Highly Efficient K0.15MnO2 Birnessite Nanosheets for Stable

Article pubs.acs.org/JPCC

Highly Efficient K0.15MnO2 Birnessite Nanosheets for Stable Pseudocapacitive Cathodes Matthew Yeager,†,⊥ Wenxin Du,†,⊥ Rui Si,‡ Dong Su,§ Nebojsa Marinković,∥ and Xiaowei Teng*,† †

Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824, United States Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, United States § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ∥ Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716, United States ‡

S Supporting Information *

ABSTRACT: In this paper, we reported a facile synthesis of Birnessite K0.15MnO2·0.43H2O nanosheets in a solution phase. The structural and electrochemical properties of the K0.15MnO2 nanosheets for supercapacitor (SC) reactions were studied, and a gravimetric capacitance of 303 F/g was obtained at a charge/discharge current of 0.2 A/g. Electrochemical kinetics showed that a non-Faradaic (electrical double layer) current existed throughout the charging potential range, while a dominant Faradaic (pseudocapacitive) current was observed at high and low potentials during anodic and cathodic scans, respectively. Asymmetric pseudocapacitive full-cells were constructed with both anodic and cathodic K0.15MnO2 composite materials and subjected to long-term galvanostatic charge/discharge analyses. A specific capacitance of 67.8 F/g was obtained for the cathodic K0.15MnO2 full-cells after 1000 cycles, with a capacitive retention of 87.8% and Coulombic and energy efficiencies of ∼100 and ∼90%, respectively. In situ X-ray absorption near edge spectroscopy further corroborated the potential-dependent Faradaic reactions, suggesting a predominant change in valence state of K0.15MnO2 to occur between 0.3 and 0.6 V (vs Ag/AgCl). The present study not only underscores the structure−function relationship of MnO2-based electrode materials for SC reactions but also provides a new approach in fabricating advanced pseudocapacitors by utilizing cost-effective transition metal oxide materials. Birnessite-type δ-MnO2 has a 2D laminar structure with an interlayer distance around 0.7 nm, permitting water molecules and alkali metal cations to easily transfer into and out of the interlayer region without causing significant structural rearrangements.9,12 Therefore, δ-MnO2 has already proven to be a useful compound for ion-exchange composites, heterogeneous catalysts, and electrode materials for both SCs and Li-ion batteries.13−20 MnO2 has been of interest for Li-ion batteries and SCs due to its theoretical capacitance of 1233 F/g. However, because of its low electronic conductivity (10−5 to 10−6 S/cm), MnO2 powders have attained only 10−20% of this theoretical limit (100−200 F/g).6,7 Implementation of MnOX-based electrode materials require a more thorough understanding of its pseudocapacitive charge transfer mechanism. Spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS) have been used to study the electronic states of Mn, from which

1. INTRODUCTION Supercapacitors (SCs) are a class of energy storage devices that fill the gap between batteries (high energy density) and electrostatic capacitors (high power density).1−3 The implementation of SCs has been hampered by the lack of costeffective electrode materials and the mechanistic understanding of charge storage that occurs at the electrode/electrolyte interface.4,5 Manganese dioxide (MnO2) has become a strong candidate for SC electrode materials due to its favorable redox activity, low cost, abundance, and environmentally friendly nature. Morphologically, MnO2 crystals consist of [MnO6] octahedra with shared vertices and edges.6 Stacking of [MnO6] octahedra enables the building of one-dimensional (1D, e.g., α-, β-, γ-MnO2) or two-dimensional (2D, e.g., δ-MnO2) tunnel structures.7 Brousse et al. first proposed that tunnel size can significantly affect the performance of manganese-based pseudocapacitance: MnO2 electrodes with larger tunnel sizes (e.g., α-, γ-, and δ-MnO2) favored the storage of alkaline cations (e.g., Na+, K+, Li+) and exhibited higher capacitance, whereas narrow tunnel sizes (e.g., β-MnO2) were unsuitable for cation storage and subsequently possessed lower capacitance.8−11 © 2012 American Chemical Society

