Ostwald Ripening Improves Rate Capability of High Mass Loading

Subscriber access provided by AUSTRALIAN NATIONAL UNIV

Letter

Ostwald Ripening Improves Rate Capability of High Mass Loading Manganese Oxide for Supercapacitors Yu Song, Tianyu Liu, Bin Yao, Mingyang Li, Tianyi Kou, Zihang Huang, Dong-Yang Feng, Fuxin Wang, Yexiang Tong, Xiao-Xia Liu, and Yat Li ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00405 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Energy Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

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

ACS Energy Letters

Ostwald Ripening Improves Rate Capability of High Mass Loading Manganese Oxide for Supercapacitors Yu Song1,2, Tianyu Liu2, Bin Yao2, Mingyang Li2,3, Tianyi Kou2, Zi-Hang Huang1,2, Dong-Yang Feng1, Fuxin Wang3, Yexiang Tong3, Xiao-Xia Liu1* and Yat Li2*

1

Department of Chemistry, Northeastern University, Shenyang, 110819, P. R. China

2

Department of Chemistry and Biochemistry, University of California- Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA

3

KLGHEI of Environment and Energy Chemistry MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China

1

ACS Paragon Plus Environment

ACS Energy Letters

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

Page 2 of 21

Abstract Realizing fast charging/discharging for high mass loading pseudocapacitive materials has been a great challenge in the field of supercapacitors, because of the sluggish electron and ion migration kinetics through the thick electrode materials. Here we demonstrate for the first time a facile hydrothermal treatment that can substantially enhance the rate capability of a highly-loaded manganese oxide electrode via Ostwald ripening process. Hydrothermal treatment improves not only the electrical conductivity of manganese oxide, but also the ion diffusion rate in the thick oxide film. At slow scan rates below 40 mV s−1, the capacitance of hydrothermally treated manganese oxide electrode increases linearly with mass loading (up to 23.5 mg cm−2) as expected for a capacitor under the non-diffusion-limited condition. At high scan rates beyond 100 mV s−1, capacitive saturation is observed only at a high mass loading of ~9 mg cm−2, which is significantly larger than the values reported for other manganese oxide electrodes. The electrode achieves an areal capacitance of 618 mF cm−2 at a high scan rate of 200 mV s−1, which is 3 times higher than that of the untreated sample. An asymmetric supercapacitor assembled with a hydrothermally treated manganese oxide cathode and a vanadium oxide/exfoliated carbon cloth anode can deliver a good volumetric energy density of 5 mWh cm−3. This value is 2-10 times higher than the values obtained from supercapacitors with comparable dimensions.

Table of contents

2


Page 3 of 21

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

ACS Energy Letters

Manganese dioxide (MnO2), a prototypical pseudocapacitive material，has received great attention as an electrode material for electrochemical capacitors because of its high theoretical capacitance (1110 F g−1 at a potential window of 1 V), environmental friendliness, low cost, etc.1-10 Indeed, MnO2 electrodes have achieved specific capacitances close to its theoretical value when the loading amount of MnO2 is small, usually less than 0.5 mg cm−2.7,11-15 Yet, electrochemical capacitors with such a small mass loading of capacitive active material may not be practically feasible. Commercially available electrodes typically require an areal mass loading of active materials about 8-10 mg cm−2.16-18 Improving the specific capacitance of highly loaded MnO2 electrode, especially at large current density (i.e., fast discharging), has been an outstanding challenge in the field. This challenge is two folded. Sluggish electron transport in a thick, non-conductive MnO2 film is problematic when the mass loading of MnO2 is high. Likewise, ion diffusion is also expected to be hindered in a densely packed and thick film.19,20 As a result, the specific capacitance and rate capability of MnO2 electrodes declined significantly as the increase of mass loading or film thickness.17,21 Strategies involving deposition of MnO2 onto three-dimensional conductive substrates (current collector) have been developed to enhance the performance of highly loaded MnO2 electrodes.19,22-24 In addition, Yu et al. reported that wrapping MnO2 with conjugated polymers and carbon nanotubes can greatly improve the performance of MnO2, due to the enhanced charge transfer through the wrapping materials.25 While these strategies improved the specific capacitance of highly loaded MnO2 electrodes at slow scan rates/current densities, their capacitances were not able to be retained at fast charge and discharge rates (Table S1, Supporting Information).3,19,22 Alternatively, the capacitive performance of MnO2 electrode can be augmented by improving the 3


