Photoproduction of Hydrogen by Decamethylruthenocene

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.

Redox flow battery and indirect water electrolysis

Véronique Amstutz

The progressive integration of renewable energy sources such as wind turbines and photovoltaic pannels in the current electrical network, and the rise of the electrical mobility, provoke a change of paradigm in the sector of energy management. The increasing variability of the electricity production and consumption profiles, together with the requirement for a reliable supply of energy, necessitates the implementation of energy storage means of different scales and for a wide range of applications. Redox flow batteries (RFBs) are well adapted for buffering the fluctuations of solar or wind energy production. They present a fast response time, can withstand a large number of charge-discharge cycles and their ouput power is independant of their energy capacity. The main limitation of RFBs resides in their low energy density. The objective of the present work was to test a mean to overcome this low energy density. A new concept was developed, which rests on the addition of a second pathway to discharge the RFB. This pathway allows to generate hydrogen and oxygen, without affecting the functioning of the RFB. This concept was called dual-circuit RFB and was patented. RFBs are based on two liquids electrolytes, each stored in a reservoir, and flowing through an electrochemical cell for their electrochemical conversion. Each electrolyte contains one redox couple and Ce(IV)/Ce(III) and V(III)/V(II) were selected as positive and negative redox couples in the RFB developed here. Charge-discharge curves of a VâCe RFB were measured for characterisation purposes and for the preparation of the charged electrolytes. The latter ones can be discharged electrochemically in the RFB to generate electricity (electrochemical discharge mode), or they can be directed in a secondary circuit where they are discharged for the production of hydrogen or oxygen (chemical discharge mode). The chemical discharge of the negative electrolyte consists of the reaction of V(II) with protons to produce hydrogen and V(III). Mo2C was selected as heterogenous catalyst for the characterisation of the reaction. A conversion close to 100% suggested no loss of current. A kinetic analysis provided some insights into the catalytic mechanism of this reaction. The chemical discharge of the positive electrolyte aimed at the conversion of Ce(IV) to Ce(III) by the oxidation of water to oxygen and also necessitates a catalyst. Iridium dioxide (IrO2) and ruthenium dioxide (RuO2) were evaluated in terms of conversion and kinetics. The composition of the positive electrolyte was shown to be of importance for this reaction. To show the feasibility of this concept, a larger-scale demonstrator system was designed based on a 10 kW (40kWh) commercially available vanadium RFB. A characterisation of this RFB was first performed. The design a suitable Mo2C catalyst and its corresponding catalytic bed is also discussed. As a conclusion, the concept developed and experimentally tested in the present work leads to a RFB system which is characterised by two discharge modes, increasing its energy storage capacity and energy density. The dual-circuit RFB also represents a crossing between electrical grid and hydrogen mobility as the hydrogen produced by surplus electricity could be delivered to fuel cell cars. The application of this system in a local distribution electrical network, close to a wind or solar source seems a promising approach.

EPFL2015

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

Decamethylruthenocene Hydride and Hydrogen Formation at Liquid|Liquid Interfaces

Hubert Girault, Manuel Alejandro Méndez Agudelo, Pekka Eero Peljo, Lucie Josette Renée Rivier, Micheal Diarmaid Scanlon, Thomas James Stockmann

The formation and the dissociation of metal hydrides are key steps within the hydrogen evolution reaction (HER) pathway for photochemical water splitting, but also impacts a wide range of other catalytic, industrial, and biochemical reactions. Herein, we describe our recent work studying HER at the interface between two immiscible electrolyte solutions (ITIES), between water|1,2-dichloroethane. This is a unique platform for evaluating the kinetics/thermodynamics for metallocene hydride formation using decamethylruthenocene. In this approach, an aqueous acid serves as the proton source and is pumped across the ITIES via an externally applied potential or the use of a phase transfer catalyst. Simulated curves developed using COMSOL Multiphysics software and compared to experimental ones, indicate a modified EC′ (electrochemical–chemical) mechanism for the decamethylruthenocene hydride formation. In the proposed pathway, decamethylruthenocene hydride is metastable in 1,2-dichloroethane and persists on the time scale of the recorded cyclic voltammograms long enough to transfer to the aqueous phase where it quickly dissociates. This is evidenced through an asymmetric, ion transfer wave observed experimentally and concluded to be hydride transfer. Shake-flask experiments with head space gas sampling demonstrated that hydrogen production was observed only when the biphasic system was positively polarized, to favor proton transfer, and decamethylruthenocene was photoactivated. This approach, combining electrochemical, simulation, and chromatographic methods, brings new insight into the factors that underlie the mechanism and rates of hydride formation/dissociation at soft interfaces.