Hybrid composite polyaniline-nickel hydroxide electrode materials for supercapacitor applications

Pristine and nanocomposite (NC) hybrid electrodes of polyaniline (PANI)-nickel hydroxide [Ni(OH)2] have been prepared by single and two-step electrodeposition processes, respectively, onto stainless-steel (SS) substrates. Enhanced reversibility and stability of amorphous PANI- Ni(OH)2 NC electrodes compared to single electrode materials have been explored. PANI has a nanofibrous morphology, Ni(OH)2 has nanoplatelet-type morphology, and the NC electrodes retain an overall nanofibrous morphology. The maximum specific capacitance (SC), obtained from integrated charge under voltammetric conditions, for PANI (electro-deposited for 5 min), NC (electrodeposition of Ni(OH)2 for 10 min and 20 min onto PANI electrode surface) and Ni(OH)2 (electrodeposited for 10 min) electrodes, are 0.59, 39.06, 32.36, and 113.8 F/g, respectively, suggesting higher electrochemical performance of Ni(OH)2 electrode compared to PANI and NC electrodes. The retention in SC values with faster scan rates from 10 to 100 mV/s for PANI, NC (10 min), NC (20 min) and Ni(OH)2 are 38.7, 61.1, 52.4, and 29.0 %, respectively, explicitly confirming a higher reversibility in NC electrodes. The retention in SC values with increase of cycle number up to 1000 for PANI, NC (10 min), NC (20 min) and Ni(OH)2 electrodes are 34.9, 61.5, 67.5, and 40.7 % respectively, demonstrating higher electrochemical stability of NC electrodes over pure-phase electrodes. Nearly 2.15, 79.36, 66.66 and 406.83 mC/cm2 charges on PANI, NC (10 min), NC (20 min) and Ni(OH)2 electrodes, respectively, are obtained. Inner to total charge and outer to total charge ratios have been used to explain contributing sites to total charge in pristine and NC electrodes.

10 min) electrodes, are 0.59, 39.06, 32.36, and 113.8 F/g, respectively, suggesting higher electrochemical performance of Ni(OH) 2 electrode compared to PANI and NC electrodes. The retention in SC values with faster scan rates from 10 to 100 mV/s for PANI, NC (10 min), NC (20 min) and Ni(OH) 2 are

