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A novel stable electrochromic thin film: a Prussian Blue analogue based on palladium hexacyanoferrate
281
J. Electroanal. Chem., 292 (1990) 281-287 EIsevier Sequoia S.A., Lausanne
Preliminary note
A novel stable electrochromic thin film: a Prussian Blue analogue based on palladium hexacyanoferrate Mian Jiang and Zaofan Zhao Department
of Chemistry,
Wuhan University
Wuhan 430072 (P.R. China)
(Received 3 July 1990; in revised form 8 August 1990)
INTRODUCTION
Modified electrodes have received a good deal of attention over the past fifteen years, largely because they have a wide range of potential applications in electrochemical technology, particularly in chemical analysis and energy conversion, but possibly also in information storage and display. The transition metal hexacyanoferrate is an important class of the insoluble mixed polynuclear compounds which has the general formula Mt(MB(CN),),. x H,O where MA and MB are transition metals with different formal oxidation numbers and k and I represent stoichiometric subscripts. The structure of Prussian Blue was first discussed by Keggin and Miles [l] in 1936 and the physical and chemical properties of these compounds have been studied extensively [2,3]. However, the investigations concerning the electrochemical behaviour of electrodes modified with thin films of these compounds have only appeared in recent years [4]. Among these compounds, Prussian Blue (PB) has been most intensively studied [5-lo]. It can be used as electrode-coating material for electrochromic displays, as electrocatalyst, in photosensing devices, and in solid-state batteries. It seems that very little work has been done on other Prussian Blue analogue modified electrodes [ll]. Two reasonable approaches in devising novel hexacyanometalate coatings can be considered. One is to consider the mixed transition metal systems through which one could exploit available information on highly insoluble bulk material precipitates [12] and produce microstructures with novel physicochemical properties, as has been done using the reduction-electrodeposition technique in preparation of an indium hexacyanoferrate modified glassy carbon electrode [13]. The other is based on the strong affinity/adsorptivity of hexacyanometalate anions on the naked surface of a transition metal substrate (sometimes accompanied by a precipitation reaction) such as nickel wire [14]. In the study of electrochemical behaviour of the palladium electrode, we found that the immersion of a freshly-abraded palladium 0022-0728/90/$03.50
0 1990 - Elsevier Sequoia S.A.
282
wire into an acidic solution containing potassium hexacyanoferrate(II1) produces a stable surface-bound electroactive system appearing in potassium sulfate solution, although the current signal of the redox couple (- 0.70 V vs. SCE) is weak. To examine this phenomenon further, cyclic voltammetric scans of some conducting substrates (glassy carbon, platinum, etc.) were applied in a mixed solution containing both palladia chloride and potassium hexa&y~ofe~at~III) and the formation of the expected surface-bound electroactive films was observed. The resulting palladium hexacyanoferrate thin film (abbreviated as PdHCF herein and after) is extremely stable in solutions containing potassium or hydrogen ion in the pH range O-10. It also has a rapid colour-switching property and ion-permselectivity towards hydrogen or potassium ion which permits its potential application in electrochrornism. We wish to present here our preliminary results.
