Controlling the corrosion and hydrogen gas liberation inside lead-acid battery via PANI/Cu-Pp/CNTs nanocomposite coating

The liberation of hydrogen gas and corrosion of negative plate (Pb) inside lead-acid batteries are the most serious threats on the battery performance. The present study focuses on the development of a new nanocomposite coating that preserves the Pb plate properties in an acidic battery electrolyte. This composite composed of polyaniline conductive polymer, Cu-Porphyrin and carbon nanotubes (PANI/Cu-Pp/CNTs). The structure and morphology of PANI/Cu-Pp/CNTs composite are detected using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Based on the H2 gas evolution measurements and Tafels curves, the coated Pb (PANI/Cu-Pp/CNTs) has a high resistance against the liberation of hydrogen gas and corrosion. Electrochemical impedance spectroscopy (EIS) results confirm the suppression of the H2 gas evolution by using coated Pb (PANI/Cu-Pp/CNTs). The coated Pb (PANI/Cu-Pp/CNTs) increases the cycle performance of lead-acid battery compared to the Pb electrode with no composite.

Indeed after 150 a long time since lead-acid battery (LAB) innovation, advancements are still being made to the lead battery performance and in spite of its inadequacies and the competition from more energy storage cells; the LAB battery still holds the lion's share of the total battery sales 1 . In brief, in the LAB battery the PbO 2 (positive plate) and Pb (negative plate) respond with the electrolyte (H 2 SO 4 ) to form energy 2,3 . The main advantages of LAB battery are low cost, low internal impedance, and easily recycled 4 .
One of the most important difficulties facing the LAB battery industry is the liberation of bubbles of hydrogen gas and corrosion of negative plate (pb) [5][6][7] . This may cause a great low in battery performance and also explosion in the LAB battery room.
The utilize of added substances (additives) within the battery electrolyte is one of the approaches which offers an increase in battery performance without much modification of other components [8][9][10][11] . The major issue lies with choosing a reasonable added substance which is chemically, thermally and electrochemically steady in exceedingly corrosive environment.
To resolve these difficulties experimentally, many researchers tried to decrease the rate of the hydrogen gas (HER) and corrosion of negative plate (pb) by applying additives such as organic compounds, surfactants and ionic liquids [12][13][14][15][16] . These additives are utilized to increase performance of the LAB battery through working as cathodic-type inhibitors.
Previous research has shown that the use of conductive polymer coatings may be a good solution to overcome the failure in battery electrodes 17,18 . Unfortunately, the stability of conductive polymers under ambient conditions is a persistent problem. To maximize the efficiency of conductive polymer coatings, the different nano-particles with unique properties such as Cu-Porphyrins (Cu-Pp) and carbon nanotubes (CNTs) will be incorporated in the texture of PANI forming new nanocomposite coating.
The low cost, ease of synthesis, high environmental reliability, and high conductivity of PANI, Cu-Pp and CNTs make them promising materials for the formation of new composites 19,20 . Methods. The experimental setup for the H 2 gas evolution measurements was described in our earlier work 23,24 . For this purpose, the Pb electrodes (dimension = 1.5 cm × 0.5 cm × 0.04 cm) were placed in 5.0 M H 2 SO 4 solution (100 ml). The period of immersion is 5 h. The rate of hydrogen evolution (HER) is calculated by dividing the volume of the hydrogen evolved (ΔV) to immersion time (t) and electrode surface area (A), as given in Eq. (1) 25 : Electrochemical tests (Tafel and EIS) were performed using glass cell (3-electrodes cell). The electrochemical responses were observed using Potentiostat instrument (model: Gill AC -947-ACM). In this system, the Pt and Hg/Hg 2 SO 4 electrodes serve as counter and reference electrodes, respectively. The EIS experiment was performed in a frequency range 1.0 Hz-30 kHz at −1.1 V vs. Hg/Hg 2 SO 4 . The Tafel experiment was performed in a potential range (−250 mV) to (250 mV) versus OCP with short scan rate (1.0 mV s −1 ).
AC electrical conductivity of PANI, PANI/CNTs and PANI/Cu-Pp/CNTs were determined by impedance analyzer in frequency range 10 Hz-1000 kHz.
The cycle performance of LAB battery was inspected by using laboratory made cells (2.0 V/2.8 Ah). This cell contains one negative electrode and two positive electrodes. The separator is poly vinyl chloride. The electrolyte is 5.0 M H 2 SO 4 . The cycle performance tests were carried out using different negative electrodes i.e. bare Pb, coated Pb (neat PANI), coated Pb (PANI/CNTs) and coated Pb (PANI/Cu-Pp/CNTs). In all cases, the tests were stopped at 1.7 V (the cut-off discharging voltage) and measured at C/5 rate and at 298 K.
The structure and morphology of PANI/Cu-Pp/CNTs composite were detected using TEM, SEM (Jeol-Jem 1200EX II) and XRD (PANIalytical X'PERT PRO) analysis.

