Observation of Thin Layer Diffusion during Electro-Oxidation of

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A New Zn-NiHCF Hybrid Electrochemically Formed on Glassy Carbon: Observation of Thin Layer Diffusion During Electro-oxidation of Hydrazine Alam Venugopal Narendra Kumar, and James Joseph J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 Oct 2014 Downloaded from http://pubs.acs.org on October 23, 2014

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The Journal of Physical Chemistry

A New Zn-NiHCF Hybrid Electrochemically formed on Glassy Carbon: Observation of Thin Layer Diffusion during Electro-Oxidation of Hydrazine Alam Venugopal Narendra Kumar† and James Joseph*†

†Electrodics and Electrocatalysis Division, CSIR-Central Electrochemical Research Institute,

Karaikudi-630 006, India.

ACS Paragon Plus Environment

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ABSTRACT

Electrochemical modification of electrodes with thin films of metal hexacyanoferrates (MHCF) enhances the interfacial properties such as electrocatalysis, electrochromism, ion transport properties. Attempts to prepare hybrids of metal hexacyanoferrates along with Au nano particles or by substituting the structure of MHCF with another metal ion were reported from various research groups including us. In this work, we report modification of Glassy carbon with a new single phase hybrid Zn-NiHCF (Hexacyanoferrate prepared in presence of Zn2+ and Ni2+) electrochemically for the first time.

The hybrid film was characterized by a single well

reversible redox process centered at 450 mV. The effect of the zinc and nickel ions in the deposition bath on the effectiveness of modification was studied in detail. The modified hybrid films were characterized by voltammetry, chrono amperometry, field emission scanning electron microscopy, X-ray diffractometry etc.,. The modified films were used for the electro-oxidation of hydrazine. The deviation from the expected behavior of the hybrid porous films provides evidence for the thin layer mass transport prevailing in the film. Thin layer diffusion behavior in MHCF films for the electrocatalytic oxidation of hydrazine is reported for the first time. The electro-oxidation of hydrazine appears to take place on the glassy carbon (GC) surface. The need for caution in analyzing the electrocatalytic properties of MHCF interfaces is reiterated.

Keywords: Modified electrodes, Hybrid hexacyanoferrate, Thin layer diffusion, Chronoamperometry, Electrocatalysis.


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Introduction: Metal hexacyanoferrates (MHCF (M= Fe, Ni, Co, Ag, Zn, Cu etc.,)) are compounds that generally forms a face centered cubic (FCC) crystal structure with a fixed arrangement of two metal atoms, one linked with carbon and other with nitrogen of the cyanide ligand.

Both

physical and chemical properties of MHCF were greatly decided by the metal ions (Fe, Ni, Cu, Co, Zn etc) that coordinate to –CN and –NC. Owing to the simple preparation procedure involved in MHCF synthesis, many research findings were documented on MHCF properties in various applications like ion sieving, electrochemical bio sensors, battery, electrochromic devices.1–7 MHCF are widely used for electroanalytical purpose due to its endowed redox potentials that varies with metal atoms sharing FCC lattices. The formal redox potentials of MHCF are mainly depend on the metal atom coordinated to -NC and its composition in the crystal structure. Attempts have been made to tune the formal redox potential of Prussian blue (PB) by incorporating nickel ions in PB lattice by both chemical and electrochemical methods.8,9 Similar electrochemical characteristics were also observed in NiHCF films prepared in different mole fractions of Pd2+ and Ni2+ ions in the modifying mixture.10

Dostel et al., have

demonstrated the lattice reconstruction of PB films on cycling the modified film in Cd ion solution.11 Further, interesting observation by James et al., have revealed that FeHCF could be converted to CuHCF either partly or fully by the electrochemically cycling of FeHCF films in Cu2+ containing solution.12

It is interesting to see that the NiHCF formed on Ni substrate

possess one redox center corresponding to the surface confined redox reaction of ferro/ferricyanide.13 As mentioned earlier, the formal potentials of MHCF are greatly influenced by its stoichiometry. NiHCF shows two sets of redox peaks on GC when prepared by potential cycling method in K+ medium with formal potentials closer to each other. The reason for the


