17595
2009, 113, 17595–17598 Published on Web 09/21/2009
Multifunctional MnO2-Carbon Nanoarchitectures Exhibit Battery and Capacitor Characteristics in Alkaline Electrolytes Jeffrey W. Long,* Megan B. Sassin, Anne E. Fischer, and Debra R. Rolison Code 6170, Surface Chemistry Branch, U.S. NaVal Research Laboratory, Washington, DC 20375
Azzam N. Mansour Systems and Materials for Power and Protection Branch, NSWC, Carderock DiVision, West Bethesda, Maryland 20817
Valencia S. Johnson, Phillip E. Stallworth, and Steve G. Greenbaum Department of Physics and Astronomy, Hunter College of the City UniVersity of New York, New York, New York 10065 ReceiVed: July 24, 2009; ReVised Manuscript ReceiVed: September 13, 2009
We demonstrate that, when distributed as nanoscale coatings on the walls of carbon nanofoam substrates, manganese oxides exhibit voltammetric signatures in LiOH-containing alkaline electrolytes that are characteristic of either electrochemical capacitors or batteries, depending on the potential range investigated. Pseudocapacitance is observed for positive potential ranges, and ex-situ X-ray absorption spectroscopy confirms that the native layered birnessite MnOx structure is retained as the Mn oxidation state is toggled between 3.72 and 3.43. When the cycling range is extended to more negative potential limits, well-defined reduction and oxidation features are observed, with an associated reversible change in the Mn oxidation state of 0.71 after 25 cycles. For these deep-discharge conditions, high charge-storage capacities are facilitated by the reversible interconversion of birnessite and γ-MnOOH forms of the nanoscale MnOx coating. Solid-state 7Li NMR is used to investigate the role of Li+ from the alkaline electrolyte in enhancing the cycling stability of the MnOx-carbon nanofoam. En route to high-performance electrochemical capacitors and other rate-critical electrochemical devices, we are developing a class of “multifunctional electrode nanoarchitectures” in which electroactive functionalities, including transition metal oxides, polymers, and electrocatalytic nanoparticles, are dispersed as conformal, nanoscale deposits at the surfaces of ultraporous, conductive carbon aerogels and nanofoams.1,2 Manganese oxides (MnOx) are a particularly attractive choice for the design of multifunctional nanoarchitectures owing to their established use as active materials for electrochemical energy storage in configurations ranging from primary Zn/MnO2 alkaline cells3 to rechargeable Li-ion batteries,4 and, more recently, electrochemical capacitors.5 We recently demonstrated that the self-limiting redox reaction of aqueous permanganate with carbon nanofoam substrates generates nanofilaments of Na+-templated birnessite-type MnOx that are homogenously distributed at the carbon walls throughout the thickness (170 µm) of the nanofoam paper (see Figure 1).6,7 In mild aqueous electrolytes (e.g., 1 M Na2SO4), the faradaic pseudocapacitance of the MnOx “paint” improves the energystorage capacity of the electrode structure by factors of 2-10 with respect to what can be achieved solely by double-layer * To whom correspondence should be addressed. Phone: (202) 404-8697. Fax: (202) 767-3321). E-mail:
[email protected].
10.1021/jp9070696 CCC: $40.75
Figure 1. nanofoam.
Transmission electron micrograph of MnOx-carbon
capacitance at the unmodified carbon nanofoam surface. Because the nanoscale oxide coating does not occlude the internal pore network, the hybrid structure retains the rapid charge-discharge response characteristics of the supporting carbon nanoarchitecture. We are now extending the electrochemical investigations of these MnOx-carbon nanoarchitectures to concentrated alkaline electrolytes, which provide higher ionic conductivity and electrolyte solubility relative to nominally neutral-pH electrolytes. Preliminary 2009 American Chemical Society
17596
J. Phys. Chem. C, Vol. 113, No. 41, 2009
Figure 2. Electrochemical measurements for a MnOx-carbon nanofoam in 1.5 M LiOH + 5.6 M KOH, showing: (a) cyclic voltammetry for the capacitor (red) and battery (blue) potential regions at 2 mV s-1; (b) representative cyclic voltammograms at cycles 1, 5, 10, and 25 of 25 consecutive cycles at 2 mV s-1; (c) galvanostatic charge-discharge curves at various applied currents ranging from 250 to 1000 mA g-1, normalized to the total electrode mass.
cyclic voltammetry measurements in a mixed LiOH/KOH electrolyte (see Figure 2a) reveal that MnOx-carbon nanofoam electrodes exhibit the characteristics of both a capacitor and a battery electrode, depending on the potential range examined. For example, when cycling the MnOx-carbon nanofoam over a potential range (+0.4 to -0.5 V vs Hg/HgO) that would mimic its operation as a positive electrode for an asymmetric EC, we observe a mirror-like voltammogram, which is characteristic of a pseudocapacitance process (red trace in Figure 2a). The average specific capacitance derived from voltammetry for the MnOxcarbon nanofoam in this potential range is 150 F g-1, which is significantly greater than the 90 F g-1 previously measured for the same electrode composition in a mild aqueous electrolyte (1 M Na2SO4).6,7 Normalizing to the oxide content of the
Letters MnOx-carbon nanofoam structure (29 wt % “MnO2”) and subtracting the estimated capacitance of the native nanofoam, we calculate that the specific capacitance of the MnOx coating itself is ∼500 F g-1, which would correspond to a Mn oxidation state change of ∼0.41 e-/Mn. When the MnOx-carbon electrode is cycled to more negative potentials (-1.2 V), we observe “battery-like” features in the form of multiple reduction peaks during the negative potential sweep, and on reversal of the scan, a single well-defined peak at +0.1 V that dominates the oxidation process. The reversibility of the associated MnOx redox process is demonstrated in Figure 2b, where after an initial loss on the first cycle the oxidation/reduction currents are relatively stable for cycles 2-25. Charge-storage capacities were calculated by integrating the voltammetric features for the 25th cycle, subtracting the estimated capacitance contribution from the carbon nanofoam, and then normalizing the resulting integrated charge to the known Mn content (18 wt %) of the composite architecture. For this particular MnOx-carbon nanofoam structure, we estimate that 0.75 electrons per Mn are reversibly stored, corresponding to a specific capacity of 230 mA h g-1 MnO2. Galvanostatic charge-discharge curves (see Figure 2c) also exhibit capacitor- and battery-like behavior in the form of both sloping and plateau regions, respectively. A series of imposed currents was applied to a series of MnOx-carbon nanofoam electrodes to assess their performance as a function of charge-discharge rate. Total electrode capacities were relatively unchanged over the range investigated, decreasing from 146 to 135 mA h g-1 as the current density was increased by a factor of 4, from 250 to 1000 mA g-1. The high MnO2-specific capacity and cycling stability combined with the retention of capacity at demanding charge-discharge rates suggest that, with increased MnOx weight loadings in the nanofoam,8 such MnOx-carbon nanofoam structures may be attractive materials for cathodes in “rechargeable” Zn/MnO2 batteries, which have been the subject of intense interest for the past 30 years.9-11 Common forms of manganese oxides (e.g., the ramsdellite-pyrolusite intergrowth γ-MnO2 or the lamellar birnessite MnO2)12 are typically rechargeable in alkaline electrolytes only if the discharge is limited to
