Polypyrrole Solid-State Supercapacitors Drawn on Paper

Abstract

Solid-state supercapacitors with areal capacitance in the order of 100 mF⋅cm^-2 are developed on paper substrates, using eco-friendly, low-cost materials and a simple technology. The electrochemically active material used as the electrode is prepared from a stable water-based ink, obtained by doping commercial polypyrrole (PPY) powder with dodecylbenzene sulfonic acid (DBSA), and characterized by optical and electrical measurements, Raman investigation and Atomic Force Microscopy. The PPY:DBSA ink can be directly applied on paper by means of rechargeable water pens, obtaining, after drying, electrically conducting solid state tracks. The PPY:DBSA layers are then interfaced to one another through a polymer gel based on potassium hydroxide and chitosan, acting both as the ion-conducting medium and as the separator. The areal capacitance of the devices developed by following such a simple rule can be improved when the PPY:DBSA ink is applied in combination with other nanostructured carbon material.

Keywords: paper substrates; polypyrrole; supercapacitor.

Figure 1

Capacitor structures used in this study: (a) interdigitated electrodes are first drawn on paper and the KOH–chitosan gel is drop-deposited on top of the interdigitated section; (b) two strips of paper with the electrode and the KOH–chitosan gel are joined together with one strip facing the other.

Figure 2

Absorption spectrum of a PPY: DBSA film in the VIS–NIR (a); Raman spectrum of PPY (red line) powder compared to PPY:DBSA (black line) (b).

Figure 3

Schematic view of the electrochemical cells used to characterize the PPY:DBSA films (a). Photos of a typical electrochemical cell, with the PPY films in the reduced (b) and oxidized (c) state.

Figure 4

Current–voltage loops measured at different scan speed on a typical device of the kind sketched in Figure 3a (a); current intensity measured in correspondence of peak A and peak B, plotted as a function of the scan speed (b).

Figure 5

Resistance measured on a track applied on paper as a function of the number of layers of PPY:DBSA ink. The inset shows how the resistance after the application of 6 layers behaves as a function of frequency.

Figure 6

Interdigitated electrodes plotted on paper using the PPY:DBSA ink (a); magnified view of a couple of about 0.8 μm thick crossing lines, drawn using the PPY:DBSA ink below the AFM tip (b).

Figure 7

AFM image (area 10 μm × 10 μm) of a typical PPY:DBSA film.

Figure 8

Raman spectra of pristine CNTs, DBSA/CNT, and PEO/CNT hybrid nanocomposites.

Figure 9

Raman spectra of pristine CNTs, DBSA/CNT, and PEO/CNT hybrid nanocomposites in the spectral region around the G’ band.

Figure 10

Relative intensity ratios of D/G, G’/G, and G’/D peaks of pristine CNTs, DBSA/CNT, and PEO/CNT hybrid nanocomposites.

Figure 11

AFM micrograph of pressed CNT powder (a) and of CNTs dispersed in PEO (b) and in DBSA (c).

Figure 12

I–V measurements performed on samples having different kind of interdigitated electrodes with the same geometry, applied on paper substrates, with a KOH–chitosan layer on top.

Figure 13

Current–voltage loop (a), voltage charge–discharge curve (b), frequency-dependent impedance (c) and photograph (d) of a typical device on paper, with a mixture of the PPY:DBSA and DBSA/CNT inks as electrodes and a KOH–chitosan layer on the top.

Figure 14

Current–voltage measurements performed at different scan speeds on a sample consisting of a couple of rectangularly shaped pieces of paper, each having a mixture of PPY:DBSA and DBSA/CNT applied on the top, interfaced through a KOH–chitosan layer (a); areal capacitance estimated from the current–voltage loops, as a function of the scan speed (b).

Figure 15

Voltage charge–discharge curves measured on the same device to which Figure 13 refers to (a); areal capacitance estimated from the charge–discharge curves, as a function of current density (b); power density versus energy density (c).

Figure 16

Cole–Cole plot of the same device to which Figure 13 and Figure 14 refer to.

Figure 17

Snapshot of an LED being supplied by three paper devices connected in series. The series of devices was previously charged to a 4 V; the LED, with a 1 kΩ resistance in series, stays on for a few seconds after the supply is removed.

Figure 18

Steady state current–voltage loop measured on a typical device with geometry as described in Figure 1b and PPY:DBSA electrodes, before and after stress (a); charge–discharge curve measured at constant current density on the same device, before and after stress (b).