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Anchoring Mn3O4 Nanoparticles on Oxygen Functionalized Carbon Nanotubes as Bifunctional Catalyst for Rechargeable Zinc-Air Battery Laiquan Li, Jun Yang, Hong bin Yang, Liping Zhang, Jinjun Shao, Wei Huang, Bin Liu, and Xiaochen Dong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00009 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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ACS Applied Energy Materials

Anchoring Mn3O4 Nanoparticles on Oxygen Functionalized Carbon Nanotubes as Bifunctional Catalyst for Rechargeable Zinc-Air Battery Laiquan Li,a,b Jun Yang,a Hongbin Yang,b,c Liping Zhang,b Jinjun Shao,a* Wei Huang,d Bin Liu,b* Xiaochen Donga* a

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211800, China. b

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62

Nanyang Drive, Singapore 637459, Singapore. c

Institute for Materials Science and Devices, Suzhou University of Science and Technology,

Suzhou, 215000, China. d

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University

(NPU), 127 West Youyi Road, Xi'an 710072, China.

E-mail:

[email protected]

(X.

C.

Dong);

[email protected]

[email protected] (J.J. Shao)

1

ACS Paragon Plus Environment

(B.

Liu);

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Transition metal oxide hybridized with carbon is promising for multi-functional electrocatalysis. In this work, Mn3O4 nanoparticles were embedded onto oxygen functionalized carbon nanotubes (Mn3O4/O-CNT) via a facile wet impregnation method followed by oxygen plasma treatment. The O-CNTs not only act as conductive support for Mn3O4 nanoparticles, but also provide catalytically active centers for the oxygen evolution reaction (OER). Benefitted from both the excellent electrocatalytic activity and the separated oxygen reduction reaction (ORR) and OER active sites, the zinc-air battery assembled from the bifunctional Mn3O4/O-CNT electrode exhibits high power density, reaching 86.6 mW cm-2 with discharge capacity up to 827.6 mAh g-1 and super cycling stability, which can be stably charged and discharged over as long as 150 hours at 2 mA cm-2. Our work demonstrates an innovative design for stable bifunctional catalysis in renewable energy applications. Keywords: Mn3O4, Carbon nanotube, Oxygen reduction, Oxygen evolution, Bifunctional electrocatalyst, Rechargeable Zn-air battery

INTRODUCTION Rechargeable zinc (Zn)-air battery has received great attention in energy storage because of its high capacity, low cost, and good environmental compatibility.1-4 The overall efficiency of a Zn-air battery is limited by the sluggish kinetics of electrocatalytic reduction and evolution of molecular oxygen.5-10 Noble metals, such as Pt and Ir, are so far the most efficient electrocatalysts for ORR and OER, but they are scarce and very expensive.11 2


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Therefore, it is highly desirable to develop low cost alternatives with comparable activity and durability to replace the noble metal based catalysts. Among many alternative replacements, manganese oxides are promising.12-14 For example, Mn3O4 is able to effectively catalyze ORR in alkaline medium, resulting from its variable valence states and coordination structure.15-17 However, the electrical poor conductivity (10-7-10-8 S cm-1) and large volume change of Mn3O4 during the cycles severely limit its application potential in energy conversion and storage devices.12 To address this problem, several strategies have been employed including morphology or crystal structure engineering,15, 18-19 doping or coating with other metals,20-22 as well as defect engineering.23 Although these methods are effective, they are typically complicated and time consuming. Hybridizing with conductive carbonaceous material (e.g. reduced graphene oxide,15,

24

carbon black,25 or carbon nanotube26-27) offers another facile and effective approach to enhance the electrocatalytic performance of manganese oxides. However, previous researches only focused on single catalytic function, ignoring the fact that the carbonaceous material could also provide different catalytic active sites besides serving as a conductive support. Herein, we report a facile wet impregnation method coupled with oxygen plasma to anchor Mn3O4 nanoparticles onto CNTs as well as to functionalize CNTs with oxygen functional groups. CNTs not only provide nucleation sites to grow and embed ultra-small Mn3O4 nanoparticles, but also electrically connect them. The oxygen functional groups on CNTs from oxygen plasma treatment shall supply plenty of active sites for OER. Therefore, the hybrid Mn3O4/O-CNT exhibits both excellent ORR and OER activities. The Zn-air battery 3



