One–pot fabrication and capacitive performances of coral-like polyaniline formed in saturated salt solution

An efficient and simple template-free approach was present to construct hierarchical coral-like polyaniline (PANI) in a saturated salt solution for scalable fabricating active material with excellent performance for supercapacitor application. The structure, morphology, and capacitance properties of coral-like PANI were investigated. The coral-like PANI with a highly crystalline structure was found to be composed of nanofibers with a diameter and length of 50–80 nm and 200–500 nm, respectively. Furthermore, the prepared PANI electrode exhibited excellent capacitive performances (465.1 F g−1 at 2 A g−1) and remained 81.0 % even at 100 A g−1, and 62.2 % retained after 2000 cycles. It showed that the coral-like PANI has great potential for development in the supercapacitor field as electrode material.


Introduction
Polyaniline (PANI), as an exceptionally conjugated polymer material with highly theoretical specific capacitance, has drawn intense attention in the flexible energy storage system with the growing demand for flexible electronic [1,2].It is demonstrated that the morphology and structure of PANI electrode materials have a significant effect on their charge storage capacity [2][3][4].Furthermore, the innovative three-dimensional micro/nanostructured PANI, especially with its unique structure and high orientation, could promote its charge storage ability [3][4][5].However, many strategies for fabricating coral-like PANI composed of well-defined nanofibers-involved using complex templates, surfactants, toxic solvents, or extreme conditions like frozen or even complex preparation processes [4][5][6].In this paper, an efficient and simple route for scalable preparation PANI with hierarchical coral-like was developed.The influences on the morphological structure, chain structure, crystallinity, and electrochemical properties were investigated.

Experimental
One-pot polymerization of aniline (Ani) in saturation aqueous solution of NaCl was applied to prepare three-dimensional micro/nanostructured PANI.Firstly, 0.093 g Ani and 0.228 g ammonium persulfate (APS) were dissolved in a 10 ml saturation salt solution with 1 M HCl.The above solutions were poured into a 50 ml beaker under an ice bath under magnetic stirring at a speed of 1000 rad min -1 .After 1 h of reaction, 10 mL HCl solutions of 186 mg of Ani and 456 mg of APS were slowly added into the reaction system, and then the stirring speed decreased to 100 rad min -1 .After 6 h of reaction, excessive acid, oligomers, and inorganic salts were removed by washing and filtration for three times and dried at 60 ℃ for 24 h.
The morphologies and micro/nanostructures were characterized on an S-04800a scanning electron microscope, Nicolet 8700 Fourier-Transform Infrared Spectrometry and Bruker D-8 Discover X-ray diffractometer with CuKα radiation, respectively.Various electrochemical property tests were carried out in a three-electrode system (1 M H 2 SO 4 ).The preparation of the working electrode by dripping 5.0 μL product dispersing solution (2 mg mL -1 ) onto the glassy carbon electrode.

Morphology and microstructure
SEM images of coral-like PANI are shown in Figure 1a and Figure 1b.More clearly, the hierarchical PANI was composed of well-defined nanofibers with a diameter and length of 50-80 nm and 200-500 nm, respectively.This unique structure is helpful to increase the area between the active material and electrolyte.Furthermore, five infrared characteristic peaks appeared at 1567, 1488, 1302, 1229, and 1114 cm -1 , as illustrated in Figure 1c, demonstrating that the PANI was in conductive emeraldine salt form [3]. The infrared peak at 1488 and 1567 cm -1 could be ascribed to the C=C stretching deformation of the benzene and the quinone ring, respectively.The oxidation degree (x) of the PANI could be calculated by the ratio of the peak intensity of the quinone ring (IQ) to the benzene ring (IB) based on the equation: x x/ IQ/IB = 1 [2].The calculated value was 0.448, which is a relatively larger value of x, implying a relatively large conjugation length and high conductivity of the PANI.Moreover, There are three broad peaks at 2 θ ~ 9.3, 15.2, and 20.5°and a sharp peak at 2 θ ~ 25.2° as observed in figure 1d, attributing to the (001), ( 010), (100), and (110) planes of PANI, respectively.It is believed that the improvement of π-π chain stacking, increased orientation, and decreased distance between polymer chains occurred when the saturation salt solution was presented in a reactive medium.

