Electrochimica Acta 47 (2001) 815 – 823 www.elsevier.com/locate/electacta In situ FTIR spectroscopy of the Zn–Br battery bromine storage complex at glassy carbon electrodes W. Kautek a, *, A. Conradi a , Ch. Fabjan b , G. Bauer b a Laboratory for Thin Film Technology, Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin, Germany b Institute for Electrochemical Technology and Solid State Chemistry, Vienna Uniersity of Technology, Getreidemarkt 9, A-1060 Vienna, Austria Received 26 October 2000; received in revised form 30 May 2001 Abstract In situ reflection absorption FTIR spectroscopy (in situ FTIR) and small spot X-ray electron emission spectroscopy (XPS) of emersed electrodes were used to examine the electrochemical double layer on glassy carbon (GC) and the anodic storage reactions of the zinc bromine battery which are the formation of a non-aqueous N-methyl-ethyl-pyrrolidinium (MEP + ) and/or N-methyl- ethyl-morpholinium (MEM + ) polybromide phase. Oxidative conversion of the GC surface to COH, CO and COO - is observed during potential cycling between the hydrogen and oxygen evolution. The extrapolation of the intensity of the bending vibration signal due to adsorbed water allowed the determination of the point of zero charge (pzc). The pzc of GC is more than 1 V more negative than that of gold in the presence of MEP-Br or MEM-Br. Above the pzc, up to the bromide oxidation potential, MEM + shows a much stronger chemical affinity to GC than MEP + because the MEM morpholinium bridge oxygen shows a stronger interaction with the oxidised carbon surface species than the pure CH-structure of the pyrrolidinium species. This study suggests that the battery charging reaction involves mainly a heterogeneous electron transfer from Br - followed by a homogeneous chemical reaction leading to MEP-polybromide, whereas the conversion of specifically adsorbed MEM-Br ion pairs to polybromide is a slow process. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: In situ analysis; Batteries; Buried interface; Fourier transform IR spectroscopy 1. Introduction The zinc bromine battery has raised vivid interest because of its promising application not only in electric vehicles or in connections with generating sources such as fuel cells, solar arrays, and wind turbines, but also in integrated systems for power quality and electric reli- ability. The zinc bromine battery utilises a liquid cathode (catholyte) and a liquid anode (anolyte), stored in small tanks adjacent to the reaction cells. The stor- age reactions of the zinc bromine battery are the ca- thodic deposition of zinc and the anodic formation of a non-aqueous polybromide phase [1 – 7]. The role of the quaternary ammonium bromide constituents in this electrode reaction, N -methyl-ethyl-pyrrolidinium- (MEP) and N -methyl-ethyl-morpholinium (MEM) bro- mide, has been studied at glassy carbon in the present investigation. First electrochemical in situ investigations of this electrolyte system at gold model electrodes with a phase-stabilised electrochemical quartz microbalance (ECQMB) and in situ reflection absorption FTIR spec- troscopy (in situ FTIR) showed that specifically ad- sorbed polybromide anions formed MEM-Br n and MEP-Br n ion pairs, and that MEM-Br n was adsorbed more strongly than MEP-Br n [7]. Therefore, it was concluded that a homogeneous chemical reaction of the dissolved MEP + cation with electrochemically gener- ated bromine leads to the storage complex MEP-Br n much more rapidly than the heterogeneous electro- chemical reaction of MEM-Br n which was strongly adsorbed at gold. In the present investigation, this mechanism was studied at carbonaceous electrode ma- terials, e.g. glassy carbon, which are chemically more * Corresponding author. Tel.: +49-30-8104-1822; fax: +49-30- 8104-1827. E-mail address: wolfgang.kautek@bam.de (W. Kautek). 0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(01)00762-9