Kinetics of oxygen evolution at α-Fe2O3 photoanodes: a study by photoelectrochemical impedance spectroscopy

Abstract

Photoelectrochemical Impedance Spectroscopy (PEIS) has been used to characterize the kinetics of electron transfer and recombination taking place during oxygen evolution at illuminated polycrystalline α-Fe(2)O(3) electrodes prepared by aerosol-assisted chemical vapour deposition from a ferrocene precursor. The PEIS results were analysed using a phenomenological approach since the mechanism of the oxygen evolution reaction is not known a priori. The results indicate that the photocurrent onset potential is strongly affected by Fermi level pinning since the rate constant for surface recombination is almost constant in this potential region. The phenomenological rate constant for electron transfer was found to increase with potential, suggesting that the potential drop in the Helmholtz layer influences the activation energy for the oxygen evolution process. The PEIS analysis also shows that the limiting factor determining the performance of the α-Fe(2)O(3) photoanode is electron-hole recombination in the bulk of the oxide.

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

“…Similar looking impedance spectra have recently been reported for hematite electrodes, and a variety of equivalent circuits put forth interpret these spectra. 11,45 In these analyses, the low frequency semicircle is generally attributed to the series arrangement of the depletion capacitance of the semiconductor SC C and the Helmholtz capacitance at the electrode surface, and the role of surface states has largely been ignored.…”

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

“…In addition, the negligible faradaic, steady state photocurrent densities at these potentials is often attributed to slow hole transfer kinetics of water oxidation. 25,26,[28][29][30] We suggest that this may be an intrinsic part of the bimolecular mechanism of water oxidation on the hematite surface. At high applied potentials, however, the photocurrent density is essentially the same for the H 2 O and [Fe(CN) 6 ] 3-/4-systems; the minor difference is attributed to Fermi level pinning in the H 2 O electrolyte.…”

“…24 The water oxidation (oxygen evolution) reaction at the hematite electrode surface is generally reported to be sluggish, however, which allows for increased recombination and a concomitant loss in efficiency. [25][26][27][28][29][30] A detailed understanding of the water oxidation reaction at the hematite electrode surface is therefore very important in devising strategies to overcome this kinetic barrier. There have been several recent studies, employing a variety of techniques, to understand the nature of the slow water splitting 4 reaction with hematite photoelectrodes.…”

“…Peter and co-workers recently used IS and intensity modulated photocurrent spectroscopy (IMPS) to study the kinetics of water oxidation at hematite electrodes which also pointed out the crucial role of surface-trapped holes. 25,26,32 We also recently employed IS at different conditions of voltage, light intensity and electrolyte pH to investigate the main steps involved in water oxidation. 21 Importantly, a general physical model, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate, was established.…”

Interpretation of Cyclic Voltammetry Measurements of Thin Semiconductor Films for Solar Fuel Applications

“…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.…”