graphene oxide hydrogel for capacitive energy storage

Journal Pre-proof Preparation of manganese oxide/graphene oxide hydrogel for capacitive energy storage You Yin Lv, Zhao Jie Huang, Hong Zhong Chi, Xin Zheng, Haiying Qin, Feng Yan PII:

S0013-4686(19)32202-9

DOI:

https://doi.org/10.1016/j.electacta.2019.135330

Reference:

EA 135330

To appear in:

Electrochimica Acta

Received Date: 24 June 2019 Revised Date:

14 October 2019

Accepted Date: 17 November 2019

Please cite this article as: Y.Y. Lv, Z.J. Huang, H.Z. Chi, X. Zheng, H. Qin, F. Yan, Preparation of manganese oxide/graphene oxide hydrogel for capacitive energy storage, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135330. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Electrodeposition conditions play a decisive role in the capacitive performance of the manganese oxide/graphene oxide hydrogel electrodes.

Preparation of manganese oxide/graphene oxide hydrogel for capacitive energy storage a

a

,a

a

a

You Yin Lv , Zhao Jie Huang , Hong Zhong Chi* , Xin Zheng , Haiying Qin , Feng Yan a

b

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China b

Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, 35487, United States of America

Abstract: Electrodepositing manganese oxide into graphene hydrogel provides a facile and effective way to make full use of structural features of the hydrogel and capacity advantage of manganese oxide. The electrodeposition conditions play a key role in the deposition of manganese oxide, but little is known about their effect on electrochemical properties of the hybrid electrodes. Here, we systematically clarify the influences of deposition parameters (including current density, deposition 2+

time, Mn concentration and bath temperature) on the capacitive performance (including apparent capacitance, specific capacitance, rate capability, cycling stability and charge storage mechanism) of manganese oxide/ graphene oxide hydrogel hybrid electrodes. Under the proper preparation conditions, the highest specific capacitance of 453.7 F g

-1

is obtained, and 92.4% capacitance

-1

retention (419.2 F g ) is maintained after 5000 charge/discharge cycles, indicating excellent charge storage capability and cycling stability.

Keywords: Manganese oxide; Graphene hydrogel; Electrodeposition condition; Capacitive performance;

Corresponding author: H. Z. Chi College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China E-mail: hzchi @ hdu.edu.cn (H. Z. Chi) 1

1. Introduction Manganese oxides (MnOx), including compositions of MnO, Mn3O4, Mn2O3, MnO2 and the like, are widely used in battery industry such as supercapacitors, primary batteries and rechargeable batteries[1, 2]. Although cheap in cost, abundant in resources, weak in toxicity and intrinsically high in -1

specific capacitance (1370F g ), low utilization and poor electronic conductivity hinder their applications [3, 4]. Strategies have been putting forward to tackle theses limits. The basic tactics involve improving conductivity and/or availability to the electrolyte. Controllable synthesis of manganese oxides with a specific morphology or microsturcture used to be the subject of extensive research [5, 6]. Doping is another viable method. The introduction of dopants, including metallic [7] and non-metallic [8] elements, can ameliorate electronic structure and electrical conductivity of the manganese oxide by affecting its band edge or forming an impurity state in its band gap. Either way, powdery MnOx are usually produced. When these powders are used as electrochemically active material to fabricate electrodes, an appropriate amount of conductive agent and binder are required. Also, a current collector, usually a nickel foam, is necessary[9, 10]. On the one hand, these additives are electrochemically inactive, causing not only an increase in internal resistance but also a decrease in the proportion of electrochemically active material [11]; on the other hand, the use of these materials complicates the manufacturing process. In contrast, depositing MnOx on conductive substrates is a facile and effective way to make high-performance electdrodes[12, 13]. If the substrate, like graphene monoliths[14, 15], can serve as a current collector, conductive agents and binders are no longer needed, which greatly simplifies the manufacturing process. More importantly, because the morphologically or structurally controlled MnOx are uniformly dispersed on the substrate, the large surface available to the electrolyte gives more active substances the opportunity to participate in charge storage[16]; because the MnOx are directly grown on the substrate, the contact resistance between the active material and the current collector is small, and the path for solid-state diffusion is short[17]. As a result of the improved utilization and conductivity, the as-prepared MnOx possess superior electrochemical properties, for instance, Donne et al. once produced MnO2 films with ultra -1

high capacitance (>2000 F g ) by chronoamperometry technique [18]. This strategy can be realized through techniques such as physical or chemical vapor deposition[19], chemical redox reaction[20], and electrodeposition[21, 22]. 2