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appearance of the two reversible redox peaks is due to the two different stoichiometric forms present in NiHCF namely, (KNi1.5[Fe(CN)6] and K2Ni[Fe(CN)6]) when prepared by potential cycling method.14,15 Likewise, electrochemically deposited films of CoHCF having two different stoichiometric forms show one anodic and two cathodic peaks (quasi reversible behaviour).16 Efforts have been taken for preparing individual phase pure forms of NiHCFs by choosing the right Fe and Ni precursors under predefined experimental conditions.17 The work by Chen et al., help out to distinguish the formal potential of Ni1.5[Fe(CN)6] and K2Ni[Fe(CN)6] forms. They also explored the stability of two stoichiometric forms and its electrocatalytic activity towards H2O2 oxidation/ reduction. The Chen et al., have identified conditions for synthesizing two compositionally different forms of NiHCF electrochemically. Chemically synthesized NiHCF, from either ferrocyanide or ferricyanide precursors exhibit distinctly different redox potentials. Recently, Boopathi et al., have demonstrated that metallic Ni impurities present in the single walled carbon nanotube (SWCNT) act as nucleating centre for ZnHCF formation.18 In the later work they claim that ZnHCF formation was occurred only on the impure SWCNT modified GC surface. However ZnHCF formation was not observed when SWCNT were allowed for strong acid wash (to remove metal impurities). In the work described in this paper, we have investigated on the effect of added impurity Ni2+ during GC electrode modification with ZnHCF. Here the electrochemical properties of MHCF prepared in presence of Ni2+ and Zn2+ (equimolar concentration) is entirely new hybrid analogue. Single phase hybrid consisting of Zn2+ and Ni2+ ions is characterized by a single redox peak.

The hybrid modified electrodes were

electrochemically characterized and compared with both NiHCF and ZnHCF film modified electrodes. Our study reveals that the formal potential of the Zn-NiHCF hybrid is totally unique from the other two individual metal hexacyanoferrates namely, NiHCF and ZnHCF. X-ray


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diffraction studies and voltammetric studies of chemically synthesized compounds were used to support our results. Morphology of Zn-NiHCF was also compared with the individual metal analogues using FESEM micrographs. Stability and electrochemical response of Zn-NiHCF films were investigated and compared with pristine NiHCF films in different cations (K+, Na+ H+) containing electrolytes. Electron catalytic properties of the hybrid film points to the occurrence of thin layer diffusion in the 3 D films of Zn-NiHCF films for the first time. The thin layer diffusion behavior of the hydrazine in the Zn-NiHCF film was analyzed in detail in this work. 2. Experimental 2.1. Instruments: The voltammetric experiments were performed in Autolab Model PGSTAT30, Eco Chimie Netherlands with three electrode cell system. Glassy carbon (GC) electrode of area 0.07 cm2 from BAS instruments (USA) was used as working electrode Platinum foil fused to a glass tube was used as auxillary electrode. Normal calomel electrode (NCE) with 0.1 M KCl as electrolyte, (potential 0.280 V vs NHE) was used as reference electrode. The Field Emission Electron Microscope (FESEM) images were collected from ZEEIS X-Max (Oxford Instruments). X-ray diffraction data were collected from PAN analytical diffractometer Model PW3040/60 X’pert PRO operated with X-ray source CuKα radiation (λ= 0⋅15406 nm) generated at 40 kV and 20 mA. Scans were done at 3° min–1 for 2θ values between 10 to 80°. 2.2 . Chemicals: Analytical grade chemicals were used for this study. Potassium ferricyanide (K3[Fe(CN6)]) and Potassium nitrate (KNO3), Zinc Nitrate (ZnNO3) and Nickel Chloride (NiCl2) and Hydrazine


