two electrons to make a chemical bond.