C as air cathode catalyst for rechargeable aluminum–air battery

Journal of Alloys and Compounds 824 (2020) 153950

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Cobalt ion intercalated MnO2/C as air cathode catalyst for rechargeable aluminumeair battery Zijie Xia a, b, Yunfeng Zhu a, b, *, Wenfeng Zhang c, d, **, Tongrui Hu a, b, Tao Chen c, d, Jiguang Zhang a, b, Yana Liu a, b, Huaxiong Ma c, d, Huizheng Fang a, b, Liquan Li a, b a

College of Materials Science and Engineering, Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, China c CCCC Tianjin Port Engineering Institute Co., Ltd., 1002 South Dagu Road, Hexi District, Tianjin, 300222, PR China d CCCC First Harbor Engineering Co., Ltd., Building No.8, Yuejin Road, Tianjin Port Bonded Zone, 300461, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 November 2019 Received in revised form 14 January 2020 Accepted 20 January 2020 Available online 23 January 2020

Overcoming the self-corrosion and surface passivation of aluminum anode, and the slow kinetics of cathodic electrochemical reactions are of great significance for the practical application of aluminumeair battery. In this study, we replaced the traditional aqueous electrolyte with AlCl3-urea ionic liquid electrolyte, and prepared CoeMnO2/C catalysts as cathode catalyst. Structures and electrocatalytic activity of the x % CoeMnO2/C (x is the mole percent of Co to Mn, x ¼ 0, 10, 20, 30, 40 and 50) catalysts have been investigated systematically. After Co ions intercalation, the specific surface area of the catalyst increased and average pore diameter decreased. The unique interaction between Co ions and MnO2 led to an increase in the catalytic activity of the catalyst in the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) compared with MnO2/C. In particular, 40% CoeMnO2/C showed the largest specific surface area (154.25 m2 g1) and the smallest average pore diameter (6.47 nm). It showed the most positive half-wave potential (0.727 V vs. RHE) and the biggest limiting current density (4.744 mA cm2) in ORR process, and also exhibited the lowest onset potential (1.593 V) and the biggest limit current density (15.177 mA cm2) in OER process. Furthermore, aluminumeair battery assembled with 40% CoeMnO2/C demonstrated excellent reversible charge and discharge performance, which had an average discharge voltage of 1.5 V and an average charge voltage of 2 V during 30 cycles at a limited battery capacity of 375 mAh g1. Our results reveal the possibility of designing a rechargeable aluminumair battery working at ambient conditions based on the CoeMnO2/C air cathode catalyst for the first time. Our work opens up a new way to achieve the rechargeability of aluminum-air batteries, and our highly active electrocatalytic materials can be used in a wider range of electrochemical energy applications. © 2020 Elsevier B.V. All rights reserved.

Keywords: Rechargeable aluminumeair battery Air cathode catalyst Ionic liquid electrolyte

1. Introduction Metaleair batteries, also known as “semi-fuel” cells, are between the primary batteries and fuel cells, which keep the advantages of fuel cells and overcome the shortcomings of fuel cells, like high cost, high technical threshold, and the demand of infrastructure. Among the various metaleair batteries, aluminumeair battery

* Corresponding author. College of Materials Science and Engineering, Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, China. ** Corresponding author. CCCC Tianjin Port Engineering Institute Co., Ltd., 1002 South Dagu Road, Hexi District, Tianjin, 300222, PR China. E-mail addresses: [email protected] (Y. Zhu), [email protected] (W. Zhang). https://doi.org/10.1016/j.jallcom.2020.153950 0925-8388/© 2020 Elsevier B.V. All rights reserved.

has the advantages of high energy density, rich aluminum resources, safety, low cost, and environmental friendliness. The theoretical specific energy and industrial specific energy of aluminumeair battery can reach 8100 Wh kg1 and 300e400 Wh kg1 [1,2], indicating that aluminumeair battery can be used as a very promising sustainable energy source. Whereas, the self-corrosion and surface passivation of aluminum anode cause the corrosion potential of aluminum anode to move positively and reduce the utilization of aluminum anode, which are the major obstacles to the application of traditional aqueous aluminumeair batteries [3e5]. To remove these defects, replacing traditional alkaline aqueous electrolyte with room temperature ionic liquids (RTILs) electrolyte has been regarded as an