The electrodepostion is a better technique for fabricating electrodes and electrode materials since it features accurate controllability, high deposition efficiency, convenient operation, and low cost[23, 24]. The deposits, films or particles with specific morphology, are affixed on substrates without sacrificing their surfaces, so the technique is especially suitable for graphene materials whose electronic properties are susceptible to their structure [20]. We once made MnO2/graphene hydrogel electrode by anodic electrodeposition[25, 26], and the electrodes competently worked in an asymmetric supercapacitor that acts as an electrical source to power a blue light emitting diode, confirming feasibility and practicability of electrodepositing manganese oxide into graphene hydrogel to fabricate hybrid electrode. In addition to the porous and electrically conductive framework, three-dimensional graphene architectures are of light weight and large surface area relative to the nickel foam[27-30]. A large surface area makes sense for the substrate[31, 32]. On the one side, graphene architectures themselves can store charges through the electric double layer at the interfaces between graphene sheets and the electrolyte; on the other side, the large surface area ensures the formation of thin MnOx films. Because electrochemical properties of MnOx films deteriorate as their thickness increases[33], a thinner film helps to improve the capacitance performance when MnOx loading in an electrode is fixed. The thickness, morphology and properties of electrodeposited MnOx films are closely related to deposition conditions. Dubal’s group studied the effects of potentiodynamic, potentiostatic and galvanostatic modes on the structure, morphology and then capacitive performance of MnO2 thin films [34]. Ivey et al. systematically investigated the effects of electrodeposition conditions on nucleation and growth processes, ultimately the morphology and structure of manganese oxide[35]. To date, however, little research has been done on MnOx electrodeposition in porous graphene architectures, and the influence of deposition conditions on the electrochemical properties of the obtained electrode is rarely studied. We previously reported the influence of deposition conditions on the electocrystallization process of manganese oxide[36]. Here, a series of MnOx/graphene oxide hydrogel (MGH) electrodes were fabricated by modifying current density, deposition time, Mn

2+

concentration and bath temperature. The relationships between preparation conditions and capacitive performance were systematically investigated and clarified. Such understanding will offer guidance for the manufacture of MnOx electrodes with outstanding electrochemical properties. 3

2. Experimental In order to fabricate MGH, graphene oxide hydrogel (GH) was electrochemically treated in a Mn(CH3COO)2 solution with different current densities, deposition times, Mn

2+

concentrations and

bath temperatures. The GH was prepared by the hydrothermal treatment of a graphene oxide -1

o

suspension (6 mg mL ) at 150 C for 15 h. The weights of the GHs and MGHs were got by weighing their freeze-dried samples. Their surface morphologies were observed using scanning electron microscopy (Sirion-100, The Netherlands), and their capacitive performances were evaluated by means of cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) on a CHI 660D electrochemical workstation (China). The electrochemical impedance spectroscopies (EIS) were conducted in the frequency range between 100 KHz and 1 Hz at 0 V (vs. SCE) with an AC voltage amplitude of 5 mV on a Gammry instrument (Interface1000, USA). A supplementary document details the reagents used in the study, preparation routes of the graphene oxide, GH as well as MGH, and methods of test and analysis for those electrochemical measurements. 3. Results and Discussion 3.1 Influence of current density 2+

In the light of the CV curve of a Mn containing solution[23], the deposition current density is the 2+

2+

most important factor determining whether or not the Mn oxidation process happens (Mn + 2H2O +

