Monday, September 29, 2008

Artificial Photosynthesis ... and misunderstandings.

I submitted this as a letter to Chemistry World... maybe they will publish it next month. Anyway, it seems salient to the topic of this blog and I have raised this subject previously. If the industry magazine for the British chemistry "industry" (mostly universities) can get it wrong, what chance do other journalists stand?


There is much misconception in the media regarding the new "breakthrough catalyst" for "artificial photosynthesis" as was also reported-on in September's issue of Chemistry World. For a start, the process described by the MIT team has nothing to do with photosynthesis per se, but involves an improved electrode for "splitting water" by electrolysis to generate oxygen at a lower potential than normal. It does not involve a direct light-induced process (and certainly not the reduction of CO2), other than the tenuous connection that light is first harvested using a PV-cell, and the electricity from that then used to electrolyse water, forming H2 and O2 which can be later combined in a fuel cell to produce electricity when the sun has stopped shining. The counter-electrode is made from platinum, to combine the residual protons and electrons (derived from H2O) to make hydrogen.

The new electrode is formed by depositing onto "tin indium oxide" a solid film containing Co and P, formed electrochemically from Co2+ and "phosphate" ions present in solution. Thus the medium is hardly "pure water". Since there is only enough indium (used among other things for LCD's) known worldwide to last for another 5 - 10 years for its current purposes, the large scale implementation of this technology is posed a resource challenge. The article also refers to work by Winther-Jensen et al. who have made a potential "fuel cell" electrode based on the organic conductor PEDOT, which might replace one of the standard platinum electrodes, but even they concede in the paper cited: "the electrode described here provides only a partial solution to some of the problems with the use of Pt... because Pt is also used in the anode (fuel) electrode in the fuel cell."

Fascinating chemistry in both papers, but neither do we have artificial photosynthesis cracked nor is the need for platinum obviated, the demand for which already exceeds the mere 200 tonnes a year of new metal that is recovered. In terms of the energy crunch and the pressing gap between demand and supply of oil we are not out of the woods yet, and it would be irresponsible to claim otherwise.

Yours sincerely,

Professor Chris Rhodes B.Sc., D.Phil., D.Sc., C.Chem., FRSC.

No comments: