Ali Kachmar

New insights in the electrocatalytic proton reduction and hydrogen oxidation by bioinspired catalysts: a DFT investigation.

In this paper, we present a DFT study of the proton reduction mechanism catalyzed by the complex [Ni(P₂(H)N₂(H))₂](2+), bioinspired from the hydrogenases. A detailed analysis of the reactive isomers is discussed together with the localizations of the transitions states and energy minima. The reactive catalytic species is a biprotonated Ni(0) complex that can show different conformations and that can be protonated on different sites. The energies of the different conformations and biprotonated species have been calculated and discussed. Energy barriers for two different reaction mechanisms have been identified in solvent and in gas phase. Frequency calculations have been performed to check the nature of the energy minima and for the calculations of entropic energetic terms and zero point energies. We show that only one conformation is mostly reactive. All the others species are nonreactive in their original form, and they have to pass through conformational barriers in order to transform in the reactive species.

Multi-scale Modeling-based Prediction of PEM Fuel Cells MEA Durability under Automotive Operating Conditions

The MEA durability in state-of-art PEMFC is one of the main shortcomings limiting the large-scale development and commercialization of this zero-emission power technology. It is largely observed that the micro-structural properties of the MEA evolve during PEMFC automotive-like operating conditions and translate into the cell potential degradation. Interpretation of the impact of the operation mode on the cell potential degradation mode in relation with the micro-structural experimental observations is difficult because of the strong coupling between the different underlying physicochemical phenomena (Figure 1). From this, the development of physical-based modeling tools is essential for the engineering community not only to elucidate the MEA degradation and failure mechanisms but also to predict the MEA durability in function of its initial material properties and operating conditions. Within this context, we have recently developed at CEA/LCPEM a multiphysics-multiscale model of the electrochemical processes taking place in Ptand Pt-M alloy-based PEMFC MEA, with M being a transition element [1-9]. This approach, dedicated to the understanding of the detailed mechanisms underlying the MEA degradation under automotive operating conditions (Figure 2), scales up ab initio data (obtained from DFT and MD simulations) into coupled/interacting elementary kinetic models –without using Butler-Volmer equations [5]of: 1) the cathodic catalyst oxidation/dissolution and electrochemical ripening [1-2, 7-8], 2) the dissolved metal ions diffusion/electro-migration/re-crystallization in the ionomer [1-2], 3) the cathodic carbon catalyst-support corrosion and the induced Pt coarsening [3-4], 4) the membrane chemical degradation, and 5) the detailed CO contamination kinetics on the anodic catalyst [9]. These aging models are coupled with non-equilibrium thermodynamics mechanistic descriptions of the MEA physicochemistry at the spatial nanoscale (detailed DFTbased HOR and ORR pathways, steric and electrochemical double layer effects...) and microscale (ionic and reactants transfers, water transport). From the algorithmic point of view the global model combines inhouse developed Kinetic Monte Carlo (e.g. for the catalyst degradation description, including the change of the catalyst nanomorphology -reconstruction-) and CFD (for transport phenomena) codes. By accounting for the numerical feedback between the calculated instantaneous local conditions (local reactant species concentrations...) and the degradation phenomena, the model allows predicting the evolution of the materials aging and its transient impact on the MEA performance degradation (cell potential evolution with the current as an input). In this paper we report new results obtained with this model applied to a Pt-based MEA including: the prediction of the impact of different current operation modes (OCV, steady-state and cycled current) on the MEA cell potential degradation and durability (defined as the time where the cell potential goes abruptly to zero) at different electrodes RH and temperatures. The interplaying between the different aging phenomena in relation with reactants humidification conditions is discussed. the impact of long-term (>500h) CO anodic injection on the membrane chemical degradation (this extends our recent result on the use of CO anode contamination as a mitigation method of the cathode carbon corrosion under power-cycled conditions [4, 9]). All the simulations are discussed in comparison with experiments carried out with dedicated single-cells and with TEM/HR-TEM observations as well as EPR and XPS micro-structural characterizations before and after operation in co-flow and counter flow operating single cells. Acknowledgments. This work is funded by the French National Research Agency (ANR) through the program “PAN-H”, within the context of the project “MAFALDA”. References [1] A. A. Franco, M. Tembely, J. Electrochem. Soc., 154 (7) B712 (2007). [2] A. A. Franco, ECS Trans., 6 (10) 1 (2007). [3] A. A. Franco, M. Gerard, J. Electrochem. Soc., 155 (4) B367 (2008). [4] A.A. Franco et al., ECS Trans., 13 (15) 35 (2008). [5] A. A. Franco et al., J. Electrochem. Soc., 153 (6) A1053 (2006). [6] A. A. Franco et al., Fuel Cells, 7, 99 (2007). [7] A. A. Franco et al., ECS Trans., 13 (17), 29 (2008). [8] A.A. Franco et al, J. Electrochem. Soc., 156 B410 (2009). [9] A.A. Franco et al., Electrochim.Acta, in press (2009).

Dynamic properties of a hexadecamolybdenum wheel: studies in solution and density functional theory calculations.

