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Ion flux during electrochemical charging of Prussian Blue films
213
J. Electroanal. Chem, 234 (1987) 213-227 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
ION FLUX DURING ELECTROCHEMICAL BLUE FILMS
B.J. FELDMAN
* and 0.R
MELROY
l
CHARGING
*
IBM Almaden Research Center, San Jose, CA 951204099 (Received
20th January
OF PRUSSIAN
1987; in revised form 16th March
(U.S.A.) 1987)
ABSTRACT Thin (approximately 100 run) films of Prussian Blue were electrcchemically deposited onto mass-sensitive oscillating quartz-crystal electrodes. Changes in electrode mass during the electrodeposition process indicated that the as deposited films were highly hydrated. Mass changes upon electrochemical reduction and oxidation of the Prussian Blue films in the nitrate salts of K+, Rb+, and Cs+ at pH 4, were measured. Some weight gain occurred upon the original reduction and reoxidation, indicating that an irreversible structural reorganization occurred. Subsequent electrochemical cycling produced a reversible mass gain upon reduction, and a reversible mass loss upon oxidation, indicating that alkali metal cations enter and exit the fihn upon reduction and oxidation, respectively. The mass/charge behavior of the films was studied in 0.5 M RbNO,, over the pH range 2.7-6.4. The pH dependence indicated that considerable proton expulsion can occur upon oxidation of the Prussian Blue, and some proton inclusion can accompany reduction. The film mass losses uponoxidationin 0.5 M RbNOs and 0.5 M RbOOCCH,, pH 4, were virtually indistinguishable, implying that the supporting electrolyte anions played little role in the electrochemistry in these media.
INTRODUCTION
Thin films of crystalline Prussian Blue [l-16] have been deposited electrochemically on a variety of electrode surfaces. Although the exact structure of these films remains controversial [ll], it is a mixed valent compound which can be generically described as a ferric ferrocyanide, in which both the C-coordinated Fe(I1) and N-coordinated FeQII) centers are electrochemically active. These films have attracted interest due to their possible use as batteries [4], electrochromic devices (in both liquid [lo] and solid [16] states), and corrosion inhibitors [17]. Additional work has been stimulated by Prussian Blue’s well known charge density, site-site distance, and ion-conducting channel size, which make it a good subject for fundamental studies of electron [15,16,18] and ion storage and conduction.
* l
Presentaddress: Western Regional * To whom correspondence should
0022-0728/87/303.50
R esearch Center be addressed.
0 1987 Elsevier Sequoia
USDA-ARS,
S.A.
Albany,
CA 94710, U.S.A.
214
Prussian Blue exhibits both reductive and oxidative waves in various aqueous media. (For the purposes of this discussion, reduction will refer to the reduction of the mixed valent Fe(III),Fe(II) Prussian Blue to the Fe(II),Fe(II) form; oxidation will refer to electrochemical creation of the Fe(III),Fe(III) form.) In the cyclic voltammetry of the Fe(II1) reduction, both El,z and Efwhm (thermodynamic and kinetic parameters, respectively) vary greatly with the nature of the supporting electrolyte cation. Reversible reductions are supported in electrolyte containing NH:, K’, Rb+ and Cs+, but not in H+, Li+, or any known divalent ions. The oxidation of the C-coordinated Fe(I1) centers is less reversible and has not been as thoroughly studied. Some controversy exists as to the nature of these two redox processes and there are indications that there may be structural changes upon the first reduction and subsequent reoxidation of the film. The stoichiometric formula of the film prior to electrochemical cycling has not been proven; the two most likely candidates are Fe,(Fe(CN),), .6 H,O [19-211 (the six water molecules are coordinated to Fe atoms), and MFe,(CN),, where M+ is a monovalent cation. (These are often classified as “insoluble” and “soluble” Prussian Blue, respectively, a historical distinction which refers to the ease of peptidization.) The above stoichiometries ignore interstitial water molecules, a considerable number of which occupy the Prussian Blue lattice under conditions of normal atmospheric humidity [19-211. Spectroscopic evidence supports the formulation Fe,(Fe(CN),), .6 H,O. Neither XPS [6], Auger [6], nor EDX [22] spectroscopy show any evidence of cations in films prior to electrochemical cycling. However, the El,* for oxidation in KC1 is observed [2] to vary in a Nernstian fashion with K+ concentration indicating the oxidation is described by eqn. (4), and that the correct formula is KFez(CN),. A possible solution was proposed by Mortimer and Rosseinsky [13], who hypothesized, on the basis of changes in the Prussian Blue optical spectra, that cycling through the electrochemical reduction results in conversion from Fe,(Fe(CN),), .6 H,O to MFe,(CN),. However, changes in the optical spectra have also been attributed to the existence of the non-reductively cycled Prussian Blue in a partially oxidized state [6,11]. EDX spectroscopy does indicate [16] the presence of K+ and Cs+ in films which have been cycled in electrolyte containing those cations, though not in the quantity required by the formula MFe,(CN),, indicating the cycled films may be a mixture of the two stoichiometries. If one assumes the Fe,(Fe(CN),), .6 H,O stoichiometry, the reduction and oxidation processes may then be formulated [ll]: Fe,(Fe(CN),),.6 Fe,(Fe(CN),),
H,O+4
e-+4
M++
M,Fe,(Fe(CN),),.6
.6 H,O - 3 e- + 3 X- + X,Fe,(Fe(CN),),
where X- is a monovalent anion. For MFe,(CN),,
H,O .6 H,O
(1) (2)
the analogous processes are:
MFez(CN),
+ e- + M+ + M,Fe,(CN),
(3)
MF%(CN),
- e- - M+ + Fq(CN),
(4)
If the formula Fe.,(Fe(CN),), - 6 Hz0 is correct, then cation (eqn. 1) and anion entrance (eqn. 2) would be required for charging to the reduced and oxidized states,
215
respectively. However, for MFe,(CN),, electrochemical charging would be accompanied only by cation motion; cations would enter upon reduction (eqn. 3), and exit upon oxidation (eqn. 4). Recently, the oscillating quartz crystal electrode has emerged [23,24] as a powerful tool for studying mass changes accompanying electr~he~cal events in situ. Sub-monolayer mass sensitivity is obtainable, and the technique has already been used to study underpotential deposition of monolayers of Pb on Au [25], electroless Cu deposition [26], and the ionic motion accompanying electrochemical cycling of poly(pyrrole) [27] and poly(aniline) [28]. In this paper, the oscillating quartz crystal electrode has been applied to the study of the reduction and oxidation of Prussian Blue films in aqueous electrolytes, in an attempt to determine the identity of the ions which move into or out of the lattice to effect charge compensation. To this end mass changes upon oxidation and reduction of Prussian Blue films in electrolyte solutions containing the NO; salts of K+, Rb+, and Cs+, and the CH,COOsalt of Rb+, at various pH values, are reported. The electrodeposition of Prussian Blue and structural changes accompanying its initial reduction were also studied. EXPERIMENTAL
Prussian Blue films were prepared by reductive electr~~stall~ation (at 0.5 V vs. Ag/AgCl) from an aqueous solution 2 mM in K,Fe(CN), and Feel,, pH 2. Alkali metal nitrate solutions were prepared with Barnstead “Nanopure” water and Aldrich reagent grade salts. Alkali metal nitrate electrolyte were 0.5 M, and were adjusted to the appropriate pH with nitric acid. The rubidium acetate solutions were adjusted to pH 4 with acetic acid. Quartz crystals ~Valpey-Fisher) were one inch diameter, AT cut, overtone polished, and had resonant frequencies of 4.9, 5, or 6 MHz. 250 nm gold electrodes were evaporated onto a 2 nm Si adhesion layer in an Edwards E306A coating system. The active electrochemical area was 0.32 cm2. The oscillator circuit, electrochemical cell and associated electronics have been described elsewhere [25]. The electrolyte was routinely degassed with N, before reductive scans although this had virtually no effect on the observed electrochemistry. A Pt coil auxiliary electrode, in conjunction with an an Ag/AgCl (3 M KCI) reference electrode (Micr~l~tr~es, Inc.), was used. All potentials are reported with respect to this reference electrode. Fresh Prussian Blue modified quartz crystals were used for experiments in electrolytes containing different cations, since irreversible film changes which are apparently cation dependent occur upon electrochemical cycling. The pH and anion dependence experiments were each conducted on single Prussian Blue modified quartz crystal electrodes. RESULTS AND DISCUSSION
Measurements at the quariz crystal electrode It has been previously shown [24] that the mass sensitivity of the quartz crystal microbalance to the deposition of rigid (elastic) films is the same in solution as in
216
vacuum. For systems in which the mass of the deposited film is less than a few percent of the crystal mass, the Sauerbrey equation [29] can be used to relate the change in oscillation frequency (Af) to the mass of the deposited film (m) by:
Af = -2 mfz/Afi
(5)
where f, is the crystal frequency prior to film deposition, pq is the shear modulus of quartz (2.947 X 10” g/cm s’), ps is the density of quartz (2.648 g/cm3), and A is the area of the oscillating surface. (Only that area of the crystal which is sandwiched between the conducting electrodes oscillates.) Immersion of one of the crystal faces (and attached Au electrode) in liquid results in the creation of a highly damped sinusoidal shear wave which travels through the liquid, away from the crystal. Since these waves are completely damped in a few hundred nm [24] (the exact distance depends upon the solution viscosity), the crystal senses only a thin layer of liquid. The net effect is to reduce the crystal oscillation frequency by a constant value, typically 1000-2000 Hz. Assuming the density and viscosity of the solution are constant, this initial frequency decrease can be treated as an offset and any processes which result in additional changes in crystal mass can then be studied. These processes include film formation [25-281 or dissolution, and the motion of ions [27,28] and solvent [28] into and out of a film. Prussian Blue electrodeposition The quartz microbalance was used to monitor the rate of deposition of Prussian Blue from a solution of ferric and ferricyanide ions. The electrode potential was stepped from the open circuit value (about 0.73 V) to 0.5 V, and the crystal frequency monitored concurrently with the deposition current. Figure 1 is the resulting plot of mass gain per unit area vs. charge density, with a least squares straight line drawn through the data. The molar mass of the deposited Prussian Blue
Fig. 1. Mass gain/unit area vs. charge plot for electrodeposltion of a Prussian Blue onto a quartz crystal electrode in 0.1 M KCI, 2 mM FeCI,, 2 mM K,Fe(CN),, pH 2, at 0.5 V vs. SCE. f, = 5.962.212 Hz. The line is a least squares fit.