Received: May 17, 2012 Revised: August 19, 2012 Published: September 4, 2012 20173

dx.doi.org/10.1021/jp304809r | J. Phys. Chem. C 2012, 116, 20173−20181

The Journal of Physical Chemistry C

Article

High-resolution transmission electron microscopy (HRTEM) was carried out with a JEOL-2100F at 200 kV. High angle annular dark field (HAADF) scanning TEM (STEM) images were collected using an aberration-corrected Hitachi HD 2700C at the Center for Functional Nanomaterials at the Brookhaven National Laboratory (BNL). Lowmagnification TEM images were obtained using a Zeiss/LEO 922 Omega TEM. Thermogravimetric analysis (TGA) was conducted on a Mettler Toledo TGA/DSC 1 by applying a 5 °C min−1 rate of temperature increase from 20 to 400 °C under a nitrogen environment. Energy dispersive X-ray spectroscopy (EDS) was conducted by an Amray 3300FE field emission SEM with a PGT Imix-PC microanalysis system. X-ray diffraction (XRD) spectra were obtained at beamline X7B (λ = 0.3196 Å) at the National Synchrotron Light Source at BNL. The powdered samples were loaded inside quartz capillary tubes (I.D. = 0.5 mm), which could be rotated during the XRD measurements to remove the preferred orientation. A PerkinElmer image plate detector was used to collect a twodimensional powder pattern, and a FIT2D code was used to integrate the powder rings. 2.2. Half-Cell Electrochemical Analyses and Electrode Preparation. A CHI 660 single-channel electrochemical workstation (CH Instruments) was used to examine the electrochemical properties of the hydrous K0.15MnO2 product in a three-electrode system, which was composed of (i) a rotating glassy carbon electrode as the working electrode (geometric surface area = 0.196 cm2), operating at 1000 rpm; (ii) a Ag/AgCl (1 M KCl) reference electrode; and (iii) a platinum wire counter electrode. The composite material to be studied was prepared by dissolving the necessary components (hydrous K 0.15 MnO 2 , poly(3,4-ethylenedioxythiophene) (PEDOT; Sigma-Aldrich, 2.2−2.6% in H2O), and/or carbon black (Vulcan XC-72, 254 m2/g)) in ethanol (Pharmaco-Aaper, ACS/USP grade, 190 proof) and drop-casting the mixtures on the working electrode in 5−10 μL volumes. The total working electrode loading of materials, if present, was 5.0 μg of hydrous K0.15MnO2, 1.25 μg of PEDOT, and 1.25 μg of carbon black. Both cyclic voltammetry (CV) and chronopotentiometry (CP) analyses were performed in a 100 mL round-bottom flask with 50 mL of argon-purged, 0.1 M Na2SO4 electrolyte (Alfa Aesar Puratronic, 99.9955%) in the potential range of 0−0.9 V. CV was performed at scan rates of 1, 2, 10, 20, 50, 100, and 200 mV/s to study the effects of scanning rate on capacitance. CP measurements were performed between 0 and 0.9 V (vs Ag/ AgCl) with current densities of 0.2, 0.4, 1.0, 2.0, 5.0, and 10 A/ g. 2.3. Full-Cell Fabrication and Galvanostatic Cycling. Full-cell pseudocapacitors were fabricated so that both selfdischarge analyses and long-term galvanostatic charge/ discharge analyses could be performed to determine cell performance, durability, and efficiency. The full-cells were fabricated from two exterior shell casings (2.0 cm (OD) × 2.5 mm (depth)) that enclosed a two-electrode system within a 1.0 M Na2SO4 (Alfa Aesar Puratronic, 99.9955%) electrolytic solution. The asymmetric cell assembly comprised a redoxactive electrode coated with approximately 6.0 mg of K0.15MnO2 and 2.0 mg of PEDOT and an opposing electrode coated with 15.0 mg of PEDOT, and the symmetric cell assembly comprised two opposing electrodes each coated with approximately 6.0 mg of PEDOT. The electrode supports consisted of 1.5 cm Toray Carbon Paper (40% wet proofing, ETek, Inc.), which were separated by a polypropylene filter

