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The effect of electrolyte cation on electrochemically induced activation and capacitive performance of Mn3O4 electrodes
Electrochimica Acta 324 (2019) 134894
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The effect of electrolyte cation on electrochemically induced activation and capacitive performance of Mn3O4 electrodes Cuiyin Liu a, b, Yanfeng Chen a, c, Xian Sun a, Biyu Chen a, Yue Situ a, Hong Huang a, * a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China School of Materials Science and Energy Engineering, Foshan University, Foshan, 528000, China c Guangdong Provincial Key Laboratory of Distributed Energy Systems, School of Chemical Engineering and Energy Technology, Dongguan University of Technology, Dongguan, 523808, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2019 Received in revised form 24 July 2019 Accepted 15 September 2019 Available online 17 September 2019
A phase transition from spinel Mn3O4 to layered birnessite has been reported through electrochemically induced activation in aqueous electrolyte for high performance supercapacitor, battery and water splitting applications. Here, Li2SO4, MgSO4 and K2SO4 aqueous electrolytes were used for the activation of Mn3O4 electrode to investigate the effect of electrolyte cation on the electrochemical activation. The Mn3O4 electrode exhibited different activated results in the three electrolytes, including the efficiency of phase transition, morphology of resulting birnessite and capacitive performance of the activated electrode. The cations affect the proceeding of Hþ insertion or the peeling-off of birnessite nanosheets during the phase transition process, resulting in the formation of nanosheet materials with different lateral dimensions. Effect of electrolyte cation on the electrochemical behavior of nanosheets electrode is further investigated. The nanosheets electrode activated in K2SO4 exhibits capable capacitive behavior with reasonable specific capacitance and cycling stability. Understanding how different cations affect the phase transition of Mn3O4 together with its capacitive performance will conduce to the development of manganese oxide electrode systems with high performance. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Mn3O4 Spinel-to-layer phase transition Nanosheets Electrolyte cation Capacitive performance
1. Introduction In the recent past, transition metal oxides with an internal tunnel or layer structure have been attracting much attention as electrode materials for energy storage device since their large tunnel cavities or interlayer spacing permits facile insertion of electrolyte ions to store charge efficiently [1e7]. Of these oxides, MnO2 is naturally abundant, environmentally benign and has various crystallographic structures in which vertice- and edgeshared MnO6 octahedra basic units are stacked to build tunnels or layers thus allowing cations such as Liþ, Naþ, Kþ or Mg2þ and water molecules to occupy [8]. Birnessite-type manganese oxides, as one type of manganese oxide material, have a two-dimensional layered structure constructed by layers of edge-shared MnO6 octahedra with interlayer space of ~7 Å [9]. This type of material has been extensively studied as a promising candidate for intercalation/ deintercalation-based charge storage, since its large interlayer was
* Corresponding author. E-mail address: [email protected] (H. Huang). https://doi.org/10.1016/j.electacta.2019.134894 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
expected to be more facile to accommodate the electrolyte ions thus generating a higher capacitance [10e13]. Recently, spinel Mn3O4 was reported to be electrochemically activated into layered birnessite through galvanostatic or voltammetric cycle in aqueous Na2SO4 electrolyte [14e17]. It is a unique phase transition achieving high-performance electrochemical energy storage [15,18,19] and electrocatalytic water splitting [20,21]. Our recent work has reported the formation of birnessite nanosheets assembled honeycomb architecture through electrochemically induced activation of Mn3O4 nanospheres electrode [14]. During the activation, Mn2þ in Mn3O4 precursor was extracted and Naþ in electrolyte was inserted into the layered structure together with H2O, driving the formation of Na-birnessite nanosheets. The role of H2O/H3Oþ molecule in the phase transition of Mn3O4 has been investigated on the basis of thermodynamic and kinetic calculations [22]. It was concluded that the insertion of H2O/H3Oþ into the spinel framework provided a key thermodynamic driving force to initiate the transition process, and meanwhile kinetically lowered the activation barrier of Mn rearrangement for the formation of layered structure. These crystal water molecules then distributed in the layered structure, stabilizing each single MnO6 layers. The
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preinsertion of Naþ and H2O into the interlayer region could facilitate the diffusion, insertion/extraction and transport process of the electrolyte ions in birnessite resulting in more efficient charge storing [10,23,24]. In addition to Naþ, other cation species such as Liþ, Kþ or Mg2þ also reportedly affect the charge storage performance of birnessite through preinsertion [25e29]. And various electrolyte cations affect the charge storage mechanism of manganese oxides electrode [30e33]. However, other aqueous electrolytes have not been used in the reported literatures for the electrochemically induced activation of Mn3O4. And the role of electrolyte cation in the spinel-to-layer phase transition of Mn3O4 is not clear. It is thus of high significance to investigate the cation effect on electrochemically induced activation of Mn3O4 together with its capacitive performance. In this work, the effect of Liþ, Mg2þ and Kþ cations in the aqueous electrolytes on electrochemically induced activation of Mn3O4 nanospheres electrode and its capacitive performance was studied. Cyclic voltammetry has been used to investigate the electrochemical behavior of the manganese oxide electrodes in different aqueous electrolytes. The relationship among the phase transition of spinel Mn3O4, morphological change of the material and capacitive performance of the electrode has been presented. Understanding how different cations affect the spinel-to-layer phase transition of Mn3O4 can conduce to the development of manganese oxide electrode systems with high performance. 2. Experimental 2.1. Materials Manganese acetate tetrahydrate (Mn(Ac)2∙4H2O), ethylene glycol and lithium sulfate monohydrate (Li2SO4$H2O) were obtained from Aladdin Co., Ltd (China), Shanghai Lingfeng Chemical Reagent Co., Ltd (China) and Guangzhou Chemical Reagent Factory (China). Magnesium sulfate heptahydrate (MgSO4$7H2O) and potassium sulfate (K2SO4) were purchased from Tianjin Damao Chemical Co., Ltd (China). Nickel foam was purchased from Taiyuan Lizhiyuan Battery Materials Co., Ltd (China). All chemicals are of analytical grades and were used as received. 2.2. Preparation of Mn3O4 nanospheres electrode Mn3O4 nanospheres were grown on nickel foam through a facile hydrothermal reaction described in our previous reports [14,34]. Typically, Mn(Ac)2∙4H2O dissolved in a mixture of ethanol and water was mixed with ethylene glycol forming a clear solution. Then, a piece of nickel foam was immersed into the solution and statically aged for 3 days. The manganese precursor was gradually oxidized and became a bright brown colour. The solution and the nickel foam were transferred into an autoclave and kept at 180 C for 5 h growth. The nickel foam sample was washed, dried and then annealed at 250 C for 2 h in air. The mass loading of Mn3O4 nanospheres on nickel foam was about 0.9e1.2 mg, which was weighed using a microbalance with sensitivity of 1 mg (Mettler Toledo MX5). 2.3. Electrochemically induced activation Mn3O4 nanospheres electrode was subjected to a galvanostatic charge/discharge cyclic treatment at 1 A g1 from 0 to 0.9 V (vs. Ag/ AgCl) for 200 cycles on a NEWARE auto-cycler. During the galvanostatic cycle, electrochemically induced activation was occurred on Mn3O4 nanospheres electrode. The cyclic process was conducted in a three-electrode configuration with a Pt wire counter electrode and an Ag/AgCl (3 M KCl) reference electrode. Respectively,
aqueous solutions of 0.5 M Li2SO4, MgSO4 and K2SO4 were used as electrolytes to investigate the cation effect on the activation and capacitive performance of Mn3O4 electrode. 2.4. Characterization X-ray diffraction (XRD, Bruker D8 Advance, Germany) was performed using Cu-Ka radiation (l ¼ 1.54178 Å). The morphologies of the samples were observed by Field emission scanning electron microscopy (FESEM, Hitachi SU8220, Japan) and field-emission transmission electron microscopy (TEM, JEOL, JEM-2100F, Japan). The phase transition of spinel Mn3O4 was determined by Raman scattering (Lab RAM Aramis, HJY, France). X-ray photoelectron spectroscopy (XPS) was carried out using Al Ka monochromatic Xray source with photon energy of 1486.6 eV (ESCALAB 250, Thermo, USA). The electrochemical measurements of cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were performed in a three-electrode cell with a Pt wire counter and an Ag/AgCl (3 M KCl) reference electrode on a CHI660E electrochemical workstation (Chenhua, China). Aqueous electrolyte of 0.5 M Li2SO4, MgSO4 or K2SO4 was used for the electrochemical measurements. 3. Results and discussion 3.1. Material characterization XRD pattern of Mn3O4 nanospheres shown in Fig. 1(a) confirmed the tetragonal spinel structure of hausmannite Mn3O4 (JCPDS 240734). Raman spectrum is shown in Fig. 1(b). Three Raman bands located at 319, 372 and 659 cm1 further confirmed the spinel structure of Mn3O4 [35]. From the SEM and TEM images shown in Fig. 1(c and d), it can be observed that Mn3O4 nanospheres in average size of ~67 nm were formed by aggregation of nanoparticles in smaller size of ~12 nm, cladding uniformly on the entire surface of nickel foam. 3.2. Cation effect on electrochemical activation of Mn3O4 electrode The effect of electrolyte cations on the electrochemically induced activation of Mn3O4 electrode was investigated by cyclic voltammetry. Fig. 2 shows the CV curves of Mn3O4 electrodes scanned in a potential range of 0e0.9 V at different scan rates using Li2SO4, MgSO4 and K2SO4 aqueous electrolytes, respectively. Significantly increasing anodic currents in high potential region were detected at different scan rates in the three electrolytes, indicating of an irreversible electrochemical oxidation of Mn3O4 electrode. This is consistent with the cyclic voltammetry results obtained in Na2SO4 electrolyte in our recent work [14], which is related to the electrochemically induced phase transition of spinel Mn3O4. It suggests that the three aqueous electrolytes of Li2SO4, MgSO4 and K2SO4 are all capable to induce the activation of Mn3O4 electrode. In addition to the irreversible electrochemical oxidation, a small reductive peak was detected distinctively in the region closed to 0 V when MgSO4 aqueous electrolyte was used, which could be attributed to an irreversible electrochemical reduction of the electrode. To further study the effect of electrolyte cations on the electrochemically induced activation of Mn3O4 electrode, the specific capacitance and coulombic efficiency (a ratio of discharge time to charge time) on different cycling stages during the 200 galvanostatic cycles in aqueous electrolyte of Li2SO4, MgSO4 or K2SO4 was calculated. The specific capacitance of the electrode exhibited different changing pattern during the galvanostatic cycle in the electrolytes of different cation species. As shown in Fig. 3(a), a slow and steady rise in specific capacitance during 200 charge/discharge
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Fig. 1. (a) XRD pattern, (b) Raman spectrum, (c and d) SEM images and (e) TEM image of Mn3O4 nanospheres.
