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Ultrafine Mn3O4 Nanowires/Three–dimensional Graphene/ Single–Walled Carbon Nanotube Composites: Superior Electrocatalysts for Oxygen Reduction and Enhanced Mg/air Batteries Chunsheng Li, Yan Sun, Weihong Lai, Jia-Zhao Wang, and Shu-Lei Chou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09013 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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ACS Applied Materials & Interfaces

Ultrafine

Mn3O4

Nanowires/Three–dimensional

Graphene/Single–Walled

Carbon

Nanotube

Composites: Superior Electrocatalysts for Oxygen Reduction and Enhanced Mg/air Batteries

Chun-Sheng Li a,b, Yan Sun a,b,*, Wei-Hong Lai c Jia-Zhao Wang c, and Shu-Lei Chou c,*

a

College of Chemical Engineering, North China University of Science and Technology,

Tangshan City, Hebei Province, 063009, P. R. China. b

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin, 300071, P. R. China c

Institute for Superconducting and Electronic Materials, University of Wollongong,

Wollongong, NSW 2522, Australia.

*Corresponding authors: E–mail: [email protected] (Yan Sun) Tel.: +86 0315 2592169; [email protected] (Shu-Lei Chou) Tel.: +61 2 4298 1405.

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ABSTRACT The exploration of highly efficient catalysts for the oxygen reduction reaction to improve sluggish kinetics still remains a great challenge for advanced energy conversion and storage in metal–air

batteries.

In

this

paper,

ultrafine

Mn3O4

nanowires/three–dimensional

graphene/single–walled carbon nanotube catalysts with an electron transfer number of 3.95 (at 0.60 V vs. Ag/AgCl) and kinetic current density of 21.7–28.8 mA cm-2 were developed via a microwave irradiation–assisted hexadecyl trimethyl ammonium bromide (CTAB) surfactant procedure to greatly enhance the overall catalytic performance in Mg/air batteries. To match the electrochemical activity of the cathode catalysts, a large–scale Mg anode prepared with micropersimmon–like particles via a mechanical disintegrator and Mg(NO3)2–NaNO2 based electrolyte containing 1.0 wt% trihexyl(tetradecyl)phosphonium chloride ionic liquid were applied. Combining the ultrafine Mn3O4 nanowires/three–dimensional graphene/single–walled carbon nanotube as an efficient electrocatalyst for the oxygen reduction reaction and an Mg micro/nanoscale anode in the novel electrolyte, the advanced Mg/air batteries demonstrated a high cell open circuit voltage (1.49 V), a high plateau voltage (1.34 V), and a long discharge time (4177 min) at 0.2 mA cm-1, showing a high power density. Therefore, it is believed that this device configuration has great potential for application in new energy storage technologies. KEYWORDS: Mn3O4 nanowires, 3D graphene/single–walled carbon nanotube catalysts, microwave irradiation strategy, Mg/air Batteries, energy storage and conversion

1. INTRODUCTION Metal–air batteries for renewable energy storage and conversion are being given first priority in current studies in the energy storage field owing to increasing concerns about worldwide environmental issues and the global energy crisis. 1-12 Among these batteries, Mg/air batteries have fascinating theoretical properties: huge energy density (3910 Wh kg−1), high theoretical voltage (3.09 V), low cost (US $1800–3000/ton for Mg in the Alibaba quotation on 8 Apr. 2016), light weight (1.738 g cm-3 for Mg), and very good safety. 13-27 In this system, only the Mg anode with a high Faradic capacity (2.2 Ah g-1) needs to be sealed in the battery, while an abundance of oxygen (O2) as an active component of the cathode can be continuously taken from the ambient air. The practical performance of Mg/air batteries, however, is not as good as those


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of commercial Zn/air cells and the promising Al/air batteries. challenges are associated with the following factors:

28-29

The fundamental scientific

1-6, 13, 14-17, 30-33

(1) the lack of highly

efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) in the cathode; (2) the formation of a Mg(OH)2 insulating film with high polarization that greatly reduces the discharge voltage and passivates the anode surface reaction; and (3) the H2 evolution in a side reaction during Mg self–discharge, which leads to a loss of the anodic efficiency. To solve these vital problems, superior bifunctional catalysts, novel electrolytes with low self–discharge rates, and Mg anode with high chemical activity need to be ultimately explored. Among the oxygen reduction catalysts, Mn3O4 with its special mixed–valence state has triggered widespread interest because of its electrocatalytic activity towards several redox reactions, 34-45 as well as its environmental friendliness, abundance in natural resources, and low cost. The major disadvantage of Mn3O4, however, are its poor electroconductivity, which is an obstacle to electron transfer for the ORR, 46-51 and its serious tendency towards agglomeration, leading to reduced catalytic durability. Fortunately, three–dimensional (3D) graphene/single– walled carbon nanotube (GN/SWCNT) sandwiches

