Investigation on the Oxidation Mechanism of Cobalt Hydroxide to

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Investigation on the Oxidation Mechanism of Cobalt Hydroxide to Cobalt Oxyhydroxide Thimmasandra Narayan Ramesh† Department of Chemistry, Central College, Bangalore UniVersity, Bangalore 560 001, India

Cobalt hydroxide and cobalt oxyhydroxide derive their crystal structure from brucite mineral. Cobalt hydroxide crystallizes in hexagonal 1H polytype while cobalt oxyhydroxide in rhombohedral symmetry. In spite of the isostructural relationship between cobalt hydroxide and oxyhydroxide, we observe different symmetries. Hexagonal and rhombohedral polytypes are related to each other by a simple translation vector. Translation of AC layers by (1/3, 2/3, 1) and (2/3, 1/3, 1) with respect to each other generates the 3R polytype. Careful evaluation of the X-ray powder diffraction (PXRD) patterns of cobalt oxyhydroxide reveals the exact stacking sequence to be AC CB BA. The structural transformation of cobalt hydroxide to cobalt oxyhydroxide is reported. Introduction Layered transition metal hydroxides and oxides have drawn considerable attention of materials scientists due to their applications in batteries, catalysts, etc. Metal oxide-hydroxides of Ni, Co, and Mn plays a critical role in the charge-discharge process of nickel hydroxide, cobalt hydroxide, and manganese dioxide electrodes, respectively.1-3 A common reaction scheme has been proposed for the charge-discharge process among the hydroxide/oxide electrodes: a

MII(OH)2 divalent hydroxide

T c

b

HMIIIO2 trivalent bronze

T d

MIVO2 quadrivalent dioxide

where M ) Ni, Co, and Mn. When the M2+ is partially or completely oxidized to the (III)/(IV) state, an equivalent number of protons are eliminated from M(OH)2.4 To restore charge neutrality, the MO(OH) and MO2 phases are formed. The oxidation-reduction reaction involves diffusion of protons during the charge-discharge process. Nickel hydroxide being a wide band gap semiconductor inhibits the complete discharge of the active material thereby affecting the electrochemical performance of the electrode.5 To overcome this, cobalt compounds are used as additives as well as coprecipitated with nickel hydroxide during the product workup to improve the electrochemical properties of the nickel hydroxide electrode.6-8 Intrinsically, these cobalt oxides act as electronic conducting phases and during the charging process they get converted to oxyhydroxide forming an effective conducting medium within the particles.9 Addition of cobalt is known to enhance the active material utilization and oxygen evolution potential and decrease the cell volume changes in the nickel hydroxide electrode.10,11 Conventionally, the electrochemical charge-discharge process of manganese dioxide and nickel hydroxide is represented by steps b/c and steps a/d. Recently there was a report on the study of the structural transformation mechanism in the nickel oxyhydroxide electrode.12 It would be of contemporary interest to investigate the phase transformation mechanism during the oxidation of cobalt hydroxide prepared by chemical methods. It will be interesting to investigate whether the phase transfor† To whom correspondence should be addressed. E-mail: [email protected].