217
can be determined from the slope of the line and was found to be 643.5 g/mol. The molar mass of Fe,(Fe(CN),), * 6 H,O (deposition involves reduction of the three ferricyanide Fe(II1) atoms) is 361.2 g/mol, while that of KFe.JCN), is 331 g/mol. These calculated molar masses are much smaller than the values measured in situ during deposition. This disagreement is probably due to water incorporated in the film. The water molecules may occupy specific sites in the Prussian Blue crystal lattice, or they may lie in “pools” of solvent in pores within the film or in pockets created by surface roughness. (Any water in surface pockets or pores in the film behaves as an elastic mass [30]). Ellipsometric data [31] indicate that growing Prussian Blue films are highly hydrated, and that the degree of hydration is dependent upon the preparation conditions. After removal from solution and equilibration in dry nitrogen, the mass of the Prussian Blue film was recalculated from the difference between pre- and post-deposition crystal frequencies, the dry molar mass was found to be 327.9 g/mol, in relatively good accord with the values for Fe,(Fe(CN),), .6 H,O (361.2 g/mol), and KFe,(CN), (331 g/mol). (We hesitate to use the molar mass to distinguish between the two probable film stoichiometries, since the efficiency of the electrodeposition process is unknown. If some reduced material escapes from the electrode surface without crystallizing, then an erroneously low molar mass will be calculated.) The initial segment of Fig. 1 (low charge, low mass) is somewhat concave, indicating that the first layers of deposited Prussian Blue are more compact (containing less interstitial and/or defect occupying solvent) than subsequent layers. The linearity of Fig. 1 at long times indicates that the Prussian Blue film is elastic (non-viscous). Thus, there should be little damping of the crystal shear wave in the film, and the use of eqn. (5) is justified. Lattice rec~ns~~e~ionupon initiaireduction It has been proposed [13] that the electrochemical reduction of the Prussian Blue crystal is accompanied by an irreversible structural change from Fe,+(Fe(CN),), .6 H,O to MFez(CN),. In K+ containing electrolyte, the cyclic voltammetric reductive waveshape is drastically changed ( Erwhmis much reduced) from the first to subsequent scans. Lundgren and Murray have observed [22] the expulsion of Fe atoms from a Prussian Blue film during the initial reduction, by me~urement of the optical absorption of Fe(bpy), (bpy is 4,4’-bipy~dine) formed by chelation of the expelled Fe with bpy molecules in the supporting electrolyte solution. Assuming that low spin (C-coordinated) Fe atoms are conserved in this reorganization, a possible reaction stoichiometry is as follows (ignoring interstitial water molecules): Fe,(Fe(CN),),
.6 H,O
-Fe3++6 Hz013 MFe,(CN), +3 M+
The net mass change observed upon reductive cycling should therefore depend upon the mass of M+, the electrolyte cation. Mass changes after reduction and subsequent reoxidation by the first cyclic voltammetric scan were measured for two different Prussian Blue films, in K+ and Rb+. This measurement was complicated by the tendency of the frequencies of
some film coated crystals to drift slowly upon immersion in solvent. The drift generally ceased after a few voltammetric scans. However, measurements of mass change upon the initial reduction and reoxidation were performed only on crystals whose frequency stabilized immediately after immersion in electrolyte. In the KNO, electrolyte, (initial dry film weight = 14.71 pg/cm2), almost no net mass change is predicted (- 0.045 pg/cm*), and little was observed (0.037 pg/cm2) upon the initial redox cycle. (This change approaches the frequency detection limit.) In RbNO, (initial dry film weight = 10.0 pg/cm2), the mass gain predicted by eqn. (6) is 0.897 pg/cm2, while the observed mass gain was 0.574 pg/cm*. These numbers suggest some (although inconclusive) support for lattice reorganization upon reduction. The reorganization may be less complete than suggested by eqn. (6) protons may be co-incorporated with alkali-metal cations, or the extent of hydration (interstitial water content) may change during structural reorganization. Stronger evidence for the incorporation of alkali metal cations (and protons) into Fe(III),Fe(II) Prussian Blue films during electrochemical cycling will be presented below. Cyclic voltammetty in K +, Rb+, and Cs ’ nitrates, pH 4 Figure 2 illustrates cyclic voltammetry of a Prussian Blue film (curve A) with concurrent crystal oscillation frequency (curve B) in 0.5 M, pH 4 KNO, solution. (Approximate film thickness was 113 nm, based on a density [19] of 1.5 g/cm3.) Both oxidative and reductive waves are observed and both processes are reversible. The reduction of the Prussian Blue occurs at approximately 0.2 V and is accompanied by a simultaneous decrease in the frequency of the oscillator indicating an increase in the mass of the film. This drop in frequency is reversible upon reoxidation of the film. Prussian Blue can also be reversibly oxidized and this is observed at about 0.9 V. Again, as in the case of Prussian Blue reduction, oxidative current is accompanied by an increase in the frequency of the oscillator (mass decrease) which returns to its original value after the subsequent rereduction. This behavior suggests strongly that cations (probably K+) are present in the Fe(III),Fe(II) film and that they exit the film (to satisfy electroneutrality) upon oxidation of the film and enter upon reduction. This point will be discussed quantitatively below. Figures 3 and 4 illustrate the analogous experiment in 0.5 M RbNO, and CsNO, (pH = 4) respectively. The cyclic voltammetric potential limits were adjusted to account for differences in E,,* for Prussian Blue reduction, which (along with E fwhm) is strongly cation dependent. Again, the observed mass changes are highly reversible, reduction is accompanied by a mass gain, and oxidation by a mass loss. The failure of the crystal to return to precisely its original value could be due to a number of factors: (1) frequency drift due to temperature instability, (2) a slight oxidation of the Au electrode at 1 V, or (3) an actual hysteresis in the film oxidation and rereduction. In any event, the discrepancy is small, and the correlation between current flow and film mass change is obvious. The molar mass of the ion which enters or exits the film upon charging can be
219
-80 -100
E/ V(vs
: 10
08
06
AgIAgCI) 0.4
02
0
-0
2
100 50 3
_
0 -50 -100 -150
Fig. 2. Cyclic voltammetric (A) and potential-frequency (B) curves for Prussian crystal electrode in 0.5 M KNO,, pH 4. v = 10 mV/s, f0 = 5,817,409 Hz.