52-55

with the SWCNTs interlinked by a

graphene framework, which also functions as a support material for the catalyst,

56-71

have

attractive advantages, owing to their high electronic conductivity, high specific surface area, good flexibility, and excellent electrochemical stability. Inspired by this strategy, the integration of Mn3O4 with GN/SWCNT is highly desirable to achieve competitive activity and prevent the aggregation of nanomaterials. Very recently, Mn3O4 nanostructures (nanoparticles,

34-36, 46-49

hierarchical network 50)/graphene hybrids have been fabricated to investigate their significant enhancement of catalytic activity and stability for the ORR. Although great progress has been made during recent years, there are few reports on Mn3O4 nanowires/3D GN/SWCNT composites with a highly efficient preparation method to further improve the catalytic performance and fast thermal/kinetic behavior in Mg/air batteries. In this contribution, we propose a new configuration for the Mg/air battery using an ultrafine Mn3O4 nanowires/3D GN/SWCNT hybrid as catalyst, with micro/nanoscale Mg as the anode, and Mg(NO3)2–NaNO2 with an ionic liquid (IL) additive as the electrolyte to avoid severe self–corrosion. The advantages of the reported research are as follows. Firstly, the composite, consisting of ultrafine Mn3O4 nanowires with a diameter of 10 nm anchored on 3D GN/SWCNT, which was fabricated through a microwave irradiation–assisted CTAB surfactant approach,



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exhibits a higher onset potential and larger kinetic current than that of pure 3D GN/SWCNT sandwiches, indicating high electrocatalytic activity and excellent durability toward the ORR in the cathode. The high electron transfer number (n) of ultrafine Mn3O4 nanowires/3D GN/SWCNT can reach ~3.95 at 0.60 V, which is much higher than that of pure 3D GN/SWCNT sandwiches (n ≈ 2.58). Moreover, the results are very near to those for precious metal Pt/C catalyst (n ≈ 4). The greatly increased catalytic properties are attributed to the improvement of electroconductivity due to the Mn3O4 nanowires and the successful prevention of agglomeration of the hybrid 3D catalysts. In addition, Mg micro–persimmon–like particles (assembled from nanospheres with a uniform diameter of 70–150 nm) were obtained via a fast mechanical synthesis, starting from commercial Mg powders in an ultra–fine friction nanogrinder running at 29000 rpm for 20 min. Finally, a novel electrolyte for Mg/air batteries containing Mg(NO3)2 (2.6 M) and NaNO2 (3.6 M) with 1.0 wt% trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]) ionic liquid additive was developed to satisfy the requirements of the Mg anode and supply high stability, a low corrosion rate, and low electrode polarization.

2. EXPERIMENTAL SECTION 2.1 Materials All of the reagents were of analytical grade and utilized without further purification. 50 wt% Mn(NO3)2 solution and KMnO4 were obtained from Tianjin Fengchuan Chemical Reagent Co. Ltd. (China). 3D GN/SWCNT sandwiches and hexadecyl trimethyl ammonium bromide (C19H42BrN, CTAB) were obtained from Beijing Northern Energy Co. Ltd. and Tianjin Bodi Chemical Holding Co. Ltd. (China), respectively. Mg powders (99.99%) were acquired from the Fine Chemical Research Institute of Tianjin Jinke (China). Trihexyl（tetradecyl）phosphonium chloride [P6,6,6,14][Cl] (97%, Cyphos) was vacuum dried at 80 °C for 12 h. Distilled water was applied to dissolve the reactants in the experimental process. All glassware was washed and dried before use. 2.2 Preparation of Mn3O4 nanomaterial/3D GN/SWCNT hybrids Microwave fabrication of Mn3O4 nanomaterial/3D GN/SWCNT composites was carried out in a modified microwave chamber (Midea PJ21C–AU, power: 700 W, frequency: 2.45 GHz) 72-73

with a refluxing system. In the preparation route, the microstructure, size dispersion, and 4 Environment ACS Paragon Plus