Introduction
Electrochemical supercapacitors (ES), have attracted considerable attention in recent years, as they are capable of providing a higher power density than batteries, in addition to high energy density compared to traditional capacitors, long cyclability and fast charge/discharge capability, which are useful in miniaturized consumer electronic products (where high operating potential is required and energy density is directly proportional to the square of operating potential window) such as memory back-up systems, electrical vehicles and industrial power/energy management etc. [1,2,3]. Based on the charge storage mode, they are divided in two categories i.e. electrical double layer capacitors and pseudocapacitors or faradaic capacitors.
In an electric double layer capacitor, energy is physically stored by means of ionic accumulation at the electrode/electrolyte interface, whereas in a pseudocapacitor, energy is stored due to reversible surface or near-surface faradaic reactions. As a result, the SC of the ES is proportional to the accessible surface area of the electrode material in contact with the electrolyte ions/redox species. The SC value of the active material can be increased by converting the bulk structure to its nanostructure form which eventually possesses a high surface area and smaller particles (which shorten the path length of ion transfer and release mechanical stress during charge/discharge process) [4,5,6,7,8]. PANI is a commonly used material for ES applications due to availability of several preparation methods for different morphologies, its low-cost and environmental and chemical stabilities etc. [9]. Due to considerable electronic conductivity and moderate SC value, nanofibers/nanowires of PANI are useful for ES [10]. The structure is based on repeating units of aniline, connected in such a way that forms a backbone of alternating nitrogen atoms and benzene rings. PANI exhibits leucoemeraldine, emeraldine, and pernigraniline states depending upon the degree of oxidation of the nitrogen atoms. Of these, only emeraldine salt is electrically conductive [11]. The main drawback of PANI electrode lies in its lower cycling stability and electrochemical inactivity in alkaline solution [12]. Hence, it is necessary to make composite of PANI with other suitable inorganic nanostructures not only to overcome its limited stability, but also to boost the overall capactiance.
Among the series of transition metal oxides/hydroxides, nickel hydroxide [Ni(OH) 2 ] is one active metal hydroxide used as an ES material. With well-defined redox activity, low-cost, layered morphology and high SC value, it has been extensively studied over the last few decades [18,19]. However, it suffers weak adhesion between the nickel-based material and the current collector (substrate), leading to the loss of active material and short cycle life as an ES electrode [20]. In order to overcome this drawback, NCs of Ni(OH) 2 with MnO 2 [21], Ni 3 S 2 /3D GN (three-dimensional graphene network) [22], graphene oxide [23], CoO/rGO [24], Co 3 O 4 /RGO [25] etc., have been studied with enhanced performance. The pioneering work on composite oxides as electrode materials for supercapacitors, such as Co 3 O 4 @MWCNT nanocable, and CoOOH nanoplates with multi-walled carbon nanotubes have proven high cyclic stability over 10,000 cycles [26,27].
PANI polymerized from aniline in aqueous acidic solution can be converted to several forms with different electrical properties by acid/base treatments and/or oxidation. When semi-oxidized PANI film is immersed in hydrazine aqueous solution/ascorbic acid (15 h), it generally reduces to leucoemeraldine state and when immersed in ammonium peroxydisulfate solution (15 h), oxidizes into pernigraniline state [28] which can also be protonated by dipping in strong acidic solution and deprotonated by immersing in alkaline solution. The electrochemical oxidation/reduction of PANI electrode can be obtained by cyclic voltammetry (CV) between -0.2 to 0.8 V in the acidic medium. In the anodic electrode potential scan of a CV, PANI demonstrates leucoemeraldine-emeraldine-pernigraniline state sequence, while in the cathoic scan PANI follows the reverse sequence i.e. pernigranilineemeraldine-leucoemeraldine states. The CV of PANI film in voltage window -0.2 to 0.6 V in alkaline solution is electrochemically inactive but mechanically stable [29]. Thus, the direction of polarization of PANI electrode at specific voltage can be used to convert a particular oxidation state of PANI. As PANI electrode is polarised at negative potential, anionic species are expelled due to electrostatic repulsion, while concentration of cationic species increases [30]. Here, single step and two-step electrodeposition methods were developed to deposit PANI, Ni(OH) 2 and NC electrodes, respectively. We have investigated the structural, morphological, and electrochemical supercapacitive properties of pristine PANI, Ni(OH) 2 and PANI-Ni(OH) 2 NC electrodes.

Experimental
Aniline, nickel nitrate hexahydrate, sulphuric acid and sodium hydroxide of analytical reagent grade were purchased from Merck and used without further purification. The working electrode i.e. pieces of SS (1 cm Â 4 cm) were cleaned with acetone and distilled water ultrasonically, polished with emery paper, and dried in air before each trial. An Epsilion potentiostat was used for electrodeposition under galvanostatic mode in a three-electrode system. The standard threeelectrode glass cell consisted of SS as working electrode (on which plastic tape was fixed except on one side 1 Â 1 cm 2 area and contact terminal), a platinum plate of 1.5 cm Â 1.5 cm as counter electrode, and a Ag/AgCl reference electrode. and 20 min (represented by (C)). All electrodes were immersed in distilled water for 2 min and dried in air at room temperature before further measurements. In summary, electrodes A and D were phase pure PANI and Ni(OH) 2 whereas B and C were their composites.
The X-ray diffraction (XRD) patterns were obtained by using a (Ultima IV, Rigaku 2500) diffractometer with CuÀKa radiation in the 2q range of 10e80 with scan rate 0.5 sec/step. The surface appearance and elemental analysis of the electrodes were investigated using field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) images obtained using a Hitachi S-4800 scanning electron microscope. All the electrochemical properties were studied in 1.0 M NaOH electrolyte solution.

