Mechanistic study of the photo‐generation of hydrogen by decamethylruthenocene

Hydrogen evolution by decamethylruthenocene (Cp2RuII) was studied in detail highlighting that metallocenes are capable of photo‐reducing hydrogen without the need of an additional sensitizer. Electrochemical, gas chromatographic and spectroscopic (UV/vis, 1H and 13C NMR) measurements corroborated by density functional theory (DFT) calculations indicate that the production of hydrogen occurs by a two‐step process. First, the decamethylruthenocene hydride ([Cp2RuIV(H)]+) is formed in the presence of an organic acid. Subsequently, [Cp2RuIV(H)]+ is reversibly reduced via a heterolytic reaction with one‐photon excitation leading to a first release of hydrogen. Thereafter, the resultant decamethylruthenocenium ion ([Cp2RuIII]+) is further reduced leading to a second release of hydrogen by deprotonation of a methyl group of [Cp2RuIII]+. Experimental and computational data show the spontaneous conversion of [Cp2RuII] to [Cp2RuIV(H)]+ in the presence of protons. Calculations highlight that the first reduction is endergonic (ΔG0 = 108 kJ·mol−1) and needs an input of energy by light for the reaction to occur. The hydricity of the methyl protons of [Cp2RuII] was also considered.

Mediated water electrolysis in biphasic systems

Hubert Girault, Pekka Eero Peljo, Lucie Josette Renée Rivier, Micheal Diarmaid Scanlon, Heron Vrubel

The concept of efficient electrolysis by linking photoelectrochemical biphasic H2 evolution and water oxidation processes in the cathodic and anodic compartments of an H-cell, respectively, is introduced. Overpotentials at the cathode and anode are minimised by incorporating light-driven elements into both biphasic reactions. The concepts viability is demonstrated by electrochemical H2 production from water splitting utilising a polarised water–organic interface in the cathodic compartment of a prototype H-cell. At the cathode the reduction of decamethylferrocenium cations ([Cp2Fe(III)]+) to neutral decamethylferrocene (Cp2Fe(II)) in 1,2-dichloroethane (DCE) solvent takes place at the solid electrode/oil interface. This electron transfer process induces the ion transfer of a proton across the immiscible water/oil interface to maintain electroneutrality in the oil phase. The oil-solubilised proton immediately reacts with Cp2Fe(II) to form the corresponding hydride species, [Cp2Fe(IV)(H)]+. Subsequently, [Cp2Fe(IV)(H)]+ spontaneously undergoes a chemical reaction in the oil phase to evolve hydrogen gas (H2) and regenerate [Cp2Fe(III)]+, whereupon this catalytic Electrochemical, Chemical, Chemical (ECC′) cycle is repeated. During biphasic electrolysis, the stability and recyclability of the [Cp2Fe(III)]+/Cp2Fe(II) redox couple were confirmed by chronoamperometric measurements and, furthermore, the steady-state concentration of [Cp2Fe(III)]+ monitored in situ by UV/vis spectroscopy. Post-biphasic electrolysis, the presence of H2 in the headspace of the cathodic compartment was established by sampling with gas chromatography. The rate of the biphasic hydrogen evolution reaction (HER) was enhanced by redox electrocatalysis in the presence of floating catalytic molybdenum carbide (Mo2C) microparticles at the immiscible water/oil interface. The use of a superhydrophobic organic electrolyte salt was critical to ensure proton transfer from water to oil, and not anion transfer from oil to water, in order to maintain electroneutrality after electron transfer. The design, testing and successful optimisation of the operation of the biphasic electrolysis cell under dark conditions with Cp2Fe(II) lays the foundation for the achievement of photo-induced biphasic water electrolysis at low overpotentials using another metallocene, decamethylrutheneocene (Cp2Ru(II)). Critically, Cp2Ru(II) may be recycled at a potential more positive than that of proton reduction in DCE.

Royal Society of Chemistry2017

Photoproduction of Hydrogen by Decamethylruthenocene

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

Hydrogen production on demand by redox-mediated electrocatalysis: A kinetic study

Hubert Girault, Sunny Isaïe Maye, Danick Reynard

Producing hydrogen from water using a redox mediator on solid electrocatalyst particles in a reactor offers several advantages over classical electrolysis in terms of safety, membrane degradation, purity and flexibility. Herein, vanadium-mediated hydrogen evolution on a commercial and low-cost Mo2C electrocatalyst is studied through the development of a reaction kinetics model. Based on a proposed mechanistic reaction scheme, we established a kinetic rate law dependent on the concentration of V2+, the state-of-charge of the vanadium electrolyte from a vanadium redox flow battery and the amount of available catalytic sites on solid Mo2C. Kinetic experiments in transient conditions reveals a first-order dependence on both the concentration of V2+ and the concentration of catalytic active sites and a power law with an exponential factor of 0.57 was measured on the molar ratio V2+/V3+, i.e. on the electrochemical driving force generated on the Mo2C particles. The kinetic rate law was validated by studying the rate of reaction in steady-state conditions using a specially developed rotating ring-disk device (RRD) methodology. The kinetic model was demonstrated to be a useful tool to predict the hydrogen production via the chemical oxidation of V2+ over Mo2C at low pH ( > 1 M H2SO4). For a perspective, the model was implemented in a semi-batch reactor. The simulations highlight the optimal state-of-charge (SOC) to carry out the reaction in an efficient way for a given demand in hydrogen.