Blue coated quartz
acquired from the slope of a plot of the integrated charge from the cyclic voltammograms (summed over a few selected potential intervals) vs. the film mass change (obtained from eqn. 5). Figure 5 is such a plot for Prussian Blue reductive voltammetry in K+, Rb+, and Cs+ electrolyte (from Figs. 2, 3, and 4). Linear least squares lines fit to the data have near zero intercepts, and fit the data well, indicating that a single charge carrier (or an unchanging ratio of multiple charge carriers) is responsible for compensation of the injected negative charge in any given electrolyte. In Table 1, columns 2 and 3, the apparent molar masses of the charge compensating cations (obtained from the slopes of Fig. 5) are compared with the molar masses of K+, Rb+, and Cs+. The apparent molar masses scale with cation molar masses, and are in each case a little lower (11.1-17.5 g/mol). Thus, electrochemical reduction of Prussian Blue films in pH 4 electrolyte is accompanied by a nearly stoichiometric incorporation of alkali metal cations. This is an unsurprising result, since both of the proposed Prussian Blue structures, Fe,(Fe(CN),), - 6 H,O and MFe,(CN),, require cation entrance upon reduction (eqns. (1) and (3), respectively). The systematic deviation of the apparent equivalent weights from the cation molar masses is interesting. The nearly constant absolute error (as opposed to a
220
100 -
i:ll ;,
.-IO0
08
10
0.4
06
0.2
80 60 40
-
20 -
Q -i -
o-
-60 Fig.
3.
-
Cyclic voltammetric (A) and ~tential-frequency
(B) curves for Prussian Blue coated quartz
crystal electrode in 0.5 M RbNO,, pH 4. u = 10 mV/s, lo = 5,981,470 Hz.
E /V(vs
-15OL 10
40 F
08
Ag/AgCI) 0.6
04
02 A
Fig. 4. Cyclic voltammetric (A) and potential-frequency (B) curves for Prussian Blue coated quartz crystal electrode in 0.5 M CSNO~, pH = 4. u = 10 mV/s, f. = 4,893,657 Hz.
221
4
n
4
2
-0
6
Charge / mC cni2 Fig. 5. Mass/unit area vs. charge plots for reduction of F’russian Blue films coated onto quartz crystal elwtrodes in KNO, (A), RbN03 (B), and CsNO, (C), cakulated from F&s. 2, 3, and 4, respectively. Lines are least squares fits to the data.
constant relative error) argues against a systematic measurement error (such as underestimation of electrode area or damping of the shear wave in the outer layers of the film). There are two plausible explanations for this systematic deviation. First, a cation which enters the Prussian Blue cage upon reduction may force some amount of water to exit the cage. If each alkali metal cation replaced one water molecule, the measured molar mass would be equal to the molar mass of the alkali metal cation, minus that of water (18 g/mol). This explanation accounts well for the observed deviation (Table 1, columns 2 and 3). An alternate explanation is that alk~-Mets cation inclusion is ~ornp~~ by proton inclusion. The measured molar mass would then be a weighted average of proton and alkali metal cation equivalent weights. To test this possibility, the pH dependency of the mass changes accompanying electrochemical cycling (in RbNO,) of Prussian Blue films was measured (see below). Plots of charge density vs. mass change/unit area were also constructed for the oxidation of Prussian Blue in KNOS, RbNO,, and CsNO,, pH 4 (Figs. 2, 3, and 4).
TABLE
1
Molar masses of charge compensating Electrolyte
’
KN03 RbNOs CsNO, ’ 0.5 M, pH b Supporting ’ From Fig. d From Fig.
ions in various electrolytes
Molar mass b /g mol-’
Reductive moIar mass ’ /g moi-’
Oxidative mofar mass d /g mol-’
39.4 85.5 132.9
23.3 68.0 121.8
23.5 29.2 57.4
4. electrolyte cation moIecular weight. 5. 6.
222
1 Charge /mC
2
3
cd
Fig. 6. Mass/unit area vs. charge plots for oxidation of Prussian Blue films coated onto quartz crystal electrodes in KN03 (A), RbNO, (B), and CsNO, (C), calculated from Figs. 2, 3, and 4, respectively. Lines are least squares fits to the data.
The plots (Fig. 6) are linear (with the exception of curve B); however, the slopes do not scale with cation molar mass like those for reduction (Fig. 5). The apparent molar masses of the exiting species are listed in Table 1, column 4. For K+, the molar mass of the species entering upon reduction, and exiting upon oxidation are almost identical, indicating that charge compensation during electrochemical charging (both oxidative and reductive) in 0.5 A4 KNO,, pH 4, is primarily accomplished by K+ exit and entrance. This observation correlates nicely with the observed [2] Nemstian (59 mV/decade) dependence of both the reductive and oxidative formal potentials on [K+] in KC1 electrolyte. For Prussian Blue oxidation in RbNO, and CsNO,, pH = 4 (curves B and C) the situation is more complicated. Firstly, some curvature is observed in curve B (voltammetry in RbNO,). There are two likely sources for this discrepancy. First, the nature of the charge compensating ion may be potential dependent, and liable to change according to the partial degree of oxidation of the Prussian Blue. Another possibility is that, at the voltammetric scan rates employed (10 mV/s), the film oxidation state lags the applied electrode potential. Then, charge which is assumed to result in Prussian blue oxidation (to the Fe(III),Fe(III) form) would in reality go towards reoxidizing the reduced [Fe(II),Fe(II)] form. Since charge compensation for the Prussian Blue oxidation and reduction apparently involves different mixtures of cations (see below), this could account for the curvature observed in curve B. A least squares straight line was drawn through the data in order to calculate the weight of the charge compensating ion (8); this line is dashed in deference to the apparent non-linearity of the data. For oxidation in Rb+ and Cs+, a mass loss is observed, indicating that oxidation results in some exit of alkali metal cations, but the mass loss is considerably smaller than that expected from the molar mass of the alkali metal ions (Table 1). There are three likely origins of this systematic discrepancy between the measured molar masses and the molar masses of the alkali metal cations for the mass losses
223
accompanying oxidation: (1) alkali metal cation exit is accompanied by water molecule entrance, (2) protons and alkali metal cations are simultaneously transported out of the film, or (3) NO; anions move into the film as alkali metal cations exit. Of course, more than one of these processes may operate simultaneously. To discriminate between the above possibilities, the pH dependence of electrochemically induced film mass changes was studied. Charge neutralization by proton transport in Prussian Blue films The pH dependence of the film mass change (both oxidative and reductive) was studied in 0.5 A4 RbNO, electrolyte, in which the pH was varied from 2.7 to 6.4. RbNO, was chosen since the molar mass (29.2 g/mol) of the species exiting the film during oxidation in this medium (at pH = 4) was found to be much less than the molar mass of Rb+ (85.5 g/mol). This leads to the conjecture that either anion inclusion or proton co-exclusion (or both) must play an important role in the oxidative electrochemistry of Prussian Blue in RbNO,. The lower limit of the pH range (2.7) was set to deter H+ reduction during the reductive scan, the upper limit (6.4) to prevent formation of Fe(OH),. The pH dependency data is summarized in Figs. 7 and 8 and Table 2. Figures 7 and 8 are charge density vs. mass change/unit area plots obtained for the Prussian Blue reduction and oxidation, respectively, at various pHs. Again, the plots have near zero intercepts and are fairly linear. The measured molar masses of the charge compensating species for reduction (Fig. 7) at various pHs are listed in Table 2, column 2. At pH 2.7, the molar mass of the entering species (51.8 g/mol) is considerably smaller than the molecular weight of Rb+ (85.5 g/mol), or the molar masses obtained in less acidic solutions. Reduction of Prussian Blue in acidic (pH = 2.7) RbNO, appears to involve proton co-inclusion. The molar masses at pH 4.6 (69.4 g/mol) and pH 6.4 (63.4 g/mol) are, within the probable limits of
Fig. 7. Mass/unit area vs. charge plots for reduction of a Prussian Blue film coated onto a quartz crystal electrode in 0.5 M RbNO,, pH 2.7 (A), pH 4.6 (B), and pH 6.4 (C). o = 10 mV/s, f,, = 5.038.459 Hz. Lines are least squares fits to the data.