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load carrying capacity of the nanohybrids can be manipulated by the concentration of precursors, as listed in Table S1 of the supporting information. Briefly, 0.1000–0.3000 g of 3D GN/SWCNT sandwiches were firstly dispersed in 24.5 mL of distilled water containing 2.5 mL of 50 wt% Mn(NO3)2 solution and 0.1000 g CTAB under strong magnetic stirring for 30 min. Subsequently, the KMnO4 solution (0.2000 g, 5 mL distilled water) was transferred dropwise into the above suspension. Finally, the resulting mixture was placed in the modified microwave refluxing system (on for 10 s, off for 10 s) for a constant irradiation time of 30 min. After the reaction, the as–formed black powders were centrifuged at 8000 rpm for 10 min, rinsed with distilled water and ethanol several times, and dried at 90 °C overnight for further electrochemical characterization. 2.3 Synthesis of Mg microscale persimmons Uniform Mg micropersimmons assembled from abundant nanoparticles were obtained via disintegration of commercial Mg powders (99.99%) in a modified ultra–fine friction nanogrinder (Shanghai Jiupin Co., Ltd., China, Model: JP–200A, power: 1200 W) operated at 29000 rpm for 20 min in an glove box (MIKROUNA Universal 2440/750) filled with high– purity argon gas (99.999%, Air Products and Chemicals (Tianjin), Inc., USA) at room temperature. The experimental dwell time was altered to control the size and shape of the high– purity Mg micro/nanostructures. 2.4 Material Characterization Powder X–ray diffraction (XRD, Rigaku MiniFlexII, Cu Kα radiation, λ = 1.54056 Å), field emission scanning electron microscopy (FE–SEM, a Hitachi S–4800 (Japan) and a ZEISS MERLIN Compact (Germany) at an accelerating voltage of 5–10 kV), and transmission electron microscopy and high–resolution transmission electron microscopy (TEM/HRTEM, Philips Tecnai G2 F20, acceleration voltage of 200 kV) were conducted to measure the microstructure and topology of the as–synthesized products. Raman spectra of Mn3O4 nanostructure/3D GN/SWCNT composites were collected on a confocal microscope laser Raman spectrometer system (Thermo Electron Corporation, model: 0013, USA) using an excitation laser with a wavelength of 532 nm. The specific surface areas of Mn3O4/3D GN/SWCNT composites were



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analyzed with N2 adsorption/desorption investigation at 77 K (Quantachrome Instruments Nova 2200 e apparatus). 2.5 Electrocatalytic activity evaluation Electrocatalytic activity was evaluated using rotating disk electrode (RDE) voltammetry with computer–controlled potentiostats (Princeton 2273 and 636, Princeton Applied Research). 74-77

The instruments used include an electrochemical workstation and a rotation speed controller

in a conventional three–electrode cell at room temperature. After analyzing the published reports about Mn3O4–based nanocatalysts, 0.1 M KOH is widely used in most of literatures. 35-36, 38-39, 4748, 50

Therefore, the electrolyte use in our study paves an easy way to compare the

electrochemical results on air–electrode with the pioneering researches. The RDE experiments were carried out under alkaline conditions, using 0.1 M potassium hydroxide (KOH) as the electrolyte, an Ag/AgCl reference electrode and a platinum wire counter electrode as well. For the RDE measurements, the glassy carbon (GC) working electrode (5 mm in diameter) was first polished with 1.00, 0.05, and 0.03 µm alumina powders. All catalyst inks were prepared by the same method. 4 mg of catalyst was dispersed in 2 mL of 4:1:0.05 v/v/v water/isopropanol/5% Nafion® to form a homogeneous ink. Then, 30 µl of the catalyst ink (containing 60 µg of catalyst) was loaded onto the glassy carbon electrode. Cyclic voltammograms (CVs) were collected in O2 saturated 0.1 M KOH solution from –1.00 V to 1.00 V at a scan rate of 50 mV/s. Linear sweep voltammograms (LSVs) to measure the ORR performance were collected in O2 saturated 0.1 M KOH solution with different rotation speeds from 400 to 2025 rpm at a potential varied from –0.70 V to 0.20 V with a scan rate of 10 mV/s. 2.6 Calculation of electron transfer number (n) On the basis of the RDE data, the electron transfer number per oxygen molecule involved in oxygen reduction can be determined from the Koutecky–Levich equation, which shows the inverse current density (j-1) as a function of the inverse of the square root of the rotation speed (ω-1/2) at different potential values: 1/j = 1/jk + 1/Bω-1/2


(1)

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Where jk is the kinetic current and ω is the electrode rotation rate. B is determined from the slope of Koutecky–Levich (K–L) plots according to the Levich equation, as given below: B = 0.2nF(DO2)2/3υ-1/6CO2

(2)

Where n is the electron transfer number gained per oxygen molecule. F is the Faraday constant (F = 96485 C mol-1), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10-5 cm2 s-1), υ is the kinetic viscosity (0.01 cm2 s-1), and CO2 is the bulk concentration of O2 (1.2 × 10-6 mol cm-3). 2.7 Discharge performance of Mg/air batteries The electrochemical properties of Mg/air batteries were investigated on a modified CR2032–type coin cell with an air cathode, a polyethylene (PE) separator, a Mg micropersimmon anode, and a novel electrolyte with a pH value of 5 (containing Mg(NO3)2 (2.6 M) – NaNO2 (3.6 M) with 1 wt% trihexyltetradecylphosphonium chloride [P6,6,6,14][Cl] ionic liquid additive). The catalyst layers were obtained by mixing Mn3O4 nanomaterial/3D GN/SWCNT composite, carbon, and poly(tetrafluoroethylene) (PTFE) binder. The Mn3O4 nanowires/3D GN/SWCNT composite is beneficial for fast access of the novel electrolyte ions to the electrodes. The discharge characteristics of the Mg/air batteries were investigated galvanostatically at a current density of 0.2 mA·cm-2 between 1.60 and 0.20 V using a LAND– CT2001A battery testing instrument (Wuhan Land Electronics Co., Ltd., China) in an air atmosphere. Parallel measurements were carried out at least four times to guarantee reliability.

3. RESULTS AND DISCUSSION 3.1 The microstructure of Mn3O4 nanomaterial/3D GN/SWCNT composites The rapid reaction of Mn(NO3)2 and KMnO4 in a mixture of CTAB surfactant and distilled water can result in the synthesis of Mn3O4/3D GN/SWCNT nanocomposite, as listed in Table S1. Figure 1a presents typical XRD patterns of the pure 3D GN/SWCNT sandwiches and the as– generated Mn3O4 nanostructures/3D GN/SWCNT composites fabricated by the microwave irradiation–assisted CTAB method with a reaction time of 30 min. The XRD pattern of the 3D GN/SWCNT (Figure 1aI) indicates that the three broad diffraction peaks at 26.34°, 42.61°, and 44.83° should be assigned to the (002), (100), and (101) planes of graphitic carbon, respectively. 7 Environment ACS Paragon Plus


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The XRD patterns in Figure 1aII and aIII of the nanocomposites obtained after altering the amounts of raw material present a sharp and broadened diffraction peak at 36.2°, which can be attributed to the (211) reflection, thus implying a highly preferred growth orientation for the Mn3O4 (ICDD–JCPDS card No. 1–1127). Other intense diffraction peaks are also well indexed to tetragonal Mn3O4 with space group I41/amd (No. 141) and lattice parameters a = 5.75 Å and c = 9.42 Å. Raman spectroscopy, as a powerful analytical method, is extensively applied to precisely distinguish the types of chemical bonding, the number of layers, and defects and disorder of graphene–based materials. Figure 1b presents the Raman spectra of bare 3D graphene and the Mn3O4/3D GN/SWCNT nanocomposites. The evident difference between the profile (I) and profiles (II–III) is a remarkable peak in the Raman shift at about 630 cm-1, originating from the presence of Mn3O4 nanosquares and ultrafine nanowires in Samples 2 and 3, respectively. Meanwhile, the two intense peaks for the D peak at 1350 cm-1 and the G peak at 1592 cm-1 are generated from the Raman shifts of 3D GN/SWCNT sandwiches. The broad D peak represents the lattice−defect–induced vibrations of the C–C bonds in the carbon nanosheets, suggesting a disorder of the 3D graphene layers. On the other hand, the sharp G peak is related to the vibrations of the sp2 hybridized carbon atoms, implying good crystallinity of the graphene. The relative intensity ratios of the D and G peak (ID/IG) are 0.532, 0.785, and 0.788 for Samples 1, 2, and 3, respectively. The prominent increase in the ID/IG ratio for the Mn3O4 nanowires/3D GN/SWCNT composite is attributed to the increased amount of defects owing to the conjugation of Mn3O4 anchored on the graphene framework and the randomness of the nanolayers.


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Figure 1 (a) Typical XRD patterns of (I) Sample 1, commercial 3D GN/SWCNT sandwiches, (II) Sample 2, Mn3O4 nanosquares/3D GN/SWCNT composite, and (III) Sample 3, ultrafine Mn3O4 nanowires/3D GN/SWCNT composite. (b) Raman curves of (I) Sample 1, pure 3D GN/SWCNT sandwiches, (II) Sample 2, Mn3O4 nanosquares/3D GN/SWCNT composite, and (III) Sample 3, ultrafine Mn3O4 nanowires/3D GN/SWCNT composite.

The surface morphology of the 3D GN/SWCNT sandwiches and the Mn3O4 nanostructures/3D GN/SWCNT composites was estimated via SEM and TEM/HRTEM, as shown in Figure 2. The 3D GN/SWCNT sandwiches (Sample 1) composed of two-dimensional (2D) interlinked nanosheets can be obviously identified (Figure 2a–d, and Figures S1, S2 in the supporting information), which have an interplanar spacing of 0.37 nm in the HRTEM image of Figure 2d, suggesting high graphitization. It is generally recognized that the successful assembly of 2D wrinkled graphene nanosheets into 3D architectures can provide a good platform for manipulating the overall electrocatalytic properties in graphene–based composites. In Sample 2, the Mn3O4 nanosquares were uniformly anchored on the surface of graphene layers after the microwave irradiation–assisted CTAB surfactant procedure for 30 min, as shown in the SEM and TEM images of Figure 2e and f. The additional HRTEM image in Figure 2g demonstrates that the resultant highly crystalline Mn3O4 nanosquares have a width of approximately 15 nm. Moreover, the clear lattice fringes measured from Figure 2h have a 0.25 nm spacing, corresponding to the (211) planes of the tetragonal Mn3O4 structure. The elemental mappings of the product are depicted on Figure S3, showing the good dispersion of the anchored nanosquares



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on the 3D GN/SWCNT sandwiches. Moreover, Figure 2i–l illustrates the topology of the Mn3O4 nanowires/3D GN/SWCNT for Sample 3 in low– and high–magnification SEM and TEM/HRTEM images. From the SEM images in Figure 2i, a large quantity of ultrafine nanowires with extraordinarily smooth surfaces is anchored on the graphene to build up a 3D nano-network architecture. These Mn3O4 nanowires have uniform diameters of 8–30 nm and are up to several micrometers in length (Figure 2i and Figure S4). The growth habits of the Mn3O4 nanowires were further studied by TEM and HRTEM. It is clear that the individual Mn3O4 nanowires with a fairly high aspect ratio tightly join two graphene sheets (Figure 2j), which can significantly enhance the electronic conductivity and thus improve the catalytic activity and stability towards the ORR. Figure 2k–l presents HRTEM images of the highly crystalline wire– like Mn3O4 on the region marked by the square in Figure 2j. The very distinct interplanar spacing of 0.25 nm is in good agreement with the theoretical value for Mn3O4 (211) planes (Figure 2l), which corresponds to the orientated growth in the XRD pattern of Figure 1a. Therefore, the Mn3O4 nanowires anchored between the 3D GN/SWCNT sheets were grown with their length along the [010] preferential direction, as shown by the white arrow in Figure 2k. From the crystal diagram of Mn3O4 in Figure S5, two neighboring (211) planes are indicated in red color in the crystallographic view of Mn3O4 along the b–axis. Furthermore, the separation between the atomic planes of 0.247 nm is consistent with the experimental results from HRTEM in Figure 2l.


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Figure 2 SEM and TEM/HRTEM images of microstructures for self–assembly of Mn3O4 nanostructures/3D GN/SWCNT fabricated via a microwave irradiation–assisted CTAB surfactant process: (a–d) Commercial pure 3D GN/SWCNT sandwiches framework for Sample 1; (e–h) Mn3O4 nanosquares/3D GN/SWCNT composite for Sample 2; and (i–l) Well–defined Mn3O4 nanowires/3D GN/SWCNT composite for Sample 3. The (i) image shows Mn3O4 nanowires radiating from the surfaces of 3D GN/SWCNT sandwiches. Representative TEM/HRTEM images (j–l) of a single–stranded Mn3O4 nanowire, showing the lattice structure of the Mn3O4 crystals along the growth direction of the nanowire. The blue squares in (c), (g), and (k) indicate the areas enlarged in (d), (h), and (l), respectively.

Due to the novel growth mechanism, the 3D GN/SWCNT sandwiches play a vital role in controlling the homogeneity of nucleation for the Mn3O4 nanosquares, because the C–C atomic distance of 0.2464 nm for graphene is very close to the 0.2480 nm spacing of the (211) planes for Mn3O4, as calculated by the equation shown in Figure 3a. After the further reaction under microwave irradiation, the ultrafine Mn3O4 nanowires/3D GN/SWCNT hybrids then can be prepared in a highly efficient way on a large scale (Figure 3b). Moreover, the Raman spectroscopy of the ultrafine Mn3O4 nanowires/3D GN/SWCNT also indicates a high ID/IG ratio (0.788) as shown in Figure 1b, which is attributed to the randomness of the nanolayers and the increased defects due to the conjugation of Mn3O4 anchored on 3D GN/SWCNT sandwiches.

Figure 3 The novel growth mechanism of the Mn3O4 nanowires/3D GN/SWCNT under high– power microwave irradiation: (a) the formation of uniform Mn3O4 nucleations on the outer surface of graphene, and (b) the controllable growth of 1D Mn3O4 nanowires on graphene in the microwave field. 11 Environment ACS Paragon Plus


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3.2 The electrochemical properties of Mn3O4 nanomaterial/3D GN/SWCNT The linear sweep voltammetry (LSV) curves in Figure 4 confirm the electrocatalytic performance of 3D GN/SWCNT, Mn3O4 nanosquares/3D GN/SWCNT, and Mn3O4 nanowires/3D GN/SWCNT with an approximately negative onset potential of –0.05 V and a half-wave potential of –0.15 V versus Ag/AgCl. Both of these two performances are much better than those of pure 3D GN/SWCNT (–0.093 V, –0.180 V), but are not good as those of commercial Pt/C, which are 0.13 V and 0.04 V, respectively. Comparison of catalyst performance and kinetic current at different potentials of commercial 20% Pt/C is illustrated on Figure S6 and Table S2 in the Supporting Information. In addition, the observed oxygen reduction peak in CV was also shifted to a higher current for Mn3O4 nanowires/3D GN/SWCNT (Figure S7). As can be seen, the electron transfer number per oxygen molecule (n) for the ORR was determined from the LSV curves (Figure 4) according to the K–L equation. The K–L relationship is valid in the mixed diffusion and kinetically limited regime. In the K–L plots, the similar slopes are an indication of the first–order reaction kinetics of the ORR. As shown in Table 1, the modified 3D graphene–based materials for Samples 2–3 have a similar slope to Pt/C, from which n was close to ~4.0, suggesting a four-electron pathway for the ORR. Nevertheless, the number of electrons transferred for the pure 3D GN/SWCNT of Sample 1 was approximately determined to be ~2.58, indicating a two electron pathway for the ORR, also implying that the modified 3D GN/SWCNT has superior performance to the pure 3D GN/SWCNT sandwiches.


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Figure 4 Electrochemical properties of Mn3O4 nanomaterial/3D GN/SWCNT for Samples 1–3: Rotating–disk voltammograms of (a) pure 3D GN/SWCNT sandwiches (Sample 1), (c) Mn3O4 nanosquares/3D GN/SWCNT composite (Sample 2), and (e) Mn3O4 nanowires/3D GN/SWCNT composite (Sample 3) in O2–saturated 0.1 M KOH at a sweep rate of 10 mV/s and different rotation rates. The catalyst loading is 306 µg cm-2. (b, d, f) Corresponding Koutecky–Levich plots (j-1 versus ω-1/2) at different potentials.



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What is more, the values of ik (Table S2) at different potentials can be obtained by taking the inverse of the y–intercept in the respective K–L plots. The highest calculated kinetic current density was observed for Mn3O4 nanowires/3D GN/SWCNT composite (Sample 3), reaching 51.8% of the Pt/C value. Furthermore, the limiting current density of the Mn3O4 nanosquares/3D GN/SWCNT composite (Sample 2) is also much larger than that of the pure 3D GN/SWCNT sandwiches. This value can reach 45.8% of the Pt/C value. Table 1 Summary of the important performances of ORR catalysis

Sample

1

2

3

Materials 3D GN/SWCNT sandwiches Mn3O4 nanosquares/3D GN/SWCNT composite Mn3O4 nanowires/3D GN/SWCNT composite

Half wave potential (V)

Onset potential (V)

–0.18

–0.093

2.38

2.38

2.43

2.50

2.58

–0.15

–0.051

3.80

3.87

3.94

4.00

4.00

–0.15

–0.048

3.78

3.80

3.83

3.90

3.95

Number of electrons transferred 0.40V 0.45V

0.50V

0.55V 0.60V

Information regarding the mechanism of O2 adsorption can be obtained through the Tafel slopes in the generated Tafel plots (Figure 5). The intervals used to determine the Tafel slopes in the low and high current density regions are from –0.60 V – 0 V and from 0 V – 0.60 V on the log scale, respectively (Figure S8). The Tafel slope in the low and high current density regions for the Pt/C is –0.10 V dec-1 and –0.19 V dec-1, respectively (Figure S8 d). The presence of two different slopes corresponds to the switch between Langmuir adsorption and Temkin adsorption of oxygen on the Pt/C catalyst. The Tafel plots of Samples 1–3 show a similar profile, with Tafel plot slopes from –0.19 to –0.22 V dec-1 in the high current density region and from 0.10 to 0.12 V/dec in the low current density region (Figure S8 a–c), indicating similar adsorption kinetics compared to Pt/C (Figure S8 d).


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Figure 5 Tafel plots of pure 3D GN/SWCNT sandwiches (Sample 1), Mn3O4 nanosquares/3D GN/SWCNT composite (Sample 2), Mn3O4 nanowires/3D GN/SWCNT composite (Sample 3), and Pt/C.

