A bi-functional cobalt-porphyrinoid electrocatalyst: balance between overpotential and selectivity.

Cobalt porphyrins have been shown to electrocatalyze O2 reduction as well as H2 evolution. A cobalt porphyrin is synthesized when two electron-withdrawing groups are used in an attempt to reduce the overpotential involved in O2 reduction and H2 evolution. The results show that in the case of O2 reduction, instead of reduction of overpotential, the selectivity for O2 reduction changes to 2e-/2H+ to produce H2O2. In the case of H2 evolution, the CoI state is incapable of catalyzing H+ reduction. Rather, a CoIII-H species needs to be reduced to CoII-H before H2 can be produced. There results add to the ongoing investigations into the factors that control the rates, overpotential and selectivity of these important cathodic processes.