-

→ MnO2 + 4H + 2e ). Therefore, the influence of deposition current density on the capacitive performance of the MGHs was investigated. The MGHs were deposited at different current densities, -2

namely, 5 (MGH-5i), 10 (MGH-10i), 15 (MGH-15i) and 20 mA cm

(MGH-20i). Other deposition

conditions include a 2 M Mn(CH3COO)2 solution, a electric charge transfer of 200 mC and a bath o

temperature of 25 C. The deposition effect is illustrated by taking MGH-10i as an example. No redox peaks appear in -1

its CV curves, and those CV curves maintain rectangular until the scan rate of 100 mV s (Fig. 1a); the charge curves are symmetric with the corresponding discharge curves (Fig. 1b). These features are similar to those of the GH in Fig. S1, but it is worthy to note that at a particular scan rate (or discharge current density), the closed area of the CV curve (or duration of the discharge) of MGH-10i is greater than that of the GH, meaning that the MGH has superior charge storage capability relative to the GH. There is little doubt about the help of manganese oxide to boost the capacitance. A particular concern 4

is the charge storage mechanism of the MGHs. Carbon materials such as graphene store charges through electrochemical double-layers, thereby rapidly completing the charge/discharge processes [37-39]; comparatively, transition metal oxides including manganese oxides store charges by reversible faradic reactions[17, 40, 41]. Although the exchange of cations or protons can be done quickly at the interface between the MnOx and the electrolyte, the charges still take time to diffuse in the solid phase to contact the internal active material. In other words, the introduction of MnOx may alter the charge storage process of the GH electrode. Ardizzone et al. once reported a method to distinguish the specific process of charge storage[42, 43]. At slower scan rates, not only surface materials but also internal materials have the opportunity to take part in the faradic reaction, which implies that if the scan rate is slow enough, all active materials can contribute to the charge storage. As a result, the total charge (qt) can be known -1

-1

1/2

by extrapolating q to v = 0 from the relationship of q and v . Conversely, at a very high scan rate, only the very external part can participate in the charge storage process, and this amount of charge -1/2

(qo) can be acquired by extrapolating q to v = ∞ from the q versus v

plot. -1

As shown by the linear fitting curves in Fig. 1c and d, the qo is 67.2 and 87.3 C g for the GH and -1

MGH-10i, respectively, and their qt is 144.6 and 581.6 C g , respectively. The GH stores charges through electrochemical double-layers, so it is still possible to store large proportion of charges at higher scan rates, and the ratio of qo to qt is 46.5%. However, MGH-10i has more qt than the GH because of the pseudocapacitance (Fig. 1d), so qo only accounts for a small fraction of qt (about 15%) though its value is larger than that of the GH. These data suggest that GH primarily stores charges on its large surface, but a portion of charge still needs time to accomplish the charge transfer process as oxygen-containing functionalities store charge through redox reactions[44]; in MGH-10i case, manganese oxide covers the surfaces of the graphene sheets to take part in the charge storage process, resulting in the graphene sheets to work mainly as a conductive substrate instead of an electrode active material. To intuitively display the influence of deposition current density on the morphology and electrochemical performance of the MGHs, their SEM images are shown in Fig. 1e-h, and their capacitive behaviors are compared in Fig. 2. Obviously, a higher overpotential is necessary to generate a higher deposition current (Fig. 2a). The closed area of the CV curve of MGH-5i is the same 5

as that of the GH, while the areas of MGH-10i and MGH-15i enlarge sequentially. Interestingly, the area of MGH-20i is a little smaller than that of MGH-15i (Fig. 2b). Their GCD curves show a similar trend in discharge duration (Fig. 2c). Their specific and apparent capacitances were calculated and -2

plotted in Fig. 2d. It can be clearly seen that the electrodes prepared at 5 and 15 mA cm have the smallest and largest specific capacitance, respectively, and the apparent capacitance always rises -2

within the research scope. These results indicate that (i) no deposition takes place at 5 mA cm , and (ii) there is an optimum current density for the deposition from the standpoint of specific capacitance. The image of MGH-5i (Fig. 1e) is almost indistinguishable from the one of GH (Fig. S2). Considering its capacitive performance, it confirms no MnOx is deposited on the graphene sheets at -2

the current density of 5 mA cm . As the deposition current increases, the MnOx coatings gradually thicken (Fig. 1f-h). Such change is in line with the relationship between deposition rate and current density (Eq. 1) [35]: V=

