© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 1, JANUARY 2, 1997
LETTERS Dynamic Neutron Reflectivity Measurements during Redox Switching of Nickel Hydroxide Films Paul M. Saville, Marylou Gonsalves, and A. Robert Hillman* Department of Chemistry, UniVersity of Leicester, Leicester, LE1 7RH, U.K.
Robert Cubitt Institut Laue-LangeVin, B.P.156, 38042 Grenoble Cedex 9, France ReceiVed: July 8, 1996; In Final Form: September 30, 1996X
We report the first dynamic neutron reflectivity measurements on redox switching of a modified electrode. Neutron reflectivity with isotopic substitution of the solvent was used to probe the dynamics of ion and solvent transfer processes in electrochemically precipitated nickel hydroxide films exposed to aqueous LiOH solution. Ion and solvent transfer rates differ during oxidation and reduction. Even at slow potential scan rates, film solvent content is kinetically controlled.
Introduction Electrodes modified with surface-confined redox functionalities1 have potential applications in sensors,2 optical devices,3 and batteries.4 Their rational design requires correlation of interfacial structure and properties, and accordingly a wide range of techniques has been used to provide insight into the chemical nature5 or, in the case of the STM and AFM,6 surface topography of the interface. An important, but relatively unexplored, factor in determining electrode performance is film spatial distribution. For example, it determines the ease with which mobile species can enter and leave the film: this governs the rate and extent of electron transport to and from film-bound redox sites and the rate of access of solution species to these sites. Most assessments of electroactive film characteristics assume spatial homogeneity, with respect to both the populations of mobile species (ions and solvent) and the matrix (polymer, metal oxide, etc) through which they move. Despite general recognition that this is an unrealistic approximation, it is commonly * Author for correspondence. Email address:
[email protected]. X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5647(96)02033-0 CCC: $14.00
used in the absence of better information, since spatial distributions of thin films can be probed by relatively few techniques, notably ellipsometry7,8 and neutron9 or X-ray reflectivity.10 Collectively, these techniques are complementary to STM in that they probe subsurface structure, rather than external surface contours. Individually, they are complementary in their requirements and the structural information they provide. Although data interpretation for these reflectivity methods is somewhat model dependent,11 neutron reflectivity does have the considerable advantages of contrast variation and, due to the highly penetrating nature of the neutron, in situ applicability. The disadvantage is that neutron reflectivity measurements commonly involve rather long data acquisition times, often precluding access to kinetic issues. The purpose of this Letter is to show that neutron reflectivity can in fact be used, and its selectivity for solvent distribution exploited, under kinetically controlled conditions. Methodology The basic principle of neutron reflectivity is to impinge a collimated neutron beam, of wavelength λ and incident angle θ, onto an interface and measure the reflected intensity, R, as a © 1997 American Chemical Society
2 J. Phys. Chem. B, Vol. 101, No. 1, 1997
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function of momentum transfer, Q (defined as [4π/λ] sin θ). The measured reflectivity, R(Q), depends on the neutron refractive index profile perpendicular to the interface (zdirection). The neutron refractive index is a function of the scattering length density, Nb, which is the product of the number density, N, and the sum of the neutron scattering lengths, b, of the different nuclei present. The neutron scattering length varies from nucleus to nucleus, so changing the isotopic composition of the system changes the reflectivity. Here H2O/D2O substitution is used in the bathing electrolyte to increase the scattering from solvent swollen regions of a nickel hydroxide film without changing the film structure.11 At a positive step in the scattering length density profile, neutrons with momentum transfers below a critical value, Qc, are totally reflected. This is seen as a region of unit reflectivity at small values of Q. Qc is related to a positive step in the scattering length density, ∆(Nb), resulting in total reflection by
Qc ) x16π∆(Nb)
(1)
Kinetic measurements using neutron reflectivity are not commonplace due to the low flux of neutron sources and the relatively weak interaction of neutrons with the sample. It is possible, however, to make reflectivity measurements as a function of time either at a single Q value (using a reactor source) or over a limited Q range (using an intense spallation source), provided the time scale of the kinetic process is sufficiently long. Electrochemical quartz crystal microbalance (EQCM) studies of a range of electroactive films5 suggest that redox-driven changes in film solvent population can occur on time scales from seconds to minutes, commensurate with neutron reflectivity measurements. Here it is shown how this provides a means of probing the dynamics of solvent and ion populations and attendant density changes in electroactive nickel hydroxide films under potentiodynamic conditions, where the rate of the charge/discharge reaction driving solvent transfer is controlled via the potential scan rate (V). This chemical system is of considerable interest due to its technological importance in batteries12,13 and electrochromic displays.3 The present work complements transient EQCM experiments on nickel hydroxide films,14,16 which suggest that solvent transfer is slower than ion transfer but offer no insight into the (temporal variation of) spatial distribution. Experimental Section The neutron reflectivity cell, incorporating the Pt counter electrode and KCl saturated calomel (SCE) reference electrode, has been described elsewhere.9 The working electrodes were evaporated gold films on single-crystal quartz blocks (10 cm path length, neutron transmission ca. 70%). Nickel hydroxide films were deposited from 0.1 mol dm-3 Ni(NO3)2 (99.999%) solution by sweeping the potential from 0.0 V to -0.7 V (vs SCE) and holding it at -0.7 V until a predetermined charge had been passed.15 The film was then transferred to 1.0 mol dm-3 LiOH. H2O (99.95%) solution in D2O, and the potential was cycled between 0.0 and 0.5 V for 30 min. Reflectivity measurements were made at the NIST reactor (Gaithersburg, MD) on the NG7 reflectometer. The neutron wavelength was 4.768 Å and full reflectivity profiles were measured over the momentum transfer range 0.005 e Q/Å-1 e 0.090. Reflectivity measurements were made on the reduced and oxidized states of the film for characterization purposes. The momentum transfer (controlled through incident angle, θ) was then set to a value where the reflectivity changed significantly on film oxidation (see below), and the potential
Figure 1. Neutron reflectivity of a nickel hydroxide film (Γ ) 310 nmol cm-2) at 0.0 V (solid line, reduced state) and 0.5 V (dashed line, oxidized state). Solution composition: 1.0 mol dm-3 LiOH / D2O. Vertical line corresponds to Q ) 0.0122 Å-1, at which kinetic measurements were made. Inset shows cyclic voltammogram, V ) 2 mV s-1.