assembled from the Mn3O4/O-CNT electrode is able to produce a peak power density of 86.6 mW cm-2 with discharge capacity up to 827.6 mAh g-1, which remains stable after 150 hours of continuous charging and discharging at 2 mA cm-2. EXPERIMENTAL SECTION CNT pretreatment: to carboxylate CNTs as well as to remove metal impurities, the as-received CNTs (NanoLab Inc.) were refluxed in a mixed solution containing 10 ml of 65 wt.% HNO3 and 30 ml of 98 wt.% H2SO4 at 70 °C for half an hour. Synthesis of hybrid Mn3O4/O-CNT: In a typical synthesis, 50 mg of purified CNTs, 0.245 g of Mn(CH3COO)2·4H2O and 0.024 g of LiOH were added into 10 mL of anhydrous ethanol under vigorous stirring to form homogeneous Mn2+ emulsion. The suspension was then transferred to a 30-mL glass flask followed by ultrasonication for 5 h. Afterwards, the resultant solid product was separated, washed by deionized water and ethanol, dried, and calcined at 300 °C in air to obtain hybrid Mn3O4/CNT. Mn3O4 nanoparticles (Mn3O4 NP) were prepared by the same procedure without adding CNTs. To functionalize CNTs with oxygen-containing functional groups, oxygen plasma treatment was conducted. Briefly, 20 mg of Mn3O4/CNT was paved onto a slice of glass substrate and placed into the chamber of a plasma cleaner (Harrick plasma, PDC-32G, 115 V) filled with oxygen. The Mn3O4/O-CNT was obtained after O2 plasma treatment at a pressure of 700 mTorr for 20 min. Materials characterization: the structural and morphological information were examined by X-ray powder diffraction (XRD) with Cu Kα radiation (Bruker AXS D8, λ = 1.5406 Å) and transmission electron microscopy (TEM, JEOL JEM-3010). Raman spectroscopy was 4


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performed on a Renishaw RM1000 microscope using a 514.5 nm excitation laser. Fourier transform infrared spectroscopy (FTIR) spectra were obtained on PerkinElmer Spectrum One FTIR Spectrometer. Nitrogen adsorption/desorption isotherms were collected on an accelerated surface area and porosimetry system (ASAP 2020) at -196 °C. Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. Chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific) at 2.4 × 10-10 mbar using a monochromatic Al Kα X-ray beam (1486.60 eV). All binding energies were referenced to the C 1s peak (284.60 eV). Electrochemical measurements: the electrochemical characterization was carried out in 1 M oxygen-saturated KOH solution on a CHI 760D electrochemical workstation. The three-electrode configuration consists of a saturated calomel electrode (filled with saturated KCl) as the reference electrode, a Pt foil as the counter electrode and a catalyst loaded glassy carbon electrode as the working electrode. The catalyst ink was prepared by dispersing the active material (5 mg) in a mixture of isopropanol (480 µL) and DI water (480 µL), followed by addition of 40 µL of 5 wt.% Nafion (Nafion 117, DuPont) and sonicated for 3 h. To prepare the working electrode, 6 µL of the catalyst ink was drop-casted onto a polished glassy carbon (GC) rotating disk electrode (RDE) and allowed to dry overnight at room temperature, giving a catalyst loading of 0.15 mg cm-2. The polarization curves were recorded at a scan rate of 5 mV s-1 and 1600 rpm and corrected with iR-drop compensation. All potentials reported in this study were referenced to the reversible hydrogen electrode (RHE) according to the Nernst equation (ERHE = ESCE + 5