µ··

(1)

ρ

where V is deposition rate, µ means current efficiency and i represents deposition current density, respectively; ρ and E are the density and electrochemical equivalent of the deposit, respectively. As long as the deposition is able to occur and the current efficiency changes little, the amount of deposited MnOx will augment along with the increasing current density - the higher the deposition current density, the more the MnOx is deposited, and consequently, the larger the apparent capacitance is obtained. However, the charge storage capability of a MnOx film deteriorates as it becomes thick [33], and thus the specific capacitances in Fig. 1d have a maximum value. This is not just a matter of specific capacitance, but the rate capability (Fig. 2e) and cycling life (Fig. 2f) worsen with the increase of the deposition current density, as well. When the discharge current density varies -1

-1

from 0.5 to 5 A g , specific capacitance decreases from 131.4 to 77.3 F g for the GH, from 135.0 to -1

-1

-1

76.8 F g for MGH-5i, from 380.4 to 197.7 F g for MGH-10i, from 534.1 to 260.9 F g for MGH-15i, -1

and from 519.2 to 209.6 F g for MGH-20i, respectively. Similarly, after 5000 charge/discharge cycles, the capacitance retentions of the GH, MGH-5i, MGH-10i, MGH-15i and MGH-20i are 96.9, 96.3, 94.1, 92.7 and 85.2%, respectively. Although both qo and qt grow greater (Fig. 2g and h), the qo/qt ratio drops due to a more significant increase in qt. A table (Table 1) is given to generalize the influence of 6

deposition current density on capacitive performance of the MGHs. 3.2 Influence of deposition time According to equation 1 and the law of conservation of charge, provided the deposition can happen, the amount of a deposit increases with the prolonging of deposition time, which inevitably affects the electrochemical properties of the as-prepared electrode. Hence, the influence of deposition time on -2

capacitive performance of the MGHs was investigated. The MGHs were fabricated at 15 mA cm and o

25 C, from a 2 M Mn(CH3COO)2 solution, and the processes lasted for 8 (MGH-8s), 17 (MGH-17s), 33 (MGH-33s) and 67 s (MGH-67s), respectively. Their deposition curves are depicted in Fig. 3a. The MGH-67s is used as an example to exhibit capacitive behaviors of the MGHs, and its CV profiles at different scan rates and GCD curves at different charge/discharge current densities are -1

presented in Fig. S3a and b, respectively. At a certain scan rate, say 20 mV s , the closed area of the CV profiles of the MGHs enlarges with the deposition time (Fig. 3b), and so does the length of discharge time (Fig. 3c), implying the gradual improvement of charge storage capability. The calculated data in Fig. 3d, both specific capacitance and apparent capacitance, support this conclusion. If a very thin film is prepared, MnOx itself can have a rather high specific capacitance[18]. Then, the capacitive performance deteriorates with the thickened films. In our study, however, though the larger surface area renders the substrate the ability to store charge, graphene hydrogel itself does not -1

have a high specific capacitance (typically 120 to 200 F g ). In the case of the deposition time of 8 s, the MnOx film only accounts for small portion of the total electrode. Consequently, the specific capacitance of MGH-8s is lifted inadequately. With the extension of the deposition time, the amount of MnOx increases correspondingly, meaning the contribution of pseudocapacitance increases. The end result is the improved charge storage capability. Nevertheless, the prolonging of deposition time or thickening of the MnOx film exacerbates the decline in utilization of active material at higher discharge rates (Fig. 3e), the loss in capacitance during cycling operation (Fig. 3f) and the reduction in proportion of qo due to the growing qt (Fig. 3g) but almost unchanged qo (Fig. 3h). The influences of deposition time on the capacitive performance of the MGHs are summarized in Table 2. 2+