Fig. 2. CV curves of Mn3O4 electrode collected in (a) Li2SO4, (b) MgSO4 and (c) K2SO4 aqueous electrolyte at the scan rates of 5 mV s1, 2 mVs1 and 1 mV s1.
cycles was observed for the electrode in Li2SO4. In the case of MgSO4, a rapid increase in specific capacitance during the initial 30 cycles and a precipitous decline during the subsequent cycles were observed. As for K2SO4, the specific capacitance increased steadily in the initial 70 cycles, and then showed a slow downward trend. It can be demonstrated that Mn3O4 electrode exhibited different electrochemically induced behavior in the aqueous electrolyte of different cations. The electrolyte cations play a major role in the efficiency of electrochemically induced activation of Mn3O4 electrode. The highest value of specific capacitance was achieved in MgSO4, followed by the value achieved in K2SO4 and Li2SO4. The results of calculated coulombic efficiency at different cycling stages in Li2SO4, MgSO4 or K2SO4 aqueous electrolyte are shown in Fig. 3(b). The coulombic efficiency could not reach 100% at the initial stage of the cycle in the three aqueous electrolytes due to the irreversible electrochemical oxidation of Mn3O4. And then the coulombic efficiency showed a gradual increase and eventually reached 100%, indicating of the reversible capacitive behavior of the electrode. The coulombic efficiencies higher than 100% were observed in MgSO4 during the cycle. It may be associated with the irreversible reduction of the electrode, which can produce additional electron transfer during the discharge process.
3.3. Electrode characterization The electrodes activated to the coulombic efficiency of 100% in the aqueous electrolytes of Li2SO4, MgSO4 and K2SO4 were denoted as Li-1, Mg-1 and K-1. And the electrodes collected after 200 charge/ discharge cycles were denoted as Li-2, Mg-2 and K-2. SEM images of the electrodes are shown in Fig. 4. For Li-1 electrode shown in Fig. 4(a and b), a small amount of nanosheets were observed on the electrode surface, retaining a large proportion of pristine nanosphere structure. And most of the nanospheres were converted into nanosheets on Li-2 electrode shown in Fig. 4(c and d). The lateral dimension of the nanosheets was measured to be in the range of 40e100 nm. The slow evolution from nanospheres to nanosheets in Li2SO4 is the reason for the slow increase in its specific capacitance. In the case of MgSO4, the pristine nanospheres were completely converted into nanosheets on Mg-1 electrode as shown in Fig. 4(e and f). The Mn3O4 electrode showed high electrochemical activity in MgSO4. The resulting nanosheets have a lateral dimension of 200e400 nm. Unfortunately, the nanosheets with large lateral dimension were seriously damaged on Mg-2 electrode. As shown in Fig. 4(g and h), a layer of nanosheets with a small lateral dimension was coated on the surface of the electrode. It can be supposed in a
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Fig. 3. (a) Specific capacitance and (b) coulombic efficiency of the electrode at different cycling stages during the electrochemical activation process using the aqueous electrolyte of Li2SO4, MgSO4 or K2SO4.
way that manganese oxide nanosheets were subjected to dissolution into aqueous electrolyte during the cycle, followed by the redeposition on the electrode surface forming nanosheets layer. The dissolved manganese oxide could not be completely redeposited on the electrode, resulting in the collapse of the nanosheet structure and sustained decline of the specific capacitance. In the case of K2SO4, the pristine nanospheres were also completely converted into nanosheets on K-1 electrode (Fig. 4(i and j)). The lateral dimension of the nanosheets was measured to be 60e200 nm. And no significant morphological change was observed on K-2 electrode (Fig. 4(k and l)). It suggests that the resulting nanosheet structure activated in K2SO4 tended to be stable during the subsequent cycle, resulting in the steady trend of specific capacitance. Raman spectroscopy was used to investigate the phase transition of Mn3O4 during the galvanostatic cycle, which would be help to further confirm the activation efficiency of the electrode. Three major Raman bands at 325, 372 and 664 cm1 characteristic of spinel Mn3O4 [35] were observed for Li-1 electrode (Fig. 5(a)). Besides, a small peak at 500 cm1 and a shoulder peak at around 580650 cm1 were detected, which could be attributed to the vibrational features of birnessite (506, 575 and 656 cm1) [35]. The characteristic peak of Mn3O4 at 664 cm1 can been still observed on Li-2 electrode, while enhanced characteristic peak signals of
birnessite at 500 cm1 and around 580-650 cm1 were detected. This prolonged process of phase transition further demonstrates a low activation efficiency of Mn3O4 electrode in Li2SO4. In the case of MgSO4, three Raman bands at 511, 591 and 618 cm1 were detected for Mg-1 electrode in Fig. 5(b). No characteristic peak of Mn3O4 was detected, indicating complete phase transition of Mn3O4 in MgSO4. The vibrational feature of Mg-1 electrode is closed to that of reported MnOx electrode electrochemically reduced at 0.3 V [36], further suggesting the electrochemical reduction of the electrode in MgSO4. After 200 charge/discharge cycles, three Raman bands characteristic of birnessite at 498, 578 and 655 cm1 were observed for Mg-2 electrode, which was derived from the redeposition of manganese oxide shown in Fig. 4(g and h). Three Raman bands characteristic of birnessite at 502, 579 and 657 cm1 were observed for K-1 electrode (Fig. 5(c)), indicating the phase transition from spinel Mn3O4 to birnessite. And no distinct change of Raman bands on K-2 electrode was observed, suggesting the high cycling stability of birnessite nanosheets activated in K2SO4. XPS was used to analyze the surface chemistry of Li-1, Li-2, Mg1, Mg-2, K-1 and K-2 electrode. Fig. 6(a) compares the Mn 3s corelevel spectra of the electrodes. The energy separations between the two Mn 3s peaks are measured to be 5.20 eV for Li-1 electrode, 4.92 eV for Li-2 electrode, 4.64 eV for Mg-1 electrode, 4.69 eV for Mg-2 electrode, 4.85 eV for K-1 electrode and 4.76 eV for K-2 electrode, respectively. Based on the relationship between Mn 3s splitting and the valence of Mn ions [37], the mean Mn valence states are estimated to be þ3.07 for Li-1 electrode, þ3.39 for Li-2 electrode, þ3.70 for Mg-1 electrode, þ3.65 for Mg-2 electrode, þ3.47 for K-1 electrode and þ3.57 for K-2 electrode, respectively. The Mn 2p core-level spectra (Fig. 6(b)) further confirm the distinction of the mixed Mn valence states on the electrodes. The increase of Mn valence in the case of the three electrolyte solutions indicates the electrochemically induced oxidation of Mn3O4 electrode. The Mn valence states of the electrodes showed the highest in MgSO4, followed by in K2SO4 and in Li2SO4. The lower Mn valence states on Li-1 and Li-2 electrode further confirmed the low activation efficiency of Mn3O4 electrode in Li2SO4. The O 1s core-level spectra of the electrodes shown in Fig. 6(c) can be deconvoluted into three bands corresponding to anhydrous Mn oxide (MneOeMn), Mn hydroxide (MneOeH) and structure water (HeOeH) [38]. Table 1 summarizes the