Comparative study of the performance of α-MnO2 and amorphous manganese dioxide air electrodes for zinc-air batteries

doi:10.21203/rs.3.rs-4874725/v1

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Comparative study of the performance of α-MnO2 and amorphous manganese dioxide air electrodes for zinc-air batteries

https://doi.org/10.21203/rs.3.rs-4874725/v1

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In order to determine the optimal crystalline form of manganese-based catalysts for zinc-air battery cathodes, this paper successfully synthesised nano α-MnO₂ and amorphous manganese dioxide (AMO) materials by hydrothermal and liquid-phase co-precipitation methods, respectively. The results showed that the spherical AMO material had larger specific surface area and more mesopores than the rod-like α-MnO₂. Moreover, AMO have abundant structural defects and short-range ordered atomic arrangements that can enhance the ion diffusion kinetics and improve the catalytic performance of the materials. Through electrochemical tests, it was found that the AMO materials have better catalytic properties compared to α-MnO₂. At a current of 10 mA/cm², its discharge specific capacity reached 575.2 mAh/g, which was 11.1 % higher than that of 517.8 mAh/g for α-MnO₂. In the long-cycle test, the initial round-trip efficiency of the electrode prepared of AMO was also better than that of α-MnO₂. However, when the AMO electrodes were charged and discharged for a long time, part of the AMO would be converted to α-MnO₂, which led to a gradual decrease in the cycling stability of the AMO electrodes. Therefore, this paper concludes that AMO materials are superior to α-MnO₂ as catalysts for zinc-air batteries.

Zinc-air battery

Manganese dioxide

Catalyst

Microstructure

Electrochemical performance

With the increasing shortage of non-renewable energy sources, research and development of new green energy sources has become an important priority[1-2]. Among them, zinc-air batteries have been widely researched due to their large theoretical specific energy, low cost, reliability, and safety [3]. However, zinc-air batteries suffer from slow kinetics during the oxidation reaction, which leads to a deficiency in the electrochemical performance of zinc-air batteries, limiting the further business application of zinc-air batteries. For this reason researchers have improved the charging and discharging performance and battery life by developing bifunctional catalysts with excellent ORR/OER performance [4-7].

Among the many catalysts for the air electrode of zinc-air batteries, manganese-based catalysts have the advantages of high activity, low manufacturing cost, and environmental friendliness, so they have attracted extensive attention from researchers [8]. Among them, manganese dioxide(MnO₂) has been highlighted for its unique structure and excellent physicochemical properties. The electrochemical properties of MnO₂ as a catalyst depend on its crystal structure and physical morphology [9]. MnO₂ has a variety of crystalline structures due to its compositional characteristics. Therefore, the crystalline structure of manganese dioxide directly affects its catalytic performance as a catalyst for air electrode.

Huang et al [10] prepared α-MnO₂, β-MnO₂ and γ-MnO₂ by a simple reaction at low temperatures. It was found that the larger specific surface favoured the oxygen reduction reaction, and the catalytic performance of α-MnO₂ was superior to that of β-MnO₂, and that of β-MnO₂ was superior to that of γ-MnO₂.

Huang et al [11] prepared composite catalysts combining Co₃O₄ with different crystalline forms of MnO₂ by hydrothermal method，and found that，the composite of Co₃O₄ with MnO₂ lowered the oxygen reduction reaction barrier, thus improving the catalytic performance of the composites. It was also found that the catalytic performance of the composite catalysts followed Co₃O₄/δ-MnO₂ > Co₃O₄/α-MnO₂ > Co₃O₄/β-MnO₂.

Meng et al [12] studied and compared the catalytic performance of different crystalline manganese dioxide catalysts and concluded that α-MnO₂ had the best catalytic performance and its catalytic activity followed the pattern of α-MnO₂ > AMO > β-MnO₂ > δ-MnO₂.

Many studies have shown that amorphous manganese dioxide (AMO) has large short-range ordered atomic arrangements and abundant structural defects. The structural defects can serve as storage and catalytically active sites for reversible ions in batteries, which can enhance the diffusion kinetics of ions and can also effectively increase the capacity of cells[13-17]. Moreover, the inherent isotropic behaviour of the amorphous structure can help to release the strain generated by the volume change during the charge/discharge cycling of the MnO₂ electrode, thus reducing the degradation of the electrode material [18]. Theoretically AMO will have better catalytic performance as catalysts. However, only a few literatures had reported that AMO had higher catalytic performance than α-MnO₂ in zinc-air batteries. Therefore it is necessary to firstly compare and study the α-MnO₂ and AMO microstructures and their electrochemical properties, and clarify their catalytic mechanisms in zinc-air batteries. This is important for the later development of manganese-based bifunctional catalysts.