ACS Energy Letters

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

Page 4 of 21

intrinsic electrical conductivity of MnO2. Electrochemical deposition methods have been commonly used for synthesis of MnO2 materials. However, most electrochemically deposited MnO2 have amorphous structures with poor electrical conductivity.8,9,11 Moreover, these MnO2 materials are often mixed with lower valence Mn species (e.g., trivalent species MnOOH), forming non-stoichiometric manganese oxides (MnOx).8,9 Previous studies showed that manganese oxides with trivalent species have much lower electrical conductivity than MnO2, which negatively affect their electrochemical performance.8,26-28 Post-growth annealing has been used to increase the electrical conductivity of MnOx materials by simultaneously increasing crystallinity and eliminating the low valence state of Mn.6-8,20,29,30 However, thermal annealing is usually accompanied with the sacrifice of surface area and structural water, which is known to be critical in promoting ion diffusion in oxide lattice.8,9,31-35 Therefore, an efficient strategy that can simultaneously improve the electrical conductivity and ion diffusion of a high mass loading manganese oxide electrode has not yet been demonstrated. Here we demonstrate for the first time a facile hydrothermal method based on Ostwald ripening that substantially enhances the capacitive performance of a highly-loaded manganese oxide electrode, by simultaneously improving its electrical conductivity and facilitating ion transfer rate. The rate capability of the electrode with a mass loading of 9 mg cm−2 is comparable to other manganese oxide with much lower mass loading reported in the literature. The electrode achieved an excellent areal capacitance of 618 mF cm−2 at a fast scan rate of 200 mV s−1, which is 3 times higher than that of the untreated electrode. In addition, an asymmetric supercapacitor assembled with a hydrothermally treated manganese oxide cathode and a vanadium oxide/exfoliated carbon cloth anode can deliver a remarkable volumetric energy density of 5 mWh cm−3. This value is 2-10 times higher than the values obtained from supercapacitors with comparable dimensions.

4


Page 5 of 21

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

ACS Energy Letters

A thick layer of manganese oxide (denoted as MnOx) was electrochemically deposited onto a piece of carbon cloth in a 0.1 M manganese acetate (MnAc2) electrolyte at a constant current density of 10 mA cm−2 for 1500 s. As shown in Figure 1a-c, a uniform MnOx coating is obtained on each carbon fiber. The rapid electrochemical nucleation process leads to the formation of poorly crystalline manganese oxide.11,36,37 The as-prepared MnOx was hydrothermally treated in a Teflon-lined autoclave at 90 ºC for 2 hours. The obtained sample is denoted as MnOx-h. The hydrothermal treatment leads to two obvious structural modifications of the manganese oxide film. First, a highly crystalline layer is formed on the film surface, assembling a core-shell like structure, as shown in Figure 1 c, e and g. Time-dependent growth experiments showed that only a small amount of crystallites were randomly distributed on the surface after the first hour hydrothermal treatment (Figure S1a and b, Supporting Information). As the increase of treatment time, the amount and size of the surface crystallites increased and eventually formed a dense crystal layer (Figure S1e and f, Supporting Information). Second, the manganese oxide film became more porous after the hydrothermal treatment (Figure 1d and e). We attribute the structural modifications to Ostwald ripening, which is a crystal growth process that involves coarsening and recrystallization. Larger crystals with smaller surface to volume ratio are favored over the energetically less stable smaller crystallites during the course of the ripening.38,39 We anticipate that the dissolution of small crystallites and the re-construction of large crystallites occurred during the hydrothermal treatment resulting in the formation of highly crystalline outer layer and interior porous structure.

5


ACS Energy Letters

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

Page 6 of 21

Figure 1. (a) Schematic illustration showing the structural evolution during the course of the hydrothermal reaction. Cross-sectional SEM images of (b) MnOx and (c) MnOx-h. Magnified SEM images of (d) MnOx and (e) MnOx-h highlighted in the dashed box of (b) and (c), respectively. Top view SEM images of (f) MnOx and (g) MnOx-h. A number of experiments were carried out to reveal the structural change of the MnOx film upon hydrothermal treatment. Burnauer-Emmett-Teller (BET) analysis of N2 gas sorption data shows that the specific surface area of MnOx is slightly increased after the hydrothermal treatment (Figure 2a). More importantly, the pore size is widened by more than 50% (from 4.3 nm to 6.5 nm) after hydrothermal treatment, confirming the increase of structural porosity. Three peaks (labeled as Mn-O-Mn, Mn-O-H, and H-O-H) are identified in the O 1s X-ray photoelectron spectroscopy (XPS) spectra of MnOx and MnOx-h (Figure 2b and c). The peak located at the binding energy of 531.5 eV has been reported for Mn-O-H originates from trivalent manganese oxy-hydroxide.26,40 Judging from the peak area, MnOx-h exhibits lower content of Mn-O-H than MnOx. According to the linear relationship between the 6


Page 7 of 21

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

ACS Energy Letters

oxidation state and the Mn 3s peak separation, the mean valence state of Mn in MnOx and MnOx-h are estimated to be 3.5 and 3.8, respectively (Figure S2, Supporting Information).31 The increased valence state of MnOx-h indicates that electrochemically deposited MnOx was oxidized during hydrothermal treatment, possibly by the dissolved oxygen in the precursor solution under the hydrothermal conditions.5,26 The peak centered at the binding energy of 532.6 eV is ascribed to structural water, including both physically adsorbed and chemically bonded water in manganese oxide.9,40 It has been reported that structural water can function as ion conductive channels because ions can migrate near manganese oxide surface by hopping between the water molecules and the −OH sites on the oxide surface.35 Significantly, MnOx and MnOx-h have similar content of structural water (Figure 2b and c). The structural water molecules are unlikely to leave the host oxide during the hydrothermal reaction because of two possible reasons. First, the structural water is not necessarily perturbed during the Ostwald ripening process. Chuah et. al reported that structural water can be retained in hydrous zirconia during the Ostwald ripening process in a hydrothermal treatment at 100 ºC.41 Second, the reaction was carried out in an aqueous environment and the hydrothermal treatment temperature was much lower than the dehydration temperature of manganese oxide in air.8 In contrast, a control MnOx sample annealed in air (denoted as MnOx-a) exhibited significantly lower concentration of two hydrous species (i.e., Mn-OH and H2O) and smaller surface area at the similar mass loading (Figure S3, Supporting Information). Therefore, our hydrothermal treatment is confirmed to be an effective method in improving the crystallinity of manganese oxide while retaining structural water and surface area, which is impossible for conventional thermal annealing methods.