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Z. Xia et al. / Journal of Alloys and Compounds 824 (2020) 153950

effective method [6,7]. Beyond that, the aluminumeair battery can be charged reversibly by applying an ionic liquid electrolyte [8e10]. At present, several reports about rechargeable aluminumeair batteries focus on the ionic liquid electrolytes synthesized from AlCl3 and other organic salts [9e12]. Specifically, the feasibility of using AlCl3-based urea, AlCl3-based acetamide and AlCl3-based 1-ethyl3-methylimidazolium chloride (EMImCl) as electrolytes for rechargeable aluminumeair batteries has been evaluated under ultra-dry atmosphere [13]. In the EMImCl/AlCl3 ionic liquid electrolyte system, non-oxides, such as TiN, TiC and TiB2 [14] ceramic materials and aluminum terephthalate-organic framework [15], are respectively used as cathode materials to suppress the production of by-products such as Al(OH)3 and Al2O3 in the air cathode of rechargeable aluminumeair battery. In addition to ionic liquid electrolyte, the smooth progress of the ORR/OER is also considerable for the normal operation of rechargeable aluminumeair battery [16e19]. Nevertheless, the sluggish kinetics of ORR/OER on the air cathode of aluminumeair battery requires efficient electrocatalyst to ensure good performance [20e23]. In the current reports on rechargeable aluminumeair batteries, the commercial Pt/C is commonly used as an air cathode catalyst on account of its excellent ORR/OER electrocatalytic performance [7,8,13]. However, the relatively high cost of commercial Pt/C materials limits its practical application. Consequently, there is an urgent need to develop low-cost and high-active ORR/OER bifunctional catalyst for rechargeable aluminumeair battery. The birnessite-type manganese dioxide (MnO2) is a layered manganese oxide composed of manganese oxygen octahedron (MnO6) in a co-edge or co-angle arrangement, which has the advantages of fine particles, large specific surface area, excellent ORR/ OER catalytic performance, and low preparation cost [24,25]. The layered MnO2 material has a special structure, and other ions or molecules can be easily intercalated in the interlayer, which brings about good ion exchange performance of layered birnessite-type MnO2 [26]. Besides, the birnessite-type MnO2 has a strong adsorption capacity for certain metals (especially for the adsorption of Co element) [27]. Moreover, the multivalent Co ions have ORR/ OER bifunctional catalytic activity since it can act as a donorreceptor adsorption site during the process of reversible adsorption-desorption of oxygen [28e30]. In this study, we developed a novel CoeMnO2/C catalyst by using ion exchange method to intercalate Co ions in the interlayer of MnO6 and dispersed the as-prepared CoeMnO2 catalyst on conductive carbon black as air cathode catalyst. We systematically investigated the structures and electrocatalytic properties of the CoeMnO2/C catalysts. The unique interaction between Co ions and MnO2 led to an increase in the catalytic activity of the catalyst in the ORR/OER compared with MnO2/C. Especially, the 40% CoeMnO2/C exhibits the highest ORR/OER catalytic activity and displays an excellent ORR performance with a four-electron pathway oxygen reduction mechanism. Furthermore, we assembled a button-type aluminumeair battery using 40% CoeMnO2/C as the air cathode catalyst, AlCl3-containing urea as the electrolyte, and 0.2 mm thick aluminum sheet as the aluminum anode, and successfully realized reversible charge/discharge with excellent cycling performance. To the best of our knowledge, this is the first report of button-type rechargeable aluminumeair battery assembled with ionic liquid electrolyte and non-Pt/C air cathode catalyst.

method. Briefly, KMnO4 (0.79 g) was dissolved in deionized water (50 mL) and stirred for 30 min, then 0.1 mol L1 Co(NO3)2 6H2O solution was added dropwise and stirred at 70  C for 1 h. Finally, carbon black (0.5 g) was added and stirring was continued until the solution turned from purple to black, and the obtained precipitate was centrifuged with deionized water for at least 3 times, and then dried at 60  C for 8 h in a blast drying oven. Different amounts of Co(NO3)2 were used to obtain the x % CoeMnO2/C (x is the mole percent of Co to Mn, x ¼ 10, 20, 30, 40 and 50) catalysts. In addition, the reference catalyst MnO2/C was obtained without adding Co(NO3)2 under the same preparation process. 2.2. Catalyst characterization Phase structures of the catalysts were characterized by a D max/ RB diffractometer with Cu-Ka radiation. All X-ray diffraction (XRD) patterns were obtained in 2q ranging from 10 to 80 with 0.02 step and scan rate of 10 /min. Morphologies and compositions of the catalysts were characterized by scanning electron microscopy (SEM, JSM-5900) and transmission electron microscopy (TEM) on a JEOL JEM 2100F configured with energy dispersive X-ray (EDX) and scanning transmission electron microscope (STEM). X-ray photoelectron spectra (XPS) was recorded by a Kratos Axis Supra™ spectroscopy. Co and Mn contents in the catalysts were determined by inductively coupled plasma (ICP) atomic emission spectrophotometry (PE optima 7000DV). The nitrogen adsorption-desorption isotherms of the catalysts were obtained on an automated gas sorption analyzer (ASAP 2020M-Physisoprtion Analyzer) at 77 K. The specific surface area and average pore size of the catalysts were calculated by the Brunauer-Emmett-Teller method. 2.3. Electrochemical activity measurements We used a computer-controlled CHI750E electrochemical workstation (Chenhua Instruments, China) and a rotating disk electrode (AFCBP1, PINE Instruments Company, USA) consisting of a speed control unit to characterize the electrocatalytic performance of the catalysts. All measurements including cyclic voltammograms (CV) and linear-sweep voltammetry (LSV) were tested with a conventional three-electrode system in 0.1 M KOH at room temperature. A platinum sheet (5  5 mm), a Ag/AgCl (KCl, 3.5 M) electrode and an electrocatalyst loaded glassy carbon electrode were used as the counter electrode, the reference electrode and the working electrode severally. To prepare the working electrode, 5 mg prepared catalyst was dissolved in 2 mL ultrapure deionized water and sonicated for 30 min, then 20 mL Nafion solution (5 wt%, DuPont, USA) was added and the ultrasonication was continued below 30  C for 30 min to obtain a catalyst ink. Finally, 10 mL prepared catalyst ink was dropped on the center of the glassy carbon electrode (5 mm in diameter), and naturally dried at room temperature overnight to obtain the working electrode with a catalyst loading of 0.126 mg cm2. The CV was tested between 0 V and 1.2 V (vs. reversible hydrogen electrode (RHE)) with a sweep rate of 50 mV s1 at rotating speed of 1600 rpm. The LSV test for ORR was performed at different rotating speeds from 100 to 2500 rpm with a sweep rate of 5 mV s1 between 0 V and 1.2 V. The potential range for OER test was between 0.96 V and 1.96 V with a sweep rate of 5 mV s1 at rotating speed of 1600 rpm. The potentials were converted to the RHE via the following equation (Eq. (1)).

