**3. Results and discussion**

### **3.1 Electropolymerization kinetics and mechanisms of of OCP and OHP 3.1.1 Electropolymerization**

Electropolymerization of OCP or OHP on platinum electrode from aqueous solution containing 0.6M H2SO4 at 303 K in the absence and presence of monomer, was studied by cyclic voltammetry at potential between -366 and +1780 mV(vs. SCE) with scan rate of 25 mVs-1.

The obtained voltammograms in absence and presence of monomer is represented in Fig. 1(a-c). The voltammogram in the absence of monomer exhibit an oxidation peak (I) which developed at -300 mV vs. SCE, which is a result of hydrogen adsorption on Pt electrode [Arslan et al, 2005]. While, the voltammograms in the presence of OCP exhibit two oxidation peaks (I and II) that progressively developed at -300 and +863 mV (vs. SCE) repetitively and one reduction peak (II') at +248 mV (vs. SCE). While in the presence of OHP ; two oxidation peaks (I and II) that progressively developed at --200 and +622 mV (vs. SCE) repetitively and one reduction peak (II') at +0.20 mV (vs. SCE). One one hand, the first oxidation peaks (I) are a result of hydrogen reduction [Arslan et al, 2005] as mentioned above where, the second oxidation peaks (II) correspond to oxidation of monomer to give phenoxy radical which adsorbed on Pt-electrode [Gattrell & Kirk,1993]. The adsorbed radicals react with

Note: peak (I) in both (b) and (c) is not shown in the figure.

Fig. 3. Cyclic voltammograms of solutions containing 0.06M H2SO4 at 303 K with scan rate 25mVs-1.

Electropolymerization of OCP or OHP on platinum electrode from aqueous solution containing 0.6M H2SO4 at 303 K in the absence and presence of monomer, was studied by cyclic voltammetry at potential between -366 and +1780 mV(vs. SCE) with scan rate of 25

The obtained voltammograms in absence and presence of monomer is represented in Fig. 1(a-c). The voltammogram in the absence of monomer exhibit an oxidation peak (I) which developed at -300 mV vs. SCE, which is a result of hydrogen adsorption on Pt electrode [Arslan et al, 2005]. While, the voltammograms in the presence of OCP exhibit two oxidation peaks (I and II) that progressively developed at -300 and +863 mV (vs. SCE) repetitively and one reduction peak (II') at +248 mV (vs. SCE). While in the presence of OHP ; two oxidation peaks (I and II) that progressively developed at --200 and +622 mV (vs. SCE) repetitively and one reduction peak (II') at +0.20 mV (vs. SCE). One one hand, the first oxidation peaks (I) are a result of hydrogen reduction [Arslan et al, 2005] as mentioned above where, the second oxidation peaks (II) correspond to oxidation of monomer to give phenoxy radical which adsorbed on Pt-electrode [Gattrell & Kirk,1993]. The adsorbed radicals react with

**3.1 Electropolymerization kinetics and mechanisms of of OCP and OHP** 

**3. Results and discussion** 

**3.1.1 Electropolymerization** 

Note: peak (I) in both (b) and (c) is not shown in the figure.

25mVs-1.

Fig. 3. Cyclic voltammograms of solutions containing 0.06M H2SO4 at 303 K with scan rate

mVs-1.

other monomer molecule via head-to-tail coupling to form predominantly a para-linked dimeric radical and so on to form oligomer and polymer film; this film is a chain of isolated aromatic rings (polyethers) without π-electrons delocalization between each unit as shown in schemes (1 and 2). The oxidation occurs at more positive values ~ + 863 and +622 mV vs. SEC where the presence of (Cl and OH) make the oxidation process difficult.

On reversing the potential scan from, the reversing anodic current is very small indicating the presence of polymer layer adhered to the Pt-surface [Sayyah et al, 2010]. One cathodic peak (II') was found which could be ascribed to the reduction of the formed polymer films.

The effects of repetitive cycling on Pt- electrode in aqueous solution containing 0.6M H2SO4 with and without monomer at 303 K are shown in Fig. 4 (a-c). The data reveal that, in absence of monomer, the repetitive cycling show the oxidation peak (I) only. The current of this peak (ipI) is almost the same and not affected with cycling up to 6 cycles (c.f. fig. 4 (a)).

Fig. 4. Repetitive cycling of electropolymerization from solution containing 0.6M H2SO4 at 303 K with scan rate 25 mVs-1

Electropolymerization of Some Ortho-Substituted Phenol Derivatives on Pt-Electrode from

H2SO4 at 303 K with scan rate from 15 to 45 mV s-1) are shown in Table 1;

Scan rate, (Vs-1)

Table 1. Calculated values of Diffusion coefficients.

