Water splitting takes place at semiconductor electrodes that absorb visible light and have the appropriate band edges positions to straddle the water redox potential. The photolysis of water is performed by photogenerated holes that are injected from the valence band or from intermediate states in the bandgap (surface states). Meanwhile the electrons are evacuated towards a cathode by diffusive transport or by a drift field (if available). The recombination of carriers competes strongly with their injection for the useful anodic and cathodic electrochemical reactions. If the fuel forming reaction at the anode surface is slow, a catalytic layer may improve the operation of the electrode, and this may occur either by increasing the reaction rate or by decreasing the recombination of surface holes with electrons in the conduction band. Materials and surface treatment for each function should be purposely designed, but it is not easy to distinguish which electronic processes dominate the electrode performance in a given case. Here we discuss the application of impedance spectroscopy experimental and theoretical tools to identify the mechanism of operation of photoanodes for solar fuel production.
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