Zn-Ce redox flow batteries and the balance problem

Dear readers,

One of the key issues of rare earth elements supply chain is that of the balance problem. Lesser used but naturally abundant materials like cerium and yttrium get stockpiled over the period of time and creates a supply-demand imbalance.

This blogpost deals with a redox flow battery which in part uses cerium as one of its active components. A redox flow battery is different from conventional rechargeable batteries (say Li-ion) in the sense that their active components are more often than not liquids stored in tanks that are pumped into the electrochemical reactor. This gives a way to decouple power and energy output of the battery: energy is how much of active components you can store in a tank and power is how fast your battery stack can delivery that energy. As you could imagine, this makes flow batteries unique in their field of applications. They can be used in large scale stationary applications like grid storage, load balancing, storage device for solar panels etc.

 zncefig

Fig.1: The main species and electrode reactions in a proton exchange membrane Zn-Ce flow battery on charge1

Zn-Ce flow batteries have a relatively high cell voltage (2.2 V) due to the nature of the choice of redox couples used. Such a high cell voltage is also achieved because of the choice of electrolyte: methanesulfonic acid which helps cross the normal water decomposition voltage (1.23 V). The chemistry in itself is easy to remember: during discharge, zinc oxidizes in the positive electrode and cerium gets reduced in the negative electrode. The reverse happens during charge as shown in Fig.1. The redox reaction happens in separate compartments separated by a cation exchange membrane.     The advantages of this system are: relatively high cell voltage for an aqueous system, moderately high energy densities  (25-35 W h/dm3), the chemistries of zinc deposition/stripping as well as cerium redox reactions are well known.

Nevertheless, there are quite a few challenges as well which are listed in a review by Frank.C.Walsh et al. 2 . The oxidation of cerium III/IV  takes place at a relatively high voltage and parasitic reactions like oxygen evolution can lower its efficiency. The anodes used for this reaction are rather expensive like Ti/Pt system and the precious metal coating can wither off over time due to acidic environments. Electrode to membrane gap gets altered due to change of shape in zin electrode during deposition and stripping and there too, hydrogen evolution during charge is a parasitic reaction.

However, the Zn-Ce flow battery research community is trying to solve these problems with different strategies such as 3D porous carbon foam or felt electrodes, electrolyte additive to reduce stress in zinc electrode, use of multiphase modelling to optimize the operating parameters etc.

In a nutshell, Zn-Ce flow batteries are very intriguing in part due to cerium’s high oxidation potential from which the battery derives its high operating voltage. It would be very interesting to observe how the challenges thrown up by the system are tackled as handling cerium is a key to addressing the balance problem.

  1. Li X, Pletcher D, Ponce de Léon C, Walsh FC, Wills RGA. (2015) Redox flow batteries for energy storage using zinc electrodes, Menictas C, Skyllas-Kazacos M, Mariana Lim T (eds), Advances in batteries for large- and medium-scale energy storage: Applications in Power systems and electric vehicles, Woodhead Publishing, 293-315.
  2. Walsh, F. C., Ponce de Léon, C., Berlouis, L., Nikiforidis, G., Arenas-Martínez, L. F., Hodgson, D. and Hall, D. (2015), The Development of Zn–Ce Hybrid Redox Flow Batteries for Energy Storage and Their Continuing Challenges. ChemPlusChem, 80: 288–311. doi: 10.1002/cplu.201402103

 

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