2.1、Experimental materials

All the reactants and solvents are analytical grade and used without any further purification. MnSO₄·H₂O (Shanghai Chemical Reagent General Factory); CTAB (Shanghai Lingfeng Chemical Reagent Co.); KMnO₄ (Sinopharm Chemical Reagent Co. Ltd.); Polyvinylpyrrolidone (PVP) (Sinopharm Chemical Reagent Co. Ltd.); H₂O₂ (Aladdin Reagent (Shanghai) Co.); NH₃·H₂O (Shanghai Lingfeng Chemical Reagent Co.); C₂H₄O₂ (Wuhan Chemical Reagent Co.); Acetylene black (Sinopharm Chemical Reagent Co., Ltd.); PTFE (60 wt.%, Shanghai Chemical Reagent General Factory); Glass fibre diaphragm (battery grade, Whatman, UK); Nickel Foam (Battery Grade, Changde Liyuan New Material Co.).

2.2、Preparation of samples

Preparation of α-MnO₂ [19]: Weighed 0.03 mol of MnSO₄·H₂O dissolved in 100 ml of distilled water and added 4 wt.% of CTAB dispersant in it. During stirring，100 ml of KMnO₄ solution at a concentration of 0.2 mol/L was slowly added. The mixed solution was reacted in a water bath for 4 h at 160 °C. After cooling naturally, dispersing by ultrasonication for 10 min. Wash several times with distilled water. Finally, the samples were dried in an oven at 80 ℃ for 12 h to obtain α-MnO₂ sample.

Preparation of AMO: Weighed 1.975 g of KMnO₄ and dissolved in 100 ml of distilled water with stirring and added 1 wt.% of polyvinylpyrrolidone (PVP). Then slowly added about 30 ml of a 30 % solution of H₂O₂. Adjust the pH with ammonia and glacial acetic acid until the solution is weakly basic. After addition, stirring was continued for 30 min and aging at room temperature for 2 days. The product was filtered, washed repeatedly with deionised water. Then the sample was dried at 80 ℃ and holding for 10 minutes to obtain the AMO sample.

2.3、Preparation of air electrodes

The structure of the self-supported air electrode prepared in this paper was shown in Figure 1. The specific process for preparing the catalytic layer in this figure was: The catalyst, acetylene black and PTFE were configured in a mass ratio of 7:2:1. Added a little anhydrous ethanol and distilled water, mix thoroughly and ultrasonic vibration for 30min. Apply the mixed paste catalyst layer evenly to one side of the air electrode. The specific preparation process for the waterproof and breathable layer in this figure was: Acetylene black and PTFE were prepared in a mass ratio of 9:1. Add a little anhydrous ethanol and distilled water, mix thoroughly and also ultrasonic vibration for 30min. Apply the mixed paste waterproof and breathable layer evenly to the other side of the air electrode. Then put it into the vacuum drying oven dried at 80 ℃ for 1 h, and press it under 5 Mpa for 2 min by using a hydraulic press to get the air electrode.

2.4、Material characterisation

The phase composition of the sample was determined using a GENESIS 2000 XMS X-ray diffractometer from EDAX, USA. The radiation source was CuKa (k = 0.154 nm), scanned continuously at a rate of 10°/ min. The morphology and primary particle size of the samples were characterized using a transmission electron microscope (FEI Talos F200x) from Thermo Fisher, USA. The Raman spectra of the materials were recorded at room temperature using a confocal HR800 Raman microscope from HORIBA JobinYvon, France, at an excitation wavelength of 514 nm. The samples were tested for BET using the ASAP2460 surface area and pore size analyser, from Micromeritics, USA. The electronic valence states and the composition of the catalyst surfaces after charging and discharging were analysed using an X-ray photoelectron spectrometer, Thermo Scientific K-Alpha, Thermo Fisher Scientific, USA.

