Operando photoelectrochemical diagnosis of iron oxide (&#945;-Fe2O3) photoelectrodes for water splitting

Operando photoelectrochemical diagnosis of iron oxide (α-Fe2O3) photoelectrodes for water splitting

Hen Dotan^a, Avner Rothschild^a, Nripan Mathews^b, Takashi Hisatomi^b, Michael Grätzel^b

^aTechnion - Israel Institute of Technology, Haifa, Israel

Proceedings of International Conference on New Advances in Materials Research for Solar Fuels Production (SolarFuel14)

Montréal, Canada, 2014 June 25th - 26th

Organizer: Thomas Hamann

Oral, Hen Dotan, presentation 019
Publication date: 16th April 2014

Iron oxide (α-Fe₂O₃, hematite) has a unique combination of visible light absorption, stability in aqueous solutions, low cost and abundance, identifying it as one of the most promising materials for conversion of solar power to hydrogen energy via solar-powered water photoelectrolysis. Intensive research efforts have led to significant improvements in the power conversion efficiency of iron oxide photoanodes for water photoelectrolysis, but nevertheless it remains quite low even for the champion photoanodes that achieve ~30% of the theoretical photocurrent limit for iron oxide. Attempts to identify the losses and suppress them by tailoring the photoanode microstructure and chemical composition are often obstructed by the complexity of the operation mechanism and lack of detailed understanding of the salient interrelationships between different steps in the overall water photoelectrolysis reaction. To overcome this difficulty we developed an advanced operando diagnosis method that combines linear sweep voltammetry measurements in dark and under illumination in different aqueous electrolytes that enable resolving different processes occurring inside the photoanode and at the photoanode/electrolyte interface. The electrochemical measurements are analyzed using a rigorous theoretical model that addresses all the underlying processes and represents them in an equivalent circuit diagram. Specific processes and their respective nodes in the equivalent circuit diagram can be selectively probed by testing the photoanode in different electrolytes. For instance, the Ferrocyanide-Ferricyanide ([Fe(CN)₆]^-3/-4) redox couple probes the photovoltaic function of the photoanode, whereas hydrogen peroxide (H₂O₂) is a hole scavenger that measures the photocurrent density that is limited only by generation and recombination processes occurring within the photoanode. Thereby, we construct the complicated water photoelectrolysis mechanism step by step, resolving and quantifying the different contributions to the overall process on step at a time. This rigorous analysis yields a set of well-defined descriptors that provide detailed quantitative information on all the different processes involved in the water photoelectrolysis reaction. This is the enabling key to improving the photoanode efficiency by rational design rather than trials and errors. Selected examples will be given in the lecture.

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