7


ACS Energy Letters

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

Page 8 of 21

Figure 2. (a) N2 sorption isotherms of MnOx and MnOx-h. Inset shows the pore size distribution derived from the corresponding adsorption curves. Core level O 1s XPS spectra of (b) MnOx and (c) MnOx-h. The black dashed lines are the experimental results, which can be deconvoluted into three synthetic peaks (blue, yellow and red curves). The black solid curves are the summation of the synthetic peaks. The peak ratios are calculated based on the areas of the synthetic peaks. Furthermore, X-ray diffraction (XRD) and transmission electron microscopy (TEM) images were performed to investigate the effect of hydrothermal treatment on the structure of MnOx. As shown in Figure S4 (Supporting Information), the crystal phase of manganese oxide changed from ε-MnO2 (MnOx) to γ-MnO2 (MnOx-h). The crystal structures of ε-MnO2 and γ-MnO2 are very similar, except that γ-MnO2 exhibits less structural defects (e.g., De Wolff faults and microtwinning) than ε-MnO2.2,42 The diffraction peaks of MnOx-h also have smaller full width at half maximum (FWHM) than that of MnOx, indicating it has higher crystallinity and/or crystal size.41 These observations are consistent with the proposed Ostwald ripening mechanism. High resolution TEM images and selected area electron diffraction (SAED) provides direct evidence for the improved crystallinity after the hydrothermal treatment (Figure 3). The highly diffusive rings indicate that the crystallinity of MnOx is rather poor (Figure 3a inset). The lattice fringe spacing observed in the high-resolution TEM image (Figure 3c) is 8


Page 9 of 21

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

ACS Energy Letters

0.264 nm, 0.242 nm, and 0.213 nm, corresponding to the d-spacing of β-MnOOH (311), ε-MnO2 (100) and (101) planes, respectively. Hydrothermal treatment converted this poorly crystalline structure to a polycrystalline structure composed of γ-MnO2 crystals (Figure 3b inset). The well-resolved lattice fringes corresponding to γ-MnO2 (131), (120), and (300) planes are observed for the MnOx-h sample, further confirming the crystallinity of MnOx was indeed increased upon hydrothermal treatment.

Figure 3. TEM images of (a) MnOx and (b) MnOx-h. Insets show the SAED patterns. (c) Lattice-resolved TEM image of MnOx. (d and e) Lattice-resolved TEM images of MnOx-h collected from the region highlighted by dashed boxes in (b). Electrochemical performances of the manganese oxide electrodes hydrothermally treated for different durations were studied using a three-electrode electrolytic cell filled with 5 M LiCl aqueous electrolyte (Figure S5 in the Supporting Information). Among the samples, MnOx-h hydrothermally 9


ACS Energy Letters

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

Page 10 of 21

treated for 2h delivered the best electrochemical performance. Here we compare the performance of MnOx with the 2h MnOx-h sample. As shown in Figure S6a (Supporting Information), both MnOx and MnOx-h electrodes exhibit pseudocapacitive behavior with a series of broad peaks observed at slow scan rates, which can be attributed to Li+ intercalation and deintercalation process.43 The negative shift of the anodic peak implies that MnOx-h has higher electronic and ionic conductivities than MnOx.44 The improved kinetics also reflected in the better rate capability of MnOx-h (Figure 4a). MnOx-h exhibits an excellent areal capacitance of 1610 mF cm−2 (equivalent to a gravimetric capacitance of 180 F g−1 normalized to the mass of active materials, i.e., ~ 9 ± 0.1 mg cm−2) at a scan rate of 5 mV s−1, which is only slightly higher than the value obtained from MnOx. Nevertheless, the capacitance differences at fast scan rates are obvious. MnOx-h yields a remarkable capacitance of 618 mF cm−2 at 200 mV s−1, which is 3 times higher than MnOx. The capacitance and rate capability of MnOx-h are among the best values reported for manganese oxide electrodes with comparable mass loading (Table S1, Supporting Information).3,10,12,19,22-24,45 In contrast, the control sample MnOx-a exhibited substantially lower capacitance than that of MnOx-h at all current densities we measured, owing to the decreased surface area and the loss of structural water (Figure S7, Supporting Information). These experimental results clearly indicate that conventional air annealing method have limitations for improving the electrochemical performance of manganese oxide electrodes with high mass loading. Figure 4b shows the areal capacitance of MnOx-h collected at different scan rates as a function of mass loading. At slow scan rates below 40 mV s−1, the capacitance increased linearly with mass loading (up to 23.5 mg cm−2) as expected for a capacitor under the non-diffusion-limited condition. It means most of the active materials participated in charge storage. At high scan rates beyond 100 mV s−1, capacitive 10