2.5 Battery assembly and electrochemical performance testing

The simulated battery prepared in this paper was a box structure assembled with Plexiglas plates as shown in Figure 2. The air electrode had a contact area of 1 cm² with the air, while the distance between the air electrode and the negative electrode of zinc flake (0.3 mm) was about 8 mm. A glass fibre diaphragm was used between the two electrodes and injected with the prepared ZnO saturated 6 M KOH + 0.2 M (CH₃COO)₂Zn alkaline electrolyte, which was then ready for electrochemical performance measurements. Using Shanghai Chenhua CHI614C electrochemical workstation, air electrode as working electrode, pure nickel strip as auxiliary electrode and Hg/HgO as reference electrode to form a three-electrode system. The three-electrode system was tested by placing it in a 0.1 mol/L KOH electrolyte. The test parameters of EIS were: scanning speed 10⁵Hz~10^-2Hz, amplitude 5mV. The test parameters of CV were: voltage -0.8V~0.8V, scanning speed 1mV/s.

Using Neware Battery Tester to perform discharge specific capacity test and cycle performance test of assembled batteries. The polished zinc sheet (0.15mm) was used as the negative electrode of the battery and the prepared air electrode was used as the positive electrode. Zinc sheets were weighed before testing and discharged at a constant current of 10 mA/cm². At the end of the test the discharged zinc tablets were weighed to obtain the amount of active substance consumed. Before cycling performance test, the batteries were rested for 5 min and then charged and discharged using a constant current of 10 mA/cm² , and the batteries would have a resting time of 1 min between the charging and discharging processes.

3.1 Microstructure comparison between α-MnO₂ and AMO

3.1.1 XRD characterisation of α-MnO₂ and AMO

Figure 3 showed the XRD patterns of α-MnO₂ and AMO. As shown in Figure 3-1, the characteristic diffraction peaks of α-MnO₂ appeared at 2θ = 28.69°, 37.51° and 56.08°, which corresponded to the (310), (211) and (521) crystal faces of α-MnO₂ (PDF#44-0141), respectively. While AMO only had two bun peaks at 2θ = 36.9° and 66.3°, the bun peak around 37° was the characteristic peak of aqueous manganese oxide. This indicated that the sample did not form a long-range ordered crystal structure, had very low crystallinity and would be amorphous manganese dioxide.

3.1.2 TEM characterization of α-MnO₂ and AMO

Figure 4 showed the TEM images of the prepared (a) α-MnO₂ and (b) AMO. As shown in the figure, the prepared α-MnO₂ was a nanorod-like crystal with pores and uniform thickness, while the prepared AMO was a nanoscale spherical product with abundant pores. Through Nanomeasure statistics, it could be determined that the average rod length of α-MnO₂ reached 490 nm and the average rod diameter reached 13 nm, while the average particle size of the prepared AMO was 117 nm.

3.1.3 BET characterization of α-MnO₂ and AMO

Figure 5 showed the comparison of (a) nitrogen adsorption and desorption curves and (b) pore size distribution of α-MnO₂ and AMO, and Table 1 showed the results of BET analysis of α-MnO₂ and AMO. As shown in Figure 5, the nitrogen adsorption and desorption curves of the five samples did not completely coincide, and all had H3-type hysteresis lines. And the pore size distribution of both materials was wide, indicating that both materials had porous structure. From Table 3-1, it could be seen that the specific surface area of AMO was 115.69 m²/g, which was 230.06 % higher compared to 34.99 m²/g with α-MnO₂. And AMO had larger pore volume and average pore size with narrower pore size distribution, which made AMO materials have more mesoporous pore structures in the 20-50 nm range. This pore structure was more favourable for oxygen transport, thus improving the catalytic properties of the material.

Table 1 BET analysis results of α-MnO₂ and AMO

Materials	Specific surface area（m²g^-1）	Pore volume（cm³g^-1）	Average pore size（nm）
α-MnO₂	34.99	0.073	8.381
AMO	115.69	0.192	6.632

3.1.4 Raman characterization of α-MnO₂ and AMO

Figure 6 showed the Raman spectra of α-MnO₂ and AMO. As shown in the figure, both samples had only one peak between 500-700 nm, which represented the stretching of Mn-O in the [MnO₆] octahedron. The Mn-O stretching vibration peak of AMO was at 623.2 cm^-1, which was blueshifted compared to 639.5 cm^-1 for α-MnO₂, suggesting that the structure of AMO was not as stable as α-MnO₂. And the AMO material had more oxygen vacancy defects, which led to more catalytically active sites in the AMO cathode. In addition, the Mn-O stretching vibration peaks of AMO were significantly wider and less intense than those of α-MnO₂, which indicated that AMO had a low degree of crystallinity, which was consistent with the results in Figure 3-1.

