Influence of co-electrodeposited Gold particles on the electrocatalytic properties of CoHCF thin films

Electrochimica Acta 139 (2014) 88–95

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Influence of co-electrodeposited Gold particles on the electrocatalytic properties of CoHCF thin films Alam Venugopal Narendra Kumar, James Joseph ∗ Electrodics and Electrocatalysis Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630 006,India

a r t i c l e

i n f o

Article history: Received 3 June 2014 Received in revised form 20 June 2014 Accepted 20 June 2014 Available online 11 July 2014 Keywords: Cobalt hexacyanoferrate Hydrazine Electrocatalysis and Modified electrodes

a b s t r a c t The electrochemical modification of solid electrodes with metal hexacyanoferrate thin films for enhancing the interfacial properties has created interest for over the past three decades. The preparation of Prussian blue (PB) Au nano composites for the enhancement in the electrocatalytic properties of PB on glassy carbon electrode has been reported by us. The incorporation of Au nano particles in Cobalt hexacyanoferrate (CoHCF) films on Glassy carbon by co-electrodeposition is expected to benefit its interfacial electron transfer properties. The present work describes the effect on the interfacial properties by incorporated Au particles in CoHCF (CoHCF(Au)) modified electrodes. The CoHCF(Au) modified electrodes were characterized by UV-Vis spectrophotometry, Cyclic Voltammetry, AC Impedance, FE-SEM etc., Influence on the electrocatalytic properties of CoHCF(Au) films have been explored by performing two important reactions i) Hydrazine elecrtro-oxidation ii) Oxygen evolution reaction. Our results reveal that CoHCF(Au) modified GC electrode perform better in terms of charge transport in the redox film and also for the electrooxidation of hydrazine in comparision with simple CoHCF modified electrodes. By using the current-transient technique (chrono method i vs t curve) the hydrazine diffusion coefficient (D0 ) were calculated. Diffusion coefficient of hydrazine was approximately three times higher on CoHCF(Au) electrode, 9.5 × 10−5 cm2 s−1 compared with simple CoHCF modified electrode, 3.3× 10−5 cm2 s−1 . Similarly, we also discuss results which reveal that CoHCF(Au) electrodes enhances electrocatalytic activity in splitting water to oxygen in 0.1 M NaOH solution compared to simple CoHCF and Au deposited on GC electrodes. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The CoHCF is one of the most widely studied metal hexacyanoferrates next to the prototype material of the family namely ‘Prussian blue (PB).[1–4] CoHCF also has face centre cubic lattice but with two different transition metal atoms Co and Fe, which differs from Prussian blue (two Fe atoms, high spin and low spin).[5] This two different transition metals combination in CoHCF make this particular material complex, very facinating to understand its chemistry. So, exploring CoHCF chemistry has become more pertinent. Many research groups have extensively contributed to show the multifunctional properties of metal hexacyanoferrates like, thermochromism, [6] ion exchange, [7–9] redox mediation [10], photoinduced magnetism, electrochromism [11,12], photocatalyst [13], biosensors [14,15] and in electrocatalysis [16]. It is also important to understand the structure and compostion of

∗ Corresponding author. E-mail address: [email protected] (J. Joseph). http://dx.doi.org/10.1016/j.electacta.2014.06.156 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

metal hexacyanoferrates to categorize them through fundamental perspective. Recently, the structural investigation of Cu, Co, and Ni hexacyanoferrates have attracted intense studies with the help of X-ray techniques [17,18]. Particularly, for CoHCF it has been demonstrated that the redox films are capable of showing cation (K+ , Na+ ) dependent electrochromic and thermochromic behaviour which make this material unique from other metal hexacyanoferrates [6,19]. Unlike other transition metal hexacyanoferrates, understanding the redox processes that takes place in CoHCF modified films is more complex. Lezna, et al., have explained the different stoichiometric forms of CoHCF formed by potentiodynamic method using insitu FT-IR measurements [2]. Later the same research group have clearly demonstrated that the CoHCF composition on gold (Au) electrode can be controlled by tuning the experimental conditions like Co2+ /Fe(CN)6 3− ratio, deposition potential window and delay time (time delay after mixing two precursors) using electrochemical impedance spectroscopy (EIS) and in situ UV-visible spectroscopy [3]. The electrochemical modification of Carbon electrodes with CoHCF films which exhibited two redox processes were first reported by James et al., [20]