2. Experimental

EðRHEÞ ¼ EAg=AgCl þ 0:059pH þ 0:197

2.1. Catalyst preparation

The number of electrons transferred (n) for ORR by LSV was calculated on the basis of Koutecky-Levich equations (Eq. (2) and (3)).

CoeMnO2/C catalysts were synthesized by a simple redox

(1)

Z. Xia et al. / Journal of Alloys and Compounds 824 (2020) 153950

1 1 1 1 1 ¼ þ ¼ þ J JL JK Bu12 JK

(2)

B ¼ 0:62nFC0 D0 2=3 v1=6

(3)

3

where, JL and JK are the limiting current for the electrode reaction of reactive species by the diffusion controlled process and the kinetic current, respectively. F is Faraday constant (96486.4 C mol1). C0 (1.2  106 mol cm3) and D0 (1.9  105 cm2 s1) are the concentration and diffusion coefficient of O2 in 0.1 M KOH solution, respectively. u (rad s1) is rotation rate, and n (0.01 cm2 s1) is the kinematic viscosity of the electrolyte [31,32]. 2.4. Fabrication and electrochemical measurement of aluminumeair battery Application of the as-prepared catalysts in rechargeable aluminumeair battery was based on a CR2032 button cell with lacunal positive pole shell. An aluminum sheet (16 mm in diameter) was used as anode, and a piece of glass fiber (whatman GF/D) loaded with 100 ml AlCl3-urea ionic liquid was used as battery separator. Briefly, AlCl3 (99%, anhydrous) and urea (99%) were weighed at a molar ratio of 1.3:1 in a glove box, and then AlCl3 was added to the urea in batches and stirred to obtain a pale yellow translucent liquid, which was used directly as the electrolyte. To prepare the air cathode, the as-prepared catalysts and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) were dissolved in N-Methyl pyrrolidone (NMP) at a mass ratio of 3:2:10 to obtain a slurry, then the slurry was brushed onto a piece of carbon cloth (16 mm in diameter) and dried in a vacuum oven at 60  C for 2 h. According to the difference in mass before and after the carbon cloth coating, the loading of the corresponding catalyst in the air cathode can be calculated to be 4  104 g cm2. The button-type aluminumeair batteries were assembled in an argon filled glove box with H2O and O2 contents below 0.1 ppm. The assembled batteries were placed in the glove box for 24 h before testing to ensure that the electrolyte was in full contact with the electrode. Galvanostatic charge and discharge performances of the assembled batteries were tested by a LAND battery test system (CT2001A). All electrochemical measurements were carried out under ambient atmospheric conditions without any forced air supply. 3. Results and discussion 3.1. Phase structure and morphology characterization Fig. 1 shows the XRD patterns of MnO2/C and CoeMnO2/C catalysts. The diffraction peaks at 12.2 , 36.8 and 65.7 correspond to the characteristic peaks of (002), (006) and (119) crystal planes of birnessite-type MnO2 (JCPDS No. 18e0892). It can be seen clearly that all the broad diffraction peaks are low in intensity, reflecting the low crystallinity and small grain size of the prepared catalyst. In addition, the straight line passing through the diffraction peaks of the (006) crystal planes of all the catalysts is slightly inclined to the left, indicating that the interlayer spacing of the (006) crystal plane of the catalyst increases slightly as the Co ions content increases. Furthermore, no diffraction peak from Co or Co compound is found in the XRD patterns. Table S1 is a summary of the results of the ICP test. The results show that the actual content of Co is slightly lower than the theoretical values. The results of the above XRD and ICP imply that cobalt ions have been intercalated into the layers of MnO2. Fig. 2a shows that the isotherms of all catalysts can be classified

