Based on the performance of hydrotalcite as anode material for a Zn–Ni secondary cell, a modification: PPY coated Zn–Al–LDH was adopted

Abstract

Polypyrrole-coated, layered double hydroxide was successfully synthesized by the polymerization of pyrrole in the slurry of hydrotalcite under ultrasonication and stirring. The structure and morphology of PPY–LDH composites were confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and the results indicate that the PPY was successfully coated onto the surface of the Zn–Al–LDH. The electrochemical performance of the PPY–LDH composites as the electrode has been evaluated for the Zn–Ni secondary cell, which proves their exceptional reversibility and superior cycle stability. Besides, the results of the electrochemical impedance spectroscopy (EIS) test indicate that PPY modification improves the conductivity of the anode and decreases the charge transfer resistance of the electrode, which greatly enhances the electrochemical performance of the PPY-LDH composites.

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Introduction

In recent years, secondary battery systems have been playing a more important role in our daily lives. The widely used secondary batteries include mainly lead-acid batteries and Li-ion batteries. However, it is unfortunate that these batteries have some problems that need to be solved; for example, the lead-acid battery energy density is low, which is inconvenient when used in moving-large energy storage systems, while Li-ion batteries have high cost and a series of safety problems. Zinc electrodes as anode materials of alkaline secondary batteries have received more and more attention, due to their excellent electrochemical performance,^1,2 such as high energy density, high open circuit voltage, low cost and environmental friendliness. However, the development of the Ni–Zn-secondary battery is restricted by its poor cycle lifetime, owing to the defects of the zinc electrode, such as shape change, dendrite growth, surface passivation and zinc self-discharge. These problems are mainly caused by the high solubility of zinc active materials in alkaline electrolyte. Hence, various additives, such as Bi(III),^3,4 In(III),^5,6 calcium zincates,^7–9 and polyaniline and polypyrrole^10,11 have been added to zinc electrodes in order to conquer the problems mentioned above.

Layered double hydroxide (LDH) compounds, owing to their nature of interlayer compensating anions and the property of compositional flexibility, have been widely designed to fulfill specific requirements in various fields, including ion conductivity,¹² adsorbents,¹³ catalysts for chemical synthesis,¹⁴ and anion exchangers.¹⁵ In our previous work, hydrotalcite, the very example of LDHs, and its modified materials, have been studied in detail as a new type of electrode in zinc–nickel secondary cells.^16–19 The results revealed that for the hydrotalcite, a novel anode material for zinc–nickel secondary cells, better electrochemical cycling stability was observed, compared with that of conventional ZnO. However, the prepared Zn–Al–LDH suffers low conductivity and the electron transfer is largely suppressed in the electrode reaction.²⁰ Therefore, in order to enhance the electrochemical properties, further modification on Zn–Al–LDH should be carried out. Electronically conducting polymers have been the focus of many intensive research programs for their low cost, in the last twenty years. In many conductive polymers, polypyrrole (PPY) has become a hot research topic for its low cost, facile synthesis by chemical or electrochemical methods, environmental stability, high conductivity, and electrochemical performance, which can be used in the biological, ionic detection, capacitor and antistatic material fields, and in the modification of electrodes of the photoelectric chemical cell, and the electrode material of the battery.^21,22 In addition, as presented by Suga et al.,²³ anode materials that contain zinc active material and a polymer layer, could suppress zinc dendrite growth and shape change.

Therefore, in this paper, we synthesized the Zn–Al–LDH/PPY composite through chemical polymerization under ultrasonication, with the aim to combine the merits of LDHs and PPY to achieve superb electrochemical performance. The electrochemical behavior of the prepared Zn–Al–LDH/PPY electrode was tested through various analytical techniques.

Results and discussion

Structure and morphology characterizations of all samples

Fig. 1 shows the XRD patterns of pure Zn–Al–LDH, sample 1 (add 0.2 mL pyrrole), sample 2 (add 0.3 mL pyrrole) and sample 3 (add 0.45 mL pyrrole). For pure Zn–Al–LDH (curve A), the characteristic diffraction peaks (2θ = 11.74° 23.57°, 34.95°, 39.35°, 60.23°) are consistent with the Zn–Al–LDH standard spectrum (JCPDS 38-0486 standard card). Compared to the XRD patterns of the three samples (curve B, curve C and curve D), no obvious difference is observed, confirming the conclusion that all samples have the typical structure of Zn–Al–LDHs. In other words, PPY may merely interact with the surface of Zn–Al–LDH particles and no new structure compound is formed. Besides, the diffraction peaks are sharp, narrow and symmetrical, with a low and stable baseline, indicating that all the as-prepared samples are well crystallized.