224
8-
1 Charge]
2
3
mC cni2
Fig. 8. Mass/unit area vs. charge plots for oxidation of a &ssian Blue film coated onto a quartz crystal electrode in 0.5 M RbNO,, pH 2.7 (A), pH 4.6 (B), and pH 6.4 (C). u = 10 mV/s, f. = 5,038,459 Hz. Lines are least squares fits to the data.
experimental error, indistinguishable. Since the measured molar mass shows no pH dependence over the pH interval 4.0 to 6.4, the deviation between these measured molar masses and Rb+ molar mass (85.5 gfmol} should probably be attributed to water exclusion accompanying alkali-metal cation exclusion. However, proton co-inclusion, even at moderate to high pH, cannot be strictly ruled out. The effect of pH on Prussian Blue film mass change during oxidative voltammetry is more striking. The charge density vs. mass loss/unit area plots for oxidation in RbNO, at various pHs are shown in Fig. 8, and the measured molar masses are given in Table 2, column 3. The molar mass of the exiting ion varies widely and systemati~lly with pH, from 15.2 g/mol (pH 2.7) to 60.4 g/mol (pH 6.4). At low pH (2.7), proton exit is apparently the primary charge compensation mechanism during film oxidation. At pH 6.4, the measured molar mass (60.4 g/mol) approaches that of Rb+ minus a water molecule (67.5 g/mol), suggesting that proton transport is not very important at this pH, and that extra water molecules probably occupy the sites vacated by exiting alkali metal cations. In summary, in RbNO, electrolyte, charge compensation during Prussian Blue oxidation is apparently accomplished entirely by proton and Rb’ exclusion, with attendant water molecule
TABLE 2 Molar masses of charge compensating
ions III 0.5 M RbNO,
at various pHs
PH
Reductive molar mass ’ /g mot-’
Oxidative molar mass ’ /g mol-’
2.7 4.6 6.4
51.8 69.4 63.4
15.2 31.9 60.4
’ From Fig. 7. b From Fig. 8.
225
inclusion. NO; inclusion need not be invoked to explain any aspect of Prussian Blue electrochemistry in RbNO, or KNO,. Protons and cations are ejected upon oxidation; apparently the population of protons in the Fe(III),Fe(II) film scales inversely with the pH of the contacting solution. Dependence of vo~tammet~c features on pH The cyclic voltammetric E,,2 values for Prussian Blue oxidation and reduction in RbNO, also exhibit a pH dependency. For reduction to the Fe(II),Fe(II) form, E,,, is relatively invariant at pH 6.4 to 4.6 (0.31 V), then increases to 0.35 at pH 2.7. This trend mirrors exactly the mass gain pH dependency depicted in Fig. 7, apparently indicating that proton incorporation into the Prussian Blue films occurs at more extreme (negative) potentials than Rb+ inco~oration. No systematic wave shape dependence ( Efwhm) upon pH was observed. For oxidation to the Fe(III),Fe(III) form, extraction of E,,z values is impossible due to the less than ideal voltammetric waveshape (see Fig. 3A). However, Prussian Blue oxidation does apparently become more difficult as the supporting electrolyte pH is reduced. If the oxidation “onset potential” is defined as the potential at which the oxidative current is equal to twice the ~~rnurn background value, then this “onset potential” varies systematically from pH 6.4 (0.78 V) to pH 4.6 (0.83 V) to pH 2.7 (0.91 V). Again, this trend parallels the pH dependency of oxidative mass loss shown in Fig. 8. Proton incorporation and expulsion, corresponding to Prussian Blue reduction and oxidation, respectively, are apparently less favored thermodynamically than the corresponding processes involving Rb+ cations.
The pH dependence experiment described above implies that anions play a very limited role in Prussian Blue oxidative electrochemistry. This inference was tested by a very limited study of the anion dependence of film mass changes during Prussian Blue oxidation. The oxidation, of a Prussian Blue film, was studied in 0.5 M RbNO, and 0.5 A4 RbOOCCH, (both pH 4). The charge density vs. mass loss/unit urea plot in these two media is shown in Fig. 9. (Curve A, from Prussian Blue oxidation in RbNO,, is reproduced from Fig. 6.) The plots are somewhat curved; the first increments of oxidative charge passed to the film result in greater mass losses than do subsequent charging. As discussed previously, this may be due to a change in the nature of the charge compensating species with film potential, or to some hysteresis in charging films of this thickness (ca. 100 nm) at 10 mV/s. In any case, the molar masses of the exiting species, calculated from the slopes of the dashed lines in Fig. 9, are 29.2 g/mol (NO;), and 26.7 g/mol (CH,COO-). Within experimental error, these values are equivalent. If anion inclusion occurred during Prussian Blue oxidation, then the bulky acetate ion would be expected to be less mobile than the smaller nitrate. The lessened CH,COOmobility would presumably result in a greater reliance on cation exclusion for charge compensation (ignoring for the moment the effects of proton motion), and therefore a greater film mass loss. The fact that the film mass
226
08
-
N
-
;OS 2 r
-
TJ 04
-
E
’ A (NON)
/’
1’ 9
: 2
,/
_ ,/‘.d , 1’ 02
,’ /’
,c
,’
,
//’
//’
//I=
,‘,A ’
/’
Fig. 9. Mass/unit area vs. charge plots for oxidation of a Prussian Blue film coated onto a quartz crystal electrode, in 0.5 M RbNO, (A) and 0.5 A4 RbOOCCH, (B), both pH 4. o = 10 mV/s, f0 = 5.981.470 Hz. Lines are least squares fits to the data.
loss exhibited no anion dependence supports the conclusion of the pH dependence study, that exit of alkali metal cations and protons is the primary charge compensation mechanism during Prussian Blue oxidation. We were precluded from testing the mobility of several smaN anions (such as CN- and the halide ions) in the Prussian Blue lattice by the instability of the Au electrode in such media at the potentials required to oxidize the Prussian Blue. CONCLUSIONS
Mass change measurements on voltametrically scanned Prussian Blue films indicate a change in the mass of the film (depending on electrolyte cation) after a single reduction and reoxidation, reinforcing the theory of lattice reorganization during the initial film reduction. Reduction of Prussian Blue films from alkali metal nitrate salts results in the incorporation of alkali metal cations, probable expulsion of water molecules, and under some circumstances (RbNO,, pH 2.7), co-incorporation of protons. The mass loss which consistently accompanies Prussian Blue oxidation is unequivocal evidence that Fe(IZi),Fe(II) Prussian Blue films, which have previously been cycled electrochemically in alkali metal cation electrolyte, contain quantities of those cations. Oxidation results in alkali metal cation expulsion, probable water incorporation, and, if the incorporated alkaii metal cation is bulky and immobile, proton expulsion. It is interesting to note that, in pH 4 KNO,, K+ transport dominates the electrochemistry, while in RbNO, and CsNO, (pH 4), protons play a larger role. Apparently, the importance of proton transport in these systems is due to the relative immobility of the Rb’ and Cs+ which have been incorporated into the Fe(III),Fe(II) films. However, these cations can be made to exit the film upon oxidation if the film proton population is sufficiently depressed.
227
No evidence was found for anion inclusion during Prussian Blue oxidation in NO; or CH,COO- salts. An important question is whether this behavior is general to all anions, or is due to the size of NO; and CH,COO-. Similar studies at Prussian Blue modified Pt coated quartz crystals in halide media will be required to answer this question. ACKNOWLEDGEMENT
Illuminating discussions with C. Lundgren (UNC, acknowledged.
