A study of the discharge process of manganese oxide in borate solution using in situ techniques

ElecfrochimicuActa, Vol. 38, No. 213,pp. 34-347, 1993 Rinted in Gmt

0013-4686p3 $6.00+ 0.00 0 1992.Pergamon Press Ltd.

Britain.

A STUDY OF THE DISCHARGE PROCESS OF MANGANESE OXIDE IN BORATE SOLUTION USING IN SITU TECHNIQUES B. A. LOPEZ

DE

M~~HIIUA*,TCJSHIAK~Omsumt

and NORIO SAT@

* Instituto de Ciencias Quimicas, Facultad de Agronomia y Agroindustrias, Universidad National de Santiago de1 Estero, UNSE. Avda Belgrano(S) 1912,420O Santiago de1 Estero, Argentina t Electrochemistry Laboratory, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received 8 April 1992) Abstract-The change of electrodeposited manganese oxide on gold during the reduction process was investigated in borate solution at pH 9.2, using in situ optical techniques. Raman spectroscopy (RS) showed five bands of manganese dioxide at 414 480,520,580 and 64Ocm-I, which decreased and disappeared during the cathodic discharge process. These changes occurred reversibly during the reductionoxidation cycle. The band at 58Ocm-’ can be assigned to the Mn-0 stretching because of the correlation found between the XPS analyses and the Raman spectroscopy. In situ Raman spectra of thermal manganese oxide were also performed, and showed the same five Raman bands at 415,485,515, 585 and 635cm-‘. Electrochemically modulated infrared spectroscopy (EMIRS) is associated with the appearance of a O-H stretching band. The changes of RS, EMIRS and XPS reveal the proton+lectron injection for the reduction reaction of electrolytic MnO, . Key words: raman, EMIRS, manganese dioxide, borate.

INTRODUCTION techniques, such as Raman (RS), surface-enhanced Raman spectroscopy (SERS), electrochemically modulated infrared reflectance (EMIRS) as well as uu-ois reflectance, were used to investigate the formation of surface layers of hydroxide and oxyhydroxides[ l-43. By uv-uis reflectance spectroscopy[l] it was possible to characterize some of the oxygenated species formed at the nickel electrode-alkaline solution interface. By ir reflectance spectroscopy[2] information was obtained about the nature of the hydroxides (strength of the bond between O-H and Ni) and the role of water. Structural changes occurring during deposition of thin (ca 10nm) MnO, films and oxidation of similar Mn(OH), films were examined using SERS[4]. The Raman bands were attributed to Mn-0 and Mn-OH vibrations in MnO,, MnOOH and Mn(OH), species. The 670 cm- ’ band was attributed tentatively to an intermediate Mn(IV)-0-Mn(II1) species formed en route to MnOOH. The in situ techniques give new insights concerning the reversibility of some of the electrochemical processes which occur on the electrode surface when varying the potential. In particular, the oxide films which undergo a process of reversible double injection of ions and electrons have been studied, mainly in connection with the electrochromic reaction. The hydrated WO, was discussed on the basis of ir and RS which change as the potential shifts from the transparent state to the coloured state and a model of this process has been proposed. The electro-

In situ spectroscopic

* Author to whom correspondence should be addressed.

chromic process may start with a reaction between the terminal W-O and the incorporated water[SJ. The reduction of MnO, is accompanied by the incorporation of protons, ie a simultaneous protonelectron insertion reaction occurs. Both manganese and oxygen ions remain in their sites while the reduction progresses by a hopping mechanism which yields MnOOH isostructural with Mn02. This single solid phase reaction is, in principle, reversible[6]. Raman spectroscopy under ex situ and in situ conditions with MnO,/Au electrodes[7] shows that MnO, films exhibit five Raman peaks while there are no distinct peaks for the reduced oxide film. On returning to the oxidized state all the peaks appear again. The changes in the Raman bands occur during the reduction process as the electrochromic reaction takes place. XPS has been used to examine electrolytic MnO, on Au at different discharge states[S]. The XPS spectra of oxygen 1s change with the reduction process and from the deconvolution of this spectra in Mn=O, Mn-OH and adsorbed water, the number of oxidation, n, of Mn could be computed. In this paper, manganese dioxide was studied by in situ Raman scattering spectroscopy to monitor the composition changes of the 8lm during the cathodic discharge process. IR reflection absorption and XPS results were employed in addition to the Raman measurements.

EXPERIMENTAL Most of the experimental set-up and procedure have been previously described[7-91. The Raman 341

342

B. A. LOPEZDE MISHIMAet al.

spectrometer used was a triple type of monochromator (JASCO-R-800T) in which the excitation was made by using a single line of 1 = 514.5 nm from an Ar ion laser. The Raman scattering light was detected by a photomultipler (HAMAMATSU R-649) and a photon counter. The manganese oxide was deposited onto a gold disc (4mm diameter) by anodization at 2.0mA cm-’ in aqueous solution containing 0.66 mol dm-3 MnSO, and 0.5moldm-’ H,SO* at 85°C. The thickness of the films was around 0.44 pm, which was calculated from the electric charge for the deposition at 77mCcm- 2 by assuming the density of the MnO, to be 1.65gcm-3[10]. Other manganese oxide samples for Raman spectra were used: International Common Sample (ICS) No. 12 (chemical MnO,) and thermal manganese oxide prepared in this laboratory. Thermal MnO, deposits on gold disc of geometrical area cu. 0.12cm2 were prepared as follows. The disc was degreased in acetone, submerged in nitric acid washed with distilled water, and allowed to dry. It was then immersed in a methanol solution containing 1.74 mol dmd3 Mn(NO,), 4H,O and annealed in air after drying for 3 h at 2OOC[ll]. The electrochemically modulated infrared reflectance (EMIRS) measurements were made with a thin layer spectroelectrochemical cell and a beam spectrometer, whose design and set-up were described elsewhere[12]. The electrode was an 8mm diameter gold disk, sealed into Pyrex glass. Solutions were prepared with Millipore water. The optical window, a 2mm thick silicon disk attached to the cell with a glass flange at its front end brought the p-polarized incident radiation onto the electrode at an incident angle of cu. 45 degrees. The electrode was polished to a mirror finish with successively finer grades of alumina down O.O5micron, then immersed in hot sulfuric acid and rinsed with distilled water. The fdm of manganese dioxide with of ca. 0.44pm thick was then electrodeposited on it. For the EMIRS the manganese oxide electrode was modulated with respect to the potential between 0.7OV and 0.9OV, -O.lOV and -0.4OV and - 0.10 V and 0.40 V. The modulation frequency was 11 Hz. The change in electrode reflectivity (AR/R) in response to the potential modulation was observed as a function of wavelength by using a synchrotrack lock-in amplifier NFLI.575. The detector employed was an InSb cooled to liquid nitrogen temperature. A signal averaging of typically 10 scans was conducted. A slit width of 5 mm was employed. The electrolyte used was a 0.1 mol dmm3 aqueous borate solution, pH9.2, which was purged by purified nitrogen gas. The electrode potential was measured by using a Ag/AgCl reference electrode. Experiments were performed at 22°C.

Au/MnO@raCSdn PH 92

b-QW.’

700

Mx)

500 Ramon Shift, Avkd

Loo

Fig. 1. In situ Raman spectrum of the MnO,/Au electrode in pH 9.2 borate solution at during the reduction-oxidation cycle. (a) E = 0.8OV,(b) E = -0.8OV, (c) E = 0.8OV (after

reoxidation).

tions under reduction-oxidation conditions were shown[7]. Figure 1 shows in situ Raman spectra of electrolytic MnO,/Au electrode at different potentials. The spectrum obtained at potential of E = 0.8OV shows five Raman peaks at 410,480,520,580 and 640 cm- ’ (Fig. la). Reducing the potential from 0.8OV, there was a Raman spectrum change. Under the reduced state at -0.8OV (Fig. lb) there were no distinguishable peaks on the Raman spectrum. On returning to 0.8OV the peaks appeared again (Fig. lc). Figure 2 shows the Raman spectra when the potential was reduced from 0.80 to 0.3OV. The spectra were obtained as follows: the electrode was initially kept at E, = 0.8OV for 1 h (Fig. 2a) then the potential was swept at u = 0.0017 Vs-’ from the initial value to 0.3OV, and the Raman spectrum obtained after keeping the electrode at this potential for 3 h (Fig. 2b). The Raman spectrum at 0.30 V is similar to that at 0.8OV. The former exhibits the five Raman bands which increased with time (Fig. 2~). Figure 3 shows the Raman spectrum when the potential was reduced from 0.80 to 0.15V. The five Raman bands of the spectrum at E = 0.15 V

Au/Mn~/Borate Saln pH92

RESULTS 600

Raman spectra Electrolytic

manganese

oxide. In

our previous study, the ability of the Raman techniques to characterize the MnO, and to detect film modifica-

Ramon

SW Shift, Adcrtl

4co

Fig. 2. Time effect of in situ Raman spectra of manganese dioxide on Au. (a) E = 0.8OV (initial), (b) reduction at E = 0.3OV during 3 h, (c) reduction at E = 0.3OV during 15h.

343

Discharge of MnO, in borate

3 .= g S c

l

700

WO Ranan

Shift,

L&cm1

400

CEO

Fig. 3. Time effect of in situ Raman spectra of manganese dioxide on Au. (a) E = O.SOV (initial), (b) reduction at E = 0.15 V during 3 h, (c) reduction at E = 0.15 during 4 h.

decreased with time (Fig. 3b). After 4 h, only broad bands were observed (Fig. 3~). The selected experience time at the different reduction potentials was 4h. Figure 4a shows the potentiodynamic polarization curve of manganese dioxide during the reductionoxidation cycle from E = 0.8OV to O.OV and on return to 0.8OV at sweep rate 0.0017 Vs-‘. Figure 4b shows the dependence of the Raman spectrum peak intensity at 58Ocm-’ with potential. The arbitrary values were taken from the Raman spectra obtained at different potential according to the prc cedure described above. The peak height of th Raman band at 580cm-’ decreased sharply after th cathodic peak on the voltammogram (arounl 0.20 V). The values of the current density at the stationar state for different potentials where the Rama spectra were performed are shown in Fig. 5. Fo potential values higher than 0.2OV, the densit current is positive, so the manganese oxide is oxi dized and the Raman spectrum shows the five band! The initial density current was negative and its valu corresponded to the potentiodynamic values of Fig. 4a. Then it evolved to positive values and remained positive until the end of the spectrum.

I/mA

Llu/MnO2/Borate

sdn

a

03

-@O

E/V

Fig. 5. Stationary polarization curve of the MnOJAu trode in borate solution at pH 9.2.

elec-

For potential values lower than 0.2OV the current becomes negative, the manganese oxide is reduced and the Raman bands decrease. The changes introduced in the Raman spectrum are only observed when the current at the stationary state corresponds to a reduction process (Fig. 5). Finally, Fig. 6 shows the time effect under the reduced state at -0.09V. There are no peaks at this potential (Fig. 6a) and the Raman spectrum does not exhibit new Raman bands at 24 h (Fig. 6b). On returning to the initial potential E = 0.8OV (Fig. 6c), the peaks appear again. Thermal manganese oxide. In order to compare with the Raman spectra of the manganese dioxide electrodeposited on gold, Raman spectra of thermal MnO, (stoichiometric and j? structure) and KS No. 12 were performed. Figure 7 shows Raman spectra for ICS No. 12 (chemical MnO,) and thermal MnO, samples measured under ex situ conditions in air. The spectrum of ICS No. 12 exhibits Raman bands at wavenumbers of 490, 523, 581 and 645 cm- ’ (Fig. 7a). The spectrum of thermal MnO, is shown in Fig. 7b. We observed Raman bands at 530, 581 and 652cm-‘. Figure 8 shows the in situ Raman spectrum of a thermal manganese oxide sample in borate solution pH9.2. The electrode was kept at potential E = 0.40 V for 1 h and then the Raman spectrum was measured. In the oxidized state at 0.4OV the MnO, shows five Raman peaks at 410, 485, 515, 585 and

001 00

AdMn BorclteSdn pH90-I

-0.01

b

Fig. 4. (a) Potentiodynamic polarization curve of the MiOJ& electrode in borate solution at pH 9.2 at sweep rate of 0.0017 V s-l between E, = 0.80 V and E,. c = 0.00 V. (b) Raman intensity of the Raman peak at 58Ocn1-~ as a function of potential.

Ramvl Shift, Awcrd

Fig. 6. Time effect of in situ Raman spectra of manganese dioxide on Au under reduced state. (a) Reduction at E = -0.09 V for 3 h, (b) reduction at E = -0.09 V for 24 h, (c) reoxidation at E = 0.8OV for 1 h.

344

B. A.

DE

LOPEZ

MISHIMAet al.

L. 700

600

Roman

Shift

500

400

nr

/cm-’

660

300

.

.

BINDING

.

8hGY

/1 $Jrjt

OS

Fig. 7. Raman spectra of MnO, under ex situ conditions in air. (a) ICS No. 12, (b) thermal manganese oxide.

Au/thermal

3

MnOpomte

bin

i‘\

4 I-

T

I I 640

/eV

//

535 530 BINDING

ENERGY

535 530 /eV

Fig. 10. XPS spectra of Mn 2p and 0 1s electrons of electrodeposited manganese oxide sample on gold in pH 9.2 borate solution at oxidized state E = 0.8V and reduced state at E = -0.8 V. I

600

750

700

650

Ramon

I

I

600

Shift

550

500

450

400

350

Or/cm-’

Fig. 8. In situ Raman spectra of thermal manganese oxide in pH 9.2 borate solution at oxidation potential E = 0.4V.

635 cm- ’ which are similar to the Raman bands of electrolytic manganese oxide (Fig. la). EMIRS

Figure 9a shows EMIRS spectrum for electrolytic MnO, obtained by calculating R/R when the manganese couple was modulated with respect to the

1 Modulated

I

-0lOV to o/lo

.

L!m4cco3m~’ WAVE NUMBER/ml

Fig. 9. EMIRS spectrum modulated at 11 Hz of the MnOJAu electrode in pH 9.2 borate solution. (a) AR/R modulated between 0.70 V and 0.90 V, (b) AR/R modulated between -O.lOV and -O&IV, (c) AR/R modulated between -O.lOV and 0.4OV.

electrode potential between 0.7OV and 0.90 V at 11 Hz. We can see a broad O-H stretching band. The band has a maximum at around 35OOcm- ’ and a shoulder at 3270cm-‘. A flat, featureless EMIRS spectrum was obtained, as shown in Fig. 9b, when the potential modulation range was restricted to the lower potential between -O.lOV and -0.40 V. For a potential modulation between -0.10 V and 0.40 V, the O-H stretching band appeared again (Fig. SC). Therefore, the structure of the water/hydroxyl ions in the films would play an important role in determining the EMIRS spectra in the O-H stretching region. XPS XPS has also been used to examine electrolytic MnO, on Au at different discharge (reduced) states in the potential region from 0.8OV to -0.80 V[S]. The spectra of Mn 2p and 0 1s photoelectrons were performed. The Mn peaks corresponding to the 2p l/2 and 2p 312 states and 0 1s peak were examined before and after partial discharge in borate solutions. The Mn 2p and 0 1s spectra at 0.8OV and -0.8OV are shown in Fig. 10. The spectra of manganese states did not change significantly with potential; however, the 0 1s peak changed during the cathodic discharge process. The 0 1s spectrum can be decomposed into three components: the Mn=O component, the Mn-OH or Mn-OH, component and the water adsorbed. The Mn=O component decreases with the potential and the Mn-OH or Mn-OH, increases during the cathodic discharge Table 1. Dependence of the Mn-OH/Mn-0 ratio and the oxidation number of Mn on potential: MnO, (OH), (OH,). film calculated from XPS results*

E/V Mn-OH/Mn-0 n (z = 0.43) * From [S].

0.80 0.33 4.00

0.10 0.72 3.45

-0.10 0.83 3.46

-0.80 0.92 3.33

Discharge of MnO, in borate process, as previously reported by us[S]. The film composition MnO, (OH),, (OH,), was estimated from the x and y + z values, assuming the z value and considering the charge balance[l3]. Also, it was possible to calculate the oxidation number n of manganese at different potentials. The molar atomic ratio Mn-OH/Mn-0, x, y + z and n values are shown in Table 1.

DISCUSSION Electrolytic manganese oxide

The in situ Raman spectra of electrolytic MnO,/Au electrode show at a potential of E = 0.8OV five Raman peaks, located at 410, 480, 520,580 and 64Ocm-‘. All the Raman peaks intensity increases with the polarization at E = 0.8OV in particular the main peak at 580cm- ’ (Fig. 11). Therefore, the changes in the height of this peak are correlated with the changes observed in the XPS spectra of the 0 1s state concerning the Mn-OH/Mn=O ratio in the tilm and the oxidation number n calculated for manganese. The sample as prepared has n = 0.389 which indicates that this sample is a mixture of Mn(II1) and Mn(IV)[8]. When the electrode is polarized at 0.8OV, n reaches a value of 4.00, thus indicating that Mn(IV) is mainly present in the oxide. The corresponding Raman spectra of these samples (Fig. ll), respectively show an increase of the peak at 580crn- ’ with polarization. During the discharge process at different potentials, the value of n decreases from 4.00 at 0.8OV to 3.30 at O.lOV and then remains almost constant (Table 1). With regard to the Raman spectra, the height of the peak at 580cm-’ sharply decreases in the same potential region (Fig. 4b). These results are well correlated with current values in the polarization curve (Fig. 5). It is worth noticing that the height of the Raman peak at SSOcm- ’ decreases when a significant amount of Mn(II1) is present in the film. Table 1 also shows the values of the atomic molar ratio Mn-OH/Mn=O where it can be observed that this relation increases during the discharge process because the Mn-0 component decreases (x) and the Mn-OH component increases (y + z).

345

From these results the Raman band at 580cm-’ might be assigned to the Mn-0 stretching. With regard to the Raman bands at 410,480, 520 and 640, at present we cannot assign them to a specific vibration. Gosztola et al. measured SER spectra of MnO, and Mn(OH),[4]. The MnO, SER spectrum exhibited a broad asymmetric Raman band at 585cm-i. During the reduction new Raman bands appeared. They assigned the 415cm-’ band to a Mn-OH vibration in MnOOH and the 470cm- ’ band to a Mn-OH vibration in Mn(OH),. The SER spectrum of Mn(OH), shows a broad band at 47Ocm-’ and bands at 415, 500 and 585 cm-’ during the oxidation. The origin of the 500 cm- ’ band appearing during Mn(OH), oxidation (Gosztola et al. attributed to a form of MnOOH other than the groutite structure. The appearance of the Raman band at 670cm-‘, assigned to Mn-0 vibration, was associated with vibration Mn(IV)-0-Mn(II1) species in route to MnOOH. In the present study we cannot reach the Mn(OH), state because the MnO, electrodeposited on Au is reduced, in borate solutions, to an Mn(III) compound (n = 3.13). On the other hand, all of our experimental conditions are different from those for the SER measurements. The thickness of MnOa film in the present paper is ten times that used in the SER study (77mCcm-’ vs. 8mCcme2) but thin enough for seeing the electrochromic process. We could observe the color change film from brown to yellow during the reduction-oxidation cycle. With polarization at 0.8OV the film color became black and it is possible the spectra suffered the deleterious effects of light absorption[l4]. However, the five Raman bands increase with the polarization time at 0.80 V (Fig. 11). We obtained Raman spectra at different potentials under stationary conditions (4 h). The films prepared by us are hydrous and non-stoichiometric. The XPS spectrum of 0 1s we can decompose into Mn-0, Mn-OH or Mn-OH, and adsorbed water components; it is possible the Raman band we obtained came from these components. Table 2 shows the arbitrary values of the Raman peak heights which were taken from the Raman spectra during the reduction process. We can observe that the bands at 640, 520 and 480cm-’ decrease with the potential in a similar way to the band at 580cm- ’ (Fig. 4b). The band at 410cm- ’ is weak and remains practically constant with potential. It is possible that this band corresponds to the Mn-OH vibration. Gosztola et al.[4] reported that the band at 415cm-’ exhibited a frequency shift to Table 2. Height of Raman bands at different potentials (in arbitrary units) E/volt

ml

SW

400

Fig. 11. Rarnan spectra at E = 0.8OVat different polarixation times: (a) as prepared, (b) oxidation during 4 h.

Raman shift Au/cm-’ 410 480 520 580 640

0.80

0.30

0.15

12

15 13 24 47 12

14 8 16 24 8

:‘: 47 15

B. A. LOPEZ

346

DE

398cm- ’ when H,O was substituted for D,O in the electrolyte. Finally, the reduction product is mainly a compound of Mn(III), according to the n value obtained from XPS analyses (Table 1). The results in Fig. 6 indicate that the spectrum neither exhibits bands, nor new Raman bands despite the fact of having been kept at the reduced state for 24 h (Fig. 6b). So, the reduction product is either inactive when subjected to Raman spectroscopy or an amorphous compound of Mn(II1). The Raman spectrum of amorphous solid materials has been reported to be extremely complex. These spectra show few bands; sometimes none can be detected[15]. Besides, the corresponding EMIRS spectrum between -O.lOV and -0.4OV is not defined. A poor organization of the structure may explain the absence of any strong O-H stretching absorption band. This fact then could support the amorphous nature for the reduction product. Arsov et al. recently obtained amorphous Ti-oxide films. In situ Raman spectroscopy showed the transformation of amorphous Ti-oxide film to the anatase, brookite and rutile mineral forms[16]. Thermal manganese oxide. The in situ Raman spectra shows the five Raman bands, similar to electrolytic manganese oxide; 415, 475, 520, 585 and 64Ocm-’ (Fig. 8). The thermal MnO, oxide is stoichiometric manganese oxide and can be described as having a pyrolusite or ramsdellite structure. The basic units in both structures, ramsdellite and pyrolusite, are (MnO,)-octahedra joined to alternate double chains (1 x 2 tunnels) in the former and to alternating single chains (1 x 1 tunnels) in the latter. On the other hand, the electrolytic MnO, , EMD, is classified as a series of intergrowth structures between ramsdellite and pyrolusite, called the nsutite-family[ 17, 181. X-ray diffraction patterns showed that the thermal manganese oxide prepared by us corresponds to pyrolusite structure. Independently of its actual crystallographic structure (tl, /I) stoichiometric MnO, may be thought of as being constituted of hexagonal close-packed layers of oxygen ions, with half the octahedral holes being occupied by Mn4+, and all tetrahedral holes being empty. For the octahedral model (point group OH) there are six normal vibrations of the following species and activities: 1 A,, (vlXR), 1 E, (v2NR), 2 F,, (v3Xv4XI), 1 F,, (v5XR) and 1 F,, (~6) (inactive)[ 19, 203. The Raman bands obtained under ex situ conditions are broad (Fig. 7 and[7]) but under in situ con-

Table 3. Raman

MISHIMA et al.

ditions they are more defined (Figs 1 and 8). The Raman band at 580 cm- ‘-585 cm- ’ is the strongest. In view of the rule that totally symmetric vibrations give rise to the strongest Raman lines, it appears certain that this strong Raman line at 580cm- ’ would correspond to the symmetric fundamental vl (A,,), stretching mode Mn-0. In Table 3 are compiled the Raman frequencies measured, both ex situ and in-situ, for the different samples. The spectra of all different samples: thermal MnO, , electrodeposited manganese oxide and International Common Sample (ICS) exhibit three Raman bands at wavenumbers of 520, 580 and 650cm-i. From the values of the frequencies for the Raman bands obtained in ex situ and in situ conditions we learn that these frequencies belong to the vibration of the octahedra MnO, in the different samples of the manganese dioxide studied by us. Referring again to the changes in both the potential of the Raman band at 580cm- ’ (Fig. 4b) and the values of the oxidation number n, the height of the Raman peak at 580cm- ’ decreases when a significant amount of Mn(II1) is present in the film (n = 3.45 at O.lOV). These experimental results are consistent with those of Maskell et aI.[21] and Ruestchi[22] who proposed that during the electrochemical reduction of MnO,, in the 0.80-O.OV potential region, the inserted electrons associated with Mn4+ to form Mn3+ are delocalized over adjacent Mn4+. The Mn3+ ions are not being formed until the second half of discharge. The valency state must be pictured as fluctuating between that of Mn4+ and Mn3+ and may be best described by a statistical average valency, depending on the degree of reduction. With regard to the results obtained with EMIRS, the O-H stretching band observed is likely due to the O-H groups produced by the proton injection.

CONCLUSIONS Our study of the behavior of manganese dioxide in borate solution by in situ Raman scattering spectroscopy, EMIRS and XPS may be summarized as follows : The Raman and EMIRS responses show that reversible composition on changes of the electrolytic MnO, surface take place during the reductionoxidation cycle.

bands of manganese

Sample ICS No. 1 (electrolytic) ICS No. 22 (chemical) ICS No. 12 (chemical) Au/MnO, (electrolytic) Au/MnO, (thermal decomposition) Au/MnO, Au/MnO,

(electrolytic)* (thermal decomposition)*

* In situ in borate,

pH 9.2.

dioxide

Raman 650 660 645 650 672 652 640 635

580 580 581 580 580 581 580 (s) 585 (s)

shift (Av/cm-i) 520 530 523 520 526 530 520 515

490 497 480 485

410 410

Discharge of MnO, in borate Thermal manganese oxide and electrolytic manganese oxide shown the same five Raman bands under in situ conditions. The Raman bands obtained from the spectra of the non-stoichiometric and stoichiometric samples suggest correspondence to the active vibrations of the octahedral MnO, present in the different structures. The peak at %Ocm- ’ can be assigned to Mn-0 stretching because of the good correlation found between the XPS analyses and Raman spectroscopy results. The reduction product of the electrolytic MnO, in borate solution is an amorphous Mn(III) compound, as evidenced by the techniques used.

REFERENCES 1. F. Hahn, B. Beden, M. J. Croissant and C. Lamy, Electrochim. Acta 31, 335 (1986). 2. F. Hahn, D. Floner, B. Beden and C. Lamy, Electrachim. Acta 32. 1631 (1987). 3. C. A. Melen&es add S.‘Xu, J. electrochem. Sot. 131, 2239 (1984).

4. D. Gosztola and M. J. Weaver, J. electroanal. Chem. 271, 141 (1989). 5. T. Ohtsuka, K. Kunimatsu, N. Goto and N. Sato, Ber. Bunsenges, phys. Chem. 91,313 (1987). 6. T. Ohzuku and T. Hirai, in Manganese Dioxide Electrode Theory and Practice for Electrochemical Applications (Edited by B. Schumm, Jr. R. L. Middaugh, M. P.

347

Grotheer and J. C. Hunter), p. 141. The Electrochemical Society, Pennington, New Jersey (1984). 7. B. A. Lopez de Mishima, T. Ohtsuka and N. Sato, J. electroanal. Chem. 243,219 (1988). 8. B. A. Lopez de Mishima, T. Ohsuka, K. Konno and N. Sato, EIectrochim. Acta 36,148s (1991). 9. T. Ohtsuka, J. Guo and N. Sato, J. electrochem. Sot. 133,2473 (1986). 10. A. A. Spricis, G. J. Slaidins, J. J. Ah&z, and J. R. Dzelme, Electrochimiya 1% 339 (1982). 11. L. D. Burke and M. J. Ahern, J. electrochem. Sot. 132, 2662 (1985). 12. K. Kunimatsu, J. phys. Chem. 8g, 2195 (1984). 13. K. Konno, S. Kobayashi, H. Takahashi and M. Nagayama, Corros. Sci. 22,913 (1982). 14. J. Desilvestro, D. A. Corrigan and M. J. Weaver, J. electrochem. Sot. 135,885 (1988). 15. J. G. Contreras, Espectroscopia Raman y Estructura Molecular Unesco, Anibal Pinto S. A. Conception, Chile (1987). 16. Lj. D.’ Ars&, C. Kormann, and W. Plieth, J. electrothem. Sot. 13% 2964 (1991). 17. R. G. Bums, in Battery Material Symposium, Vol. 1 (Edited by A. Kozawa and N. Nagayama), p. 197. 18. S. Turner and P. R. Buseck, Nature 304,143 (1983). 19. S. D. Ross, Inorganic Infrared and Raman Spectra. McGraw-Hill, New York (1972). 20. K. Nakamoto, Infrared and Ramon Spectra of Inorganic and Coordination Compounds, 3rd ed. Wiley, New York, 1978. 21. W. C. Maskell, J. A. E. Shaw and F. L. Tye, J. appl. Electrochem. 12, lOl(i982). 22. P. Ruetschi, J. electrochem. Sot. 135,2663 (1988).