All precursor materials (PdC12 from Wuhan Smelting Factory, K,Fe(CN), and K,Fe(CN), from Tianjing Chemical Reagent Factory) were of analytical-reagent grade and used without further purification. Solutions were prepared with triply distilled water, the final distillation was made from potassium permanganate. Glassy carbon slabs of 2 mm diameter and ca. 20 mm length (The Artifical Crystal M~ufactu~ng Institute of Beijing) were fixed in a Teflon tube, and the surface (0.0314 cm*) to be exposed to the solution was polished to a mirror-like finish using 0.5 pm alumina. The polished working electrode was cleaned ultrasonically in 1 M NaOH solution, alcohol, and water successively, and was then pretreated electrochemically in 1 M H,SO, solution by cyclic voltammetric scanning in the potential range of -0.2 to 1.1 V vs. SCE until a steady state cyclic voltammogram would appear. The same procedure as for glassy carbon was used for platinum disk (diameter 0.5 mm) and gold disk (diameter 3 mm) substrates, respectively. All experiments employed a one compartment el~tr~he~cal cell, a platinum foil counter electrode and a SCE reference electrode. All potentials cited in this text are referred to SCE. Cyclic voltammetry and potentiostatting was performed using a Princeton Applied Research Model 173 potentiostat/galvanostat with a Model 175 universal programmer. The ex-situ reflection FT-IR measurements were performed on a glassy carbon substrate (0 = 10 mm) using a Nicolet 170 SX FT-IR spectrophotometer (Nicolet Instrumental Co., Wisconsin) equipped with a MCT detector. A Herrick Scientific Corporation diffuse reflectance cell was placed inside the spectrometer to collect the reflection radiation. Thirty-two scans were co-added at a resolution of 4 cm-l. Although the PdHCF thin film could be formed simply by dipping conducting substrates (Ir, Pd, Au, Pt, and glassy carbon) into the mixed solution containing PdCl, and K,Fe(CN), for at least one hour, the highly reproducible surfaces with known coverage have been obtained by potential cyclic scans of these substrates in the modification solution.
283
In general, the electrode modification did not require very careful pretreatment of the glassy carbon substrate in the “dipping” and “reductive electrodeposition” experiments. Unless otherwise stated, the electrode modification with PdHCF film was accomplished by cycling the potential (at 100 mV s-‘) of a glassy carbon electrode starting from 1.0 to 0 V for 5-30 min in the solution mixture of 5 mM PdCl,, 5 mM K,Fe(CN),, 0.5 M KCl, and 1 M HCI. Freshly prepared, de-aerated solutions are strongly re~~ended for the modification. By varying the number of CV scans, different film thicknesses from ca. 10e9 to low7 mol cm-* can be obtained. The analogue approach can be applied for the modification of other conducting substrates such as gold, platinum, iridium, graphite, and tin oxide-coated glass slides. All experiments were conducted at room temperature. Although prepurified nitrogen gas was used for deaeration of solutions, no influence of oxygen from the air could be detected in the potential range O-l.0 V. RESULTS AND DISCUSSION
The PdHCF thin film modified electrode was characterized by cyclic voltammetry and optical spectroscopy. Representative voltammograms for PdHCF film are shown in Fig. 1 in 1 M HCI and 1 M KC1 electrolytes containing no deliberately added electroactive material. The observation of persistent cyclic voltammetric waves provides evidence for prolonged surface attac~ent. We found that these waves are not affected by stirring the electrolyte, offering more proof that the material is irreversibly associated with the electrode surface. As a single and
E/V
“3.
SCE
Fig. 1. Cyclic voltammograms of the PdHCF film modified glassy carbon electrode in 1 M HCI (A) and 1 M KC1 (B) electrolytes. Scan rate: 100 mV s- ‘. Prepared condition as mentioned in the text (10 min).