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were purchased from (MERCK). Stock solutions were prepared using Milli Q water of resistivity 18 M ohm cm2. 2.3. Modified Electrode Preparation: GC electrode was first cleanly polished using alumina coated polishing paper of grade (5/0) until the mirror finish was attained. The polished electrode was subjected to pretreatment in ultra-sonic bath for 5 minutes in Milli Q water. Finally the electrode was used for modification through potential cycling between 1.0 and 0.0V for 20 cycles at the scan rate of 50 mV/s. The modifying mixture initially contains 0.1 M KNO3 containing 0.5 mM K3[Fe(CN)6]. Electrodeposition was started after instantaneous addition of either ZnNO3 (0.5 mM) or NiCl2 (0.5 mM) or their mixture. After 20 continuous growth cycles, the GC electrode was removed and rinsed with copious amount of Milli Q water and dried. Finally the modified electrode was used for further analysis. 3. Results and Discussions. 3.1. Voltammetric investigation of Zn-NiHCF films. The electrodes were modified with films of Nickel hexacyanoferrate or Zinc hexacyanoferrate conventionally by cycling the electrode potential between 1.0 V and 0.0 V in a 0.1 M KNO3 medium containing corresponding metal chloride and potassium ferricyanide.

The cyclic

voltammetric growth pattern on GC in a medium containing 0.1M KNO3 containing 0.5 mM K3Fe(CN)6 and 0.5 mM of metal salts a) NiCl2 + ZnNO3 b) NiCl2 and c) ZnNO3 are shown in Fig 1(a-c). The first cycle in each growth pattern (Fig.1d-f) show indications of the growth of the films on GC surface by forming corresponding metal ferrocyanide on the electrode. It is worth mentioning that the metal ferrocyanide complexes are known to have lower solubility product than corresponding metal ferricyanide complex.19 The observed deposition potential for


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Zn-NiHCF was in between NiHCF and ZnHCF signifying the formation of hybrid. The redox response noted for the modified electrode in the 0.1 M KNO3 medium containing no electroactive substances are given in Fig.2.

The two redox processes were observed for NiHCF

modified electrodes in K+ containing electrolytes Fig.2a and are attributed to the presence of two stoichiometric forms of NiHCFs in the film see equation (1&2).20 . ( ) + + . ( ) → (1) ( ) + + ( ) → (2) 50

1

a

0 -0.5

5

I / µA

I / µA

20

-1

-1.5

-10 -25

-2

-2.5

-40

-3

-55

-3.5

0

0.2

0.4

0.6

0.8

0

1

b

0.4

0.6

0.8

1

e

6

25

0.2

E vs NCE / V

8

E vs NCE / V

40

4

I / µA

10

I / µA

d

0.5

35

-5

2 0 -2

-20

-4

-35

-6 -8

-50 0

0.2

0.4

0.6

0.8

0

1

0.2

0.4

0.6

0.8

1

E vs NCE / V

E vs NCE / V 105

3

c

80

f 1

55 30

I / µA

I / µA

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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 -20

-1 -3

-45 -70

-5

-95

-7

-120 0

0.2

0.4

0.6

E vs NCE / V

0.8

1

0

0.2

0.4

0.6

0.8

1

E vs NCE / V

Figure 1. Comparision of voltammetric patterns corresponding to individual MHCF. a) voltammagram obtained for Zn-NiHCF modified GC electrode b) corresponds to the NiHCF


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modified films and c) corresponds to the ZnHCF modified electrode response in 0.1 M KNO3 at 50 mV/s.

(d,e & f) shows the corresponding first cycle responds during film growth

respectively. The NiHCF film growth exhibited a steady growth of redox peaks in anodic as well as cathodic directions.

The redox behaviour during voltammetric growth of NiHCF film on electrode

indicates that there are not much dynamic changes happening in the film. In the case of the hybrid film growth, the anodic and cathodic peaks occurring during the growth exhibited shift up to 5-10 potential cycles. This observation represents some dynamic changes occurring in the film during film growth. The shift in the redox potential during the growth cycle may indicate changes in the film composition during the cycling process. The inter-conversion of one metal hexacyanoferrate to another during electrochemical cycling is reported by us and by Dostel et al.,.11,12 The authors have reported complete conversion of PB film to CdHCF film on cycling in a medium containing Cd2+ ions. Similar inter-conversion of MHCFs happening during growth cycles may explain the shift in the potentials during initial growth cycles. More studies are in progress to throw more light on these aspects. Fig.2f shows the response of the ZnHCF modified electrode in KNO3. The single set of redox peak is due to the redox reaction described below equation (3). ( ) ( ) + + → (3) The two stoichiometric forms of NiHCF were separately deposited either chemically or electrochemically by varying the experimental parameters

by Wei Chen et al.,.21 They have

established that the potential controlling technique can be employed to synthesize composition and morphology tunable NiHCF nano-interfaces. The stability of ZnHCF formed on GC to electrochemical cycling was very poor as seen from the gradual reduction of peak currents on


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potential cycling (Figure not shown). The stable response of electrochemically formed ZnHCF films on wax impregnated graphite is reported,22 whose redox behavior resembles with the redox behavior observed for films deposited on GC. The lower stability of ZnHCF films on GC may be attributed by the smooth surface of GC. Recently Boopathi et al., have reported their inability to form ZnHCF on GC electrochemically.