the average Mn oxidation state was determined to alternate between (III) and (IV) with requisite charge-compensation from the intercalation/deintercalation of electrolytic ions.21−23 However, XPS has to be conducted under vacuum conditions, precluding in situ analyses. In addition to XPS, X-ray absorption near edge spectroscopy (XANES) is another element-specific charge state probe and is ideally suited to study the electronic structure of nanomaterials involving charge transfers.24−26 XANES can be performed under normal atmospheric conditions in aqueous electrolytes, facilitating in situ studies. Although XANES has been previously used to study the electronic structure of MnOX under different conditions,21,27 the use of in situ XANES to study the charge transfer processes of MnOX during redox reactions has only been reported in limited cases.23,28−31 In addition to the fundamental understanding of the charge storage mechanism, the synthesis of homogeneous MnO2 electrode nanomaterials, particularly layered δ-MnO2 nanostructures, is often complex and intensive. A common δ-MnO2 synthetic method is via exfoliation,22,32−40 which involves the protonation of bulk MnO2 or manganate precursors in the presence of a variety of intercalators (e.g., dodecyl sulfate ions and quaternary ammonium salts). Intercalators and/or water molecules introduce electrostatic repulsion to bulk Mn precursors, facilitating the formation of finely layered MnOx. However, exfoliation methods typically require high temperature pretreatment and long reaction times (a few days), and are unable to yield uniform layered structures without a wide thickness distribution. To our knowledge, a facile, room temperature synthesis of uniform nanoscale Birnessite-type materials has not yet been reported. Here we present a simple and low-cost synthesis of Birnessite-type MnO2. We studied the electrochemical capabilities of MnO2 in a three-electrode cell via cyclic voltammetry and chronopotentiometry, and further fabricated asymmetric full-cells to study the electrode stability and both the Coulombic and energy efficiencies of K0.15MnO2-based systems via long-term galvanostatic cycling. In situ XANES was conducted in an electrochemical cell to elucidate the valence state evolution of MnO2 during charging processes, which confirmed the presence of potential-dependent Faradaic processes as predicted via electrochemical kinetics analyses from CV experiments.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis and Characterization. The hydrous K0.15MnO2 nanosheet synthesis was conducted with precise drop-casting at room temperature, enabling the fine control over the morphology and particle size. A solution of 1.5 g of KOH (Alfa Aesar, 99.99%) in 150 mL of deionized (DI) water was first prepared in a round-bottom flask. Then, 27.5 mg of MnCl2·4H2O (Alfa Aesar, 99%) was dissolved in 10 mL of DI water and placed into a 20 mL plastic syringe. A Sage Instruments Model 355 syringe pump operating at 1/3 mL min−1 was used for the slow injection of the Mn2+ precursor into the KOH aqueous solution under an open environment. Following a 1 min Mn2+ precursor injection, the resulting brown-colored mixture was stirred for an additional 60 min before the precipitate was centrifuged and washed multiple times with copious amounts of DI water to remove residual solvated ions. The final wash was performed with ethanol (Pharmaco-Aaper, ACS/USP grade, 190 proof), and the K0.15MnO2 product was vacuum-dried overnight. 20174



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Figure 1. (a, c) Bright field TEM, (b) dark field HAADF, and (c) high-resolution TEM images of K0.15MnO2 nanosheets.

membrane (Whatman, 0.45 μm pore size). Placed between the hydrophobic electrodes and polypropylene filter membrane were nonwoven porous cloths, which permitted for ample electrolyte wetting and facile cell fabrication (see Figure S5, Supporting Information). An eight-channel MTI Battery Analyzer (MTI Corporation) was used to perform both selfdischarge analyses over a 24 h period and galvanostatic cycling analyses between 0 and 1.2 V for 1000 cycles. 2.4. In Situ XANES Measurements. An electrochemical cell was constructed for in situ XANES measurements (Figure 7). The working electrode (WE) comprised K0.15MnO2 ink deposited onto a Toray Carbon Paper support (E-Tek). The ink was composed of hydrous K0.15MnO2 and carbon black (20:80 by weight, respectively) dissolved in ethanol (Pharmaco-Aaper, ACS/USP grade, 190 proof), with approximately 30−40 mg of dried ink being deposited onto each WE. The counter electrode (CE) was naked Toray Carbon Paper. The WE and CE were separated with two PTFE rings and a porous cloth. Two platinum foils contacted the electrodes to complete the electrochemical circuit, and a carbon cloth was placed on the outer side of the WE to enhance current collection. A MI402 Ag/AgCl reference microelectrode (Microelectrodes, Inc.) was used. In situ XANES was conducted at beamline X19A at the National Synchrotron Light Source at BNL. The in situ experiments were carried out at the Mn K edge (6539 eV), and the spectra were calibrated by mounting a reference Mn foil sample between the transmission and reference ionization chambers. A constant potential was applied to the cell during XANES measurements via chronoamperometry analyses at various potentials (from 0 to 1.2 V vs Ag/AgCl).