284
well-defined redox couple (peak potential: E, = 0.70 V; EC = 0.68 V) can be observed in HCl solution (Fig. 1A) and Pd(I1) is not expected to be electroactive in the potential range studied, O-l.0 V, according to our diagnostic experiments and previous literature [15,16], the reaction should be attributed to the reversible redox behaviour of hexacyanoferrate(II1, II) in the film. In this respect, the PdHCF system resembles somewhat the Cu(II)-hexacyanoferrate(II1, II) and Cr(III)hexacyanoferrate(II1, II) microstructures [17,18]. Another unique property of PdHCF film is seen from the observation that, although the reductive-electrodeposition of palladous ions on the glassy carbon electrode in acidic media was observed to start at -0.1 V and reach the maximum at -0.2 V accompanied by great evolution of hydrogen gas, Pd(I1) immobilized in the film cannot be reduced to Pd(0) in aqueous electrolytes in the potential range of 1.2 to - 1.0 V. Therefore, PdHCF can be considered as a PB analogue in which the mixed-valent Fe(II1, II) has replaced Pd(I1). The peak currents of Fig. 1A are linearly proportional to the scan rate up to 500 mV s-l, as expected for a surface-type behaviour. This also indicates that the heterogeneous electron transfer reaction at the interface between electrode substrate and PdHCF film, the electron self-exchange reaction and the migration of counterions (H+) with the PdHCF film are fast, hence the whole redox process proceeds swiftly. At sweep rates of 5 to 500 mV s-l, the peak potentials are almost unchanged, and the peak potential separation is lo-20 mV. At higher sweep rates, a wider splitting appears; this deviation may reflect limitations associated with charge propagation in the film. The reversibility of the redox processes in PdHCF film is illustrated in another way by the ratios QJQ, of the charge consumed in the positive (a) and negative (c) sweeps, respectively. Q,/Q, remains almost equal to unity. Some observations concern the strong dependence of the surface voltammetric responses on the choice of supporting electrolytes. To provide electroneutrality during the redox processes, an unimpeded flux of the cations must be assured. Unlike the PB film which has the order of ionic permeability NH: > K+ > Cs+ > Na+> Li+> Ba’+> Ca2+> H+ [19,20], H+ was found to penetrate the PdHCF film more readily than other cations do (Fig. 1A). The peak currents of Fig. 1A increase with the concentrations of hydrochloric acid in the range 0.01-6 M whereas the mid-peak potentials shift positively from 0.57 to 0.80 V. This phenomenon is similar to that of CrHCF [18] and gives it potential as a sensor for hydrogen ions as these concentrations since a pH glass electrode generally has an “acidic deviation” in strong acidic media (pH < 2). The K+ effect on PdHCF film however, is of a complicated nature, as can be seen in Fig. 1B. The film in the potential range from 0 to 1.0 V undergoes mainly two redox transitions (labelled as peaks Pi (0.70 V) and P2 (0.58 V). Sometimes a third redox transition could be seen as a shoulder on the negative side of peaks P2c,2a depending on the modification conditions. Nevertheless, prolonged CV scans (typically scans at 100 mV s-l for 2 h) would make peaks P2 disappear whereas peaks Pr are unchanged. The peak currents of PdHCF film in 1 M KC1 are lower than those in 1 M HCl, but are higher than those in solutions of
285
other cations. We found that the electroactivity of PdHCF film is stable in supporting electrolyte containing NH:, alkali metal cation, or alkaline-earth metal cation, whereas the voltammetric peaks of the modified electrode in KC1 are sharper than in solutions of other cations. In conclusion, the ion transportation in the modified film is very sophisticated, and cannot be explained simply in terms of hydrated’ionic radii and the system’s channel size, as was done for the PB modified electrode 1211. The lack of physical and chemical data for the bulk PdHCF precipitate in previous literature also limits the clarification of the mechanism. There must be other factors, such as chemical interaction between the ion and the modified film, electrostatic factors, and polarizabilities, influencing the cation penetration in or out of PdHCF film. The cause of the order H+> K+ in permeability is not clear at present. A detailed understanding of the ion transportation in PdHCF film may be of considerable help in the study of other zeolitic, membrane and ion exchange systems. Another important feature of the data in Fig. 1B is that the two redox transitions are very symmetrical ( AEp = 0 mv), the redox behaviour of peaks Pr fits the nemstian