Recent Developments of the Solution-Processable and Highly Conductive Polyaniline Composites for Optical and Electrochemical Applications

Solution-processable conducting polymers (CPs) are an effective means for producing thin-film electrodes with tunable thickness, and excellent electrical, electrochemical, and optical properties. Especially, solution-processable polyaniline (PANI) composites have drawn a great deal of interest due to of their ease of film-forming, high conductivity up to 103 S/cm, excellent redox behaviors, processability, and scalability. In this review, basic principles, fabrication methods, and applications of solution-processable PANI composites will be discussed. In addition, recent researches on the PANI-based electrodes for solar cells (SCs), electrochromic (EC) windows, thermoelectric (TE) materials, supercapacitors, sensors, antennas, electromagnetic interference (EMI) shielding, organic field-effect transistors (OFETs), and anti-corrosion coatings will be discussed. The presented examples in this review will offer new insights in the design and fabrication of high-performance electrodes from the PANI composite solutions for the development of thin-film electrodes for state-of-art applications.


Conductivity Enhancement of PANI:CSA
In the past, studies on improving the conductivity of PANI:CSA have been carried out by controlling the solvent [5,10], temperature [7,11], film-forming time [11], and film-thickness [12]. It was evident that the optimum electrical conductivity was achieved when using a solvent consisting of a higher amount of m-cresol and an appropriate amount of CHCl 3 [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. According to the method of Kaner et al., more uniform PANI nanofibers (NFs) were readily formed at a water/CHCl 3 interface when compared to the conventional synthesis of aniline while using a single aqueous phase [2]. In addition, performing the interfacial polymerization at less than −30 • C promotes para-coupling of aniline monomers, which resulted in PANI nanostructures with fewer structural defects [7,8]. The secondarily doped of the PANI prepared by the interfacial polymerization has shown the maximum conductivity close to 10 3 S/cm, which is approximately six times higher than the conventional PANI:CSA synthesized Polymers 2019, 11,1965 4 of 17 by conventional single-phase polymerization, due to the enhanced crystallinity [9]. Furthermore, Lee et al. reported that the PANI chains can be better aligned through the thickness-controlled drop-casting (TCDC) method [12]. The structural defects in the PANI:CSA were even reduced by using CNDs as nucleating agents during the polymerization of aniline ( Figure 2) [17]. Furthermore, PANI:CSA that was grown on the CNDs provides greater surface areas when compared to the conventional PANI:CSA, which results in improved device performances [17]. Tremendous efforts have been made to utilize the PANI:CSA as electrode materials in various devices because of the excellent electrical conductivity of PANI:CSA.
Polymers 2019, 11,1965 4 of 17 crystallinity [9]. Furthermore, Lee et al. reported that the PANI chains can be better aligned through the thickness-controlled drop-casting (TCDC) method [12]. The structural defects in the PANI:CSA were even reduced by using CNDs as nucleating agents during the polymerization of aniline ( Figure  2) [17]. Furthermore, PANI:CSA that was grown on the CNDs provides greater surface areas when compared to the conventional PANI:CSA, which results in improved device performances [17]. Tremendous efforts have been made to utilize the PANI:CSA as electrode materials in various devices because of the excellent electrical conductivity of PANI:CSA.   [13][14][15]17], organic solar cells (OSCs) [12,18], and perovskite solar cells (PSCs) [19]. In particular, the PANI:CSA has been studied as counter electrodes (CEs) in the DSSCs [13][14][15]17]. The PANI:CSA is considered to be one of the promising substitutes for platinum-coated transparent conductive oxides (Pt-coated TCOs). This is because the Pt-coated TCOs suffer from mechanical brittleness of TCOs, and it is difficult to cover the large areas of TCOs while using the Pt-sputtering process [14,15]. The PANI:CSA CE has demonstrated a transmittance of 72.9% at 600 nm, and the PCE of DSSCs while using the PANI:CSA CEs was changed by different protonation levels of the PANI structure [13]. Furthermore, the PANI:CSA can be used as a CE in a bifacial DSSC [14]. The PANI:CSA CEs with controllable sizes and shapes of pores could be realized by using different porogens, which results in the relative efficiency of 101.0% (PCE of 6.23%) when compared to the conventional Pt-coated TCO cell (PCE of 6.17%) [15]. Although the thermal decomposition of porogens within the PANI:CSA were helpful in increasing the surface areas of CEs, it was inevitable to avoid undesirable conductivity losses due to the increased surface roughness of the PANI:CSA film [15,16]. After applying the PANI:CSA that was grown on the CNDs as a CE, larger surface areas (43.6 m 2 g −1 ), higher electrical conductivity (774 S/cm), and superior PCE (7.45%) of the DSSC could be simultaneously achieved [17].