A.V.N. Kumar, J. Joseph / Electrochimica Acta 139 (2014) 88–95

Most of the MHCF modified electrodes showed reversible redox peaks in K+ containing electrolytes. However, unlike other metal hexacyanoferrates, the CoHCF modified films showed high quasi reversibility for the first redox process in potassium containing electrolytes. Thangavel and Ramaraj [21] have reported the effect of incorporation of citrate stabilized Au nanoparticles (Aunano ) on the properties of CoHCF. The Aunano co-deposited with CoHCF modified electrode by them have shown a similar voltammetric pattern as pristine CoHCF films but they report the improvement in charge transport rate and electrocatalytic oxidation of H2 O2 . In this report, we demonstrated the effect of Au particles coelectrodeposition on the increased reversibility of GC modified CoHCF films. We have also shown that the charge transfer process in the CoHCF film depends on Au3+ concentration in the modification mixture during deposition. Electrochemical Impedance Spectroscopy (EIS) measurements and Field Emission Scanning Electron Microscopy (FESEM) were used to characterize the modified CoHCF(Au) films. Au incorporated CoHCF redox films were grown on GC electrode with optimized experimental conditions and used for studying the hydrazine electro-oxidation. The enhanced electrocatalytic performance and electrode kinetics of CoHCF(Au) electrode towards hydrazine oxidation were compared with that of CoHCF modified electrode without Au incorporation.

2. Experimental 2.1. Materials Chemicals such as cobalt chloride (CoCl2 ), potassium nitrate (KNO3 ), hydrazine were purchased from (MERCK), Potassium ferricyanide (K3 Fe(CN)6 ) (Ranbaxy chemicals), Auric acid (HAuCl4 ) from (Sigma Aldrich) and all salt solutions were prepared in acid cleaned glassware using Milli Q water of resistivity 18 ohm cm2 . Potential cycling method using three electrode system was adopted for depositing cobalt hexacyanoferrate thin films. Glassy carbon (GC) was employed as working electrode (area 0.07 cm2 Bio Analytical systems USA), platinum and normal calomel electrode (NCE) were used as counter and reference electrodes respectively. 2.2. CoHCF(Au) film preparation Both CoHCF and CoHCF(Au) redox films were grown on GC electrode by potential cycling method. Here, we choose 0.1 M KNO3 as a supporting electrolyte in all our electrochemical experiments. The reaction mixture containing 0.5 mM of K3 [Fe(CN)6 ] and 0.5 mM CoCl2 was potential cycled between 1.0 V to 0.0 V vs NCE (1 M KCl solution used) for 20 continuous cycles at the sweep rate of 50 mV/s. The Au and CoHCF deposition take place at potential more cathodic to 1.0 V and at starting potential, no electrochemical reaction take place on the electrode surface. In the case of CoHCF(Au) redox films, we varied the concentration of HAuCl4 in the reaction bath from 0.2 mM to 0.8 mM in an equi molar mixture of CoCl2 and K3 [Fe(CN)6 ] (0.5 mM each).[20] After modification the working electrode was rinsed with Milli Q water and dried at room temperature and used for further electrochemical and physical characterization. 2.3. Material characterization Physical characterisation like FESEM for a CoHCF and CoHCF(Au) film electrodes were done using ZEISS X-MAX Oxford instruments. Cyclic voltammetry and chronoamperometry experiments were carried out using Autolab model PGSTAT 30, (Eco Chemie Netherlands). Electrochemical Impedance spectroscopy (EIS) of the

