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J. Phys. Chem. B 2001, 105, 8712-8717
Spectroelectrochemical Investigations of Cation-Insertion Reactions at Sol-Gel-Derived Nanostructured, Mesoporous Thin Films of Manganese Oxide† Jeffrey W. Long,*,‡ Lala R. Qadir,‡ Rhonda M. Stroud,§ and Debra R. Rolison*,‡ Surface Chemistry and Surface Modification Branches, NaVal Research Laboratory, Washington, D.C. 20375 ReceiVed: April 6, 2001; In Final Form: July 14, 2001
Sol-gel-derived manganese oxide can readily be prepared as highly porous, nanostructured materials, including ambigel or aerogel monoliths and thin films supported on conductive, transparent glass. The electrochromic properties of manganese oxide, a mixed electron-cation conductor, provide an independent measure of the electronic state of the manganese in the oxide during the electrochemically driven cation-insertion process. Sol-gel-derived thin films of birnessite-type manganese oxide, NaδMnO2‚xH2O, which is a layered polymorph of MnO2, were characterized by spectroelectrochemistry. During nonaqueous electroinsertion reactions of Li+, Mg2+, and tetrabutylammonium cation, the spectral and electrochemical signatures are temporally distinguished and electrochemical site specificity can also be discerned. The rate of coloration of the oxide (recovery of the Mn(IV) state) is approximately 4-fold more rapid than the rate of decoloration as Mn(III) centers are electrogenerated and electrolyte-derived cations are concomitantly inserted. The electrochromic efficiency for Li+ insertion into thin films of birnessite-type manganese oxides is 36 cm2 C-1.
Introduction Transition metal oxides that are mixed conductors of electrons and cations are much studied materials for charge-storage applications.1,2 One important example is manganese oxide, which has the desirable properties of low cost and low toxicity. As with other electrically conductive metal oxides, manganese oxide stores electrochemical charge by a double insertion of electrons and cations into the solid state. Typical insertion cations are protons, as with the aqueous Zn/MnO2 chemistry that is the basis of the alkaline power cell,3 or lithium, as with lithium or lithium-ion batteries:4
MnIVO2 + xe- + xH+ h HxMnIIIxMnIV1-xO2 MnIVO2 + xe- + xLi+ h LixMnIIIxMnIV1-xO2 Cation-insertion processes underlie important technological applications of the electrically conductive metal oxides in devices besides batteries,1,2 such as ultracapacitors5 and direct methanol fuel cells.6 Electron/cation insertion is also responsible for the electrochromism of many metal oxides, including manganese oxide,7-11 where the electronic structure (and therefore the optical absorbance) of the oxide changes as a function of the oxidation state of the metal in the oxide.12 The charge-storage properties of metal oxides can be optimized when the oxides are prepared as nanostructured architectures that contain interconnected (i.e., bicontinuous) networks of solid and pore.13 Preparing a charge-insertion oxide as a nanostructured material maximizes its surface area, which in turn maximizes the electrical interface across which charge insertion occurs, and minimizes the distances across which †
Part of the special issue “Royce W. Murray Festschrift”. * To whom correspondence should be addressed. E-mail: jwlong@ ccf.nrl.navy.mil;
[email protected]. ‡ Surface Chemistry Branch, Naval Research Laboratory. § Surface Modification Branch, Naval Research Laboratory.
10.1021/jp0112830
cations must transport to convert the oxide to its protonated or lithiated form. A continuous mesoporous network, in turn, serves to facilitate mass transport of solvent and cations to the electroreacting metal oxide surface. Sol-gel chemistry provides a synthetic route for the preparation of nanostructured oxides. Under the appropriate reaction conditions a sol-to-gel transition occurs, resulting in a threedimensional network of nanoscale domains. The continuous porosity and high surface area of the wet gel are retained in the dry state if the pore fluid is removed as a supercritical fluid to form an aerogel. Alternatively, if the pore fluid has a low surface tension, such as liquid alkanes have, it can be evaporated with only moderate densification of the solid-pore architecture to form an ambigel (a term that designates a gel dried at ambient pressures13). The successful synthesis and charge-insertion character of electrically conductive metal-oxide aerogels and ambigels have been reported for V2O5,14-16 MoO3,17 LiMnOx,18 and MnO2.19,20 In this report, nanoscale, highly porous manganese oxide (MnOx) aerogels and ambigels are prepared in both monolithic and thin-film forms and characterized by physical and spectroelectrochemical methods. The electrochemical response at such high-surface-area, strongly oxidizing electrodes may be complicated by interfacial contributions such as double-layer charging and electrolyte decomposition. The electrochromic response provides a complementary in situ method that temporally monitors the effect of electrochemically driven charge insertion and deinsertion on the electronic state of nanostructured manganese oxide. We also explore the effects of the cation on the reversibility and insertion rates at these nanoscale manganese oxide domains in nonaqueous electrolytes. Experimental Section Chemicals. Fumaric acid (Aldrich), NaMnO4‚H2O (Aldrich), hexane (Aldrich), and cyclohexane (Aldrich) were used as received. The electrolyte salts, LiClO4 (Aldrich), Mg(ClO4)2
This article not subject to U.S. Copyright. Published 2001 by the American Chemical Society Published on Web 08/02/2001
Investigations of Manganese Oxide (Aldrich), and tetrabutylammonium perchlorate (TBAClO4; Fluka), were dried under vacuum at 60 °C for 48 h. Propylene carbonate (Aldrich) was dried over 4 A molecular sieves (Linde) and then vacuum distilled at 70 °C. The initial NaMnO4‚H2O solutions are prepared with 18 MΩ cm H2O (Barnstead Nanopure). Sample Preparation. Monolithic manganese oxide gels were prepared by a method similar to that described previously.19-21 In brief, solid fumaric acid is added to a vigorously stirred solution of 0.20 M NaMnO4‚H2O in a 1:3 molar ratio. The resulting manganese oxide sol (brown in color) is degassed under a moderate vacuum for 8 min to facilitate the evolution of CO2 and then poured into 5 mL polypropylene molds. Gelation occurs in 1-1.5 h. The gels are aged for 24 h, during which time some moderate syneresis is observed. The aged gels are rinsed with several aliquots of fresh H2O to remove soluble byproducts and then processed to form xerogels, ambigels, or aerogels by varying the rinsing and drying procedure. These gels differ from those previously prepared20 in that a reactive rinse with 1 M H2SO4 is not employed, as this step destabilizes the mechanical ruggedness of the supported thin films of MnOx. Xerogels are obtained by decanting excess H2O and then drying in air at 60 °C over 2 days. Ambigels are prepared by soaking the H2O-wet gels in several aliquots of acetone over 2 days, followed by pore-fluid exchange with several aliquots of either hexane or cyclohexane. Excess liquid is decanted, and the gels are dried at 60 °C over 2 days. Aerogels are obtained by transferring acetone-wet gels to an autoclave and thoroughly rinsing with CO2(l) to replace the acetone. The autoclave temperature is raised to 40 °C, where the CO2 exists as a supercritical fluid (SCF). The SCF CO2 is then slowly vented to yield fragile, low-density, but monolithic aerogels. Thin films are derived from the same manganese oxide sol as the monoliths. Films are cast on pre-cut indium-doped tin oxide coated glass slides, ITO (Delta Technologies). Fluorglas tape was used to demark an area on the ITO electrode of 15 mm × 7 mm, onto which is deposited a 4 µL aliquot of the manganese oxide sol. A silanized glass cover slip is placed on top of the sol, so that a smooth layer of sol lies between the ITO slide and cover slip. The sol-coated ITO slides are then placed in a hydration chamber for 6 h, during which time gelation occurs. After 6 h, the slides are immersed in H2O; after a few hours, the cover slip falls free of the film, leaving a continuous manganese oxide film. At this point, the ITOsupported films are processed to xerogel, ambigel, or aerogel forms using the same solvent-exchange and drying procedures described above for the monolithic samples. All monolith and thin-film manganese oxide samples are heated in air at 2 °C min-1 to 300 °C, held at temperature for 2 h, and then cooled to ambient temperature at 2 °C. This heating protocol was previously shown by simultaneous thermogravimetric analysis and differential scanning calorimetry to bring the MnOx gels to constant weight without inducing a phase change to Mn2O3.19,20 The heated MnOx gels retain some structural water and are more properly designated as NaδMnO2‚ xH2O. Spectroelectrochemistry. Spectroelectrochemical experiments were performed in a glass-cuvette cell with a 1 cm path length and a MnOx gel-coated ITO slide as the working electrode. Both reference and counter electrodes (FLEXIREF Ag/AgCl electrode (World Precision Instruments) and Pt mesh, respectively) were placed outside of the spectrometer beam (Agilent model 8453 UV-visible spectrophotometer). Electrolytes were thoroughly degassed with N2 prior to experiments,
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8713
Figure 1. High-resolution TEM for a birnessite NaδMnO2‚xH2O ambigel.
and the spectroelectrochemical cell was maintained under a blanket of N2 through all measurements. Electrochemical control was achieved with a Radiometer Analytical Voltalab 40 computer-controlled potentiostat. Prior to spectroelectrochemical measurements, all ITO-supported thin films were first equilibrated for 24 h in a vacuum desiccator with propylene carbonate vapor at reduced pressure and 60 °C. Physical and Elemental Characterization. Transmission electron microscopy and electron diffraction (300 kV Hitachi H9000 transmission electron microscope) were performed on specimens prepared by pipetting suspensions from methanol of the desired MnOx aerogel, ambigel, or xerogel powder (derived from the monoliths) onto a holey carbon grid. Physisorption measurements were performed with a Micromeritics ASAP 2010 system, with surface areas derived using the multipoint BrunauerEmmett-Teller (BET) method and cumulative pore volumes determined using the Barrett-Joyner-Halenda (BJH) method. The film thickness of ITO-supported MnOx was determined by profilometry (Tencor P-10 Surface Profiler). The manganese content of MnOx monolithic xerogels, ambigels, aerogels, and representative thin films was determined by elemental analysis; the sodium content was also determined for monolithic samples (Galbraith Laboratories). The oxygen content could not be determined by elemental analysis because of the presence of sodium in the samples. Average Mn oxidation states were determined by titration according to published procedures.22 Results and Discussions Structural Characterization of Monoliths. High-resolution transmission electron microscopy (Figure 1) shows that the manganese oxide gels consist of oblong particles of approximately 3-5 nm (c axis) by 10-20 nm (a-b plane) and appear to be the same heavily disordered, pseudohexagonal layered birnessite phase we reported previously for gels derived from the Na-based, H2SO4-reacted preparation.20 From the diffraction pattern of the xerogel sample, we estimate lattice constants of a ) 2.93 Å and c ) 8.14 Å. The c (interlayer) spacing is not well defined, partly because of the heavily disordered nature of the samples and also possibly resulting from vacuum dehydration in the electron microscope. This 8.14 Å interplanar spacing is larger than that typically observed for birnessite (7.1 Å),23 which suggests that a high fraction of the interlayer space is filled with H2O and cation dopants. The surface areas, pore volumes, and pore-size distributions determined for the monolithic manganese oxide samples using
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TABLE 1: Physisorption and Profilometry Results for Birnessite-Type MnOx Gelsa
MnOx gel xerogel ambigel (hexane) ambigel (cyclohexane) aerogel
monolithsb
thin films
BJH cumulative BET surface area/ pore volume/ cm3 g-1 m2 g-1
mean film thickness/ nm
210 230 220 230
0.50 0.64 1.2 1.0
500 ( 200 1000 ( 400 1300 ( 400 1500 ( 400
TABLE 2: Results for Chemical Composition of Birnessite-Type MnOx Gels as Determined by Elemental Analysis for Mn and Na Contentsa MnOx gel
total Mn/wt %
Na/Mn atom ratio
average Mn oxidation state
xerogel ambigel (cyclohexane) aerogel
52.8 54.6 57.7
0.129 0.134 0.125
3.92 3.84 3.81
a Average Mn oxidation state is determined based on chemical titration with ammonium iron(II) sulfate.22
a Surface area and pore volume results are measured using N 2 physisorption methods with monolithic samples. Film thickness values are determined using profilometry on ITO-supported MnOx films. b Replicate error is within (2%; batch-to-batch error for surface area is (10%.
Figure 3. Visible absorption spectra for a 1.6 µm thick birnessite NaδMnO2‚xH2O ambigel film in 1 M LiClO4/propylene carbonate as a function of electrode potential. Absorbance at 500 nm (‚‚‚) denotes monitoring wavelength used for insertion studies.
Figure 2. Plots of pore-size distribution for various gel forms of birnessite NaδMnO2‚xH2O. Distributions are derived on the basis of a cylindrical pore model.
N2 physisorption are summarized in Table 1. All forms exhibit high specific surface areas of approximately 220 m2 g-1, in agreement with the nanocrystalline nature of the manganese oxide domains observed in the TEM imaging. The variously dried manganese oxide gels are differentiated both by cumulative pore volumes and by pore-size distribution (Figure 2). Xerogels exhibit the lowest pore volume and a pore size restricted to less than 20 nm in diameter. For ambigels, the nature of the low-surface-tension fluid that evaporates from the wet MnOx gels affects the number of mesopores centered at ∼25 nm but not the general distribution of represented pore sizes. As seen in Figure 2 and Table 1, the greater cumulative pore volume that results for ambigel obtained by drying from cyclohexane, rather than hexane (which exceeds even that derived for the aerogels), arises from pores sized from 15 to 30 nm. Aerogels exhibit a wide distribution of pore sizes, even extending into the macropore (>50 nm24,25) range. The various degrees of pore collapse for the xerogel, ambigel, and aerogel forms are also reflected in the measured thickness of the ITO-supported films (Table 1). Films prepared as ambigels and aerogels also exhibit less cracking than their xerogel counterparts and generally have better optical transmissive quality. Monolithic MnOx samples are also examined for their chemical composition by elemental analysis and chemical titration. The various gel forms do contain varying amounts of Mn, but the Na/Mn ratios are very similar (Table 2). Although the H2SO4-reaction step was not employed in the preparation of these MnOx gels, the average Mn oxidation states, as determined by chemical titration, range from 3.81 to 3.92 and are similar to those measured previously for H2SO4-reacted MnOx gels with a cryptomelane structure (3.88-3.92).19
Spectroelectrochemistry. A typical set of visible absorption spectra as a function of electrode potential is shown in Figure 3 for an ITO-supported MnOx ambigel thin film. These spectra exhibit a monotonic decrease in absorptivity as the wavelength increases from 400 to 800 nm, with no distinct peaks observed. These features are qualitatively similar to those observed previously for MnO2 films in both aqueous7-10 and nonaqueous11 electrolytes. The visible absorbance of MnOx has been attributed to the Mn(IV) state, so that a decrease in the absorbance upon application of a reducing potential corresponds to formation of Mn(III) centers with the accompanying stoichiometric insertion of the charge-compensating cation, in this case Li+.8,10 This anodically coloring electrochromic behavior of MnOx is confirmed in the present case for nanoscale birnessite MnOx. The fact that the absorption spectrum for the as-processed MnOx film at the initial open-circuit potential, Eoc, lies between that for the film under reducing (E ) -1.0V) and oxidizing (E ) +1.4V) conditions (see Figure 3) is consistent with the fact that mixed-valent Mn(III/IV) oxide is initially obtained from sol-gel processing,21 in qualitative agreement with the average Mn oxidation states obtained by chemical titration (see Table 2). The electrochromic properties are further investigated as a function of the degree of Li+ insertion by applying current steps (10 µA for 180 s) to the thin-film MnOx electrode, followed by recording the spectrum and the open-circuit potential after 15 min. Each current step should coincide with the insertion of 0.036 Li per MnO2, as based on the Mn content of the final film.26 Prior to the application of current pulses, the film was poised at 1.4 V for 300 s to oxidize Mn(III) centers in the asprocessed mixed-valent MnOx to Mn(IV). The spectroscopic response, in terms of the change in optical density, ∆OD, correlates linearly with respect to the degree of lithiation (χLi) of the birnessite MnOx film up to the insertion of ∼0.36 Li per MnO2 (Figure 4).
Investigations of Manganese Oxide
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Figure 4. Plots of ∆OD at 500 nm and of Eoc as a function of Li insertion at a birnessite NaδMnO2‚xH2O ambigel film. Measurements of ∆OD have a sample error of (5%.
The oxidation state for the as-prepared mixed-valent MnOx film could ideally be determined based on its initial ∆OD. Extrapolation from this ∆OD value (0.50) on the plot in Figure 4 to the x axis yields an initial average Mn oxidation state of 3.92, which is higher than the oxidation state determined from chemical titration (3.84). Such a quantitative discrepancy would arise if manganese cation vacancies are present, in that the vacancies should lower the effective chemical valency without affecting the quantity of color centers. The electrochromic efficiency parameter, η, defined as ∆OD/ Q, where Q is the charge density,11 is 36 cm2 C-1 for the birnessite ambigel (as derived from the slope of the linear region in Figure 4). This value is higher than a previously reported electrochromic efficiency of 7.2 cm2 C-1, also in a nonaqueous Li+-containing electrolyte, for MnOx films prepared by electronbeam evaporation.11 Electrochromic efficiencies as high as 130 cm2 C-1 have been reported for MnOx films in aqueous borate buffer solution.9 The influence of the insertion cation on the electrochemical and electrochromic properties of birnessite NaδMnO2‚xH2O ambigel films is shown in Figure 5. The voltammetric response is plotted in terms of both the electrochemical current and the change in absorbance at 500 nm, ∂A500/∂E. With Li+ or Mg2+ as the cation, the electrochemical and spectroscopic response of the birnessite MnOx film correlates fairly well as gauged by the shape of the current-potential (i - E) and differential absorbance-potential (∂A500 - E) curves. An offset in the positions of the peaks is noted on the potential (and time) axis during the reduction process, in which the spectroscopic response precedes the electrochemical response. In both cases, the features are fairly broad, with comparable charge passed for both Li+ and Mg2+ insertion/deinsertion. The spectroscopic and electrochemical responses do diverge in the potential range -0.5 to -1.0 V, where the continuing electrochemical current is not reflected by further changes in the spectroscopic response, which indicates that further reactions are occurring which are not accompanied by changes in the electronic state of Mn in the MnOx electrode. The electrochemical and spectroscopic responses also diverge at positive potentials (>+1.4V) with the onset of electrolyte decomposition reactions. The kinetics of the lithium-insertion reaction are investigated using potential-step chronocoulometry, while simultaneously monitoring the spectroscopic response (chronoabsorptometry).27 Upon application of a potential step from an oxidizing (E ) +1.4 V) to a reducing potential (E ) -1.0 V), the Li+ insertion reaction is accompanied by a rapid decrease in the absorbance,
Figure 5. Voltammograms recorded at ITO-supported thin-film electrodes of birnessite NaδMnO2‚xH2O ambigel in propylene carbonate electrolytes (scan rate ) 4 mV s-1). Dashed lines denote the voltammetric current and solid lines denote the change in absorbance as a function of time (and potential) at λ ) 500 nm for (a) 1 M LiClO4 and (b) 0.5 M Mg(ClO4)2 electrolytes. The dotted line (‚‚‚) denotes zero on both y axes. These plots represent the 2nd full voltammetric cycle.
although a steady-state normalized absorbance of 0.54 is reached only after ∼4 min (Figure 6a). A steady-state response for the charge is not reached within 10 min. A more rapid electrochromic response is observed on the Li ion deinsertion step (E ) +1.4 V), where steady state is reached by 60 s (Figure 6b). The coulometric deinsertion is also more rapid, requiring 4-5 min to reach steady state. In an effort to discriminate among the various cationassociation sites at/within the nanoscale domains of these disordered birnessite gels, we introduce a bulky organic cation tetrabutylammonium (TBA+, ionic diameter, d ∼ 9 Å28) into the supporting electrolyte instead of the smaller insertion cations, Li+ and Mg2+ (ionic diameters, 1.18 and 1.44 Å, respectively). The chemical intercalation (ion exchange) of tetraalkylammonium cations in birnessite-type manganese oxide has been previously reported, where the intercalation of tetrabutylammonium was accommodated by an expansion of the interplanar spacing from 7 to either 22 Å29 or 16 Å,30 depending on reaction conditions. For electrochemically driven insertion/association of TBA+ into the birnessite-type ambigel film, multiple voltammetric peaks are observed in the presence of TBA+ both on the negative and positive potential sweeps (Figure 7a). Distinctive peaks are more clearly evident in the electrochromic response, ∂A500/∂E, with cathodic peaks at ∼ -0.2 and -0.8 V, and anodic peaks at 0 and +0.9 V (Figure 7b). The more densified xerogel film exhibits only one well-defined cathodic peak in TBA+ electrolyte at ∼ -1.1 V and a smaller anodic peak at ∼+0.9 V. The electrochromic efficiencies, in terms of absorbance rather than ∆OD, can also be estimated based on the ratio of the spectroscopic response (∂A/∂t) to the electrochemical response (current, or ∂Q/∂t) at one of the voltammetric peaks. Using this estimation, similar values for the electrochromic efficiency of
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Figure 6. Absorbance at 500 nm (A500) and coulometric traces for potential-step experiments at birnessite NaδMnO2‚xH2O ambigel films: (a) step to E ) -1.0 V and (b) step to E ) +1.4 V, in 1 M LiClO4/propylene carbonate; and (c) step to E ) -1.4 V and (d) step to E ) +1.4 V in 0.1 M TBAClO4. These measurements were recorded on the 2nd discharge/charge cycle.
Figure 7. The spectroelectrochemical response at thin-film electrodes of birnessite NaδMnO2‚xH2O in 0.1 M TBAClO4/propylene carbonate (scan rate ) 4 mV s-1). Dashed lines denote the voltammetric current, and solid lines denote the change in absorbance as a function of time (and potential) at a wavelength of 500 nm for (a) ambigel film and (b) xerogel film. The dotted line denotes zero on both y axes. These plots represent the 1st full voltammetric cycle, initiated at Eoc.
12 and 14 cm2 C-1 are obtained for the xerogel and ambigel, respectively, when measured at the TBA+ insertion peak. The kinetics of the TBA+ insertion reaction at thin films of MnOx ambigel are also investigated by simultaneous chronocoulometry and chronoabsorptometry. As in the case of Li+
insertion, upon application of a reducing potential step (E ) -1.4 V), the absorbance exhibits a decrease comparably rapid to that seen for Li+ insertion, presumably corresponding to TBA+ insertion at electrogenerated Mn(III) centers (Figure 6c); the coulometric response is even more sluggish than that seen for Li+ insertion. When the potential is stepped to +1.4 V to facilitate TBA+ deinsertion, a rapid increase in the absorbance is observed up to ∼50 s, but the absorbance of the initial oxidized form of the MnOx film is not achieved, indicating poor reversibility for the TBA+ insertion/deinsertion process. The decoloration that occurs while poised at +1.4 V (see Figure 6d), which is a potential that should maintain Mn as Mn(IV), may indicate that the alkyl functionalities on the TBA cation are being catalytically oxidized by Mn(IV). The energetically distinct cation-association events seen by spectroelectrochemistry for TBA+/e- reactions at the birnessitetype MnOx ambigel need to be attributed to processes beyond those expected of cation intercalation in the interlayer space of the birnessite structure. Possible reactions of TBA+ with nanostructured mesoporous birnessite include surface adsorption mechanisms,31 or association with vacancies.32 Reutschi has previously demonstrated the association of H+ with electrogenerated Mn(III) centers as well as with cation vacancy sites in electrolytic MnO2.33 The association of Li+ with cation vacancies has also been proposed as a possible explanation for the large Li ion capacities observed for V2O5 aerogels.13 Such alternate mechanisms should be amplified in these nanostructured forms of manganese oxide and more pronounced for the more disordered ambi/aerogel forms than for xerogel forms of MnOx. Conclusions Nanostructured manganese oxide monoliths and ITO-supported films have been prepared by sol-gel chemistry, with control of the pore-solid architecture on the nanoscale. The electrochromic properties of these materials prepared as thin films permit rapid spectroelectrochemical characterization of cation-insertion reactions at these nanoscale solids. The spectroscopic response provides a method that selectively monitors
Investigations of Manganese Oxide the electronic state of Mn in the manganese oxide during the insertion process. The use of bulky organic cations in the supporting electrolyte suggests the presence of multiple energetic sites for cation association with the MnOx domains, as confirmed by changes in the optical absorbance. Acknowledgment. This work was supported by the Office of Naval Research. The authors acknowledge Amanda Young (NRL summer student) for performing the chemical titration of the nanostructured manganese oxides. D.R.R. and J.W.L. also wish to acknowledge R.W. Murray for his guidance during their respective graduate careers at UNC and for the continuing inspiration of his insightful and joyous approach to science. References and Notes (1) Desilvestro, J.; Haas, O. J. Electrochem. Soc. 1990, 137, 5C. (2) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Nova´k, P. AdV. Mater. 1998, 10, 725. (3) Chabre, Y.; Pannetier, J. Prog. Solid State Chem. 1995, 23, 1. (4) Thackeray, M. Prog. Solid State Chem. 1997, 25, 1. (5) Conway, B. E. J. Electrochem. Soc. 1991, 138, 1539. (6) (a) Rolison, D. R.; Hagans, P. L.; Swider, K. E.; Long, J. W. Langmuir 1999, 15, 774. (b) Long, J. W.; Stroud, R. M.; Swider-Lyons, K. E.; Rolison, D. R. J. Phys. Chem. B 2000, 104, 9772. (7) Burke, L. D.; Murphy, O. J. J. Electroanal. Chem. 1980, 109, 373. (8) Kanoh, H.; Hirotsu, T.; Ooi, K. J. Electrochem. Soc. 1996, 143, 905. (9) Cordoba de Terresi, S. I.; Gorenstien, A. Electrochim. Acta 1992, 37, 2015. (10) Lee, C.-H.; Cahan, B.; Yeager, E. J. Electrochem. Soc. 1973, 120, 1689. (11) Seike, T.; Nagai, J. Solar Energy Mater. 1991, 22, 107. (12) Granqvist, C. G. Appl. Phys. 1993, A57, 3. (13) Rolison, D. R.; Dunn, B. J. Mater. Chem. 2001, 11, 963. (14) Owens, B. B.; Passerini, S.; Smyrl, W. H. Electrochim. Acta 1999, 45, 215.
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8717 (15) Dunn, B.; Fuqua, P.; Salloux, K. J. Non-Cryst. Solids 1995, 188, 11. (16) Dong, W.; Rolison, D. R.; Dunn, B. Electrochem. Solid-State Lett. 2000, 3, 457. (17) Harreld, J. H.; Dong, W.; Dunn, B. Mater. Res. Bull. 1998, 33, 561. (18) Passerini, S.; Coustier, F.; Giorgetti, M.; Smyrl, W. H. Electrochem. Solid-State Lett. 1999, 2, 483. (19) Long, J. W.; Swider-Lyons, K. E.; Stroud, R. M.; Rolison, D. R. Electrochem. Solid-State Lett. 2000, 3, 453. (20) Long, J. W.; Stroud, R. M.; Rolison, D. R. J. Non-Cryst. Solids 2001, 285, 288. (21) Bach, S.; Henry, M.; Baffier, N.; Livage, J. J. Solid-State Chem. 1990, 88, 325. (22) Katz, M. J.; Clarke, R. C.; Nye, W. F. Anal. Chem. 1956, 28, 507. (23) Post, J. E.; Veblen, D. R. Am. Miner. 1990, 75, 477. (24) The names given to distinguish nanometer-sized porosity are defined by IUPAC to be microporous (pores sized 50 nm); in light of the IUPAC definitions, nanoporous or nanopore are nonsensical terms. (25) Sing, K. S. W.; Everett, D. H.; Haul, R. H. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (26) The ratio of Li inserted per MnO2 is based on a Mn content of 30 ( 2 µg per film, as determined by elemental analysis of representative films. (27) Heineman, W. R.; Kissinger, P. T. In Laboratory Techniques in Analytical Chemistry; Heineman, W. R., Kissinger, P. T., Eds.; Marcel Dekker: New York, 1984; pp 64-69. (28) Barrer, R. M.; James, S. D. J. Phys. Chem. 1960, 64, 421. (29) Liu, Z.; Ooi, K.; Kanoh, H.; Tang, W.; Tomida, T. Langmuir 2000, 16, 4154. (30) Gao, Q.; Giraldo, O.; Tong, W.; Suib, S. L. Chem. Mater. 2001, 13, 778. (31) Lyon, L. A.; Hupp, J. T. J. Phys. Chem. 1995, 99, 15718. (32) (a) Ruetschi, P. J. Electrochem. Soc. 1984, 131, 2737. (b) Ruetschi, P. J. Electrochem. Soc. 1988, 135, 2657. (33) Ruetschi, P.; Giovanoli, R. J. Electrochem. Soc. 1988, 135, 2663.