Lee et al. demonstrated the OSC while using the PANI:CSA layer and the PEDOT:PSS layer as an anode and a buffer layer, respectively [12]. In the comparative studies on the OSCs using PANI:CSA and PEDOT:PSS as HTLs, the OSC cell that was based on the PANI:CSA exhibited a superior stability as compared to that of the PEDOT:PSS cell (Figure 3a) [18]. PANI:CSA was  PANI:CSA has been used as electrode materials for dye-sensitized solar cells (DSSCs) [13][14][15]17], organic solar cells (OSCs) [12,18], and perovskite solar cells (PSCs) [19]. In particular, the PANI:CSA has been studied as counter electrodes (CEs) in the DSSCs [13][14][15]17]. The PANI:CSA is considered to be one of the promising substitutes for platinum-coated transparent conductive oxides (Pt-coated TCOs). This is because the Pt-coated TCOs suffer from mechanical brittleness of TCOs, and it is difficult to cover the large areas of TCOs while using the Pt-sputtering process [14,15]. The PANI:CSA CE has demonstrated a transmittance of 72.9% at 600 nm, and the PCE of DSSCs while using the PANI:CSA CEs was changed by different protonation levels of the PANI structure [13]. Furthermore, the PANI:CSA can be used as a CE in a bifacial DSSC [14]. The PANI:CSA CEs with controllable sizes and shapes of pores could be realized by using different porogens, which results in the relative efficiency of 101.0% (PCE of 6.23%) when compared to the conventional Pt-coated TCO cell (PCE of 6.17%) [15]. Although the thermal decomposition of porogens within the PANI:CSA were helpful in increasing the surface areas of CEs, it was inevitable to avoid undesirable conductivity losses due to the increased surface roughness of the PANI:CSA film [15,16]. After applying the PANI:CSA that was grown on the CNDs as a CE, larger surface areas (43.6 m 2 g −1 ), higher electrical conductivity (774 S/cm), and superior PCE (7.45%) of the DSSC could be simultaneously achieved [17].
Lee et al. demonstrated the OSC while using the PANI:CSA layer and the PEDOT:PSS layer as an anode and a buffer layer, respectively [12]. In the comparative studies on the OSCs using PANI:CSA and PEDOT:PSS as HTLs, the OSC cell that was based on the PANI:CSA exhibited a superior stability as Polymers 2019, 11, 1965 5 of 17 compared to that of the PEDOT:PSS cell (Figure 3a) [18]. PANI:CSA was introduced as a hole transport layer (HTL) to promote hole extraction ability and improve the efficiency and stability of PSCs, and the PANI:CSA exhibited higher power conversion efficiency (PCE) of 15.42% when compared to the PEDOT:PSS with PCE of 14.11% (Figure 3b) [19]. These results indicate that the PANI:CSA can be a promising candidate for HTLs in both PSC and OSC [18,19].
The TE performances of PANI:CSA were improved by combining the advantages of PANI:CSA with inorganics and carbons [54,55,58,59,61]. The maximum PF (µW·m −1 ·K −2 ) of the PANI:CSA combined with tellurium (Te), CNT, and GS were 146, 401, and 55, respectively (Figure 4a,b) [54,58,61]. Especially, the high TE performances of PANI:CSA/CNT and PANI:CSA/GS are attributable to following reasons [58,59,61]. 1) The phonon scattering at the contact surfaces between PANI and carbons significantly reduces the thermal conductivity of the composites. 2) The high electrical conductivity of carbons and PANI:CSA results in improved TE performances since the mobility of charge carriers in PANI:CSA is still maintained. 3) Synergistic effects between m-cresol solvent and carbons also contributes to the formation of highly ordered PANI:CSA chains in the TE device. Interestingly, multilayer structures that were composed of PANI:CSA and PEDOT:PSS were prepared while using a layer-by-layer deposition (Figure 4c,d) [20]. Hole diffusion from the PANI:CSA to the PEDOT:PSS resulted in the maximum PF of 49 µW·m −1 ·K −2 [20]. These results suggest that the PANI:CSA is one of fascinating candidates for the TE generators.