relationship well, as it has been found that the mid-peak potential decreases by ca. 60 mV per decade of the decreasing potassium ion concentration (T = 28” C) whereas the peak current stays almost constant. On the other hand, the oxidation and reduction peaks are not perfectly symmetrical in Fig. 1A (A Ep = 20 mv), the oxidation peak is also somewhat narrower than the reduction one. The latter difference apparently results from certain kinetic notations during reduction or cation inclusion required for charge balance. The difference in breadth of the oxidation and reduction peaks is an effect also observed for organic polymer films with redox centers on electrodes [22]. It can also arise from interactions between sites or from non-equivalence of sites. As pH variations (pH O-10) and the choice of the supporting electrolyte (chloride, nitrate, sulphate, phosphate or acetate) hardly seem to play a role in PdHCF electrochemistry, the system may serve as a good model for the investigation of structural reorganization of the redox centres during the film reaction. Some attention has been paid to possible changes in PdHCF film microstructure upon prolonged use. Apart from the effect of prolonged CV scans on PdHCF film in KC1 solution as mentioned above, we found that, after deposition of the PdHCF thin film on glassy carbon, the redox response when it is used immediately in a CV experiment is different from that of the film stored in air for at least 4 h. A 30% increase of the voltammetric current was observed for the latter. Our preliminary results also show that upon further potential cycling the rate of film change decreases. After lo4 CV scans, both the cathodic and anodic peak currents in Fig. 1 decrease only slightly (typically 10%). Potential excursions up to 1.1 V and down to - 1.4 V do not result in film degradation either. The electrochromic effect of the PdHCF thin film can be observed clearly on glassy carbon under potential scan or potential step during the overall oxidation. The film is orange at potentials positive of 1.0 V, yellow-green at potentials negative of 0.2 V. Furthermore, it is possible to switch rapidly (in about 2-3 s) between the
286
#i”-‘l \ Tr-----> 3200
1600
2400
800
v /cm’
Fig. 2. Reflection FT-IR spectra of PdHCF coating in the reduced state (A) and upon oxidation (B) in a separate cell. The film was dried in a vacuum desiccator before recording the spectra.
two colour states (using the selecting switch on the potentiostat). Electrochromism appeared to be reversible during at least 200 switch steps between 1.0 and 0 V. As the process must involve penetration/ transportation of H+ or K+ of the supporting electrolyte, it can be expected that the above behaviour is consistent with fairly high ionic conductivity, making PdHCF film potentially attractive for application in a polymeric-type battery. Additional structural information was obtained from reflectance IR measurements of PdHCF films on glassy carbon slides. A typical infrared spectrum of the reduced system (Fig. 2A) clearly indicates the presence of a -CN functional group (a strong absorbance band at 2061 cm-‘), characteristic ferrocyanide peaks in the range from 400 to 800 cm-’ [23], as well as interstitial water with its response near 1650 and 3000 cmp2. What is probably even more important in the IR data is that, upon oxidation, the -CN group peak undergoes splitting (Fig. 2B); the latter result suggests cyanide bridging between Fe and Pd or stronger interaction between -CN and Pd (when compared to the reduced form); nevertheless, it implies structural changes upon film oxidation. This should also be correlated with the cation storage and its release during ferrocyanide oxidation. The above IR experiments were performed ex situ. It is noteworthy that the reduced film, both in the wet and dried state, is not oxidized by oxygen from the air; the latter factor simplifies experimental approaches, particularly spectroscopic ones.
287
Further mechanistic investigations on PdHCF and other novel PB analogues are currently under way. Full descriptions will be published later. ACKNOWLEDGEMENTS
Helpful discussions with Dr. B. Zhou of Shaoguan Qualification Institute of Chemicals and Industrial Products (Guangdong Province, P.R. China) are acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