Results and discussion
Structure and morphology of PANI/Cu-Pp/CNTs. TEM and SEM were conducted on the surface of the PANI/Cu-Pp/CNTs nanocomposite to detect the morphology of nanocomposite, as shown in Fig. 1. The relevant TEM image of CNTs can be seen in Fig. 1a, which illustrates that CNTs are made up of homogeneous tubes. The TEM image in Fig. 1b shows that the Cu-Pp particles have a nanoplate shape.The TEM images in It is evident that new nanocomposites suppressed the hydrogen gas evolution reaction. Also, the considerable low HER was detected in coated Pb (PANI/Cu-Pp/CNTs).
The HER for coated and uncoated Pb can be investigated via EIS experiments at − 1.1 V vs. Hg/Hg 2 SO 4 . Then, the EIS method can be used for extracting the impedance parameters for cathodic reaction (i.e. Hydrogen gas evolution reaction) 26 . Figure  The Nyquist parts exhibit a similar trend (i.e. charge transfer trend) for all uncoated and coated electrodes 27 . The best suitable equivalent circuit (EC) for Nyquist parts was inserted in Fig. 5a. This EC contains R ct (charge transfer resistance of HER on Pb), C dl (electrical double layer capacitor) and R s (electrolyte resistance) 28,29 . All these elements are presented in Table 1. Compared with the bare Pb, the coated electrodes with various coatings (neat PANI, PANI/CNTs and PANI/Cu-Pp/CNTs) showed higher R ct and lower C dl (see Table 1). The coated  www.nature.com/scientificreports/