deconvolution of O 1s spectra for the electrodes. Since the electrochemical activation of Mn3O4 electrode is initiated by the insertion of H2O/ H3Oþ [22], the MneOeH and HeOeH signals of Li-1, Mg-1 and K-1 electrode characteristic of hydrous species may be influenced by the activation of the electrodes. Mg-1 electrode exhibited the highest content of hydrous species, followed by K-1 and Li-1 electrode. Recently, through stochastic surface walking calculation method, Li et al. revealed a atomic-level mechanism of spinel-tolayer phase transition of Mn3O4 [39]. A transient phase of spinel H0.5MnO2 was formed by Hþ embedding into the tetrahedral sites of spinel after the extraction of Mn2þ, offering a low barrier for the transition process. Mn3O4 electrode exhibited an incomplete phase transition in aqueous electrolyte of Li2SO4. Since Liþ has a small ionic radius, it could steadily occupy the tetrahedral site of spinel structure [40]. This hindered the formation of transient H0.5MnO2, leading to a low phase transition efficiency. In the case of MgSO4, Mn3O4 electrode exhibited a complete phase transition in spite of the small ionic radius of Mg2þ. This is probably associated with the strong solvation of Mg2þ with water molecules in aqueous electrolyte, which would lead to a larger ionic radius of hydrated Mg2þ together with higher desolvation energy before ion insertion [41]. As for K2SO4, the ionic radius of Kþ is too large to occupy the tetrahedral site of spinel structure [39], enabling a complete phase
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Fig. 4. SEM images of (a and b) Li-1, (c and d) Li-2, (e and f) Mg-1, (g and h) Mg-2, (i and j) K-1, and (k and l) K-2 electrode.
Fig. 5. The comparison of Raman spectra for the electrodes activated in (a) Li2SO4: Li-1 and Li-2, (b) MgSO4: Mg-1 and Mg-2 and (c) K2SO4: K-1 and K-2.
Fig. 6. Comparisons of (a) Mn 3s, (b) Mn 2p and (c) O 1s core-level XPS spectra of (1) Li-1, (2) Li-2, (3) Mg-1, (4) Mg-2, (5) K-1 and (6) K-2 electrode.
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Table 1 The deconvoluted results of O 1s spectra for the electrodes. Sample
Li-1 Li-2 Mg-1 Mg-2 K-1 K-2
Binding energy/eV
Peak area/%
MneOeMn
MneOeH
HeOeH
MneOeMn
MneOeH
HeOeH
529.92 529.73 529.82 529.72 529.86 529.74
531.36 531.11 531.30 531.32 531.41 531.54
532.40 529.40 532.40 532.40 532.40 532.40
64.54 35.22 33.90 44.44 61.01 58.75
18.04 43.50 47.23 32.61 24.11 30.50
17.42 21.28 18.87 22.95 14.88 10.75
transition of Mn3O4 electrode. Kim et al. observed birnessite nanosheets peeled off from the edge of spinel Mn3O4 for phase transition using scanning transmission electron microscope (STEM) [42]. It is reasonable to believe that the phase transition efficiency of Mn3O4 directly affects the structure of the peeled-off nanosheet, particularly the lateral dimension. The aqueous electrolyte of Li2SO4 developed the smallest lateral dimension of nanosheets due to the low phase transition efficiency caused by competition between Liþ and Hþ on structural insertion. Besides, the nanosheets with full phase transition activated in different aqueous electrolytes also exhibited different lateral dimensions, including MgSO4 and K2SO4, and Na2SO4 using in our previous work [14]. The aqueous electrolyte of MgSO4 developed the largest lateral dimension of nanosheets, followed by Na2SO4 and K2SO4. It suggests that the electrolyte cations play a role on regulation of lateral dimension for the nanosheets during the peeling-off process, which was induced by the strain partially built from the insertions of water molecules and electrolyte cations into the layered structure [39]. The nanosheets with larger lateral dimension then delivered a higher specific capacitance, since increasing the surface area of materials could generally provide more reactive sites so as to enhance the activity [43e45]. In addition, the nanosheets with smaller lateral dimension tended to have higher stability during charge/discharge cycle. For example, the specific capacitance of the electrode cycled in Li2SO4 exhibited no downward trend during the 200 cycles of charge/discharge process. With the increase of lateral dimension for the nanosheets, 94% retention of the maximum capacitance was obtained for K-1 electrode, but 69% retention for the electrode using Na2SO4 in our previous work and only 37% retention for Mg-1 electrode. It was reported that reducing the size of electrode materials could help to enhance the stability in structure during the charge and discharge process [46e48]. The reduction of lateral dimension for the manganese oxide nanosheets would enhance the cycling stability of the electrode to some extent.
Fig. 7. CV curves of Li-2 electrode scanned in Li2SO4, of Mg-1 electrode scanned in MgSO4 and of K-1 electrode scanned in K2SO4.
valence change between Mn(II) and Mn(III); while b1 and b2 redox peaks can be attributed to the valence change between Mn(III) and Mn(IV). A larger current of a2 peak was detected, indicating an irreversible reduction reaction from Mn(III) to soluble Mn(II). This irreversible reduction reaction is the reason for the dissolution of nanosheets during the cycle together with subsequent fading on the specific capacitance of the electrode in MgSO4. In the case of K-1 electrode, a quasi-rectangular shape of CV curve was obtained in K2SO4, indicating a capable pseudo-capacitive behavior of the electrode. To accurately understand the effect of electrolyte cation on the electrochemical behavior of the nanosheets electrode, K-1 electrode with capable pseudo-capacitive property was further investigated by cyclic voltammetry in aqueous electrolyte of Li2SO4, MgSO4 or K2SO4. As shown in Fig. 8, the CV curve of K-1 electrode scanned in Li2SO4 had no difference with that scanned in K2SO4. No broad redox peak as shown in Fig. 7 was detected in this case with Li2SO4, since K-1 electrode exhibited a complete phase transition form spinel to birnessite. This further indicates the broad redox peaks shown in Fig. 7 was attributed to the spinel structure for insertion and extraction of Liþ. In the case of MgSO4, an irreversible
3.4. Electrochemical behavior of the nanosheets electrode The effect of electrolyte cation on the electrochemical behavior of the nanosheets electrode was further investigated. Fig. 7 displays the CV curves of Li-2 electrode scanned in Li2SO4, of Mg-1 electrode scanned in MgSO4 and of K-1 electrode scanned in K2SO4. The three nanosheets electrodes exhibited strictly different redox characteristics in the aqueous electrolyte of different cations. A pair of broad redox waves with reversible properties can be identified at the range of 0.6e0.9 V on Li-2 electrode, which may be associated with the insertion and extraction of Liþ on tetrahedral site of the spinel structure. It is similar to the typical redox peaks of spinel LiMn2O4 cathode in lithium ion battery [40,49]. From the CV curve of Mg-1 electrode, a pair of redox peaks denoted as a1 and a2 at 0e0.4 V and another pair of redox peaks denoted as b1 and b2 at 0.45e0.75 V were observed. According to the Pourbaix diagram of Mn [50], the redox peaks of a1 and a2 can be attributed to the
Fig. 8. CV curve of K-1 electrode scanned in Li2SO4, MgSO4 or K2SO4.