3.1.5 XPS characterization of α-MnO₂ and AMO

Figures 7 showed the full XPS spectra, Mn2p spectrograms and O1s spectrograms of α-MnO₂ and AMO materials. From the Mn2p spectra in Figure 3-5, it could be seen that the Mn2p peaks of α-MnO₂ were centred at 653.88 and 642.18 eV, respectively, with a spin spacing of 11.7 eV. The Mn2p peaks of AMO were centred at 654.08 and 642.18 eV, respectively, with a spin spacing of 11.9 eV, which was slightly larger than that of α-MnO₂. The Mn⁴⁺ peak area of the AMO material was slightly smaller than that of α-MnO₂, suggesting that the structural stability of the AMO material might not be as good as that of α-MnO₂. This corresponded to the Raman test above. From the O1s spectrograms, it could be seen that O^2-, O vacancies and H₂O were present in both materials. O^2- was the oxygen ion bound to the metal and H₂O was the water adsorbed on the surface of the material.

3.2、Comparison of electrochemical properties of α-MnO₂ and AMO

3.2.1 AC impedance (EIS) test of α-MnO₂ and AMO

Figure 8 showed the Nyquist curves of α-MnO₂ and AMO. As can be seen from the figure, their Nyquist curves could be divided into two parts: the high-frequency region was semicircular, controlled by the kinetics, and formed by the charge-transfer impedance inside the electrodes. The low frequency region was approximated as a straight line and was the Warburg impedance. The AC impedance curves of α-MnO₂ and AMO had similar shapes, suggesting that the redox reaction at the solution interface with both catalysts was the same process. The presence of the second arc in the first quadrant indicated that the reaction between the electrolyte and the catalyst was a stepwise process. The process was as follows:

O₂+e^-→O₂^-

O₂^-+3e^-+HO₂→4OH^-

The process of O₂ getting an electron to transform into O₂^- corresponds to the high frequency region in Figure 8. And the process of O₂^- getting three electrons to convert to OH^- corresponds to the low frequency region in Figure 8. From the figure, it could be seen that the AMO material had a larger Warburg impedance and therefore AMO had better catalytic properties.

3.2.2 CV test of α-MnO₂ and AMO

Figure 9 showed the CV curves of α-MnO₂ and AMO. Both materials showed an oxidation peak around 0.4 V and a reduction peak around -0.5 V. It was more likely that this peak corresponds to redox of the MnO₂ itself (as a cation de-insertion reaction). The corresponding reduction peak was likely attributed to some combination of oxygen reduction and MnO₂ redox (cation insertion). For further comparison, it could be clearly found that the area enclosed by AMO was larger than the area enclosed by α-MnO₂. This indicated that the AMO cathode cell might have a higher discharge specific capacity than α-MnO₂cathode.

3.2.3 Comparison of discharge curves of α-MnO₂ and AMO electrodes

Figure 10 showed the comparative discharge curves of zinc-air batteries assembled with air electrodes prepared from α-MnO₂ and AMO. From the figure, it could be seen that the open-circuit voltage of the α-MnO₂ air electrode was 1.40 V, while the open-circuit voltage of AMO reached 1.63 V, which was significantly increased. The discharge specific capacity of the α-MnO₂ air electrode was 517.8 mAh/g. The discharge specific capacity of the AMO air electrode reached 575.2 mAh/g, which was 11.1 % higher than the α-MnO₂ air electrode. The analysis was due to the fact that AMO had more catalytically active sites due to more defects (see Figure 6) and therefore had a higher open-circuit voltage. The increase in open-circuit voltage could effectively prolong the discharge time, which led to an increase in its discharge specific capacity.

3.2.4 Comparison of the cycling performance of α-MnO₂ and AMO electrodes

Figures 11 showed a comparison of the long cycle performance of zinc air batteries assembled with air electrodes prepared from α-MnO₂ and AMO. It could be seen that both material air electrodes were able to perform long cycles. The electrode prepared with α-MnO₂ had an initial round trip efficiency of 65.6 %. The initial round-trip efficiency of the AMO-prepared electrode reached 77.4 %, which was 18.0 % higher than that of α-MnO₂. The AMO prepared electrodes maintained good round trip efficiency for the first 600 cycles, but the subsequent round trip efficiency decreased dramatically to 30.7 % at 2200 cycles. The analysis might be attributed to the prolonged charging and discharging which resulted in a crystalline transition of the AMO material, leading to a significant decrease in the catalytic stability of the material.

