Cathodic shift in onset potential of solar oxygen evolution on hematite by 13-group oxide overlayers

Hisatomi, Takashi; Formal, Florian Le; Cornuz, Maurin; Brillet, Jérémie; Tétreault, Nicolas; Sivula, Kevin; Grätzel, Michaël

doi:10.1039/c1ee01194d

Cited by 273 publications

(302 citation statements)

References 38 publications

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“…This calls into question the strategy of passivating surface states on hematite electrodes which has been pursued recently since they might be an inherent feature of oxidizing water, and not necessarily deleterious. 48,50 It is well known that water oxidation proceeds by an innersphere mechanism, thus a proposed mechanisms should account for this; 59 describing this reaction as a charge transfer from a valence band hole is misleading. In addition, the negligible faradaic, steady state photocurrent densities at these potentials is often attributed to slow hole transfer kinetics of water oxidation.…”

Section: Resultsmentioning

confidence: 99%

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Klahr

Giménez

Fabregat‐Santiago

et al. 2012

Energy Environ. Sci.

456

561

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Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe 2 O 3 ) on transparent conductive substrates for photocatalytic water oxidation studies.Comparison of the oxidation of water to the oxidation of a fast redox shuttle allowed for new insight in determining the rate limiting processes of water oxidation at hematite electrodes. It was found that an additional overpotential is needed to initiate water oxidation compared to the fast redox shuttle. A combination of electrochemical impedance spectroscopy, photoelectrochemical and electrochemical measurements were employed to determine the cause of the additional overpotential. It was found that photogenerated holes initially oxidize the electrode surface under water oxidation conditions, which is attributed to the first step in water oxidation. A critical number of these surface intermediates need to be generated in order for the subsequent hole-transfer steps to proceed. At higher applied potentials, the behavior of the electrode is virtually identical while oxidizing either water or the fast redox shuttle; the slight discrepancy is attributed to a shift in potential associated with Fermi level pinning by the surface states in the absence of a redox shuttle. A water oxidation mechanism is proposed to interpret these results.

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Section: Resultsmentioning

confidence: 99%

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Klahr

Giménez

Fabregat‐Santiago

et al. 2012

Energy Environ. Sci.

456

561

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“…It should also be remarked that in other recent studies, the capacitive response of surface states has been found to be reduced in the presence of Ga 2 O 3 and Al 2 O 3 overlayers. 36,37 It has been suggested that 13-group oxide overlayers passivate surface states by releasing lattice strain of the hematite layer. On the other hand, the deposition of cobaltbased catalyst layers also leads to a decrease of surface state capacitance by accelerating the charge-transfer rate from surface states.…”

Section: Irect Transformation Of Solar Energy Into Chemical Energymentioning

confidence: 99%

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

Bertoluzzi

Badia-Bou

Fabregat‐Santiago

et al. 2013

J. Phys. Chem. Lett.

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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 ...

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“…Surface modification with a range of catalytic and noncatalytic materials have been shown to reduce the overpotential for water photo-oxidation, including cobalt-based treatments (16), IrO 2 nanoparticles (17), and Ga 2 O 3 and Al 2 O 3 overlayers (18)(19)(20). Particular interest has recently focused upon in situ deposition of cobalt oxo/hydroxo-based (CoO x ) overlayers on several water oxidation photoanodes (15,(21)(22)(23)(24)(25), building upon reports that amorphous CoO x layers, formed from Co(II) salts, are effective water oxidation electrocatalysts operating at moderate overpotentials (26).…”

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confidence: 99%

Dynamics of photogenerated holes in surface modified α-Fe ₂ O ₃ photoanodes for solar water splitting

Barroso

Mesa

Pendlebury

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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418

429

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This paper addresses the origin of the decrease in the external electrical bias required for water photoelectrolysis with hematite photoanodes, observed following surface treatments of such electrodes. We consider two alternative surface modifications: a cobalt oxo/hydroxo-based (CoO x ) overlayer, reported previously to function as an efficient water oxidation electrocatalyst, and a Ga 2 O 3 overlayer, reported to passivate hematite surface states. Transient absorption studies of these composite electrodes under applied bias showed that the cathodic shift of the photocurrent onset observed after each of the surface modifications is accompanied by a similar cathodic shift of the appearance of long-lived hematite photoholes, due to a retardation of electron/hole recombination. The origin of the slower electron/hole recombination is assigned primarily to enhanced electron depletion in the Fe 2 O 3 for a given applied bias.

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Cathodic shift in onset potential of solar oxygen evolution on hematite by 13-group oxide overlayers

Cited by 273 publications

References 38 publications

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

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

Dynamics of photogenerated holes in surface modified α-Fe ₂ O ₃ photoanodes for solar water splitting

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Cathodic shift in onset potential of solar oxygen evolution on hematite by 13-group oxide overlayers

Cited by 273 publications

References 38 publications

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

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

Dynamics of photogenerated holes in surface modified α-Fe 2 O 3 photoanodes for solar water splitting

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Dynamics of photogenerated holes in surface modified α-Fe ₂ O ₃ photoanodes for solar water splitting