Lucie Josette Renée Rivier
Splitting water to produce hydrogen (H2) fuel appears very promising to address the challenges of solar energy storage and global warming as the only by-product generated is oxygen (O2). An alternative strategy, called batch water-splitting, based on Interfaces between Two Immiscible Electrolyte Solutions (ITIES) and artificial light-induced water-splitting is presented in the following thesis. This approach consists of two biphasic systems for the photo-production of H2 and O2. The following thesis focuses on the realisation of the H2 side. In first instance, the photo-induced hydrogen evolution reaction (HER) by decamethylruthenocene (Cp2Ru(II)) is reported as a strategy to facilitate water splitting in biphasic systems. Hydrogen evolution by Cp2Ru(II) was studied in detail. The study highlights that Cp2Ru(II) is an attractive molecule capable of photo-reducing hydrogen without the need for an additional sensitizer. Electrochemical, gas chromatographic and spectroscopic (UV/vis, 1H and 13C NMR) measurements indicate that the production of hydrogen occurs by a two-step process. First, the decamethylruthenocene hydride ([Cp2Ru(IV)(H)]+) is formed in the presence of acids, followed by the reduction of this complex via a hetero-dissociation reaction leading to a first release of hydrogen. Thereafter, the resultant decamethylruthenocenium ion ([Cp2Ru(III)]+) is further reduced leading to a second release of hydrogen by subtraction of a proton from a methyl group of [Cp2Ru(III)]+. Experimental results showed an excitation of [Cp2Ru(IV)(H)]+ at ï¬ = 243 nm to evolve H2 for the first oxidation. [Cp2Ru(III)]+ was produced from the reduction of protons by Cp2Ru(II) at ï¬ = 365 nm and electrochemically regenerated in situ on a Fluorinated Tin Oxide (FTO) electrode surface. A promising internal quantum yield of 25 % was obtained for HER by Cp2Ru(II) combined with electrochemical recycling. Thereafter, HER by Cp2Ru(II) was performed at ITIES. Shake-flask experiments demonstrated the production of H2 only when the biphasic system was positively polarized, to favor proton transfer. Kinetics/thermodynamics for decamethylruthenocene hydride formation were electrochemically evaluated at liquidÇliquid interface. Simulated curves developed using COMSOL Multiphysics software and compared to experimental data, indicate a modified EC (electrochemicalâchemical) mechanism for the [Cp2Ru(IV)(H)]+ formation at polarised interfaces. In the proposed pathway, [Cp2Ru(IV)(H)]+ is sufficiently stable in dichloroethane to transfer at negative potentials to the aqueous phase where it quickly dissociates. Additionally, the SHG response of [Cp2Ru(IV)(H)]+ as function of the polarisation applied confirmed this mechanism. Finally, an alternative method using homogeneous catalysts, Co(dmgh)2(py)Cl and Fe2(ï-SCH2C6H4CH2S)(CO)6 at liquidÇliquid interfaces was investigated. Coupled with a sacrificial electron donor and a sensitizer, H2 production was achieved for both catalysts. However, results showed the polarisation was not responsible for the proton transfer and the electron donor was identified as the dominant proton source. This study represents major progress in the development of the batch water splitting process as it overcomes the use of sacrificial electron donors. Moreover, these investigations provide substantial improvement in the general understanding of the photo-production of H2 by metallocenes and reactions and characterisations at ITIES.
EPFL2017