J.F. Keggin and F.D. Miles, Nature, (1936) 577. D.F. f&river, Struct. Bonding (Berlin), 1 (1966) 32. H.E. Williams, Cyanogen Compounds, 2nd ed., Arnold, London, 1948. V.D. Neff, J. Electrochem. Sot., 125 (1978) 886. K. Itaya, T. Ataka and S. Toshima, J. Am. Chem. Sot., 104 (1982) 4767. K.P. Rajan and V.D. Neff, J. Phys. Chem., 86 (1982) 4361. F. Li and S. Dong, Electrochim. Acta, 32 (1987) 1511. K. Itaya, I. Uchida and S. Toshima, J. Phys. Chem., 87 (1983) 105. D.W. Deberry and A. Viehbeck, J. Electrochem. Sot., 130 (1983) 249. M. Kaneko and T. Okada, J. Electroanal. Chem., 255 (1988) 45. P.J. Kulesza and M. Faszynska, Electrochimica Acta, 34 (1989) 1749. I.V. Tanaev, G.B. Seifer, Yu. Ya. Kharitinov, V.G. Kuznetsov and A.P. Korol’kov, Ferrocyanide Chemistry, Nauka, Moscow, 1971 (in Russian). P.J. Kulesza and M. Faszynska, J. Electroanal. Chem., 252 (1988) 461. A.B. Bocarsly and S. Sinha, J. Electroanal. Chem., 137 (1982) 157. R.K. Astakhova and B.S. Krasikov, Vestn. Leningrad. Univ., Ser. Fiz. Khim., 4 (1969) 116. J.A. Cox, SE. Gadd and B.K. Das, J. Electroanal. Chem., 256 (1988) 199. L.M. Siperko and T. Kuwana, J. Electrochem. Sot., 130 (1983) 396. M. Jiang, X. Zhou and Z. Zhao, J. Electroanal. Chem., 287 (1990) 389. S. Dong and F. Li, J. Electroanal. Chem., 210 (1986) 31. F. Li and S. Dong, Scientia Sinica, 8 (1986) 72. K. Itaya, I. Uchida and V.D. Neff, Act. Chem. Res., 19 (1986) 162. J.C. LaCroix and A.F. Diaz, J. Electrochem. Sot., 135 (1988) 1457 and references therein. R.A. Nyquist and R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971.
J. Electroanal. Chem., 292 (1990) 281-287 EIsevier Sequoia S.A., Lausanne
Preliminary note
A novel stable electrochromic thin film: a Prussian Blue analogue based on palladium hexacyanoferrate Mian Jiang and Zaofan Zhao Department
of Chemistry,
Wuhan University
Wuhan 430072 (P.R. China)
(Received 3 July 1990; in revised form 8 August 1990)
INTRODUCTION
Modified electrodes have received a good deal of attention over the past fifteen years, largely because they have a wide range of potential applications in electrochemical technology, particularly in chemical analysis and energy conversion, but possibly also in information storage and display. The transition metal hexacyanoferrate is an important class of the insoluble mixed polynuclear compounds which has the general formula Mt(MB(CN),),. x H,O where MA and MB are transition metals with different formal oxidation numbers and k and I represent stoichiometric subscripts. The structure of Prussian Blue was first discussed by Keggin and Miles [l] in 1936 and the physical and chemical properties of these compounds have been studied extensively [2,3]. However, the investigations concerning the electrochemical behaviour of electrodes modified with thin films of these compounds have only appeared in recent years [4]. Among these compounds, Prussian Blue (PB) has been most intensively studied [5-lo]. It can be used as electrode-coating material for electrochromic displays, as electrocatalyst, in photosensing devices, and in solid-state batteries. It seems that very little work has been done on other Prussian Blue analogue modified electrodes [ll]. Two reasonable approaches in devising novel hexacyanometalate coatings can be considered. One is to consider the mixed transition metal systems through which one could exploit available information on highly insoluble bulk material precipitates [12] and produce microstructures with novel physicochemical properties, as has been done using the reduction-electrodeposition technique in preparation of an indium hexacyanoferrate modified glassy carbon electrode [13]. The other is based on the strong affinity/adsorptivity of hexacyanometalate anions on the naked surface of a transition metal substrate (sometimes accompanied by a precipitation reaction) such as nickel wire [14]. In the study of electrochemical behaviour of the palladium electrode, we found that the immersion of a freshly-abraded palladium 0022-0728/90/$03.50
0 1990 - Elsevier Sequoia S.A.
282
wire into an acidic solution containing potassium hexacyanoferrate(II1) produces a stable surface-bound electroactive system appearing in potassium sulfate solution, although the current signal of the redox couple (- 0.70 V vs. SCE) is weak. To examine this phenomenon further, cyclic voltammetric scans of some conducting substrates (glassy carbon, platinum, etc.) were applied in a mixed solution containing both palladia chloride and potassium hexa&y~ofe~at~III) and the formation of the expected surface-bound electroactive films was observed. The resulting palladium hexacyanoferrate thin film (abbreviated as PdHCF herein and after) is extremely stable in solutions containing potassium or hydrogen ion in the pH range O-10. It also has a rapid colour-switching property and ion-permselectivity towards hydrogen or potassium ion which permits its potential application in electrochrornism. We wish to present here our preliminary results.