In order to further analysed the reason for the decrease in round trip efficiency of α-MnO₂ and AMO materials, the physical phase composition of the electrodes surface after long cycling were tested. Figure 12 showed the XRD patterns of α-MnO₂ and AMO electrodes after 2000 cycles. From this figure, it could be seen that ZnCO₃, KCO₃ and ZnO were present on the surface of both electrodes after long cycles. The presence of ZnCO₃ and KCO₃ was due to the electrodes involved in the reaction with CO₂ from the air. ZnO was formed due to the combination of oxygen in the air with Zn²⁺. This led to passivation of the material surface and affected the catalytic properties of the material. Mn₃O₄ was due to the disproportionation of Mn³⁺ which formed Mn₃O₄, an electrochemically inert substance. Further analysis showed that diffraction peaks of α-MnO₂ existed on the surface of the electrode after 2000 cycles of the AMO electrode. This indicated that some of the AMO had been transformed to α-MnO₂ during the long charge/discharge cycles, which led to a significant decrease in its round-trip efficiency after 2000 cycles. This also further proved that the catalytic performance and stability of AMO electrode in zinc-air batteries are higher than that of α-MnO₂ electrode.

Figure 13 showed the comparison of the full XPS spectra, Mn2p spectrograms and O1s spectrograms of the α-MnO₂ electrode and the AMO electrode after cycling for 2000 cycles. From the full XPS spectra (a, d), it could be seen that the elements K and Zn were also present on the surface of both electrodes after long charging and discharging cycles, indicated the presence of compounds of these two elements. From the Mn2p spectra (b, e), it was obvious that the Mn⁴⁺ peak areas of both materials were much reduced compared to the uncharged and discharged electrodes (Figure 7). This indicated that the stability of the material became worse, which was due to passivation and deformation of the material due to prolonged charging and discharging. As can be seen from the O1s spectrograms, O^2- and H₂O were present in both electrode materials, and O vacancies were absent or present in very small amounts. O^2- was the O element in metal oxides and H₂O was the water in the air adsorbed by the electrodes. This was also further proved that the abundance of oxygen defects in AMO materials was important to improve the catalytic performance of their electrodes.

(1）Nano-rod-like α-MnO₂ and spherical nano AMO materials were successfully prepared, and the microstructure study of the two materials showed that，AMO materials had a larger specific surface area and more mesoporous with a narrow pore size distribution, which was favourable for the transport of ions and air. Furthermore, AMO materials had rich structural defects and short-range ordered atomic arrangements, which can enhance the ion diffusion kinetics and improve the catalytic performance of the materials.

(2）The study of the electrochemical properties of the two electrodes showed that, the AMO electrode had greater Warburg impedance and better redox properties. The AMO electrode had an open-circuit voltage of 1.63 V, which was higher than that of the α-MnO₂ air electrode of 1.40 V. The AMO air electrode achieved a discharge specific capacity of 575.2 mAh/g, which was 11.1 % higher than that of the α-MnO₂ air electrode of 517.8 mAh/g.

(3）The study of the long cycle performance of the two electrodes showed that, the AMO prepared electrode achieved a round-trip efficiency of 77.4 % at the initial time, which was 18.0 % higher than that of the α-MnO₂ prepared electrode. It was found that some of the AMO materials would transformed to α-MnO₂ after prolonged charging and discharging. The crystalline transition of AMO materials led to a decrease in the cycling stability of AMO electrodes.

Therefore, in zinc-air batteries, this paper considered that AMO materials have better catalytic performance as catalysts compared to α-MnO₂.

Acknowledgments

This work was supported by Jiangsu University Advantage Discipline Construction Project (Jiangsu Gov. Office issued 2018-10) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Data availability statement

The authors will supply the relevant data in response to reasonable requests.

Conflict of interest disclosure

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

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You are reading this latest preprint version

Comparative study of the performance of α-MnO2 and amorphous manganese dioxide air electrodes for zinc-air batteries

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Abstract

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1、Introduction

2、Experimental materials and methods

3、Results and Discussion

4、Conclusion

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