Abstract
Polyaniline (PANI) is a common electrically conducting polymer with numerous potential applications, including use in energy storage and electrochromic devices. In this communication, we describe the critical parameters for the deposition of PANI onto ITO-coated plastic, both under flat and curved conditions, and we explore the effects that the electrode curvature has on the electrochemical and electrochromic behavior of PANI. We found a simple water rinsing process after deposition was critical for good film adhesion and electrochemical activity. Additionally, PANI deposition and electrochemical performance was unaffected at a bending radius of 10 and 7.5 mm, but at 5 mm the electrochemical deposition was significantly impacted due to the strain on the underlying ITO.
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Polyaniline (PANI) is one of the most commonly studied and utilized conducting polymers because of its easy and inexpensive synthesis, environmental stability, reversible doping/de-doping, and various possible oxidation states. PANI can be synthesized through the chemical oxidative polymerization (COP)1,2 or electrochemical oxidative polymerization (ECOP) of aniline.3,4 While COP is used to synthesize bulk quantities of PANI in industrial settings, ECOP allows for localized deposition of this conductive polymer onto an active substrate. Electrochemically deposited PANI films have applications in a wide array of fields, including sensors,5 photocatalysis,6 solid-state supercapacitors,7 and electrochromic devices.8–11
Two general methods are utilized for the ECOP of aniline. The potentiodynamic method utilizes the cyclic voltammetry (CV) technique, where the electrode potential is cycled between oxidizing and reducing potentials and the polymer alternates between its conducting and insulating forms, which ultimately results in a more disordered polymer chain with a compact structure.12 At the end of a CV experiment, the polymer is in its insulating form, requiring further doping to produce the conductive form of the polymer needed for most applications. The potentiostatic method involves the use of chronoamperometry (CA), where the electrode potential is held at a desired value for a predetermined time. After a CA experiment, the polymer is in its conducting form, requiring no further doping. Due to its simplicity and lack of doping requirements, we have focused on the CA method of ECOP in this work.
The electrode material used during the ECOP process can also have a significant impact on the resulting performance of the PANI film.13 For instance, it was found that PANI deposited on platinum and gold electrodes behaved differently during hydrogen detection depending on which electrode was used as the substrate during PANI deposition.14 Furthermore, the beginning of the electropolymerization mechanism (formation of oligomers and oligomer deposition on the electrode) strongly depends on the reactivity of the electrode used;15 therefore, large differences in polymer properties can be found depending on whether a traditional metal electrode is used, such as a platinum, or whether a transparent semiconducting electrode is used, such as indium-tin oxide (ITO).
While PANI has been extensively studied on ITO-coated glass, ITO-coated plastics (such as polyethylene terephthalate, PET) are also commercially available. The advantages of ITO- PET include not only a dramatic decrease in cost, but also use in flexible electronic devices. Likely due to the lower conductivity and transmittance of ITO-coated plastics, there are fewer reports of the ECOP method of synthesizing PANI on these flexible substrates16–20 and none that we are aware of that investigate how this flexibility impacts either the deposition or the performance of PANI-modified ITO-PET. Of particular interest to our group was the impact of the electrode curvature on the ECOP of PANI and the subsequent electrochemical performance.
Experimental
Materials
All chemicals were used as received unless specifically stated. Aniline (CAS# 62-53-3) was purchased from TCI America. H2SO4 (CAS# 7664-93-9) and 60 Ω/sq ITO-PET (product # 639303-5EA) were purchased from Sigma-Aldrich. The Ag/AgCl reference electrode was purchased from CH Instruments (product # CHI 111) and the platinum mesh counter electrode was purchased from Alfa Aesar (CAS# 7440-04-4). High purity water (18 MΩ) was used to prepare all aqueous solutions.
Electropolymerization of aniline
A Biologic SAS SP300 Potentiostat was used for all electrochemical experiments. 3D printed electrochemical cells were used for the electropolymerization and electrochemical characterization of samples. Additional information regarding the 3D printing parameters and access to the designs can be found in the supplementary material. The electropolymerization solution consisted of 100 mM aniline in 1 M H2SO4. CA experiments were performed in order to deposit PANI on ITO electrodes by applying +2.0 V vs Ag/AgCl for 45 s. Following deposition, films were gently rinsed with high purity water and allowed to dry overnight.