0.241+ 0.0592×pH). Calculation: Koutecky-Levich plot (J-1 vs ω-1/2) was used to calculate the number of electrons transferred (n). The rotating disk electrode measurement was conducted at a scan rate of 5 mV s-1 under various rotation speed (400, 625, 900, 1225, 1600, and 2025 rpm, Pine Instrument Co.). The number of electrons transferred (n) can be obtained from the slope of the Koutecky-Levich plot based on the Koutecky-Levich equation: 1 1 1 1 1 = + = + , = 0.62 ⁄ ∙ ⁄ ∙ (1) J B ∗ √ where J is the measured current density; JL and JK are the diffusion-limiting current density, and the kinetic-limiting current density, respectively; B is the reciprocal of the slope; ω is the rotation speed in rpm; F is the Faraday constant (96485 C mol-1), D is the oxygen diffusion coefficient in 1 M KOH (1.3 × 10-5 cm2 s-1); υ is the kinetic viscosity (5.45×10-3 cm2 s-1), and C0 is the bulk concentration of oxygen (9.3 × 10-7 mol cm-3). Rotating ring-disk electrode (RRDE) measurement was performed to determine the total electron-transfer number (n) and hydrogen peroxide yield (%H2O2) based on: n=

4$%&'( (2) $)&*+ ⁄, + $%&'(

%. / =

2$)&*+ /, × 100% (3) $)&*+ ⁄, + $%&'(

Where Idisk and Iring are the voltammetric currents at the disk and ring electrode, respectively. N is the RRDE collection efficiency, which was determined to be 0.31. Assembly of rechargeable Zn-air battery: rechargeable Zn-air battery in the two-electrode configuration was assembled according to the following procedure: first, a piece of pre-treated carbon cloth was dipped into a bottle filled with 3 ml catalyst ink (2.5 mg ml-1), followed by gently shake overnight. This process was repeated twice until the catalyst 6


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loading reach about 0.5 mg cm-2. A slice of gas diffusion layer (GDL, AvCarb P75T, Fuel Cell Store) was then attached to the catalyst loaded carbon cloth. Subsequently, the hybrid electrode was used directly in the rechargeable Zn-air battery. The electrolyte used was 6 M KOH filled with 0.2 M ZnCl2. RESULTS AND DISCUSSION The hybrid Mn3O4/O-CNT catalyst was prepared by a facile wet impregnation method followed by O2 plasma treatment as schematically displayed in Scheme 1. Mn2+ ions were first adsorbed on the surface of carbon nanotubes through carboxylate functional groups (Figure S1) under continuous ultrasonication. Subsequently, the Mn2+ species on CNTs were oxidized to form Mn3O4 nanoparticles when calcined in air at 300 oC. Finally, active sites of OER were implanted onto the Mn3O4 nanoparticles decorated CNTs by an oxygen plasma treatment, resulting an OER and ORR bifunctional catalyst. Figure 1a shows a TEM image of the hybrid Mn3O4/O-CNT, from which, it is clear to see that the Mn3O4 nanoparticles are uniformly distributed on the surface of CNTs. The size of Mn3O4 nanoparticles is uniform in the range of 7 to 9 nm (inset of Figure 1a). Removing CNTs during synthesis significantly increases the particle size of Mn3O4 (Figure S2a). The Mn3O4 nanoparticles are tightly embedded onto the CNTs as shown in the high resolution TEM image (Figure 1b), which provides strong interaction with the CNT support. The interplanar spacing of 0.49 nm and 0.27 nm can be indexed to the (101) and (103) planes of hausmannite Mn3O4. The crystal structure of the as-prepared samples was analyzed by XRD. Figure 1c shows the XRD pattern of Mn3O4/O-CNT, in which all diffraction peaks well match with those from the JCPDS card No. 24-0734, indicating formation of 7



tetragonal hausmannite Mn3O4.31 The diffraction peak at 2θ = 26° can be indexed to the (002) plane of CNTs,32 which is identical to that of the pristine CNTs (violet line in Figure 1c), suggesting that oxygen plasma treatment did not damage the crystal structure of CNTs. To further explore the structural characteristics, Raman spectra were collected. As shown in Figure 1d, the Raman peak at 633 cm-1 assigned to the A1g symmetric stretching of M-O bond of the MnO6 octahedra appears on both spectra of Mn3O4/O-CNT and Mn3O4,33-34 while no such signal is observed at the same position in the Raman spectrum of CNTs, demonstrating the growth of spinel Mn3O4 on CNTs. Furthermore, both Mn3O4/O-CNT and pristine CNTs show similar D and G bands, which further supports the fact that oxygen plasma exerts little change on the bulk structure of CNTs. To probe detailed surface states, XPS measurements were performed. Figure 2a compares the XPS survey spectra of Mn3O4/O-CNT and CNTs, in which, Mn3O4/O-CNT shows additional Mn 2p peaks. The high-resolution Mn 2p spectrum as displayed in Figure 2b gives a Mn 2p3/2-2p1/2 doublet at 642.0 and 653.8 eV with a splitting width of 11.8 eV, which is consistent with the reported Mn3O4.35-36 Figure 2 c and 2d shows the C 1s spectra of Mn3O4/O-CNT and Mn3O4/CNT, which could be deconvoluted into four components: C-C/C=C (284.6 eV), C-O (285.9 eV), C=O (287.8 eV) and O-C=O (289.9 eV).37 Notably, the relative content of C=O in Mn3O4/O-CNT is ca. 7.1 at.% (Figure 2c & Table S1), which is obviously higher than that in pristine CNTs (4.9 at.%, Figure 2d & Table S1). Similarly, the O2 plasma treated CNTs (O-CNT) possesses a higher concentration of surface C=O group than the pristine CNT (Figure S3 & Table S1). The increase in the C=O content should be associated with the O2 plasma treatment.38 8