3.4 The Mg micropersimmon anode, and the effects of novel ionic liquid based electrolytes on the discharge of Mg–air batteries made from 3D GN/SWCNT sandwiches

To meet the requirements of electrochemical catalysts, Mg anode and a new ionic liquid based electrolyte were systematically selected. For Mg anode micro/nanomaterials, an effective way to promote the anode performance in practical Mg–air batteries is to prepare Mg micro/nanostructures to enhance the chemical activity. Therefore, the Mg micropersimmons were prepared via disintegrating commercial Mg powders in an ultra–fine friction nanogrinder at 29000 rpm for 20 min (Figure S9). The close relationship between the grinding time and shape evolution of the Mg products was also ascertained in detail (Figure S10 and S11).



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For the electrolyte, the mixture of Mg(NO3)2 (2.6 M) and NaNO2 (3.6 M) solution in the Mg/air batteries exhibits a low corrosion rate for Mg anode and a low anodic polarization. Sathyanarayana’s group reported that the Mg(NO3)2–NaNO2 based electrolyte also had a 90% Faradaic efficiency for Mg anode with no serious passivation behavior at a discharge current density of 20 mAcm-2.

78-79

In the cheapest NaCl electrolyte in Mg/air batteries, the anodic

dissolution current density dramatically increases at initial potential window (–1.5 to –1.2 V) but greatly decreases at the following scanning potential between –1.2 and –0.5 V. Moreover, the vital disadvantage of the NaCl electrolyte is the formation of a passive Mg(OH)2 insulating film, which leading to the high polarization from the Mg electrode and the loss of discharge voltage. 14 Therefore, the electrolyte of Mg(NO3)2–NaNO2 based electrolyte is a good choice for Mg/air battery. To

further

improve

the

performance

of

ionic

transportation,

trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]) ionic liquid has outstanding features as an additive in electrolyte to a high ionic conductivity for Mg2+,

80-81

electrochemical stability

over a wide potential window, a low melting point, and a low viscosity. The most critical criterion to employ a room temperature ionic liquid in primary battery system is its excellent ionic conductivity. In order to combine the merits of the two electrolytes, Figure 6 illustrates the discharge curves of 3D GN/SWCNT sandwiches (Sample 1) electrode in a mixed electrolyte of Mg(NO3)2 (2.6 M) and NaNO2 (3.6 M) with the additive of 0.5, 1.0, and 2.0 wt% [P6,6,6,14][Cl] ionic liquid at a discharge current density of 0.2 mA cm-2. These three electrolytes have a similar open voltage of 1.22 V and a plateau voltage of 1.20 V. However, after 1.0 wt% [P6,6,6,14][Cl] ionic liquid was applied in the electrolyte (Figure 6, curve b), an obvious discharge time of about 220 min can be obtained because the appropriate amount of additive builds up an expressway for enhancing the Mg2+ charge transfer. Consequently, the novel Mg(NO3)2–NaNO2–based electrolyte consisting of 1.0 wt% [P6,6,6,14][Cl] ionic liquid was selected to measure the following electrochemical properties of Mn3O4 nanostructures/3D GN/SWCNT nanohybrids.


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Figure 6 Galvanostatic discharge profiles for 3D GN/SWCNT sandwiches (Sample 1) electrode at a current density of 0.2 mA cm-2 in a mixed electrolyte of Mg(NO3)2 (2.6 M) and NaNO2 (3.6 M) with different amounts of [P6,6,6,14][Cl] ionic liquid: (a) 0.5 wt%, (b) 1.0 wt%, and (c) 2.0 wt%.

3.4 The discharge of Mg/air batteries made from Mn3O4 nanomaterial/3D GN/SWCNT The effects of morphology changes on the electrochemical performance of the Mn3O4 nanostructures/3D GN/SWCNT were further examined. Figure 7 presents typical discharge curves of Mg/air batteries made from Samples 1–3 at a constant current of 0.2 mA cm-2. These profiles of cell potential (V) versus discharge time (t) have a similar shape, but the serving time and voltage plateau have a big difference. For pure 3D GN/SWCNT sandwiches (Sample 1), it delivers a short discharge time (218 min) at a current density of 0.2 mA cm-2 and a lowplateau of 1.20 V. This behavior is related to the ORR property with a low electron transfer number of only 2.58 (at 0.60 V vs. Ag/AgCl) as listed in Table 1. Fortunately, Mn3O4 nanosquares/3D GN/SWCNT composite (Sample 2) exhibits a higher plateau of 1.22 V and a better life–time of



