Langmuir 1994,10, 3933-3936
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In Situ Atomic Force Microscopy Imaging of Electroprecipitated Nickel Hydrous Oxide Films in Alkaline Electrolytes Rong-rong Chen, Yibo Mo, and Daniel A. Scherson* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 441 06 Received June 13, 1994@ In situ atomic force microscopy images of nickel hydrous oxide films electrodepositedon the basal plane of highly oriented pyrolytic graphite in alkaline electrolytes have shown that a stepwise oxidation leads to irreversible formation of wide crevices throughout the material. Upon subsequent stepwise reduction, the gaps close leaving a hairline type crack which followsthe profile of the crevice. These potential induced structural rearrangements have been attributed to stresses induced by differencesin the densities of the nickel hydrous oxide in the two oxidation states.
Introduction
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Hydrated forms of nickel oxide have found wide application as electrode materials in electrical energy storage and energy generation devices, including NVCd and NV metal hydride rechargeable batteries and2" fuel cells.' As evidenced by rather recent in situ X-ray absorption fine structure (XAFS)measurements: and in agreement with earlier X-ray diffraction studies of the crystalline forms of these c o m p ~ u n d s the , ~ Ni(I1) hydrous oxide (discharged state), denoted hereafter as Ni(OH),(hyd.), undergoes a contraction ofca. 15%along the brucite layers upon oxidation to yield the corresponding Ni(II1) hydrous oxide (charged state) or NiOOH(hyd.). These changes in oxidation state are accompanied by the incorporation or release of solvated electrolyte ions into and from the interlayer spacing to achieve charge electrone~trality.~ Such a n effect modifies markedly the effective density of the hydrous oxide5(and, thereby, the macroscopic dimensions of the material) and is believed to be responsible for the gradual loss in electrode capacity observed after extensive charge-discharge cycling. Despite its technological importance, however, no attempts have been made to probe these electrochemically driven structural modifications in situ, that is, with the electrode immersed in the electrolyte under potential control. This Letter presents in situ atomic force microscopy (AFM)6r7images of thin films of Ni(OH)z(hyd.) electroprecipitated on the basal plane of highly oriented pyrolytic
* Abstract published inAdvanceACSAbstracts, October 15,1994.
(1) McBreen, J. In Modern Aspects of Electrochemistry, Conway, B., White, R., Bockris, J. O'M, Eds.; Plenum Press: New York, 1991;Vol. 23. (2) (a)McBreen, J.; O'Grady, W. E.; Pandya, K. I.; Hoffman, R. W.; Sayers, D. E. Langmuir 1987,3, 428. (b) Pandya, K. I.; OGrady, W. E.; Conigan,D. A.; McBreen, J.;Hoffman, R. W. J.Phys. Chem. 1990, 94,21. ( c ) Pandya, K. I.; Hoffman, R. W.; McBreen, J.;OGrady, W. E. J. Electrochem. SOC.1990,137,383. (d) McBreen, J.; O'Grady,W. E.; Tourillon, G.; Dartyge, E.; Fontaine, A.; Pandya, K. I. J. Phys. Chem. 1989,93,6308. (e) Guay, D.; Tourillon, G.; Dartyge, E.; Fontaine, A.; McBreen, J.;Pandya, K. I.; OGrady, W. E. J.Electroanal. Chem. 1991, 305, 83. (3) Areview in the general area ofthe structure and electrochemistry of nickel hydroxides and oxyhydroxides may be found in Oliva, P.; Leonardi, J.;Laurent, J. F.; Delmas, C.; Braconnier, J. J.; Figlarz, M.; Fievet, F.; de Guibert, A. J. Power Sources 1982, 8, 229. (4) Evidence in support of ion migration in and out of the lattice has been obtained from in situ quartz crystal microbalance measurements. See, for example, (a)Bernard, P.; Gabrielli, C.;Keddam, M.; Takenouti, H.; Leonardi, J.; Blanchard, P. Electrochem. Acta 1991, 36, 743. (b) Cheek,G. T.; O'Grady, W. E.ExtendedA6stract.s;Electrochemical Society Meeting, Fall 1990, Abstract 259; p 374. (5) Bode, H.; Dehmelt, K.; Witte, J. 2.Anorg.Allg. Chem. 1969,366, 1. ( 6 )For a recent review in the area of AFM see: Quate, F. Surf. Sci. 1994,299/300, 980.
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Potential / V vs. SCE Figure 1. Cyclic voltammogram of a Ni(OH)2film electrodeposited on the basal plane of highly oriented pyrolytic graphite (HOPG(bp))obtained in the open atmosphere in a 1M KOH solution: electrode area, -0.3 cm2;scan rate, 2 mV/s.
graphite HOPWbp) in an alkaline electrolyte as a function of the extent of oxidation and the rates at which the oxidation-reduction process is carried out. The interactions between the Ni(OH)2(hyd.)/NiOOH(hyd.)film and the underlying support are expected to be very weak, due primarily to the unreactive character of the graphitic planes; hence, the effects observed may be ascribed entirely to the intrinsic properties of the metal hydrous oxide overlayer.
Experimental Section Nickel hydrous oxide films were prepared by cathodic electrodepositions by polarizing the HOPG(bp)electrode of an active cross-sectional area ca. 0.3 cm2(as defined by the Viton O-ring used in the Digital Instruments, fluid cell) at -1.0 V vs SCE in 0.05 M Ni(N03)~ aqueous solutions for either 30 or 60 s. This methodology has been found t o yield the a form of Ni(OH)2.6 After deposition, the films were rinsed with ultrapurifiedwater and immediately examined by cyclic voltammetry in the same cell in 1.0 M KOH in the open atmosphere, yielding curves such as that shown in Figure 1. The charge associated with the characteristicNi(II)/Ni(III)redox couple for these films was in (7) For applications of AFM to the study of electrochemicalsystems see, for example, (a) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. Science 1991,251,133. (b)C.-H. Chen, C.-H.;Gewirth, A. J.Am. Chem. SOC.1991,113, 6049. (8)Conigan, D. A. J. Electrochem. Soc. 1987, 134, 377.
0 1994 American Chemical Society
3934 Langmuir, Vol. 10,No.11, 1994
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B C A Figure 2. In situ atomic force microscopy(AFM)images of a 0.1pm x 0.1 pm area of a Ni(OH)z(hyd.)film (ca. 3.2 mC) supported on HOPG(bp) obtained at a potential of 0.15 V vs SCE, i.e. fully reduced state, in 1.0M KOH in the fluid cell after transfer from the preparation cell (frame A). Frames B and C were obtained at 0.33 and 0.015V in the same area after stepping the potential twice between these two values. The arrow on the left side of the pictures indicates the direction of the AFM vertical scan (see Figure 3 caption and text for details).
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Figure 3. In situ AFM images acquired during the potential steps specified in the Figure 2 caption. The lower section of frame A was recorded at 0.015 V immediately following acquisition of the image shown in frame A, Figure 2, whereas the upper section of this frame was obtained after stepping the potential to 0.33V. The upper section of frame B was acquired after stepping the potential to 0.015V,immediately following acquisition of frame A in this figure, whereas the lower section was collected aRer stepping the potential to 0.33 V. the range 2.2-3.8 mC. After a single voltammogram, the Ni(OH)z(hyd.)/HOPG(bp)specimenswere transferredto the in situ cell for AFM measurements (fluid cell, Digital Instruments). All in situ AFM images were obtained with a Nanoscope I1 microscope(Ditigal Instruments) using a scanner type A and a microfabricated cantilver with a spring force of 0.12N/m. Two Pt wires placed at the inlet and outlet of the cell were employed as counter and pseudoreference electrodes, respectively. The stability of the Pt pseudoreference electrode was monitored periodically using the peak potentials of the Ni(OH)z(hyd.)/ NiOOH(hyd.) redox couple as a reference point. Images were collected at a fured potential either by stepping or scanning the voltage to the desired value to render the film in a mixed or fully oxidized or fully reduced state. Potential control was achieved with a PAR potentiostat (Model 173)and a PAR Universal Programmer (Model 175).
Results and Discussion Frame A, Figure 2 shows an atomic force microscope (AFM) image of a Ni(OH)z(hyd.)film with an integrated reduction peak of 3.2 mC obtained in situ a t a potential of 0.15 V vs SCE, i.e. fully reduced state, in 1.0 M KOH. Except for the three clearly visible bright objects aligned along the diagonal, the surface, at this rather low magnification, appears for the most part featureless. Serendipitously,these objects serve as markers to compare spatially images obtained of the same general area a t different potentials. Modifications in the film microstructure induced by changes in the oxidation state can be better explored by measurements in which the potential is stepped to the
desired value in the middle of the image scan. Images obtained in a series of experiments of this kind are shown in Figure 3, where the arrow on the left side of each picture indicates the direction of the AFM vertical scan. In particular, the lower section of frame A, Figure 3, was obtained a t 0.015 V following acquisition of the image shown in frame A, Figure 2. After slightly less than half of the complete area had been imaged, the film was fully oxidized by stepping the potential to 0.33 V, producing a clearly defined horizontal line in the scan. The upper half portion of this image revealed the presence of a large crevice on the left-hand side. Because of its large aspect ratio, however, no information regarding the depth of this feature could be obtained from a detailed analysis of these data. In fact, the rather featureless appearance of the line scan in the crack (not shown here) indicates that the side of the AFM tip was most likely resting on the walls of the crevice while scanning through this area. A subsequent potential step in the opposite direction (0.33 0.015 V) applied immediately after the image in frame A in this figure had been acquired, restored to a large extent the features observed in frame A, Figure 2 (see upper half, frame B, Figure 3). This electrochemically induced structural modification was found to be almost (see below) reversible, as judged by the results obtained by stepping the potential once again to 0.33 V. As shown in the lower image of panel B, Figure 3, the crevice,which extended down to the lower right corner of the imaged section, reappeared. Evidence that the entire crevice was formed during the first stepwise film oxidation is given in frame B, Figure 2 obtained at 0.33 V after the lower section of the image in frame B, Figure 3 had been recorded. Reduction of the film by a subsequent potential step (0.33 0.015 V, see frame C, Figure 2) gave rise to the formation of a hairline crack, precisely in the same region of the crevice. This clearly indicates that under the conditions of this experiment, the crevicecreated during the original full oxidation is not fully repaired once the film is subsequently reduced. It is rather remarkable that the detailed features of the boundaries of the two regions generated by the initial oxidation remain invariant after the oxidation state of the film is changed. Although no attempt was made to establish the number, shape, or length of the cracks, about 50% of the areas imaged on a given film electrode were found to contain these features. Figure 4 shows images obtained with a much thinner film, i.e. 2.4 mC for experiments in which the potential was scanned a t a slow rate (1mV/s), as opposed to stepped, in the range in which the redox transition occurs. The
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Langmuir, Vol. 10, No. 11, 1994 3935
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Figure 4. In situ atomic force microscopy (AFM) images of 0.1pm x 0.1p m area of a thin Ni(OH)Z(hyd.)film (ca. 0.24 mC) supported on HOPabp) in 1 M KOH acquired at potentials E = 0.065 V (frame A), E = 0.242 V (frame B), E = 0.267 V (frame C),E = 0.305 V (frame D), E = 0.373 V (frame E), and E = 0.05 V (frame F). These images were obtained after scanning as opposed to steping the potential to the desired value.
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Figure 5. In situ AFM images obtained for a different area of the same thin film described in the Figure 4 caption in the reduced state before (frame A) and after (frame B) a potential scanning excursion into the fully oxidized region. The images in frames C and D were obtained followingtwo successive oxidation and reduction steps in the sequence 0.05 V- 0.35 (lower and upper sections of frame C, respectively) and 0.05 0.35 V (upper and lower sections of Frame D, respectively). Frame E was recorded in the oxidized state after the sequence of steps in the previous frames had been completed.
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main aim of this set of measurements was to examine correlations between the extent of oxidation and the detailed structure of the film. Frames A through F were collected a t a fixed potential during the first oxidation of the film in the fluid cell by scanning the voltage a t the specified rate from 0.065 (fully reduced state, frame A) to 0.242 (frame B); 0.267 (frame C), 0.305 (frame D), 0.373 (fully oxidized, frame E), and back to the fully reduced
state, 0.05 V vs SCE (frame F). Based on these images, the gradual oxidation of the film leads to a monotonic widening of the gap first observed on the center left part of frame B to yield in the fully oxidized state (see frame E in this figure) a rotated “T”-type crevice. As was the case with the thicker film in Figures 2 and 3, the reduction of the film does not heal the crack but leaves instead a hairline on the structure.
3936 Langmuir, Vol. 10, No. 11, 1994 Asimilar type of behavior was observed in another area of this thin film (see Figure 5)) as evidenced by a comparison between images obtained for the reduced film before (frame A) and after (frame B) a potential scanning excursion into the fully oxidized region. A very different effect was found, however, for this same film area upon stepping, as opposed to scanning the potential between the two oxidation states. This is illustrated in frame C in this figure which shows in the lower and upper sections images of the film before (0.05 V) and after full oxidation (0.35V), respectively. Particularly striking is the formation of a wide open area on the upper part of the image, which remains unmodified upon subsequent reduction of the film (see upper section of frame D in this figure). Upon reoxidation, the film undergoes further breakdown as evidenced by the appearance of new wide cracks in the lower section of frame D. Such structural effects are for the most part irreversible, as little or no changes could be observed after reducing the film (see frame E). The most likely explanation for the observed structural instability of the film upon oxidation may be found in the large variations in the densities associated with the lattice in the two redox states. In particular, the electroprecipitation method is believed to yield a-Ni(OH)z,for which the density Q is in the range 2.5-2.8g/cm3and, therefore, much lower than those associated with both b-NiOOH (e ca. 4.7 g/cm3) and y-NiOOH (e ca. 3.8 g / ~ m ~ More ).~ specifically, the oxidation of the film may be envisaged to start from discrete points in the sheetlike structure leading to a localized contraction. As the process continues these areas will grow, pulling the lattice in many different directions, creating mechanical stresses which would eventually lead to ruptures along weakly bound sections
Letters of the film. When the process is reversed, each of these newly formed, disconnected patches, will expand along essentially the same contraction path to occupy the previous area without re-forming, however, the original structure and leaving instead a space along the fractured path. The much more severe damage caused by changing oxidation states with a step compared to a scan may be due to differences in the relative rates of phase nucleation and growth. In particular, in situ S7FeMossbauer effect spectroscopy measurements of closely related Fe(0H)Z(hyd.) dispersed on high area carbonghave shown that a step-type oxidation generates numerous small particles of FeOOH(hyd.), whereas a slow scan produces fewer, albeit larger particles of the same material. On this basis, one may expectthat the likelihood of a crevice being formed will increase in proportion to the number of nuclei formed. As the results presented here appear to indicate, that would be the case when the oxidation of the film is carried out by a potential step a s opposed to a scan. Experiments of the type described in this work are now underway involving nickel hydrous oxide films containing co-electroprecipitated cobalt to examine the role this commonly used metal additive may play in promoting structural stability of the material.
Acknowledgment. This work was supported by the Department of Energy, Basic Energy Science. Additional funding was provided by Eveready Battery Company, Westlake, OH. (9) Fierro, C.F.;Carbonio, R.E.;Scherson,D.; Yeager, E.B.J.Phys. Chem. 1987,91,6579.
