Description and performance of a novel aqueous all-copper redox flow battery Laura Sanz a, b, * , David Lloyd a , Eva Magdalena b , Jesús Palma b , Ky € osti Kontturi a a Aalto University, Department of Chemistry, Espoo, Finland b Institute IMDEA Energy, M ostoles, Madrid, Spain highlights  A novel, aqueous chemistry based on copper for use in redox flow cells is presented.  Comparable energy density to vanadium systems due to high solubility of copper (3 M).  Uses low cost, less toxic, easily recyclable and abundant materials.  Simplicity: single step preparation of electrolyte and no catalysts required.  Elimination of heat exchangers since the operational temperature range is extended. article info Article history: Received 31 January 2014 Received in revised form 16 May 2014 Accepted 3 June 2014 Available online 12 June 2014 Keywords: Redox flow batteries Aqueous electrolytes Copper-chloride complexes Cost reduction Hybrid flow cells abstract In this paper we present a novel aqueous redox flow battery chemistry based on copper chloro com- plexes. The energy density (20 Wh L 1 ) achieved is comparable to traditional vanadium redox flow batteries. This is due to the high solubility of copper (3 M), which offsets the relatively low cell potential (0.6 V). The electrolyte is cheap, simple to prepare and easy to recycle since no additives or catalysts are used. The stack used is based on plain graphite electrode materials and a low-cost microporous separator. The system can be operated at 60  C eliminating the need for a heat exchanger and delivers an energy efficiency of 93, 86 and 74% at 5,10 and 20 mA cm 2 respectively. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The growing demand for electricity expected during the coming decades has increased interest in the development of new tech- nologies for energy production from renewable power sources, such as wind and solar. However, the success of these new renewable power sources needs to be coupled with the introduc- tion of competitive energy storage devices for load-levelling and peak-shaving such that these renewable sources could be tied to the grid. In this fashion, the problem of the unpredictable and intermittent energy production behaviour of renewable power sources may be overcome. For electrical energy storage, electrochemical devices such as batteries and supercapacitors have been shown to provide higher efficiencies compared to other en- ergy storage systems currently utilized [1,2]. Within the wide variety of electrochemical devices for energy storage, redox flow batteries (RFB) are one of the best options for massive storage due to their higher capacity for massive storage in comparison with other battery technologies. RFBs typically employ two soluble redox couples at high concentrations in aqueous or organic media which are stored in two external tanks and pumped into an electrochemical reactor, where one of the species of the redox couple is transformed into the other, storing or delivering energy depending upon whether the device is charging or dis- charging. The reactor is composed of a stack of two-electrode cells. The two electrodes are typically composed of graphite bipolar plates and carbon felts. These electrodes are separated by an ionic exchange membrane, typically Nafion, to avoid mixing of the pos- itive and negative half-cell electrolytes [3]. * Corresponding author. Institute IMDEA Energy, M ostoles, Madrid, Spain. Tel.: þ34 646369231. E-mail addresses: laura.sanz@aalto.fi, laurasanzrubio@gmail.com, laura.sanz@ imdea.org (L. Sanz), david.lloyd@aalto.fi (D. Lloyd). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2014.06.008 0378-7753/© 2014 Elsevier B.V. All rights reserved. Journal of Power Sources 268 (2014) 121e128