3.3 Influence of Mn concentration Ivey et al. once demonstrated that the supersaturation ratio (S) of Mn ion containing solutions had an direct impact on the crystal structure and morphology of the deposited MnOx and could be adjusted 7

2+

by changing deposition parameters [35, 45]. On the basis of the formation reaction of MnOx (Mn + +

-

2H2O → MnO2 +4H +2e ), the S value can be expressed as[35, 45]: S=

[α  ]·[α  ] [α  , ]·[α , ]

(2) 2+

-

where α and α e represent actual and equilibrium activities of Mn and OH , respectively. Since the 2+

ion activity is closely related to its concentration, the variation of Mn concentration will alter the S value, affect the electrocrystalization mode of the MnOx and ultimately determine its capacitive -2

o

performance. To understand this influence, MGHs were fabricated at 15 mA cm and 25 C in 0.5 (MGH-05c), 1 (MGH-1c), 2 M (MGH-2c) and saturated Mn(CH3COO)2 solution (MGH-Sc) for a deposition time of 67 s. As a result of the decrease of charge carriers, or, the increase of electrolyte resistance, the deposition potential rises reasonably with the reduction of Mn

2+

concentration (Fig. S4a). The

capacitance characteristics of the MGHs fabricated from various concentration of Mn(CH3COO)2 solution are elucidated by the CV and GCD tests of MGH-1c (Fig. S3b and c), and lateral comparisons of performance of the MGHs are given in Fig. 4. Except for MGH-Sc, the closed area of CV profiles -1

2+

obtained at 20 mV s contracts as the Mn concentration is reduced (Fig. 4a); and the discharge time -1

of GCD curves obtained at 1 A g also follows a similar trend (Fig. 4b). In accordance with equation 2, a low concentration of Mn(CH3COO)2 solution has a small S value, which means the nucleation rate is sluggish, and subsequent nucleation tends to proceed on the previous ones rather than to re-nucleate new nucleus (instantaneous nucleation). Its deposit is apt to form discrete clusters or fibrous 2+

morphology in the growth process because only a minute amount of Mn can be replenished at the deposition interface due to the lower Mn

2+

concentration and the smaller concentration gradient

between the deposition interface and bulk of the electrolyte (Fig. 4g). Hence, in spite of the minimal apparent capacitance (and qt in Fig. 4f and Fig. S4e) deriving from the slower deposition rate (Eq. 1), MGH-05c has an acceptable specific capacitance (Fig. 4c) since such morphology is conducive to the acquirement of the electrolyte [35], which raises the utilization of the MnOx (and and qo in Fig. 4f and 2+

Fig. S4f). With the increase of Mn concentration, the electrodeposition process becomes easier (Fig. 4h and i), the deposition rate is accelerated, and more MnOx is deposited in the MGHs. Accordingly, charge storage capability of MGH-1c and MGH-2c is improved (Fig. 4c). As for MGH-Sc, its drop in 8

specific capacitance may result from thick MnOx films or coarse MnOx grains (Fig. 4j). Similar to the 2+

MGHs deposited at different deposition times, increasing Mn concentration spoils the high rate capability (Fig. 4d), aggravates the capacitance fading after cycling (Fig. 4e) and reduces the qo/qt ratio 2+

(Fig. 4f). Table 3 lists the influence of Mn concentration on capacitive performance of the MGHs. 3.4 Influence of bath temperature In order to account for the influence of bath temperature, GHs were electrodeposited in saturated -2

Mn(CH3COO)2 solutions for 67 s at 15 mA cm and at various temperatures, namely, 5 (MGH-5d), 15 o

(MGH-15d), 25 (MGH-25d) and 35 C (MGH-35d), respectively. Higher temperatures can (i) raise the 2+