All precursor materials (PdC12 from Wuhan Smelting Factory, K,Fe(CN), and K,Fe(CN), from Tianjing Chemical Reagent Factory) were of analytical-reagent grade and used without further purification. Solutions were prepared with triply distilled water, the final distillation was made from potassium permanganate. Glassy carbon slabs of 2 mm diameter and ca. 20 mm length (The Artifical Crystal M~ufactu~ng Institute of Beijing) were fixed in a Teflon tube, and the surface (0.0314 cm*) to be exposed to the solution was polished to a mirror-like finish using 0.5 pm alumina. The polished working electrode was cleaned ultrasonically in 1 M NaOH solution, alcohol, and water successively, and was then pretreated electrochemically in 1 M H,SO, solution by cyclic voltammetric scanning in the potential range of -0.2 to 1.1 V vs. SCE until a steady state cyclic voltammogram would appear. The same procedure as for glassy carbon was used for platinum disk (diameter 0.5 mm) and gold disk (diameter 3 mm) substrates, respectively. All experiments employed a one compartment el~tr~he~cal cell, a platinum foil counter electrode and a SCE reference electrode. All potentials cited in this text are referred to SCE. Cyclic voltammetry and potentiostatting was performed using a Princeton Applied Research Model 173 potentiostat/galvanostat with a Model 175 universal programmer. The ex-situ reflection FT-IR measurements were performed on a glassy carbon substrate (0 = 10 mm) using a Nicolet 170 SX FT-IR spectrophotometer (Nicolet Instrumental Co., Wisconsin) equipped with a MCT detector. A Herrick Scientific Corporation diffuse reflectance cell was placed inside the spectrometer to collect the reflection radiation. Thirty-two scans were co-added at a resolution of 4 cm-l. Although the PdHCF thin film could be formed simply by dipping conducting substrates (Ir, Pd, Au, Pt, and glassy carbon) into the mixed solution containing PdCl, and K,Fe(CN), for at least one hour, the highly reproducible surfaces with known coverage have been obtained by potential cyclic scans of these substrates in the modification solution.
283
In general, the electrode modification did not require very careful pretreatment of the glassy carbon substrate in the “dipping” and “reductive electrodeposition” experiments. Unless otherwise stated, the electrode modification with PdHCF film was accomplished by cycling the potential (at 100 mV s-‘) of a glassy carbon electrode starting from 1.0 to 0 V for 5-30 min in the solution mixture of 5 mM PdCl,, 5 mM K,Fe(CN),, 0.5 M KCl, and 1 M HCI. Freshly prepared, de-aerated solutions are strongly re~~ended for the modification. By varying the number of CV scans, different film thicknesses from ca. 10e9 to low7 mol cm-* can be obtained. The analogue approach can be applied for the modification of other conducting substrates such as gold, platinum, iridium, graphite, and tin oxide-coated glass slides. All experiments were conducted at room temperature. Although prepurified nitrogen gas was used for deaeration of solutions, no influence of oxygen from the air could be detected in the potential range O-l.0 V. RESULTS AND DISCUSSION
The PdHCF thin film modified electrode was characterized by cyclic voltammetry and optical spectroscopy. Representative voltammograms for PdHCF film are shown in Fig. 1 in 1 M HCI and 1 M KC1 electrolytes containing no deliberately added electroactive material. The observation of persistent cyclic voltammetric waves provides evidence for prolonged surface attac~ent. We found that these waves are not affected by stirring the electrolyte, offering more proof that the material is irreversibly associated with the electrode surface. As a single and
E/V
“3.
SCE
Fig. 1. Cyclic voltammograms of the PdHCF film modified glassy carbon electrode in 1 M HCI (A) and 1 M KC1 (B) electrolytes. Scan rate: 100 mV s- ‘. Prepared condition as mentioned in the text (10 min).