Chapel Hill) are gratefully
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
V.D. Neff, J. Electrochem. Sot., 125 (1978) 886. D. Ellis, M. Eckhoff and V.D. Neff, J. Phys. Chem., 85 (1985) 1225. K.P. Rajan and V.D. Neff, J. Phys. Chem., 86 (1982) 4361. V.D. Neff, J. Electrochem. Sot., 131 (1985) 1382. K. Itaya, H. Akahoshi and S. Toshima, J. EIectrochem. Sot., 129 (1982) 1498. K. Itaya, T. Ataka and S. To&ma, J. Am. Chem. Sot., 104 (1982) 4767. K. Itaya, T. Ataka, S. Toshima and T. Shinohara, J. Phys. Chem., 86 (1982) 2415. K. Itaya, I. Uchida and S. Toshima, J. Phys. Chem., 87 (1983) 105. K. Itaya, I. Uchida, S. Toshima and R.M. De La Rue, J. Electrochem. See., 131 (1984) 2086. K. Itaya, K. Shibayama, H. Akahoshi and S. Toshima, J. Appl. Phys., 53 (1982) 804. K. Itaya, I. Uchida and V.D. Neff, Act. Chem. Res., 19 (1986) 162. R.J. Mortimer and D.R. Ross&sky, J. ElectroanaI. Chem., 131 (1983) 133. R.J. Mortimer and D.R. Rosseinsky, J. Chem. Sot. Dalton Trans., (1984) 2059. C.E.D. Chidsey, B.J. Feldman, C. Lundgren and R.W. Murray, Anal. Chem., 58 (1986) 601. B.J. Feldman and R.W. Murray, Anal. Chem., 58 (1986) 2844. B.J. Feldman and R.W. Murray, Inorg. Chem., 26 (1987) 1702. H. Rubin, B.D. Humphrey and A.B. Bocarsly, Nature (London), 308 (1984) 5957, A. Viehback and D.E. Deberry, J. Electrochem. Sot., 132 (1985) 1369. H.J. Buser, D. Schwarxenback and W.A. Petter, Inorg. Chem., 16 (1977) 2704. F. Herren, P. Fischer, A. Ludi and W. HaIg, Inorg. Chem., 19 (1980) 956. A. Ludi and H.U. Gudel, Struct. Bonding (Berlin), 14 (1973) 1. C. Lundgren and R.W. Murray, University of North Carolina, Chapel Hill, personal communication, 1986. S. Bruckenstein and M. Shay, 1983 Pittsburgh Conference and Exposition, Atlantic City, NJ, Abstracts, Paper 763. K.K. Kanazawa and J.G. Gordon, Anal. Chem., 57 (1985) 1770. O.R. Melroy, K.K. Kanazawa, J.G. Gordon and D.A. Buttry, Langmuir, 6 (1986) 697. R. Schumacher, J.J. Pesek and O.R. Melroy, J. Phys. Chem., 85 (1985) 4338. J.H. Kaufman, K.K. Kanaxawa and G.B. Street, Phys. Rev. Lett., 53 (1984) 2461. D. Orata and D.A. Buttry, J. Am. Chem. Sot., 109 (1987) 3574. G. Sauerbrey, Z. Phys., 155 (1959) 206. R. Schumacher, J.G. Gordon and O.R. Melroy, J. Electroanal. Chem., 216 (1987) 127. A. Hamnett, University of Oxford, personal communication, 1987.
J. Electroanal. Chem, 234 (1987) 213-227 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
ION FLUX DURING ELECTROCHEMICAL BLUE FILMS
B.J. FELDMAN
* and 0.R
MELROY
l
CHARGING
*
IBM Almaden Research Center, San Jose, CA 951204099 (Received
20th January
OF PRUSSIAN
1987; in revised form 16th March
(U.S.A.) 1987)
ABSTRACT Thin (approximately 100 run) films of Prussian Blue were electrcchemically deposited onto mass-sensitive oscillating quartz-crystal electrodes. Changes in electrode mass during the electrodeposition process indicated that the as deposited films were highly hydrated. Mass changes upon electrochemical reduction and oxidation of the Prussian Blue films in the nitrate salts of K+, Rb+, and Cs+ at pH 4, were measured. Some weight gain occurred upon the original reduction and reoxidation, indicating that an irreversible structural reorganization occurred. Subsequent electrochemical cycling produced a reversible mass gain upon reduction, and a reversible mass loss upon oxidation, indicating that alkali metal cations enter and exit the fihn upon reduction and oxidation, respectively. The mass/charge behavior of the films was studied in 0.5 M RbNO,, over the pH range 2.7-6.4. The pH dependence indicated that considerable proton expulsion can occur upon oxidation of the Prussian Blue, and some proton inclusion can accompany reduction. The film mass losses uponoxidationin 0.5 M RbNOs and 0.5 M RbOOCCH,, pH 4, were virtually indistinguishable, implying that the supporting electrolyte anions played little role in the electrochemistry in these media.
INTRODUCTION
Thin films of crystalline Prussian Blue [l-16] have been deposited electrochemically on a variety of electrode surfaces. Although the exact structure of these films remains controversial [ll], it is a mixed valent compound which can be generically described as a ferric ferrocyanide, in which both the C-coordinated Fe(I1) and N-coordinated FeQII) centers are electrochemically active. These films have attracted interest due to their possible use as batteries [4], electrochromic devices (in both liquid [lo] and solid [16] states), and corrosion inhibitors [17]. Additional work has been stimulated by Prussian Blue’s well known charge density, site-site distance, and ion-conducting channel size, which make it a good subject for fundamental studies of electron [15,16,18] and ion storage and conduction.
* l
Presentaddress: Western Regional * To whom correspondence should
0022-0728/87/303.50
R esearch Center be addressed.
0 1987 Elsevier Sequoia
USDA-ARS,
S.A.
Albany,
CA 94710, U.S.A.
214
Prussian Blue exhibits both reductive and oxidative waves in various aqueous media. (For the purposes of this discussion, reduction will refer to the reduction of the mixed valent Fe(III),Fe(II) Prussian Blue to the Fe(II),Fe(II) form; oxidation will refer to electrochemical creation of the Fe(III),Fe(III) form.) In the cyclic voltammetry of the Fe(II1) reduction, both El,z and Efwhm (thermodynamic and kinetic parameters, respectively) vary greatly with the nature of the supporting electrolyte cation. Reversible reductions are supported in electrolyte containing NH:, K’, Rb+ and Cs+, but not in H+, Li+, or any known divalent ions. The oxidation of the C-coordinated Fe(I1) centers is less reversible and has not been as thoroughly studied. Some controversy exists as to the nature of these two redox processes and there are indications that there may be structural changes upon the first reduction and subsequent reoxidation of the film. The stoichiometric formula of the film prior to electrochemical cycling has not been proven; the two most likely candidates are Fe,(Fe(CN),), .6 H,O [19-211 (the six water molecules are coordinated to Fe atoms), and MFe,(CN),, where M+ is a monovalent cation. (These are often classified as “insoluble” and “soluble” Prussian Blue, respectively, a historical distinction which refers to the ease of peptidization.) The above stoichiometries ignore interstitial water molecules, a considerable number of which occupy the Prussian Blue lattice under conditions of normal atmospheric humidity [19-211. Spectroscopic evidence supports the formulation Fe,(Fe(CN),), .6 H,O. Neither XPS [6], Auger [6], nor EDX [22] spectroscopy show any evidence of cations in films prior to electrochemical cycling. However, the El,* for oxidation in KC1 is observed [2] to vary in a Nernstian fashion with K+ concentration indicating the oxidation is described by eqn. (4), and that the correct formula is KFez(CN),. A possible solution was proposed by Mortimer and Rosseinsky [13], who hypothesized, on the basis of changes in the Prussian Blue optical spectra, that cycling through the electrochemical reduction results in conversion from Fe,(Fe(CN),), .6 H,O to MFe,(CN),. However, changes in the optical spectra have also been attributed to the existence of the non-reductively cycled Prussian Blue in a partially oxidized state [6,11]. EDX spectroscopy does indicate [16] the presence of K+ and Cs+ in films which have been cycled in electrolyte containing those cations, though not in the quantity required by the formula MFe,(CN),, indicating the cycled films may be a mixture of the two stoichiometries. If one assumes the Fe,(Fe(CN),), .6 H,O stoichiometry, the reduction and oxidation processes may then be formulated [ll]: Fe,(Fe(CN),),.6 Fe,(Fe(CN),),
H,O+4
e-+4
M++
M,Fe,(Fe(CN),),.6
.6 H,O - 3 e- + 3 X- + X,Fe,(Fe(CN),),
where X- is a monovalent anion. For MFe,(CN),,
H,O .6 H,O
(1) (2)
the analogous processes are:
MFez(CN),
+ e- + M+ + M,Fe,(CN),
(3)
MF%(CN),
- e- - M+ + Fq(CN),
(4)
If the formula Fe.,(Fe(CN),), - 6 Hz0 is correct, then cation (eqn. 1) and anion entrance (eqn. 2) would be required for charging to the reduced and oxidized states,
215
respectively. However, for MFe,(CN),, electrochemical charging would be accompanied only by cation motion; cations would enter upon reduction (eqn. 3), and exit upon oxidation (eqn. 4). Recently, the oscillating quartz crystal electrode has emerged [23,24] as a powerful tool for studying mass changes accompanying electr~he~cal events in situ. Sub-monolayer mass sensitivity is obtainable, and the technique has already been used to study underpotential deposition of monolayers of Pb on Au [25], electroless Cu deposition [26], and the ionic motion accompanying electrochemical cycling of poly(pyrrole) [27] and poly(aniline) [28]. In this paper, the oscillating quartz crystal electrode has been applied to the study of the reduction and oxidation of Prussian Blue films in aqueous electrolytes, in an attempt to determine the identity of the ions which move into or out of the lattice to effect charge compensation. To this end mass changes upon oxidation and reduction of Prussian Blue films in electrolyte solutions containing the NO; salts of K+, Rb+, and Cs+, and the CH,COOsalt of Rb+, at various pH values, are reported. The electrodeposition of Prussian Blue and structural changes accompanying its initial reduction were also studied. EXPERIMENTAL