Mn concentration of a saturated Mn(CH3COO)2 solution or (ii) enhance ionic activity owing to the increased activity coefficient (Eq. 3). Thus, as stated in the Nernst equation (Eq. 4), the deposition potential lowers with the elevating temperature (Fig. S5a): k  = k  + ak  T − 25 + bT − 25 E = E" −

#

%$ln

[α ]·[α ]

[α  ]·[α  ]

(Eq. 3)

(Eq. 4) o

in equation 3, kT and K25 are activity coefficient at T and 25 C, respectively; a and b are constants; and in equation 4, E, E0, R, z and F are the cell potential, standard cell potential, the gas constant, the number of moles of transferred electrons and Faraday’s constant. CV and GCD curves of MGH-15d are depicted in Fig. S5b and c to display the typical capacitive behaviors of the MGHs. Both the closed area of CV profiles (Fig. 5a) and the discharge duration of GCD curves (Fig. 5b) indicate that MGH-35d has a smaller specific capacitance with respect to MGH-25d. The data in Fig. 5c further confirm that the apparent capacitance of MGH-35d also drops slightly. It is inappropriate just to attribute the capacitance decay to the thickness of MnOx film. Because the deposition time (67 s) -2

and current density (15 mA cm ) are constant, the number of electrons for the depositions should be the same, and then the deposition amount of the MnOx should be the same, too. However, the bath temperature has rather complex effects on the electrocrystallization process of MnOx. On the one side, a higher temperature benefits the electrocrystallization of MnOx. According to van’t Hoff equation, the equilibrium constant of the deposition reaction (Mn

2+

+

-

+ 2H2O → MnO2 +4H +2e )

increases with the elevating temperature, meaning a reduction in denominator or an increase in S value in equation 2. The rising S value lets the nucleation process transform from an instantaneous 9

mode to a progressive mode that facilitates rapid nucleation. Also, the enhanced ion activity (Eq. 3) helps ions diffuse to the deposition interfaces, and thereby promoting the growth of MnOx crystalline. o

In an extreme case (for example, 35 C), the higher temperature can result in an abnormal grain growth. So, besides the thickened film, the superposed and coarse MnOx particles (Fig. S6) cut down the availability of the electrolyte, further deteriorating the capacitive performance of the electrode. -

-

Furthermore, at high temperatures, side reactions are significant (4OH →2H2O+O2+4e ) [46]. In o

practice, when the electrodeposition was carried out at 35 C, a large number of bubbles were observed on the surface of the deposited GH. That is, some transferred electrons are used for the formation of oxygen instead of the deposition of MnOx. The high-rate capability, cycling capacitance retention and qo/qt ratio of the MGHs fabricated at different bath temperatures are shown in Fig. 5d to f, respectively. As a semiconductor material, MnOx inclines to undergo the pseudocapacitive reaction on or near surfaces. A slow charge rate +

ensures enough time for Na to diffuse from the surface to the interior, so most active substances have the opportunity to participate in charge storage, leading to a high qt. In the opposite way, it is +

impossible for Na to achieve a complete diffusion, so only the active materials on or near the surface can contribute to capacitance (qo), while internal materials are “dead”[47]. That is why the specific capacitance declines as the discharge rate increases, and the more the amount of deposition, the smaller the capacitance retention (Fig. 5d). Although the actual causes for the capacitance fading 2+

after long term cycling remain unclear, it is often blamed to the formation of soluble Mn through 3+

4+

2+

the disproportionation reaction of 2Mn →Mn + Mn . Since the porous structure of the GH is still reserved after the electrodeposition, some dissolved Mn

2+

is detained in the pores and may be

re-oxidized to MnOx during the charge/discharge operation. We thus believe that it is the reason why the MGHs possess superior cycling stability relative to other types of MnOx electrodes[35]. EIS tests were performed to interpret the electrochemical behaviors of the MGHs (Fig. 5g). No matter the GH or the MGHs fabricated at different deposition conditions, their Nyquist plots are consistent with the frequency response characteristics of porous materials. Nevertheless, the presence of MnOx brings about an arc in intermediate frequencies, implying another resistance is introduced into the charge storage process. The pseudocapacitive reaction of MnOx in a Na2SO4 +