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The ORR activity of hybrid Mn3O4/O-CNT catalyst was evaluated on rotating disk electrode (RDE) in 1 M oxygen saturated KOH aqueous solution. For comparison, polarization curves of pristine CNTs, Mn3O4 nanoparticles and commercial 20 wt.% Pt/C were also collected. The Mn3O4/O-CNT exhibits an ORR onset potential and a half-wave potential (E1/2) at 0.92 and 0.85 V (vs. RHE), respectively, which are very close to the values of commercial Pt/C (0.95 and 0.87 V vs. RHE) and remarkably better than those for pristine CNTs and Mn3O4 nanoparticles. The ORR limiting current density reaches 3.26 mA cm-2 for Mn3O4/O-CNT (at 0.5 V vs. RHE), which is also comparable to that of Pt/C (3.38 mA cm-2) but much higher than the ORR current density for CNTs (1.85 mA cm-2) and Mn3O4 nanoparticles (2.80 mA cm-2) (Figure 3b). It should be noted that the hybrid catalyst showed similar ORR activity before and after O2 plasma treatment (Figure S4), suggesting negligible influence of the O2 plasma on Mn3O4 nanoparticles. To further study the

ORR

kinetics,

rotating

ring-disc

electrode

(RRDE)

measurements

and

Koutecky-Levich analyses were conducted. As shown in Figure S5, the H2O2 yield on Mn3O4/O-CNT electrode during ORR is below 5%, which is comparable to that of the commercial 20 wt.% Pt/C. The electron transfer number for the ORR process on the Mn3O4/O-CNT catalyst was further confirmed using Koutecky-Levich plots. Figure 3c shows the linear sweep voltammetry (LSV) curves of Mn3O4/O-CNT measured at rotating speed from 400 to 2025 rpm. All LSV curves display obvious current plateau resulting from limited oxygen diffusion. Based on the limiting current density, the Koutecky-Levich plots at various potentials can thus be obtained (Figure 3d), which give the electron transfer number to be ~3.95, suggesting a four-electron ORR pathway for the hybrid 9



Mn3O4/O-CNT catalyst (/ + 43 + 2. / → 4/. ). Besides good catalytic activity, long-term stability and high methanol tolerance are also critical for practical applications of ORR catalysts. Herein, the catalytic stability of Mn3O4/O-CNT was evaluated by chronoamperometry. Figure 3e displays the chronoamperometric response of Mn3O4/O-CNT and Pt/C for ORR at 0.7 V (vs. RHE) with a rotation speed of 900 rpm in 1 M KOH solution. After 10,000 s of continuous operation, the Mn3O4/O-CNT maintained 93% of its initial ORR current density, while commercial Pt/C suffered from 20% decay in ORR activity. After long-time stability test, it was noticed that Pt/C experienced a remarkable activity loss with a much more negative half-wave potential, while almost no variation in half-wave potential was observed on Mn3O4/O-CNT (Figure 3f). Additionally, the hybrid Mn3O4/O-CNT catalyst also exhibited much improved methanol tolerance as compared to the commercial Pt/C (inset in Figure 3e). Besides ORR, we also tested the OER performance of Mn3O4/O-CNT in 1 M KOH aqueous solution (Figure 4). Among various catalysts, the hybrid Mn3O4/O-CNT catalyst shows the highest oxygen-evolution current density at all anodic potentials (Figure 4a). The overpotential required to drive a current density of 10 mA cm-2 for Mn3O4/O-CNT is 410 mV, which is obviously smaller than that for Mn3O4-CNT (470 mV), CNTs (470 mV) and Mn3O4 nanoparticles (550 mV). It is noteworthy to mention that the OER activity of CNTs could be significantly enhanced by O2 plasma treatment (Figure S9), resulting from the increased contents of electron-withdrawing C=O functional groups on the surface of CNTs (Figure 2c and d), which was found to be crucial in altering the electron clouds around the 10