23

The Fig.2d shows the voltammetric response of GC

electrode modified by thin films of Zn-NiHCF in 0.1 M KNO3 medium. It is interesting to note that there is only one well reversible redox peak at around 0.500 V unlike the two redox peaks observed for NiHCF films. There is a marked shift from the ZnHCF film redox potentials to cathodic direction by more than 0.280 V. Both the redox peaks obeyed characteristics of ideal surface reactions, linear peak current to scan rate relationship obeyed upto scan rate 0.15 Vs-1 (Figure.S1 Supporting Information (SI)).24 The shift from the redox potentials of ZnHCF/GC and NiHCF/GC indicates the formation the hybrid Zn-NiHCF film on GC. Electrochemically modified films of NiHCF and Zn-NiHCF were cycled in alkali (0.1 M KOH) for 15 continuous cycles at 50 mV/s. NiHCF modified film shows an increasing oxygen evolution reaction activity in each cycles due to the conversion of NiHCF to Ni(OH)2. Similar conversion of NiHCF films to stable Ni(OH)2 were consistent with the earlier observation by us and Sheela et al.,25–27 However in the case of Zn-NiHCF modified electrode show a poor stability by decreasing the columbic charge in 10 cycles under same experimental conditions (Figure.S2(A&B) SI), Later observation presume us that Ni(OH)2 conversion is more feasible through pristine NiHCF films than that of Zn-NiHCF films. To understand the effect of film composition and ratio of zinc and nickel ion on the electrochemical behavior of the modified electrodes, modification of GC was carried out at various metal ion’s ratio. The responses of the modified electrodes are depicted in Fig 2(a-f). When the modification mixture contained no zinc ions, the electrochemical response


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of NiHCF modified electrode showed two redox peaks centered on 0.4 & 0.54 V. On increasing the Zinc ions in the modification mixture, the relative peak current corresponding to the first redox peak (designated as a1c1) increased.

The peak current due to the first redox couple

increased further on increasing the zinc ion concentration in the bath. When the zinc /nickel ion concentration ratio reaches 1, there is only one set of redox peaks observed even at very slow scan rates indicating the occurrence of a single redox process.

c1 5 µA

-0.1

0.1

d

a1

a

0.3

Ni - 0.5 mM Zn - 0.0 mM

a1 0.5

0.7

Ni – 0.5 mM Zn – 0.5 mM

10 µA

0.9

1.1

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

E / V vs NCE

e

b

Ni – 0.4 mM Zn – 0.6 mM

5 µA

Ni - 0.7 mM Zn - 0.3 mM

10 µA

c1

c 10 µA

0

0.2

f

Ni – 0.6 mM Zn - 0.4 mM

0.4

0.6

E vs NCE / V

0.8

15 µA

1

0

0.2

Ni – 0.0 mM Zn – 1.0 mM 0.4

0.6

0.8

1

E vs NCE / V

Figure 2. Represents the change in voltammetric response of NiHCF redox films prepared in presence of Zn2+ in the reaction bath. Voltammograms of all composition were recorded in 0.1 M KNO3 at 50 mV/s scan rate. The composition of Ni2+ and Zn2+ used were ( concentration in mM) a) 0.5 & 0 b) 0.7 & 0.3 c) 0.6 & 0.4 d) 0.5 & 0.5 e) 0.4 & 0.6 and f) 0 & 1 mM.