Page 11 of 21

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

ACS Energy Letters

saturation was observed only at a high mass loading of ~9 ± 0.1 mg cm−2. Notably, this is currently the best performance ever obtained from carbon supported MnO2 electrode with such high mass loading. This performance is substantially better than other previously reported MnO2-based electrodes, such as MnO2-carbon nanotube textile (capacitance saturated at ~3.8 mg cm−2),3 3D graphene/MnO2 composite (capacitance saturated at 3.31 mg cm−2)19 and graphene foam/carbon nanotube/MnO2 film (capacitance saturated at ~2.1 mg cm−2).22 Besides, the electrode with mass loading of 9 mg cm−2 is considered to be feasible for practical applications, as most commercial supercapacitors have active mass loading around 8-10 mg cm−2.46 An ultra-high areal capacitance of 4.2 F cm−2 was obtained from a MnOx-h electrode with a mass loading of 23.5 mg cm−2. MnOx-h also exhibited good cycling stability with 91% capacitance retention in 6000 cycles (Figure S8, Supporting Information). SEM studies confirmed that there is no obvious morphological change in MnOx-h after the cycling stability test (Figure S9, Supporting Information). To understand the interplay between the structural modifications and the enhanced rate performance, electrochemical impedance spectroscopy (EIS) studies were conducted for MnOx and MnOx-h. As shown in Figure 4c, MnOx-h exhibits considerably smaller equivalent series resistance (Rs) and charge transfer resistance (Rct) than that of MnOx, confirming the electrical conductivity of MnOx is enhanced after hydrothermal treatment. In addition, MnOx-h also has narrower Warburg resistance region than MnOx,47 meaning ionic transport is more efficient in MnOx-h.48 We also performed potentiostatic intermittent titration technology (PITT) to qualitatively compare the Li+ diffusion coefficient (D) in the two electrodes (Figure S10, Supporting Information).49 Indeed, PITT results show that the diffusion coefficient increased after hydrothermal treatment, which is consistent with EIS 11


ACS Energy Letters

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

Page 12 of 21

data.49 On the contrary, MnOx-a presents the widest low-frequency Warburg region (the linear part with a slope angle of ~45o), proving that MnOx-a has the largest ion diffusion resistance (Figure S11, Supporting Information), again supporting the importance of the presence of structural water and pores for ion diffusion. Electrode kinetic and quantitative analysis using Trasatti method1,50 were used to understand the influence of hydrothermal treatment on (1) capacitive controlled (surface) capacitance including electrical double layer effect and fast faradaic contribution from the oxide surface/subsurface; and (2) diffusion controlled capacitance arisen from Li-ion intercalation and deintercalation in the oxide bulk structure (Figure S12, Supporting Information). The results are summarized in Figure 4d. At 5 mV s−1, capacitive-controlled capacitance contributed only 45% to the total capacitance of MnOx. Whilst after the hydrothermal treatment, this value increased substantially to 75%. We believed the change was due to the enhanced electrical conductivity and ion diffusion accompanied with the structural modifications. The dominant capacitive controlled charge storage mechanism enables MnOx-h for fast charging and discharging, which explains its improved rate capability and capacitance at high scan rates, even though the surface area is almost the same between MnOx and MnOx-h.

12


Page 13 of 21

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

ACS Energy Letters

Figure 4 (a) Areal capacitances of MnOx and MnOx-h electrodes with mass loading of ~9 mg cm−2 obtained at different scan rates. (b) Areal capacitance of MnOx-h with different mass loading collected at different scan rates. (c) Nyquist plots of MnOx and MnOx-h collected at open circuit potential with a perturbation of 5 mV from 10 mHz to 20 kHz. Dashed lines highlight Warburg impedance region. (d) Histogram illustration of capacitive and diffusive capacitance contribution in MnOx and MnOx-h at a scan rate of 5 mV s−1. A highly crystalline oxide layer was created after the hydrothermal treatment. To understand the role of this layer, we etched the surface layer and investigated the structural properties and electrochemical performance of the remaining core (denoted as E-MnOx-h). The shell layer was completely removed after immersing in a 6 M hydrochloric acid aqueous solution for 30s (Figure S13, Supporting Information). The core level Mn 3s and O 1s XPS spectra show that the chemical composition of E-MnOx-h is identical to MnOx-h (Figure S14a and b, Supporting Information), 13