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adjacent carbon atoms and facilitating the adsorption of OER intermediates.29,39,40 In this way, we have developed a bifunctional electrocatalyst with separated ORR and OER active sites. Additionally, long-term OER stability of Mn3O4/O-CNT was also tested at a fixed potential of 1.65 V vs. RHE. As shown in Figure 4c, the Mn3O4/O-CNT electrode exhibits superior durability with hardly any current decay over 30 hours of continuous electrolysis, which is consistent with the nearly overlapped polarization curves before and after the durability test (Figure 4d). To

further

understand

the

electrocatalytic

performance

of

Mn3O4/O-CNT,

electrochemically active surface area (ECSA) was measured based on the double-layer method as shown in Figure S11. ECSA can be determined by electrochemical double-layer capacitance (Cdl).41-42 The calculated Cdl of Mn3O4/O-CNT, CNTs and Mn3O4 NP are 11.7, 7.1 and 3.6 mF cm-2, respectively. Obviously, Mn3O4/O-CNT possesses much larger active surface area than CNTs and Mn3O4 nanoparticles, which shall provide more efficient contact between catalyst and the electrolyte.43 The higher ECSA of the hybrid catalyst should originate from its larger surface area (Figure S12). Additionally, the strong interaction between Mn3O4 nanoparticles and O-CNTs can also boost electron transfer during oxygen evolution/reduction processes and benefit the electrochemical stability. To demonstrate the practical application potential of the Mn3O4/O-CNT catalyst in energy storage and conversion, rechargeable Zn-air battery was assembled, which consists of a Zn electrode, a separator and an air electrode made of Mn3O4/O-CNT bifunctional catalyst (Figure S13). Figure 5a displays the open-circuit voltage (OCV) of the Zn-air battery, from which, the OCV of the Zn-air battery with Mn3O4/O-CNT air electrode reaches as high as 11



1.45 V, which is even higher than the one assembled from the state-of-the-art Pt/C + Ir/C catalyst (1.42 V). Figure 5b shows the polarization and power density curves, which show that the Mn3O4/O-CNT electrode can supply a charging and discharging current density of 50 mA cm-2 at 2.33 and 0.88 V, respectively, comparable to the Pt/C + Ir/C (2.31 and 0.94 V) air electrode. The peak power density of the Zn-air battery with Mn3O4/O-CNT electrode reaches to 86.6 mW cm-2, higher than that of Pt/C + Ir/C electrode (73.4 mW cm-2). Galvanostatic discharge measurement was conducted at current densities of 5, 10 and 20 mA cm-2. As shown in Figure 5c, the discharge voltages at all current densities were maintained stable during the discharging process. Notably, the discharging duration at current density of 5 mA cm-2 is nearly four times longer than that at the current density of 20 mA cm-2, demonstrating the excellent rate performance. The specific capacity normalized to the mass of zinc for the Zn-air battery with Mn3O4/O-CNT electrode reaches as high as 827.6 mAh gZn-1 (Figure 5d). Furthermore, the battery also showed good rechargeability, which was tested by galvanostatic charging and discharging at the current density of 2 mA cm-2. No obvious increase of the voltage gap between the charging and discharging curves was observed throughout more than 150 cycles of continuous charging/discharging processes over a period of 150 h. Even at high current densities (e.g. 10 mA cm-2), the Zn-air battery with Mn3O4/O-CNT electrode could be repeatedly charged and discharged, which showed better stability than the one assembled from the Pt/C + Ir/C electrode (Figure S14). CONCLUSION In summary, we have developed a facile scalable wet impregnation method coupled with 12


