TY - THES ID - 134841933 TI - Synthesis and electrochemistry of liquid cobalt salts for use in redox flow batteries AU - Geysens, Pieter AU - Binnemans, Koen. AU - Fransaer, Jan. AU - KU Leuven. Faculteit Wetenschappen. Opleiding Master in de chemie PY - 2015 PB - Leuven : KU Leuven. Faculteit Wetenschappen DB - UniCat UR - https://www.unicat.be/uniCat?func=search&query=sysid:134841933 AB - Abstract New liquid cobalt salts were synthesized and characterized during this master thesis. In order to synthesize these liquid cobalt salts, cobalt(II) was coordinated by neutral ligands and combined with weakly coordinating anions. Different types of neutral ligands were used. The most important ones were 1-alkylimidazoles with a varying alkyl chain length from C1 to C12, amides (N,N-dimethylacetamide, 1-ethyl-2-pyrrolidone) and diamines (1,10-phenantroline). Two different anions were used: bis(trifluoromethylsulfonyl)imide (bistriflimide, Tf2N-) and methanesulfonate (mesylate, OMs-). Bistriflimide was the most appropriate anion since the liquid cobalt salts with this anion had the lowest melting points and viscosities. Complexes with many different ligands and bistriflimide or mesylate anions were synthesized and characterized with CHN, DSC, IR, viscometry, XRD. One of the goals of this thesis was to investigate the influence of the alkyl chain length of the 1-alkylimidazole ligands on the melting point of [Co(AlkIm)6][Tf2N]2- and [Co(AlkIm)6][OMs]2-complexes A general trend can be observed for the melting points of the [Co(AlkIm)6][Tf2N]2-complexes. The melting points first decrease in the series (C1 to C8) because as the alkyl chain on the ligand becomes longer, the crystal packing becomes less compact and the cation-anion interactions become weaker. If the alkyl chains becomes even longer, the melting point increases again due to the Van der Waals interactions between those chains. A second trend that is observed is an alternating odd-even effect for the melting points of the [Co(AlkIm)6][Tf2N]2- and [Co(AlkIm)6][OMs]2-complexes. The complexes with odd-numbered alkyl chains systematically have a lower melting point than the complexes with even-numbered alkyl chains. For instance, the complexes with 1-propylimidazole ligands have a significantly lower melting point than the complexes with 1-ethylimidazole and1-butylimidazole ligands. For the [Co(AlkIm)6][Tf2N]2-complexes, the odd-even effect fades out at higher chain lengths. The liquid cobalt salts with the lowest melting points and viscosities were characterized with electrochemical methods. The possibility to deposit cobalt layers from these liquid cobalt salts was investigated as well as the reversibility of the Co(II)/Co(III) couple to test their potential use as electrolytes in redox flow batteries. Two liquid cobalt salts with amide ligands were characterized: [Co(NEP)6][Tf2N]2 and [Co(DMAc)6][Tf2N]2. It was possible to deposit smooth and homogeneous thick cobalt layers from these electrolytes. However, it was not possible to oxidize Co(II) to Co(III). Instead, the anodic decomposition of the ligand or anion was observed. Three liquid cobalt salts with 1-alkylimidazole ligands were characterized: [Co(PrIm)6][Tf2N]2, [Co(HeIm)6][Tf2N]2 and [Co(DoIm)6][Tf2N]2. It was possible to reduce Co(II) to Co(0) but only very thin and inhomogeneous cobalt layers could be obtained from these electrolytes. Instead, most of the cathodic current was used to form cobalt nanoparticles, which was verified by TEM experiments It was not possible to observe the Co(II)/Co(III) couple in [Co(HeIm)6][Tf2N]2 and [Co(DoIm)6][Tf2N]2. In [Co(PrIm)6][Tf2N]2, Co(II)/Co(III) couple could be observed but it was only partially reversible. ER -