89

modified films were measured using IVIUM CompactStat instrument. 3. Results and Discussions 3.1. Substrate effect on CoHCF redox film Fig. 1 A&B depict the responses of CoHCF film modified GC and Gold electrodes respectively in 0.1 M KNO3 medium. Both the voltammograms are qualitatively similar indicating no serious substrate effect on the redox response of CoHCF films. The modification of carbon electrodes with CoHCF were first described by James et al., [20] who report two sets of redox processes occurring during electrochemical cycling. The first redox couple a1c1 appear to be highly quasi reversible with a large peak to peak separation of around (Ep) 50 mV whereas, the second redox couple appears to be reversible Ep ≤ 10 mV at the scan rate of 5 mV/s († ESI Fig.S1&S2). The anodic processes were merged to yield only one anodic peak at sweep rates above 0.02 V/s. The quasi-reversibility may be attributed to the increased association of K+ with CoHCF films. The charge transport in the Metal hexacyanoferrate films were in general thought of as ion dependent electron transport [22]. The charge transport in CoHCF film modified electrodes can be assigned to the K+ dependant charge transport as given in equation 1 &2. During electro-oxidation of the film, the electrons from the film move to the electrode side and the ions move to the solution side. Similarly during electro-reduction, the electrons move to the solution side from the electrode and the ions move from the solution side to the electrode. The incompatibility in these movements results in a resistive potential drop leading to quasireversible response in the film. To circumvent this problem, we have attempted to incorporate the gold particles in to the CoHCF film by co electro-deposition. 3.2. Au incorporation in CoHCF redox film The bath used for the electro-deposition (described in detail in experimental section) contain CoCl2 , K3 [Fe(CN)6 ], KNO3 and HAuCl4 . Senthil et al., have reported that the potential cycling of the GC in KNO3 medium containing AuCl4 − and K3 Fe(CN)6 results in the formation of Au-Prussian blue nano composite films [23]. The formation of Prussian blue is identified as due to the decomposition of potassium ferricyanide to free ferric ions on gold nuclei electrodeposited on GC [24–26]. Surprisingly we have noticed that the decomposition of potassium ferricyanide to form PB is prevented when the modifying mixture contain equi-molar Co2+ ions, which was again confirmed through invariant–CN stretching frequency of CoHCF and CoHCF(Au) at 2143 cm−1 († ESI Fig.S3) [2]. Further results reveal that the CoHCF(Au) modified electrode also exhibit a similar voltammetric pattern to that of ordinary CoHCF modified electrode in Na+ containing electrolyte († ESI Fig.S5B).[20] This is in contrast to the observation by Manoj et al., who reported that it is not possible to form CuHCF alone without PB in presence of gold chloride in solution [27]. Their modification bath contained stoichiometrically excess ferricyanide with respect to Cu2+ and perhaps this is one possible reason for their failure to observe the stability of potassium ferricyanide in presence of transition metal ion during potential cycling in presence of gold chloride in solution. From Fig. 2B it is apparent that the CV pattern leads to growth of two distinct redox couples in presence of AuCl4− indicating deposition of CoHCF(Au) films on GC. Fig. 2 shows distinct features in the growth pattern of CoHCF and CoHCF(Au) films on GC. The increased reversibility of the redox processes may be attributed to the incorporation of Au particles in the film during growth. Fig. 2B voltammetric pattern resembles with that seen in

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40

a1

A

40

I / μΑ

20 I / μΑ

a1

B

30

0

10 0

c2

-20

20

-10

-40

c1 0.0

0.2

0.4 0.6 E vs NCE / V

c2

-20 0.8

c1 0.0

1.0

0.2

0.4 0.6 E vs NCE / V

0.8

1.0

Fig. 1. Cyclic voltammograms corresponding to CoHCF films deposited on two substrates (A) on GC electrode and (B) on Au-electrode containing 0.1 M KNO3 medium at the scan rate of 50 mV/s.

the Fig. 1A. i.e apart from the solution ferro/ferricyanide redox reaction, one anodic process and two cathodic processes were appeared. To study the effect of Au incorporation on the electrochemical deposition of CoHCF on GC, composition of AuCl4 − in the modification mixture was varied from 0.1 mM to 0.8 mM. The voltammetric response of CoHCF(Au) film modified electrodes in 0.1 M KNO3 containing no other electroactive species in solution show two well reversible redox processes corresponding to the reactions [21] as shown below. KCoII 1.5[FeII (CN)6 ] ↔ CoII 1.5[FeIII (CN)6 ] + K + + e−