mation mechanism during oxidation is similar to that of nickel hydroxide reported.12 A thorough structural characterization of β-CoOOH is not reported in spite of many reports on the synthesis of cobalt oxyhydroxide.13-15 The oxide-hydroxides can also be formulated as bronzes of the type HxMO2 (x ) 1) and represent the end member of the series, which are welldefined hydroxides of the brucite structure. The crystal structure of brucite mineral is comprised of hexagonally close packed hydroxyl ions with an alternate layer of octahedral interstitial sites occupied by divalent metal ions. The composition of brucite is with [M(OH)6] (M ) Co, Ni, Mn) layers separated by an interlayer distance of 4.6 Å. The OH bond is parallel to the stacking direction in the structure. The layer is saturated as all the oxygen atoms are bound by protons. The stacking sequence of metal hydroxide, i.e., AbC AbC AbC, where A and C represent the anions and b denotes the cation present in the octahedral interstitial.16 Oxidation appears to occur by proton extraction, and when this happens vacancies are created in the proton sites, as some oxygens are no longer bound to a proton. The protons either become delocalized or get positioned at sites which are less well-defined than an earlier phase.17 Chemical oxidation of cobalt hydroxide results in the formation of the β-CoOOH phase. On the basis of PXRD simulation studies, we show that the polytype transformation takes place via a different stacking sequence compared to the nickel hydroxide electrode. Experimental Section Synthesis. Cobalt hydroxide was obtained from Aldrich Chemical Co. and used as such. Cobalt oxyhydroxides were prepared as follows. (i) Cobalt hydroxide powder was heated at 100 °C for 3 days in a hot air oven. (ii) Cobalt hydroxide powder was added to a solution of sodium hydroxide-hydrogen peroxide mixture and aged for 18 h at 80 °C. (iii) Cobalt nitrate solution (1 M, 50 mL) was added dropwise to sodium hydroxide solution (2 M, 200 mL) under constant stirring. After complete addition of metal nitrate solution, calculated quantities of hydrogen peroxide were added to oxidize the product. After the reaction ceased, the product was divided into two parts. Part I was aged in mother liquor at 80 °C for 18 h, and the other portion was hydrothermally treated in mother liquor at 160 °C for 5 days. The products were filtered, washed with water, rinsed with acetone, and dried at room temperature.

10.1021/ie901714v  2010 American Chemical Society Published on Web 01/12/2010

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Figure 1. Observed cobalt hydroxide (solid lines) with DIFFaX simulated PXRD patterns (O).

Characterization. All the samples were characterized by PXRD pattern using a Bruker D8 diffractometer (CuKR source λ ) 1.5418 Å). Data was collected at a scan rate of 2°θ min-1 with 2θ steps of 0.05°. Thermogravimetric experiments were performed with a Mettler Toledo model 851e TG/SDTA system (heating rate of 10 °C min-1 under air up to 750 °C. Thermogravimetric studies of these samples show one step weight loss with the formation of Co3O4. DIFFaX simulations studies were carried out using DIFFaX code. The details about the PXRD simulations have been explicitly defined in the literature.18

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Figure 2. DIFFaX-simulated powder XRD patterns of pure 1H, 2H1, 2H2, 2H3, 3R1, and 3R2 polytypes of β-cobalt hydroxide.

Results and Discussion Figure 1 shows the PXRD pattern of cobalt hydroxide sample obtained from Aldrich Chemical Co. The peak positions in the PXRD pattern matches with the reported powder diffraction database (JCPDS card no. 002-0925) having an interlayer spacing of 4.6 Å. In the class of layered hydroxide phases derived from brucite mineral, the exact crystal structure is dictated by the stacking sequence of the hydroxyl ions. Polytypism is a 1-D polymorphism in which structural changes manifest exclusively along the stacking direction.19 The study of different polytypes is a subject of contemporary interest in materials chemistry. In such an instance, it is illuminating to describe the overall structure of the solid in terms of the packing of anions. With the use of symbols A, B, and C to represent hydroxyl ion positions and symbols a, b, and c to represent cation positions, the metal hydroxide slab, [M(OH)2] (M ) Ni, Co, Mn) can be represented by the symbol AbC or more simply as AC. The brucite-like structure would then comprise a hexagonal stacking of these slabs as AC AC AC-. We use the nomenclature of Bookin and Drits to characterize this stacking sequence as 1H. The “1” denotes the single layered periodicity and “H/R” stands for the hexagonal/rhombohedral stacking.20 Other stacking sequences are also possible. Three different stacking sequences are envisaged for a structure with a two layer periodicity, AC CA AC CA--2H1, AC AB AC AB-2H2, and AC BA AC BA AC---2H3. Structures with 3 layer periodicity having rhombohedral symmetry can be generated by 2 sequences, AC CB BA AC 3R1 and AC BA CB AC---3R2. DIFFaX code enables us to simulate the PXRD patterns and provides information on the relative peak positions and intensities of the (h0l)/(0kl) reflections. This acts as a fingerprint to distinguish one polytype from other. Figure 2 shows the simulated PXRD patterns of 1H, 2H1, 2H2, 2H3, 3R1, and 3R2 polytypes of the cobalt hydroxide sample.