Fig. 1. XRD patterns of MnO2/C and CoeMnO2/C catalysts with different Co/Mn mole ratio.

as type IV isotherm and capillary condensation appears at relative pressure (0.4e0.9), reflecting the small pore size of the catalysts. The pore size distribution of the catalysts (Fig. 2b) indicates that the pore diameter of the catalysts is mostly less than 10 nm after the addition of Co ions. The specific surface areas and average pore diameter of the as-synthesized catalysts are listed in Table 1. It shows that the specific surface area of CoeMnO2/C is nearly 6 times larger than that of MnO2/C, and the corresponding average pore size is reduced to almost 1/10 of that of MnO2/C, wherein the 40% CoeMnO2/C shows the largest specific surface area (154.25 m2 g1) and the smallest average pore diameter (6.47 nm). Large specific surface area can make oxygen, catalyst and electrolyte contact more fully during the charge and discharge process of aluminumeair battery, which promotes the ORR/OER process. Furthermore, the large specific surface area can provide more active sites at the three-phase interface, which also facilitates the ORR/OER process. SEM images of the MnO2/C and CoeMnO2/C catalysts are presented in Fig. 3 and Fig. S1. The MnO2/C is composed of a large number of irregularly shaped small fragments which are disorderly and clumped together. Fig. 3bef shows that those irregularly shaped fragments gradually agglomerate closely to form large particle agglomerates with the addition of Co ions, and the particle diameter becomes larger as the Co ions increases. In addition, Figs. S1be1f show that the surface of the catalyst becomes denser as the content of Co ions increases, especially the 40% CoeMnO2/C. This may be due to the fact that there is an electrostatic attraction between the positive Co ions and negative MnO6 sheets after the introduction of Co ions into the interlayers of MnO6 sheets, which can be in favor of the stabilization for laminated structure [33]. It should be noted that when the Co ion content increases to 50%, some large pore depressions appear on the surface of the 50% CoeMnO2/C compared with that of 40% CoeMnO2/C. Microstructure and morphology of the 40% CoeMnO2/C catalyst was further characterized by TEM. Fig. 4a and b are TEM images of the 40% CoeMnO2/C catalyst. Fig. 4a shows many curved MnO2 sheets are closely intertwined and dispersed on the amorphous carbon. This specific several-layer structures of the birnessite-type MnO2 can be seen more clearly in Fig. 4b. HRTEM image obtained from the area highlighted by the rectangle in image (b) is shown in Fig. 4c. The lattice fringe spacing of 0.751 and 0.753 nm corresponds to the (002) crystalline plane of birnessite-type MnO2, which is slightly larger than the interplanar spacing (0.72 nm) of the (002) crystalline plane in the birnessite-type MnO2 standard PDF card, indicating a delicate spacing broadening of the layers after the intercalation of Co ions. Besides, the SAED pattern with a diffused

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Z. Xia et al. / Journal of Alloys and Compounds 824 (2020) 153950

Fig. 2. (a) N2 adsorption-desorption isotherms at 77 K of the MnO2/C and CoeMnO2/C catalysts and (b) the corresponding pore size distributions.

Table 1 Summary of BET surface areas and average pore diameter of the as-synthesized catalysts. Sample

Surface area (m2 g1)

Average pore diameter (nm)