Variable temperature (1)H NMR studies of the host-guest complex [Mo(16)O(16)S(16)(OH)(16)(H(2)O)(4)(PDA)(2)](4-) (1 ; PDA(2-) = phenylenediacetate) previously carried out in D(2)O have revealed a complex behavior in solution, involving a gliding motion of both parallel phenyl rings of the PDA(2-) ligands. In the present work, we present new NMR spectra carried out in the aprotic solvent CD(3)CN, which allow the observation of the proton signals associated with the bridging hydroxo groups of the inorganic host. The new spectra provide detailed information about the concerted reorganization of the guest components, that is, PDA(2-) and water molecules. The existence of an equilibrium between two distinct isomers differing in the linking mode between the inorganic host and the two equivalent PDA(2-) ligands is evidenced. This equilibrium appears strongly dependent upon the temperature, leading to a complete inversion of the distribution between 300 and 226 K. The thermodynamic data related to the isomerization reaction have been determined (Delta(r)H = -50.5 kJ mol(-1) and Delta(r)S = -215 J mol(-1) K(-1)). Furthermore, at low temperature, one of the isomers exists in two conformations, only differing in the H-bond network involving the inner water molecules. Density functional theory calculations were carried out to push ahead the interpretations obtained from experiment, identify the isomers of 1, and specify the role and the positions of the guest water molecules. Among the various structures that have been calculated for 1, three fall in a narrow energy range and should correspond to the species characterized by variable-temperature (1)H NMR experiments in CD(3)CN. Finally, this study shows how the internal disposition of the ligands affects the ellipticity of the Mo(16) ring which varies from one isomer to the other in the 0.73-1 range and highlights solvation of the ring as one of the key parameters for the conformational design of these flexible host-guest systems.

Conformational changes in a flexible, encapsulated dicarboxylate: evidence from density functional theory simulations.

The dynamical behavior of the [Mo12O12S12(OH)12{O2C-(CH2)N-CO2}]2- complexes is analyzed via first-principles molecular dynamics simulations. Experimental X-ray data play the role of initial configurations for our search in the configuration space. We show that different stable and metastable conformers are possible, and these are thermally accessible at relatively low temperatures. This is the main outcome of our first-principles molecular dynamics approach in which the temperature allowing for thermal activation has been set to T = 500 K, which is consistent with the variable temperature 1H NMR spectra of these complexes in solution taken at comparable, although moderately lower, temperature. This implies that a relatively large manifold of folding configurations is available to the encapsulated guest species. Moreover, the high flexibility of both the host cage and the inserted guests allows for the accommodation of a rather wide variety of functional groups with potential applications in several fields.

Mapping the Free Energy of Lithium Solvation in the Protic Ionic Liquid Ethylammonuim Nitrate: A Metadynamics Study

Understanding lithium solvation and transport in ionic liquids is important due to their possible application in electrochemical devices. Using first-principles simulations aided by a metadynamics approach we study the free-energy landscape for lithium ions at infinite dilution in ethylammonium nitrate, a protic ionic liquid. We analyze the local structure of the liquid around the lithium cation and obtain a quantitative picture in agreement with experimental findings. Our simulations show that the lowest two free energy minima correspond to conformations with the lithium ion being solvated either by three or four nitrate ions with a transition barrier between them of 0.2 eV. Other less probable conformations having different solvation pattern are also investigated.

An efficient and cyclic hydrogen evolution reaction mechanism on [Ni(PH2NH2)2]2+ catalysts: a theoretical and multiscale simulation study

In this paper we report a theoretical and a multiscale simulation study of the hydrogen evolution reaction (HER) on the [Ni(PH2NH2)2]2+ catalyst in acidic media (2H+ + 2e− → H2). First, at the DFT calculations level, a cyclic pathway for the HER is proposed highlighting the shuttling of electrons with protons on the conformationally flexible catalyst. The theoretical calculation gives a better understanding of the efficient cyclic pathway of [Ni(PH2NH2)2]2+, and the effect of solvent on the mechanism has been discussed. The σ-donating and π-accepting nature of H2–Ni bond has been identified in the H2 complex. The oxidation state of the Ni centre and geometrical changes of the catalyst in the reaction coordinate are also identified. Then a mean-field kinetic model incorporating the calculated DFT data has been developed. This model allows us to simulate the behaviour of these catalysts in electrochemical conditions representative of polymer electrolyte membrane water electrolyzers operation. Calculated results include experimental observables such as polarization curves showing good agreement with available experimental data. Competitive phenomena between the different electrochemical mechanisms, the protons and H2 transport, and their relative impact on the overall cell performance are particularly discussed.

New insights into the atomic structure of amorphous TiO2 using tight-binding molecular dynamics

Amorphous TiO2 (a-TiO2) could offer an attractive alternative to conventional crystalline TiO2 phases for photocatalytic applications. However, the atomic structure of a-TiO2 remains poorly understood with respect to that of its crystalline counterparts. Here, we conduct some classical molecular dynamics simulations of a-TiO2 based on a selection of empirical potentials. We show that, on account of its ability to dynamically assign the charge of each atom based on its local environment, the second-moment tight-binding charge equilibration potential yields an unprecedented agreement with available experimental data. Based on these simulations, we investigate the degree of order and disorder in a-TiO2. Overall, the results suggest that a-TiO2 features a large flexibility in its local topology, which may explain the high sensitivity of its structure to the synthesis method being used.