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3.2. Study of Zn-NiHCF film characteristics using XRD The XRF analysis on modified Zn-NiHCF films also confirms the presence of Zn, Ni and Fe in the modified film (Figure.S3 SI). The NiHCF formed chemically has FCC structure as revealed from the XRD data shown in the Fig 3a.28 The ZnHCF film has rhombohedral structure

29,30

,

entirely different lattice `structure compared to NiHCF. 5000 4000 3000 2000 1000 0

Intensities (a.u)

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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

a 20

30

40

50

60

70

80

3000

a

2000 1000

b

0

10

20

30

40

50

60

70

b

d

80

12000

15

18

9000

21

a

24

2θ (Cu Kα )

6000

c

3000 0

10

20

30

40

50

60

70

b

80

2θ (Cu Kα )

30

40

50

60

2θ ( Cu Kα )

Figure 3. XRD patterns for chemically synthesised HCF scanned between angle 10 – 80˚ a) ZnNiHCF b) NiHCF c) ZnHCF and d) overlay graph of NiHCF and Zn-NiHCF showing the shift in 2θ values to the lower angles. The XRD patterns of the chemically synthesized Zn-NiHCF resemble NiHCF with the observed 2θ shift of 0.8 degree for all the planes. This observation clearly indicates that the zinc ions are able to get substituted to the NiHCF lattice during electrochemical modification. By the appearance of single redox peak for the Zn-NiHCF hybrid films electrochemically and from the FCC structure of chemically formed Zn-NiHCF hybrid (Figure.S4 SI), it is reasonable to believe that the structure of electrochemically formed hybrid also might possess FCC structure. We have


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earlier reported the tuning of redox potentials of the surface ferro/ferricyanide redox couple by changing the ratio of Iron/Nickel ions in the modification mixture.8 It is worth mentioning that both FeHCF and NiHCF are face centered cubic structures and can form hybrids in all ratios.31 On increasing the Zinc/Nickel ion ratio exceeds 1, the single redox peak corresponding to ZnNiHCF was retained. There was an extra peak with small magnitude obtained occurring at more anodic value, origin of which is not exactly clear. The small peak is arising due to the excess zinc ion in the bath. The observed instability of the modified electrode response prompted us to believe that there may be some ZnHCF crystals formed along with Zn-NiHCF hybrid which is unstable on electrochemical cycling. 3.3. Surface and morphology of MHC (Ni, Zn and Zn-Ni) Fig 4A shows the FESEM picture for the modified films on GC electrode. The fig 4B and 4C are FESEM of the Zn-NiHCF and NiHCF respectively. The charge deposited for ZnHCF films are higher than that observed for NiHCF films under similar experimental condition. The ZnHCF films were electrochemically crystallized as bigger cube like particles as seen from the inset in the Fig 4D. The clear difference in the morphologies between NiHCF and ZnHCF is consistent with the two XRD pattern observed (Fig.3). The single phase Zn-NiHCF films on GC exhibited the redox peak potential at 0.5 V which is less anodic than the redox potentials of electrochemically formed ZnHCF. Similar shift in the redox peak potentials of ZnHCF grown on Ni impurities were observed by Boopathi et al.,.18 In their case the ZnHCF film also would have been formed by the nickel ion (impurity in SWCNT) incorporation in the MHCF lattice during oxidation-reduction cycle during ZnHCF formation.

Zn-NiHCF films were formed on

GC as continuous film which shows uniform and dense film. The Zn-NiHCF films appear as


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more compact film than simple NiHCF films as evident from the FE-SEMs shown in Fig.4B. On magnifying the FE-SEM, the hybrid film exhibited uniform cracks of dimension 30-40 nm.

Figure 4. FESEM images of three different HCF modified electrodes taken with electron beam source of 2 eV A) bare GC electrode. B) Zn-NiHCF ( Inset B magnified view of Zn-NiHCF) C) NiHCF and d) Zn-HCF and (Inset.D shows the magnified images of ZnHCF) Usually cracks are formed when there is volume expansion during oxidation-reduction cycles in thin compact films.