Figure 3. Powder XRD patterns of cobalt oxyhydroxide samples obtained by (a) oxidation in air, (b) oxidation using hydrogen peroxide, (c) direct precipitation of cobalt hydroxide followed by oxidation using hydrogen peroxide at 80 °C, and (d) direct precipitation of cobalt hydroxide followed by oxidation using hydrogen peroxide and hydrothermal treatment at 160 °C.

It is clearly evident from the results of simulation studies that cobalt hydroxide crystallizes in 1H polytype. Figure 1 shows the observed PXRD pattern overlaid with simulated 1H polytype. Figure 3 shows the PXRD pattern of cobalt oxyhydroxide samples obtained by different methods. Cobalt oxyhydroxide obtained by air oxidation of cobalt hydroxide powder at 100 °C for 3 days is shown in Figure 3a. Figure 3b shows the cobalt oxyhydroxide obtained by the oxidation cobalt hydroxide powder using hydrogen peroxide as an oxidizing agent. In Figure 3c is shown the cobalt oxyhydroxide prepared by precipitation of cobalt hydroxide using sodium hydroxide followed by the oxidation using hydrogen peroxide and the slurry was treated at 80 °C for 18 h. In these cobalt oxyhydroxide samples, the first peak appears at 4.37 Å indicating that the interlayerspacing decreased by 0.2 Å compared to β-cobalt hydroxide and the peaks in the mid-2θ region are relatively broad. In order to improve the crystallinity of the cobalt oxyhydroxide sample, the freshly precipitated cobalt hydroxide slurry was subjected to hydrothermal treament at 160 °C. In Figure 3d is shown the PXRD pattern of cobalt hydroxide precipitated and oxidized by using hydrogen followed by hydrothermal treatment of the slurry for the 160 °C sample. The reflections in the cobalt

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Figure 4. DIFFaX-simulated powder XRD patterns of pure 1H, 2H1, 2H2, 2H3, 3R1, and 3R2 polytypes of β-cobalt oxyhydroxide.

Figure 5. Observed powder XRD pattern of (a) cobalt oxyhydroxide obtained at 80 °C sample and simulated powder XRD patterns of the (b) 3R1 and (c) 3R2 polytypes.

oxyhydroxide sample obtained by the precipitation of cobalt hydroxide followed by oxidation using hydrogen peroxide and aging at 80 °C are very broad indicating the smaller crystallite size (see Figure 3c). DIFFaX simulations were carried out, and the corresponding patterns of 1H, 2H1, 2H2, 2H3, 3R1, and 3R2 polytypic modifications of cobalt oxyhydroxide are shown in Figure 4. Parts b and c of Figure 5 show the simulated PXRD patterns of 3R1 and 3R2 polytypes with a restricted crystallite size of 100 Å. Even though the peak positions of the observed PXRD pattern match well with 3R1 and 3R2 polytypes, the relative intensities of the 3R2 polytype is much closer to the observed pattern compared to the 3R1 polytype. It can also be a mixture of 3R2 and 3R1 polytypes, respectively. Since the X-ray diffraction pattern is poorly ordered, it is difficult to conclude which polytype is dominant. There are also reports by other authors on β-CoOOH and the diffraction patterns of the cobalt oxyhydroxide compounds were relatively broad in the mid-2θ region, indicative of poorly ordered samples. To improve the crystallinity, we prepared the β-CoOOH sample by the hydrothermal method at 160 °C for 5 days as shown in Figure 6. The X-ray diffractogram shows the first peak position at 4.37 Å in the PXRD pattern of hydrothermally prepared β-CoOOH (a ) 2.85 Å and c ) 13.2 Å), and the pattern matches with the JCPDS

Figure 6. PXRD pattern of observed cobalt oxyhydroxide prepared by hydrothermal treatment at 160 °C (solid lines) with DIFFaX simulation (O).