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2857 min, corresponding to the enhanced electroconductivity of nanosquares for improving the electron transfer number (~4.0) and the prevention of agglomeration to some extent. Surprisingly, the ultrafine Mn3O4 nanowires/3D GN/SWCNT hybrid (Sample 3) shows a higher open circuit discharge voltage (1.49 V), a higher voltage plateau of 1.34 V, and a longer service time (4177 min), thus implying excellent electrocatalytic properties towards the ORR in a high–energy density battery. The outstanding electrochemical behavior of the ultrafine Mn3O4 nanowires/3D GN/SWCNT composite is ascribed to the following reasons: (1) The ultrafine Mn3O4 nanowires with their special mixed–valence state have an average diameter of 10 nm and feature a high specific surface area of 396 m2 g-1 (Figure S12), with an especially large number of continuous interfaces between the hybrid materials that were generated during the fast microwave fabrication. (2) The cross–linked network structure can successfully prevent aggregation and facilitate electron transportation for promising electrocatalytic activity, with an electron transfer number of 3.95 (at 0.60 V vs. Ag/AgCl) and high kinetic current density (21.7–28.8 mA cm-2), because the ultrafine Mn3O4 nanowires are tightly anchored onto the conducting substrate of 3D GN/SWCNT sandwiches, undoubtedly producing easy pathways for highly efficient electrical conductivity. (3) The novel Mg(NO3)2–NaNO2 electrolyte with a 1.0 wt% [P6,6,6,14][Cl] ionic liquid additive and the Mg micropersimmon anode were beneficial for greatly enhanced ionic conductivity and anode activity. In summary, the innovation of this new configuration was the result of scientific basic research on Mn3O4 nanowires/3D GN/SWCNT composite to dramatically improve the energy conversion efficiency of Mg/air batteries.


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Figure 7 Typical discharge curves of the Mg/air batteries measured with a current density of 0.2 mA cm-2: (a) Sample 1, commercial pure 3D GN/SWCNT sandwiches, (b) Sample 2, Mn3O4 nanosquares/3D GN/SWCNT composite, and (c) Sample 3, ultrafine Mn3O4 nanowires/3D GN/SWCNT composite. Electrolyte: a mixed electrolyte of Mg(NO3)2 (2.6 M) and NaNO2 (3.6 M) with the additive of 1.0 wt% [P6,6,6,14][Cl] ionic liquid.

4. CONCLUSIONS Ultrafine Mn3O4 nanowires grown on 3D GN/SWCNT sandwiches have been fabricated by a rapid microwave irradiation–assisted CTAB surfactant process using Mn(NO3)2 and KMnO4 as the precursors. It was found that samples of 3D GN/SWCNT modified with Mn3O4 have better electrocatalytic performance in regards to their limiting current density, electron transfer number (n ~ 3.95), kinetic current density (21.7–28.8 mA cm-2), and onset potential (–0.048 V), as well as their half-wave potential (–0.15 V). These products have similar O2 adsorption kinetics to Pt/C. The ultrafine Mn3O4 nanowires/3D GN/SWCNT hybrids exhibit an excellent 3D architecture with uniform particle diameters and high electronic conductivity, leading to superior



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electrocatalyst for the ORR with a high electron transfer number and enhanced performance in Mg/air batteries.

■ ASSOCIATED CONTENT Supporting Information (1) Experimental parameters for the hybrids. (2) The microstructure of 3D GN/SWCNT sandwiches and Mn3O4 nanomaterials/3D GN/SWCNT composites. (3) ORR properties of commercial 20% Pt/C. (4) Cyclic voltammograms of Mn3O4/3D GN/SWCNT nanocomposites. (5) Linear fits of Mn3O4/3D GN/SWCNT nanocomposites. (6) Preparation of microscale Mg anode. (7) Effects of novel ionic liquid based electrolytes on the discharge of Mg–air batteries made from commercial 3D GN/SWCNT sandwiches. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding authors * Yan Sun. E–mail: [email protected]. Tel.: +86 0315 2592169. * Shu-Lei Chou. E–mail: [email protected]. Tel.: +61 2 4298 1405. Notes: The authors declare no competing for financial interest.

■ ACKNOWLEDGMENTS This work was fully supported by funding from the National Natural Science Foundation of China (No. 21203051), the Training Program Foundation for the Excellent Youth Talents of Hebei Province, the Natural Science Foundation of Hebei Province (B2014209318 and B2014209319), the Research Fund of the Excellent Youth Fund for Higher Education of Hebei Province (YQ2013014 and Y2012031), and the Doctoral Foundation of North China University of Science and Technology (GP201310). The work is also supported by the Australian Research Council through a Linkage Project (LP120200432). All authors were involved in carefully


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revising the paper. Our group also acknowledges Dr. Tania Silver for scientific discussions and critical reading of the manuscript.

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Graphical Abstract


Single

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