Battery | EPFL Graph Search

In a battery system, an electrochemical cell has a first compartment containing a positive electrolyte with an electrode 2 at least partially immersed in the positive electrolyte containing the first redox active material, forming a positive half cell. A second compartment contains a negative electrolyte with an electrode 1 at least partially immersed in the negative electrolyte containing the second redox active material, forming a negative half -cell. A third electrolyte is substantially immiscible and in contact with the positive and the negative electrolyte. The third electrolyte contains a common ion with the positive electrolyte and another common ion with the negative electrolyte, establishing a polarisation between the positive and negative electrolytes and enhancing the potential difference between the positive and negative electrode.

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

Alternative anode materials for methane oxidation in solid oxide fuel cells

Joseph Sfeir

Fuel Cells are electrochemical devices that are able to directly convert chemical energy to electrical energy, without any Carnot limitation. Hence, their energy efficiencies are relatively high. Among the various types of fuel cells, solid oxide fuel cells (SOFC) are operated at high temperatures and in principle can run on various fuels such as natural gas and hydrogen. As natural gas is sought to become one of the main fuels of the next decades, its direct feed to a SOFC is desirable as the reaction free energy is high CH4 + 2O2 = CO2 + 2H2O, ΔrG°800°C = -800.8kJ/mol At present, conventional SOFC are operated with pure hydrogen or partially to fully reformed natural gas implying an efficiency penalty. SOFC anodes are generally made of electrocatalytically active Ni-YSZ cermet. Pure CH4 is not fed directly to the anode, because of problems associated with the anode deactivation and coking as CH4 leads to the detachment of Ni particles from the YSZ support and their encapsulation by carbon. To overcome this problem, partially oxidized or reformed CH4 is used. For SOFCs running on direct CH4 feed, alternative anode materials to Ni-YSZ, or new anode formulations are necessary. However, in this case, several parameters influence the anode performance and stability. The anodes should withstand reduction at a low PO2 of around 10-24 atm, be compatible with the SOFC electrolyte, possess an acceptable conductivity and thermal expansion coefficient, and appropriate catalytic and electrocatalytic properties along with low coking activity. LaCrO3 and CeO2-based materials as well as some metal catalysts were studied here for their potential application as anodes in direct oxidation of CH4. This choice of materials was made as lanthanum chromite and ceria-based materials are mixed conductors known to resist quite well to the very reducing conditions in SOFC as they are commonly used for interconnect and electrolyte materials respectively. Moreover, these compounds possess some catalytic activity for CH4 activation and combustion. LaCrO3 and CeO2-based materials show rather low electrical conductivity (on the order of 1 S/cm in reducing conditions), necessitating the application of an extra material for proper current collection. Nevertheless, these oxides possess many of the stringent characteristics cited above. For SOFC anode purposes, LaCrO3-based compounds, substituted with Ca, Sr, Mg, Mn, Fe, Co and Ni, were synthesized by a modified citrate route from nitrate precursors. An optimal calcination temperature of 1100°C was found to be necessary to obtain XRD-pure yet sinteractive compounds. Conductivity measurements in air and in reducing atmospheres of 10-21 atm showed that the La A-site substituents (Ca and Sr) influenced the conductivity more than the Cr B-site substituents (Mg, Mn, Fe, Co and Ni). In humidifed H2, the total conductivity was maximally of 6.5 S/cm. Surface reaction with YSZ electrolyte was evidenced by SEM, XPS and SIMS, in reducing conditions, especially under current load, when these powders were applied as anodes. A current induced demixing effect was noted for heavily substituted LaCrO3. Ceria-based compounds, doped with Y, Nb, Pr and Gd, were synthesized through 3 different techniques: the solid-state method, coprecipitation and the NbCl5 route. The coprecipitation route was found to be most appropriate for Y, Pr and Gd, whereas for Nb a modified precipitation and the NbCl5 technique were the most efficient. Co-doping of Nb and Gd or Pr was studied in order to increase the electronic as well as the ionic conductivity in CeO2. Among all dopants, Nb was found to promote most the adhesion to YSZ electrolyte sheets. Adding Nb to Gd-doped ceria improved the electrode adhesion on YSZ sheets, but deteriorated its conductivity in oxidizing conditions. No interfacial reaction with YSZ was observed by HRTEM when Nb-doped ceria anodes were sintered on YSZ at 1200°C/4h. Catalytic measurements were undertaken with these materials in CH4 rich atmospheres. Different reaction mixtures were chosen to simulate the various SOFC operating conditions: partial oxidation, CO2 reforming by recycling and H2O reforming. Among the different elements, Sr and Ni were found to be the most active substituents for LaCrO3, as they promote all three types of reactions. The combination of both elements is favorable not only for catalytic use, but also for electrode application as they tend to increase the electronic conductivity of the material. Temperature programmed oxidation (TPO) and TEM made on the catalysts after runs in CH4, showed a very low carbon coverage over these oxides, except in the case of Fe substitution, over which carbon deposition was promoted. XPS analysis showed also that all B-site substituted compounds did not segregate after the catalytic runs whereas Ca and Sr (A-site substituents) tended to segregate in H2 and H2O rich atmospheres. Thermodynamic calculations, made using correlations developed in literature, show that Ca and Sr are expected to segregate in H2 and H2O rich atmospheres as they tend to form volatile hydroxyl species. Also, the thermodynamic stability of the LaCrO3 was observed to depend much on the substitution. For CeO2-based oxides, the activity for steam reforming increased from Nb

