Multihole water oxidation catalysis on haematite photoanodes revealed by operando spectroelectrochemistry and DFT

Michael Graetzel, Yimeng Ma, Matthew Thomas Mayer
NATURE PUBLISHING GROUP, 2020
Article

Résumé

Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal-oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits low activation energy (E-a approximate to 60meV). Density functional theory is used to determine the energetics of charge accumulation and O-O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface. The oxidation of water remains the kinetic bottleneck of solar-to-fuel synthesis. Now, spectroelectrochemical evidence together with density functional theory calculations show that charge accumulation determines the reaction mechanism on metal-oxide photoanodes. These insights reveal features that are common to the mechanisms of water oxidation carried out by other inorganic and biological systems.

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https://infoscience.epfl.ch/record/275829?ln=fr

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Michael Graetzel, Yimeng Ma, Matthew Thomas Mayer
NATURE PUBLISHING GROUP, 2020
Article

Résumé

Official source

https://infoscience.epfl.ch/record/275829?ln=fr

À propos de ce résultat

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Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

The direct transformation of sunlight into fuels and chemicals has attracted considerable attention for the potential impact on the storage of solar energies at large scales and the reduction of CO2 emissions. More than 80% of our current global energy consumption comes from the burning of non-renewable fossil fuels which produces enormous quantities of CO2 and contributes to global warming. We believe today that the key to reduce our dependence on exhaustible natural resources and limit the emission of greenhouse gases is the transition to CO2-free renewable energy sources. However, harvesting of energy from renewable sources (i.e. solar, wind, tidal) is intermittent and the deployment of devices for their transformation into useful forms of energy (i.e. electricity, chemicals, fuels, biomass) in a global scale is conditioned upon the improvement in efficiency of the energy storage mechanisms. A promising approach to store energy is the use of sunlight to drive the production of hydrogen fuel from water. Hydrogen produced from water and sunlight can be transformed back to electricity on demand or be used to generate mechanical energy in cars to power the transportation industry. The oxidation of hydrogen generates useable energy and at the same time exhaust clean, carbon-free water as the only product. Therefore, the production of hydrogen using a renewable energy source in an efficient and inexpensive device has the potential to discourage the continued use of fossil fuels and improve our environment. The viability of any mechanism for the storage of sunlight as fuel will depend on the reduction of energy penalties during the transformation process. Electrocatalysts are advanced materials that bring into one active site electrons and molecules in the appropriate orientation to lower activation energy barriers, accelerate rate of transformations and improve overall efficiencies for chemical reactions. The best catalysts for water splitting are precious metals such as Pt, Ir and Ru, which show superior rates of reaction at low overpotentials. However, these noble metals are too expensive and scarce for large scale applications. This thesis summarizes our recent research in the preparation, characterization, and application of Earth-abundant electrocatalysts for solar water splitting. Although in the last decade a great number of efficient and inexpensive electrocatalysts have been reported, methods for the deposition of these materials on the surface of photoabsorbers are rarely reported. This work shows various simple and scalable techniques for the deposition of hydrogen and oxygen evolving electrocatalysts on the surface of photocathodes and photoanodes, respectively. Surface-protected photocahodes were activated for solar hydrogen production in alkaline conditions with MoS2+x, Ni-Mo and Mo2C. A novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst is also reported. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

EPFL2016

Chargement

Atomic Scale Observation of Chemical and Electronic Properties of Metal-Oxide Surfaces