Fig. 1 XRD patterns of pure Zn–Al–LDH (curve A), sample 1 (curve B), sample 2 (curve C), sample 3 (curve D).

Fig. 2 shows the morphology images of pure LDH and sample 2. As can be seen in Fig. 2a, Zn–Al–LDH presents a hexagonal layer structure, which is the typical morphology of hydrotalcite. The particle size of Zn–Al–LDH is distributed around 200–400 nm and the lamellar thickness is about 50 nm. Fig. 2b shows the TEM image of pure LDH, the surface of pure LDH looks very clean and smooth. The structure of PPy coated Zn–Al–LDH can be clearly seen in Fig. 2c, where the surface of Zn–Al–LDH is covered by PPy. PPy was immobilized on the Zn–Al–LDH, based on a simple chemical polymerization using ammonium persulphate (APS) as the oxidant and sodium p-toluenesulfonate as the doping agent. Under ultrasonication, pyrrole will deposit homogeneously on the surface of Zn–Al–LDH; once the oxidizing agent is added to the above system, the polymerization of pyrrole will start. With increasing time, the chain length of the polypyrrole is longer. Finally, the PPY coated LDH structure is formed.²⁴ Fig. 2d presents the magnified image of PPY coated LDH, from which we can clearly get that the thickness of the PPY layer is about 15 nm. Compared with pure LDH, the PPy particles that are distributed on the Zn–Al–LDH have high stretching conjugated π bonds that will form a conductive net to improve the conductivity of the Zn–Al–LDH electrode. It is through this effect that the coated electrode presents a better electrochemical performance than the pure LDH electrode. The following electrochemical tests can provide more evidence to illustrate the advantages of the coating structure. Similarly, with the XRD analysis, the SEM and TEM images of pure LDH and sample 2 also indicate the reservation of the original structure of Zn–Al–LDH, which further demonstrates that no other chemical reactions between PPY and hydrotalcite occurred; the PPY simply coated the surface of the hydrotalcite.

Fig. 2 SEM and TEM images of pure Zn–Al–LDH and sample 2: (a) SEM image of pure LDH, (b) TEM image of pure LDH, (c) TEM image of sample 2, (d) magnified TEM of sample 2.

The FT-IR spectra of pure PPY, Zn–Al–LDH and sample 2 are presented in Fig. 3. The special bonds of pure PPY (curve A) located at 1556, 1361, 1311, 1189, 1052, 921 cm⁻¹ are consistent with the recorded data.^25,26 The stretching vibration at 1556 cm⁻¹ belongs to the C=C double bond, the stretching vibration at 1361 cm⁻¹ is attributed to the C–C bond, and peaks at 1311, 1189, 1052, 921 cm⁻¹ are attributed to the pyrrole ring. For Zn–Al–LDH (curve B), the broad band around 3446 cm⁻¹ is assigned to the O–H stretching modes of interlayer water molecules and H-bonded of OH groups. The peaks at 1474 and 792 cm⁻¹ belong to vibrational and bending modes of CO₃²⁻. Other absorption bands below 800 cm⁻¹ are associated with metal–oxygen (M–O) stretching and bending modes.²⁷ Obviously, for sample 2, the peaks at 3349, 1373, 809, 619, 581 cm⁻¹ are consistent with Zn–Al–LDH, while peaks at 1573, 1199, 1037, 925 cm⁻¹ belong to pure PPY. Therefore, the FT-IR spectra provide strong evidence for the successful combination of PPY and LDH, which is consistent with the TEM test above (Fig. 2).

Fig. 3 FT-IR pattern of (A) pure PPY, (B) Zn–Al–LDH, (C) sample 2.

