Role of the I₂/I^– Equilibrium in the Photocatalysis of Oxygen Reduction at Ag/AgI

Photocatalysis of the oxygen reduction reaction at a silver electrode coated with silver iodide is examined with particular attention to the role of the I 2 /I – equilibrium in the mechanism of catalysis. We demonstrate the dependence of the photovoltage on the standard reduction potentials for the O 2 /H 2 O and the I 2 /I – equilibria. Iodide, present only via photon-driven charge transfer from silver into the AgI film, oxidizes to iodine in coupling with the oxygen reduction reaction. The photovoltage at the cathode is found tied to that redox couple. The photovoltage is nearly pH-independent owing to a balance between proton consumption during oxygen reduction and the shifting ratio ([I 2 ]/ a I – 2 ). The catalytic cycle is maintained by regeneration of AgI by the thermodynamically allowed reaction of iodine with silver.