Prussian Blue films were prepared by reductive electr~~stall~ation (at 0.5 V vs. Ag/AgCl) from an aqueous solution 2 mM in K,Fe(CN), and Feel,, pH 2. Alkali metal nitrate solutions were prepared with Barnstead “Nanopure” water and Aldrich reagent grade salts. Alkali metal nitrate electrolyte were 0.5 M, and were adjusted to the appropriate pH with nitric acid. The rubidium acetate solutions were adjusted to pH 4 with acetic acid. Quartz crystals ~Valpey-Fisher) were one inch diameter, AT cut, overtone polished, and had resonant frequencies of 4.9, 5, or 6 MHz. 250 nm gold electrodes were evaporated onto a 2 nm Si adhesion layer in an Edwards E306A coating system. The active electrochemical area was 0.32 cm2. The oscillator circuit, electrochemical cell and associated electronics have been described elsewhere [25]. The electrolyte was routinely degassed with N, before reductive scans although this had virtually no effect on the observed electrochemistry. A Pt coil auxiliary electrode, in conjunction with an an Ag/AgCl (3 M KCI) reference electrode (Micr~l~tr~es, Inc.), was used. All potentials are reported with respect to this reference electrode. Fresh Prussian Blue modified quartz crystals were used for experiments in electrolytes containing different cations, since irreversible film changes which are apparently cation dependent occur upon electrochemical cycling. The pH and anion dependence experiments were each conducted on single Prussian Blue modified quartz crystal electrodes. RESULTS AND DISCUSSION
Measurements at the quariz crystal electrode It has been previously shown [24] that the mass sensitivity of the quartz crystal microbalance to the deposition of rigid (elastic) films is the same in solution as in
216
vacuum. For systems in which the mass of the deposited film is less than a few percent of the crystal mass, the Sauerbrey equation [29] can be used to relate the change in oscillation frequency (Af) to the mass of the deposited film (m) by:
Af = -2 mfz/Afi
(5)
where f, is the crystal frequency prior to film deposition, pq is the shear modulus of quartz (2.947 X 10” g/cm s’), ps is the density of quartz (2.648 g/cm3), and A is the area of the oscillating surface. (Only that area of the crystal which is sandwiched between the conducting electrodes oscillates.) Immersion of one of the crystal faces (and attached Au electrode) in liquid results in the creation of a highly damped sinusoidal shear wave which travels through the liquid, away from the crystal. Since these waves are completely damped in a few hundred nm [24] (the exact distance depends upon the solution viscosity), the crystal senses only a thin layer of liquid. The net effect is to reduce the crystal oscillation frequency by a constant value, typically 1000-2000 Hz. Assuming the density and viscosity of the solution are constant, this initial frequency decrease can be treated as an offset and any processes which result in additional changes in crystal mass can then be studied. These processes include film formation [25-281 or dissolution, and the motion of ions [27,28] and solvent [28] into and out of a film. Prussian Blue electrodeposition The quartz microbalance was used to monitor the rate of deposition of Prussian Blue from a solution of ferric and ferricyanide ions. The electrode potential was stepped from the open circuit value (about 0.73 V) to 0.5 V, and the crystal frequency monitored concurrently with the deposition current. Figure 1 is the resulting plot of mass gain per unit area vs. charge density, with a least squares straight line drawn through the data. The molar mass of the deposited Prussian Blue
Fig. 1. Mass gain/unit area vs. charge plot for electrodeposltion of a Prussian Blue onto a quartz crystal electrode in 0.1 M KCI, 2 mM FeCI,, 2 mM K,Fe(CN),, pH 2, at 0.5 V vs. SCE. f, = 5.962.212 Hz. The line is a least squares fit.
217
can be determined from the slope of the line and was found to be 643.5 g/mol. The molar mass of Fe,(Fe(CN),), * 6 H,O (deposition involves reduction of the three ferricyanide Fe(II1) atoms) is 361.2 g/mol, while that of KFe.JCN), is 331 g/mol. These calculated molar masses are much smaller than the values measured in situ during deposition. This disagreement is probably due to water incorporated in the film. The water molecules may occupy specific sites in the Prussian Blue crystal lattice, or they may lie in “pools” of solvent in pores within the film or in pockets created by surface roughness. (Any water in surface pockets or pores in the film behaves as an elastic mass [30]). Ellipsometric data [31] indicate that growing Prussian Blue films are highly hydrated, and that the degree of hydration is dependent upon the preparation conditions. After removal from solution and equilibration in dry nitrogen, the mass of the Prussian Blue film was recalculated from the difference between pre- and post-deposition crystal frequencies, the dry molar mass was found to be 327.9 g/mol, in relatively good accord with the values for Fe,(Fe(CN),), .6 H,O (361.2 g/mol), and KFe,(CN), (331 g/mol). (We hesitate to use the molar mass to distinguish between the two probable film stoichiometries, since the efficiency of the electrodeposition process is unknown. If some reduced material escapes from the electrode surface without crystallizing, then an erroneously low molar mass will be calculated.) The initial segment of Fig. 1 (low charge, low mass) is somewhat concave, indicating that the first layers of deposited Prussian Blue are more compact (containing less interstitial and/or defect occupying solvent) than subsequent layers. The linearity of Fig. 1 at long times indicates that the Prussian Blue film is elastic (non-viscous). Thus, there should be little damping of the crystal shear wave in the film, and the use of eqn. (5) is justified. Lattice rec~ns~~e~ionupon initiaireduction It has been proposed [13] that the electrochemical reduction of the Prussian Blue crystal is accompanied by an irreversible structural change from Fe,+(Fe(CN),), .6 H,O to MFez(CN),. In K+ containing electrolyte, the cyclic voltammetric reductive waveshape is drastically changed ( Erwhmis much reduced) from the first to subsequent scans. Lundgren and Murray have observed [22] the expulsion of Fe atoms from a Prussian Blue film during the initial reduction, by me~urement of the optical absorption of Fe(bpy), (bpy is 4,4’-bipy~dine) formed by chelation of the expelled Fe with bpy molecules in the supporting electrolyte solution. Assuming that low spin (C-coordinated) Fe atoms are conserved in this reorganization, a possible reaction stoichiometry is as follows (ignoring interstitial water molecules): Fe,(Fe(CN),),
.6 H,O
-Fe3++6 Hz013 MFe,(CN), +3 M+
The net mass change observed upon reductive cycling should therefore depend upon the mass of M+, the electrolyte cation. Mass changes after reduction and subsequent reoxidation by the first cyclic voltammetric scan were measured for two different Prussian Blue films, in K+ and Rb+. This measurement was complicated by the tendency of the frequencies of
some film coated crystals to drift slowly upon immersion in solvent. The drift generally ceased after a few voltammetric scans. However, measurements of mass change upon the initial reduction and reoxidation were performed only on crystals whose frequency stabilized immediately after immersion in electrolyte. In the KNO, electrolyte, (initial dry film weight = 14.71 pg/cm2), almost no net mass change is predicted (- 0.045 pg/cm*), and little was observed (0.037 pg/cm2) upon the initial redox cycle. (This change approaches the frequency detection limit.) In RbNO, (initial dry film weight = 10.0 pg/cm2), the mass gain predicted by eqn. (6) is 0.897 pg/cm2, while the observed mass gain was 0.574 pg/cm*. These numbers suggest some (although inconclusive) support for lattice reorganization upon reduction. The reorganization may be less complete than suggested by eqn. (6) protons may be co-incorporated with alkali-metal cations, or the extent of hydration (interstitial water content) may change during structural reorganization. Stronger evidence for the incorporation of alkali metal cations (and protons) into Fe(III),Fe(II) Prussian Blue films during electrochemical cycling will be presented below. Cyclic voltammetty in K +, Rb+, and Cs ’ nitrates, pH 4 Figure 2 illustrates cyclic voltammetry of a Prussian Blue film (curve A) with concurrent crystal oscillation frequency (curve B) in 0.5 M, pH 4 KNO, solution. (Approximate film thickness was 113 nm, based on a density [19] of 1.5 g/cm3.) Both oxidative and reductive waves are observed and both processes are reversible. The reduction of the Prussian Blue occurs at approximately 0.2 V and is accompanied by a simultaneous decrease in the frequency of the oscillator indicating an increase in the mass of the film. This drop in frequency is reversible upon reoxidation of the film. Prussian Blue can also be reversibly oxidized and this is observed at about 0.9 V. Again, as in the case of Prussian Blue reduction, oxidative current is accompanied by an increase in the frequency of the oscillator (mass decrease) which returns to its original value after the subsequent rereduction. This behavior suggests strongly that cations (probably K+) are present in the Fe(III),Fe(II) film and that they exit the film (to satisfy electroneutrality) upon oxidation of the film and enter upon reduction. This point will be discussed quantitatively below. Figures 3 and 4 illustrate the analogous experiment in 0.5 M RbNO, and CsNO, (pH = 4) respectively. The cyclic voltammetric potential limits were adjusted to account for differences in E,,* for Prussian Blue reduction, which (along with E fwhm) is strongly cation dependent. Again, the observed mass changes are highly reversible, reduction is accompanied by a mass gain, and oxidation by a mass loss. The failure of the crystal to return to precisely its original value could be due to a number of factors: (1) frequency drift due to temperature instability, (2) a slight oxidation of the Au electrode at 1 V, or (3) an actual hysteresis in the film oxidation and rereduction. In any event, the discrepancy is small, and the correlation between current flow and film mass change is obvious. The molar mass of the ion which enters or exits the film upon charging can be
219
-80 -100
E/ V(vs
: 10
08
06
AgIAgCI) 0.4
02
0
-0
2
100 50 3
_
0 -50 -100 -150
Fig. 2. Cyclic voltammetric (A) and potential-frequency (B) curves for Prussian crystal electrode in 0.5 M KNO,, pH 4. v = 10 mV/s, f0 = 5,817,409 Hz.