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oxygen plasma to anchor Mn3O4 nanoparticles on oxygen functionalized CNTs (Mn3O4/O-CNT) as a bifunctional OER and ORR electrocatalyst. The oxygen plasma treated CNTs not only act as conductive support for Mn3O4 nanoparticles, but also at same time provide catalytically active centers for OER, resulting in separated OER and ORR active sizes. The Zn-air battery thus assembled from the bifunctional Mn3O4/O-CNT electrocatalyst could be stably charged and discharged over as long as 150 hours at 2 mA cm-2. During discharging, the device exhibited an open-circuit voltage of 1.45 V, a peak power density of 86.6 mW cm-2 and a high specific capacity of 827.6 mAh g-1. This work demonstrated a facile approach to construct bifunctional high-performance catalyst for energy storage and conversion applications. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Additional experimental results of the performance and characterization of catalysts, including Figures S1-S14 and Tables S1. AUTHOR INFORMATION Corresponding Authors (X.C.D) E-mail: [email protected] (B.L) E-mail: [email protected] (J.J. S) E-mail: [email protected] ORCID: Xiaochen Dong: 0000-0003-4837-9059 Notes 13



The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by NNSF of China (61525402, 5161101159), Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: RG10/16 and RG111/15, Singapore A*Star Science and Engineering Research Council-Public Sector Funding (PSF): 1421200075.

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Scheme 1. Schematic illustration showing the formation process of Mn3O4/O-CNT.

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Figure 1. (a, b) TEM and HRTEM images of the as-prepared Mn3O4/O-CNT. Inset shows the size histogram. (c) XRD patterns of Mn3O4/O-CNT (red) and pristine CNTs (violet). (d) Raman spectra of Mn3O4/O-CNT, CNTs and Mn3O4 nanoparticles.

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Figure 2. (a) XPS spectra for Mn3O4/O-CNT and CNT. (b) Mn 2p spectrum. (c, d) C1s spectra of Mn3O4/O-CNT and Mn3O4/O-CNT.

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Figure 3. ORR performance. (a) LSV curves collected at a scan rate of 5 mV s-1. (b) Comparison of ORR limiting current density at 0.5 V (vs. RHE). (c) LSV curves of Mn3O4/O-CNT collected on RDE in O2-saturated 1 M KOH at different rotation speed from 400 to 2025 rpm with a scan rate of 5 mV s-1. (d) Koutecky-Levich plot of Mn3O4/O-CNT at different applied potential from 0.3 to 0.7 V. (e) Chronoamperometric response of 24


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Mn3O4/O-CNT and Pt/C at 0.7 V (vs. RHE) with rotation speed of 900 rpm in 1 M KOH. Inset shows the methanol crossover effect at 0.7 V vs. RHE. (f) LSV curves of the Mn3O4/O-CNT and Pt/C electrodes before and after stability tests.

Figure 4. (a) LSV curves collected on RDE for Mn3O4/O-CNT, Mn3O4-CNT, CNTs and Mn3O4 nanoparticles in O2-saturated 1 M KOH at a rotation speed of 1600 rpm and a scan rate of 5 mV s-1. (b) The corresponding Tafel plots. (c) Chronoamperometric responses at a fixed potential of 1.65 V vs. RHE. (d) LSV curves collected before and after long-term stability tests for Mn3O4/O-CNT.

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Figure 5. (a) Open circuit voltage profiles of the Zn-air battery with Mn3O4/O-CNT and Pt/C + Ir/C air electrode. (b) Galvanodynamic charging/discharging profiles and power density curves of Zn-air battery assembled from Mn3O4/O-CNT and mixed Pt/C + Ir/C air electrode. (c) Discharge curves of Zn-air battery with Mn3O4/O-CNT electrode at 5, 10 and 20 mA cm-2 discharging rate (measured in a sealed plastic box in which oxygen was aerated gently). (d) The corresponding specific capacity profiles. (e) Charging/discharging cycles at the current density of 2 mA cm-2. Insets show the initial and after long time cycling testing charging/discharging curves of Zn-air battery with Mn3O4/O-CNT electrode. 26


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381x319mm (96 x 96 DPI)


Anchoring Mn3O4 Nanoparticles on Oxygen ... - ACS Publications

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