Capacitance Performance
The CV curves at different scan rates of coral-like PANI are shown in Figure 2a.There are three pairs of peaks on 0.10/0.22V,0.52/0.55V,and 0.75/0.79Vcorresponding to the redox transition between the leucoemeraldine (LEB) form and emeraldine (EB) form, the transition between benzenoid to quinoid and the redox transition between the EB and pernigraniline (PB), respectively [3].With the increase in scanning rate, the CV curves showed a transfer of the oxidation and reduction peaks to the positive and negative potentials, respectively.It can be attributed to the effect of internal resistance.With the increase to 100 mV s -1 , C-PANI retained 72.2 % of its initial capacitance (from 857.7 to 619.6 F g -1 ).
Furthermore, the total capacitive of the PANI could be separated into the capacitive and diffusion controlled component based on the equation: i v = k v k v , where i(v) and v represent the current and scan rate, respectively, and k 1 and k 2 is the constant [2].The shaded area in Figure 2c corresponds to the contribution of capacitive of coral-like PANI at 10 mV s -1 , which accounts for 68.7 %.It is further found that the proportion of pseudocapacitive charge storage increased with the increasing rate, as plotted in Figure 2d.More clearly, the proportion of capacitive contribution varied from 63.9 % to 89.4 % with the increase from 5 to 100 mV s −1 , showing that even at a high scanning rate, surface redox reaction also had a significant contribution to the electrochemical behavior.It indicates that the coral-like PANI has a reasonable surface control capacitance behavior during the rapid charging-discharging process and can respond quickly to redox when the current changes [7].The GCD test was further performed at various densities, as shown in Figure 3a.Obviously, coral-like PANI exhibited near triangular-shaped curves, which can be attributed to the reversible redox reactions.The specific capacitance of coral-like PANI reduced from 465.1 to 376.7 F g -1 (from 2 to 100 A g -1 ).The high electrochemical performance of coral-like PANI could be benefited from the distinct surface area, efficient utilization of active materials, and fast reversible pseudocapacitive storage mechanism [4].
Subsequently, the change of specific capacitance for coral-like PANI during 2000 cycles, as displayed in Figure 3c, drops rapidly during the first 500 cycles and then almost keeps constant.A relatively large capacitance retention of 62.2 % could be beneficial to high chain orientation and structure stability [4].
Finally, to better understand the relationship between resistance behavior and electrochemical performance, the Nyquist plot of C-PANI is shown in Figure 3d.The coral-like PANI exhibited good charge storage behavior on account of an almost perpendicular line in the low frequency.And the equivalent series resistance of the PANI is about 4.4 Ω, and the relaxation time (ԏ 0 , ԏ = , f 0 is the frequency when the virtual capacitance is at its maximum) constant can be calculated according to the peak of the imaginary part of capacitance.The ԏ 0 of coral-like PANI is about 12.3 ms, ԏ 0 is closely related to the number of carriers, and the lower ԏ 0 indicates that it has more carriers, further demonstrating the unique coral-like PANI is advantageous to enhance the ion-transport rate [3].IOP Publishing doi:10.1088/1742-6596/2592/1/0120105 2 A g -1 ), and the attainability of capacitance is 62.2 % after 2000 charge-discharge cycles.This method is hopeful to provide an effective route to accelerate the development of PANI in supercapacitors and improve charge storage capacity.

Figure 1 .
Figure 1.(a, b) SEM image and enlarged image, (c) FT-IR spectrum, and (d) XRD pattern for the coral-like PANI.

Figure 2 .
Figure 2. (a) CV curves, (b) the change of capacitance, (c) fitted curve, and (d) the change of capacitance contribution for the coral-like PANI.

Figure 3 .
Figure 3. (a) GCD curves, (b) the change of specific capacitance with current density, (c) cyclic performance at 20 A g -1 , and (d) the plot of Nyquist and real and imaginary capacitance for the coral-like PANI.
A simple and template-free approach was presented to scalable prepare micro/nanostructured coral-like PANI(C-PANI) composed of well-defined nanofibers by one-pot chemical polymerization of Ani in saturation salt solution.This hierarchical micro/nanostructured C-PANI with relatively high orientation and improved mesoporous structure, which could promote charge transfer and ion diffusion between electrode material and electrolyte.The specific capacitance of C-PANI is up to 465.1 F g -1 (at NESP-2023 Journal of Physics: Conference Series 2592 (2023) 012010