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• RESEARCH TOPICS EMA2 AREAS+ (SOUTH EAST ASIA)

For Molecular Design and Synthesis we are looking for a PhD student to work on a research project related to the development of new non-aqueous electrolytes for redox flow battteries.

Ionic liquid redox flow batteries

This is a collaborative research project between the groups of the supervisor Prof. Koen Binnemans (KU Leuven, Department of Chemistry) and the co-supervisor Prof. Jan Fransaer (KU Leuven, Department Metallurgy and Materials Engineering, MTM). The expertises of the Binnemans group are the synthesis and characterization of new ionic liquids, whereas the Fransaer group is focused on the electrochemical characterization of the ionic liquids and the construction of demo batteries.

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Project

A secure electricity supply is indispensable for modern society, but it is becoming more and more difficult due to phasing out of nuclear power plants or generation of electricity from fossil fuels. More and more, we have to rely on electricity produced by wind turbines and photovoltaic (solar) cells. Unfortunately, the wind is not always blowing and it is evident that solar cells do not produce electricity at night. However, it is currently not possible to store large amounts of electricity. All electricity that is produced has to be consumed immediately. The storage of electricity is a major challenge for the energy sector. During the last week of 2012, the demand for electricity in Western Europe was low due to unseasonally warm weather and low economic activity, and there was so much power generated by wind turbines that wind farms in Germany had to pay to put their electricity on the grid while in Belgium wind energy was given away for free. Hence it would be benefial if there was a way to store this energy until the demand for electricity was higher. An obvious solution is to store electricity under the form of chemical energy, i.e. in batteries. Only a limited number of types of batteries have a storage capacity that is compatible with the production capacity of a large wind turbine or a solar cell farm. Lead-acid batteries have a large storage capacity, but have only a limited lifetime and consume large amounts of lead. One of the most promising types of battery for large capacity storage of electricity is the redox flow battery [1,2].

Redox flow batteries store chemical energy in the form of dissolved redox couples. Electricity is generated in a separate power module. During discharge, the two electrolytes flow from separate storage tanks to the electrolysis cell for the redox reaction, with ions transferred between the two electrolytes across an ion-exchange membrane. During the recharging step, the process is reversed. Advantages are that flow batteries are easy to scale up (kWh to 100s MWh), have high power density and fast response time. Several types of redox flow batteries have been described in literature, with the all-vanadium, the zinc-bromine and the zinc-cerium batteries as the best known ones [3]. The former two types of redox flow batteries are commercially available, but still suffer from several problems (limited solubility of metal salts, stability of redox couples, low energy density). All of the current commercial redox flow batteries use water as a solvent, which strongly limits the maximum energy densities that can be obtained due to the limited voltage range between the different oxidation states (for vanadium redox flow batteries this is 1.26 V, for manganese batteries it is 1.1 V) and the maximum solubility of these metals (2.5 M of vanadium in a H2SO4/HCl mixture of sulfuric acid and hydrochloric acid). Hence, current state-of-the-art vanadium redox flow batteries have a theoretical energy density of 40 Wh/liter. In practice (due to inherent losses, etc.) such batteries have an energy density of 25 Wh/liter, which is lower than the energy density of lead acid batteries (60-75 Wh/l) but due to their lower cost, suitable for stationary power storage.

