The performance of PEMFCs mainly depends on the proton conductivity of the polymer membrane used as electrolyte. An improvement of the knowledge of mechanisms involved in proton transfer can be very useful in order to develop new materials. Where the experimental knowledge is until insufficient, molecular modeling represent avery interesting way to study proton conductivity.
Our research mainly focus on the ab-initio modeling (essentially based on Density Functional Theory, DFT) of dye-sensitized solar cells, whose operating principles rely on electronic process localized at the dye semi- conductor interface, where an ultrafast electron injection takes place between the dye excited state and the semiconductor conduction band. An efficient computational protocol has been developed to quantitatively describe both isolated and adsorbed systems. To this end, UV-Visible spectra of dyes are simulated using time-dependent DFT and a polarizable continuum solvation model, while both the isolated semiconductor and combined dye/semiconductor systems are investigated using large supercells under periodic boundary conditios. Electron injection times are computed using a simple orbital-based model. Large clusters are also extracted from obtained periodic structures in order to study the semiconductor influence on the dyes' UV-Visible spectra.
Our main aims are:
(i) to in silico characterize and design either organometallic or purely organic dyes, in order to improve both the dye light response and the semiconductor surface coverage
(ii) to characterize the dye/semiconductor interface, and in particular to take into account the electron injection process, in order to optimize the energy levels matching between dye and semiconductor, and thus to improve the ultrafast electron transfer.
More details about optimization of dyes for ZnO-based Dye-Sensitized Solar Cells (DSCs) by a joined theoretical and experimental approach.
[1] T. Le Bahers, F. Labat, T. Pauporté, I. Ciofini, Phys. Chem. Chem. Phys., (2010)
[2] F. Labat, A.H. Fuchs and C. Adamo, "Toward an Accurate Modeling of the Water-Zeolite Interac- tion: calibrating the DFT Approach", J. Phys. Chem. Lett., 1, 763–768 (2010)
[3] T. Le Bahers, T. Pauporté, G. Scalmani, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys., 11, 11276 (2010)
[4] F. Labat, I. Ciofini, H.P. Hratchian, M. Frisch, K. Raghavachari and C. Adamo, "First Principles Modeling of Eosin-Loaded ZnO Films: a Step toward the Understanding of Dye-Sensitized Solar Cell Per- formances", J. Am. Chem. Soc., 131, 14290–14298 (2009)
[5] F. Labat, I. Ciofini and C. Adamo, "Modeling ZnO phases using a periodic approach: from bulk to surface and beyond", J. Chem. Phys., 131, 044708–044719 (2009)
[6] F. Labat and C. Pouchan, "Adsorption of cyanodiacetylene on ice: a periodic approach", Phys. Chem. Chem. Phys., 11, 5833–5842 (2009)
[7] A. Prestianni, A. Martorana, F. Labat, I. Ciofini and C. Adamo,"A DFT investigation of CO oxida- tion over neutral and cationic gold clusters", J. Mol. Struct. (Theochem), 903, 34–40 (2009)
[8] A. Prestianni, A. Martorana, I. Ciofini, F. Labat and C. Adamo, "CO Oxidation on cationic gold clusters: a theoretical study", J. Phys. Chem. C, 112, 18061–18066 (2008)
[9] S.Hazebroucq,F.Labat,D.LincotandC.Adamo,"Theoreticalinsightsontheelectronicproperties of eosin Y, an organic dye for photovoltaic applications", J. Phys. Chem. A 112, 7264–7270 (2008)
[10] F. Labat, Ph. Baranek and C. Adamo, "Structural and electronic properties of selected rutile and anatase TiO2 surfaces : an ab initio investigation", J. Chem. Theory Comput. 4, 341–352 (2008)
A great number of organic compounds spontaneously decompose, by a free-radical reaction of the carbon chain with molecular oxygen, in a self-propagating process of auto-oxidation, which may generate a wide variety of peroxide molecules. Many laboratory accidents have been ascribed to the presence of peroxides in solvents or reagents. Among the chemicals widely used in a lot of laboratories and industries as solvent or reactive, ethers are the most notorious peroxide formers causing several accidents. However, few of works propose mechanistic study of their reaction of oxidation.