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reduction peak similar to that of Fig. 7 was detected for K-1 electrode. It suggests that the irreversible reduction of the electrode was ascribed to MgSO4 aqueous electrolyte, since the hydrolyzation of Mg2þ in aqueous electrolyte would release Hþ for the electrochemical reduction of manganese oxides into soluble Mn2þ [51]. The irreversible reduction of the electrode in MgSO4 resulted in the dissolution of manganese oxide nanosheets together with the fast fading of the specific capacitance. Cyclic voltammetry and galvanostatic charge/discharge measurements were used to investigate the capacitive performances of Li-2, Mg-1 and K-1 electrode. As shown in Fig. 9(a), significant redox peaks were observed in the high potential region of the CV curve at various scan rates for Li-2 electrode, which were attributed to the redox between Mn(IV) and Mn(III) associated with the insertion/extraction of Liþ into/from the spinel structure. GCD curves in various current densities of Li-2 electrode are shown in Fig. 9(b). The GCD curves showed distortion in the high potential region corresponding to the broad redox peaks in the CV curves. The symmetries of the CV and GCD curves demonstrate the reversibility of charge storage process for Li-2 electrode. The specific capacitance of Li-2 electrode was calculated to be 271.9 F g1 (68.0 mAh) at 1 A g1 and remained as 197.0 F g1 (49.3 mAh) at 20 A g1, delivering a high retention of 72.4%. CV curves at various scan rates of Mg-1 electrode tested in MgSO4 are shown in Fig. 9(c).
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An irreversible reduction peaks emerged in the low potential region at each scan rate. Still, distortion for the discharge process towards 0 V in the GCD curves was observed for Mg-1 electrode. The specific capacitance of Mg-1 electrode was calculated to be 521.7 F g1 at 1 A g1 and remained to be 326.9 F g1 at 20 A g1, delivering a retention of 62.7%. The CV curves and GCD curves of K1 electrode tested in K2SO4 aqueous electrolyte are shown in Fig. 9(e and f), respectively. The quasi-rectangular shape of CV curves at all scanning rates and the quasi-linear shape of GCD curves at all current densities exhibit good symmetry, further indicating capable pseudo-capacitive behavior of K-1 electrode. The specific capacitances of K-1 electrode was calculated to be 319.7 F g1 at 1 A g1 and remained to be 218.8 F g1 at 20 A g1, delivering a retention of 68.4%. The K-1 electrode tended to be more competent for capacitive charge storage compared with Li-2 and Mg-1 electrode due to its reasonable capacitance and cycling stability. 4. Conclusions In this work, the effect of electrolyte cations on the electrochemically induced activation of Mn3O4 nanospheres electrode together with its capacitive performance was investigated by using Li2SO4, MgSO4 or K2SO4 aqueous electrolyte. A low efficiency of
Fig. 9. (a) CV curves at different scan rates and (b) GCD curves at different current densities of Li-2 electrode in Li2SO4. (c) CV curves at different scan rates and (d) GCD curves at different current densities of Mg-1 electrode in MgSO4. (e) CV curves at different scan rates and (f) GCD curves at different current densities of K-1 electrode in K2SO4.
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phase transition from spinel Mn3O4 to birnessite was obtained in Li2SO4 due to the competitive insertion of Liþ with Hþ, while a full transition was achieved in MgSO4 or K2SO4. Besides, different lateral dimensions of birnessite nanosheets were electrochemically synthesized in electrolytes of different cations, since the cations affected the phase transition process of birnessite nanosheets peeling off from the edge of spinel Mn3O4. The aqueous electrolyte of MgSO4 developed the largest lateral dimension of nanosheets with highest specific capacitance. However, a poor cycling stability was obtained due to the irreversible reductive reaction of the electrode caused by hydrolyzation of Mg2þ. The aqueous electrolyte of K2SO4 developed a moderate lateral dimension of nanosheets with reasonable specific capacitance and cycling stability, being an advisable aqueous electrolyte for capacitive charge storage. Acknowledgment The authors gratefully thank the “National Natural Science Foundation of China (No. 51573058)” for financial support of this work. References [1] H.-S. Kim, J.B. Cook, H. Lin, J.S. Ko, S.H. Tolbert, V. Ozolins, B. Dunn, Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3x, Nat. Mater. 16 (2016) 454. https://doi.org/10.1038/nmat4810. [2] Y. Zhu, L. Peng, D. Chen, G. Yu, Intercalation pseudocapacitance in ultrathin VOPO4 nanosheets: toward high-rate alkali-ion-based electrochemical energy storage, Nano Lett. 16 (2016) 742. https://doi.org/10.1021/acs.nanolett. 5b04610. [3] C. Xia, Z. Lin, Y. Zhou, C. Zhao, H. Liang, P. Rozier, Z. Wang, H.N. Alshareef, Large intercalation pseudocapacitance in 2D VO2(B): breaking through the kinetic barrier, Adv. Mater. 30 (2018) 1803594. https://doi.org/10.1002/adma. 201803594. [4] P. Iamprasertkun, W. Hirunpinyopas, A.M. Tripathi, M.A. Bissett, R.A.W. Dryfe, Electrochemical intercalation of MoO3-MoS2 composite electrodes: charge storage mechanism of non-hydrated cations, Electrochim. Acta 307 (2019) 176e187. https://doi.org/10.1016/j.electacta.2019.03.141. [5] D. Li, W. Guo, Y. Li, Y. Tang, J. Yan, X. Meng, M. Xia, F. Gao, Tunnel structured hollandite K0.06TiO2 microrods as the negative electrode for 2.4 V flexible allsolid-state asymmetric supercapacitors with high performance, J. Power Sources 413 (2019) 34e41. https://doi.org/10.1016/j.jpowsour.2018.11.088. [6] Y. Liu, X. Liu, F. Bu, X. Zhao, L. Wang, Q. Shen, J. Zhang, N. Zhang, L. Jiao, L.Z. Fan, Boosting fast and durable sodium-ion storage by tailoring well-shaped Na0.44MnO2 nanowires cathode, Electrochim. Acta 313 (2019) 122e130. https://doi.org/10.1016/j.electacta.2019.04.140. [7] Y. Yang, Y. Tang, S. Liang, Z. Wu, G. Fang, X. Cao, C. Wang, T. Lin, A. Pan, J. Zhou, Transition metal ion-preintercalated V2O5 as high-performance aqueous zincion battery cathode with broad temperature adaptability, Nano Energy 61 (2019) 617e625. https://doi.org/10.1016/j.nanoen.2019.05.005. [8] Q. Feng, K. Yanagisawa, N. Yamasaki, Hydrothermal soft chemical process for synthesis of manganese oxides with tunnel structures, J. Porous Mater. 5 (1998) 153e162. https://doi.org/10.1023/a:1009657724306. [9] P.L. Goff, N. Baffier, S. Bach, J.-P. Pereira-Ramos, Structural and electrochemical properties of layered manganese dioxides in relation to their synthesis: classical and sol-gel routes, J. Mater. Chem. 4 (1994) 875e881. https://doi.org/ 10.1039/JM9940400875. [10] S.-C. Lin, Y.-T. Lu, Y.-A. Chien, J.-A. Wang, P.-Y. Chen, C.-C.M. Ma, C.-C. Hu, Asymmetric supercapacitors based on electrospun carbon nanofiber/sodiumpre-intercalated manganese oxide electrodes with high power and energy densities, J. Power Sources 393 (2018) 1e10. https://doi.org/10.1016/j. jpowsour.2018.05.019. [11] S. Zhu, L. Li, J. Liu, H. Wang, T. Wang, Y. Zhang, L. Zhang, R.S. Ruoff, F. Dong, Structural directed growth of ultrathin parallel Birnessite on b-MnO2 for highperformance asymmetric supercapacitors, ACS Nano 12 (2018) 1033e1042. https://doi.org/10.1021/acsnano.7b03431. [12] W. He, C. Wang, F. Zhuge, X. Deng, X. Xu, T. Zhai, Flexible and high energy density asymmetrical supercapacitors based on core/shell conducting polymer nanowires/manganese dioxide nanoflakes, Nano Energy 35 (2017) 242e250. https://doi.org/10.1016/j.nanoen.2017.03.045. [13] Y.-Q. Li, X.-M. Shi, X.-Y. Lang, Z. Wen, J.-C. Li, Q. Jiang, Remarkable improvements in volumetric energy and power of 3D MnO2 microsupercapacitors by tuning crystallographic structures, Adv. Funct. Mater. 26 (2016) 1830e1839. https://doi.org/10.1002/adfm.201504886. [14] C. Liu, Y. Chen, W. Huang, Y. Situ, H. Huang, Birnessite manganese oxide nanosheets assembled on Ni foam as high-performance pseudocapacitor electrodes: electrochemical oxidation driven porous honeycomb architecture formation, Appl. Surf. Sci. 458 (2018) 10e17. https://doi.org/10.1016/j.apsusc.