The redox reactions of metal hexacyanoferrate thin films need

inercalation/de-intercalation of supporting electrolyte cations to maintain charge neutrality in the film.32–34 The uptake and exit of the supporting electrolyte cations and occurrence of lattice


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exchanges from the compact film might cause cracks in the Zn-NiHCF film during electrochemical deposition process( inset Fig.4B). The hybrid film modified electrode show a single redox peak with cathodic shift in the formal redox potential in supporting electrolytes (K+, Na+ and H+) (Figure.S5(A) SI) similar to that observed for the corresponding MHCF modified electrodes.35 Likewise, the stability of Zn-NiHCF films on electrochemical cycling were tested in K+, Na+ and H+ containing electrolytes. Stability test results reveals that Zn-NiHCF modified electrode show high stability to electrochemical cycling in K+ containing electrolyte with 95 ± 4% charge retention compared to that of NiHCF films with charge retention 80 ± 5% (Figure.S5(B) SI).

3.4. Voltammetric study of hydrazine oxidation. The Fig.5(a) show the electro-oxidation behavior of 1 mM hydrazine in 0.1 M KNO3 on GC. From the voltammogram, it is evident that the electro-oxidation takes place at 0.32 V (peak potential) with very low magnitude of current. The Fig.5 (b&c) show the electro-oxidation behaviour of 1 mM hydrazine in 0.1 M KNO3 at GC modified with NiHCF and Zn-NiHCF hybrid respectively. It is thought-provoking to note that the electro-oxidation of hydrazine take place on the above modified electrode at the same potential where it happened on GC. The magnitude of the electro-oxidation current on NiHCF and Zn-NiHCF modified electrodes were crudely 3 & 9 times (at the scan rate 50 mV/s) respectively. The electro-oxidation peak for hydrazine occurs at much lower redox potentials than that of Zn-NiHCF films indicating that the redox center in the film is not participating in the mediated electro-oxidation of hydrazine. This observation prompted us to believe that the electro-oxidation takes place on the surface of GC. The enhancement of the electro-oxidation current to nine times was intriguing. We initially thought that the GC surface might get activated on electrochemical cycling during


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electrochemical modification of GC with Zn-NiHCF.

Our control experiments on GC

electrochemically cycled between 0.0 and 1.0 V revealed that the hydrazine oxidation current did not show any notable change by the above electrochemical pre-treatment.

Figure 5. Cyclicvoltammetry response for hydrazine oxidation on GC electrode surface (a), voltammetric patterns obtained for NiHCF (b) and Zn-NiHCF(c) in presence of 1 mM hydrazine in 0.1 M KNO3 scan rate 50 mV/s. 3.5. Evaluating the apparent rate constant (k) for hydrazine oxidation. Here, chronoamperometry technique was employed to study the kinetics of hydrazine oxidation reaction on Zn-NiHCF electrode. Zn-NiHCF modified GC electrode performances were compared with bare GC electrode in this study. Chronoamperometric experiments were carried out by double potential step method, by holding the electrode potential at 0.0 V for 10 s and forcing it to the hydrazine oxidation potential i.e 0.5 V until it get a steady state response that appears in curve Fig.6, which indicates hydrazine oxidation reaction is a diffusion controlled electron transfer process. The apparent rate constant (k) for hydrazine oxidation reaction can be determined by the method described in the literature.36


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= ! / #$/ erf(! / ) + exp(−!) /! /

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(4)

The terms present in the above equation ./ ⁄.0 is the ratio between the faradaic current measured after and before hydrazine addition. Here, ! = 2 3 , ( 3 ) is the concentration of hydrazine in the bulk in (M) and ( 2) is the catalytic rate constant in (M-1.s-1) and () is the time elapsed. Equation (4) has been simplified accordingly, when erf(! / ) argument of error function exceeds 2, the function is almost equals to 1 as described in.37 So, the equation becomes

= $ / ! / = $ / (2 3 )/ (5)

On plotting the values of ./ ⁄.0 / the linear relationship was observed and the rate constant (k) was calculated from the slope (see inset Fig.6). Interestingly, the apparent rate constant (k) calculated were 9.096×103 M-1 s-1, 1.955×103 M-1 s-1 8.25×102 M-1 s-1 and 7.06×102 M-1 s-1 for Zn-NiHCF, ZnHCF, GC, and NiHCF electrodes respectively. The calculated values reveal that hydrazine oxidation reaction is approximately ten times higher on Zn-NiHCF electrode surface than that on GC see Table.1.