This thesis work, carried out in the domain of the INERIS research project named RIPER (for "study of Risk linked to the Peroxidation of chemical products"), proposes the theoretical study, at the DFT level of theory, of all the available mechanisms of diethyl ether decomposition and will lead to develop a detailed chemical kinetic model of this auto-oxidation process to better understand his accidental risks.
Solid Oxide Fuel Cells (SOFCs) are a promising technology for direct or indirect conversion of a wide range of hydrocarbons into electrical energy, enabling increased efficiency over traditional power generation systems. Moreover, SOFCs cause reduced pollution such as NOx and SOx upon operation. One of the main limitation is however the need of efficient catalysts at the anode for hydrocarbons oxidation that also match all conductivity and compatibility with other materials requirements. The high working temperature of SOFCs (800-1000K) enables the use of cheaper catalysts such as cerium oxide to oxidize the fuel, avoiding the use of expensive noble metals. Cerium oxide (CeO2) crystallizes in the face-centred cubic form (Fm3m). Oxygen vacancies (VO) can be easily formed and healed, leading to the well-know "oxygen storage capacity" of ceria (OSC). Each oxygen vacancy leaves behind a pair of electrons that have been shown to localize into atomic-like Ce4f orbitals of two neighbouring Cerium. Electronic structure of ceria shows two band gaps: (i) from O 2p states (valence band) to localized, atomic-like Ce 4f states and (ii) from O 2p to Ce 5d states (conduction band). Qualitative correct description of the electronic structure of both oxidation states of ceria with a single DFT functional is challenging. Pure DFT calculations tend to over delocalize these electrons, predicting an incorrect conducting state for reduced ceria. Hybrid approaches such as PBE0 manage to predict the correct insulating states for both stoichiometric and reduced ceria and gives quantitatively reasonable values for both band gaps. Moreover, PBE0 can also be used to describe molecular species such as hydrogen, methane, enabling the study of catalytic properties of ceria for fuels oxidation.
H2 adsorption on a (110) slab unit cell
In pharmaceutical chemistry, UV-visible absorption spectra are routinely used in the quantitative determination of organic compounds in solution. When a UV-visible spectrometer is employed as a detector for HPLC-UV analysis, the presence of analyte gives a response which is assumed to be proportional to the concentration, using chemical standards to quantify the substance. However, the quantitative analysis of impurities requires its isolation or its synthesis, a time demanding and costly procedure.
The simulation of UV-visible spectra by computational chemistry tools is particularly appealing since modern approaches are able to provide results with an accuracy comparable to that obtained by experiments (about 0.1 eV). In this sense, methods based on Time-Dependent Density Functional Theory (TD-DFT) provide very accurate results.
Such approaches (as well as other quantum chemical methods) allow for the calculation of electronic transitions between the ground state and the different excited states which give the energies (hence the wavelength) of the corresponding radiations. The intensities are then evaluated from the oscillator strengths, these latter being obtained from the transition moments. However, quantum mechanics provides UV-visible spectrum as ray spectrum, but due to different factors such as natural line width, Doppler effect, pressure, vibronic effect and also thermal excitement, each transition can be enlarged with a gaussian shape.
In this context an integrated approach has been developed for the simulation of UV-visible absorption spectra. Starting from the ray transitions obtained by TD-DFT calculation, the spectra are then obtained by independent optimization of width for each gaussian function. Such computational procedure is able to significantly increase the agreement between experimental and theoretical spectra.
[1] Éric A. G. Brémond, Jérôme Kieffer, and Carlo Adamo, THEOCHEM, 954, 52 (2010)