was cycled between 0.0 and 0.5 V (scan rate 2 mV s-1). Reflectivity data were collected in short time bins equivalent to potential intervals of 28 mV. Results and Discussion The cyclic voltammetric response of the nickel hydroxide film exposed to 1.0 mol dm-3 LiOH / D2O is inset into Figure 1. Under the conditions employed, the anodic and cathodic peak potentials are 0.430 and 0.145 V, respectively. Integration of the anodic current response yields a surface coverage Γ ) 310 nmol cm-2. Comparison with the deposition charge (corresponding to 470 nequiv cm-2) indicates a deposition efficiency of 65%. Sections of the reflectivity profiles for the oxidized and reduced states of the nickel hydroxide film are shown in Figure 1. In the plateau region from 0.008 e Q/Å-1 e 0.010 neutrons, undergo total reflection. On oxidation, the critical edge shifts to smaller Q, indicating a decrease in the scattering length density of the film (cf. eq 1). The fringes observed in the reflectivity profiles arise from reflection from the interfaces of the gold electrode; from their separation a gold film thickness (given by d ) 2π/∆Q) of 350 nm is deduced. For the kinetic experiment, a value of Q is required for which the film reflectivity differs substantially in oxidized and reduced states; hence, the momentum transfer was set to Q ) 0.0122 Å-1, indicated by the vertical line of Figure 1. At this value simulations show an approximately linear relationship between the reflectivity and scattering length density (i.e. water content) over the Nb range calculated from the measured values for the critical edges. Subsequent data correspond to dynamic measurements of R as a function of applied potential at this fixed Q value. Figure 2 shows the reflectivity and electrochemical charge (q) as functions of the applied potential (E) during a slow scan cyclic voltammetric experiment on an electrode previously equilibrated in the reduced state at 0.0 V. From the individual responses, two key features are identified. Firstly, there is substantial hysteresis in both R(E) and q(E). Secondly, the responses fail to return to their initial values at the end of the
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J. Phys. Chem. B, Vol. 101, No. 1, 1997 3
a
Figure 3. Plot of reflectivity (at Q ) 0.0122 Å-1) as a function of film charge for the data of Figure 2.
b
Figure 2. Plots of (a) reflectivity at Q ) 0.0122 Å-1 and (b) electrochemical charge as functions of applied potential for the nickel hydroxide electrode of Figure 1; conditions as in the inset to Figure 1.