-

solution is MnO2 + Na + e →NaMnO2 [48]. Obviously, the insertion/desertion process of Na 10

+

generates a reactive resistance (Rct) that associates with the arc. With the elevating bath temperature, the Rct and the Warburg impedance (WR), which corresponds to the slope of the straight line in the low-frequency region, become larger gradually. As described above, the MnOx film becomes thicker or the MnOx grains grow coarser as the bath temperature elevates. These changes make the Na

+

insertion/desertion process or electron diffusion process difficult. The Rct and Warburg impedance were evaluated based on the equivalent circuit in the inset of Fig. 5g, and their data along with other capacitive performance data are tabulated in Table 4. 4. Conclusion In summary, electrodeposition conditions play a decisive role in capacitive performance of the MGH electrodes. Improving the deposition current density, prolonging the deposition time, increasing the 2+

Mn concentration or elevating the bath temperature are all beneficial to MnOx deposition. However, +

the decline in qo/qt ratio reveals that thick films or coarse grains make the Na insertion/desertion process or electron diffusion process difficult, leading to the deterioration in high-rate capability. Besides, despite loss in capacitance with cycling, the MGHs exhibit superior cycling stability due to the confinement effect of pores of the GH that force some dissolved Mn

2+

to be re-precipitated.

Specifically: (I) A proper current density is the prerequisite for the fabrication of a high-performance MGH electrode. Neither of too small or too large a current density is conducive to charge storage. It is 15 -2

mA cm in this study. 2+

(II) The deposition rate of MnOx is slower at a lower Mn concentration, but the deposit is of fibrous morphology that contributes the electrolyte contact and charge transport. (III) The dominant process of electrocrystallization is transformed from instantaneous nucleation to progressive nucleation to abnormal growth with the elevating bath temperature, causing that the +

insertion/desertion and diffusion processes of Na become laborious as evidenced by the EIS tests. -1

(IV) The optimum specific capacitance (453.7 F g ) is derived from the electrode fabricated from a 2 -2

o

M Mn(CH3COO)2 at 15 mA cm and 25 C for 67 s. After 5000 charge/discharge cycles, 92.4% of its initial capacitance is still remained. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.electacta.xxxx.xx.xxx. 11

Acknowledgements The authors acknowledge financial supports from the Natural Science Foundation of Zhejiang Province (grant number LY18B060005).

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16

Table 1. The influences of deposition current density on the capacitive performance of the MGHs

Electrode

Specific capacitance (F g-1)

Apparent capacitance (F)

Capacitance retention* (%)

Capacitance retention** (%)

Qt (C g-1)

qo (C g-1)

MGH-5i MGH-10i MGH-15i MGH-20i

124.2 332.8 457.1 413.9

0.31 0.88 1.45 1.59

56.9 52.0 48.8 40.4

96.3 94.1 92.7 85.2

157.5 581.6 860.3 889.7

64.6 87.3 94.7 93.0

* The ratio of the capacitance obtained by discharging at 5 A g-1 to the one at 0.5 A g-1. ** The ratio of the capacitance obtained at very beginning of the cycling test to the one at 5000 cycles (1 A g-1).

17

Table 2. The influences of deposition time on the capacitive performance of the MGHs

Electrode

Specific capacitance (F g-1)

Apparent capacitance (F)

Capacitance retention* (%)

Capacitance retention** (%)

Qt (C g-1)

qo (C g-1)

MGH-8s MGH-17s MGH-33s MGH-67s

273.4 353.9 420.6 452.1

0.60 0.92 1.26 1.46

55.7 52.7 49.5 47.2

95.1 94.3 94.1 92.6

440 662 769 859.1

79.4 86.4 89.6 93.0

* The ratio of the capacitance obtained by discharging at 5 A g-1 to the one at 0.5 A g-1. ** The ratio of the capacitance obtained at very beginning of the cycling test to the one at 5000 cycles (1 A g-1).