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The effect of electrolyte cation on electrochemically induced activation and capacitive performance of Mn3O4 electrodes Cuiyin Liu a, b, Yanfeng Chen a, c, Xian Sun a, Biyu Chen a, Yue Situ a, Hong Huang a, * a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China School of Materials Science and Energy Engineering, Foshan University, Foshan, 528000, China c Guangdong Provincial Key Laboratory of Distributed Energy Systems, School of Chemical Engineering and Energy Technology, Dongguan University of Technology, Dongguan, 523808, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2019 Received in revised form 24 July 2019 Accepted 15 September 2019 Available online 17 September 2019
A phase transition from spinel Mn3O4 to layered birnessite has been reported through electrochemically induced activation in aqueous electrolyte for high performance supercapacitor, battery and water splitting applications. Here, Li2SO4, MgSO4 and K2SO4 aqueous electrolytes were used for the activation of Mn3O4 electrode to investigate the effect of electrolyte cation on the electrochemical activation. The Mn3O4 electrode exhibited different activated results in the three electrolytes, including the efficiency of phase transition, morphology of resulting birnessite and capacitive performance of the activated electrode. The cations affect the proceeding of Hþ insertion or the peeling-off of birnessite nanosheets during the phase transition process, resulting in the formation of nanosheet materials with different lateral dimensions. Effect of electrolyte cation on the electrochemical behavior of nanosheets electrode is further investigated. The nanosheets electrode activated in K2SO4 exhibits capable capacitive behavior with reasonable specific capacitance and cycling stability. Understanding how different cations affect the phase transition of Mn3O4 together with its capacitive performance will conduce to the development of manganese oxide electrode systems with high performance. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Mn3O4 Spinel-to-layer phase transition Nanosheets Electrolyte cation Capacitive performance
1. Introduction In the recent past, transition metal oxides with an internal tunnel or layer structure have been attracting much attention as electrode materials for energy storage device since their large tunnel cavities or interlayer spacing permits facile insertion of electrolyte ions to store charge efficiently [1e7]. Of these oxides, MnO2 is naturally abundant, environmentally benign and has various crystallographic structures in which vertice- and edgeshared MnO6 octahedra basic units are stacked to build tunnels or layers thus allowing cations such as Liþ, Naþ, Kþ or Mg2þ and water molecules to occupy [8]. Birnessite-type manganese oxides, as one type of manganese oxide material, have a two-dimensional layered structure constructed by layers of edge-shared MnO6 octahedra with interlayer space of ~7 Å [9]. This type of material has been extensively studied as a promising candidate for intercalation/ deintercalation-based charge storage, since its large interlayer was
* Corresponding author. E-mail address: [email protected] (H. Huang). https://doi.org/10.1016/j.electacta.2019.134894 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
expected to be more facile to accommodate the electrolyte ions thus generating a higher capacitance [10e13]. Recently, spinel Mn3O4 was reported to be electrochemically activated into layered birnessite through galvanostatic or voltammetric cycle in aqueous Na2SO4 electrolyte [14e17]. It is a unique phase transition achieving high-performance electrochemical energy storage [15,18,19] and electrocatalytic water splitting [20,21]. Our recent work has reported the formation of birnessite nanosheets assembled honeycomb architecture through electrochemically induced activation of Mn3O4 nanospheres electrode [14]. During the activation, Mn2þ in Mn3O4 precursor was extracted and Naþ in electrolyte was inserted into the layered structure together with H2O, driving the formation of Na-birnessite nanosheets. The role of H2O/H3Oþ molecule in the phase transition of Mn3O4 has been investigated on the basis of thermodynamic and kinetic calculations [22]. It was concluded that the insertion of H2O/H3Oþ into the spinel framework provided a key thermodynamic driving force to initiate the transition process, and meanwhile kinetically lowered the activation barrier of Mn rearrangement for the formation of layered structure. These crystal water molecules then distributed in the layered structure, stabilizing each single MnO6 layers. The
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preinsertion of Naþ and H2O into the interlayer region could facilitate the diffusion, insertion/extraction and transport process of the electrolyte ions in birnessite resulting in more efficient charge storing [10,23,24]. In addition to Naþ, other cation species such as Liþ, Kþ or Mg2þ also reportedly affect the charge storage performance of birnessite through preinsertion [25e29]. And various electrolyte cations affect the charge storage mechanism of manganese oxides electrode [30e33]. However, other aqueous electrolytes have not been used in the reported literatures for the electrochemically induced activation of Mn3O4. And the role of electrolyte cation in the spinel-to-layer phase transition of Mn3O4 is not clear. It is thus of high significance to investigate the cation effect on electrochemically induced activation of Mn3O4 together with its capacitive performance. In this work, the effect of Liþ, Mg2þ and Kþ cations in the aqueous electrolytes on electrochemically induced activation of Mn3O4 nanospheres electrode and its capacitive performance was studied. Cyclic voltammetry has been used to investigate the electrochemical behavior of the manganese oxide electrodes in different aqueous electrolytes. The relationship among the phase transition of spinel Mn3O4, morphological change of the material and capacitive performance of the electrode has been presented. Understanding how different cations affect the spinel-to-layer phase transition of Mn3O4 can conduce to the development of manganese oxide electrode systems with high performance. 2. Experimental 2.1. Materials Manganese acetate tetrahydrate (Mn(Ac)2∙4H2O), ethylene glycol and lithium sulfate monohydrate (Li2SO4$H2O) were obtained from Aladdin Co., Ltd (China), Shanghai Lingfeng Chemical Reagent Co., Ltd (China) and Guangzhou Chemical Reagent Factory (China). Magnesium sulfate heptahydrate (MgSO4$7H2O) and potassium sulfate (K2SO4) were purchased from Tianjin Damao Chemical Co., Ltd (China). Nickel foam was purchased from Taiyuan Lizhiyuan Battery Materials Co., Ltd (China). All chemicals are of analytical grades and were used as received. 2.2. Preparation of Mn3O4 nanospheres electrode Mn3O4 nanospheres were grown on nickel foam through a facile hydrothermal reaction described in our previous reports [14,34]. Typically, Mn(Ac)2∙4H2O dissolved in a mixture of ethanol and water was mixed with ethylene glycol forming a clear solution. Then, a piece of nickel foam was immersed into the solution and statically aged for 3 days. The manganese precursor was gradually oxidized and became a bright brown colour. The solution and the nickel foam were transferred into an autoclave and kept at 180 C for 5 h growth. The nickel foam sample was washed, dried and then annealed at 250 C for 2 h in air. The mass loading of Mn3O4 nanospheres on nickel foam was about 0.9e1.2 mg, which was weighed using a microbalance with sensitivity of 1 mg (Mettler Toledo MX5). 2.3. Electrochemically induced activation Mn3O4 nanospheres electrode was subjected to a galvanostatic charge/discharge cyclic treatment at 1 A g1 from 0 to 0.9 V (vs. Ag/ AgCl) for 200 cycles on a NEWARE auto-cycler. During the galvanostatic cycle, electrochemically induced activation was occurred on Mn3O4 nanospheres electrode. The cyclic process was conducted in a three-electrode configuration with a Pt wire counter electrode and an Ag/AgCl (3 M KCl) reference electrode. Respectively,
aqueous solutions of 0.5 M Li2SO4, MgSO4 and K2SO4 were used as electrolytes to investigate the cation effect on the activation and capacitive performance of Mn3O4 electrode. 2.4. Characterization X-ray diffraction (XRD, Bruker D8 Advance, Germany) was performed using Cu-Ka radiation (l ¼ 1.54178 Å). The morphologies of the samples were observed by Field emission scanning electron microscopy (FESEM, Hitachi SU8220, Japan) and field-emission transmission electron microscopy (TEM, JEOL, JEM-2100F, Japan). The phase transition of spinel Mn3O4 was determined by Raman scattering (Lab RAM Aramis, HJY, France). X-ray photoelectron spectroscopy (XPS) was carried out using Al Ka monochromatic Xray source with photon energy of 1486.6 eV (ESCALAB 250, Thermo, USA). The electrochemical measurements of cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were performed in a three-electrode cell with a Pt wire counter and an Ag/AgCl (3 M KCl) reference electrode on a CHI660E electrochemical workstation (Chenhua, China). Aqueous electrolyte of 0.5 M Li2SO4, MgSO4 or K2SO4 was used for the electrochemical measurements. 3. Results and discussion 3.1. Material characterization XRD pattern of Mn3O4 nanospheres shown in Fig. 1(a) confirmed the tetragonal spinel structure of hausmannite Mn3O4 (JCPDS 240734). Raman spectrum is shown in Fig. 1(b). Three Raman bands located at 319, 372 and 659 cm1 further confirmed the spinel structure of Mn3O4 [35]. From the SEM and TEM images shown in Fig. 1(c and d), it can be observed that Mn3O4 nanospheres in average size of ~67 nm were formed by aggregation of nanoparticles in smaller size of ~12 nm, cladding uniformly on the entire surface of nickel foam. 3.2. Cation effect on electrochemical activation of Mn3O4 electrode The effect of electrolyte cations on the electrochemically induced activation of Mn3O4 electrode was investigated by cyclic voltammetry. Fig. 2 shows the CV curves of Mn3O4 electrodes scanned in a potential range of 0e0.9 V at different scan rates using Li2SO4, MgSO4 and K2SO4 aqueous electrolytes, respectively. Significantly increasing anodic currents in high potential region were detected at different scan rates in the three electrolytes, indicating of an irreversible electrochemical oxidation of Mn3O4 electrode. This is consistent with the cyclic voltammetry results obtained in Na2SO4 electrolyte in our recent work [14], which is related to the electrochemically induced phase transition of spinel Mn3O4. It suggests that the three aqueous electrolytes of Li2SO4, MgSO4 and K2SO4 are all capable to induce the activation of Mn3O4 electrode. In addition to the irreversible electrochemical oxidation, a small reductive peak was detected distinctively in the region closed to 0 V when MgSO4 aqueous electrolyte was used, which could be attributed to an irreversible electrochemical reduction of the electrode. To further study the effect of electrolyte cations on the electrochemically induced activation of Mn3O4 electrode, the specific capacitance and coulombic efficiency (a ratio of discharge time to charge time) on different cycling stages during the 200 galvanostatic cycles in aqueous electrolyte of Li2SO4, MgSO4 or K2SO4 was calculated. The specific capacitance of the electrode exhibited different changing pattern during the galvanostatic cycle in the electrolytes of different cation species. As shown in Fig. 3(a), a slow and steady rise in specific capacitance during 200 charge/discharge
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Fig. 1. (a) XRD pattern, (b) Raman spectrum, (c and d) SEM images and (e) TEM image of Mn3O4 nanospheres.
