Hematite photoelectrodes for water splitting: evaluation of the role of film thickness by impedance spectroscopy

The electrochemical behavior of alpha-Fe2O3 photoelectrodes prepared by spray pyrolysis with different thicknesses was examined under dark and illumination conditions. The main charge transport phenomena occurring in the PEC cell photoelectrodes were characterized by electrochemical impedance spectroscopy (EIS) operating under dark conditions. The impedance spectra were fitted to an equivalent electrical circuit model for obtaining relevant information concerning reaction kinetics and charge transfer phenomena occurring at the semiconductor/electrolyte interface. A three-electrode configuration was used to carry out the electrochemical measurements allowing a detailed study concerning the double charged layer at the semiconductor/electrolyte interface that arises under dark conditions. The model parameters determined by EIS were then related to the film thickness to assess the role of electronic conduction in the performance of the cell. Moreover, by correlating the sample thickness differences with their electrochemical impedance spectroscopy response, it was possible to discriminate the two main phenomena occurring on semiconductor/electrolyte interfaces of photoelectrochemical systems under dark conditions: the space charge layer and the electrical double layer.

Back Electron-Hole Recombination in Hematite Photoanodes for Water Splitting

Maurin Cornuz, Michael Graetzel, Florian Le Formal, Stanley David Tilley

The kinetic competition between electron-hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC), and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular, all three methods show the. presence of a recombination process on the 10 ms to 1 s time scale, with the time scale and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor/electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor/electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long-lived holes (milliseconds) at the semiconductor surface. However, under such modest bias conditions, the energetic barrier generated by the space charge layer field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale faster than water oxidation. Preventing this back electron-hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically similar to 500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.

American Chemical Society2014

Understanding the Role of Underlayers and Overlayers in Thin Film Hematite Photoanodes

Michael Graetzel, Ludmilla Steier, Stanley David Tilley

Recent research on photoanodes for photoelectrochemical water splitting has introduced the concept of under-and overlayers for the activation of ultrathin hematite films. Their effects on the photocatalytic behavior were clearly shown; however, the mechanism is thus far not fully understood. Herein, the contribution of each layer is analyzed by means of electrochemical impedance spectroscopy, with the aim of obtaining a general understanding of surface and interface modifications and their influence on the hematite photoanode performance. This study shows that doping of the hematite from the underlayer and surface passivation from annealing treatments and an overlayer are key parameters to consider for the design of more efficient iron oxide electrodes. Understanding the contribution of these layers, a new design for ultrathin hematite films employing a combination of a gallium oxide overlayer with thin niobium oxide and silicon oxide underlayers is shown to achieve a photocurrent onset potential for the photoelectrochemical oxidation of water more negative than 750 mV versus the reversible hydrogen electrode (RHE) at pH 13.6, utilizing Co-Pi as a water oxidation catalyst. It is demonstrated that multilayer hematite thin film photoanodes are a strategy to reduce the overpotential for this material, thereby facilitating more efficient tandem cells.

Wiley-V C H Verlag Gmbh2014

Back Electron-Hole Recombination in Hematite Photoanodes for Water Splitting

Maurin Cornuz, Michael Graetzel, Florian Le Formal, Stanley David Tilley

American Chemical Society2014