Blue coated quartz
acquired from the slope of a plot of the integrated charge from the cyclic voltammograms (summed over a few selected potential intervals) vs. the film mass change (obtained from eqn. 5). Figure 5 is such a plot for Prussian Blue reductive voltammetry in K+, Rb+, and Cs+ electrolyte (from Figs. 2, 3, and 4). Linear least squares lines fit to the data have near zero intercepts, and fit the data well, indicating that a single charge carrier (or an unchanging ratio of multiple charge carriers) is responsible for compensation of the injected negative charge in any given electrolyte. In Table 1, columns 2 and 3, the apparent molar masses of the charge compensating cations (obtained from the slopes of Fig. 5) are compared with the molar masses of K+, Rb+, and Cs+. The apparent molar masses scale with cation molar masses, and are in each case a little lower (11.1-17.5 g/mol). Thus, electrochemical reduction of Prussian Blue films in pH 4 electrolyte is accompanied by a nearly stoichiometric incorporation of alkali metal cations. This is an unsurprising result, since both of the proposed Prussian Blue structures, Fe,(Fe(CN),), - 6 H,O and MFe,(CN),, require cation entrance upon reduction (eqns. (1) and (3), respectively). The systematic deviation of the apparent equivalent weights from the cation molar masses is interesting. The nearly constant absolute error (as opposed to a
220
100 -
i:ll ;,
.-IO0
08
10
0.4
06
0.2
80 60 40
-
20 -
Q -i -
o-
-60 Fig.
3.
-
Cyclic voltammetric (A) and ~tential-frequency
(B) curves for Prussian Blue coated quartz
crystal electrode in 0.5 M RbNO,, pH 4. u = 10 mV/s, lo = 5,981,470 Hz.
E /V(vs
-15OL 10
40 F
08
Ag/AgCI) 0.6
04
02 A
Fig. 4. Cyclic voltammetric (A) and potential-frequency (B) curves for Prussian Blue coated quartz crystal electrode in 0.5 M CSNO~, pH = 4. u = 10 mV/s, f. = 4,893,657 Hz.
221
4
n
4
2
-0
6
Charge / mC cni2 Fig. 5. Mass/unit area vs. charge plots for reduction of F’russian Blue films coated onto quartz crystal elwtrodes in KNO, (A), RbN03 (B), and CsNO, (C), cakulated from F&s. 2, 3, and 4, respectively. Lines are least squares fits to the data.
constant relative error) argues against a systematic measurement error (such as underestimation of electrode area or damping of the shear wave in the outer layers of the film). There are two plausible explanations for this systematic deviation. First, a cation which enters the Prussian Blue cage upon reduction may force some amount of water to exit the cage. If each alkali metal cation replaced one water molecule, the measured molar mass would be equal to the molar mass of the alkali metal cation, minus that of water (18 g/mol). This explanation accounts well for the observed deviation (Table 1, columns 2 and 3). An alternate explanation is that alk~-Mets cation inclusion is ~ornp~~ by proton inclusion. The measured molar mass would then be a weighted average of proton and alkali metal cation equivalent weights. To test this possibility, the pH dependency of the mass changes accompanying electrochemical cycling (in RbNO,) of Prussian Blue films was measured (see below). Plots of charge density vs. mass change/unit area were also constructed for the oxidation of Prussian Blue in KNOS, RbNO,, and CsNO,, pH 4 (Figs. 2, 3, and 4).
TABLE
1
Molar masses of charge compensating Electrolyte
’
KN03 RbNOs CsNO, ’ 0.5 M, pH b Supporting ’ From Fig. d From Fig.
ions in various electrolytes
Molar mass b /g mol-’
Reductive moIar mass ’ /g moi-’
Oxidative mofar mass d /g mol-’
39.4 85.5 132.9
23.3 68.0 121.8
23.5 29.2 57.4
4. electrolyte cation moIecular weight. 5. 6.
222
1 Charge /mC
2
3
cd
Fig. 6. Mass/unit area vs. charge plots for oxidation of Prussian Blue films coated onto quartz crystal electrodes in KN03 (A), RbNO, (B), and CsNO, (C), calculated from Figs. 2, 3, and 4, respectively. Lines are least squares fits to the data.
The plots (Fig. 6) are linear (with the exception of curve B); however, the slopes do not scale with cation molar mass like those for reduction (Fig. 5). The apparent molar masses of the exiting species are listed in Table 1, column 4. For K+, the molar mass of the species entering upon reduction, and exiting upon oxidation are almost identical, indicating that charge compensation during electrochemical charging (both oxidative and reductive) in 0.5 A4 KNO,, pH 4, is primarily accomplished by K+ exit and entrance. This observation correlates nicely with the observed [2] Nemstian (59 mV/decade) dependence of both the reductive and oxidative formal potentials on [K+] in KC1 electrolyte. For Prussian Blue oxidation in RbNO, and CsNO,, pH = 4 (curves B and C) the situation is more complicated. Firstly, some curvature is observed in curve B (voltammetry in RbNO,). There are two likely sources for this discrepancy. First, the nature of the charge compensating ion may be potential dependent, and liable to change according to the partial degree of oxidation of the Prussian Blue. Another possibility is that, at the voltammetric scan rates employed (10 mV/s), the film oxidation state lags the applied electrode potential. Then, charge which is assumed to result in Prussian blue oxidation (to the Fe(III),Fe(III) form) would in reality go towards reoxidizing the reduced [Fe(II),Fe(II)] form. Since charge compensation for the Prussian Blue oxidation and reduction apparently involves different mixtures of cations (see below), this could account for the curvature observed in curve B. A least squares straight line was drawn through the data in order to calculate the weight of the charge compensating ion (8); this line is dashed in deference to the apparent non-linearity of the data. For oxidation in Rb+ and Cs+, a mass loss is observed, indicating that oxidation results in some exit of alkali metal cations, but the mass loss is considerably smaller than that expected from the molar mass of the alkali metal ions (Table 1). There are three likely origins of this systematic discrepancy between the measured molar masses and the molar masses of the alkali metal cations for the mass losses
223
accompanying oxidation: (1) alkali metal cation exit is accompanied by water molecule entrance, (2) protons and alkali metal cations are simultaneously transported out of the film, or (3) NO; anions move into the film as alkali metal cations exit. Of course, more than one of these processes may operate simultaneously. To discriminate between the above possibilities, the pH dependence of electrochemically induced film mass changes was studied. Charge neutralization by proton transport in Prussian Blue films The pH dependence of the film mass change (both oxidative and reductive) was studied in 0.5 A4 RbNO, electrolyte, in which the pH was varied from 2.7 to 6.4. RbNO, was chosen since the molar mass (29.2 g/mol) of the species exiting the film during oxidation in this medium (at pH = 4) was found to be much less than the molar mass of Rb+ (85.5 g/mol). This leads to the conjecture that either anion inclusion or proton co-exclusion (or both) must play an important role in the oxidative electrochemistry of Prussian Blue in RbNO,. The lower limit of the pH range (2.7) was set to deter H+ reduction during the reductive scan, the upper limit (6.4) to prevent formation of Fe(OH),. The pH dependency data is summarized in Figs. 7 and 8 and Table 2. Figures 7 and 8 are charge density vs. mass change/unit area plots obtained for the Prussian Blue reduction and oxidation, respectively, at various pHs. Again, the plots have near zero intercepts and are fairly linear. The measured molar masses of the charge compensating species for reduction (Fig. 7) at various pHs are listed in Table 2, column 2. At pH 2.7, the molar mass of the entering species (51.8 g/mol) is considerably smaller than the molecular weight of Rb+ (85.5 g/mol), or the molar masses obtained in less acidic solutions. Reduction of Prussian Blue in acidic (pH = 2.7) RbNO, appears to involve proton co-inclusion. The molar masses at pH 4.6 (69.4 g/mol) and pH 6.4 (63.4 g/mol) are, within the probable limits of
Fig. 7. Mass/unit area vs. charge plots for reduction of a Prussian Blue film coated onto a quartz crystal electrode in 0.5 M RbNO,, pH 2.7 (A), pH 4.6 (B), and pH 6.4 (C). o = 10 mV/s, f,, = 5.038.459 Hz. Lines are least squares fits to the data.