Characterization of PANI films
Electrochemical performance of the films was evaluated using cyclic voltammetry (CV) in an electrolyte consisting of 1 M H2SO4 in a voltage range of –0.5 to 1.2 V vs Ag/AgCl at a scan rate of 10 mV/s. Electrochemical experiments performed under curved conditions were accomplished using 3D printed electrochemical cells and are described in detail in the supplementary material. UV-vis spectra were performed using a Shimadzu UV-1800 spectrometer with a 3D printed adapter previously described.21
Results and Discussion
The ECOP of PANI on ITO substrates commonly includes a wide variety of pre- and post-deposition treatments of the electrode and electrolyte solution. After evaluation of several different treatment processes, gentle rinsing of the electrode with water immediately following the deposition was the only parameter we found to be critical for the formation of a well-adhered and electrochemically active PANI film onto ITO-PET electrodes (Figure 1). Rinsing has previously been attributed to the removal of unreacted monomers and oligomers that can hinder the performance of PANI.6 In experiments performed without post-deposition rinsing, the PANI film would crack or fall off the electrode entirely upon placement in a new electrolyte solution for electrochemical evaluation. Other parameters, such as electrode pre-treatment and aniline distillation, resulted in slightly improved performance but were not found to be necessary (see supplementary material). Following deposition, the PANI modified electrode showed the characteristic redox peaks and electrochromic behavior in a 1 M H2SO4 electrolyte (Figures 1B and 1C). The electrochromic behavior seen in these films is not as dramatic as those observed in other studies,8–11 though we note that the films described here are composed solely of PANI rather than a composite.
Figure 1. Electrochemical deposition and evaluation of PANI on ITO-PET substrates. A) Chronoamperometric deposition of PANI with photo of a typical film after rinsing (inset). B) CV evaluation of PANI on ITO-PET and a bare ITO-PET electrode in 1 M H2SO4. C) Electrochromic evaluation of PANI on ITO-PET. Bias after reduction (inset left) and oxidation (inset right).
Analysis of the electrochemical performance of the films was then evaluated under curved conditions using 3D printed electrochemical cells that held the electrodes at different bending radii (10, 7.5, or 5 mm). As seen in Figure 2A, the curvature of the electrode has no significant effect on the redox activity of the electrodes until a bending radius of 5 mm. Under this more extreme strain, the normalized reduction peaks are shifted to more negative values, suggesting more reducing strength is required under these conditions. Importantly, the electrochromic behavior is still observed under curved conditions (Figure 2B), though slight differences were observed at a 5 mm bending radius, as expected based on the CV results. Furthermore, a PANI modified electrode that underwent 100 bending cycles at 7.5 mm still demonstrated typical CV and electrochromic behavior (see supplementary material). The ability to use these PANI modified electrodes in curved conditions may enable the use of these films in new settings that require this kind of flexibility. Furthermore, this flexible electrode system will provide opportunities to explore the strain induced changes in the electrochemical and electrochromic performance of PANI in future investigations.
Figure 2. PANI on ITO-PET deposited under flat conditions and analyzed under curved conditions. A) CV evaluation in 1 M H2SO4. B) Photographs of electrochromic behavior under 5 mm bending radius at different voltages during CV experiment.
In addition to evaluating the PANI films under curved conditions, we also explored the ECOP of PANI under curved conditions. As seen in Figure 3, the deposition proceeded normally until a bending radius of 5 mm, at which point the deposition of PANI localized in the region of greatest strain. Importantly, the electrochemical and electrochromic behavior of the films was similar to that of PANI deposited under flat conditions, though in the case of the film deposited at a bending radius of 5 mm the electrochromic behavior was only observed in the region where the film deposited. This provides a new opportunity to selectively deposit an electrochemically active material based on the physical strain made possible by using a flexible electrode like ITO-PET.
Figure 3. Photos of PANI film deposited under different bending radii during the deposition (top) and after rinsing (bottom).
Summary
In this study, we have evaluated the deposition of PANI using inexpensive and flexible ITO-PET electrodes. Of the deposition parameters studied, rinsing the PANI film immediately after electrochemical deposition was found to be the only critical parameter. Experiments were then performed to investigate how the flexibility of the substrate would impact the performance of the PANI modified ITO-PET. In these experiments, the electrochemical and electrochromic performance was unaffected until a bending radius of 5 mm, at which point the performance or deposition significantly deviated due to the high levels of strain on the underlying ITO. The ability to perform electrodeposition on these flexible and transparent electrodes will provide opportunities for new types of sensors, photovoltaics, and windows. Future studies that take advantage, or eliminate the effects, of the different performances at a small bending radius will further the understanding of these flexible and electrochemically active composite materials.
Acknowledgments
We gratefully acknowledge the financial support from The University of Tulsa. Author D.M.W. was supported by a Bellwether Fellowship as well as a Student Research grant Program through the Office of Research and Sponsored Programs at The University of Tulsa. Author M.P. was supported through both the Chemistry Summer Undergraduate Program and the Tulsa Undergraduate Research Challenge offered through The University of Tulsa.
ORCID
Gabriel LeBlanc 0000-0002-3331-9734