284
well-defined redox couple (peak potential: E, = 0.70 V; EC = 0.68 V) can be observed in HCl solution (Fig. 1A) and Pd(I1) is not expected to be electroactive in the potential range studied, O-l.0 V, according to our diagnostic experiments and previous literature [15,16], the reaction should be attributed to the reversible redox behaviour of hexacyanoferrate(II1, II) in the film. In this respect, the PdHCF system resembles somewhat the Cu(II)-hexacyanoferrate(II1, II) and Cr(III)hexacyanoferrate(II1, II) microstructures [17,18]. Another unique property of PdHCF film is seen from the observation that, although the reductive-electrodeposition of palladous ions on the glassy carbon electrode in acidic media was observed to start at -0.1 V and reach the maximum at -0.2 V accompanied by great evolution of hydrogen gas, Pd(I1) immobilized in the film cannot be reduced to Pd(0) in aqueous electrolytes in the potential range of 1.2 to - 1.0 V. Therefore, PdHCF can be considered as a PB analogue in which the mixed-valent Fe(II1, II) has replaced Pd(I1). The peak currents of Fig. 1A are linearly proportional to the scan rate up to 500 mV s-l, as expected for a surface-type behaviour. This also indicates that the heterogeneous electron transfer reaction at the interface between electrode substrate and PdHCF film, the electron self-exchange reaction and the migration of counterions (H+) with the PdHCF film are fast, hence the whole redox process proceeds swiftly. At sweep rates of 5 to 500 mV s-l, the peak potentials are almost unchanged, and the peak potential separation is lo-20 mV. At higher sweep rates, a wider splitting appears; this deviation may reflect limitations associated with charge propagation in the film. The reversibility of the redox processes in PdHCF film is illustrated in another way by the ratios QJQ, of the charge consumed in the positive (a) and negative (c) sweeps, respectively. Q,/Q, remains almost equal to unity. Some observations concern the strong dependence of the surface voltammetric responses on the choice of supporting electrolytes. To provide electroneutrality during the redox processes, an unimpeded flux of the cations must be assured. Unlike the PB film which has the order of ionic permeability NH: > K+ > Cs+ > Na+> Li+> Ba’+> Ca2+> H+ [19,20], H+ was found to penetrate the PdHCF film more readily than other cations do (Fig. 1A). The peak currents of Fig. 1A increase with the concentrations of hydrochloric acid in the range 0.01-6 M whereas the mid-peak potentials shift positively from 0.57 to 0.80 V. This phenomenon is similar to that of CrHCF [18] and gives it potential as a sensor for hydrogen ions as these concentrations since a pH glass electrode generally has an “acidic deviation” in strong acidic media (pH < 2). The K+ effect on PdHCF film however, is of a complicated nature, as can be seen in Fig. 1B. The film in the potential range from 0 to 1.0 V undergoes mainly two redox transitions (labelled as peaks Pi (0.70 V) and P2 (0.58 V). Sometimes a third redox transition could be seen as a shoulder on the negative side of peaks P2c,2a depending on the modification conditions. Nevertheless, prolonged CV scans (typically scans at 100 mV s-l for 2 h) would make peaks P2 disappear whereas peaks Pr are unchanged. The peak currents of PdHCF film in 1 M KC1 are lower than those in 1 M HCl, but are higher than those in solutions of
285
other cations. We found that the electroactivity of PdHCF film is stable in supporting electrolyte containing NH:, alkali metal cation, or alkaline-earth metal cation, whereas the voltammetric peaks of the modified electrode in KC1 are sharper than in solutions of other cations. In conclusion, the ion transportation in the modified film is very sophisticated, and cannot be explained simply in terms of hydrated’ionic radii and the system’s channel size, as was done for the PB modified electrode 1211. The lack of physical and chemical data for the bulk PdHCF precipitate in previous literature also limits the clarification of the mechanism. There must be other factors, such as chemical interaction between the ion and the modified film, electrostatic factors, and polarizabilities, influencing the cation penetration in or out of PdHCF film. The cause of the order H+> K+ in permeability is not clear at present. A detailed understanding of the ion transportation in PdHCF film may be of considerable help in the study of other zeolitic, membrane and ion exchange systems. Another important feature of the data