A ZnO/PVA/CHI Composite Electrode for Rechargeable Zinc-air Battery

doi:10.21203/rs.3.rs-2752718/v1

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A ZnO/PVA/CHI Composite Electrode for Rechargeable Zinc-air Battery

https://doi.org/10.21203/rs.3.rs-2752718/v1

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ZnO/PVA/CHI composite electrode with internal pore structure has been successfully prepared by cross-linking polymerization of polyvinyl alcohol (PVA) and chitosan (CHI). The resulting composite electrode exhibits a maximum anode dissolution current of 0.34A, 4.86 times higher than that of the pure ZnO electrode (0.07A).More importantly, the R_ctof the composite electrode is 1.01Ω·cm²for the composite electrode, which is lower than that of pure ZnO electrode (2.805Ω·cm²), attesting that the composite electrode possess a more favorable reaction rate. Moreover, the rechargeable battery assembled with composite electrode demonstrates a great improvement in charge/discharge efficiency and cycle stability. It remains over 80% of capacity retention even after 100 cycles, while that of pure electrode can only maintain this level less than 40 cycles. This proposed composting strategy herein may pay a way for the exploration of novel zinc anode for rechargeable zinc-air batteries with superior electrochemical performance.

ZnO/PVA/CHI composite

Polyvinylalcohol

Chitosan

Rechargeable zinc-air battery

Zinc anode

Rechargeable zinc-air battery possesses several advantages such as high energy, abundant resources, superior safety performance, good environmental compatibility, and easy storage, which endow it with incomparable advantages over other metal-air batteries ^[1]. However, in addition to the sluggish mechanisms including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics of the oxygen-involved reactions^[2–3], the dissolution of active materials and zinc dendrite formation of zinc electrode, especially at high charge-discharge rate, cause its discharge capacity fading and short cycle-life, which limits its commercialization^[4–5].

To overcome this issue and stabilize the zinc electrode, methods have been made to solve these problems ^[6–9].For example, Vatsalarani et al. reported that the zinc anode could be activated by polyethylene^[10].Heet al. synthesized N-doped carbon cloth (NC) via magnetron sputtering and explored as the substrate to induce the uniform nucleation of zinc metal in order to suppress the dendrite growth ^[11]. Zuo and his coworkers reported a new strategy for optimizing the solid-state electrolyte/electrode interface coupling by combining porous zinc electrode and thermal-sensitive solid-state electrolyte F127 to improve the areal-capacity of solid-state zinc-air batteries^[12]. Benefiting from the porous structure and excellent contact, the SZAB using porous zinc anode and the solid-state electrolyte F127 exhibits high areal capacity of 133 mAh·cm^− 2 at a current density of 4 mA·cm^− 2, which is 100 times higher than that of the solid zinc-air battery using zinc foil as the anode. In our team’s previous work^[13], a ZnO/PVA/PAADDA composite electrode for rechargeable zinc-air battery was prepared by chemical cross-linked PVA (polyvinyl alcohol) with poly(acrylamide-co-diallyldimethylammonium chloride) (PAADDA).The three-dimensional network is of benefit to sustaining the stability of zinc electrode, restricting the shape change and zinc dendrite growth, which is useful for keeping high activity and prolonging cycle life of the electrode.

Chitosan(CHI), as a highly biocompatible, biodegradable, low in toxicity, non-antigenic, and absorbable linear amino hetero-polysaccharide, has been widely applied in textile, medicine, food, chemical industry and other fields ^[14].It’s reported that CHI can strongly adsorbs heavy metal ions such as Cu²⁺, Cr³⁺, Cd²⁺, Ni²⁺, Ph²⁺, Zn²⁺, and Hg^2+[15], and the adsorption capacity is linked to the deacetylation degree^[16–17]. Additionally, CHI in combination with other polymers has been frequently explored to ameliorate the physicochemical as well as biological properties. A synthetic, non-toxic, water-soluble, and biocompatible polymer with good characteristics is polyvinyl alcohol (PVA) ^[18]. Besides, PVA/CHI scaffolds offer benefits of improved porosity, improved mechanical properties, and higher biocompatibility ^[19].

