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Volume IO, number 11,12 MATERIALS LETTERS March 1991

Infrared spectroscopic studies of the oxide-hydroxides of Ni, Co and Mn

P. Vishnu Kamath Department sf Chemistry, Central College, Bangalore University, Bangalore 560 001, India

and

S. Ganguly I.D.L. Nitro Nobel Basic Research Institute, P.O. Box 397, Malleswaram, Bangalore 560 003, India

Received 29 August 1990; in final form 25 November 1990

Metal oxide-hydroxides of the general formula MO(OH) (M=Ni’+, Co3+ and Mn’+ ) are actually bronzes of the correspond- ing quadrivalent metal oxides and should be represented as H,M02 (x= 1). Infrared spectroscopic studies show that these com- pounds do not exhibit any hydroxyl stretch. The protons seem to be delocalized between slabs of MOz.

Metal oxide-hydroxides of Ni3+, Co3+ and Mn3+ All infrared spectra were recorded on a Perkin- are known to play a major role in the charge-dis- Elmer model 684 IR spectrometer with KBr pellets charge cycle of nickel hydroxide [ 11, cobalt hydrox- on CsI windows at a resolution of 3 cm- ‘. ide [ 2,3 ] and manganese dioxide [ 41 electrodes re- In fig. 1 are shown the infrared spectra of Ni (OH ) 2 spectively. A complete characterization of these and NiO( OH). The OH stretching region and the phases is therefore essential to our understanding of metal oxygen stretching region ( 800-200 cm- ’ ) are the electrode reactions taking place at these oxide both shown in detail. electrodes. In particular the nature of the hydroxyl Ni(OH), shows a sharp peak at 3630 cm-’ fol- species in the oxide-hydroxides has not been clearly lowed by a broad band from 3500 to 3200 cm-’ in- understood. Melendres et al. [ 5 1, from a study of the dicating the presence of both free and hydrogen- Raman spectra of the higher oxides of nickel, have bonded hydroxyl groups. Both the a and p forms of opined that the formulation of the oxide-hydroxide Ni(OH)* have varying amounts of water of hydra- of nickel as NiO(OH) is incorrect. In this commu- tion [ 8,9] and the broad band in the OH stretching nication we report our infrared spectroscopic studies region has a significant contribution from the water of the oxide-hydroxides of Ni3+, Co3+ and Mn3+ of hydration. NiO( OH), on the other hand, shows and compare the results with those of the corre- only a broad weak absorption in the range 3600-3 100 sponding divalent hydroxides. cm- ’ precluding the presence of free hydroxyl groups All compounds studied here were prepared by the in this compound. method of Brauer [6] and characterized by X-ray In the low-frequency (800-200 cm- ’ ) region, diffraction. The X-ray diffractograms were found to Ni( OH), shows a complex spectrum with broad match with those reported for these compounds in bands at 525 and 450 cm-’ and a sharp peak at 350 the JCPDS powder diffraction file [ 71. As observed cm-‘. NiO(OH) shows a broad band at 570 cm-’ by other authors [ 5,8], the diffraction patterns of and a shoulder at 460 cm-‘. the nickel compounds were broad, indicative of In fig. 2 are shown similar results for the cobalt poorly ordered samples. system. Co(OH), shows a sharp OH stretch at 3620

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Fig. I. Infrared spectra of Ni(OH)I (A) and NiO(OH) (B). cm-’ followed by a broad band in the 3500-3000 structure exhibit a sharp peak in the OH stretching cm-’ region. In the low-frequency region it shows a frequency region [ 5 1, indicating the presence of free spectrum similar to that of Ni( OH)z with bands at hydroxyl groups. The trivalent oxide-hydroxides, 480 and 440 cm-’ and a sharp peak at 300 cm-‘. which are known to form by the oxidation of the di- CoO(OH), on the other hand, shows a weak broad valent hydroxides at the oxide electrodes do not show absorption band in the 3600-3 100 cm-’ region and any well-defined OH stretching modes. Free hy- there is clearly no indication of the existence of any droxyl groups are therefore absent in the oxide-hy- free hydroxyl group. In the low-frequency region, the droxides. It may therefore be more appropriate to spectrum of Co0 (OH) is very simple with a single formulate these compounds as oxides rather than ox- band at 580 cm-‘. There is a pronounced asym- ide-hydroxides. However, the X-ray diffraction pat- metry on the low-frequency flank. terns as well as the infrared spectra of these trivalent In fig. 3 is shown the spectrum of MnO (OH ) . Once phases are different from those of the well-known di- again the 3600-3000 cm-’ region shows no free OH valent oxides of Ni, Co and Mn, indicating that they stretching frequency, while the low-frequency region are not the same. Melendres et al. [ 5 1, from a group is very complex with bands at 6 10,490,400 and 330 theoretical analysis of their Raman spectra, have in- cm-‘, respectively. terpreted the trivalent nickel oxide-hydroxide phase Structurally well-defined hydroxides of brucite as derived from the quadrivalent NiOz. A similar

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Fig. 2. Infrared spectra ofCo(OH)Z (A) and CoO(OH) (B). 538 Volume 10, number 11,12 MATERIALS LETTERS March 1991

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Fig. 3. Infrared spectrum of MnO (OH ) conclusion can be extended to the case of Co and Mn be 3.6 -t 0.1, a result compatible with the identifi- as well. The oxide-hydroxides can then be formu- cation of the phase as HXNi02 (x=0.4). Buss et al. lated as bronzes of the type H,M02 (x= 1). x= 2 [ 12 ] in similar work on cobalt hydroxide electrodes represents end members of the series which are well- have achieved an oxidation state of 3.7. (2) The de- defined hydroxides of the brucite structure. Brucite localization of the protons also explains the high pro- structure consists of layers of MO* with the protons ton diffusion coefficients [ 13 ] of the nickel hydrox- sticking out into the interleaving space [ lo]. The OH ide electrodes. bond axis is parallel to the c axis of the crystal. The Further investigations on the oxide-hydroxides of proton layer is saturated as all the oxygens are bound other metals are currently in progress. to a proton each. Oxidation appears to occur by proton extraction and when this happens vacancies are created in the References proton layers, as some oxygens are no longer bound to a proton. The protons either become delocalized [ 1] P. Oliva, J. Leomarde, J.F. Laurent, C. Delmas, J.J. or get positioned at sites which are less well defined Braconnie, M. Figlarz, F. Fievert and A. de Guibert, J. Power than earlier leading to the disappearance of the free Sources 8 (1982) 229. [2] R.D. Armstrong and E.A. Charles, J. Power Sources 25 hydroxyl stretching mode in these compounds and (1989) 89. the phase acquires a bronze-like character. Fig. 4 il- [3] J. Batter, D.H. Buss, H.J. Harms and 0. Glemser, J. lustrates this model pictorially. This model qualita- Electrochem. Sot. 137 (1990) 173. tively explains the two observed phenomena: ( 1) [4] B.D. Desai, J.B. Femandes and V.N. Kamat Dalal, J. Power Corrigan and Knight [ 111 have evaluated the charge Sources 16 (1985) 1. [ 51C.A. Melendres, W. Paden, B. Tani and W. Walczak, J. on Ni on an oxidized nickel hydroxide electrode to Electrochem. Sot. 134 (1987) 762. [ 61 G. Brauer, Handbook of preparative inorganic chemistry (Academic Press, New York, 1965 ) . [ 71 J.V. Smith, ed., Powder diffraction tile, Joint Committee on Powder Diffraction Standards, Pennsylvania ( 1967). [S] B. Mani and J.P. de Neufville, J. Electrochem. Sot. 135 (1988) 800. [9] H. Bode, K. Dehmelt and J. Witte, Electrochim. Acta 11 (1966) 1079. [lo] H. Krebs, Fundamentals of inorganic crystal chemistry (McGraw-Hill, New York, 1968). [ 1 I ] D.A. Corrigan and S.L. Knight, J. Electrochem. Sot. 136 (1989) 613. M b-d2 “MO(OH)” HMOa [ 12 ] D.H. Buss, J. Bauer, W. Diembeck and 0. Glemser, J. Chem. Fig. 4. Schematic model to explain the bronze-like behaviour of Sot. Chem. Commun. (1985) 81. the oxide-hydroxides. The hatched region represents slabs of [ 13) D.M. MacArthur, J. Electrochem. Sot. 117 ( 1970) 729. MO*, 0 represents a proton, 0 represents a vacancy.

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