in Fig. 1B is that the two redox transitions are very symmetrical ( AEp = 0 mv), the redox behaviour of peaks Pr fits the nemstian relationship well, as it has been found that the mid-peak potential decreases by ca. 60 mV per decade of the decreasing potassium ion concentration (T = 28” C) whereas the peak current stays almost constant. On the other hand, the oxidation and reduction peaks are not perfectly symmetrical in Fig. 1A (A Ep = 20 mv), the oxidation peak is also somewhat narrower than the reduction one. The latter difference apparently results from certain kinetic notations during reduction or cation inclusion required for charge balance. The difference in breadth of the oxidation and reduction peaks is an effect also observed for organic polymer films with redox centers on electrodes [22]. It can also arise from interactions between sites or from non-equivalence of sites. As pH variations (pH O-10) and the choice of the supporting electrolyte (chloride, nitrate, sulphate, phosphate or acetate) hardly seem to play a role in PdHCF electrochemistry, the system may serve as a good model for the investigation of structural reorganization of the redox centres during the film reaction. Some attention has been paid to possible changes in PdHCF film microstructure upon prolonged use. Apart from the effect of prolonged CV scans on PdHCF film in KC1 solution as mentioned above, we found that, after deposition of the PdHCF thin film on glassy carbon, the redox response when it is used immediately in a CV experiment is different from that of the film stored in air for at least 4 h. A 30% increase of the voltammetric current was observed for the latter. Our preliminary results also show that upon further potential cycling the rate of film change decreases. After lo4 CV scans, both the cathodic and anodic peak currents in Fig. 1 decrease only slightly (typically 10%). Potential excursions up to 1.1 V and down to - 1.4 V do not result in film degradation either. The electrochromic effect of the PdHCF thin film can be observed clearly on glassy carbon under potential scan or potential step during the overall oxidation. The film is orange at potentials positive of 1.0 V, yellow-green at potentials negative of 0.2 V. Furthermore, it is possible to switch rapidly (in about 2-3 s) between the
286
#i”-‘l \ Tr-----> 3200
1600
2400
800
v /cm’
Fig. 2. Reflection FT-IR spectra of PdHCF coating in the reduced state (A) and upon oxidation (B) in a separate cell. The film was dried in a vacuum desiccator before recording the spectra.
two colour states (using the selecting switch on the potentiostat). Electrochromism appeared to be reversible during at least 200 switch steps between 1.0 and 0 V. As the process must involve penetration/ transportation of H+ or K+ of the supporting electrolyte, it can be expected that the above behaviour is consistent with fairly high ionic conductivity, making PdHCF film potentially attractive for application in a polymeric-type battery. Additional structural information was obtained from reflectance IR measurements of PdHCF films on glassy carbon slides. A typical infrared spectrum of the reduced system (Fig. 2A) clearly indicates the presence of a -CN functional group (a strong absorbance band at 2061 cm-‘), characteristic ferrocyanide peaks in the range from 400 to 800 cm-’ [23], as well as interstitial water with its response near 1650 and 3000 cmp2. What is probably even more important in the IR data is that, upon oxidation, the -CN group peak undergoes splitting (Fig. 2B); the latter result suggests cyanide bridging between Fe and Pd or stronger interaction between -CN and Pd (when compared to the reduced form); nevertheless, it implies structural changes upon film oxidation. This should also be correlated with the cation storage and its release during ferrocyanide oxidation. The above IR experiments were performed ex situ. It is noteworthy that the reduced film, both in the wet and dried state, is not oxidized by oxygen from the air; the latter factor simplifies experimental approaches, particularly spectroscopic ones.
287
Further mechanistic investigations on PdHCF and other novel PB analogues are currently under way. Full descriptions will be published later. ACKNOWLEDGEMENTS
Helpful discussions with Dr. B. Zhou of Shaoguan Qualification Institute of Chemicals and Industrial Products (Guangdong Province, P.R. China) are acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
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