MnO2/C 10%CoeMnO2/C 20%CoeMnO2/C 30%CoeMnO2/C 40%CoeMnO2/C 50%CoeMnO2/C

23.56 121.75 131.47 134.29 154.25 153.81

72.00 9.52 8.66 7.23 6.47 6.62

pattern characteristic in the inset of Fig. 4c shows the poor crystallinity of 40% CoeMnO2/C. Low magnification TEM image of the 40% CoeMnO2/C in Fig. 4d and dark field TEM image obtained from the red rectangle area of Fig. 4d in Fig. 4e further reveal clearly an agglomerated structure formed by closely intertwined curved MnO2 flakes. Fig. 4f shows the distribution state of each element in the 40% CoeMnO2/C, and it is apparent that all the elements are uniformly distributed on the carbon black substrate. In addition, the uniform distribution of Co element in MnO2 can further explain the successful intercalation of Co ions between the MnO2 layers. XPS was used to study the elemental compositions and valence states of the catalyst. Fig. 5 shows the XPS spectra of MnO2/C and 40% CoeMnO2/C catalysts. The spectra of MnO2/C and 40% CoeMnO2/C in Fig. 5a show clearly the peaks of Mn2p, C1s, O1s, Mn3s and Mn3p. In addition, the spectrum of 40% CoeMnO2/C also shows the peak of Co2p, revealing that Co ions are doped into MnO2/C. The O1s spectra of the two catalysts in Fig. 5b can be fitted by Oa (530.0 eV), Ob (531.4 eV) and Og (532.6 eV). Specifically, Oa is lattice oxygen (O2), Ob is hydroxyl adsorption oxygen (OH), and Og is oxygen in water molecules [34,35]. The corresponding fitting data are shown in Table 2. Compared with MnO2/C, the proportion of Ob in 40% CoeMnO2/C increases, and the increase of Ob is beneficial to the improvement of catalytic performance of the catalyst in the ORR process [36]. In addition, the decrease of Oa means that more oxygen vacancies generate, so that the catalyst provides more active reaction sites, which is also beneficial to the progress of the catalytic reaction. The peaks at the binding energies of around 654 eV and 642 eV in Fig. 5c are the peaks of Mn2p1/2 and Mn2p3/2, respectively. Taking the Mn4þ satellite structure into account, the highresolution of the Mn 2p3/2 peak was mainly fitted into three peaks at about 641.2 eV, 642.2 eV and 643.3 eV, which were assigned to Mn3þ, Mn4þ and Mn4þ, respectively [34]. The corresponding fitting data are shown in Table 3, which shows that the ratio of Mn3þ/Mn4þ of 40% CoeMnO2/C is higher than that of MnO2/ C, indicating that the introduction of Co ions can increase the ratio of Mn3þ/Mn4þ in the catalyst. The higher Mn3þ/Mn4þ ratio is

beneficial to increase the catalytic activity of the catalyst in the ORR process [19]. Fig. 5d further reveals the valence states of the introduced Co ions, and the peaks at the binding energies of about 795 eV and 780 eV correspond to Co2p1/2 and Co2p3/2 [37], respectively. The splitting value of the binding energy of Co2p1/2 and Co2p3/2 is 15 eV, indicating that Co ions exist mostly in Co3þ [38]. This may be due to the fact that the added Co2þ was oxidized 3þ to Co3þ by MnO may 4 during the catalyst preparation. While Co play a major catalytic role in the OER process [39e41]. In addition, the accompanying peak at the binding energy of 781.8 eV indicates that a small amount of Co ions are present in Co2þ [42,43].

3.2. Electrochemical measurements Although the non-aqueous ionic liquids electrolyte was used in our actual aluminumeair battery assembly, the electrocatalytic activity of the catalyst in the aqueous electrolyte still has important reference significance regarding the ORR/OER catalytic activity of the catalysts during the charge and discharge process of the aluminumeair battery. Fig. 6a shows the CV curves of the MnO2/C and CoeMnO2/C catalysts. 40% CoeMnO2/C catalyst shows an obvious anodic peak at 0.93 V and a cathodic peak at 0.6V. The anodic peak may include two consecutive steps including conversion of Mn3þ to Mn4þ and conversion of Co2þ to Co3þ [19], and the cathode peak corresponds to the reduction of oxygen. The catalytic activity of the catalysts toward the ORR was investigated, and Fig. 6b shows the LSV polarization curves of the catalysts in O2-saturated 0.1 M KOH solution at a rotating speed of 1600 rpm with a scan rate of 5 mV s1. The detailed electrochemical parameters obtained from Fig. 6b are shown in Table 4. It shows that the onset potential (the potential value at the current density of 0.5 mA cm2), half-wave potential (the potential value at the current density of 1/2 id (limiting current density)) and limiting current density all increase first and then decrease with the increase of Co ion content. Wherein, the 40% CoeMnO2/C catalyst exhibits the most positive onset potential (0.859 V), the most positive half-wave potential (0.727 V) and the biggest limiting current density (4.744 mA cm2), reflecting the most excellent ORR catalytic activity. This result is consistent with the results of the CV test. There are many factors affecting the ORR catalytic activity of the catalysts, such as microstructure, specific surface area and elements valence states. On the one hand, the specific surface area of the catalysts increases and the average pore diameter decreases as the Co ion content increases up to 40%. On the other hand, compared with MnO2/C, the intercalated Co ions increase the ratio of Mn3þ/Mn4þ and hydroxyl adsorption oxygen in the catalysts. The above reasons are beneficial to the improvement of catalytic performance of the catalyst in the ORR process.