Fig. 2. CV curves of Mn3O4 electrode collected in (a) Li2SO4, (b) MgSO4 and (c) K2SO4 aqueous electrolyte at the scan rates of 5 mV s1, 2 mVs1 and 1 mV s1.
cycles was observed for the electrode in Li2SO4. In the case of MgSO4, a rapid increase in specific capacitance during the initial 30 cycles and a precipitous decline during the subsequent cycles were observed. As for K2SO4, the specific capacitance increased steadily in the initial 70 cycles, and then showed a slow downward trend. It can be demonstrated that Mn3O4 electrode exhibited different electrochemically induced behavior in the aqueous electrolyte of different cations. The electrolyte cations play a major role in the efficiency of electrochemically induced activation of Mn3O4 electrode. The highest value of specific capacitance was achieved in MgSO4, followed by the value achieved in K2SO4 and Li2SO4. The results of calculated coulombic efficiency at different cycling stages in Li2SO4, MgSO4 or K2SO4 aqueous electrolyte are shown in Fig. 3(b). The coulombic efficiency could not reach 100% at the initial stage of the cycle in the three aqueous electrolytes due to the irreversible electrochemical oxidation of Mn3O4. And then the coulombic efficiency showed a gradual increase and eventually reached 100%, indicating of the reversible capacitive behavior of the electrode. The coulombic efficiencies higher than 100% were observed in MgSO4 during the cycle. It may be associated with the irreversible reduction of the electrode, which can produce additional electron transfer during the discharge process.
3.3. Electrode characterization The electrodes activated to the coulombic efficiency of 100% in the aqueous electrolytes of Li2SO4, MgSO4 and K2SO4 were denoted as Li-1, Mg-1 and K-1. And the electrodes collected after 200 charge/ discharge cycles were denoted as Li-2, Mg-2 and K-2. SEM images of the electrodes are shown in Fig. 4. For Li-1 electrode shown in Fig. 4(a and b), a small amount of nanosheets were observed on the electrode surface, retaining a large proportion of pristine nanosphere structure. And most of the nanospheres were converted into nanosheets on Li-2 electrode shown in Fig. 4(c and d). The lateral dimension of the nanosheets was measured to be in the range of 40e100 nm. The slow evolution from nanospheres to nanosheets in Li2SO4 is the reason for the slow increase in its specific capacitance. In the case of MgSO4, the pristine nanospheres were completely converted into nanosheets on Mg-1 electrode as shown in Fig. 4(e and f). The Mn3O4 electrode showed high electrochemical activity in MgSO4. The resulting nanosheets have a lateral dimension of 200e400 nm. Unfortunately, the nanosheets with large lateral dimension were seriously damaged on Mg-2 electrode. As shown in Fig. 4(g and h), a layer of nanosheets with a small lateral dimension was coated on the surface of the electrode. It can be supposed in a
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Fig. 3. (a) Specific capacitance and (b) coulombic efficiency of the electrode at different cycling stages during the electrochemical activation process using the aqueous electrolyte of Li2SO4, MgSO4 or K2SO4.
way that manganese oxide nanosheets were subjected to dissolution into aqueous electrolyte during the cycle, followed by the redeposition on the electrode surface forming nanosheets layer. The dissolved manganese oxide could not be completely redeposited on the electrode, resulting in the collapse of the nanosheet structure and sustained decline of the specific capacitance. In the case of K2SO4, the pristine nanospheres were also completely converted into nanosheets on K-1 electrode (Fig. 4(i and j)). The lateral dimension of the nanosheets was measured to be 60e200 nm. And no significant morphological change was observed on K-2 electrode (Fig. 4(k and l)). It suggests that the resulting nanosheet structure activated in K2SO4 tended to be stable during the subsequent cycle, resulting in the steady trend of specific capacitance. Raman spectroscopy was used to investigate the phase transition of Mn3O4 during the galvanostatic cycle, which would be help to further confirm the activation efficiency of the electrode. Three major Raman bands at 325, 372 and 664 cm1 characteristic of spinel Mn3O4 [35] were observed for Li-1 electrode (Fig. 5(a)). Besides, a small peak at 500 cm1 and a shoulder peak at around 580650 cm1 were detected, which could be attributed to the vibrational features of birnessite (506, 575 and 656 cm1) [35]. The characteristic peak of Mn3O4 at 664 cm1 can been still observed on Li-2 electrode, while enhanced characteristic peak signals of
birnessite at 500 cm1 and around 580-650 cm1 were detected. This prolonged process of phase transition further demonstrates a low activation efficiency of Mn3O4 electrode in Li2SO4. In the case of MgSO4, three Raman bands at 511, 591 and 618 cm1 were detected for Mg-1 electrode in Fig. 5(b). No characteristic peak of Mn3O4 was detected, indicating complete phase transition of Mn3O4 in MgSO4. The vibrational feature of Mg-1 electrode is closed to that of reported MnOx electrode electrochemically reduced at 0.3 V [36], further suggesting the electrochemical reduction of the electrode in MgSO4. After 200 charge/discharge cycles, three Raman bands characteristic of birnessite at 498, 578 and 655 cm1 were observed for Mg-2 electrode, which was derived from the redeposition of manganese oxide shown in Fig. 4(g and h). Three Raman bands characteristic of birnessite at 502, 579 and 657 cm1 were observed for K-1 electrode (Fig. 5(c)), indicating the phase transition from spinel Mn3O4 to birnessite. And no distinct change of Raman bands on K-2 electrode was observed, suggesting the high cycling stability of birnessite nanosheets activated in K2SO4. XPS was used to analyze the surface chemistry of Li-1, Li-2, Mg1, Mg-2, K-1 and K-2 electrode. Fig. 6(a) compares the Mn 3s corelevel spectra of the electrodes. The energy separations between the two Mn 3s peaks are measured to be 5.20 eV for Li-1 electrode, 4.92 eV for Li-2 electrode, 4.64 eV for Mg-1 electrode, 4.69 eV for Mg-2 electrode, 4.85 eV for K-1 electrode and 4.76 eV for K-2 electrode, respectively. Based on the relationship between Mn 3s splitting and the valence of Mn ions [37], the mean Mn valence states are estimated to be þ3.07 for Li-1 electrode, þ3.39 for Li-2 electrode, þ3.70 for Mg-1 electrode, þ3.65 for Mg-2 electrode, þ3.47 for K-1 electrode and þ3.57 for K-2 electrode, respectively. The Mn 2p core-level spectra (Fig. 6(b)) further confirm the distinction of the mixed Mn valence states on the electrodes. The increase of Mn valence in the case of the three electrolyte solutions indicates the electrochemically induced oxidation of Mn3O4 electrode. The Mn valence states of the electrodes showed the highest in MgSO4, followed by in K2SO4 and in Li2SO4. The lower Mn valence states on Li-1 and Li-2 electrode further confirmed the low activation efficiency of Mn3O4 electrode in Li2SO4. The O 1s core-level spectra of the electrodes shown in Fig. 6(c) can be deconvoluted into three bands corresponding to anhydrous Mn oxide (MneOeMn), Mn hydroxide (MneOeH) and structure water (HeOeH) [38]. Table 1 summarizes the