Water oxidation: Two metals better than one

J. Am. Chem. Soc. 138, 3623–3626 (2016)

Electrolysing water allows electrical energy to be stored as chemical energy in the form of molecular hydrogen. For the process to be sustainable, water must be oxidized simultaneously to oxygen in the half-cell reaction at the anode of the electrolyser. Catalysts based on porphyrin molecules — ring-shaped biological pigments in which metal ions typically sit in the centre — have been investigated for this oxygen evolution reaction (OER), but relatively few examples exist where two different metal ions are incorporated into the structure. Now, Doris Grumelli at INIFTA in Argentina and colleagues in Germany and Switzerland have prepared porphyrin-based structures containing two different metals, in some cases finding that they are almost two orders of magnitude more active for OER than related structures containing just one metal ion per porphyrin unit.

The researchers prepared monolayers of metal–porphyrin complexes on Au(111) surfaces. In each structure, two sites can bind metal ions — one in the centre of the molecule and one on the outside of the ring, which is bound by a total of four porphyrin molecules. In one example where Fe²⁺ ions were in the centre of the porphyrin ring, the addition of Co²⁺ ions as a secondary metal increased the amount of oxygen evolved by a factor of 86. Switching the positions of the metals decreased the activity significantly, highlighting the importance of the specific coordination environment of the metal centres.