(1)

K2 CoII [FeII (CN)6 ] ↔ KCoII [FeIII (CN)6 ] + K + + e−

(2)

Fig. 3 (A-D) show voltammetric response of CoHCF(Au) modified GC in 0.1 M KNO3 solution. From Fig. 3(A(a)) it is clear that the redox processes become more reversible. Both the redox processes obeys characteristics of ideal surface reaction as evident from the linear ip vs scan rate dependence as observed in († ESI Fig.S4). Increase in reversibility of the redox processes are attributed to the enhancement of electronic conductivity of CoHCF films during Au co-deposition. The relative peak current heights of a2c2 were found to increase when the concentration of Au3+ in the deposition bath was up to 0.4 mM Fig. 3(A-B). For higher concentration of Au3+ 0.6 and 0.8 mM, Au0 deposition rate overtakes CoHCF deposition and we see a drop in the peak height corresponding to a2c2 as seen from Fig. 3C&D. This is confirmed as due to the simultaneous oxidation/reduction reactions of co-deposited Au particles along with CoHCF Fig. 3A(b). The voltammetric pattern for the CoHCF(Au) films become featureless when the Au concentration in modification mixture is above 1 mM. This observation clearly suggests that the excess gold particles deposited along with CoHCF film may affect the charge transport during redox process. This may be due

Table 1 Rct and Equilibrium exchange current density (j0 ) for both CoHCF and CoHCF(Au) with [Au3+ ] used for film preparation. Rct (cm2 )

j0 (A cm−2 )

(1) (2) (3) (4)

0 2×10−4 4×10−4 6×10−4

568 52 171 180

4.550×10−5 4.970×10−4 1.511×10−4 1.435×10−4

3.3. Charge transport in CoHCF with different Au content The Niquist plot for CoHCF films with and without Au incorporation gave a semi circular arc followed by Warburg line Fig. 4(A-D). The charge transfer resistance Rct as a function of Au in the CoHCF films and the exchange current density were tabulated in Table 1. The amount of Au present in the redox film governs the charge transport characteristics in CoHCF(Au). The exact values of Rct for both CoHCF and CoHCF(Au) films were tabulated in Table 1. From

20

B

10

4

0

2

I / μΑ

I / μΑ

Concentration of Au3+ (M)

to the fact that the ion dependent charge transport channels in CoHCF might be blocked by bigger Au particles. The codeposited Au also would undergo redox reactions along with CoHCF as clearly indicated from the higher relative peak height for the CoHCF films having higher proportion of Au. The CoHCF(Au) films prepared by taking Co2+ :K3 [Fe(CN)6 ] ratio (2:1) gave the response qualitatively similar to that observed when the ratio is 1:1 († ESI Fig.S5). But, when the modification mixture containing 1:3 ratio of Co2+ :K3 [Fe(CN)6 ] in reaction bath a non-faradaic current appears in the voltammogram at the potential 0.0 - 0.3 V. We presume it is probably due to the formation of small quantities of PB or iron containing mixed analogues[28] [18] of (Co-Fe) hexacyanoferrates.

A

6

S.No

0 -2

-10 -20

-4

-30 0.0

0.2

0.4 0.6 0.8 E vs NCE / V

1.0

0.0

0.2

0.4 0.6 0.8 E vs NCE / V

1.0

Fig. 2. The voltammetric growth pattern of ordinary CoHCF (A) and CoHCF(Au) (B) on GC electrode in the ‘modification mixture’ as described in the experimental.