ACS Energy Letters

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

Page 14 of 21

indicating the structural water molecules are uniformly distributed in the manganese oxide film instead of concentrating in the outer shell. Likewise, E-MnOx-h has considerably narrower XRD peaks than that of MnOx, suggesting that the crystal size in the core also increased after the hydrothermal treatment (Figure S14c and d, Supporting Information).51 The results provide a strong evidence that Ostwald ripening occurred inside the core as well. By eliminating the crystal shell, the areal capacitance of E-MnOx-h dropped slightly compared to MnOx-h, due to the loss of active material (Figure S15a, Supporting Information). Nevertheless, the rate capability and the specific gravimetric capacitance of E-MnOx-h are still considerably better than that of MnOx (Figure S15b, Supporting Information). Besides, electrochemical impedance spectra (EIS) displays that E-MnOx-h has much smaller charge transfer resistance and Warburg impedance than those of MnOx (Figure S15c, Supporting Information), indicating the presence of pores in the core is beneficial for facilitating ion diffusion. These results confirm that both the surface crystalline shell and the porous core are important for improving the capacitive performance of highly loaded manganese oxide electrodes. An asymmetric supercapacitor (dimension: 10 mm × 10 mm × 1 mm) was assembled using MnOx-h as cathode and a vanadium oxide grown on exfoliated carbon cloth (VOx/ECC) as anode. Mixed-valence vanadium oxide was deposited on exfoliated carbon cloth using an electrochemical method reported elsewhere (Figure S16, Supporting Information).52 The quasi-rectangular CV and symmetric constant-current charging/discharging curves obtained at different scan rates suggest the device has near-ideal capacitive behavior (Figure 5a and b). When the operation voltage was extended to 2.2 V, the device was still able to deliver a high coulombic efficiency of 94% at a current density of 5 mA cm−2, indicating the redox reactions in the voltage window is highly reversible. This high voltage 14


Page 15 of 21

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

ACS Energy Letters

window can be attributed to the high polarization of the cathode with highly loaded manganese oxide.53,54 This device delivered a remarkable volumetric capacitance of 7.46 F cm−3 at a current density of 2 mA cm−2 (Figure 5c). More importantly, it achieved a good rate capability of 40% when the current density increased from 2 to 100 mA cm−2. The Ragone plot shown in Figure 5d compares the performance of our device with other energy storage devices with comparable size. Our device exhibits an excellent volumetric energy density of 5 mWh cm−3 at a power density of 22 mW cm−3, and 2 mWh cm−3 at a power density of 1100 mW cm−3. These values are significantly higher than the values obtained from other electrochemical capacitor devices (Figure 5d).20,49,55-58 The device is also stable with 96% capacitance retained after 10000 charge-discharge cycles (Figure S17, Supporting Information).

Figure 5. (a) CV and (b) constant current charging/discharging curves of VOx/ECC//MnOx-h collected at different scan rates. (c) Volumetric capacitance and capacitance retention of the device obtained at 15


ACS Energy Letters

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

Page 16 of 21

different current densities. (d) Ragone plot showing the volumetric energy density of VOx/ECC//MnOx-h as a function of volumetric power density. The values reported for other energy storage devices are added for comparison. In summary, we have demonstrated a facile hydrothermal treatment to boost the capacitance of high mass loading (~9 mg cm−2) manganese oxide electrode at high scan rates. The hydrothermal treatment based on Ostwald ripening induced structural modifications has several important impacts on the capacitive performance of manganese oxide. First, the treatment increases the crystallinity of manganese oxide and eliminates the low valence state of Mn, and thus, improved the electrical conductivity of the thick manganese oxide film. Second, the heating process conducted in aqueous environment preserves the structural water in the oxide lattice, ensuring fast ion diffusion. Third, the treatment facilitates ion transport through increasing the porosity of manganese oxide film via Ostwald ripening process. Kinetics analysis showed that the capacitive-controlled mechanism dominates the charge storage process of the treated manganese oxide, effectively improve its capability for fast charging. A remarkable areal capacitance of 618 mF cm−2 was obtained at a high scan rate of 200 mV s−1, which is 3 times higher than that of the untreated sample. The assembled asymmetric supercapacitor device with MnOx-h as cathode and VOx/ECC as anode delivered an outstanding volumetric energy density of 5 mW h cm−3, which is 2-10 times higher than that of other reported supercapacitors with comparable dimension. We believe this hydrothermal strategy is a general strategy that can be further extended to other metal oxide and hydroxide electrodes, thus opening up new opportunities for improving the performance of high mass loading supercapacitor electrodes.

16


Page 17 of 21

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

ACS Energy Letters

Supporting Information Calculations, Experimental section, SEM images, XRD and XPS spectra, and electrochemical data. AUTHOR INFORMATION Corresponding Author Xiao-Xia Liu: E-mail: [email protected] Yat Li: E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Yat Li acknowledges support by National Aeronautics and Space Administration (NASA) grant no. NNX15AQ01. Xiao-Xia Liu gratefully acknowledges financial support from National Natural Science Foundation of China (No. 21273029 and No. 21673035). Yu Song acknowledges the financial support from China Scholarship Council. Tianyu Liu thanks the financial support from the Chancellor Dissertation-year Fellowship granted by the University of California, Santa Cruz. We thank Dr. Teng Zhai from Nanjing University of Science and Technology for his helpful discussion on PITT. We acknowledge Dr. Tom Yuzvinsky from the W. M. Keck Center for Nanoscale Opto-fluidics, University of California-Santa Cruz for SEM image acquisition.