One way to increase the energy density of redox flow batteries is to increase the potential differences between the two redox couples. This is possible with redox couples dissolved in organic solvents [5], but volatile organic solvents have safety issues and most metals salts have a poor solubility in organic solvents. We therefore propose to dispense with the solvent all together and use ionic liquids as the redox couple and solvent combined. Of special interest are those ionic liquids where the metal ion is part of the cation, because these allow reaching very high metal ion concentrations (up to 6.5 M or three times higher than possible with vanadium). Moreover, these solvents have a much larger electrochemical window than water, allowing to increase the energy density even further and are inherently safe due to their very low vapor pressures. Only a handful of studies report on the use of metal salts of ionic liquids dissolved as electrolytes for redox flow batteries, but these are very promising [6-8]. The Binnemans and Fransaer groups at KU Leuven have contributed to the development of such metal-containing ionic liquids (liquid metal salts) with viscosities that are amongst the lowest reported [9,10] and are prime candidates for applications in redox flow batteries. The current solution for the ion-exchange membrane in redox flow batteries has been to use cation exchange membranes, as these membranes have the lowest resistivity (due to proton transport) [4]. However, this means that one has to use the same metal species in the anolyte and catholyte since cations migrate through cation exchange membranes. This strongly limits the possible redox couples that can be used in redox flow batteries to a handful of elements that have at least three stable oxidation (redox) states, i.e. vanadium, tungsten, molybdenum, chromium, manganese and uranium. We propose to use anion exchange membranes which would allow much greater flexibility in the possible redox couples.

Hence, the current project aims at introducing a novel type of redox flow battery, based on liquid metal salts in combination with anion exchange membranes. The objectives of the project are:

1. To synthesize new liquid metal salts containing metals with different accessible oxidation states. These metals include cerium, samarium, ytterbium, vanadium, chromium, iron, manganese, cobalt and copper. The liquid metal salts should have a low viscosity and a good electrical conductivity at or near room temperature so that reasonable current densities can be obtained with minimal Ohmic losses. The anion should preferably be small so that the Ohmic drop over the anion exchange membranes is as small as possible.
2. To study the electrochemistry of the metal-containing ionic liquids (liquid metals salts), with special emphasis of the redox potentials, reversibility and kinetics of the redox couples.
3. To select pairs of liquid metal salts that have the same anion, have widely different redox potentials, have fast electrochemical and reversible kinetics. For the positive half-cell, the following redox couples are of particular interest: Ce4+/Ce3+ and Fe3+/Fe2+. For the negative half-cell, the following redox couples are of interest: Sm3+/Sm2+, Yb3+/Yb2+, Cr3+/Cr2+, V3+/V2+ and Ti4+/Ti3+. The anion should preferably be small so that the Ohmic drop over the anion exchange membranes is as small as possible.
4. To develop homogeneous or heterogeneous catalysts to accelerate the electron transfer, especially for the Ce4+/Ce3+ redox couple.
5. To build a demonstration model of cerium redox flow batteries. The electrodes should have good electrochemical stability and have fast redox kinetics and excellent reversibility. To measure the performance of these batteries.

References:
[1] D.H. Doughty, P.C. Butler, A.A. Akhil, N.H. Clark, J.D. Boyes, The Electrochemical Society, Interface, Fall 2010, 49.
[2] T. Nguyen, R.F. Savinell, The Electrochemical Society, Interface, Fall 2010, 54.
[3] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M. Saleem, J. Electrochem. Soc. 158 (2011) R55.
[4] X. Li, H. Zhang, Z. Mai, H. Zhang, I. Vankelecom, Energy & Environmental Sci. 4 (2011) 1147.
[5] Q.H. Liu, A.E.S. Sleightholme, A.A. Shinkle, Y.D. Li, L.T. Thompson, Electrochem. Commun. 11 (2009) 2312.
[6] Y. Katayama, I. Konishiike, T. Miura, T. Kishi, J. Power Sources 109 (2002) 327.
[7] H.D. Pratt III, A.J. Rose, C.L. Staiger, D. Ingersoll, T.M. Anderson, Dalton Trans. 40 (2011) 11396.
[8] D.P. Zhang, Q.H. Liu, X.S. Shi, Y.D. Li, J. Power Sources 203 (2012) 201.
[9] S. Schaltin, N.R. Brooks, K. Binnemans, J. Fransaer, J. Electrochem. Soc. 158 (2011) D21.
[10] N.R. Brooks, S. Schaltin, K. Van Hecke, L. Van Meervelt, K. Binnemans, J. Fransaer, Chem. Eur. J. 17 (2011) 5054.