Z. Xia et al. / Journal of Alloys and Compounds 824 (2020) 153950

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Fig. 3. SEM images of (a) MnO2/C, (b) 10% CoeMnO2/C, (c) 20% CoeMnO2/C, (d) 30% CoeMnO2/C, (e) 40% CoeMnO2/C and (f) 50% CoeMnO2/C catalysts.

The Tafel plots obtained from the corresponding LSV curves are shown in Fig. 6c, and the slope of each plot obtained by linear fitting is marked on the corresponding plot, while the value of the slope determines the rate of electron transfer during the ORR process. Fig. 6c shows the slope values of Tafel plots of the CoeMnO2/C catalysts become larger than that of the MnO2/C catalyst, which indicates that the electron transfer rate becomes faster during the ORR process. Among them, the slope value of the Tafel plot of the 40% CoeMnO2/C catalyst is the largest, indicating that it has more excellent ORR catalytic activity. It has been reported that O2 has two reaction mechanisms in the aqueous electrolyte during the ORR process. One is that O2 is converted into H2O by four electrons in one step, and the other is that O2 is converted into H2O by two electrons in two steps, and hydrogen peroxide is an intermediate product [44]. The fourelectron pathway can increase the output voltage of the battery compared to the two-electron pathway. Fig. S2 shows the LSV curves of various catalysts in an O2-saturated 0.1 M KOH solution with a negative sweep rate of 5 mV s1 at electrode rotation rates from 100 rpm to 2500 rpm. It shows that the current density increases steadily with the increase of the rotation rate due to the fact that the higher the rotation rate, the faster the mass transmission. The Koutecky-Levich (K-L) plots of various catalysts at different potentials are illustrated in Fig. S3, and the linearity and near

parallelism of the K-L plots suggest the first-order reaction kinetics and similar electron transfer numbers for ORR at different potentials [45]. Furthermore, based on the K-L equation, the number of electron transfer in O2 reduction of each catalyst at different potentials is calculated and shown in Fig. S4. The fit value of electron transfer number in O2 reduction of different catalysts in the ORR process fluctuates between 4.3 and 5. The average number of electron transfer in O2 reduction of different catalysts during the ORR process is calculated and shown in Fig. 6d, and the average electron transfer number ranges from 4.31 to 4.55. Wherein, the average electron transfer number in O2 reduction of the 40% CoeMnO2/C catalyst is closest to 4, indicating that oxygen is reduced mainly through a four-electron pathway. Similarly, the OER catalytic activity of the catalysts was characterized. Fig. 6e shows the LSV polarization curves of the catalysts in an 0.1 M KOH solution at a rotating speed of 1600 rpm with a scan rate of 5 mV s1, and the corresponding electrochemical parameters are shown in Table 4. Similar to the ORR, the 40% CoeMnO2/C catalyst exhibits much better OER catalytic activity than that of other catalysts, which has the lowest onset potential (1.593 V), lowest overpotential (634 mV) and the biggest limiting current density (15.177 mA cm2). The excellent OER catalytic activity of the 40% CoeMnO2/C is further demonstrated by the Tafel plots of the corresponding LSV curves in Fig. 6f. The slope of each

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Z. Xia et al. / Journal of Alloys and Compounds 824 (2020) 153950

Fig. 4. (a) High magnification TEM image of 40% CoeMnO2/C, (b) Enlarged TEM image obtained from the rectangle in image (a). (c) HRTEM image obtained from the area highlighted by the rectangle in image (b) (the inset is the corresponding SAED pattern), (d) Low magnification TEM images of 40% CoeMnO2/C. e) Dark field TEM image obtained from rectangle area in image (d), (f) Elemental mapping of 40% CoeMnO2/C indicating the elemental distribution of Co, Mn, C and O.

plot obtained by linear fitting is marked on the corresponding plot, and the smaller slope value means the faster reaction rate during the OER process. Obviously, the Tafel plot of the 40% CoeMnO2/C catalyst has the smallest slope value (163 mV dec1), indicating that it has the most excellent OER catalytic activity. 3.3. Performance of rechargeable aluminumeair battery The main reason why the traditional aluminumeair battery cannot be charged after discharging is that the discharge products of the battery, like Al(OH)3 or Al(OH)e 4 , are difficult to be reduced to Al. It has been reported that some urea and acetamide based deep eutectic solvents show appreciable reversible activity for aluminum dissolution/deposition [13]. The amide group in urea and acetamide can react with AlCl3 and construct a positively charged complex and a negatively charged tetrachloroaluminate anion via the following equation (Eq. (4)) [45].