deconvolution of O 1s spectra for the electrodes. Since the electrochemical activation of Mn3O4 electrode is initiated by the insertion of H2O/ H3Oþ [22], the MneOeH and HeOeH signals of Li-1, Mg-1 and K-1 electrode characteristic of hydrous species may be influenced by the activation of the electrodes. Mg-1 electrode exhibited the highest content of hydrous species, followed by K-1 and Li-1 electrode. Recently, through stochastic surface walking calculation method, Li et al. revealed a atomic-level mechanism of spinel-tolayer phase transition of Mn3O4 [39]. A transient phase of spinel H0.5MnO2 was formed by Hþ embedding into the tetrahedral sites of spinel after the extraction of Mn2þ, offering a low barrier for the transition process. Mn3O4 electrode exhibited an incomplete phase transition in aqueous electrolyte of Li2SO4. Since Liþ has a small ionic radius, it could steadily occupy the tetrahedral site of spinel structure [40]. This hindered the formation of transient H0.5MnO2, leading to a low phase transition efficiency. In the case of MgSO4, Mn3O4 electrode exhibited a complete phase transition in spite of the small ionic radius of Mg2þ. This is probably associated with the strong solvation of Mg2þ with water molecules in aqueous electrolyte, which would lead to a larger ionic radius of hydrated Mg2þ together with higher desolvation energy before ion insertion [41]. As for K2SO4, the ionic radius of Kþ is too large to occupy the tetrahedral site of spinel structure [39], enabling a complete phase
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Fig. 4. SEM images of (a and b) Li-1, (c and d) Li-2, (e and f) Mg-1, (g and h) Mg-2, (i and j) K-1, and (k and l) K-2 electrode.
Fig. 5. The comparison of Raman spectra for the electrodes activated in (a) Li2SO4: Li-1 and Li-2, (b) MgSO4: Mg-1 and Mg-2 and (c) K2SO4: K-1 and K-2.
Fig. 6. Comparisons of (a) Mn 3s, (b) Mn 2p and (c) O 1s core-level XPS spectra of (1) Li-1, (2) Li-2, (3) Mg-1, (4) Mg-2, (5) K-1 and (6) K-2 electrode.
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Table 1 The deconvoluted results of O 1s spectra for the electrodes. Sample
Li-1 Li-2 Mg-1 Mg-2 K-1 K-2
Binding energy/eV
Peak area/%
MneOeMn
MneOeH
HeOeH
MneOeMn
MneOeH
HeOeH
529.92 529.73 529.82 529.72 529.86 529.74
531.36 531.11 531.30 531.32 531.41 531.54
532.40 529.40 532.40 532.40 532.40 532.40
64.54 35.22 33.90 44.44 61.01 58.75
18.04 43.50 47.23 32.61 24.11 30.50
17.42 21.28 18.87 22.95 14.88 10.75
transition of Mn3O4 electrode. Kim et al. observed birnessite nanosheets peeled off from the edge of spinel Mn3O4 for phase transition using scanning transmission electron microscope (STEM) [42]. It is reasonable to believe that the phase transition efficiency of Mn3O4 directly affects the structure of the peeled-off nanosheet, particularly the lateral dimension. The aqueous electrolyte of Li2SO4 developed the smallest lateral dimension of nanosheets due to the low phase transition efficiency caused by competition between Liþ and Hþ on structural insertion. Besides, the nanosheets with full phase transition activated in different aqueous electrolytes also exhibited different lateral dimensions, including MgSO4 and K2SO4, and Na2SO4 using in our previous work [14]. The aqueous electrolyte of MgSO4 developed the largest lateral dimension of nanosheets, followed by Na2SO4 and K2SO4. It suggests that the electrolyte cations play a role on regulation of lateral dimension for the nanosheets during the peeling-off process, which was induced by the strain partially built from the insertions of water molecules and electrolyte cations into the layered structure [39]. The nanosheets with larger lateral dimension then delivered a higher specific capacitance, since increasing the surface area of materials could generally provide more reactive sites so as to enhance the activity [43e45]. In addition, the nanosheets with smaller lateral dimension tended to have higher stability during charge/discharge cycle. For example, the specific capacitance of the electrode cycled in Li2SO4 exhibited no downward trend during the 200 cycles of charge/discharge process. With the increase of lateral dimension for the nanosheets, 94% retention of the maximum capacitance was obtained for K-1 electrode, but 69% retention for the electrode using Na2SO4 in our previous work and only 37% retention for Mg-1 electrode. It was reported that reducing the size of electrode materials could help to enhance the stability in structure during the charge and discharge process [46e48]. The reduction of lateral dimension for the manganese oxide nanosheets would enhance the cycling stability of the electrode to some extent.
Fig. 7. CV curves of Li-2 electrode scanned in Li2SO4, of Mg-1 electrode scanned in MgSO4 and of K-1 electrode scanned in K2SO4.
valence change between Mn(II) and Mn(III); while b1 and b2 redox peaks can be attributed to the valence change between Mn(III) and Mn(IV). A larger current of a2 peak was detected, indicating an irreversible reduction reaction from Mn(III) to soluble Mn(II). This irreversible reduction reaction is the reason for the dissolution of nanosheets during the cycle together with subsequent fading on the specific capacitance of the electrode in MgSO4. In the case of K-1 electrode, a quasi-rectangular shape of CV curve was obtained in K2SO4, indicating a capable pseudo-capacitive behavior of the electrode. To accurately understand the effect of electrolyte cation on the electrochemical behavior of the nanosheets electrode, K-1 electrode with capable pseudo-capacitive property was further investigated by cyclic voltammetry in aqueous electrolyte of Li2SO4, MgSO4 or K2SO4. As shown in Fig. 8, the CV curve of K-1 electrode scanned in Li2SO4 had no difference with that scanned in K2SO4. No broad redox peak as shown in Fig. 7 was detected in this case with Li2SO4, since K-1 electrode exhibited a complete phase transition form spinel to birnessite. This further indicates the broad redox peaks shown in Fig. 7 was attributed to the spinel structure for insertion and extraction of Liþ. In the case of MgSO4, an irreversible
3.4. Electrochemical behavior of the nanosheets electrode The effect of electrolyte cation on the electrochemical behavior of the nanosheets electrode was further investigated. Fig. 7 displays the CV curves of Li-2 electrode scanned in Li2SO4, of Mg-1 electrode scanned in MgSO4 and of K-1 electrode scanned in K2SO4. The three nanosheets electrodes exhibited strictly different redox characteristics in the aqueous electrolyte of different cations. A pair of broad redox waves with reversible properties can be identified at the range of 0.6e0.9 V on Li-2 electrode, which may be associated with the insertion and extraction of Liþ on tetrahedral site of the spinel structure. It is similar to the typical redox peaks of spinel LiMn2O4 cathode in lithium ion battery [40,49]. From the CV curve of Mg-1 electrode, a pair of redox peaks denoted as a1 and a2 at 0e0.4 V and another pair of redox peaks denoted as b1 and b2 at 0.45e0.75 V were observed. According to the Pourbaix diagram of Mn [50], the redox peaks of a1 and a2 can be attributed to the
Fig. 8. CV curve of K-1 electrode scanned in Li2SO4, MgSO4 or K2SO4.
