Credit: © 2010 AAAS

Developing a cheap and effective water-oxidation catalyst (WOC) is one of the key breakthroughs needed to produce clean energy from artificial photosynthesis. Although heterogeneous WOCs have some advantages over homogeneous ones, the latter are easier to study and this increased understanding makes them easier to improve. Many homogeneous WOCs, however, have organic ligands that undergo oxidative degradation, or include expensive and rare metals, such as ruthenium.

Now, Craig Hill and colleagues from Emory University in Atlanta have developed1 a homogeneous WOC with a cobalt oxide core, stabilized by polytungstate ligands. The catalyst is made from cheap and readily available starting materials in a fairly simple one-pot synthesis. Although Hill and co-workers made a range of similar cobalt-based polyoxometalate compounds, only one showed catalytic activity. This had a turnover frequency — the number of molecules of water oxidized by a molecule of the catalyst — of 5 s−1, much higher than a similar heterogeneous cobalt phosphate catalyst.

To show that their catalyst was stable and did not break down into aqueous cobalt ions — which can themselves act as WOCs — Hill and colleagues performed a range of experiments. As well as UV–visible and NMR spectroscopy showing no changes over a month, adding a ligand known to inhibit the catalytic activity of aqueous cobalt ions had no effect on the catalyst. Furthermore, computational studies showed that the highest occupied molecular orbitals of the catalyst were located on the cobalt core, with no contribution from the polytungstate, showing that the ligands are effectively inert.