2H2 + O2 --> 2H2O
Researchers in this field tend to fall into one of two groups. The first group wants to use hydrogen to generate energy, usually in the form of electricity via a fuel cell, and devotes its energies into driving the above reaction as far to the right as possible. The second group wants to use energy to generate hydrogen, usually by electrolysis, possibly using solar photons, and strives to drive the reaction as far to the left as possible. (A third group is concerned with hydrogen storage, using high surface area materials such as MOFs, but that’s a topic for another discussion). Although these the two groups would appear to be diametrically opposed, they have at least one thing in common. In both cases, the efficiencies of the processes are often dependent on the oxygen side of the reaction. During electrolysis, forming the O2 is the hard part, which explains why most of the advancements in this area are related to the anode. The cobalt phosphate electrode coating announced by MIT last year would be one example. And in fuel cells, it’s the cathode that causes most of the headaches, since it’s more difficult to reduce O2 then it is to oxidize H2 (at the anode).
In an attempt to negate the need for electrodes, much work has been devoted to identifying transition metal complexes which might catalyze the photochemical splitting of water in solution. The results have been generally disappointing. In many cases, sacrificial reagents are required, usually to facilitate the formation of O2, obviously limiting the usefulness of the process. In addition, since the H2 and O2 are usually co-generated at the same location, an additional step is required to isolate the H2. Not good at all.
In a recent article in Science , David Milstein describes some ruthenium chemistry which may have some implications in the solar energy field. When water was added to a ruthenium compound they’ve been working with, a new hydrido-hydroxo complex was formed.
Upon heating, this new complex continues to react with water to produce a dihydroxo ruthenium complex along with free H2. Irradiating this dihydroxo complex with a halogen lamp causes it to revert back to the original hydrido-hydroxo complex, along with the formation of O2. Catalytic photochemical splitting of water without the use of sacrificial reagents. Not bad. Even better, since the H2 and O2 are produced during different steps, there are no separation issues to be dealt with. This process is a loooong way from being commercially viable, but I enjoy any chemistry where an organometallic compound reacts constructively with water without simply igniting or decomposing into an ugly pile of goo.