The content of PPY in the LDH/PPY has been estimated through the TG test in Fig. 4. The samples were heated from 30 °C to 600 °C at a rate of 10 °C min ⁻¹. As displayed in the picture, almost all samples had weight loss below 600 °C. A slight weight loss for all samples can be seen below 180 °C, which corresponds to the evaporation of adsorbed water in Zn–Al–LDH. Besides, a rapid mass loss occurred in the range of 180–210 °C for sample 1, sample 2 and sample 3, which could be ascribed to the degradation of PPY in the composite²⁸ and the elimination of interlayer water in the LDH flakes. However, for pure LDH, there is a weight loss between 200–250 °C, which corresponds to the second step of weight loss. This is probably due to the elimination of interlayer water in the LDH flakes, which can be expressed as the following reaction equation:²⁹

[Zn₈Al₂(OH)₂₀](CO₃)·mH₂O → [Zn₈Al₂(OH)₂₀](CO₃) + mH₂O (1)

Fig. 4 The TG curves of pure LDH and three samples of PPY coated LDH.

A considerable mass loss corresponding to the elimination of interlayer water and the accompanying decomposition of CO₃²⁻ in the interlayer spaces for pure LDH at the range of 250–550 °C can be described by the following equation:³⁰

[Zn₈Al₂(OH)₂₀](CO₃) → 8ZnO·Al₂O₃ + CO₂ + 10H₂O (2)

However, for sample 1, sample 2 and sample 3, except for the elimination of water and decomposition of CO₃²⁻ in the composites in the range of 250–550 °C, PPY decomposition around 300–450 °C can explain the differences among the three samples and pure LDH. In the TG test, both pure LDH and PPy lost weight during the heating process, so the TG curves are complex; however, we obtained the weight loss of pure LDH as 58%, and 62%, 64%, 67% for sample 1, sample 2 and sample 3, respectively. By simple calculation, the 4%, 6% and 9% content of PPy can be obtained for sample 1, sample 2 and sample 3.

Cyclic voltammetry (CV) comparison of zinc electrodes

In order to investigate the effect of PPY on the electrochemical performance of LDH, CV testing is essential. The comparisons of cyclic voltammograms between bare LDH and PPY-coated-LDH after 50 cycles are shown in Fig. 5. All of them have two obvious peaks, representing anode and cathode peaks, respectively. The cathodic peaks at −1.450 V, −1.476 V, −1.453 V and −1.456 V for pure LDH, sample 1, sample 2 and sample 3, respectively, can be observed in Fig. 5. The cathodic peak corresponds to the charging process. During the charging process, the zinc ion will escape from the structure of the hydrotalcite in the first step and then be reduced into zinc metal after obtaining two electrons. The process can be represented by eqn (3)–(5).

[Zn_1−xAl_x(OH)₁₂](CO₃)_X/2²⁻·mH₂O → (1 − x)Zn²⁺ + xAl³⁺ + 2OH⁻ + X/2(CO₃)²⁻ + mH₂O (3)

Zn²⁺ + 4OH⁻ → Zn(OH₄)²⁻ (4)

Zn(OH₄)²⁻ + 2e → Zn + 4OH⁻ (5)

Fig. 5 Cyclic voltammograms of bare LDH (curve A), sample 1 (curve B), sample 2 (curve C) and sample 3 (curve D).

The XRD pattern of the Zn–Al–LDH electrode is provided in Fig. 6 after cycling 20 times which was in the fully charged state just right. The diffraction peaks of metal Zn can be clearly seen, which presents the highest peak. This illustrates that the charging product of the Zn–Al–LDH electrode is zinc metal. As everyone knows, Zn metal is very easily oxidized into ZnO in air atmosphere, so there are diffraction peaks of ZnO in the plot. The peaks of Cu come from the copper mesh that served as the current collector in the electrode.

Fig. 6 The XRD pattern of the Zn–Al–LDH electrode after being fully charged after 20 cycles.

The anodic peak corresponds to the discharging process. If we watch carefully, except for the big peak, we can find there is a little peak at a more positive direction. The two anodic peaks in the CV plot can be ascribed to the multistep anodic dissolution, and this process can be described by the following eqn. (6) and (7). This phenomenon has been reported previously;⁶ Fan has illustrated that the two anodic peaks in pure Zn–Al–LDH could be induced by the aluminum ions. After being coated by PPY, the Zn–Al–LDH electrodes also present two anodic peaks, which can be attributed to the influence of PPY.