2AlCl3 þ nAmide 4 ½AlCl2 ,nAmideþ þ AlCl4 

(4)

The composition and pH value of the chloroaluminate melt

system depend on the molar ratio of AlCl3 to X (X represents organic matter such as EMImCl, urea and acetamide). When the molar ratio is less than 1, the melt is basic and the anion is mainly  AlCl 4 and Cl ; when the molar ratio is equal to 1, the melt is neutral and the anions are only AlCl 4 ; when the molar ratio is bigger than 1,  the melt is acidic and the anions are mainly AlCl 4 and Al2Cl7 . However, the following reversible reaction (Eq. (5)) can occur in an acidic melt [13,46], which is also a key factor in making the aluminumeair battery rechargeable.

4Al2 Cl7  þ 3e 4Al þ 7AlCl4 

(5)

In addition, the reaction mechanisms of ORR/OER in ionic liquid electrolytes are complex and have not yet been fully elucidated. It has been reported that oxygen reduction in ionic liquid electrolytes is restricted to form superoxide anion radicals via a single electron pathway, as follows (Eq. (6), (7) and (8)) [47e49].

O2 þ e /O2 

(6)

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Fig. 5. (a) XPS spectra of MnO2/C and 40% CoeMnO2/C. (b) Fitted O1s spectra for MnO2/C and 40% CoeMnO2/C. (c) Fitted Mn2p spectra for MnO2/C and 40% CoeMnO2/C. (d) Fitted Co2p spectra for 40% CoeMnO2/C.

Table 2 O1s fitting data of MnO2/C and 40% CoeMnO2/C. Sample

Oa (530.0 eV)/%

Ob (531.4 eV)/%

Og (532.6 eV)/%

MnO2/C 40% CoeMnO2/C

77.4 70.5

12.2 16.0

10.4 13.5

O2  þ H þ þ e /HO2 

(7)

HO2  þ Hþ /H2 O2

(8)

Based on the above reactions, we speculate that the reaction mechanism of the current rechargeable aluminumeair battery is as follows (Eq. (9)).

2Al þ 14AlCl4  þ 3O2 þ 6H þ 48Al2 Cl7  þ 3H2 O2

(9)

Following this reaction mechanism, we applied AlCl3-urea ionic liquid as electrolyte, the as-synthesized catalysts as air cathode catalyst, and assembled them into a button-type aluminumeair battery for further charge and discharge cycling tests. The capacity-limited approach is usually used to avoid larger capacity depth so that the battery performance can be well evaluated. The battery cycling test protocol consisted of (i) charge at 0.2 mA for 1.5 h, (ii) rest at an open circuit voltage for 5 min, (iii) discharge at

0.2 mA for 1.5 h and (iv) rest at an open circuit voltage for 5 min. The assembled battery was cycled from the discharge step. Fig. 7 shows the change of charge and discharge voltage with battery capacity or time when the MnO2/C and 40% CoeMnO2/C catalysts were respectively applied in the air cathode of aluminumeair battery. Similarly, the performances of other catalysts in aluminumeair battery were also tested and shown in Fig. S5, which shows the cycle charge and discharge performances of the aluminumeair batteries with CoeMnO2/C catalysts are improved compared with that of the aluminumeair battery with MnO2/C. Wherein the aluminumeair battery with 40% CoeMnO2/C exhibits the best cycle charge and discharge performance. Specifically, it exhibits longer cycle life and better cycle stability at a defined capacity of 375 mAh g1. Besides, in nearly 30 charge and discharge cycles, the discharge voltage of the aluminumeair battery with 40% CoeMnO2/C is always above 1V, and the charge voltage does not exceed 2.5V. In contrast, the batteries using other catalysts exhibit too low discharge voltage and excessive charge voltage. It is known that the smaller the voltage difference between the charge and discharge plateaus, the more stable the battery charge and discharge and the better the battery performance. Furthermore, the inset of Fig. 7c shows a red LED with a minimum operating voltage of 1.8V can be successfully illuminated by the aluminumeair battery with 40% CoeMnO2/C. The excellent charge and discharge performance of aluminumeair battery with 40%

Table 3 Mn2p3/2 fitting data of MnO2/C and 40% CoeMnO2/C. Sample

Mn3þ (641.2 eV)/%

Mn4þ (642.2 eV)/%

Mn4þ (643.3 eV)/%

Mn3þ/Mn4þ

MnO2/C 40% CoeMnO2/C

25.8 31.0

44.1 40.0

30.1 29.0

0.35 0.45

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Z. Xia et al. / Journal of Alloys and Compounds 824 (2020) 153950

Fig. 6. (a) CV curves of the MnO2/C and CoeMnO2/C catalysts in O2-saturated 0.1 M KOH solution with a sweeping rate of 50 mV s1 at rotating speed of 1600 rpm. (b) LSV curves of various catalysts in O2-saturated 0.1 M KOH solution with a negative sweep rate of 5 mV s1 at rotating speed of 1600 rpm. (c) Tafel plots of various catalysts in ORR process. (d) The average electron transfer number in O2 reduction of various catalysts calculated on the basis of K-L equation. (e) LSV curves of various catalysts in 0.1 M KOH solution with a negative sweep rate of 5 mv s1 at rotating speed of 1600 rpm. (f) Tafel plots of various catalysts in OER process.