Figure 2. (a) XRD and (b) TGA/DTA patterns of K0.15MnO2·0.43H2O nanosheets. SA, small angle pattern; WA, wide angle pattern.

(JCPDS No. 80-1098, monoclinic, C2/m). The small angle (SA) diffraction patterns are indexed as the (00l) basal reflections of a layered Birnessite phase, with the largest basal spacing from the (001) reflection (d001) of 0.73 nm. Two other basal spacings, d002 and d004, were calculated to be 0.36 and 0.18 nm, respectively. The three evenly spaced reflections follow the correlation 0.73 nm d00l = (1) l

3. RESULTS AND DISCUSSION 3.1. Structure Analysis. Large pieces of K-intercalated δMnO2 nanosheets with a lateral dimension of several hundred nanometers were observed with weak but uniform contrast, implying that the nanosheets possessed a narrow distribution of thickness (Figure 1a). HAADF imaging was performed using an aberration-corrected STEM, as shown in Figure 1b. The polycrystalline nature of the manganese oxide nanosheets is shown in different regions, as a similar polycrystalline nature can be observed from the HRTEM (Figure 1c). An average lattice spacing of 2.86 ± 0.09 Å was calculated from HRTEM images, which is the doubling of lattice spacing of 1.41 Å that can be assigned to the (110) plane of MnO2. The chemical compositions of as-made nanosheets were determined by EDS, from which 87% Mn and 13% K have been identified for an empirical formula of K0.15MnOX (Figure S1, Supporting Information). Figure 2a shows the XRD pattern of K0.15MnOX nanosheets, which can be identified as a layered Birnessite-type MnO2

where l is equal to 1, 2, or 4, indicating a periodic layered structure. In addition to SA diffraction, wide angle (WA) diffraction of the (111) and (311) peaks corresponding to the in-plane diffraction of monoclinic MnO2 crystal were also observed. The absence of the (003) SA diffraction peak is due to the overlapping of (111) WA diffraction. Our reported interlayer spacing (0.73 nm) is identical to that of hydrous Birnessite.12,22,34 From the combination of TEM, EDS, and XRD data, we concluded that the as-made K 0.15 MnO 2 nanosheets consisted of flocculated polycrystalline MnO2 layers with intercalated water molecules and K+ cations between layers. Since the extent of oxidation of Mn is theoretically determined by the amount of K+ incorporated, the valence of Mn in K0.15MnO2·nH2O was thus calculated to be +3.85. The content of structured water was further determined by thermogravimetric/differential thermal analysis (TGA/DTA) from 25 to 400 °C under nitrogen flow. Most of weight loss 20175



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(∼12%) was observed from 25 to 280 °C due to the evaporation of interlayered and adsorbed water. The DTA curve contained two endotherms: the first between 25 and 87 °C was attributed to the evaporation of adsorbed water (4.6% by weight), whereas the second endotherm (at temperatures greater than 87 °C) was attributed to the removal of structured water (7.8% by weight). Therefore, the final chemical formula of as-made nanosheets may be expressed as K0.15MnO2·0.43H2O. The formation mechanism of the hydrous K0.15MnO2 nanosheets was determined to be (Figure 3)

Faradaic transition of Mn oxide. The mass-specific capacitance (CMS) as a function of scan rates for MP was calculated from CMS =

KOH

O2

→ MnO2 K + and H 2O

dV m dt

→ CMS =

∫t

o

tF

I dt m(Vt − Vo)

(3)

where I is the measured current at a time of t, m is the total mass of the loaded material (MP), Vo and Vt are applied potentials applied at the initial and at time of t, and to and tF are the respective times at the initial and final potentials. The average CMS was calculated to be 249 F/g based on the mass of total active material (MP) and 283 F/g based on the mass of K0.15MnO2 only (subtracting the PEDOT effect) at a scan rate of 1 mV/s. CVs of PEDOT/carbon (PC) and PEDOT-only (P) electrodes were also measured (Figure S3, Supporting Information). Data showed that the MP composite exhibited much higher capacitance than that of the PC, or P-only electrodes, indicating that the pseudocapacitive behavior of K0.15MnO2 was mainly responsible for the CMS of the MP composites. CP was conducted between 0 and 0.9 V at various charge/discharge current densities from 0.2 to 10 A/g (Figure 4b), and the CMS was calculated from

MnCl 2 ⎯⎯⎯⎯→ Mn(OH)2

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ K 0.15MnO2 ·0.43H 2O

I

(2)

CMS = Figure 3. Scheme of “bottom-up” synthesis of K0.15MnO2·0.43H2O nanosheets.