Effects of PANI/Cu-Pp/CNTs on the corrosion rate.
Tafel experiments were used to examine the corrosion rate for various electrodes containing bare Pb, coated Pb (neat PANI), coated Pb (PANI/CNTs) and coated Pb (PANI/Cu-Pp/CNTs ) in 5.0 M H 2 SO 4 (see Fig. 6). The Tafel elements such as corrosion potential (E corr ) and corrosion current density (j corr ) are presented in Table 2.
Indeed, the new nanocomposites affect on the Tafel lines (anodic and cathodic reactions) for Pb in 5.0 M H 2 SO 4 . Compared with the bare Pb, the coated electrodes with various coatings (neat PANI, PANI/CNTs and PANI/Cu-Pp/CNTs) showed lower j corr (see Table 2).
Results indicated that the j corr decreased from 5.01 mA cm −2 to 0.04 mA cm −2 when the coated Pb (PANI/ Cu-Pp/CNTs) was used. Furthermore, the E corr moved to more positive direction for coated Pb electrodes. The change in E corr values reflects a change in a corrosion system 31 . Results confirmed that PANI/Cu-Pp/CNTs nanocomposite caused a significant reduction in the corrosion rate for Pb in 5.0 M H 2 SO 4 . Furthermore, both the cathodic and anodic branches of the Tafel curves have shifted to lower current density values, suggesting that both hydrogen evolution and Pb dissolution reactions have been inhibited.
AC electrical conductivity experiments were used to prove that the low j corr for coated Pb is not due to the poor conductance of nanocomposites.
In comparison, PANI/Cu-Pp/CNTs have the highest conductivity due to the fast electron delocalization along the PANI because of the combination of Zn-Pp and CNTs 34-37 . LAB battery performances. Battery performances of LAB battery using different negative electrodes i.e.
bare Pb, coated Pb (neat PANI), coated Pb (PANI/CNTs) and coated Pb (PANI/Cu-Pp/CNTs) were examined. The cycle performance of LAB battery was recorded in Fig. 7. The LAB battery showed an open circuit potential nearly 2.10 V. All electrodes showed the depleting in discharge voltage with an increase in cycle number. Here the discharge voltage at 1.7 V represents the battery shortage and the end of discharge.  Mechanism and explanation. When discharging a LAB battery, the following reactions at the negative electrode occurs: Pb + H 2 SO 4 ↔ PbSO 4 + 2H + + 2e and 2H + + 2e ↔ H 2 38,39 . Hydrogen evolution and formation of PbSO 4 on the surface of the negative electrode can induce the loss in the battery life 40,41 .
With regards to the above results, it is clear that the using of coated negative electrodes with PANI/Cu-Pp/ CNTs composite can significantly decrease the HER and corrosion rates.
The most important mechanisms with respect to the role of PANI/Cu-Pp/CNTs composite can be based on the following aspects: 1. PANI polymer can form the physical barrier on the surface of Pb electrode. This barrier protects the surface of electrode from corrosive solution 42,43 . 2. Due to the conductivity property of the PANI polymer, the cathodic reaction (i.e. hydrogen evolution) that occurred on the surface of Pb electrode will be replaced with the PANI /electrolyte interface 44 . Therefore, we noted a significant reduction in the hydrogen evolution. 3. According to Ahmad and MacDiarmid 45 , the coating of PANI causes the moving of corrosion potential to the passive area, leading to protection of Pb electrode. Additionally, this moving in E corr gives a significant physical property against corrosion products on the Pb electrode 46 . 4. By introducing the CNTs and Cu-Pp nanoparticles in the PANI polymer matrix, that formed composite (PANI/Cu-Pp/CNTs) becomes more effective in the suppression of both hydrogen gas evolution and corrosion of Pb electrode than PANI alone 22,34 . 5. The presence of CNTs and Cu-Pp nano-particles shrink the electrolyte pathway of PANI and hence reducing the risks of corrosive solution. Moreover, both nanoparticles improve the mechanical and conductivity properties of PANI 47-50 . 6. The use of PANI/Cu-Pp/CNTs composite was effective for reducing the formation of PbSO 4 on the surface of the battery negative electrode during the cycling process. This led to the supreme performance of the LAB battery. 7. The high conductivity of CNTs and Cu-Pp nanoparticles enhanced the cycling performance of LAB battery [51][52][53] .

Conclusions
In the research, we have offered the promising composite (PANI/Cu-Pp/CNTs) coating to protect the negative plate (Pb) of LAB battery. PANI/Cu-Pp/CNTs nanocomposite was compared with neat PANI, PANI/ CNTs coatings to determine the performance of new nanocomposite. In comparison, coated Pb (neat PANI), coated Pb (PANI/CNTs) and coated Pb (PANI/Cu-Pp/CNTs) revealed the HER around 0.25 ml min −1 cm −2 , 0.02 ml min −1 cm −2 and 0.015 ml min −1 cm −2 , respectively. Compared with the bare Pb, the coated Pb electrodes showed higher R ct and lower C dl and I corr . The presence of CNTs and Cu-Pp nano-particles improve the mechanical and conductivity properties of PANI. The coated Pb (PANI/Cu-Pp/CNTs) presented a better cyclic performance compared with bare Pb electrode. This means that the use of composite (PANI/Cu-Pp/CNTs) is effective coating to enhance the performance of LAB battery. This outcome opens up magnificent opportunities for nanocomposite research that is applied to lead-acid batteries.