In our present work, the purpose of this study was to develop a new type of composite based on PVA, CHI and ZnO, in which the cross-linked structure was obtained through the reaction of hydroxyl groups (OH⁻) of PVA with the aldehyde groups (-CHO) of glutaraldehyde (GA) in the presence of acid^[13].As a result, compared to the pure ZnO electrode, the composite exhibits high cycle stability and excellent electrochemical activity, which is expected to achieve the goal of reducing zinc dendrite and improving the cycle performance.

2.1 Preparation of zinc anode and air cathode

All chemicals, except chitosan, were of analytically pure grade, and the doubly distilled water was used for preparing all solutions. After the chitosan(food grade, Qingdao Yuda Century Trade Co.), glutaraldehyde (AR, Sinopharm Chemical Reagent Co.) and PVA(AR, 99% hydrolyzed, average molecular weight 3600–4400, Wuxi Minfeng Reagent Factory) with the mass ratio of 5:1:1 were mixed evenly and proportionally, 0.30 g ZnO(Tianjin Kemiou Chemical Reagent Co.) and 0.02 g carbon fiber were added to the 0.08 g mixture to make zinc paste and evenly coated on the tinning wire copper, which is rolled and flattened after drying at room temperature. The ZnO/PVA/CHI composite electrode was thus obtained by chemical cross-linking at room temperature for 3 h after being soaked in acetone solution, containing a small amount of hydrochloric acid. For comparison, the pure ZnO electrode was made in the same way by soaking the dry electrode in acetone solution for 3 h.

Using the FeCo/C as a bi-functional catalyst, which was synthesized and described in our group’s previous work ^[20].The air electrode was constructed of a catalyst, a gas diffusion layer, and a current collector. To prepare the catalyst layer, a mixture containing 40% FeCo/C, 40% Vulcan XC-72 and 20% PTFE were mixed and grounded in suitable amount of anhydrous ethanol at room temperature, then the mixture was rolled pressed to form a film as the catalyst layer. The gas-diffusion layer containing 50 wt% PTFE and 50% wt% Vulcan XC-72 was made by the same way. Finally, nickel foam was used as a current collector. The diffusion layer, the current collector and the catalytic layer were pressed at 3×10⁷ kg·cm^− 2 for 5 min and dried in an oven at 85^oC for 24 h.

2.2Physical characterization

Fourier transform infrared (FTIR) spectra of the samples were recorded on a Nicolet Avatar 360 Fourier-IR spectrometer with KBr disks. Scanning electron microscopy (SEM) images of the samples were obtained with a JEOL JSM-6360LV microscope.

2.3 Electrochemical characterization

The electrochemical characterization was recorded on a CHI760D electrochemical workstation (Shanghai Chenhua) using a three electrode system. The ZnO/PVA/CHI composite electrode served as the working electrode, the air electrode (2 cm×2 cm) was used as the count electrode, and a zinc wire was the reference electrode. The membrane was PPAT composite membrane. A 6 mol·L^− 1 KOH solution was used as electrolyte. The polarization curves of the anode process was scanned using the slow linear scanning method (scanning speed 1 mV·s^− 1). The range of scanning potential was 0 ~ 1V.Electrochemical impedance spectroscope (EIS) measurements were conducted at the potential of 0.1 V vs. Zn/Zn(OH)₂, with its frequency range ranging from 100 to 0.01 kHz. The amplitude of AC signal was 5 mV. The galvanostatic charge-discharge tests were performed on a BTS-5V3A Battery Test System (Shenzhen Netware). The cells were charged at 25 mA·cm^− 2 for 1 h, followed by discharging down to a cutoff voltage of 0.2 V at the same current density and were cycled automatically when the process set for charge or discharge was completed. All the tests were performed at room temperature.

As illustrated inFig.1 (a), the FTIR spectra of ZnO/PVA/CHI composite were recorded from 4000 to 400 cm^− 1.The absorption peak around 3415 cm^− 1 in the spectrum belongs to the stretching vibrations of surface O-H bonds from PVA and N-H bonds from CHI^[21–22].The bands located at 2926,1421, and 1116cm^− 1 are respectively put down to the aliphatic C-H stretching vibration, the CH-OH stretching vibration, and the -CH₂stretching vibration in the PVA^[22–23].The stretching vibration absorption peak appeared at 1635 cm^− 1 in the spectra can be attributed to the O-H bond in the CHI. Among the peak assigned to ZnO, the peak located at 437 cm^− 1 is attributed to the O-Zn stretching vibrations on Zn atoms^[24]. It is worth noting that the composite spectrum shows all the absorption bands of CHI (3415 and 1635 cm^− 1), of PVA (2926, 1421, and 1116 cm^− 1) and of ZnO (437cm^− 1), confirming the existence of CHI, PVA and ZnO in the composite.