A.V.N. Kumar, J. Joseph / Electrochimica Acta 139 (2014) 88–95

30

A

40

b

a1

a2

10 0 a

-10 c1 0.2

0.0

30

c2

0.4 0.6 0.8 E vs NCE / V

C

20

c1 0.0

0.2

0.4

c2 0.6

0.8

1.0

E vs NCE / V

20

a2

a1

a1

D

a2

10

I / μΑ

I / μΑ

a2

0

-40

1.0

10 0 -10

0

-10

-20 -30

a1

-20

-20 -30

B

20

I / μΑ

I / μA

20

91

c1 0.0

0.2

0.4

0.6

c1

-20

c2 0.8

1.0

0.0

0.2

0.4

c2 0.6

0.8

1.0

E vs NCE / V

E vs NCE / V

Fig. 3. Voltammograms obtained for the GC electrode modified with CoHCF(Au) films prepared from modification mixture containing different HAuCl4 concentration (A) (a)) 0.2 mM (red) and (b) Response of Au electrodeposited GC in 0.1 M KNO3 (blue), (B) 0.4 mM, (C) 0.6 mM and (D) 0.8 mM in 0.1 M KNO3. Scan rates 50 mV/s.

these values the kinetic parameter, exchange current densities (j0 ) were calculated by using equation (3) [29]. j0 = RT/nAFRct

transport associated with the charge transport in CoHCF film is hindered by bigger Au particles. The effect of Au incorporation on the impedance characteristics in CoHCF film was schematically represented in Scheme 1. For the elecctro-catalytic oxidation studies, the CoHCF film and CoHCF(Au) film deposited from medium containing 0.2 mM AuCl4 − were compared in detail. The UV-Vis spectra († ESI Fig.S6) of CoHCF film co-electrodeposited from bath containing 0.2 mM and above AuCl4 − indicate signs of high background scattering. From the impedance data, it is possible to conclude that the charge transport in the film has improved when concentration of AuCl4 − in the modifying mixture is upto 0.2 mM.

(3)

In the above equation the symbols (R) is a gas constant 8.314 J mol−1 K−1 , (T) temeperature in Kelvin 298 K, (n) is the number of electron involved in the redox process 1, (F) is the Faradays constant 96487 C mol−1 , and (Rct ) charge transfer resistance in . Initially CoHCF film shows Rct of 563 ohm cm2 . On incorporation of Au nanoparticles the Rct become 52 ohm cm2 when the Gold chloride composition is 0.2 mM. At higher concentration of AuCl4 − , the Rct show increase because the concomitant ion

-150

-175

A

-150

2 Z" / Ωcm

2 Z" / Ω cm

-125 -100 -75 -50

-100 -75 -50 -25

-25 CoHCF

0 0

20

40

60 80 2 Z' / Ω cm

100

Au-0.2 mM

0 0

120

-150

-150

C

-125 2 Z" / Ω cm

B

-125

40 60 2 Z' / Ω cm

80

100

D

-125 -100

2 Z" / cm

-100

20

-75 -50 -25

-75 -50 -25

Au 0.4 mM

0 0

20

40 60 2 Z' / Ω cm

80

Au 0.6 mM

0 100

0

20

40

60 2 Z' / Ω cm

80

100

Fig. 4. Nyquist plots obtained for (A) CoHCF modified GC electrode, (B) for CoHCF(Au) electrode prepared using 0.2 mM Au3+ solution.(C) and (D) are plots corresponding to CoHCF(Au) prepared using 0.4 mM & 0.6 mM Au3+ respectively.

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Rct Value (Ω)

K+ K+ K+ K+ K+

e-

e-

e-

CoHCF

Charge transport In CoHCF

Au

e-

K+ K+ K + K+ K+

GC electrode

0 mM

K+

K+

e- e-e- e- e-

K+

e-

3+ ]

K+ e-

e- e-e- e-

e-

e- e- e-

e-

e-

e-

e-

0.2 mM

[Au

K+

K+ K+ K+ K+ e-

e-

K+

0.4 mM

0.6 mM

used for CoHCF(Au) film preparation

Scheme. 1. Illustrating the change of Rct value as a function of Au content in COHCF(Au) from CoHCF redox films.