Zn + 4OH⁻ → Zn(OH₄)²⁻ + 2e (6)

Zn + 3OH⁻ → Zn(OH₃)⁻ + 2e (7)

Obviously, eqn (6) occurs first and corresponds to the first anode peak; the concentration of OH⁻ around the surface of the electrode will decrease with the process depicted in eqn (6). As the reaction proceeds and the velocity of ion diffusion is lower than the electrochemical reaction, the polypyrrole (coated on zinc electrode surface) leads to an inadequate contact between the active material and OH⁻ at the reaction layer, then the anodic reaction takes place according to eqn (7), which leads to the appearance of the small peaks in Fig. 5.

After losing two electrons, the metal Zn is oxidized into zinc ions, which will enter into the structure of the Zn–Al–LDH. Therefore, during the charge and discharge process, the hydrotalcite will realize its decomposition and reconstruction. However, apart from hydrotalcite, there was some ZnO obtained as the by-product in the discharge process, which we have proven.³¹

For bare LDH electrodes (curve A), the anode peak appears at −1.214 V, while the anode peaks of sample 1 (curve B), sample 2 (curve C) and sample 3 (curve D) are −1.247 V, −1.238 V, −1.251 V, respectively. Obviously, compared with pure LDH, the electrodes of PPY-coated-LDHs present more negative anode peaks. This results from the fact that the high electrical conductivity of the polypyrrole lowered the electrochemical polarization of the Zn–Al–LDH electrode and in turn, benefited the progression of the oxidation. It was found that the electrodes of PPY-coated-LDHs present more negative cathodic peaks, compared with pure LDH. The PPY layer on Zn–Al–LDH greatly improved the conductivity of the Zn–Al–LDH electrode, which benefitted the progress of the redox reaction of the Zn–Al–LDH electrode. Therefore, in the CV plot, the PPY/LDH electrode presents a more negative anodic peak than the pure LDH electrode. However, in the charge process, the zinc ion will be reduced to Zn metal, but the coordination between N and Zn²⁺ will hinder this process, which will need more energy to achieve the reaction between Zn²⁺ and Zn. Therefore, the electrodes of PPY-coated-LDHs present more negative cathodic peaks than the pure LDH electrode.

As is known, the potential interval between the oxidation and reduction peaks, ΔE_a,c, is taken as an estimate of the reversibility of the redox reaction;³² where the value of ΔE_a,c is smaller, the reversibility of the electrode is better.^6,33 As seen from Fig. 5, the values of ΔE_a,c for PPY coated LDH are obviously smaller than that of the bare LDH, which certifies that the redox reaction of LDH exhibits better reversibility after being coated with PPY. Besides, after the introduction of PPY, compared to pure LDH, no additional peak appears, indicating that PPY does not participate in redox reactions, so it should be stable and not contribute to the capacity of the corresponding electrode.

Electrochemical impedance spectroscopy of the zinc electrode

To further understand the effect of coating, electrochemical impedance measurements were carried out. As shown in Fig. 7, all the Nyquist diagrams contain a capacitive semicircle and a Warburg diffusion tail. The high frequency arc is represented for the double-layer capacitance (CPE) in parallel with the charge-transfer resistance (R_ct) capacitance. At lower frequencies, there is a straight line having an angle of 45° with the real axis, corresponding to the Warburg impedance (Z_W), which is characteristic of the semi-infinite diffusion. Randles equivalent circuit can be seen in Fig. 7, which was used to analyze the electrochemical impedance response of the system. It includes the ohmic resistance (R_s), which contains electrolyte resistance, the current collector, the double layer capacitance (CPE), the charge transfer resistance (R_ct) and the Warburg impedance (Z_W). Generally, a capacitive semicircle at high frequency is characteristic of the charge transfer resistance in parallel with the double layer capacitance, and the larger diameter of the capacitive semicircle means the larger charge transfer resistance.³⁴ The parameters of different electrodes listed in Table 1 are obtained according to the impedance diagrams and equivalent circuit. As shown in Table 1, the charge-transfer resistance of PPY coated LDH electrodes are much lower than that of pure LDH electrode. The conductivity of hydrotalcite behaves poorly, so during the charge and discharge process, the charge transfer rate is slow, which can be represented as a large R_ct for pure LDH as shown in Fig. 7. As is known, polypyrrole has significant electrical conductivity and strong charge storage capacity resulting from the high stretching, conjugated π bond. Therefore, the conductivity of sample 1, sample 2 and sample 3, which have been coated by PPY is greatly increased. It is the high stretching conjugated chain system of the PPY that results in the promotion of electron transfer and facilitation of charge transportation, at the same time, decreasing the value of R_ct. This is very favorable for the smooth progression of the electrochemical reaction of the electrode. Contrary to the charge-transfer resistance, when the pure LDH is coated by PPY, it presents a higher Warburg resistance compared to the pure LDH, and the value of the Warburg resistance increases when the content of PPY in the composite increases. The above phenomena indicate that the appearance of PPY accelerates the electrode reaction, while slowing down the ion diffusion. Therefore, the addition of PPY should be controlled to give full play to its advantage of reducing the charge transfer resistance and simultaneously confining the Warburg resistance.