Polymers 2019, 11,1965 6 of 17 device. Interestingly, multilayer structures that were composed of PANI:CSA and PEDOT:PSS were prepared while using a layer-by-layer deposition (Figure 4c,d) [20]. Hole diffusion from the PANI:CSA to the PEDOT:PSS resulted in the maximum PF of 49 μW•m −1 •K −2 [20]. These results suggest that the PANI:CSA is one of fascinating candidates for the TE generators.

Supercapacitor Application
As PANI provides relatively higher electrochemical and pseudocapacitance when compared to the PEDOT and PPy, PANI-based electrodes have attracted a great deal of interest for use in energy storage applications [16,60,62,67]. PANI:CSA significantly improves current collections in the supercapacitors due to its two or three orders of magnitude greater conductivity. Especially, the PANI:CSA electrode can act as a free-standing electrode without using any metallic substrate, such as stainless steel, gold (Au), platinum (Pt), and so forth [16,60,62,67]. However, the low surface area of PANI:CSA films limits the overall performance of supercapacitors [16,60,62,67]. Thus, various efforts have been made to increase the surface area of PANI:CSA electrodes.
A method for incorporating the porous structures into the solution-processable CSA-doped films was conducted to increase the surface area of PANI:CSA electrodes [16]. Porous PANI:CSA electrodes were used working electrodes (WEs) in three-electrode capacitors, and pores of different sizes and shapes formed on the surface of PANI:CSA were effective in enhancing the contact between the PANI:CSA electrodes and the electrolyte ions. However, the capacitance losses that were caused by swelling and degradation of the PANI:CSA films during a number of charge/discharging processes were inevitable. According to Kim et al., the synergistic effects of PANI:CSA and reduced graphene oxide (RGO) were effective in improving the overall performance of supercapacitors [62]. Unique pseudocapacitive behaviors of the PANI:CSA enhance the total  [20]. Copyright 2016, RSC Publishing.

Supercapacitor Application
As PANI provides relatively higher electrochemical and pseudocapacitance when compared to the PEDOT and PPy, PANI-based electrodes have attracted a great deal of interest for use in energy storage applications [16,60,62,67]. PANI:CSA significantly improves current collections in the supercapacitors due to its two or three orders of magnitude greater conductivity. Especially, the PANI:CSA electrode can act as a free-standing electrode without using any metallic substrate, such as stainless steel, gold (Au), platinum (Pt), and so forth [16,60,62,67]. However, the low surface area of PANI:CSA films limits the overall performance of supercapacitors [16,60,62,67]. Thus, various efforts have been made to increase the surface area of PANI:CSA electrodes.
A method for incorporating the porous structures into the solution-processable CSA-doped films was conducted to increase the surface area of PANI:CSA electrodes [16]. Porous PANI:CSA electrodes were used working electrodes (WEs) in three-electrode capacitors, and pores of different sizes and shapes formed on the surface of PANI:CSA were effective in enhancing the contact between the PANI:CSA electrodes and the electrolyte ions. However, the capacitance losses that were caused by swelling and degradation of the PANI:CSA films during a number of charge/discharging processes were inevitable. According to Kim et al., the synergistic effects of PANI:CSA and reduced graphene oxide (RGO) were effective in improving the overall performance of supercapacitors [62]. Unique pseudocapacitive behaviors of the PANI:CSA enhance the total capacitance of energy storage devices, while the electron-rich RGO sheets enable significantly improved cycling stability, enlarged surface area, and the higher electric conductivity of electrode materials.