*For OCP; ipII (mA) = 0.207 v1/2 (mV s-1)1/2 – 0.60 : r2*=*0.90 and, For OHP; ipII (mA) = 0.228 v1/2 (mV s-1)1/2 – 0.53: r2*=*0.99* 

Fig. 6. Relation between ipII and square root of scan rate

These linear regression equations are;

Aqueous Acidic Solution; Kinetics, Mechanism, Electrochemical Studies and Characterization of… 29

 ipII = 0.4463 n F A C ( n F *v* D / R T )1/2 (4) where n is the number of exchanged electron in the mechanism, F is Faraday's constant (96485 Cmol-1), A is the electrode area (cm2), C is the bulk concentration, D is the analyst diffusing coefficient (cm2s-1), and v is the scan rate (Vs-1). R is the universal gas constant (8.134 Jmol-1K-1), T is the absolute temperature (K).The calculated values of D (at 0.6 M

Diffusing coefficient, (m2s-1)

The values of D are seen to be constant for both cases over the range of sweep rates, which

Figure 6 (a and b) shows the linear dependence of the anodic current peak, (ipII) versus ν1/2.

From Fig. 6 and the above equations we notice that; 0.9 < correlation coefficient (r2) <1. So we suggest that the electroformation of both POCP and POHP may be described partially by a diffusion-controlled process (diffusion of reacting species to the polymer film/solution interface). [Ardakani et al, 2009] It seems that, initially the electroformation of radical cations

0.015 1.83676 x 10-11 4.41282 x 10-11 0.020 2.15245 x 10-11 8.26886 x 10-11 0.025 7.16612 x 10-11 1.09351 x 10-10 0.030 9.11262 x 10-11 1.15739 x 10-10 0.045 1.07765 x 10-10 1.59405 x 10-10

again shows that the oxidation process is diffusion-controlled [sayyah et al, 2010].

OCP→POCP OHP→POHP

This means that, surface area of electrode is not affected by the H2 adsorption where, in presence of monomers (OCP or OHP) the data reveal that, during the second cycle both the oxidation and the reduction peak currents decrease significantly with repetitive cycling. This behavior is observed elsewhere as a result of fouling of electrode [Arslan et al, 2005] where phenolic products block the electrode surface and the formed film hinders diffusion of further phenoxide ions to the electrode surface, thereby causing a significant decrease in the anodic peak current and also decrease the cathodic peak current. The potential position of the redox peaks does not shift with increasing number of cycles, indicating that the oxidation and the reduction reactions are independent on the polymer thickness [Sayyah et al, 2010].

Figure 5 (a and b) illustrates the influence of the scan rate (15 – 45 mV s-1) on the potentiodynamic anodic polarization curves for OCP and OHP from aqueous solution containing 0.6 M H2SO4 at 303 K on platinum electrode. It is obvious that both the anodic and cathodic peak current densities (*ipII* and *ipII')* in the two cases increases with the increasing of the scan rate. This behavior may be explained as follows, when an enough potential is applied at Pt- surface causing oxidation of species in solution, a current arises due to the depletion of the species in the vicinity of the Pt- surface. As a consequence, a concentration gradient appears in the solution. The current (ip) is proportional to the gradient slope, dc/dx, imposed (i = dc/dx). As the scan rate increase the gradient increase and consequently the current (ip).

Fig. 5. Effect of Scan rate on electropolymerization on Pt electrode from solution containing 0.6M H2SO4 at 303 K.

Figure 6 (a and b) shows the linear dependency of the anodic peak current densities ( *ipII* ) which is corresponding to the formation of the polymer films POCP and POHP repetitivelyversus the square root of scan rate (*v*1/2). This linear relation suggests that the oxidation of OCP to POCP and/or OHP to POHP may be described by a partially diffusion-controlled process (diffusion of reacting species to the polymer film / solution interface) where the correlation coefficients (r2) is higher than 0.9 but not equal to 1.0 suggesting the non-ideal simulation relation (i.e: the process is not completely diffusion control but it is exactly a partially diffusion control. Values of *i*p are proportional directly to *v*1/2 according to Randless [Randless, 1984] and Sevick [Sevick, 1948] equation:

$$\mathbf{i\_{pll}} = 0.4463 \,\text{n} \,\text{F} \,\text{A} \,\text{C} \,\text{(n} \,\text{F} \,\text{v} \,\text{D} \,/\text{R} \,\text{T} \,\text{)}^{1/2} \tag{4}$$

where n is the number of exchanged electron in the mechanism, F is Faraday's constant (96485 Cmol-1), A is the electrode area (cm2), C is the bulk concentration, D is the analyst diffusing coefficient (cm2s-1), and v is the scan rate (Vs-1). R is the universal gas constant (8.134 Jmol-1K-1), T is the absolute temperature (K).The calculated values of D (at 0.6 M H2SO4 at 303 K with scan rate from 15 to 45 mV s-1) are shown in Table 1;


Table 1. Calculated values of Diffusion coefficients.

