Experimental and Theoretical Study of the Effect of Functionalized Pyrene Polymerization on Carbon Electrode Surfaces for Electrochemical Storage

Abstract We report the effect of −(CH₂)_n−COOH (n=0,1,3) functional groups on pyrene adsorption and associated electropolymerization mechanisms on carbon nanotube model electrodes, as well as resulting electrochemical charge‐storage properties in a Li metal supercapacitor configuration. The impact of pyrene functional groups is tested by varying the alkyl chain length as well as the composite electrode formulation protocol. From experiment and DFT calculations we conclude that (i) all pyrene derivatives bind strongly to the MWCNT (multiwall carbon nanotube) surface with submonolayer loadings. Intermolecular forces and/or packing configurations rather than charge transfer between MWCNT and substrate likely play a major role in the extent of self‐adsorption process (ii) trends of predicted ionization energies (‐HOMO levels) and Hammett substituent constants match with (measured) oxidation potentials of adsorbed monomers. However, this correlation fails for unsubstituted pyrene presumably because of molecular aggregation, an argument supported by excimer formation in supernatent solutions, (iii) fusion rather than linear‐type oligomerization takes place and is described for the first time and, (iv) longer alkyl chains lead to more extended functionalized oligomers which in turn enhances p‐doping (reversible capacity). In a Li metal hybrid supercapacitor configuration, best capacity‐power compromise was found with 55 wt% 1‐pyreneacetic, resulting in a 5‐fold capacity increase from 13 mAh/g_electrode (19 F/g_electrode) for untreated MWCNT to 68 mAh/g_electrode (102 F/g_electrode) between 2–4.4 V vs. Li⁺/Li.