REFERENCES 17


ACS Energy Letters

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

Page 18 of 21

(1) Augustyn, V.; Simon, P.; Dunn, B. Pseudocapacitive oxide materials for high-rate electrochemical energy storage. Energy Environ. Sci. 2014, 7, 1597-1614. (2) Ding, Y. S.; Shen, X. F.; Gomez, S.; Luo, H.; Aindow, M.; Suib, S. L. Hydrothermal Growth of Manganese Dioxide into Three-Dimensional Hierarchical Nanoarchitectures. Adv. Funct. Mater. 2006, 16, 549-555. (3) Hu, L.; Chen, W.; Xie, X.; Liu, N.; Yang, Y.; Wu, H.; Yao, Y.; Pasta, M.; Alshareef, H. N.; Cui, Y. Symmetrical MnO2-carbon nanotube-textile nanostructures for wearable pseudocapacitors with high mass loading. ACS Nano 2011, 5, 8904-8913. (4) Lei, Z.; Zhang, J.; Zhao, X. S. Ultrathin MnO2 nanofibers grown on graphitic carbon spheres as high-performance asymmetric supercapacitor electrodes. J. Mater. Chem. 2012, 22, 153-160. (5) Ma, R.; Bando, Y.; Zhang, L.; Sasaki, T. Layered MnO2 Nanobelts: Hydrothermal Synthesis and Electrochemical Measurements. Adv. Mater. 2004, 16, 918-922. (6) Reddy, A. L.; Shaijumon, M. M.; Gowda, S. R.; Ajayan, P. M. Coaxial MnO2/carbon nanotube array electrodes for high-performance lithium batteries. Nano Lett. 2009, 9, 1002-1006. (7) Song, M. K.; Cheng, S.; Chen, H.; Qin, W.; Nam, K. W.; Xu, S.; Yang, X. Q.; Bongiorno, A.; Lee, J.; Bai, J. and et al. Anomalous pseudocapacitive behavior of a nanostructured, mixed-valent manganese oxide film for electrical energy storage. Nano Lett. 2012, 12, 3483-3490. (8) Wang, G.; Zhang, L.; Zhang, J. A review of electrode materials for electrochemical supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (9) Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. Manganese oxide-based materials as electrochemical supercapacitor electrodes. Chem. Soc. Rev. 2011, 40, 1697-1721. (10) Zhai, T.; Wang, F.; Yu, M.; Xie, S.; Liang, C.; Li, C.; Xiao, F.; Tang, R.; Wu, Q.; Lu, X.; Tong, Y. 3D MnO2-graphene composites with large areal capacitance for high-performance asymmetric supercapacitors. Nanoscale 2013, 5, 6790-6796. (11) Song, Y.; Feng, D. Y.; Liu, T. Y.; Li, Y.; Liu, X. X. Controlled partial-exfoliation of graphite foil and integration with MnO2 nanosheets for electrochemical capacitors. Nanoscale 2015, 7, 3581-3587. (12) Zhu, C.; Yang, L.; Seo, J. K.; Zhang, X.; Wang, S.; Shin, J.; Chao, D.; Zhang, H.; Meng, Y. S.; Fan, H. J. Self-branched α-MnO2/δ-MnO2 heterojunction nanowires with enhanced pseudocapacitance. Mater. Horiz. 2017, 4(3), 415-422. (13) Kim, J. H.; Lee, K. H.; Overzet, L. J.; Lee, G. S. Synthesis and electrochemical properties of spin-capable carbon nanotube sheet/MnO(x) composites for high-performance energy storage devices. Nano Lett. 2011, 11, 2611-2617. (14) Lang, X.; Hirata, A.; Fujita, T.; Chen, M. Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors. Nat. Nanotechol. 2011, 6, 232-236. (15) Chen, W.; Rakhi, R. B.; Hu, L.; Xie, X.; Cui, Y.; Alshareef, H. N. High-performance nanostructured supercapacitors on a sponge. Nano Lett. 2011, 11, 5165-5172. (16) Song, Y.; Liu, T.-Y.; Xu, G.-L.; Feng, D.-Y.; Yao, B.; Kou, T.-Y.; Liu, X.-X.; Li, Y. Tri-layered graphite foil for electrochemical capacitors. J. Mater. Chem. A 2016, 4, 7683-7688. (17) Stoller, M. D.; Ruoff, R. S. Best practice methods for determining an electrode material's performance for ultracapacitors. Energy Environ. Sci. 2010, 3, 1294-1301. (18) Gogotsi, Y.; Simon, P. Materials science. True performance metrics in electrochemical energy storage. Science 2011, 334, 917-918. 18