18

2+

Table 3. The influences of Mn concentration on the capacitive performance of the MGHs

Electrode

Specific capacitance (F g-1)

Apparent capacitance (F)

Capacitance retention* (%)

Capacitance retention** (%)

Qt (C g-1)

qo (C g-1)

MGH-05c MGH-1c MGH-2c MGH-Sc

356.1 424.5 453.7 439.2

0.79 1.32 1.44 1.47

55.1 54.3 47.7 44.8

94.8 94.0 92.4 89.7

571.5 806.4 854.9 882.1

99.9 92.5 94.7 92.4

* The ratio of the capacitance obtained by discharging at 5 A g-1 to the one at 0.5 A g-1. ** The ratio of the capacitance obtained at very beginning of the cycling test to the one at 5000 cycles (1 A g-1).

19

Table 4. The influences of bath temperature on the capacitive performance of the MGHs Electrode

Specific capacitance (F g-1)

Apparent capacitance (F)

Capacitance retention* (%)

Capacitance retention** (%)

Qt (C g-1)

qo (C g-1)

Rct (Ω)

WR (Ω)

MGH-5d MGH-15d MGH-25d MGH-35d

333.8 417.6 441.3 421.5

0.66 1.23 1.46 1.55

54.9 53.3 45.5 42.1

94.6 93.3 90.3 87.4

523.6 617.3 892.8 909.1

91.2 92.8 93.5 91.6

7.2 7.9 8.8 10.7

3.0 3.8 4.9 6.2

* The ratio of the capacitance obtained by discharging at 5 A g-1 to the one at 0.5 A g-1. ** The ratio of the capacitance obtained at very beginning of the cycling test to the one at 5000 cycles (1 A g-1).

20

Figure caption

Fig. 1. (a) CV profiles at different scan rates and (b) GCD curves at different charge/discharge rates of the MGH-10i; the -1

1/2

charge density (q) with respect to voltammetric scan rate (v) of the GH and MGH-10i: (c) the q vs. v

plot and (d) the

-1/2

q vs. v

plot; SEM images of (e) MGH-5i, (f) MGH-10i, (g) MGH-15i and (h) MGH-20i.

-1

-1

Fig. 2. (a) Deposition curves, (b) CV profiles at 20 mV s , (c) GCD curves at 1 A g , (d) specific and apparent -1

capacitances, (e) rate capability curves, (f) cycling life curves, (g) q vs. v

1/2

-1/2

plots and (h) q vs. v

plots of the GH and

MGHs fabricated at different deposition current densities.

-1

-1

Fig. 3. (a) Deposition curves, (b) CV profiles at 20 mV s , (c) GCD curves at 1 A g , (d) specific and apparent -1

1/2

capacitances, (e) rate capability curves, (f) cycling life curves, (g) q vs. v

-1/2

plots and (h) q vs. v

plots of the MGHs

fabricated at different deposition times.

-1

-1

Fig. 4. (a) CV profiles at 20 mV s , (b) GCD curves at 1 A g , (c) specific and apparent capacitances, (d) rate capability curves, (e) cycling capacitance retentions, (f) charge densities of the MGHs fabricated from different concentrations of Mn(CH3COO)2 solution and their SEM images of (g) MGH-05c, (h) MGH-1c, (i) MGH-2c and (j) MGH-Sc.

-1

-1

Fig. 5. (a) CV profiles at 20 mV s , (b) GCD curves at 1 A g , (c) specific and apparent capacitances, (d) rate capability curves, (e) cycling capacitance retentions, (f) charge densities and (g) Nyquist plots of the MGHs fabricated at different bath temperatures.

21

Fig. 1.

22

Fig. 2.

23

Fig. 3.

24

Fig. 4.

25

Fig. 5.

26

Declaration of interests ☑The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: