Langmuir 1997, 13, 6845-6849
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Electrochemical Intercalation of Alkali-Metal Ions into Birnessite-Type Manganese Oxide in Aqueous Solution Hirofumi Kanoh,* Weiping Tang, Yoji Makita, and Kenta Ooi Shikoku National Industrial Research Institute, 2217-14 Hayashi-cho, Takamatsu, 761-03 Japan Received July 10, 1997. In Final Form: September 22, 1997X A thin layer electrode of birnessite-type manganese oxide was prepared by brushing a mixed solution of KOCOCH3 and Mn(OCOCH3)2 on a platinum substrate, followed by heating at 1073 K. The chemical composition of the electrode was KxMnOy (x ) 0.33 and y ∼ 2) with an interlayer spacing of c0 ) 0.697 nm. The positive-potential going sweep on the electrode in an aqueous phase caused the deintercalation of K+ with an increase in c0. The quasi-reversible intercalation of K+ occurred with a subsequent negativepotential going sweep in a 0.2 mol/dm3 KCl solution. The electrochemical measurements suggested that K+ is not electrochemically active in the deintercalation/intercalation reaction but H+ is. The reaction proceeds based on a mechanism consisting of an electrochemical reaction (the redox reaction between Mn3+ and Mn4+) and an ion-exchange reaction between K+ and H+. The intercalation experiments in various alkali-metal chloride solutions showed the intercalation capacity to be in the order of Na ∼ K > Li > Rb > Cs.
Introduction Electrochemical insertion/extraction reactions of various kinds of manganese oxides with Li+ in non-aqueous systems have been extensively studied with a view to the development of alternative materials for secondary batteries.1-7 The reaction in an aqueous phase has also been studied to develop recovery and separation methods for alkali-metal ions8-13 as well as materials for secondary batteries.14,15 Manganese oxide forms various structures including spinel, layered (birnessite, buserite), one-dimensional tunnel (hollandite, todorokite), and so on and also possesses different valences of manganese dependent on the mole ratio of manganese to oxygen, defects in the crystals, or the content of other metal ions such as alkali metals.16 We have studied the selective sorption of alkali-metal ions into manganese oxides and shown their ion selectivity and ion-sieve effects: spinel for Li+,8, 10 birnessite for K+, and Rb+,12 hollandite for Rb+,11 and todorokite for an ion * To whom correspondence should be addressed. E-mail: kano@ sniri.go.jp. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Ohzuku, T.; Kato, J.; Sawai, K.; Hirai, T. J. Electrochem. Soc. 1991, 138, 2556. (2) Thackeray, M. M.; de Kock, M.; Rossouw, M. H.; Liles, D.; Bittihn, R.; Hoge, D. J. Electrochem. Soc. 1992, 139, 363. (3) Guyomard, D; Tarascon, J. M. J. Electrochem. Soc. 1992, 139, 937. (4) Shokoohi, F. K.; Tarascon, J. M.; Wilkens, B. J.; Guyomard, D.; Chang, C. C. J. Electrochem. Soc. 1992, 139, 1845. (5) Pistoia, G.; Wang, G.; Wang, C. Solid State Ionics 1992, 58, 285. (6) Momchilov, A.; Manev, V.; Nassalevska, A.; Kozawa, A. J. Power Sources 1993, 41, 305. (7) Thackeray, M. M. Prog. Batteries Battery Mater. 1995, 14, 1. (8) Ooi, K.; Miyai, Y.; Katoh, S.; Maeda, H.; Abe, M. Langmuir 1989, 5, 150. (9) Tsuji, M.; Komarneni, Y.; Tamaura, Y.; Abe, M. Mater. Res. Bull. 1991, 138, 2556. (10) Feng, Q.; Miyai, Y.; Kanoh, H.; Ooi, K. Langmuir 1992, 8, 1861. (11) Feng, Q.; Miyai, Y.; Kanoh, H.; Ooi, K. Chem. Mater. 1995, 7, 148. (12) Feng, Q.; Miyai, Y.; Kanoh, H.; Ooi, K. Chem. Mater. 1995, 7, 1226. (13) Feng, Q.; Miyai, Y.; Kanoh, H.; Ooi, K. Chem. Mater. 1995, 7, 1722. (14) Li, W.; McKinnon, W. R.; Dahn, J. R. J. Electrochem. Soc. 1994, 141, 2310. (15) Li, W.; Dahn, J. R. J. Electrochem. Soc. 1995, 142, 1742. (16) Burns, R. G.; Burns, V. M. In Manganese Dioxide Symposium; Tokyo, I. C. MnO2 Sample Office: Cleveland, OH, 1980; Vol. 2, p 97.
S0743-7463(97)00767-1 CCC: $14.00
with a radius of 0.27 nm.13 We have also studied electrochemically the insertion of Li+ into a spinel-type manganese oxide electrode (Pt/λ-MnO2) in an aqueous phase17-20 and shown the possibility of recovering lithium from a dilute solution such as geothermal water.21 Birnessite-type manganese oxides have a layered structure which contains two-dimensional sheets of edgeshared MnO6 octahedra, with water molecules and metal ions between the sheets. Studies of birnessite-type manganese oxides have indicated that their properties are dependent on the preparation conditions due to the facile change in the manganese oxidation state.22,23 Several kinds of birnessite-type manganese oxide are reportedly used as the starting materials and transformed into various tunnel structures.24 This reflects the variety of chemistry in birnessite-type manganese oxide. In the present study, we report the topotactic electrointercalation of alkali-metal ions into a birnessite-type manganese oxide electrode, prepared by an electrochemical deintercalation of K+ from a potassium birnessitetype manganese oxide electrode in aqueous solution. Experimental Section A thin layer electrode of birnessite-type manganese oxide was prepared as follows. A small amount of a mixed solution of 2 mol/dm3 KOCOCH3 and 2 mol/dm3 Mn(OCOCH3)2 (K/Mn mole ratio ) 0.5) was brushed on a platinum substrate (10 × 10 × 0.3 mm3), followed by heat treatment at 1073 K in air for 2-4 min. After the sample was allowed to cool to room temperature, the brushing-heating treatment was repeated 10 times to form a uniform thin film of manganese oxide. The structure of the oxide film was characterized with a Rigaku RINT1200 X-ray diffractometer and its chemical composition analyzed with a Shimadzu (17) Kanoh, H.; Ooi, K.; Miyai, Y.; Katoh, S. Langmuir 1991, 7, 1841. (18) Kanoh, H.; Feng, Q.; Miyai, Y.; Ooi, K. J. Electrochem. Soc. 1993, 140, 3162. (19) Kanoh, H.; Feng, Q.; Miyai, Y.; Ooi, K. J. Electrochem. Soc. 1995, 142, 702. (20) Kanoh, H.; Feng, Q.; Miyai, Y.; Ooi, K. J. Electrochem. Soc. 1996, 143, 2610. (21) Kanoh, H.; Ooi, K.; Miyai, Y.; Katoh, S. Sep. Sci. Technol. 1993, 28, 643. (22) Strobel, P.; Mouget, C. Mater. Res. Bull. 1993, 28, 93. (23) Strobel, P.; Charenton, J. C.; Lenglet, M. Rev. Chim. Miner. 1987, 24, 199. (24) Feng, Q.; Yamasaki, K.; Yanagisawa, K; Ooi, K. J. Mater. Sci. Lett. 1996, 15, 963.
© 1997 American Chemical Society
6846 Langmuir, Vol. 13, No. 25, 1997
Figure 1. XRD patterns of the Pt/orgBirMnO (a) and Pt/ deBirMnO (b) electrodes. Circles denote the peaks due to the Pt substrate. AA-670 atomic absorption spectrometer after the oxide layer was dissolved with a mixed solution of HCl and H2O2. Electrochemical measurements were performed in 0.05 mol/ dm3 borate buffer solutions (pH 7.5) containing alkali-metal chlorides on an automatic polarization system HZ-1A (Hokuto Denko Co., Ltd.) using a calomel electrode and a Pt plate as the reference and counter electrodes, respectively.
Kanoh et al.
Figure 2. Voltammograms: (a) positive going sweep from 200 to 1000 mV on the Pt/orgBirMnO electrode; (b) negative going sweep from 1000 to 0 mV on the Pt/deBirMnO electrode. Electrolyte: 0.2 mol/dm3 KCl BBS. Scan rate: 1mV/s.
Results and Discussion Preparation and Characterization of Pt/orgBirMnO and Pt/deBirMnO Electrodes. A thin layer electrode of birnessite-type manganese oxide (Pt/orgBirMnO) was prepared by the brushing-heating treatment. Its XRD pattern shows two main peaks of d001 ) 0.697 nm and d002 ) 0.349 nm with minor peaks of d100 ) 0.252 nm, d101 ) 0.240 nm and d102 ) 0.210 nm, as shown in Figure 1a. This pattern is characteristic of a layered structure with an interlayer spacing of c0 ) 0.697 nm, indicating a structure similar to those of Na4Mn14O27‚9H2O,25,26 Na0.45MnO2.14‚0.76H2O,27 KxMnO2(y‚zH2O,28 and Na0.40MnO2.15‚0.6H2O.12 The K/Mn mole ratio of birnessite-type manganese oxide obtained in this work was found to be 0.33 ((0.04). The birnessite-type manganese oxide is not likely to include any water molecules because the electrode was heated at the high temperature of 1073 K in the preparation process and c0 is narrower than those described in the reports cited above. The electrode potential was swept from 200 to 1000 mV (vs SCE) at the scan rate of 1 mV/s using the Pt/orgBirMnO electrode as a working electrode in a 0.2 mol/dm3 KCl BBS (Figure 2a). The K/Mn mole ratio decreased to 0.18 ((0.04) after the positive going sweep. The voltammogram has two components of current peaks at 450 and 650 mV respectively, suggesting that the K+ deintercalation proceeds in two steps. This result agrees with the two sites of sodium birnessite-type manganese oxide for Li+.12 The K+-deintercalated electrode (Pt/deBirMnO) showed an XRD pattern similar to that of the Pt/orgBirMnO electrode except for a slight increase in c0 to 0.717 nm (Figure 1b). This result shows a topotactic electrochemical deintercalation of K+ from the Pt/orgBirMnO electrode. The increase in c0 is thought to be caused by an intercalation of water molecules because a similar result with proven intercalation of water molecules was obtained in (25) Golden, D. C.; Dixon, J. B.; Chen, C. C. Clays Clay Miner. 1986, 34, 511. (26) Golden, D. C.; Chen, C. C.; Dixon, J. B. Clays Clay Miner. 1987, 35, 271. (27) Le Goff, P.; Baffier, N.; Bach, S.; Pereira-Ramos, J. P.; Messina, R. Solid State Ionics 1993, 61, 309. (28) Chin, S.; Landrigan, J. A.; Jorgensen, M. L.; Duan, N.; Suib, S. L. Chem. Mater. 1995, 7, 1604.
Figure 3. Time courses of the potential of the two electrodes, Pt/orgBirMnO (O) and Pt/deBirMnO (b). Electrolyte: 0.2 mol/ dm3 KCl BBS.
the ion-exchange reaction of H+ with a powder sample of sodium birnessite-type manganese oxide.12 The subsequent negative going sweep from 1000 to 0 mV after the positive going sweep brought about the K+ intercalation with a broad current peak around 305 mV (Figure 2b). The K/Mn mole ratio of the electrode was found to be 0.27 ((0.05), indicating the electrochemical intercalation of K+ into the Pt/deBirMnO electrode. The XRD pattern of the K+-intercalated electrode (Pt/inBirMnO) was very similar to that of the Pt/orgBirMnO electrode with c0 ) 0.705 nm. The K+ intercalation is accompanied by the shrinkage of the layered structure, probably because of the partial deintercalation of water molecules from the interlayer space. These results show that a quasi-reversible electrochemical intercalation of K+ into the birnessite-type manganese oxide occurs topotactically. The detailed mechanism will be discussed below. Equilibrium Potentials. Time courses of the potential of the two electrodes Pt/orgBirMnO and Pt/deBirMnO are shown in Figure 3. The potential of the Pt/orgBirMnO electrode became constant relatively quickly (t ) 20 h), whereas the potential of the Pt/deBirMnO electrode showed a slow change, and finally stabilized at t ) 150 h. The c0 value of the Pt/deBirMnO electrode after equilibration in the aqueous solution showed little change, but the c0 and K/Mn mole ratio of the Pt/orgBirMnO electrode slowly varied after t > 20 h and finally reached 0.708 nm and 0.27 ((0.05) respectively. This seems due to a slow reaction of the intercalation into the interlayer space and the incorporation of H2O to the site. Thus, the
Electrochemical Intercalation of Alkali-Metal Ions
Figure 4. Equilibrium potential (E) vs log CKCl and pH: E vs log CKCl (O) and E vs pH (0) of the Pt/aqBirMnO electrode; E vs log CKCl (b) of the Pt/deBirMnO electrode. The pH dependence was measured in 0.03 mol/dm3 KCl BBS.
electrode after equilibration differs from the Pt/orgBirMnO and is abbreviated as Pt/aqBirMnO in the following sections. Equilibrium potentials of the two electrodes, Pt/deBirMnO and Pt/aqBirMnO, were measured under constant pH after equilibration. The results are plotted vs a logarithm of the KCl concentration in Figure 4. The potentials of the Pt/deBirMnO and Pt/aqBirMnO electrodes were found to be constant at 469 and 334 mV respectively. The potential of the Pt/deBirMnO electrode is higher than that of the Pt/aqBirMnO electrode, which shows that the oxidation state of the former oxide is higher than that of the latter. In general, an electrode potential should show a near-Nernstian response to a concentration of an electrochemically active species just as the Pt/λMnO2 electrode does to Li+, where the reaction xLi+ + λ-MnO2 + xe- f LixMnO2 occurs electrochemically.18 That is, a slope of about 59 mV/decade must be observed in the plot of an equilibrium potential vs a logarithm of a concentration of an electrochemically active species. These birnessite-type manganese oxide electrodes indicate constant potentials irrespective of the K+ concentration. This suggests that K+ is not electrochemically active in the deintercalation reaction. In contrast, pH dependence of the equilibrium potential of the Pt/aqBirMnO electrode shows a near-Nernstian response with a slope of -58.3 mV/pH (Figure 4). This result shows that H+ is electrochemically active according to the reaction xH+ + BirMnO + xe- f HxBirMnO. The standard potential of the Li+-intercalation/deintercalation reaction with various kinds of birnessite manganese oxide was reported to be 3.4 V vs Li/Li+ at its highest.22 The potential value is reduced to 0.1 V vs SCE using the standard potential of -3.05 V vs SHE for the half-cell reaction of the Li/Li+ system. If the standard potential value is applied to the intercalation of K+ into the Pt/deBirMnO electrode, K+ is not involved in the potential determining reaction, because the standard potential for the H+ intercalation is much higher in the system. This must be the reason why K+ is not electrochemically active. Cyclic Voltammetry. The cyclic voltammogram of the Pt/deBirMnO electrode shows a peak at 290 mV on the negative going sweep but two peaks on the positive going sweep as shown in Figure 5. The voltammogram indicated only a slight change after eight cycles. This shows that the electrochemical reaction occurs steadily
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Figure 5. Cyclic voltammograms of the Pt/deBirMnO electrode in a 0.2 mol/dm3 KCl BBS at a scan rate of 1mV/s. Key: solid line; first scan; broken line, eighth scan.
Figure 6. Voltammograms in the negative going sweep from 1000 to 0 mV on the Pt/deBirMnO electrode in a 0.2 mol/dm3 MCl BBS. M: Li (s); Na (- -); K (--); Rb (- ‚ -); Cs (-‚‚‚-).
in the system. The votammogram on the positive going sweep is similar to that in the first deintercalation of K+ shown in Figure 2a, although the two peaks shifted to lower potentials (from 450 to 370 mV and from 650 to 630 mV), whereas the voltammogram on the negative going sweep showed only one peak. These results suggest that the mechanism or kinetics differ between the deintercalation and intercalation processes. Since it was shown that the chemical compositions of the electrode in the two extreme stages at 0 and 1000 mV in the voltammetry were close to those of the Pt/inBirMnO and Pt/deBirMnO electrodes, respectively, we conclude that aqBirMnO is the same as inBirMnO. Alkali-Metal Intercalation. The negative going sweep was carried out in various alkali-metal chloride solutions using the Pt/deBirMnO electrodes. The voltammograms, XRD, and the alkali-metal uptake are shown in Figures 6 and 7 and Table 1, respectively. A peak appears at the potential range of 250-300 mV in the voltammograms irrespective of metal species, whereas the peak area depends on the species in the order of Na > K > Rb > Cs > Li (Figure 6). The chemical composition analysis of each electrode after the negative going sweep showed the following sequence in the amount of alkali metals intercalated into the electrode: Na ∼ K > Li > Rb > Cs (Table 1). Thus, no correlation was obtained between the metal amounts intercalated and the voltammograms. These results, as discussed below, suggest that the deintercalation/intercalation of alkali-metal ions is not related directly to the electrochemical reactions. It is notable that the c0 of the Rb+- and Cs+-intercalated
6848 Langmuir, Vol. 13, No. 25, 1997
Kanoh et al. Scheme 1. Electrochemical Intercalation/ Deintercalation Reactions of K+ with Birnessite-Type Manganese Oxide
Figure 7. XRD patterns after the electrochemical intercalation of alkali-metal ions into the Pt/deBirMnO electrode. The potential was swept from 1000 to 0 mV at a scan rate of 1 mV/s in a 0.2 mol/dm3 MCl BBS. M: (a) Li; (b) Na; (c) K; (d) Rb; (e) Cs. Table 1. Values of Each Parameter after the Cathodic Sweep in Various Alkali-Metal Chloride Solutions electrolytea
c0/nm
M/Mnb
K/Mnc
LiCl NaCl KCl RbCl CsCl
0.713 0.708 0.711 0.719 0.728
0.190 0.244 0.237 0.163 0.095
0.074 0.052 0.237 0.023 0.020
a Concentration of each electrolyte was 0.2 mol/dm3. b Mole ratio of alkali metals to manganese. c Mole ratio of potassium to manganese.
electrodes are greater than that of the Pt/deBirMnO electrode (0.717 nm) while their K/Mn mole ratios are much smaller. These results show that the intercalation of Rb+ or Cs+, the ion radii of which are greater than that of K+, facilitates the K+ deintercalation and the increase in c0, probably due to H2O intercalation. Mechanism of the Deintercalation/Intercalation Reaction. Summarizing the results obtained above, (1) orgBirMnO is not stable in aqueous solution, (2) the positive going sweep causes K+ deintercalation with H2O intercalation, (3) the negative going sweep causes K+ intercalation, and (4) K+ is not electrochemically active but H+ is active, thus realizing the deinteralation/ intercalation reaction of K+ from/into birnessite-type manganese oxide. Taking into account all the results, we propose the following mechanism for the changes from orgBirMnO to aqBirMnO and deBirMnO, for the K+ deintercalation from aqBirMnO and inBirMnO, and for the K+ intercalation into deBirMnO (Scheme 1). An H2O intercalation, indifferent to the electrochemical behavior, occurs in process i in Scheme 1 to stabilize the oxide phase when in contact with the aqueous phase. The species aqBirMnO is probably a stable one in the time scale for which we observed the process. Process ii is essentially the same as processes iii and v. In these processes, K+ deintercalation seems to occur electrochemically accompanied by H2O intercalation. However, since the K+ deintercalation is not an electrochemical reaction according to result (4), the process should be associated with the mechanism, “Deintercalation”, described below. The H2O intercalation seems to be necessary both to compensate the destabilization caused by the K+ deintercalation and to maintain the layered structure of the crystal. The R value should be nearly equal to zero in process i because of the only slight water content in the Pt/orgBirMnO electrode. Accordingly, the K+ intercala-
tion should be coupled with the H2O deintercalation according to the mechanism “Intercalation”. Processes iv and v are described by mechanisms “Intercalation” and “Deintercalation,” respectively, and exist in a state of equilibrium. The species aqBirMnO is quite similar to inBirMnO. Such intercalation/deintercalation would occur if a kind of activation energy were sufficiently supplied to the system. However, energy such as heat might give rise to another behavior (collapse or decomposition of the solid phase). The electrochemical activation causes a local pH change in the subnanospace between the layers in the solid phase, followed by the ion-exchange reaction between H+ in the local space and metal ions in the aqueous phase. In the process, water molecules play an important role in the space.
“Deintercalation” KMn3+Mn4+Oz‚RH2O + H2O(aq) f KMn4+2Oz(OH)‚RH2O + H+ + e- (positive going) KMn4+2Oz(OH)‚RH2O + H+ f HMn4+2Oz(OH)‚RH2O + K+(aq) (ion exchange) net: KMn3+Mn4+Oz‚RH2O + H2O(aq) f HMn4+2Oz(OH)‚RH2O + K+(aq) + e“Intercalation” HMn4+2Oz(OH)‚RH2O + e- f HMn3+Mn4+Oz‚RH2O + OH- (negative going) HMn3+Mn4+Oz‚RH2O + OH- + K+(aq) f KMn3+Mn4+Oz‚RH2O + H2Oaq (ion exchange) net: HMn4+2Oz(OH)‚RH2O + K+(aq) + e- f KMn3+Mn4+Oz‚RH2O + H2O(aq) In the above reactions, since we are focusing on the sites for K+, the stoichiometry of each reaction does not correspond to that obtained from the experimental data, because of simplification. The notation (aq) represents a species in the aqueous phase. The reactions are illustrated in Figure 8. Effects of Ion Species. The disagreement between the sequences of the current peak area and the intercalated metal amounts could not be explained if the alkali-metal ions were electrochemically intercalated, because the two
Electrochemical Intercalation of Alkali-Metal Ions
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Figure 8. Schematic diagrams of electrochemical intercalation/deintercalation reactions of K+ with birnessite-type manganese oxide.
sequences would have to agree with each other. In Li+ intercalation, the current peak area is smaller than that in the Rb+ intercalation. Nevertheless the intercalated amount of Li+ is greater than that of Rb+. Since the ion radius of Li+ is small and its intercalation is easy, the ion-exchange reaction between K+ and Li+ may proceed even if the concentration gradient is the only driving force. In fact, the K/Mn ratio decreases to 0.074 (Table 1), smaller than that of the Pt/deBirMnO electrode, after the Li+ intercalation, whereas the total amount of K plus Li is almost equal to the amount of K in the Pt/deBirMnO electrode. Since the c0 after Li+ intercalation is greater than those of the Na+- and K+-intercalated electrodes, it is suggested that smaller amounts of H2O are deintercalated in the Li+ intercalation than in the Na+ or K+ intercalation. In Rb+ intercalation, the appreciable peak current observed in Figure 6 guarantees the progress of the cathodic reaction described in the mechanism “Intercalation”. However, the total amount of Rb plus K is almost the same as the amount of K in the Pt/deBirMnO electrode. This indicates that not all of the OH- formed in the potential-negative going process is consumed in the subsequent ion-exchange reaction in the mechanism, “Intercalation”. Consequently, a part of the OH- formed
in the reaction should be deintercalated from the solid phase to the aqueous phase. In fact, the pH increased after the Rb+-intercalation in a nonbuffered system, different from the results for Li+, Na+, or K+. This result supports the validity of our model. Acknowledgment. This study was supported by a Grant in Aid for Scientific Research from the Special Coordination Fund (Frontier Ceramics Project) of the Science and Technology Agency of Japan. Abbreviations SCE, saturated calomel electrode SHE, standard hydrogen electrode BBS, 0.05 mol/dm3 borate buffer solution (pH 7.5) Pt/orgBirMnO, electrode originally prepared by the brushing-heating treatment Pt/aqBirMnO, electrode after equilibration of the Pt/ orgBirMnO electrode in a KCl BBS Pt/deBirMnO, partly K+-deintercalated electrode from the Pt/orgBirMnO or Pt/aqBirMnO electrodes by positivepotential going (positive going) sweep Pt/inBirMnO, K+-intercalated electrode into the Pt/ deBirMnO by negative-potential going (negative going) sweep LA970767D