redox cycle. Comparison of the charges under the oxidation and reduction curves indicates a 30% discrepancy. Although this may be partly due to oxygen evolution from the oxidized film,12 visibly incomplete film decolorization at the end of the steady state cycling experiment leaves no doubt that it is at least partly due to charge trapping in the oxidized state. Given the experimental uncertainties, the reflectivity change is less clear, but replicate measurements consistently show lower reflectivity for the reduced film at the end of the cycle, as compared to the start of the cycle. Both the hysteresis and “trapping” features signal kinetic control of film ion and solvent populations. We now explore how neutron reflectivity data provide insight into the coupling between these ion and solvent transfers. To do this, one must look in more detail at the responses between the potential extrema. During the oxidation half-cycle (0.30 e E /V e 0.50), there is charge flow and the reflectivity steadily decreases. During the reduction half-cycle, the reflectivity remains essentially constant until ca. 0.20 V, even though about 40% of the electrochemical charge injected in the oxidation half-cycle has been recovered. Thereafter, the reflectivity increases rather sharply, toward (although not attaining) its initial value. Neutron and electrochemical data are correlated in Figure 3. The ∆Ep value for the current peaks (see Figure 1) of 0.285 V shows that redox equilibrium is not maintained. A plot of R vs q also shows pronounced hysteresis, indicating that film solvation is not in equilibrium with film redox state. During
both oxidation and reduction, two distinct regions are identified on the basis of instantaneous slopes, dR/dq, of Figure 3. Within individual regions, ion and solvent transfers are roughly linear functions of charge passed. In both half-cycles, the larger dR/dq is associated with the second stage, indicating a lag of solvent flux behind electrochemically driven ion flux. This effect is particularly pronounced during reduction. At the end of the cycle, the reflectivity is the same as in the positive-going scan at that charge leVel but differs from the value at the start of the cycle due to incomplete charge recovery. This indicates similar film structures at this redox level, although the final (redox and solvation) state differs from the initial state. Through the electroneutrality condition, the electrochemical charge is a measure of the exchange of charged species between an electroactive film and its bathing electrolyte. The reflectivity also responds to the exchange of neutral species, primarily solvent, between the film and the solution. On the basis of isotope scattering lengths and of film and solution compositions, it is relatively straightforward to show that reflectivity changes are dominated by solvent transfer. Using D2O solvent, the solvent contribution to changes in R is further highlighted. Thus the reflectivity vs charge correlation allows us to compare the solVent population within the film with the ion flux that drives it. A clearer understanding of the dynamic response of the reflectivity awaits a complete analysis of the full reflectivity profiles. However, we note that reflectivity decreases could result from three processes: D+ expulsion, D2O expulsion, and Li+ insertion, on the basis of the scattering length densities of these species. Expulsion of D2O is expected to be a consequence of phase transformations that result in film dehydration.15 Some alkali ion is also found in the oxidized state.15 The slope changes in Figure 3 suggest a change in redox-switching mechanism, which is consistent with EQCM data14,16 that show nonmonotonic mass changes on oxidation of the nickel hydroxide film. There too, the change in mechanism occurs at about 40% charging.14 The new insight that neutron reflectivity brings, compared to nonselective detection by the EQCM, is a means of distinguishing solvent and lithium ion transfers. Conclusion The mechanism and overall stoichiometry of nickel hydroxide redox switching are generally acknowledged to be complex; sensitivity to film-based, electrolyte, and other experimental variables complicates matters further. Neutron reflectivity offers
4 J. Phys. Chem. B, Vol. 101, No. 1, 1997 a new tool for probing this problem, through separation of ion and solvent fluxes during redox cycling. Film oxidation and reduction proceed in two steps, in each of which film reflectivity (composition) is a different linear function of film charge state. Solvent trapping accompanies charge trapping under kinetically controlled conditions. In a full paper, we will extend this to a complete picture of the spatial distributions of film components as functions of electrochemically controlled charge state under both thermodynamically and kinetically controlled conditions. Acknowledgment. We thank J. A. Dura and S. K. Satija of the Reactor Radiation Division at NIST for experimental assistance, Robert Wilson for preparatory work, and Andrew Glidle for evaporating the gold electrodes. We acknowledge the EPSRC for funding (GR/J30516 and GR/K19068) and The National Institute of Standards and Technology for neutron beam time. References and Notes (1) Murray, R. W. Molecular Design of Electrode Surfaces; Wiley: New York, 1992.
Letters (2) Abruna, H. D.; Pariente, F.; Alonso, J. L.; Lorenzo, E.; Trible, K.; Cha, S. K. ACS Symp. Ser. 1994, 561, 230. (3) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism; VCH: Weinheim, 1995. (4) Megahed, S.; Scrosati, B. J. Power Sources 1994, 51, 79. (5) Hubbard, A. T. The Handbook of Surface Imaging and Visualization; CRC Press: Boca Raton, FL, 1995. (6) Bonnell, D. A. Scanning Tunneling Microscopy and Spectroscopy: Theory, Techniques and Applications; VCH: New York, 1993. (7) Gottesfeld, S. Electroanalytical Chemistry: A Series of AdVances; Marcel Dekker: New York, 1989; Vol. 15, p 143. (8) Hamnett, A. J. Chem. Soc., Faraday Trans. 1993, 89, 1593. (9) Richardson, R. M.; Swann, M. J.; Hillman, A. R.; Roser, S. J. Faraday Discuss. 1992, 94, 295. (10) Foster, M. D. Crit. ReV. Anal. Chem. 1993, 24, 179. (11) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. (12) McBreen, J. In Modern Aspects of Electrochemistry; Plenum: New York, 1990; p 29. (13) Corrigan, D. A.; Zimmerman, A. H. Nickel Hydroxide Electrodes; The Electrochemical Society: Pennington, NJ, 1990; Proceedings Volume 90-4. (14) Gonsalves, M.; Hillman, A. R. Manuscript in preparation. (15) Bode, H.; Dehmelt, K.; Witte, J. Electrochim. Acta 1966, 11, 1079. (16) Mo, Y.; Hwang, E.; Scherson, D. A. J. Electrochem. Soc. 1996, 143, 37.