Fig. 7 Typical Nyquist diagrams and Randles equivalent circuit for the zinc electrodes with bare LDH and PPY-coated LDH. Curve A: pure Zn–Al–LDH, curve B: sample 1, curve C: sample 2, curve D: sample 3.

Table 1 The electrochemical impedance of pure LDH and three samples obtained from the Nyquist plots

Samples	ZW (ohm g)	Rct (ohm g)
Pure LDH	0.0289	10.93
Sample 1	0.1075	3.176
Sample 2	0.2262	2.010
Sample 3	0.3206	3.054

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The cycle performance analysis of PPY/LDH electrodes

The presence of the conductive film, PPY, can significantly reduce the charge transfer resistance in the electrode reaction process, which has been proved by the AC impedance. The decrease in charge transfer resistance can accelerate the electron transfer rate, reduce the electrochemical polarization and improve the cycle stability of the battery. We therefore investigated the electrochemical cycle behavior of the four electrodes. As can be seen in Fig. 8A, for the pure LDH electrode, although it presents high discharge capacity in the first several cycles, compared to the PPY/LDH composites, its whole cycle performance is poor. As shown in Fig. 8A for the first 100 cycles, the discharge capacity of pure LDH is higher than the three samples, which can be attributed to the presence of PPY decreasing the weight ratio of the effective active material, and with the content of PPY increasing, the initial discharge capacity of the corresponding electrode decreases. After 270 cycles, the retention rate of the discharge capacity of pure LDH was only 68%, while the retention rate of discharge capacity of the three samples were all above 90%. After 500 cycles, the discharge capacity of bare LDH dropped to 240 mA h g⁻¹, while the PPY coated LDHs still remained above 250 mA h g⁻¹. In particular, sample 2 was maintained at 350 mA h g⁻¹ and showed the best stability in the whole cycle. In fact, it was Fan who first used Zn–Al–LDH as the anode material of the Zn–Ni secondary cell. However, he found that when Zn–Al–LDH was used as electrode active material, it suffered poor conductivity, which largely suppressed the electron transfer for the electrode reaction. The low conductivity of Zn–Al–LDH inhibits the excellent performance of Zn–Al–LDH as the anode material for the Zn–Ni secondary cell. Next, Fan added La element to hydrotalcite and obtained Zn–Al–La–LDH as the anode material for the Zn–Ni cell. However, La did not have an obvious effect on improving the conductivity of the electrode. Therefore, the conducting polymer, PPY, became the target to solve the above problem. The fact is, in the cycle test, the retention rate of the Zn–Al–La–LDH electrode was only around 79% after 400 cycles, while in this paper, sample 2 and sample 3 are both above 90%. Although the retention rate of sample 1 is only 77%, which is lower compared with sample 2 and 3, it still can be compared with Zn–Al–La–LDH with a highest retention rate of 79.0%. For PPY-coated Zn–Al–LDHs, the improvement of cycle stability can be ascribed to surface PPY. As we all know, the poor cycle stability of the zinc electrode is caused by the dissolution of discharged product in the alkaline electrolyte. Though the Zn–Al–LDH electrode can restrain the above problem to some extent, it suffers from poor conductivity. The presence of PPY increases the conductivity of the Zn electrode; at the same time, the PPY film can avoid direct contact between zinc active material and the electrolyte. In addition, for the layered PPY//LDH composite, the PPY layer contains N atom, which has coordination interaction with Zn²⁺ in the Zn–Al–LDH. This effect will prevent the oxidized product from dissolving in electrolyte.¹¹ Sample 1 showed poor cycling performance, compared to sample 3 and sample 2, which is mainly attributed to the low PPY content in sample 1. This is detrimental to improving the electrode conductivity and decreasing the direct contact between the electrode and electrolyte. Compared to sample 2, the PPY content in sample 3 is higher, which leads to a decrease in the active substance in the PPY coated LDH electrode, so the discharge capacity of sample 3 is lower than sample 2.

