State-of-the-art silicon water splitting photoelectrochemical cells employ oxide protection layers that exhibit electrical conductance in between that of dielectric insulators and electronic conductors, optimizing both built-in field and conductivity.

Iridium has long been recognized as one of the best oxygen evolution catalysts in terms of activity and stability over a wide range of pH. Despite exhibiting initially high activity for the oxygen evolution reaction (OER), a rapid and reversible activity decay is observed during continuous operation. The potential dependence of recovery of the activity is explored, and 0.0 V vs NHE is found to be an effective potential to recover the initial water oxidation performance. Iridium thin films on rotating disk electrodes are used to show that this OER activity decay is neither explained by progressive oxidation of the iridium nor by reduced mass transport to/from the electrode surface. Careful examination of the time dependence of the activity decay reveals that it is well described by a t −1/4 functional dependence across multiple electrochemical cell geometries. Tafel behavior is analyzed by normal pulse voltammetry, suggesting that, after 10 minutes of activity decay, the catalyst exhibits a five-fold decrease in active site density while the mechanism of water oxidation is not altered. We hypothesize that this decay may result from a loss of active sites capable of forming the Ir(V) = O species, possibly via progressive cross-linking of iridium sites by bridging mu-oxides.