224
8-
1 Charge]
2
3
mC cni2
Fig. 8. Mass/unit area vs. charge plots for oxidation of a &ssian Blue film coated onto a quartz crystal electrode in 0.5 M RbNO,, pH 2.7 (A), pH 4.6 (B), and pH 6.4 (C). u = 10 mV/s, f. = 5,038,459 Hz. Lines are least squares fits to the data.
experimental error, indistinguishable. Since the measured molar mass shows no pH dependence over the pH interval 4.0 to 6.4, the deviation between these measured molar masses and Rb+ molar mass (85.5 gfmol} should probably be attributed to water exclusion accompanying alkali-metal cation exclusion. However, proton co-inclusion, even at moderate to high pH, cannot be strictly ruled out. The effect of pH on Prussian Blue film mass change during oxidative voltammetry is more striking. The charge density vs. mass loss/unit area plots for oxidation in RbNO, at various pHs are shown in Fig. 8, and the measured molar masses are given in Table 2, column 3. The molar mass of the exiting ion varies widely and systemati~lly with pH, from 15.2 g/mol (pH 2.7) to 60.4 g/mol (pH 6.4). At low pH (2.7), proton exit is apparently the primary charge compensation mechanism during film oxidation. At pH 6.4, the measured molar mass (60.4 g/mol) approaches that of Rb+ minus a water molecule (67.5 g/mol), suggesting that proton transport is not very important at this pH, and that extra water molecules probably occupy the sites vacated by exiting alkali metal cations. In summary, in RbNO, electrolyte, charge compensation during Prussian Blue oxidation is apparently accomplished entirely by proton and Rb’ exclusion, with attendant water molecule
TABLE 2 Molar masses of charge compensating
ions III 0.5 M RbNO,
at various pHs
PH
Reductive molar mass ’ /g mot-’
Oxidative molar mass ’ /g mol-’
2.7 4.6 6.4
51.8 69.4 63.4
15.2 31.9 60.4
’ From Fig. 7. b From Fig. 8.
225
inclusion. NO; inclusion need not be invoked to explain any aspect of Prussian Blue electrochemistry in RbNO, or KNO,. Protons and cations are ejected upon oxidation; apparently the population of protons in the Fe(III),Fe(II) film scales inversely with the pH of the contacting solution. Dependence of vo~tammet~c features on pH The cyclic voltammetric E,,2 values for Prussian Blue oxidation and reduction in RbNO, also exhibit a pH dependency. For reduction to the Fe(II),Fe(II) form, E,,, is relatively invariant at pH 6.4 to 4.6 (0.31 V), then increases to 0.35 at pH 2.7. This trend mirrors exactly the mass gain pH dependency depicted in Fig. 7, apparently indicating that proton incorporation into the Prussian Blue films occurs at more extreme (negative) potentials than Rb+ inco~oration. No systematic wave shape dependence ( Efwhm) upon pH was observed. For oxidation to the Fe(III),Fe(III) form, extraction of E,,z values is impossible due to the less than ideal voltammetric waveshape (see Fig. 3A). However, Prussian Blue oxidation does apparently become more difficult as the supporting electrolyte pH is reduced. If the oxidation “onset potential” is defined as the potential at which the oxidative current is equal to twice the ~~rnurn background value, then this “onset potential” varies systematically from pH 6.4 (0.78 V) to pH 4.6 (0.83 V) to pH 2.7 (0.91 V). Again, this trend parallels the pH dependency of oxidative mass loss shown in Fig. 8. Proton incorporation and expulsion, corresponding to Prussian Blue reduction and oxidation, respectively, are apparently less favored thermodynamically than the corresponding processes involving Rb+ cations.
The pH dependence experiment described above implies that anions play a very limited role in Prussian Blue oxidative electrochemistry. This inference was tested by a very limited study of the anion dependence of film mass changes during Prussian Blue oxidation. The oxidation, of a Prussian Blue film, was studied in 0.5 M RbNO, and 0.5 A4 RbOOCCH, (both pH 4). The charge density vs. mass loss/unit urea plot in these two media is shown in Fig. 9. (Curve A, from Prussian Blue oxidation in RbNO,, is reproduced from Fig. 6.) The plots are somewhat curved; the first increments of oxidative charge passed to the film result in greater mass losses than do subsequent charging. As discussed previously, this may be due to a change in the nature of the charge compensating species with film potential, or to some hysteresis in charging films of this thickness (ca. 100 nm) at 10 mV/s. In any case, the molar masses of the exiting species, calculated from the slopes of the dashed lines in Fig. 9, are 29.2 g/mol (NO;), and 26.7 g/mol (CH,COO-). Within experimental error, these values are equivalent. If anion inclusion occurred during Prussian Blue oxidation, then the bulky acetate ion would be expected to be less mobile than the smaller nitrate. The lessened CH,COOmobility would presumably result in a greater reliance on cation exclusion for charge compensation (ignoring for the moment the effects of proton motion), and therefore a greater film mass loss. The fact that the film mass
226
08
-
N
-
;OS 2 r
-
TJ 04
-
E
’ A (NON)
/’
1’ 9
: 2
,/
_ ,/‘.d , 1’ 02
,’ /’
,c
,’
,
//’
//’
//I=
,‘,A ’
/’
Fig. 9. Mass/unit area vs. charge plots for oxidation of a Prussian Blue film coated onto a quartz crystal electrode, in 0.5 M RbNO, (A) and 0.5 A4 RbOOCCH, (B), both pH 4. o = 10 mV/s, f0 = 5.981.470 Hz. Lines are least squares fits to the data.
loss exhibited no anion dependence supports the conclusion of the pH dependence study, that exit of alkali metal cations and protons is the primary charge compensation mechanism during Prussian Blue oxidation. We were precluded from testing the mobility of several smaN anions (such as CN- and the halide ions) in the Prussian Blue lattice by the instability of the Au electrode in such media at the potentials required to oxidize the Prussian Blue. CONCLUSIONS
Mass change measurements on voltametrically scanned Prussian Blue films indicate a change in the mass of the film (depending on electrolyte cation) after a single reduction and reoxidation, reinforcing the theory of lattice reorganization during the initial film reduction. Reduction of Prussian Blue films from alkali metal nitrate salts results in the incorporation of alkali metal cations, probable expulsion of water molecules, and under some circumstances (RbNO,, pH 2.7), co-incorporation of protons. The mass loss which consistently accompanies Prussian Blue oxidation is unequivocal evidence that Fe(IZi),Fe(II) Prussian Blue films, which have previously been cycled electrochemically in alkali metal cation electrolyte, contain quantities of those cations. Oxidation results in alkali metal cation expulsion, probable water incorporation, and, if the incorporated alkaii metal cation is bulky and immobile, proton expulsion. It is interesting to note that, in pH 4 KNO,, K+ transport dominates the electrochemistry, while in RbNO, and CsNO, (pH 4), protons play a larger role. Apparently, the importance of proton transport in these systems is due to the relative immobility of the Rb’ and Cs+ which have been incorporated into the Fe(III),Fe(II) films. However, these cations can be made to exit the film upon oxidation if the film proton population is sufficiently depressed.
227
No evidence was found for anion inclusion during Prussian Blue oxidation in NO; or CH,COO- salts. An important question is whether this behavior is general to all anions, or is due to the size of NO; and CH,COO-. Similar studies at Prussian Blue modified Pt coated quartz crystals in halide media will be required to answer this question. ACKNOWLEDGEMENT
Illuminating discussions with C. Lundgren (UNC, acknowledged.
Chapel Hill) are gratefully
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