Christian Alexander Stephan Dette

To meet the future global energy demand - an estimated additional 10 terawatts (TWs) per year in 2050 - a diversification of energy sources and fuels is needed. Solar energy represents a prominent alternative energy source. An important material for these solar-based applications is titanium dioxide (TiO2), thanks to its stability, band alignment and abundance. TiO2 is either used as a scaffold or to create photogenerated electrons and holes to perform catalytic reactions. However, the large bandgap of TiO2 yields device efficiencies that are too low to be economically sound. To make TiO2-based devices competitive, the electronic and chemical properties of TiO2 need to be clearly understood. In this work, using scanning tunneling microscopy (STM) together with spectroscopy techniques (tunneling spectroscopy (STS) and inelastic tunneling spectroscopy (IETS)), we study the electronic and structural properties of pristine TiO2 anatase (101), the most technologically relevant polymorph of TiO2. In particular, STM-IETS was applied for the first time to obtain chemical identification of adsorbed species on the semiconducting TiO2 surface. For each step, density functional theory (DFT)-based calculations were performed to support our findings. Using STM, we showed that the high reactivity of step edges along the [-111] direction stems from oxygen vacancies (VOs). As studied with STS, this non-stoichiometric step edge exhibits a bandgap reduced by 2 eV. Furthermore, a higher amount of adsorbates are present on this step edge, suggesting a higher chemical reactivity. Going one step further, we created a novel surface phase consisting of undercoordinated Ti atoms, increasing the amount of VOs over the whole surface phase. This new surface phase exerts the same behavior as the step edges, reducing bandgap and enhancing reactivity. On the other hand, by exposing the anatase surface to excess oxygen at elevated temperatures, we reduced the overall surface reactivity. This reduction was achieved by formation of an oxygen network, acting as a passivating layer on top of the TiO2 anatase (101) surface. Additionally, the excess oxygen fills vacant positions at the highly-reactive step edges along the [-111] direction, reducing the overall surface reactivity even further. It is important to note that the preparation procedures of both surface phases with enhanced and reduced surface reactivity are cheap and reversible, only modifying standard ultra-high vacuum (UHV) cleaning methods without any additional materials. Improved characterisation of the interaction between water and the TiO2 anatase (101) surface represents another important aspect of this work. Although photocatalytic water-splitting on TiO2 has been used for over 40 years, fundamental insights of the reaction mechanisms are still missing. In this thesis work, we labeled individual H2O and OH molecules on the semiconducting surface by detecting their vibrational modes with STM-IETS. Through clear identification of adsorbed species, we demonstrated that water can thermally dissociate on the TiO2 anatase (101) substrate without an additional light source. Furthermore, for the first time, we could structurally identify formation of a well-ordered water monolayer on TiO2 anatase (101). The work presented here opens new paths towards fundamental understanding of surface reactions on metal oxides, especially water on TiO2 anatase (101), to improve future solar energy conversion devices.

EPFL2016

Chargement

Photoelectrochemical water splitting is a promising source of clean, renewable fuel in the form of hydrogen. Despite extensive research endeavors, the widespread adoption of this technology is impeded due to suboptimal catalysts for the oxygen evolution reaction (OER) taking place on the anode. Studies have shown that linear scaling relationships exist between the binding energies of the OER intermediates. These relationships are responsible for the emergence of an overpotential which limits the catalytic efficiency of electrochemical cells. This work is devoted to the study of the OER, the emergence of linear scaling relationships, and possible avenues for overcoming them. We employ a selection of ab-initio computational free energy methods based on density functional theory in order to investigate the OER at atomistic model semiconductor interfaces. An approach based on thermodynamic integration and \textit{ab-initio} molecular dynamics simulations is used to isolate the effect of the solvent on the OER free energy steps. The observed effect is twofold. First, the presence of explicit water affects the equilibrium configurations of the reaction intermediates. Second, explicit water modifies the OER free energy steps by up to 0.5 eV. This effect is rationalized in terms of electrostatic interactions at the interface. Next, we verify the existence of the linear scaling relationships across a set of semiconductor and metal interfaces. We establish the robustness of the scaling relationships with respect to the adopted energy functional. Two potential approaches of breaking the linear scaling are considered. First, a bifunctional mechanism that involves two close but functionally different active sites is investigated. The performance of the proposed mechanism is studied on a set of model interfaces, and the results indicate that specific combinations of catalysts may overcome the limitations imposed by the linear scaling relationship. We were able to establish general trends governing the efficiency of the bifunctional mechanism. In particular, a correlation between the valence band maximum and the hydrogen binding energy may be used as a descriptor in future searches for suitable hydrogen acceptors within the bifunctional scheme. Next, a detailed study of NiOOH/FeOOH catalysts identifies features most favorable to the bifunctional mechanism. Overpotentials as low as 0.13 eV are found for specific configurations. These results support the bifunctional mechanism as a means to break the linear scaling relationships. Second, hole polarons at the TiO

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surface induced by electronegative adsorbates in the OER intermediates are studied. The computational hydrogen electrode method in conjunction with a hybrid density functional is employed to quantify the effect of the surface polarons on the OER free energy steps. We find that the hole bipolarons reduce the overpotential of the reaction-determining step leading to good agreement with experiment. The stability of the polarons is further confirmed at the hydrated surface through a free energy study involving the explicit treatment of the solvent. Finally, the polaron energy level is aligned with respect to the redox levels in water to better understand the role of the polarons in the OER. Since the occurrence of surface hole polarons is unrelated to the scaling relationships, it offers an additional handle in the search for improved catalysts.

EPFL2020

Chargement

Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

EPFL2016

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EPFL2020

Atomic Scale Observation of Chemical and Electronic Properties of Metal-Oxide Surfaces

Christian Alexander Stephan Dette

EPFL2016