3.4. Surface analysis using FESEM To understand the effect of Au co-electrodeposition on the morphology of the modified films, FESEM micrographs of CoHCF films with and without gold were taken as shown in Fig. 5(B-D). The CoHCF modified GC showed smooth crystalline films of CoHCF. From the FESEM of CoHCF(Au) film, it was very clear that the Au spherical particles are of average particle size of 200 nm. In addition, Au0 particles of smaller dimensions (25- 200 nm) were also observed in the CoHCF(Au) films as seen from the magnified FESEM images .The Au-PB films prepared by Senthil et al., showed a clear cauli flower like morphology with average particle size of 300 nm clearly indicating inter dependent PB and Au formation [23]. In this case, the FESEM clearly indicate that the Au particles exist in CoHCF film as separate phase and has key role in reducing the resistance (iR drop) of the CoHCF film. The cracks observed in the FESEM of CoHCF(Au) films also point to the fact that the CoHCF and Au particles may have two different growth kinetics. FSEM shown in Fig. 5(C&D) reveal that the average particle size of Au particles increases as a function of Au3+ content in the modifying mixture. Though, the existence of larger size gold nanoparticles in CoHCF enhances the electronic conductivity in the redox film. In fact, the Au particles present in CoHCF(Au) film prepared using low concentration of Au3+ (0.2 mM) reveal that CoHCF deposition equally takes place on GC surface as well as on Au nanoparticles (yellow circles in Fig. 5 C). The redox films prepared using higher concentration of Au3+ (0.6 mM) Fig. 5D show CoHCF growth predominantly on GC surface (orange circles in Fig. 5D). On the other hand it would block the ion transport dependent movement of charges in the film during oxidation/reduction process. The electrocatalytic properties of CoHCF and CoHCF(Au) film on GC were compared using Hydrazine oxidaton as probe reaction in what follows. 3.5. Voltammetric study of CoHCF(Au) for hydrazine oxidation CoHCF modified films are known to be an excellent redox mediators for the electro-oxidation of hydrazine molecule.[30] Here we have illustrated the mediated hydrazine electro-oxidation characteristics of GC modified CoHCF(Au) redox films and compared

it with that of CoHCF films. Hydrazine oxidation is a pH dependant reaction [31–34]. The electrocatalytic performance of the CoHCF(Au) films at various pHs is shown in Fig. 6B. It shows the plot of oxidation peak current ip and onset potential vs pH. From the plot (Fig. 6B) it is clear that the electrocatalytic acitivity of CoHCF(Au) is optimum between the pH-5 and 7 (high oxidation peak current ipa and low onset potential) for hydrazine oxidation. Though the metal hexacyanoferrates show excellent stability in acidic pH and in K+ containing electrolyte, CoHCF(Au) is less active for hydrazine oxidation at pH-2. We found that the highly protonated form of hydrazine at pH-2 is not getting oxidized at CoHCF(Au) interface. Similarly the activity drops at alkaline pH-9 [35]. We attribute this is due to the instability of CoHCF(Au) redox films in OH− environment. In addition to hydrazine oxidation the possible conversion of CoHCF to cobalt oxy species by OH− ions suppress the electrocatalytic activity of the redox film. The Fig. 6A show the voltammetric responses of the electro-oxidation of hydrazine on modified electrodes with ordinary CoHCF film and CoHCF(Au) film. The presence of Au in the redox film shifts the overpotential for hydrazine oxidation by about 60 mV more anodic to that of CoHCF thin film electrode. In addition to that we calculated charge (Q) from area under the curve of a voltammogram during hydrazine oxidation with Au-CoHCF modified GC, which is found to be 580 ␮C whereas Q of CoHCF films was about 304 ␮C. So, its natural to expect that activity of CoHCF(Au) is almost twice to that of CoHCF films under same experimental conditions Fig. 6A. 3.6. Chronoamperometric studies In order to evaluate the electrocatalytic activity of the redox films, we have calculated the diffusion coefficient (D0 ) of hydrazine molecule using both the electrodes modified with films of CoHCF and CoHCF(Au). Here, i vs t curve experiments were performed in 0.1 M KNO3 (pH-5) electrolyte solution. Double step potential method was adopted for this analysis due to the multiple redox states present in modified film. Initial potential was kept at 0.2 V for 30s as first step to reduce the film completely and then the modified electrode potential was stepped to 0.7 V where the redox

A.V.N. Kumar, J. Joseph / Electrochimica Acta 139 (2014) 88–95

93

Fig. 5. FESEM micrographs of modified electrodes captured at 100 kX magnification, bare GC electrode (A), CoHCF modified GC electrode (B), CoHCF electrochemically modified from medium containing 0.2 and 0.6 mM HAuCl4 (C) & (D) respectively.

s−1 and 3.3× 10−5 cm2 s−1 respectively. The ability of CoHCF(Au) modified electrode to oxidise hydrazine is superior to that of CoHCF modified films. We presume that the high activity of CoHCF(Au) is due to reasons i) the high surface area of CoHCF(Au) because of Au nanoparticles present in it as seen from FESEM ii) due to lower charge transport resistance of CoHCF(Au) film which improves hydrazine oxidation rate. Improvement in the kinetics of hydrazine oxidation is approximately 3 times on Au incorporated CoHCF film modified electrodes.

reaction (hydrazine oxidation) takes place. Current vs time profiles were obtained for both the electrodes in the absence of hydrazine and in presence of 0.25 mM hydrazine in each addition. Current vs time response in presence and absent of hydrazine for CoHCF and CoHCF(Au) modified electrodes were represented in Fig. 7A&B. Hydrazine diffusion coefficients were calculated using Cottrell plots (current vs square root of time (i vs t−1/2 ) from the curve obtained from chronoamperometry i vs t. i = nFADC/(t.␲)1/2

(4)

In the above equation ‘n’ is number of electrons involved in the reaction ie (4), Faradays constant (F), geometric area of the electrode (A). The average diffusion coefficients of hydrazine for CoHCF(Au) and CoHCF electrode were found to be 9.5 × 10−5 cm2

3.7. Electrocatalysis towards Oxygen Evolution Reaction Oxygen evolution reaction were studied in 0.1 M NaOH using both electrocatalysts like CoHCF(Au) and CoHCF films modified GC

200

100 50 0

76

0.50

72

0.48

68

0.46

64

0.0

0.2

0.4 0.6 E vs NCE / V

0.8

1.0

0.54 0.52

80

CoHCF

ipa / µA

I / µA

CoHCF(Au)

B

84

Onset potential vs NCE / V

150

A

0.44 2

4

pH

6

8

10

Fig. 6. (A) Cyclic voltammetric response of CoHCF(Au) (red line) and CoHCF (black line) modified GC in presence of 1 mM hydrazine. (B) Plot of hydrazine oxdiation peak current, ipa (black stars) and onset potential for hydrazine oxidation filled blue squares vs supporting electrolyte pH.

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300

800

80

120

60

200

100

I / μA

600 400

I / μA

80

I / μA

I / μA

B

A

60 40 20

200

40 20

100

0

0 0.4

0.5

0.6

0.7

0.4

0.8

0.5

0.6

0.7

0.8

Time-1/2 / s-1/2

Time-1/2 / s-1/2

0

0 0

20

40 60 Time / s

80

100

0

20

40 60 Time / s

80

100

Fig. 7. Current (i) vs time (s) transient curves obtained for (A) CoHCF(Au) and (B) CoHCF modified electrode with increasing hydrazine concentration with 0.25 mM in each addition. Inset of (A&B) show their corresponding Cottrell plots (i vs 1/t1/2 ).

10

I / mA

8 6

(CoHCF modified GC electrode) (CoHCF(Au) modified GC electrode) (Au deposited GC electrode) 0.25 0.20 0.15

4

0.10

that of CoHCF films. We presume that electrochemically converted Co and Fe double oxides from CoHCF with Au nanoparticles are the reason for the shift, approximately 60 mV showing high electrocatalytic behavior. The detailed investigation on OER kinetics with CoHCF(Au) electrode and the responsibility for its activity are currently underway in our laboratory.

