1 Room Temperature Ionic Liquid Electrolytes for Redox Flow Batteries Andinet Ejigu, Peter Greatorex-Davies and Darren A. Walsh* School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, UK *darren.walsh@nottingham.ac.uk; Tel: 0044 115 8467495; Fax: 0044 115 9513562 ABSTRACT Redox flow batteries (RFBs) usually contain aqueous or organic electrolytes. The aim of this communication is to explore the suitability of room temperature ionic liquids (RTILs) as solvents for RFBs containing metal complexes. Towards this aim, the electrochemistry of the metal acetylacetonate (acac) complexes Mn(acac) 3 , Cr(acac) 3 , and V(acac) 3 was studied in imidazolium-based RTILs. The V 2+/ V 3+ , V 3+ /V 4+ , and V 4+ /V 5+ redox couples are quasi- reversible in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C 2 C 1 Im][N(Tf 2 )]. The Mn(acac) 3 and Cr(acac) 3 voltammetry, on the other hand, is irreversible in [C 2 C 1 Im][N(Tf 2 )] at glassy carbon (GC) but the rate of the Mn 2+ /Mn 3+ reaction increases if Au electrodes are used. Charge-discharge measurements show that a coulombic efficiency of 72% is achievable using a V(acac) 3 /[C 2 C 1 Im][N(Tf 2 )]/GC cell. Keywords: Ionic Liquids; Energy Storage; Cyclic Voltammetry; Electrochemical Impedance Spectroscopy; Redox Flow Battery