The cycling stability of PANI:CSA was significantly improved by applying the PANI:CSA electrode in a two-electrode configuration [62]. Nanomaterials, such as CNTs and Pt-decorated carboxyl polypyrrole nanoparticles (Pt-CPPy NPs), were combined with the PANI:CSA, and these composite electrodes were used as two-electrode cells [60,67]. It was found that the combination of PANI:CSA with CNT was effective to realize a flexible integrated electrode for a symmetric supercapacitor [60]. Conductive pathways for delocalizing electrons were readily formed in the PANI:CSA through strong π−π interactions between PANI:CSA and CNT. Relatively high gravimetric capacitance and excellent retention rate of 98% after 13000 cycles due to the synergistic effects between PANI:CSA and CNT (Figure 5a) [60]. Interestingly, Pt-CPPy NPs were used as nucleating agents to induce para-polymerization of aniline, and the secondary doping process of the PANI grown on Pt-CPPy NPs could significantly improve the electrical conductivity (Figure 5b) [67]. The composite material that was composed of PANI:CSA and Pt-CPPy was used as a two-electrode symmetric supercapacitor, and exhibited significantly improved electrical conductivity (814 S cm −1 ), specific capacitance (325.0 F g −1 ), and cyclic stability (84% of retention rate after 5000 cycles) when compared to conventional PANI:CSA [67]. Despite such improvements, the potential window of symmetric cells based on PANI:CSA is usually less than 1.2V [60,67]. This problem can be solved by choosing an asymmetric cell or non-aqueous electrolyte. Rapid and reversible doping/dedoping processes make PANI:CSA suitable as efficient sensors in detecting ammonia (NH 3 ) gases [56,65,70]. When the PANI:CSA were combined with tin (II) oxide nanoparticles (SnO 2 NPs) and GNFs, the composite electrodes demonstrated higher sensitivity, faster response, and better selectivity toward NH 3 when compared with the pure PANI:CSA [56,65]. Furthermore, the SnO 2 NPs and GNFs could greatly improve the lifetime and structural stability of the PANI:CSA. The PANI:CSA/SnO 2 composite exhibited superior sensing performances when compared with the pristine PANI and pristine SnO 2 NPs due to the synergistic effects of the PANI:CSA and SnO 2 NPs [56]. The PANI:CSA combined with carbon nanomaterials provides higher electrical conductivity and improved flexibility compared to pristine PANI:CSA due to the π−π interactions between the PANI:CSA chains and carbon nanomaterials [6,17,20,[58][59][60][61][62][63]65,66]. These advantages of PANI:CSA/carbon composites are suitable for antenna applications. In addition, these PANI:CSA/carbon composites can be readily formed into various patterns with different sizes and shapes while using screen-printing technique [63,66]. The PANI:CSA/GS composite was applied to a monopole antenna, and peak gain, directivity, and radiation efficiency of the monopole antenna based on PANI:CSA/GS were 3.60 dBi, 3.91 dBi, and 92.12%, respectively (Figure 6a) [63]. PANI:CSA NFs that were embedded with Pt-coated carbon nanoparticles (Pt-CNPs) exhibited about 1.37 times higher electrical conductivity (792 S cm −1 ) than that of pristine PANI:CSA NFs (580 S cm −1 ) (Figure 6b,c) [66]. The PANI:CSA/Pt-CNP composite was applied to a dipole tag-antenna that displayed a wide bandwidth of 0.55 GHz and a transmitted power efficiency of 99.6% [66].