Fig. 8 (A) Electrochemical cycle behaviors of nickel–zinc secondary batteries with bare LDH (curve A), sample 1 (curve B), sample 2 (curve C) and sample 3 (curve D). (B) Typical charge discharge curves of pure LDH (curve A), sample 1 (curve B), sample 2 (curve C) and sample 3 (curve D) at the current rate of 1C.

The galvanostatic charge/discharge curve analysis of zinc electrodes

From Fig. 8B, it can be seen that the Ni–Zn-secondary battery with PPY coated LDH electrode presents a lower charge and higher discharge plateau voltage than pure LDH. A lower charge plateau voltage can effectively suppress the hydrogen evolution reaction, so the charging efficiency is improved, which indicates that the PPY coated LDH samples have better electrochemical cycling performance, compared to pure LDH. Besides, for the pure Zn–Al–LDH electrode, the potential fading is much faster than for the three coated Zn–Al–LDH electrodes. It can also be clearly seen that there is more than one plateau in the discharge curves of all electrodes, which is consistent with the two anodic peaks in CV plots and is illustrated in the CV plot. The conductive properties of pure LDH are poor, which contributes to the lower charge transfer efficiency of the pure LDH electrode. As the conducting polymer, PPY can improve the electrical conductivity of the electrodes, as well as the discharge platform. More importantly, the PPY coated on the surface of the LDH electrode effectively reduces the contact between the electrolyte and the electrode active material. In summary, the cyclic stability of the PPY coated LDH electrode is improved by the two aspects of the charge and discharge.

Experimental section

Material preparation

The preparation of the PPY-coated Zn–Al–LDH involves two parts: the Zn–Al–LDH was synthesized first and then the PPY-coated Zn–Al–LDH was prepared via the polymerization of the pyrrole monomer in the presence of Zn–Al–LDH particles, accompanied by continuous ultrasound and mechanical agitation. The details are as follows: The Zn(NO₃)₂·6H₂O (11.88 g) and Al(NO₃)₃·9H₂O (3.75 g) were dissolved in 50 mL of water. This aqueous solution and a 50 mL solution containing NaOH (4.06 g) and Na₂CO₃ (2.17 g) were then added to water at a speed of 1 drop per s. The solution obtained above was stirred for 30 min at 65 °C at a constant pH value of 10. Subsequently, the resulting slurry was filtered and washed with deionized water, and then the precipitate was dispersed in deionized water and the slurry transferred into a Teflon-lined autoclave at 120 °C for 12 h. The precipitate was filtered, washed, dried under vacuum at 65 °C, and ground into fine powder for further use.

The above sample (1 g) was dispersed in 50 mL of water under ultrasound, then 0.32 g sodium p-toluene sulfonate (obtained from sodium hydroxide and p-toluenesulfonic acid) and 0.2 mL (for sample 1), 0.3 mL (for sample 2) or 0.45 mL (for sample 3) of pyrrole monomer were added to the above suspension under constant sonication, and then continuously stirred for 10 min. Afterwards, 0.21 g of ammonium persulfate was promptly added to the above solution and the resulting solution was kept under ultrasound and stirring continued for 30 min. The products were filtered and washed thoroughly with water and ethanol, and the filtered samples were dried for further analysis.