EPFL2002

Effectiveness factor of thin-layer IrO2 electrocatalyst

Erika Herrera Calderon

Dimensionally Stable Anodes (DSA®) are electrodes composed of a metal support and an oxide coating (catalyst). Particularly in this thesis, the Ti/IrO2 electrode was used for the study of the fraction of the surface which participates effectively in the investigated reactions (effectiveness factor, Ef ). Four reactions with different kinetics have been investigated: (a) Fe3+/Fe2+ (fast reaction), (b) O2 and (c) Cl2 evolutions (slow reactions), and (d) isopropanol oxidation (complex reaction involving redox catalysis). This study has allowed us to apply for the first time in electrocatalysis the term, effectiveness factor, Ef , used before in other fields like in heavy metals recovery with 3D electrodes and in heterogeneous catalysis. On the other hand, in order to achieve the main objective of this thesis, an analytical and a qualitative approach was proposed. Furthermore, the Ti/IrO2 electrodes were characterized electrochemically using the cyclic voltammetry (CV) technique in which the voltammetric charge was measured at different potential scan rates, potential windows, and electrolyte temperatures using various Ti/IrO2 loadings. From the voltammetric charge, two contributions were found. The first contribution is related to the double layer capacitance and the second with the redox couples present on the electrode surface. The first reaction investigated in this work was the Fe3+/Fe2+ redox couple. In the first part of the investigation of this reaction, the CV technique was used; however, some issues related to the uncompensated electrolyte resistance were encountered. Therefore the effectiveness factor for this particular case was not possible to obtain. On the other hand, using rotating Ti/IrO2 disk electrodes (RDE) for the same reaction, the effectiveness factor, Ef , has been estimated, however, this value is much lower than the predicted one. The other investigated reactions in this work were O2 and Cl2 evolutions. The investigated kinetics of both reactions showed that the evolution of chlorine is much faster than that of oxygen. The effectiveness factor, Ef , for the oxygen evolution was close to the unity for all IrO2 loadings, contrary to the chlorine reaction that decreased with the loading. This behaviour was predicted according to what is given in the analytical approach. The last reaction investigated in this work was the oxidation of isopropanol. In this case a model was proposed based on three main reactions: Electrochemical IrO2 oxidation to IrO3, chemical oxidation of isopropanol via IrO3, and chemical decomposition of IrO3 to IrO2. The effectiveness factor, for the electrochemical reaction seemed not to correlate with the loading, whereas for the chemical one, it diminished with the loading.

EPFL2008