Page 19 of 21

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

ACS Energy Letters

(19) He, Y.; Chen, W.; Li, X.; Zhang, Z.; Fu, J.; Zhao, C.; Xie, E. Freestanding three-dimensional graphene/MnO2 composite networks as ultralight and flexible supercapacitor electrodes. ACS Nano 2013, 7, 174-182. (20) Zhai, T.; Xie, S.; Yu, M.; Fang, P.; Liang, C.; Lu, X.; Tong, Y. Oxygen vacancies enhancing capacitive properties of MnO2 nanorods for wearable asymmetric supercapacitors. Nano Energy 2014, 8, 255-263. (21) Hu, L.; Choi, J. W.; Yang, Y.; Jeong, S.; La Mantia, F.; Cui, L. F.; Cui, Y. Highly conductive paper for energy-storage devices. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 21490-21494. (22) Liu, J.; Zhang, L.; Wu, H. B.; Lin, J.; Shen, Z.; Lou, X. W. High-performance flexible asymmetric supercapacitors based on a new graphene foam/carbon nanotube hybrid film. Energy Environ. Sci. 2014, 7, 3709-3719. (23) Xu, C.; Li, Z.; Yang, C.; Zou, P.; Xie, B.; Lin, Z.; Zhang, Z.; Li, B.; Kang, F.; Wong, C. P. An Ultralong, Highly Oriented Nickel-Nanowire-Array Electrode Scaffold for High-Performance Compressible Pseudocapacitors. Adv. Mater. 2016, 28, 4105-4110. (24) Yang, J.; Lian, L.; Ruan, H.; Xie, F.; Wei, M. Nanostructured porous MnO2 on Ni foam substrate with a high mass loading via a CV electrodeposition route for supercapacitor application. Electrochim. Acta 2014, 136, 189-194. (25) Yu, G.; Hu, L.; Liu, N.; Wang, H.; Vosgueritchian, M.; Yang, Y.; Cui, Y.; Bao, Z. Enhancing the supercapacitor performance of graphene/MnO2 nanostructured electrodes by conductive wrapping. Nano Lett. 2011, 11, 4438-4442. (26) Hu, C.-C.; Wu, Y.-T.; Chang, K.-H. Low-Temperature Hydrothermal Synthesis of Mn3O4 and MnOOH Single Crystals: Determinant Influence of Oxidants. Chem. Mater. 2008, 20, 2890-2894. (27) Wang, Z.; Qin, Q.; Xu, W.; Yan, J.; Wu, Y. Long Cyclic Life in Manganese Oxide-Based Electrodes. ACS Appl. Mater. Interfaces 2016, 8, 18078-18088. (28) Lee, J.-H.; Yang, T.-Y.; Kang, H.-Y.; Nam, D.-H.; Kim, N.-R.; Lee, Y.-Y.; Lee, S.-H.; Joo, Y.-C. Designing thermal and electrochemical oxidation processes for δ-MnO2 nanofibers for high-performance electrochemical capacitors. J. Mater. Chem. A 2014, 2, 7197. (29) Luo, J.-Y.; Li, X.-L.; Xia, Y.-Y. Synthesis of highly crystalline spinel LiMn2O4 by a soft chemical route and its electrochemical performance. Electrochim. Acta 2007, 52, 4525-4531. (30) Chang, J.-K.; Chen, Y.-L.; Tsai, W.-T. Effect of heat treatment on material characteristics and pseudo-capacitive properties of manganese oxide prepared by anodic deposition. J. Power Sources 2004, 135, 344-353. (31) Toupin, M.; Brousse, T.; Bélanger, D. Influence of Microstucture on the Charge Storage Properties of Chemically Synthesized Manganese Dioxide. Chem. Mater. 2002, 14, 3946-3952. (32) Ragupathy, P.; Park, D. H.; Campet, G.; Vasan, H. N.; Hwang, S.-J.; Choy, J.-H.; Munichandraiah, N. Remarkable Capacity Retention of Nanostructured Manganese Oxide upon Cycling as an Electrode Material for Supercapacitor. J. Phys. Chem. C 2009, 113, 6303-6309. (33) Chang, J.-K.; Huang, C.-H.; Lee, M.-T.; Tsai, W.-T.; Deng, M.-J.; Sun, I. W. Physicochemical factors that affect the pseudocapacitance and cyclic stability of Mn oxide electrodes. Electrochim. Acta 2009, 54, 3278-3284. (34) Sumboja, A.; Foo, C. Y.; Wang, X.; Lee, P. S. Large areal mass, flexible and free-standing reduced graphene oxide/manganese dioxide paper for asymmetric supercapacitor device. Adv. Mater. 19