card no. 007-0169. Among the three layered polytypes, 3R1 polytype generates only prismatic interlayer sites, while the 3R2 generates only octahedral interlayer sites. The only way to distinguish one polytype from the other is to examine the relative intensities and peak position of different (h0l)/(0kl) reflections. These two polytypes can be generated by translation of layers by a vector of (1/3, 2/3) with respect to the other. As expected, the PXRD patterns are similar, except for the relative intensities of the (101), (012), (104), (015), (107), and (018) nonbasal reflections. On the basis of the PXRD simulation data, the 3R1 polytype (012), (015), and (018) are relatively strong and in the case of the 3R2 polytype, the (101), (104), and (107) reflections will be relatively strong. Figure 6 shows the simulated PXRD pattern of the 3R1 polytype of β-CoOOH (O) stacked over the observed pattern. On the basis of peak positions, it was found that the sample crystallizes in the three layered polytype (3R1). We observed similar results for other cobalt oxyhydroxide samples prepared by air oxidation as well as hydrogen peroxide oxidation. This clearly shows that the cobalt oxyhydroxide obtained by different routes lead to 3R1 polytype only. Our results show that in spite of similar chemical composition, i.e., CoOOH in relationship to NiOOH, the oxidation mechanism of cobalt hydroxide to cobalt oxyhydroxide is quite different, i.e., their stacking sequence of layers are completely distinct.12 Similar results have been reported for the heterogenite CoOOH even though the mechanism of transformation has not been reported.21 The results may mimic the transformation mechanism during the electrochemical cycling of cobalt hydroxide. Conclusions The crystal structure analysis in most of the cases has been limited to match the peak positions in their respective crystal system. Careful examination of the relative intensities in the PXRD patterns can provide better insight into a precise stacking sequence of the layers. On the basis of DIFFaX simulation studies, we show that β-CoOOH prepared by different chemical methods converge to the 3R1 polytype. The oxidation mechanism in cobalt hydroxide takes place from the 1H to 3R1 polytypic transformation and is completely different from that of β-NiOOH observed from β-Ni(OH)2. The stacking sequence can also affect the physical and chemical properties of the material.

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Acknowledgment T.N.R. thanks the Council of Scientific and Industrial Research, GOI for the award of a Research Associate Fellowship. The author gratefully thanks Professor P. Vishnu Kamath for providing laboratory facilities to carry out the work and providing an opportunity to publish the results. The author also thanks B. E. Prasad for recording the XRD patterns and the reviewers for their useful comments Literature Cited (1) Falk, S. U.; Salkind, A. J. In Alkaline Storage Batteries; Wiley: New York, 1969. (2) Ramesh, T. N.; Kamath, P. V. Bi2O3 Modified Cobalt Hydroxide as an Electrode for Alkaline Batteries. Electrochim. Acta 2008, 53, 4721. (3) Kordesch, K.; Weissenbacher, M. Rechargeable Alkaline Manganese Dioxide/Zinc Batteries. J. Power Sources 1994, 51, 61. (4) Ismail, J.; Ahmed, M. F.; Kamath, P. V. Cyclic Voltammetric Studies of Pure and Doped Films of Cobalt Hydroxide in 1 M KOH. J. Power Sources 1991, 36, 507. (5) McBreen, J. In Modern Aspects in Electrochemistry; White R. E., Bokris, J. O. M., Conway, B. E., Eds.; Plenum Press: New York, 1990; Vol. 21. (6) Armstrong, R. D.; Charles, E. A. Some Effects of Cobalt Hydroxide Upon the Electrochemical Behaviour of Nickel Hydroxide. J. Power Sources 1989, 25, 89. (7) Wang, X.; Yan, J.; Yuan, H.; Zhou, Z.; Song, D.; Zhang, Y.; Zhu, L. Surface Modification and Electrochemical Studies of Spherical Nickel Hydroxide. J. Power Sources 1998, 72, 221. (8) Pralog, V.; Delahaye-Vidal, A.; Beaudoin, B.; Gerand, B.; Leriche, J. B.; Tarascon, J.-M. Electrochemical Behavior of Cobalt Hydroxide Used as Additive in the Nickel Hydroxide Electrode. J. Electrochem. Soc. 2000, 147, 1306. (9) Jayahsree, R. S.; Kamath, P. V. Modified Nickel Hydroxide Electrodes, Effect of Cobalt Metal on the Different Polymorphic Modifications. J. Electrochem. Soc. 2002, 149, A761.

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ReceiVed for reView October 31, 2009 ReVised manuscript receiVed December 20, 2009 Accepted December 24, 2009 IE901714V