I ΔE m Δt

(4)

where I is the constant charge or discharge current, ΔE is the potential sweep window (0.9 V), Δt is the discharge time, and m is the mass of electroactive material. The CMS values as a function of charge/discharge current densities are summarized in Figure 4c. The highest specific capacitances of the K0.15MnO2/PEDOT electrode were 249 F/g (CV) and 303 F/g (CP) from half-cell measurements. These values are higher than or comparable to a variety of SC electrode materials that were measured under similar conditions, including Fe3O4/active carbon (38−90 F/ g),42 graphene/MnO2-textile (∼300 F/g),43 Au-MnO2/carbon nanotubes (∼70 F/g),44 Li+, Na+, and K+ intercalated δ-MnO2 (140−160 F/g),36 δ-MnO2 nanoplates (180−210 F/g),37 highly crystalline δ-MnO2 powder (110−130 F/g),8,45 poorly crystalline δ-MnO2 nanostructures (∼250 F/g),21 a mixture of amorphous and crystalline MnO2 nanoparticles (72−168 F/ g),46 and Ni2+ intercalated δ-MnO2 (225 F/g).47 The enhanced SC performance of MP nanomaterials relative to these materials (or to the baseline PC or P-only electrodes) can be attributed to a number of unique characteristics, including (i) the 2D layered architectures of K0.15MnO2 that offered large electrochemically active surface areas and a reduced ion and charge diffusion length during charge/discharge processes and

In this synthesis scheme, Mn2+ ions initially reacted with KOH in aqueous solution to form Mn(OH)2 precipitate, after which the oxidation of Mn2+ to Mn4+ occurred in the presence of O2, forming the [Mn(IV)O6] octahedral building units that subsequently self-assembled into ultrathin-layered MnO2 structures.10 The formation of the MnO2 layered structures was completed with the intercalation of K+ and water molecules, yielding the K0.15MnO2·0.43H2O nanosheets. Fine control over the growth kinetics was imperative to forming the nanosheet structure, which was achieved by controllably injecting MnCl2 into KOH aqueous solution. When the injection and reaction times were increased approximately 10fold, an immediate transition from nanosheets to bulk laminated structures was observed (Figure S2, Supporting Information).41 3.2. Electrochemical Measurements. Figure 4a shows the CVs of the mixture of K0.15MnO2/PEDOT (MP) recorded at potential scan rates from 2 to 200 mV/s between 0 and 0.9 V (vs Ag/AgCl) in 0.1 M Na2SO4 solution. The CV response represented a pseudocapacitive behavior and a reversible

Figure 4. (a) CVs of K0.15MnO2/PEDOT, (b) charge/discharge cycles of K0.15MnO2/PEDOT at various current densities, and (c) mass-specific capacitance of K0.15MnO2/PEDOT from CV and CP scans after six full cycles from 0 to 0.9 V (vs Ag/AgCl); working electrode surface area = 0.196 cm2. 20176



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(ii) a conductive PEDOT coating that provided excellent interfacial contacts and highly conductive paths throughout the K0.15MnO2 nanosheets for rapid electronic transport. 3.3. Electrochemical Kinetics. The total charge stored in the electrode during SC operation is dependent on (i) the Faradaic contribution from the redox reaction (i.e., pseudocapacitance), which is promoted by the intercalation/deintercalation processes of electrolytic ions and (ii) the non-Faradaic contribution from the electrical double layer (i.e., EDL capacitance) that forms at the surface via the adsorption/ desorption of electrolytic ions. In general, the former process is limited by the diffusion of electrolytic ions (e.g., Na+, Li+, K+, or H+) into the electrode lattice, which can be considered a strictly diffusion-limited redox reaction. Therefore, the rate of charge transfer of Faradaic reactions (iF) is proportional to the square root of the scan rate (ν) according to following equation:48 ⎛ DanFν ⎞1/2 ⎟ iF = 0.495FAc ⎜ ⎝ RT ⎠