Polymers 2019, 11,1965 8 of 17 [67]. The composite material that was composed of PANI:CSA and Pt-CPPy was used as a two-electrode symmetric supercapacitor, and exhibited significantly improved electrical conductivity (814 S cm −1 ), specific capacitance (325.0 F g −1 ), and cyclic stability (84% of retention rate after 5000 cycles) when compared to conventional PANI:CSA [67]. Despite such improvements, the potential window of symmetric cells based on PANI:CSA is usually less than 1.2V [60,67]. This problem can be solved by choosing an asymmetric cell or non-aqueous electrolyte.  (Figure 7b) [72]. The saturated hole mobility, threshold voltage, external quantum efficiency, photo-sensitivity, and photo-responsivity of OFET device were 9.5 × 10 −5 , cm 2 /V·s, −1.72 V, 1.16 × 10 2 A/W, and 7.33 × 10 4 A/W, respectively [72]. Furthermore, the PANI:CSA/SiO 2 core-shell can be used for the corrosion protection coating for carbon steel substrates [45]. The PANI:CSA/SiO 2 core-shell has shown nearly five orders magnitude higher corrosion resistance (2.24 × 10 7 Ω cm 2 ) when compared with the pristine silicon coating composite was applied to a monopole antenna, and peak gain, directivity, and radiation efficiency of the monopole antenna based on PANI:CSA/GS were 3.60 dBi, 3.91 dBi, and 92.12%, respectively (Figure 6a) [63]. PANI:CSA NFs that were embedded with Pt-coated carbon nanoparticles (Pt-CNPs) exhibited about 1.37 times higher electrical conductivity (792 S cm −1 ) than that of pristine PANI:CSA NFs (580 S cm −1 ) (Figure 6b,c) [66]. The PANI:CSA/Pt-CNP composite was applied to a dipole tag-antenna that displayed a wide bandwidth of 0.55 GHz and a transmitted power efficiency of 99.6% [66].  . Furthermore, the PANI:CSA/SiO2 core-shell can be used for the corrosion protection coating for carbon steel substrates [45]. The PANI:CSA/SiO2 core-shell has shown nearly five orders magnitude higher corrosion resistance (2.24 × 10 7 Ω cm 2 ) when compared with the pristine silicon coating (5.37 × 10 2 Ω cm 2 ) [73]. This result is indicative of the improved physical barrier behavior of the PANI:CSA.

Water-Soluble PANI:PSS for Optical and Electrochemical Applications
The PSS, a vinyl polymer having sulfonate (−SO3 − ) groups promotes para-directed polymerization of aniline, which resulted in the formation of PANI with lower structural defects
Polymers 2019, 11, 1965 11 of 17 [35]. A radio frequency identification (RFID)-based wireless sensor while using the PANI:PSS composite combined with multidimensional Fe2O3 hollow nanoparticles (M_FeHNPs) was demonstrated to detect NO2 at the lowest concentration of 0.5 percent to parts per million (ppm) (Figure 9b) [36]. The application range of PANI:PSS in the field of state-of-art devices will be expanded because of its excellent solution-processability and unique redox behaviors.

Other Water-Soluble PANI Solutions for Optical and Electrochemical Applications
Other hydrophilic polymers, such as carboxymethylcellulose (CMC) [41,42,44,45], styrene-butadiene rubber (SBR) [43][44][45], polyacrylic acid (PAA) [46][47][48], polyethylene glycol (PEG) [47], polyethylene oxide (PEO) [49][50][51], poly(vinyl pyrrolidone) (PVP) [52], and polyvinlyl alcohol (PVA) [53], and these water-soluble polymers significantly improve the dispersion of PANI in the aqueous phase. In addition, when these water-soluble polymers are used as binders for electrodes or electrolyte membranes, the suppression of undesirable volumetric expansion and durability of the device can be improved [43][44][45][46][47][48][49][50][51][52][53]. According to Bilal et al., the gravimetric capacitance of a three-electrode capacitor while using the GO-PANI/CMC composite was 1721 F g −1 , which suggested that the dispersion of active materials highly affect the resulting electrochemical performances [41]. Moreover, a mixture of CMC/SBR composite as a water-soluble binder offers the effective suppression of significant volume variations and the interface maintenance of electrode materials, which led to significant improvements in the electrochemical performances of both the Li-ion battery and supercapacitor [44,45]. The solid-state carbon cloth supercacitor based on PANI/CNTs/PAA composites demonstrated an energy density of 5.8 Wh/kg at a power density of 1.1 kW/kg and a rate capability of 81% in the current range from 1 to 10 A/g (Figure 10a−e) [46]. A quasi solid state DSSC (QS-DSSC) was assembled with the PANI/PAA-g-PEG graft composite was used as a gel electrolyte, and the QS-DSSC that was based on the gel electrolyte exhibited a PCE of 6.38% under a solar illumination of 100 mW cm −2 (AM 1.5) [47]. In addition, the PANI/PAA film that was immobalized by glucose oxidase (GOx) was effective in detecting glucose molecules, and