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reduction peak similar to that of Fig. 7 was detected for K-1 electrode. It suggests that the irreversible reduction of the electrode was ascribed to MgSO4 aqueous electrolyte, since the hydrolyzation of Mg2þ in aqueous electrolyte would release Hþ for the electrochemical reduction of manganese oxides into soluble Mn2þ [51]. The irreversible reduction of the electrode in MgSO4 resulted in the dissolution of manganese oxide nanosheets together with the fast fading of the specific capacitance. Cyclic voltammetry and galvanostatic charge/discharge measurements were used to investigate the capacitive performances of Li-2, Mg-1 and K-1 electrode. As shown in Fig. 9(a), significant redox peaks were observed in the high potential region of the CV curve at various scan rates for Li-2 electrode, which were attributed to the redox between Mn(IV) and Mn(III) associated with the insertion/extraction of Liþ into/from the spinel structure. GCD curves in various current densities of Li-2 electrode are shown in Fig. 9(b). The GCD curves showed distortion in the high potential region corresponding to the broad redox peaks in the CV curves. The symmetries of the CV and GCD curves demonstrate the reversibility of charge storage process for Li-2 electrode. The specific capacitance of Li-2 electrode was calculated to be 271.9 F g1 (68.0 mAh) at 1 A g1 and remained as 197.0 F g1 (49.3 mAh) at 20 A g1, delivering a high retention of 72.4%. CV curves at various scan rates of Mg-1 electrode tested in MgSO4 are shown in Fig. 9(c).
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An irreversible reduction peaks emerged in the low potential region at each scan rate. Still, distortion for the discharge process towards 0 V in the GCD curves was observed for Mg-1 electrode. The specific capacitance of Mg-1 electrode was calculated to be 521.7 F g1 at 1 A g1 and remained to be 326.9 F g1 at 20 A g1, delivering a retention of 62.7%. The CV curves and GCD curves of K1 electrode tested in K2SO4 aqueous electrolyte are shown in Fig. 9(e and f), respectively. The quasi-rectangular shape of CV curves at all scanning rates and the quasi-linear shape of GCD curves at all current densities exhibit good symmetry, further indicating capable pseudo-capacitive behavior of K-1 electrode. The specific capacitances of K-1 electrode was calculated to be 319.7 F g1 at 1 A g1 and remained to be 218.8 F g1 at 20 A g1, delivering a retention of 68.4%. The K-1 electrode tended to be more competent for capacitive charge storage compared with Li-2 and Mg-1 electrode due to its reasonable capacitance and cycling stability. 4. Conclusions In this work, the effect of electrolyte cations on the electrochemically induced activation of Mn3O4 nanospheres electrode together with its capacitive performance was investigated by using Li2SO4, MgSO4 or K2SO4 aqueous electrolyte. A low efficiency of
Fig. 9. (a) CV curves at different scan rates and (b) GCD curves at different current densities of Li-2 electrode in Li2SO4. (c) CV curves at different scan rates and (d) GCD curves at different current densities of Mg-1 electrode in MgSO4. (e) CV curves at different scan rates and (f) GCD curves at different current densities of K-1 electrode in K2SO4.
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phase transition from spinel Mn3O4 to birnessite was obtained in Li2SO4 due to the competitive insertion of Liþ with Hþ, while a full transition was achieved in MgSO4 or K2SO4. Besides, different lateral dimensions of birnessite nanosheets were electrochemically synthesized in electrolytes of different cations, since the cations affected the phase transition process of birnessite nanosheets peeling off from the edge of spinel Mn3O4. The aqueous electrolyte of MgSO4 developed the largest lateral dimension of nanosheets with highest specific capacitance. However, a poor cycling stability was obtained due to the irreversible reductive reaction of the electrode caused by hydrolyzation of Mg2þ. The aqueous electrolyte of K2SO4 developed a moderate lateral dimension of nanosheets with reasonable specific capacitance and cycling stability, being an advisable aqueous electrolyte for capacitive charge storage. Acknowledgment The authors gratefully thank the “National Natural Science Foundation of China (No. 51573058)” for financial support of this work. References [1] H.-S. Kim, J.B. Cook, H. Lin, J.S. Ko, S.H. Tolbert, V. Ozolins, B. Dunn, Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3x, Nat. Mater. 16 (2016) 454. https://doi.org/10.1038/nmat4810. [2] Y. Zhu, L. Peng, D. Chen, G. Yu, Intercalation pseudocapacitance in ultrathin VOPO4 nanosheets: toward high-rate alkali-ion-based electrochemical energy storage, Nano Lett. 16 (2016) 742. https://doi.org/10.1021/acs.nanolett. 5b04610. [3] C. Xia, Z. Lin, Y. Zhou, C. Zhao, H. Liang, P. Rozier, Z. Wang, H.N. Alshareef, Large intercalation pseudocapacitance in 2D VO2(B): breaking through the kinetic barrier, Adv. Mater. 30 (2018) 1803594. https://doi.org/10.1002/adma. 201803594. [4] P. Iamprasertkun, W. Hirunpinyopas, A.M. Tripathi, M.A. Bissett, R.A.W. Dryfe, Electrochemical intercalation of MoO3-MoS2 composite electrodes: charge storage mechanism of non-hydrated cations, Electrochim. Acta 307 (2019) 176e187. https://doi.org/10.1016/j.electacta.2019.03.141. [5] D. Li, W. Guo, Y. Li, Y. Tang, J. Yan, X. Meng, M. Xia, F. Gao, Tunnel structured hollandite K0.06TiO2 microrods as the negative electrode for 2.4 V flexible allsolid-state asymmetric supercapacitors with high performance, J. Power Sources 413 (2019) 34e41. https://doi.org/10.1016/j.jpowsour.2018.11.088. [6] Y. Liu, X. Liu, F. Bu, X. Zhao, L. Wang, Q. Shen, J. Zhang, N. Zhang, L. Jiao, L.Z. Fan, Boosting fast and durable sodium-ion storage by tailoring well-shaped Na0.44MnO2 nanowires cathode, Electrochim. Acta 313 (2019) 122e130. https://doi.org/10.1016/j.electacta.2019.04.140. [7] Y. Yang, Y. Tang, S. Liang, Z. Wu, G. Fang, X. Cao, C. Wang, T. Lin, A. Pan, J. Zhou, Transition metal ion-preintercalated V2O5 as high-performance aqueous zincion battery cathode with broad temperature adaptability, Nano Energy 61 (2019) 617e625. https://doi.org/10.1016/j.nanoen.2019.05.005. [8] Q. Feng, K. Yanagisawa, N. Yamasaki, Hydrothermal soft chemical process for synthesis of manganese oxides with tunnel structures, J. Porous Mater. 5 (1998) 153e162. https://doi.org/10.1023/a:1009657724306. [9] P.L. Goff, N. Baffier, S. Bach, J.-P. Pereira-Ramos, Structural and electrochemical properties of layered manganese dioxides in relation to their synthesis: classical and sol-gel routes, J. Mater. Chem. 4 (1994) 875e881. https://doi.org/ 10.1039/JM9940400875. [10] S.-C. Lin, Y.-T. Lu, Y.-A. Chien, J.-A. Wang, P.-Y. Chen, C.-C.M. Ma, C.-C. Hu, Asymmetric supercapacitors based on electrospun carbon nanofiber/sodiumpre-intercalated manganese oxide electrodes with high power and energy densities, J. Power Sources 393 (2018) 1e10. https://doi.org/10.1016/j. jpowsour.2018.05.019. [11] S. Zhu, L. Li, J. Liu, H. Wang, T. Wang, Y. Zhang, L. Zhang, R.S. Ruoff, F. Dong, Structural directed growth of ultrathin parallel Birnessite on b-MnO2 for highperformance asymmetric supercapacitors, ACS Nano 12 (2018) 1033e1042. https://doi.org/10.1021/acsnano.7b03431. [12] W. He, C. Wang, F. Zhuge, X. Deng, X. Xu, T. Zhai, Flexible and high energy density asymmetrical supercapacitors based on core/shell conducting polymer nanowires/manganese dioxide nanoflakes, Nano Energy 35 (2017) 242e250. https://doi.org/10.1016/j.nanoen.2017.03.045. [13] Y.-Q. Li, X.-M. Shi, X.-Y. Lang, Z. Wen, J.-C. Li, Q. Jiang, Remarkable improvements in volumetric energy and power of 3D MnO2 microsupercapacitors by tuning crystallographic structures, Adv. Funct. Mater. 26 (2016) 1830e1839. https://doi.org/10.1002/adfm.201504886. [14] C. Liu, Y. Chen, W. Huang, Y. Situ, H. Huang, Birnessite manganese oxide nanosheets assembled on Ni foam as high-performance pseudocapacitor electrodes: electrochemical oxidation driven porous honeycomb architecture formation, Appl. Surf. Sci. 458 (2018) 10e17. https://doi.org/10.1016/j.apsusc.
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