28 Electropolymerization

This means that, surface area of electrode is not affected by the H2 adsorption where, in presence of monomers (OCP or OHP) the data reveal that, during the second cycle both the oxidation and the reduction peak currents decrease significantly with repetitive cycling. This behavior is observed elsewhere as a result of fouling of electrode [Arslan et al, 2005] where phenolic products block the electrode surface and the formed film hinders diffusion of further phenoxide ions to the electrode surface, thereby causing a significant decrease in the anodic peak current and also decrease the cathodic peak current. The potential position of the redox peaks does not shift with increasing number of cycles, indicating that the oxidation and the

Figure 5 (a and b) illustrates the influence of the scan rate (15 – 45 mV s-1) on the potentiodynamic anodic polarization curves for OCP and OHP from aqueous solution containing 0.6 M H2SO4 at 303 K on platinum electrode. It is obvious that both the anodic and cathodic peak current densities (*ipII* and *ipII')* in the two cases increases with the increasing of the scan rate. This behavior may be explained as follows, when an enough potential is applied at Pt- surface causing oxidation of species in solution, a current arises due to the depletion of the species in the vicinity of the Pt- surface. As a consequence, a concentration gradient appears in the solution. The current (ip) is proportional to the gradient slope, dc/dx, imposed (i = dc/dx). As the scan rate increase the gradient increase

Fig. 5. Effect of Scan rate on electropolymerization on Pt electrode from solution containing

Figure 6 (a and b) shows the linear dependency of the anodic peak current densities ( *ipII* ) which is corresponding to the formation of the polymer films POCP and POHP repetitivelyversus the square root of scan rate (*v*1/2). This linear relation suggests that the oxidation of OCP to POCP and/or OHP to POHP may be described by a partially diffusion-controlled process (diffusion of reacting species to the polymer film / solution interface) where the correlation coefficients (r2) is higher than 0.9 but not equal to 1.0 suggesting the non-ideal simulation relation (i.e: the process is not completely diffusion control but it is exactly a partially diffusion control. Values of *i*p are proportional directly to *v*1/2 according to

Randless [Randless, 1984] and Sevick [Sevick, 1948] equation:

reduction reactions are independent on the polymer thickness [Sayyah et al, 2010].

and consequently the current (ip).

0.6M H2SO4 at 303 K.

The values of D are seen to be constant for both cases over the range of sweep rates, which again shows that the oxidation process is diffusion-controlled [sayyah et al, 2010].

Figure 6 (a and b) shows the linear dependence of the anodic current peak, (ipII) versus ν1/2. These linear regression equations are;

*For OCP; ipII (mA) = 0.207 v1/2 (mV s-1)1/2 – 0.60 : r2*=*0.90 and, For OHP; ipII (mA) = 0.228 v1/2 (mV s-1)1/2 – 0.53: r2*=*0.99* 

From Fig. 6 and the above equations we notice that; 0.9 < correlation coefficient (r2) <1. So we suggest that the electroformation of both POCP and POHP may be described partially by a diffusion-controlled process (diffusion of reacting species to the polymer film/solution interface). [Ardakani et al, 2009] It seems that, initially the electroformation of radical cations

Fig. 6. Relation between ipII and square root of scan rate

Electropolymerization of Some Ortho-Substituted Phenol Derivatives on Pt-Electrode from

Fig. 7. Cyclic voltammetry curves showing the effect of monomers concentration on

electropolymerization and its related double logarithmic plot.

Aqueous Acidic Solution; Kinetics, Mechanism, Electrochemical Studies and Characterization of… 31

is controlled by charge transfer process. When the polymers become thick, the diffusion of reactant inside the film becomes the slowest step, the process changed to diffusion transfer, which confirms the data in Figure 4.

The intercepts in Figure 6 are small and negative, -0.60 and -0.53 for OCP and OHP respectively, which could be attributed to a decrease of the active area of the working electrode during the positive scan [Zanartu et al , 2002] or the increase of the covered area of working electrode by the adhered polymer sample, which confirm the data of Figure 4 .
