Electrochimica Acta 80 (2012) 7–14 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al hom epa ge: www.elsevier.com/locate/electacta The influence of operational parameters on the performance of an undivided zinc–cerium flow battery P.K. Leung ∗,1 , C. Ponce de Leon, F.C. Walsh Electrochemical Engineering Laboratory, Energy Technology Research Group, Faculty of Engineering and the Environment, University of Southampton, Highfield, Southampton, Hampshire SO17 1BJ, UK a r t i c l e i n f o Article history: Received 22 March 2012 Received in revised form 13 June 2012 Accepted 14 June 2012 Available online 30 June 2012 Keywords: Cerium Mixed electrolyte Redox flow battery Single flow circuit Undivided Zinc a b s t r a c t An undivided zinc–cerium redox flow battery was studied under a wide range of operational conditions, including: (i) electrolyte composition; the concentrations of ([Zn 2+ ], [Ce 3+ ] and [H + ]), (ii) current density (0–80 mA cm -2 ), (iii) electrolyte flow linear velocity (0.64–7.0 cm s -1 ) and (iv) temperature (20–60 ◦ C). The charge efficiency increased at higher current densities and at higher electrolyte flow velocities. Unlike the divided zinc–cerium system, the charge–discharge performance decreased at higher temperature, since oxygen evolution became increasingly favored at the positive electrode. The use of a low acid con- centration led to a poor conversion of Ce(III) to Ce(IV) ions during the discharge cycle. Mixed electrolytes containing methanesulfonate and sulfate anions have been evaluated at a high Ce(III) ion concentration, e.g. 0.4 mol dm -3 . After charging the battery for 4 h, the conversion of Ce(III) to Ce(IV) ions became less efficient over time due to a greater fraction of the current being used in oxygen evolution. Critical aspects for improvements in the battery performance are considered. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Redox flow batteries are a relatively new rechargeable battery technology for energy storage. The energy can be contained in chemical species within the electrolyte at a utility scale and the devices can store electricity from intermittent renewable energy sources, such as solar and wind power. The technology can also be used for load-leveling [1] and in applications such as uninter- ruptible power supplies (UPS) [2] and electric vehicles [3]. Various redox flow batteries have been proposed but only all-vanadium [4–6], zinc–bromine [7,8] and bromine–polysulfide [9,10] have been commercialized or demonstrated at a large-scale [1]. Most of these conventional flow battery systems require expensive ion- exchange membranes or microporous separators to divide the negative and positive electrolytes that may have different chem- ical compositions. Aiming to reduce costs and simplify the cell design, several membrane-free systems have been proposed: sol- uble lead acid [11], zinc–nickel [12,13], copper–lead dioxide [14] cadmium–chloranil [15] and, more recently, zinc–cerium [16] flow batteries. ∗ Corresponding author. Tel.: +44 23 80598931; fax: +44 23 80597051. E-mail address: mepkleung@ust.hk (P.K. Leung). 1 Present address: Department of Mechanical Engineering, Hong Kong University of Science & Technology (HKUST), Hong Kong, China. The undivided zinc–cerium flow battery was developed from the existing membrane-divided configuration using zinc and cerium redox couples [17–19], which resulted in the highest open-circuit cell potential difference (c.a. 2.3 V) among other flow battery chemistries. The self-discharge of zinc is slow at a low concentration of oxidizing Ce(IV) ions but increases at a higher proton concentration. Instead of using a high acid concentration as described in previous patents [17,18], the undivided system utilizes a relatively low methanesulfonic acid concentration, e.g., 0.5 mol dm -3 , to avoid excessive hydrogen evolution and to ensure that zinc electrodeposition takes place on the composite carbon polyvinyl ester negative electrode [20,21]: Zn (II) + 2e - charge ⇋ discharge Zn E  + = -0.76 V vs. SHE (1) At the compressed carbon felt positive electrode, Ce(III) ions oxidize to Ce(IV) ones [22]: Ce (III) - e - charge ⇄ discharge Ce (IV) (2) The standard electrode potential of reaction (2) lies between 1.28 and 1.72 V vs. SHE, depending on the supporting electrolyte [23]. The carbon felt positive electrode can facilitate the cerium redox reaction and facilitates a wider operational range, such as 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.06.074