Owing to the surface coating treatment, the ZnO particles can be uniformly encapsulated and maintain good porosity and structural stability, which can be confirmed by the SEM images. As seen in Fig. 1(b),the surface of the composite electrode remains smooth, with a number of round holes in the size of 1 ~ 20 µm even after 80 cycles of charge and discharge process. There is no dense zinc oxide build-up. Moreover, Figs. 1(c) to (d) shows the internal morphology of the voids. The inside of the voids maintain good interfaces flatness. This observation indicates that the ZnO particles are uniformly embedded. Compared to smooth surface of the composite electrode (Fig. 1 (b-d), the surface of the pure ZnO electrode(Fig. 1(e))is rougher due to the coating of a large amount of ice-like dendrite clusters after 80 cycles under the same charge and discharge conditions. This may be attributed to the unique architecture and the close bonding between the PVA/CHI copolymer. On the one hand, the existence of PVA/CHI copolymer may retain ZnO in its initial position, which inhabits the dissolution of its discharge products and facilitates the homogeneous distribution of active material. On the other hand, the copolymer has good alkaline resistance and maintains well mechanical property, which provides reasonable space for zinc volume change during charge/discharge process. Additionally, a large number of voids inside the electrode provide an advantageous channel for ionic (OH⁻) transfer channels, thus making the composite electrode exhibit a longer cycle life and higher activity.

To estimate the electrochemical performance of the electrodes, polarization curves and electrochemical impedance spectroscopy (EIS) measurements of the pure and the composite electrodes were performed. As shown in Fig. 2(a), the anode active dissolution areas of the pure electrode and the composite electrode are 0 ~ 0.2 V and 0 ~ 0.4 V after the first cycle respectively, showing that the ZnO/PVA/CHI electrode has higher activity than the pure ZnO electrode at the beginning of the cycle. Such a performance improvement may be attributed to a unique highly cross-linking and interpenetrating porous three-dimensional network structure, which facilitates the penetration of electrolyte, thus resulting in a higher electrode activity. Moreover, the unique structure of the composite electrode is beneficial to delaying the cathode passivation process. As expected, the active dissolution areas of the anode for the pure and composite electrode are 0 ~ 0.25 V and 0 ~ 0.53 V, respectively, even after 80 cycles. Satisfyingly, the composite electrode shows the maximum anode dissolution current of 0.34 A, which is 4.86 times higher than that of the pure ZnO electrode (0.07 A).This may lie in the fact that the porous network structure formed inside the composite is preferable for zinc electrode, because it is conductive to the migration of OH⁻, thus facilitating the utilization of interior surface of zinc electrode.

Electrochemical impedance spectroscopy measurements were carried out to further investigated the effect of the porous network stricture made from the PVA/CHI copolymer on electrochemical properties. As seen in Fig. 2 (b), the impedance response of both the pure and composite electrodes after 80 cycles is characterized by the presence of a high-frequency semicircle region and a low-frequency sloped line region. The activity of the electrode can be reflected in the R_ct, which is related to the charge transfer resistance and can be calculated from the diameter of the semicircle. As expected, the R_ct of the composite electrode is much smaller than that of the pure electrode, attesting that the composite electrode posses a more favorable reaction rate. Moreover, the electric equivalent circuit diagrams for both of the pure and composite electrode are shown in Fig. 2(c) and (d), respectively, and the fitting parameters of the electric components are shown in Table 1.

As seen from Table 1, the R_a of the pure and composite electrodes, which is related to the total ohm resistance of the solution, is 0.2049 and 0.2887 Ω·cm², respectively, indicating a lower internal resistance for the composite electrode after 80cycles.Moreover, the value of R_ct is 1.01Ω·cm²for the composite electrode, which is much lower than that of pure ZnO electrode (2.805Ω·cm²).This can be ascribed to the unique porous network stricture formed inside the composite electrode, which facilitates the transfer of electrons.

