The iridium complex [Cp*Ir(H 2 O) 3 ](SO 4 ) was used as an organometallic source for the electrodeposition of iridium oxide onto Fe 2 O 3 . The new iridium-containing electrode allowed us to study the coupling between the photocatalytic properties of hematite and the electrocatalytic properties of the iridium-based material. A cathodic shift of the photocurrent for water oxidation upon electrodeposition of the iridium complex was observed, which increased with increasing surface concentration of IrO x on Fe 2 O 3 . The shift for the highest surface concentration of iridium tested amounts to 300 mV at 200 μA·cm −2 current density. The catalytic mechanism of the IrO x layer was unveiled by impedance spectroscopy measurements fitted to a physical model and can be explained on the basis of a highly capacitive layer, which enhances charge separation and stores photogenerated holes at Fe 2 O 3 , subsequently oxidizing water. These findings improve our understanding of the mechanism of water oxidation by heterogeneous Ir-based catalysts coupled to semiconductor electrodes.

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A simple model is proposed that allows interpretation of the cyclic voltammetry diagrams obtained experimentally for photoactive semiconductors with surface states or catalysts used for fuel production from sunlight. When the system is limited by charge transfer from the traps/catalyst layer and by detrapping, it is shown that only one capacitive peak is observable and is not recoverable in the return voltage scan. If the system is limited only by charge transfer and not by detrapping, two symmetric capacitive peaks can be observed in the cathodic and anodic directions. The model appears as a useful tool for the swift analysis of the electronic processes that limit fuel production. SECTION:Energy Conversion and Storage; Energy and Charge Transport D irect transformation of solar energy into chemical energy by hydrogen production through water splitting with semiconductor materials in a photoelectrochemical cell constitutes an attractive solution to our energy needs. However, despite the intense efforts carried out in the last decades, no single material has been identified satisfying all of the efficiency, stability, and cost conditions needed for industrial deployment of this technology. 1−4 Hematite (α-Fe 2 O 3 ) has emerged as a promising candidate 5−9 due to its abundance in the earth crust, visible light absorption, and good stability in the harsh environmental conditions needed for operation, although the obtained solarto-fuel efficiencies still remain low for commercial exploitation. One of the main causes of the low performance of hematite is related to the large overpotentials required for water oxidation (around 500 mV), and surface treatments have proven to enhance notably water splitting performances. 10−12 It has been suggested that the reasons for these large overpotentials are related to sluggish hole transfer to the electrolyte 13,14 and to the existence of traps in the bulk and at the semiconductor/ electrolyte interface, 15−17 leading to high recombination. 18,19 Clearly, the separation of the different processes that constitute the oxidative current and the identification of the main kinetic bottlenecks are complex tasks. Therefore, the accurate interpretation of the results provided by characterization techniques constitutes a key tool to rationalize materials development and device optimization. Recently, we have proposed a simple physical model that allows the interpretation of impedance spectroscopy (IS) spectra for water splitting applications. 20 In this model, we have considered a monoenergetic level of surface states where both electron and holes can recombine or transfer from/to the solution.In the present study, we propose a complementary simple model to predict the curves obtained by cyclic voltammetry (CV). This characterization technique allows a quick test of the faradic behavior associated with charge transfer and the capacitive behavior associated with the separated modes of carrier storage, which depend on the thermodynamics and kinetics of the system at stake. 21 ...