Self-assembled α/β-dipeptides as dielectrics to improve the behavior of multilayered conducting polymer pseudocapacitors

Multilayered films composed of two different materials, where the less conducting one acts as a dielectric separating the layers made of the other, are known to function as efficient electrodes for electrochemical supercapacitors. In this work, we extend this concept, which was previously applied only to soft supercapacitors made from two or more conducting polymers with markedly different electrochemical activities, to examine for the first time the utilization of α/β-dipeptides in conducting polymer electrodes for symmetric supercapacitors. Specifically, we investigate the α/β-dipeptides formed by L-alanine and syn-3-amino-2-(2-fluorophenyl)-3-phenylpropanoic acid in two forms: (1) an α/β-dipeptide with –NH2 as the free terminal amino group; and (2) an α/β-dipeptide with a protonated amino group stabilized through an ionic pair with trifluoroacetate (TFA·DP1). After examining the self-assembly of DP1 and TFA·DP1 on electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) films, we fabricated three-layered electrodes consisting of two PEDOT layers separated by an intermediate layer of self-assembled DP1 or TFA·DP1, and subsequently characterized them. The best electrochemical response was achieved with the TFA·DP1-containing three-layered electrodes. These electrodes were then used to assemble symmetric supercapacitors, which were characterized by cyclic voltammetry, galvanostatic charge–discharge analysis and electrochemical impedance spectroscopy. The specific capacitance of devices incorporating TFA·DP1 between submicrometric PEDOT layers, which performed better than those in which the self-assembled peptide was omitted, was around 47 mF cm−2. In these optimized devices, the intermediate TFA-DP1 assemblies act as ultrathin, randomly distributed dielectrics connected in series with one another and in parallel with the PEDOT layers.