ACS Energy Letters

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

Page 20 of 21

2013, 25, 2809-2815. (35) Qiu, Y.; Zhao, Y.; Yang, X.; Li, W.; Wei, Z.; Xiao, J.; Leung, S. F.; Lin, Q.; Wu, H.; Zhang, Y. and et al. Three-dimensional metal/oxide nanocone arrays for high-performance electrochemical pseudocapacitors. Nanoscale 2014, 6, 3626-3631. (36) Yang, M.; Zhang, T.; Schulz, P.; Li, Z.; Li, G.; Kim, D. H.; Guo, N.; Berry, J. J.; Zhu, K.; Zhao, Y. Facile fabrication of large-grain CH3NH3PbI3-xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening. Nat. Commun. 2016, 7, 12305-12314. (37) Li, J.; Zeng, H. C. Hollowing Sn-doped TiO2 nanospheres via ostwald ripening. J. Am. Chem. Soc. 2007, 129, 15839-15847. (38) Liu, B.; Zeng, H. C. Symmetric and asymmetric Ostwald ripening in the fabrication of homogeneous core-shell semiconductors. Small 2005, 1, 566-571. (39) Yec, C. C.; Zeng, H. C. Synthesis of complex nanomaterials via Ostwald ripening. J. Mater. Chem. A 2014, 2, 4843-4851. (40) Toupin, M.; Brousse, T.; Bélanger, D. Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor. Chem. Mater. 2004, 16, 3184-3190. (41) Chuah, G. K.; Jaenicke, S.; Pong, B. K. The Preparation of High-Surface-Area Zirconia. J. Catal. 1998, 175, 80-92. (42) Hill, L. I.; Verbaere, A. On the structural defects in synthetic γ-MnO2. J. Solid State Chem. 2004, 177, 4706-4723. (43) Mai, L.; Li, H.; Zhao, Y.; Xu, L.; Xu, X.; Luo, Y.; Zhang, Z.; Ke, W.; Niu, C.; Zhang, Q. Fast Ionic Diffusion-Enabled Nanoflake Electrode by Spontaneous Electrochemical Pre-Intercalation for High-Performance Supercapacitor. Sci. Rep. 2013, 3, 1718-1726. (44) Ma, M.; Zhang, K.; Li, P.; Jung, M. S.; Jeong, M. J.; Park, J. H. Dual Oxygen and Tungsten Vacancies on a WO3 Photoanode for Enhanced Water Oxidation. Angew. Chem. Int. Ed. 2016, 128, 11998-12002. (45) Su, Z.; Yang, C.; Xu, C.; Wu, H.; Zhang, Z.; Liu, T.; Zhang, C.; Yang, Q.; Li, B.; Kang, F. Co-electro-deposition of the MnO2–PEDOT:PSS nanostructured composite for high areal mass, flexible asymmetric supercapacitor devices. J. Mater. Chem. A 2013, 1, 12432. (46) Li, X.; Shen, J.; Sun, W.; Hong, X.; Wang, R.; Zhao, X.; Yan, X. A super-high energy density asymmetric supercapacitor based on 3D core–shell structured NiCo-layered double hydroxide@carbon nanotube and activated polyaniline-derived carbon electrodes with commercial level mass loading. J. Mater. Chem. A 2015, 3, 13244-13253. (47) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Graphene-based ultracapacitors. Nano Lett. 2008, 8, 3498-3502. (48) Song, Y.; Xu, J.-L.; Liu, X.-X. Electrochemical anchoring of dual doping polypyrrole on graphene sheets partially exfoliated from graphite foil for high-performance supercapacitor electrode. J. Power Sources 2014, 249, 48-58. (49) Zhai, T.; Lu, X.; Ling, Y.; Yu, M.; Wang, G.; Liu, T.; Liang, C.; Tong, Y.; Li, Y. A new benchmark capacitance for supercapacitor anodes by mixed-valence sulfur-doped V6O(13-x). Adv. Mater. 2014, 26, 5869-5875. (50) Baronetto, D.; Krstajić, N.; Trasatti, S. Reply to “note on a method to interrelate inner and outer electrode areas” by H. Vogt. Electrochim. Acta 1994, 39, 2359-2362. 20


Page 21 of 21

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

ACS Energy Letters

(51) Li, Z. Y.; Lam, W. M.; Yang, C.; Xu, B.; Ni, G. X.; Abbah, S. A.; Cheung, K. M.; Luk, K. D.; Lu, W. W. Chemical composition, crystal size and lattice structural changes after incorporation of strontium into biomimetic apatite. Biomaterials 2007, 28, 1452-1460. (52) Song, Y.; Liu, T. Y.; Yao, B.; Kou, T. Y.; Feng, D. Y.; Liu, X. X.; Li, Y. Amorphous Mixed-Valence Vanadium Oxide/Exfoliated Carbon Cloth Structure Shows a Record High Cycling Stability. Small 2017, 13, 1700067-1700075. (53) Khomenko, V.; Raymundo-Piñero, E.; Béguin, F. Optimisation of an asymmetric manganese oxide/activated carbon capacitor working at 2V in aqueous medium. J. Power Sources 2006, 153, 183-190. (54) Singh, A.; Chandra, A. Enhancing Specific Energy and Power in Asymmetric Supercapacitors A Synergetic Strategy based on the Use of Redox Additive Electrolytes. Sci. Rep. 2016, 6, 25793-25806. (55) Xiao, X.; Ding, T.; Yuan, L.; Shen, Y.; Zhong, Q.; Zhang, X.; Cao, Y.; Hu, B.; Zhai, T.; Gong, L. and et al. WO3−x/MoO3−x Core/Shell Nanowires on Carbon Fabric as an Anode for All-Solid-State Asymmetric Supercapacitors. Adv. Energy Mater. 2012, 2, 1328-1332. (56) Lu, X.; Zeng, Y.; Yu, M.; Zhai, T.; Liang, C.; Xie, S.; Balogun, M. S.; Tong, Y. Oxygen-deficient hematite nanorods as high-performance and novel negative electrodes for flexible asymmetric supercapacitors. Adv. Mater. 2014, 26, 3148-3155. (57) Chen, L.-F.; Yu, Z.-Y.; Wang, J.-J.; Li, Q.-X.; Tan, Z.-Q.; Zhu, Y.-W.; Yu, S.-H. Metal-like fluorine-doped β-FeOOH nanorods grown on carbon cloth for scalable high-performance supercapacitors. Nano Energy 2015, 11, 119-128. (58) Yang, P.; Ding, Y.; Lin, Z.; Chen, Z.; Li, Y.; Qiang, P.; Ebrahimi, M.; Mai, W.; Wong, C. P.; Wang, Z. L. Low-cost high-performance solid-state asymmetric supercapacitors based on MnO2 nanowires and Fe2O3 nanotubes. Nano Lett. 2014, 14, 731-736.

21


Ostwald Ripening Improves Rate Capability of High Mass Loading

Recommend Documents