Results and discussion
The electrodeposition of PANI was carried out at constant potential of 0.75 V for 5 min, since above this potential the pernigraniline (non-conducting) state of PANI is dominant [31]. The electrodeposition of green PANI was obtained from the mixed homogenous solution of aniline and sulphuric acid. At applied potential of 0.75 V in a three-electrode system, a current through the conducting substrate (SS) initiated the deposition of a compact layer/film of PANI i.e. lateral growth. With completion of lateral growth i.e. a compact layer covering the surface of SS by PANI, the aniline monomer continues to grow vertically in 1D on the already present active nuclei sites in compact layer instead of forming new nuclei to form 2D planar thin film-type morphology. The eventual deposited film comprises a nanofiber-type morphology of PANI [32]. The electrodeposition of Ni(OH) 2 and NC film electrodes from the Ni(NO 3 ) 2 precursor was achieved by reduction of the nitrate ions on the SS/PANI electrode surface to produce hydroxide ions [33]. The local p H of solution was increased (on account of the generation of the OH‾ ions) and there was the precipitation of Ni(OH) 2 at the electrode surface, In Ni(OH) 2 electrodeposition, the concentration of the Ni(NO 3 ) 2 precursor (0.05 M) was low in order to ensure efficient utilization of electrochemically generated OH ions because at high concentration, Ni 4 (OH) 4 4þ could be formed at the electrode surface blocking OH ions from the reaction interface before deposition occurs [34,35]. At the time of electrodeposition of Ni(OH) 2 onto PANI, PANI undergoes reduction from emeraldine state to leucoemeraldine state with time, as its colour was observed to change from green to greenish yellow or pale yellow. Fig. 1 shows the XRD patterns of the PANI and PANI/Ni(OH) 2 NC and Ni(OH) 2 electrodes. The peaks with asterisks '*' are of SS. All clearly discernible peaks were from the SS substrate and there was no diffraction from either PANI or Ni(OH) 2 , confirming an amorphous nature.
The FESEM images of the PANI, NC for 10 min and 20 min, and Ni(OH) 2 are presented in Fig. 2 (AeD). The FESEM of PANI [ Fig. 2(A)] electrode surface confirms the nanofiber-type morphology. These nanofibers were non-uniform in size and shape. Instantaneous nucleation is a process of nuclei creation at a high rate on a small number of active sites and progressive nucleation is a process of nuclei creation at a low rate but on a large number of active sites. In the present case, the concentration of the aniline was higher (0.5 M), and we assume an instantaneous nucleation process might be dominant. This resulted in increase of current density, which increased the rate of deposition. This process is a likely cause for the increase in length of the nanofibers with time [36,37]. In Fig. 2     electrode active material [38]. For Ni(OH) 2 , in alkaline medium, the surface faradaic reaction follows [39]; From the observations of CV curves, it was inferred that, this capacitance was mainly from the Ni(OH) 2 rather than PANI in NC electrodes. The SC values of pristine and NC electrodes were calculated from the areas of CV curves according to equation, where, C s is the SC (F/g), A ¼ unprecedented. The SC value obtained for Ni(OH) 2 electrode was greater than that of PANI as well as NC electrodes.
The CVs of pristine and NC electrodes for 10e100 mV/s scan rates are presented in Fig. 5 (AeD). With increasing scan rate, the current density also increased and the oxidation peaks shifted to a more positive potential and the reduction peaks to a more negative potential. This was assigned to an increase of the internal diffusion resistance within the electro-active material with an increase in scan rate [40]. The increase in current density with scan rate was non-linear [ Fig. 6(a)], suggesting the accessible active sites of electrode material by electrolyte species (cations/anions) were surface and diffusion-based. The dependence of SC with scan rate is represented in Fig. 6(b), where with increase of scan rate from 10 to 100 mV/s the retention of SC values of AeD electrodes, respectively, were 38.7, 61.1, 52.4 and 29.0%, suggesting higher SC retention in NCs electrodes than both PANI and Ni(OH) 2 electrodes.