Table 4 Summary of electrochemical kinetic parameters of the ORR/OER process for the as-synthesized catalysts in 0.1 M KOH. Sample

MnO2/C 10%Co eMnO2/C 20%Co eMnO2/C 30%Co eMnO2/C 40%Co eMnO2/C 50%Co eMnO2/C

ORR

OER

Onset potential (V vs. RHE)

Half-wave potential (V vs. RHE)

Limiting current density (mA cm2)

Onset potential (V vs. RHE)

Overpotential (mV, hj ¼ 10 mA cm2)

Limiting current density (mA cm2)

0.811 0.814

0.637 0.639

3.652 3.916

1.679 1.659

e e

5.083 9.015

0.829

0.658

3.922

1.675

714

10.848

0.849

0.714

4.512

1.624

690

11.408

0.859

0.727

4.744

1.593

634

15.177

0.857

0.719

4.610

1.654

653

14.006

CoeMnO2/C can be correlated with the excellent ORR/OER catalytic activity of the 40% CoeMnO2/C catalyst. More specifically, the 40% CoeMnO2/C catalyst has larger specific surface area, more microporous structure and smaller pore size. On one side, the larger

specific surface area provides more active sites, increasing the contact area of oxygen with the catalyst and electrolyte, and on the flip side, microporous structure provides more oxygen transport channels.

Z. Xia et al. / Journal of Alloys and Compounds 824 (2020) 153950

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Fig. 7. Galvanostatic chargeedischarge curves and voltage versus time plots of the aluminum-air battery with the as-synthesized catalysts at an applied current of 0.2 mA: (a) and (b) MnO2/C, (c) and (d) 40% CoeMnO2/C (the inset in (c) is a red LED lightened by the battery). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

However, it is an undeniable fact that the discharge voltage of all aluminumeair batteries tends to decrease with time, and the charging voltage shows an upward trend with time, suggesting that the battery is gradually degraded. There are three possible main reasons for the battery degradation. Firstly, the gradual accumulation of discharge by-products at the air cathode may hinder the diffusion of oxygen and electron transport. Secondly, since the battery is cycled in air for a long time, the gradual decomposition of the ionic liquid electrolyte reduces the function of the electrolyte. Thirdly, the catalysts may be poisoned during long time cycling, leading to the decreased electrocatalytic activity. These obstacles are also the key targets that need to be solved in later work. Nevertheless, our results do reveal the possibility of designing a rechargeable aluminum-air battery working at ambient conditions based on the CoeMnO2/C air cathode catalyst. 4. Conclusions In summary, CoeMnO2/C catalysts were prepared by intercalating Co ions between MnO2 layers, and the microstructures and electrocatalytic properties of the catalysts have been systematically studied in this work. The 40% CoeMnO2/C catalyst showed the largest specific surface area (154.25 m2 g1) and the smallest average pore diameter (6.47 nm). In terms of electrocatalytic performance, the 40% CoeMnO2/C catalyst showed the most positive onset potential (0.859 V vs RHE), the most positive half-wave potential (0.727 V) and the biggest limiting current density (4.744 mA cm2) in the ORR process, and it also showed the lowest onset potential (1.593 V) and the biggest limit current density (15.177 mA cm2) in the OER process. In addition, the rechargeable aluminumeair battery with the 40% CoeMnO2/C catalyst showed the best cycle charge and discharge performances. At a limited battery capacity of 375 mAh g1, during nearly 90 h of charge and discharge, the battery discharge voltage is always above 1V, and the

charge voltage does not exceed 2.5V. Our results suggest that the 40% CoeMnO2/C catalyst is promising for the application in rechargeable aluminumeair battery. Declaration of competing interest The authors declare no conflict of interest. CRediT authorship contribution statement Zijie Xia: Conceptualization, Methodology, Writing - original draft, Funding acquisition. Yunfeng Zhu: Conceptualization, Resources, Writing - review & editing, Supervision, Funding acquisition. Wenfeng Zhang: Validation, Writing - original draft, Supervision. Tongrui Hu: Methodology, Formal analysis. Tao Chen: Writing - review & editing, Validation. Jiguang Zhang: Formal analysis, Investigation. Yana Liu: Formal analysis, Data curation. Huaxiong Ma: Writing - review & editing, Validation. Huizheng Fang: Methodology, Validation. Liquan Li: Project administration, Writing - review & editing, Funding acquisition. Acknowledgements This work was supported by the National Natural Science Foundation of China (51771092, 21975125), Six Talent Peaks Project in Jiangsu Province (2018, XNY-020), Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX19_0842) and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153950.

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