Additive manufacturing of polyaniline-based electrodes for energy storage and memristive devices

Abstract

The purpose of the present PhD research project was to develop and characterise an additive manufacturing (AM) process capable of 3D printing polyaniline (PANI) in its emeraldine salt form (PANI-ES), as well as polyaniline-based composites, and to showcase the potential of the printed structures in a proof-of-concept study. The process had to be capable of printing electrodes possessing the following characteristics:

 High PANI content  High electrical conductivity  Good accuracy/ shape conformity of the electrodes with high reproducibility

Overall, this project was structured in four main parts: First, a 3D printing process based on the thermal doping of PANI in its emeraldine base form (PANI-EB) with dodecyl benzene sulfonic acid (DBSA) was developed. For this purpose, a commercially available fused deposition modelling (FDM) 3D printer was modified to allow for the extrusion of a viscous PANI-EB/DBSA ink. Polyaniline was synthesised using ultrasound assisted chemical oxidation and de-doped in order to allow for the thermal doping and solidification of PANI-EB with DBSA during the printing process, obtaining 3D printed PANI-ES/DBSA structures. Printing parameters such as starting height, hotbed temperature and post-treatment time were investigated; their influence on the printability, electric conductivity and other material properties was characterised using techniques such as 4-point probe measurements, Fouriertransform infrared spectroscopy – attenuated total reflection (FTIR-ATR), X-ray powder diffraction (XRPD), thermo-gravimetric analysis (TGA) and scanning electron microscopy (SEM). The accuracy and conductivity of the printed structures showed a significant dependence on printing parameters with samples post-treated for short times and printed at low hotbed temperatures exhibiting excellent conductivities up to 20,000 mS cm⁻¹ but low accuracy and shape conformity. On the other hand, samples that were printed at high hotbed temperatures showed decreased conductivity but improved accuracy.

In the next phase, the feasibility of the developed AM process to print PANI-ES/DBSA electrodes for electrochemical processes was evaluated using a proof-of-concept. For this, 3D printed interdigitated symmetrical electrochemical capacitors were fabricated and the influence of the printing parameters on the performance was evaluated by using cyclic voltammetry (CV), charge-discharge experiments and electrochemical impedance spectroscopy (EIS). This study showcased the straightforward integration of the printed PANI-ES/DBSA electrodes in electrochemical storage devices. Although the fabricated devices did not show a superior specific capacitance, they did exhibit a memristive behaviour allowing for the potential integration of the printed electrodes in further applications such as artificial neural networks.

The influence of the synthesised PANI-EB on the printing process and the printed material was then investigated by improving the effect of ultrasound during the synthesis. This was done by upgrading the set-up from a small U300 Ultrawave ultrasonic bath with 35 W of ultrasonic power being cooled by ice cubes to a larger Ultrawave QS12 bath operating at 200 W and using a chiller with an immersed coil to control the temperature. The influence of the improved synthesis was evaluated by analysing the as synthesised PANI-ES using 4-point-probe measurements and SEM. In addition the influence on the printability, material properties and electrochemical performance in the proof-of-concept electrochemical capacitors was characterised by using 4-point-probe measurements, SEM, TGA and CV. A significant change in the behaviour of the improved PANI-EB/DBSA inks was observed, allowing not only for a larger printing window of hotbed temperatures but also improving the conductivity of the printed structures. In addition, an improvement in electrochemical performance could be observed, indicating that the synthesis method of the used PANI-EB has a significant effect on the performance and printability of PANI-ES/DBSA structures.

In the last part of the research, the feasibility of 3D printing PANI-ES/DBSA composites was evaluated by incorporating metal oxides such as barium titanate (BaTiO3), manganese oxide (MnO2) and vanadium oxide (V2O5) into the PANI-EB/DBSA ink to improve the electrochemical properties of the printed electrodes. The printability of the composite inks was evaluated and the resulting electrodes characterised by using 4- point-probe measurements, TGA, SEM as well as CV of the proof-of-concept electrochemical capacitors. While these metal oxides were successfully incorporated in various amounts into the PANI-EB/DBSA ink, they had only limited beneficial (and sometimes detrimental) effects on the material properties of the 3D printed electrodes.