0.05

2

0.00 0.5

0

0.6 0.7 E vs Hg/HgO / V

0.8

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 E vs Hg/HgO / V Fig. 8. Cyclic Voltammetric curve for OER of Au deposited GC electrode (blue line), CoHCF electrode modified GC electrode (black line), CoHCF(Au) modified GC electrode (red line) in 0.1 M KOH medium. CoHCF(Au) has high electrocatalytic activity compared with CoHCF and Au electrode towards water oxidation. Inset shows the corresponding graphs in magnified scale.

electrodes. Since transition metal (Ni,Fe,Co) oxides and phosphate structures [36–40] play a prominent role in water oxidation process than noble metals like Au, and Pt electrocatalyst in alkaline condition. Interest in exploring the metal oxides structures for energy application particularly in the field of OER was raised rapidly. Recently Pintado et al., showed that Co-Fe hexacyanoferrate polymer serves as a good electrocatalyst for water oxidation in phosphate buffer (pH = 7) solution [16]. Here, in this work we convert the metal hexacyanoferrate CoHCF and CoHCF(Au) films to their corresponding metal oxy species to investigate their OER electrocatalytic activity through voltammetric cycling in 0.1 M NaOH solution. The advantage of converting metal hexacyanoferrates like CoHCF and CoHCF(Au) containing two different metal ion leads to the formation of double (Co/Fe) oxide/hydroxides, which is an intrinsic factor for high OER electrocatalytic activity [41]. Although the noble metals Au and Pt does not work to the extent of transition metal oxides in alkaline medium but the water oxidation ability (electrocatalytic activity) of metal oxide is highly influenced by metal oxide coated substrates [42,43]. In Fig. 8 the obtained voltammetric curves for CoHCF(Au) modified electrode (red line) show a significant enhancement in the oxidizing current compared with CoHCF Fig. 8 (black line) and Au modified (blue line) GC electrodes. It is apparent that from the inset graph of Fig. 8 water oxidation onset potential is unique for all three investigated electrodes. Among these CoHCF(Au) modified electrode is gifted with minimum overpotential and high oxidation current for OER to take place. So, co-deposition of gold with CoHCF shows a remarkable effect in electrocatalytic activity by shifting the onset potential from 640 mV to 580 mV vs Hg/HgO to the cathodic side to

4. Conclusions In this paper, we have demonstrated two perfectly reversible redox processes for CoHCF modified electrodes by incorporating Au particles in the redox film during electrode modification. Similar experiments by us reveal that the doping of noble metal Pt fails in splitting the redox processes occur in CoHCF films. In addition, we also proved that the noble metal (Au) doped films show fast charge transport kinetics compared to simple CoHCF redox films. EIS results reveal that the CoHCF(Au) redox films’ resistivity or Rct becomes a tuneable property with Au3+ composition. The reasons for the change in Rct values as a function of Au3+ concentration were discussed in terms of ion dependent charge transport. Owing to low Rct of the redox mediator, CoHCF(Au) show better performance in oxidising hydrazine in contrast with films with high Rct values (CoHCF). The calculated diffusion coefficient for hydrazine using cortrell equation was found to be three orders higher on CoHCF(Au) modified electrode. The CoHCF(Au) has shown high oxygen evolution reactiom (OER) catalysis in alkaline medium compared to the CoHCF modified electrodes. This result has great relevance to the preparation of films which may find application in the electrocatalytic electrodes for energy systems. Studies on the OER kinetics of the CoHCF(Au) in alkaline media are under detailed investigation in our laboratory. Hence we conclude that doping of noble metal Au in CoHCF redox film show promise in improving the electron transport characteristics and will have applications in the area of electroanalysis.

Notes †Electronic Supplementary Information (ESI)

Acknowledgements Authors thank CSIR Network Project (Molecules to Materials to Devices CSC 0134). One of the authors A. V. Narendra Kumar acknowledges CSIR-India for the award of Senior Research Fellowship.

A.V.N. Kumar, J. Joseph / Electrochimica Acta 139 (2014) 88–95

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