The charge (q*) with a scan rate (v ¼ dV/dt) is given by [41]; where, R IðVÞdV represents an area of the respective CV. Fig. 7 illustrates the dependence of charge (q*) on the scan rate to detect the consequence of the cation/anion diffusion resistance on as prepared individual and NCs electrodes. The charge is widely used to quantify the electrochemically active sites of the electrode materials accessed by the aqueous electrolyte that contribute to capacitance [42]. The electrochemically active sites can be categorized as the outer electrochemical active sites that account for outer charge -related to the surface of a thin film, as opposed to the physical outer surface or the entire porous material) (q 0 *) contribution and the inner active sites (surfaces of material comprising internal porosity) responsible for inner charge (q i *) contribution to the total charge. It is known that outer and inner charge contributions come from electrode surface/electrolyte and bulk electrode/electrolyte interfaces, respectively, where for porous materials outer charges contribute less than inner charges, which is opposite in relatively compact materials. The estimation of outer charge (q 0 *) can be made by the extrapolation of the charge q* to v ¼ N (v is a scan rate) from the graph plot of 1/v 1/2 vs. q* [Fig. 7a]. The estimation of total charge (q t *) can be obtained by the extrapolation of charge q* to v ¼ 0, from the plot of v 1/2 vs. 1/q* [43,44,45] [Fig. 7b]. The enlarged view of plots for (B), (C), and (D) electrodes are given in Fig. 7 (c), as the variations in Fig. 7(a) are not clearly distinguished. The q i * is thus defined as the  difference between total charge and outer charge. Table 1 represents calculated charges and their charge ratios for four electrodes. The ratios of q i */q t * and q 0 */ q t * represent the contribution of electrochemical charge in the SC due to inner and outer active sites, respectively. From the Table 1 it was concluded that the contribution of total (due to an excess inner charge) charge in D electrode was higher than other electrodes.
As for real supercapacitor operations, besides a high SC value, excellent long term cycle stability has prime importance. The CV (at a scan rate of 40 mV/s) of (A) electrode (-0.2e0.5 V) and of (B), (C) and (D) electrodes (0.1e0.5 V) over 1000 cycles are presented in Fig. 8, confirming chemical stability and mechanical robustness of an individual electrode. The variations of SC as a function of cycle number are given in Fig. 9 for all electrodes. It is seen from data of Fig. 9 that all electrodes could withstand over 1000 cycles with a retention of 34.9, 61.54, 67.5 and 40.7% in SC values  obtained here is an indication of a better reversibility [33,46,47] in electrode (B) and (C). In the NC of PANI-Ni(OH) 2 , the fading with cycling is observed due to small but finite difference of oxidation and reduction peak potential of electrodes for 1 to 1000 cycles and expected conducting paths are not provided by PANI networks as it acts as insulator in alkaline solution.

Conclusions
Room temperature electrochemical deposition synthesis of amorphous PANI, PANI-Ni(OH) 2 and Ni(OH) 2 electrodes was carried out. For synthesizing pristine electrodes a single-step electrodeposition method was used and for NC electrodes, a two-step electrodeposition method was applied. Due to electrochemically inactive nature of PANI, its CV area in 1 M NaOH electrolyte and associated capacitance was consierably smaller than those of NC and Ni(OH) 2 electrodes. The SC values, estimated from the CV areas, of four electrodes PANI (A), NC electrodes (B), (C) and Ni(OH) 2 (D) measured at 10 mV/s scan rate were 0.593, 39.06, 32.36 and 113.8 F/g respectively. A larger SC in the Ni(OH) 2 electrode was due to presence

Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.