Figure 5. Calculated b-values as a function of potential for the cathodic (Na+ intercalation) and anodic scans (Na+ deintercalation).

or K+-intercalated δ-MnO2 systems, and were in good agreement with similar work on Li+-intercalated TiO2 for SC reactions.48−51 Similarly, the b-values for anodic scan were in the range 0.8−1.0 between 0 and 0.5 V, and rapidly reached the lowest value of 0.65 as the potential increased to 0.75 V. These data also indicated that the anodic currents at higher potentials arose primarily from Faradaic reactions due to the deintercalation of Na+ in the K0.15MnO2 lattice. 3.4. Full-Cell Galvanostatic Cycling and Efficiency. In order to determine the long-term cycling capability, charge storage capacity, propensity for self-discharge, and material efficiency of K0.15MnO2-based PCs, full-cell capacitors were constructed and subjected to 1000 galvanostatic charge/ discharge cycles and 24 h self-discharge analyses. Two types of asymmetric cells (anode/cathode, MP/P; anode/cathode, P/ MP) and one type of symmetric cells (anode/cathode, P/P) were constructed. Figure 6a shows the initial cycles of a cathodic MnO2 cell during galvanostatic charge/discharge analyses, in which the linear symmetry of the curves demonstrated a highly reversible and desirable capacitive behavior.

(5) +

where c is the concentration of alkaline cations (e.g., Na ) in the accumulation layer, α is the charge transfer coefficient, D is the diffusion coefficient of Na+ inside the electrode materials, n is the number of electrons involved in the Faradaic reaction, A is the surface area of the electrode materials, F is Faraday’s constant, R is the molar gas constant, and T is the temperature. On the other hand, the EDL capacitive current of non-Faradaic reactions (iNF) has a linear dependence on the scan rate according to eq 6: iNF = ACdν

(6)

where Cd is the double-layer capacitance and A is a constant. Accordingly, for the current response (i) at a given potential containing the combination of two separate mechanisms, namely, pseudo- or EDL capacitance, the following equation can be applied: i = a1ν + a 2ν1/2

(7)

Therefore, at higher scan rates, the current is dominated by EDL capacitive charge/discharge, due to the stronger linear dependence in eq 7, while the current is dominated by pseduocapacitive charge/discharge at lower scan rates. In this context, the overall current (i) is usually described by a simple power law:49−51

i = aν b

(8)

where a is an adjustable parameter and b is equal to either 0.5 or 1 when the currents are strictly dominated by pseudocapacitance or EDL capacitance, respectively. Figure 5 shows the plots of b-values as a function of scan rate (ν) for both anodic and cathodic scans from 0 to 0.9 V (vs Ag/AgCl). Although the EDL capacitive term was dominant with increasing scan rate, as shown in eq 7, simple power functions fitted the data well (Figure S4, Supporting Information). In the potential window between 0.8 and 0.4 V for cathodic scans, the b-values were in the range 0.8−1.0, whereas the b-values decreased dramatically between 0.4 and 0 V and reached the lowest value (0.65) at around 0−0.1 V. These data indicated that the cathodic currents at lower potentials arose primarily from the intercalation of Na+ into the K0.15MnO2 lattice, although a contribution from non-Faradaic current (EDL capacitance) existed throughout, since the b-values were always greater than 0.5. These results have not been reported in Na+-

Figure 6. (a) Initial galvanostatic charge/discharge cycles of cathodic K0.15MnO2 pseudocapacitive full-cell, operating at 340 mA/g; (b) longterm mass-specific capacitances (CMS) of cathodic K0.15MnO2 (340 mA/g), anodic K0.15MnO2 (360 mA/g), and symmetric PEDOT (175 mA/g) full-cells; (c) Coulombic and energy efficiencies of a cathodic K0.15MnO2 cell over 1000 galvanostatic charge/discharge cycles at 340 mA/g; (d) 24-h self-discharge analysis of the cathodic K0.15MnO2 pseudocapacitive full-cell. 20177