Table:1 Apparent rate constant (k) values for hydrazine oxidation reaction on different modified electrodes

S.No 1) 2) 3) 4)

Modified electrodes Zn-NiHCF ZnHCF GC NiHCF

Slope

k values ( M-1. s-1 )

11.401 2.456 1.249 1.035

9.096 × 103 1.955 × 103 8.250 × 102 7.060 × 102


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Figure 6. Show the chronoamperometric curve for Zn-NiHCF film with (b) and without (a) 1 mM of hydrazine 0.1 M KNO3. Inset show Ic/Il vs t1/2 plot obtained for Zn-NiHCF, ZnHCF , NiHCF and GC electrodes.

3.6. Hydrazine oxidation scan rate dependence. Henstridge et al., have found that the electro-oxidation currents obtained for the electrooxidation of dopamine and nicotine on MWCNT coated electrodes cannot be explained on the basis of only semi-infinite linear diffusion of analytes to the modified electrodes.38,39 They have found evidence for ‘a mass transport regime that includes thin-layer (within the MWCNT layers) as well as semi-infinite (from bulk solution) diffusional signatures, proving that the ‘electrocatalytic’ properties claimed of many porous and multi-walled carbon nanotube-based modified electrodes will have mass transport effects

arising from

electronic or structural

peculiarities of the modifying layers’.40 They have shown from the analysis of the logarithmic


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plot of the peak current vs scan rate that the slope of the above plot is above 0.62. The slope value associated with the pure semi-infinite diffusion of electroactive species is about 0.5 and the slope expected for the adsorbed electroactive species is 1.0.39 Fig.7A show the scan rate dependence of the peak currents for the electrooxidation of hydrazine at Zn-NiHCF modified GC. The slope obtained from the logarithmic plot of scan rate to peak current is 0.64 indicating the mass transport in the film deviates from the expected semiinfinite linear diffusion Fig.7B. To understand the origin of the thin layer diffusion behavior inside the modified film, it is necessary to have idea about how the analyte can get in to the hybrid film. The existence of cracks in the film provides a pathway for the incorporation of the hydrazine solution by capillary forces. Electro-reduction of the MHCF and the hybrid films necessitate the intake of cation in the supporting electrolyte to maintain the electro neutrality in the film. At the pH prevailing in the medium is weekly acidic, the hydrazine is likely exist as protonated form and likely to compete with K+ ion during cation intake during electro-reduction..

95

-4.3

A

70

B -4.5

log10 ipa / A

45

I / µA

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20 -5 -30

-4.7 -4.9 -5.1

-55

Gradient - 0.640

-5.3

-80 0

0.2

0.4

0.6

0.8

1

-2.1

E vs NCE / V

-1.8

-1.5

log10 ʋ /

-1.2

-0.9

-0.6

Vs-1

Figure 7. shows cyclic voltammogram of scan rate dependence of peak current during hydrazine oxidation hydrazine oxidation (A), (B) the logarithmic plot of oxidation peak current (ipa) vs scan rate (ʋ) from 10 – 230 mV/s with gradient of 0.64.


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3.7. Investigation of thin layer diffusion using Cottrell equation. Keeley and Lyons have proved thin layer diffusion behaviour of analytes in SWCNT films cast on GC electrode.41 The current transients were used for investigating the dopamine oxidation behavior on bare GC as well as SWCNT modified GC. They have observed simple Cottrell linearity for dopamine electro-oxidation at the unmodified glassy carbon electrode. However, on GC/SWCNT, drastic deviation was found from the expected semi-infinite Cottrell model. Indeed the Cottrell plot showed expected linearity at short times whereas marked deviation from linearity was observed at longer times.

135 Zn-NiHCF hybrid

110

b

GC Electrode

85

I / µA

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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 35

a 10 -15 0

0.5

1

1.5

2

2.5

3

3.5

t -1/2 / s -1/2 Figure 8. Cottrell plot obtained for hydrazine oxidation reaction on GC surface (a) and on ZnNiHCF modified electrode (b) at applied potential step of 0.6 V. From Cottrell equation the boundary value problem (i.e. x=0) for polymer/electrolyte interface was solved using laplace transformation method and the expression for modified Cottrell equation of bounded diffusion space was derived.42 Hence the latter derived equation has high


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significance in determining the diffusion limiting current (i) for oxidizing species at t

Observation of Thin Layer Diffusion during Electro-Oxidation of

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