The characterization of PPY-coated Zn–Al–LDHs

The surface morphologies of PPY-coated Zn–Al–LDHs and pure Zn–Al–LDHs were determined by scanning electron microscope (SEM) (JSM-6360LV) and transmission electron microscope (TEM) (JEOL-2010). The X-ray diffraction (XRD) pattern was obtained on a D500 (Siemens) diffractometer (36 kV, 30 mA) using Cu Kα radiation at a scanning rate of 2θ = 8° min⁻¹. Fourier transform infrared (FT-IR) spectroscopy was conducted on a Nicolet Nexus-670 FT-IR spectrometer (as KBr discs, in the range 400–4000 cm⁻¹). Thermogravimetric analyses (TGA) of all samples were carried out on a NETZSCH STA 449C instrument to determine the PPy content.

The preparation of the Zn–Al–LDHs electrodes

For fabrication of Zn–Al–LDH/PPY electrodes, the as-prepared PPY-coated Zn–Al–LDHs samples, acetylene black and polytetrafluoroethylene were ground with an agate mortar until they were well combined. The weight ratio of Zn–Al–LDHs, acetylene black and polytetrafluoroethylene (serving as conductive agent and binder, respectively) was 90 : 5 : 5. Then the uniform mixture was put on a copper mesh substrate (of size 1.2 cm × 1.2 cm), which served as the current collector. Afterwards, the pasted electrodes were pressed to a thickness of 0.30 mm and dried at 80 °C under a vacuum drying oven. For comparison, the pristine Zn–Al–LDH electrode was also fabricated in the same way. A two-electrode cell was adopted for pre-activated testing of the cell and the electrolyte was the solution of 6.0 M dm⁻³ KOH, which was saturated by ZnO. The positive electrode was the sintered Ni(OH)₂ electrode whose capacity was larger than that of the zinc electrodes for the purpose of making full use of the zinc electrodes during cycling. All the cells were pre-activated as follows: the cells were charged at a constant current of 0.1 C for 12 h, and discharged at constant current of 0.1 C to a cut-off voltage of 1.2 V, two times at room temperature. The cells were then charged at a constant current of 1 C for 1 h, and discharged at a constant current of 1 C to a cut-off voltage of 1.2 V, three times at room temperature.

Electrochemical tests

A three-electrode system was adopted for the cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS), with the Hg/HgO electrode serving as the reference electrode, the sintered Ni(OH)₂ as counter electrode, and the working electrode was the pre-activated pasted Zn–Al–LDH/PPY electrode. The CV plot was tested on an electrochemical workstation (CS310) and the voltage shifted from −0.85 V to −1.65 V, at a scanning rate of 10 mV s⁻¹.

The EIS test was carried out on a RST 5000 (Zhengzhou Shiruisi Technology Co. Ltd.) electrochemical workstation in the frequency range from 0.01 Hz to 100 kHz and the ac potential amplitude was 10 mV. For the galvanostatic charge–discharge cycle test, we used a two-electrode system containing the pre-activated pasted Zn–Al–LDH/PPY and pure Zn–Al–LDH electrode as working electrodes, and sintered Ni(OH)₂ as the counter electrode. During the cycling process, the cells were charged at 1 C for 1 h and discharged at 1 C down to 1.2 V cut-off voltages, 500 times. The cycle test was performed on a BTS-5V/10 mA battery-testing instrument (Neware, China) at room temperature. All of the reagents above were A.R. grade, and were used without further purification; the electrolyte was prepared using deionized water.

Conclusions

The LDH was prepared by the co-precipitation–hydrothermal method and then the PPY/LDH composites were synthesized with the assistance of ultrasound. XRD, SEM, FT-IR and TEM analyses clearly confirm that PPY was successfully coated onto LDH, and the structure of the LDH was not destroyed. PPY coated LDH with different PPY content, especially the sample containing 0.3 mL PPY, employed as the negative electrode of the Zn–Ni battery shows better reversibility and superior cycle stability, compared to the pure Zn–Al–LDH electrode. In short, PPY can improve the electrochemical performance of the pure Zn–Al–LDH electrode, which can be used as novel, active materials for Zn–Ni secondary cells.

Acknowledgements

This work was supported by the Natural Science Foundation of China (no. 21371180), Science and technology project of Changsha city (no. K1303015-11) and Specialized Research Fund for the Doctoral Program of Higher Education (no. 20130162110018).

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