Table 1

Fitting parameters of the electrical components with equivalent circuit for the pure and composite electrode after 80 cycles.
Electrode	L	R_a	CPE		R_ct	R_w
Electrode	H·cm^− 2	Ω·cm²	Y₀/S·secⁿ·cm^− 2	n	Ω·cm²	Ω·cm²
Pure ZnO electrode	1.268×10^− 6	0.2887	0.06304	0.6167	2.805	9.392
Composite electrode	9.827×10^− 7	0.2049	0.03599	0.7282	1.010	13.18

The cell performance of electrodes was scrutinized by assembling Zn-air batteries, and their electrochemical properties are presented in Fig. 3. To be specific, Fig. 3 (a) depicts the typical galvanostatic charge-discharge curves of zinc-air batteries with pure ZnO and ZnO/PVA/CHI tested at the first circle. The pure and composite electrodes have nearly the same charge and discharge plateau at the beginning. However, as shown in Fig. 3 (b), the composite electrode exhibited a higher discharge plateau, which is about0.2 V higher than that of the pure ZnO electrode after 80 cycles. It also demonstrates a lower charge plateau than pure ZnO electrode. The charge/discharge process of zinc anode is accompanied with redox reaction, dissolution, and electrodepositing. For the charge process, zinc ions are more likely to deposit to the uneven parts of zinc electrode, which causes the shape change and dendrite formation and in turn increase the electrochemical polarization of the ZnO anode. When it comes to the composite electrode, the unique three-dimensional network structure inhibits the diffusion of discharge products. In addition, the complex action of zinc ion by CHI enables zinc ions to be reduced in situ, which maintains the internal stability of the electrode ^[15]. In this situation, the shape change and zinc dendrite formation can be restricted, making the charge voltage plateau more stable. What’s more, lower polarization can enhance the utilization of active material and thus increase the discharge voltage plateau.

For the practical applications of zinc-air battery, the charge/discharge efficiency is the key performance indicator. As shown in Fig. 4, the pure ZnO electrode exhibit a little higher efficiency at the initial5cycles compared with the ZnO/PVA/CHI composite electrode, which can be ascribed to the fact that the content of active material (ZnO) in the pure zinc electrode is higher than that composite electrode. In despite of its higher discharge efficiency at first several cycles, the current efficiency fades quickly in the subsequent cycles. In comparison with pure ZnO electrode, although the highest discharge efficiency is little lower, it remains over 80% of capacity retention even after 100 cycles, while that of pure electrode can only maintain this level less than 40 cycles. This is explained by the fact that the ZnO/PVA/CHI composite electrode has a large number of pores, and active materials are tightly wrapped, while the pure electrode produces distinct dendrites, which can also be seen from the SEM images mentioned above.

In summary, ZnO/PVA/CHI composite electrode with internal pore structure has been successfully prepared by cross-linking polymerization of PVA and CHI. Compared to the pure ZnO electrode, the composite electrode exhibits a maximum anode dissolution current of 0.34A, which is 4.86 times higher than that of the pure ZnO electrode (0.07A).The battery assembled with ZnO/PVA/CHI electrode exhibits great improvement in charge/discharge efficiency. More importantly, the R_ct of the composite electrode is 1.01Ω·cm² for the composite electrode, which is much lower than that of pure ZnO electrode (2.805Ω·cm²), attesting that the composite electrode possess a more favorable reaction rate. This work is to develop a new type of alkaline copolymer based on PVA, CHI and ZnO composite for high-performance zinc electrode, and the high cycle stability and excellent electrochemical activity indicate that this efficient strategy will pay the way for innovative zinc anode modification.

Acknowledgements

This work was financially supported by Provincial Natural Science Foundation of Hunan (No.2020JJ4236), Key Research Project of Hunan (No.2022NK2042),and the scientific research start-up fund from Hunan Institute of Engineering for the scientific research platform (No.22RC007).

Conflict of Interest

The authors declare that they have no conflict of interest.

Data Availability Statement

The data used to support the findings of this study are included with the article.

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A ZnO/PVA/CHI Composite Electrode for Rechargeable Zinc-air Battery

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Abstract

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1. Introduction

2. Experimental

2.1 Preparation of zinc anode and air cathode

2.3 Electrochemical characterization

3. Results And Discussion

4. Conclusion

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