Other Water-Soluble PANI Solutions for Optical and Electrochemical Applications
Other hydrophilic polymers, such as carboxymethylcellulose (CMC) [41,42,44,45], styrene-butadiene rubber (SBR) [43][44][45], polyacrylic acid (PAA) [46][47][48], polyethylene glycol (PEG) [47], polyethylene oxide (PEO) [49][50][51], poly(vinyl pyrrolidone) (PVP) [52], and polyvinlyl alcohol (PVA) [53], and these water-soluble polymers significantly improve the dispersion of PANI in the aqueous phase. In addition, when these water-soluble polymers are used as binders for electrodes or electrolyte membranes, the suppression of undesirable volumetric expansion and durability of the device can be improved [43][44][45][46][47][48][49][50][51][52][53]. According to Bilal et al., the gravimetric capacitance of a three-electrode capacitor while using the GO-PANI/CMC composite was 1721 F g −1 , which suggested that the dispersion of active materials highly affect the resulting electrochemical performances [41]. Moreover, a mixture of CMC/SBR composite as a water-soluble binder offers the effective suppression of significant volume variations and the interface maintenance of electrode materials, which led to significant improvements in the electrochemical performances of both the Li-ion battery and supercapacitor [44,45]. The solid-state carbon cloth supercacitor based on PANI/CNTs/PAA composites demonstrated an energy density of 5.8 Wh/kg at a power density of 1.1 kW/kg and a rate capability of 81% in the current range from 1 to 10 A/g ( Figure 10a−e) [46]. A quasi solid state DSSC (QS-DSSC) was assembled with the PANI/PAA-g-PEG graft composite was used as a gel electrolyte, and the QS-DSSC that was based on the gel electrolyte exhibited a PCE of 6.38% under a solar illumination of 100 mW cm −2 (AM 1.5) [47]. In addition, the PANI/PAA film that was immobalized by glucose oxidase (GOx) was effective in detecting glucose molecules, and the sensitivity toward glucose increased with increasing PAA content [48]. The results suggest that the PANI/PAA composites are suitable for fabricating various electrochemical and optical devices [46][47][48]. Furthermore, PANI composites that were combined with PEO and PVP were also utilized as gel electrolytes for offering catalytic and hole-transporting properties on the QS-DSSCs (Figure 10f) [49][50][51][52]. While considering these results, it was evident that the PANI composites combined with water-soluble polymers are appropriate for constructing high-performance and solid-state energy storage and energy conversion devices [43][44][45][46][47][48][49][50][51][52]. According to Li et al., a PANI-PVA hydrogel with a tensile strength of 5.3 MPa can be readily produced by crosslinking reactions between PANI and PVA chains through boronate bonds [53]. The flexible solid-state supercapacitor based on the PANI-PVA hydrogel provided large gravimetric capacitance (928 F g −1 ) and excellent capacitance retention (90% after 1000 charge/discharge cycles) [53].

Conclusions
In this review, recent researches on solution-processable PANI composites and their applications were discussed. In addition to intrinsic advantages of the PANI, such as facile synthesis, unique redox behavior, reversible doping/dedoping, and low cost, PANI:CSA that was prepared by secondary doping enables facile formation of free-standing thin films with significantly improved electrical and electrochemical performances. The total performances of PANI:CSA could be reinforced by combining it with inorganics, carbons, CPs, and so forth, as the PANI:CSA solution enables hydrogen bonding, dipole-dipole, and ion-dipole forces with various compounds. For this reason, the PANI:CSA and its composites have been widely used in a variety of applications, such as SCs, TE materials, supercapacitors, chemical sensors, antennas, EMI shielding, OFETs, and anti-corrosion coatings. PANI:CSA should overcome several problems, such as difficulty in controlling gelation time and odor characteristic of m-cresol solvent, to replace expensive PEDOT:PSS in a wider range of applications. The advantages of the water-soluble PANI composites, such as, low cost, low toxicity, and eco-friendliness, will quickly increase the demand for water-soluble PANI in the fields of high-tech devices. Especially, these water-based PANI composites are highly advantageous for realizing solid-state devices. Developing the fabricating procedures for solution-processable PANI composites will remarkably improve the performances of various state-of-art devices, such as wireless sensors, wireless energy storage/conversion system, smart windows, and so forth.