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Table 1. Compilation of Reported Gravimetric Capacitance and Energy Density Values for Asymmetric MnO2-Based Pseudocapacitors59 cathode

anode

current collector

electrolyte

cell voltage (V)

specific capacitance (F/ g)

energy density (Wh/ kg)

reference

K0.15MnO2/PEDOT PEDOT MnO2 LiMnO2 MnO2 MnO2 MnO2 MnO2 MnO2

PEDOT K0.15MnO2/PEDOT active carbon active carbon active carbon MnO2 active carbon Fe3O4 active carbon

Ni foam Ni foam Ni foam Ni grid titanium stainless steel stainless steel stainless steel stainless steel

Na2SO4 Na2SO4 LiOH Li2SO4 KCl K2SO4 K2SO4 K2SO4 K2SO4

1.2 1.2 1.5 1.8 2.0 1.0 2.2 1.8 2.0

77.2 53.6 62.4 56 52 36 31 21.5 21

15.4 10.7 19.5 10.0 28.8 3.3 17.3 8.1 11.7

this study this study 53 54 55 52 52 52 56

The total cell capacitances from galvanostatic analyses were calculated in accordance with eq 4 but were gravimetrically normalized as CMS = 2 ×

C mActive Material

The energy efficiency was calculated from the ratio of area under the discharge curve to the area under the preceding charge curve as follows:

(9)

Energy Efficiency =

=

C m Total

IDischarge·t Discharge ICharge·tCharge t Discharge tCharge

Charge

I · V (t ) d t

I · V (t ) d t V (t ) d t

V (t ) d t

(12)

The Coulombic efficiencies over the 1000 cycles varied between 95 and 105%, suggesting a superb conservation of charge transfer for the cathodic K0.15MnO2 pseudocapacitor. The energy efficiencies, however, stabilized at approximately 90% after 50 cycles but never improved thereafter. Although competitive, the loss of energy between charging/discharging processes may be primarily attributed to internal cell resistance, as evidenced by the IR drops in the galvanostatic charging/ discharging processes (Figure 6a). Self-discharge analyses were performed by galvanostatically charging the anodic K0.15MnO2 full-cell to 1.2 V and holding the cell idle for 24 h, during which an external voltmeter measured the cell potential. Figure 6d shows a typical selfdischarge curve over a 24 h period. After 6 h, the cell recorded cell voltage was 0.7335 V and decaying at a rate of 6.77 μV s−1, and after the full 24 h analysis period, the recorded cell voltage was 0.5254 V and decaying at 0.867 μV s−1. 3.5. In Situ XANES. The K0.15MnO2·0.43H2O nanosheets were examined using Mn K-edge XANES at beamline X19A at the NSLS at BNL using a three-electrode cell fabricated for in situ analyses (Figure 7).57 All the spectra included the pre-edge features (P1) and white line (WL) features (P2), as shown in Figure 8. The former reveals the electronic excitation of Mn from the core state (1s) to an unoccupied orbital (3d), indicating the stabilization of Mn in the octahedral site.27−29 The main absorption peak (P2) represents the allowable dipole transition from the core electron (1s) to an unoccupied orbital (4p), originating from changes in valence of Mn. The XANES spectra of K0.15MnO2 showed an analogous pattern to that of a MnO2 standard powder, consistent with the conclusion that the Mn valence was +3.85. The K0.15MnO2·0.43H2O nanosheets were further studied via in situ XANES measurements at different potentials. The in situ spectra showed that all the K0.15MnO2 did not exhibit considerable differences compared to MnO2 powder, revealing a similarity in the structural characteristics of Mn under various potentials between 0 and 1.2 V. However, a definitive shift of

Coulombic Efficiency Q Discharge = Q Charge

=

∫

Charge

Discharge

∫

(10)

for symmetric cells, where mTotal is the mass of material loaded on both electrodes. The resulting CMS values for both asymmetric cells and the PEDOT-based symmetric cells are shown in Figure 6b. The competitive capacitive stability of the cathodic K0.15MnO2 asymmetric and PEDOT-based symmetric cells were 87.8% (77.2 to 67.8 F/g) and 86.0% (14.6 to 12.5 F/ g), respectively, over 1000 cycles. However, the anodic K0.15MnO2 asymmetric cell suffered extreme capacitive instability during these analyses, losing 61.8% of its initial mass-specific capacitance (53.6 to 20.4 F/g). This rapid loss in performance may be attributed to a decay of the anodic material, where the Mn(III/IV) atoms are over-reduced during deleterious charging processes to Mn(II) and thus become susceptible to electrolytic dissolution, a phenomenon previously reported by Cottineua et al.52 The initial and final massspecific capacitances of the cathodic K0.15MnO2 full-cell corresponded to energy densities of 15.4 and 13.6 Wh/kg, respectively. Both of these capacitive and energy density values are highly competitive with previously reported MnO2-based cells (see Table 1). Both the Coulombic and energy efficiencies of the anodic K0.15MnO2 cell are shown in Figure 6c during the galvanostatic measurements. Because the charging/discharging processes were conducted with a constant current, the Coulombic efficiency calculations were reduced to the necessary charge/ discharge times as follows:

=

Discharge

∫

for asymmetric cells, where mActive Material is the mass of the cathodic/anodic redox-active material, or as CMS = 4 ×

∫

(11) 20178



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Figure 7. Images of electrochemical cell for in situ XANES measurements.

the absorption edge to higher energies was observed at increasing applied potentials (Figure 8c): Mn K-edge spectra at lower potentials (0 and 0.3 V) had very similar WL peak positions (∼6561 eV), whereas the WL peak shifted to 6562.4 eV when the applied potential increased from 0.3 to 0.6 V, and retained similar peak positions even as the potentials were increased to 1.2 V. The shifting of the WL peaks to a higher energy is related to an increase in binding energy of the core electrons due to an increasing oxidation state, thus representing the Na+ deintercalation Faradaic reaction:6,7,58 K 0.15NaδMnO2 ·0.43H 2O → K 0.15MnO2 ·0.43H 2O + δ Na + + δ e−

Figure 8. In situ XANES spectra (Mn K edge) of K0.15MnO2 ·0.43H2O nanosheets at potentials from 0 to 1.20 V.

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The potential-dependent valence evolution from in situ XANES indicated that a Faradaic oxidation reaction (Na+ deintercalation) occurred around a potential of 0.6 V. This result was consistent with the ex situ CV measurements, where the bvalues decreased toward 0.5 (i.e., approaching Faradaic processes) after 0.6 V during the anodic scans.

strong charge retention over a 24 h period. In situ XANES confirmed that the pseudocapacitive behavior of these nanosheets was associated with variations in the oxidation states of Mn with respect to an externally applied potential. The major changes of the Mn electronic state occurred at potentials between 0.3 and 0.6 V, as predicted by electrochemical kinetics analyses, offering insight into the practical design, operation, and voltage-dependency of a MnO2-based SC system. Coupling K0.15MnO2·0.43H2O with other additives to promote multielectron transfer while concurrently increasing the capacitance and energy density will be our future focus.

4. CONCLUSIONS K0.15MnO2·0.43H2O birnessite nanosheets were facilely prepared at room temperature by controllably reacting Mn2+ with KOH. A maximum gravimetric capacitance of 303 F/g was obtained in 0.1 M Na2SO4 electrolyte via chronopotentiometry and cyclic voltammetry. Full-cell galvanostatic charge/discharge analyses of a cathodic K0.15MnO2 pseudocapacitor showed a competitive initial gravimetric capacitance of 77.2 F/g, which decayed marginally to 67.8 F/g after 1000 cycles (87.8% capacitive retention). Steady Coulombic and energy efficiencies of ∼100 and ∼90%, respectively, were obtained through the galvanostatic measurements, and self-discharge analyses showed

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ASSOCIATED CONTENT

S Supporting Information *

EDS spectrum of K0.15MnO2 nanosheets; TEM images of K0.15MnO2 with different morphologies; CVs of K0.15MnO2/ PEDOT, PEDOT/carbon, and PEDOT-only electrode materials; sample b-value calculations; full-cell fabrication schematic; 20179



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and initial and final galvanostatic cycling stages. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-603-862-4245. Fax: 1-603-862-3747. E-mail: xw. [email protected]. Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Nancy Cherim, Dr. Dale Barkey, and Dr. Jillian Goldfarb at UNH for assistance in TEM, EDS, impedance, and TGA/DTA measurements. This work is supported in part by the University of New Hampshire (X.T., M.Y., W.D.). Research was carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the NSLS was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Beamlines X19A are partly supported by Synchrotron Catalysis Consortium (DE-FG02-05ER15688).

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Highly Efficient K0.15MnO2 Birnessite Nanosheets for Stable

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