Redox flow battery and indirect water electrolysis

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.

Renewable hydrogen generation from a dual-circuit redox flow battery

Véronique Amstutz, Christos Comninellis, Hubert Girault, Xile Hu, Francis Powlesland, Kathryn Ellen Toghill, Heron Vrubel

Redox flow batteries (RFBs) are particularly well suited for storing the intermittent excess supply of renewable electricity; so-called “junk” electricity. Conventional RFBs are charged and discharged electrochemically, with electricity stored as chemical energy in the electrolytes. In the RFB system reported here, the electrolytes are conventionally charged but are then chemically discharged over catalytic beds in separate external circuits. The catalytic reaction of particular interest generates hydrogen gas as secondary energy storage. For demonstration, indirect water electrolysis was performed generating hydrogen and oxygen in separated catalytic reactions. The electrolyte containing V(II) was chemically discharged through proton reduction to hydrogen on a molybdenum carbide catalyst, whereas the electrolyte comprising Ce(IV) was similarly discharged in the oxidation of water to oxygen on a ruthenium dioxide catalyst. This approach is designed to complement electrochemical energy storage and may circumvent the low energy density of RFBs especially as hydrogen can be produced continuously whilst the RFB is charging.

Royal Soc Chemistry2014

Molybdenum Based Electrocatalysts for Hydrogen Evolution in Water

Heron Vrubel

The limited fossil fuel resources and the increasing levels of greenhouse gases in the atmosphere are the driving forces in the research of alternative ways of energy production and storage. The use of solar energy is expected to grow as our society claims for a renewable and sustainable source of energy. Renewable energy is intermittent. Efficient ways of storing energy must be developed to allow a smooth supply of electricity. Hydrogen is considered as one alternative, since it can be made from water electrolysis during periods of overproduction of electricity and it can be converted back when needed, by the use of fuel cells. Efficient and low-cost catalysts for the hydrogen evolution reaction (HER) are needed to increase the efficiency of the process and allow the widespread use of hydrogen. Chapter two is dedicated to the electrochemical characterization of an amorphous molybdenum sulfide film, prepared by electrodeposition using cyclic voltammetry from ammonium tetrathiomolybdate solution. The mechanism of film formation is rather complex since several redox processes are present. Gravimetric studies on the film formation during electrodeposition were performed by using an electrochemical quartz crystal microbalance that was built by us. The nature of the film was studied by X-ray photoelectron spectroscopy (XPS) prior and after electrocatalysis. A fair comparison in the activity of several materials towards HER was made possible by depositing similar amounts of catalyst with the help of the quartz microbalance. In Chapter three, we investigated the possibility to prepare by chemical methods a similar material to the electrodeposited films studied in Chapter two. MoS3 has proved to be a good “pre-catalyst” for HER; the material requires an activation process prior to hydrogen evolution. The nature of this activation was probed by XPS. Furthermore, a limited electronic conductivity was observed for MoS3. To overcome this problem, the pre-catalyst was synthesized on the surface of highly porous carbon black support. An important improvement in the catalytic activity was observed. A novel form of molybdenum sulfide was prepared from ammonium tetrathiomolybdate by reduction with sodium borohydride. This catalyst has improved catalytic activity and electronic conductivity when compared to pure MoS3. XPS analysis revealed that the chemical composition of this new catalyst is very similar to the one formed upon electroreduction of MoS3. Chapter four is dedicated to the electrochemical investigation of the catalytic properties of Molybdenum Carbide and Molybdenum Boride towards HER. Both materials exhibit good performances for HER in both acidic and basic solutions. XPS analyses have shown interesting transformations for the samples subjected to catalytic tests. Appendix I relates in detail the technical improvements of instrumental methods developed during this thesis. A full description of the equipment built as well as the design of glassware, software and signal acquisition techniques is given.

EPFL2013

Redox Flow Batteries, Hydrogen and Distributed Storage

Véronique Amstutz, Christopher Raymond Dennison, Hubert Girault, Pekka Eero Peljo, Kathryn Ellen Toghill, Heron Vrubel

Social, economic, and political pressures are causing a shift in the global energy mix, with a preference toward renewable energy sources. In order to realize widespread implementation of these resources, large-scale storage of renewable energy is needed. Among the proposed energy storage technologies, redox flow batteries offer many unique advantages. The primary limitation of these systems, however, is their limited energy density which necessitates very large installations. In order to enhance the energy storage capacity of these systems, we have developed a unique dual-circuit architecture which enables two levels of energy storage; first in the conventional electrolyte, and then through the formation of hydrogen. Moreover, we have begun a pilot-scale demonstration project to investigate the scalability and technical readiness of this approach. This combination of conventional energy storage and hydrogen production is well aligned with the current trajectory of modern energy and mobility infrastructure. The combination of these two means of energy storage enables the possibility of an energy economy dominated by renewable resources.

Schweizerische Chemische Gesellschaft2015

Molybdenum Based Electrocatalysts for Hydrogen Evolution in Water

Heron Vrubel

EPFL2013