Electrochemical Impedance Spectroscopy Study

Electrochemical impedance spectroscopy

An EIS study was carried out at different dc potentials in order to study the mechanism of ORR in O₂ saturated alkaline solutions on prepared GDEs. The Nyquist plots of GDEs under different polarization potentials are shown in Fig. 5.

The impedance diagrams show two different behaviors which depend on the applied dc potentials. In the other words, the shape of plots changed at different potentials, suggesting different electrochemical processes occurring on the electrode.

The impedance spectra acquired at the potential lower than 0.7 V show one loop in the high frequency region associated with the time constant of a charge transfer process and semi-infinite diffusive manner related to Warburg component in the low frequency region. This semi-infinite diffusive character is related to the adsorption of reactants and intermediate products.

The Nyquist plots of O₂ reduction on GDEs at the E≥0.7 show two loops. The processes that could be involved on the electrode surface that would produce these changes include [37]:

(1) Diffusion of O₂ through the gas phase in the pores (of porous carbon supported catalyst) and the electrolyte to the reaction site.

(2) Adsorption or heterogeneous surface reaction of the oxygen, together with oxygen diffusion.

(3) Charge transfer.

(4) Diffusion of reduction products into the bulk electrolyte

Many reaction mechanisms have been proposed to describe ORR in aqueous electrolytes. Among these models, the Damjanovic model (Fig. 6) is one of the most extensively employed models, due to its applicability over a wide potential region. Damjanovic model describes the ORR as a multi-electron reaction which O₂ molecules in the vicinity of the electrode are irreversibly reduced directly to H₂O through 4-electron transfer (with a constant rate, k₁) or to H₂O₂ through 2-electron transfer (constant rate, k₂). The H₂O₂ formed can be reduced to H₂O through 2-electron transfer (constant rate, k₃) or diffuse into the bulk solution [38]. The ORR mechanism on transition metals has also been investigated by theoretical calculation based on the electronic structure [39-40]. The dissociative mechanism and the associative mechanism are proposed for a low current density range (more positive potentials) and a high current density range (more negative potentials), respectively [41].

1. Dissociative Mechanism (E≥0.7 V):

In this mechanism, no H₂O₂ is produced. On a metal surface, O₂ adsorption breaks the O-O bond and forms adsorbed atomic O, which further gains two electrons in the two consecutive steps, forming hydroxide ions. Since there is no adsorbed O₂ on the catalyst surface, H₂O₂ cannot be formed. This mechanism can be considered a detailed form of the direct 4-electron pathway and can be written as follows:

1/2O₂ + M(metal active sites) → O^º_ads (11)

O^º_ads + e^– + H₂O → OH_ads + OH^–_aq (12)

OH_ads + e^– → OH^–_ads → OH^–_aq (13)

Associative Mechanism (EË‚0.7 V):

Since adsorbed O₂ is present, the O-O bond may not be broken in the following steps, resulting in the formation of H₂O₂. The H₂O₂ could either be further reduced to H₂O or be a final product. Therefore, the mechanism can be written as follows:

O₂,_ads + 2H₂O + 2e^– → H₂O₂ + 2OH^– (14)

H₂O₂ + 2e^– → 2OH^– (15)

The further reduction of H₂O₂(ads) to hydroxide ions occurs only once the enough overpotential has been reached and before the formed H₂O₂ diffuse into the bulk solution [41].

The two loop manner of GDEs in Nyquist plots may be related to two basic steps. On the other hand, for E ≥ 0.7 V, two time constants are detected during the impedance measurements (Fig. 5). The first time constant at high frequencies is associated with the charge transfer reaction according to Eq. 12, while the second may be associated with the further reduction of OH_ads to produce OH^– based Eq. 13.

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On the other hand, for EË‚0.7 V, the first time constant is related to H₂O₂ intermediate formation according to Eq. 14 and further semi-infinitive diffusive manner in the low frequency region can be explained by adsorption and diffusion of this intermediate into the bulk solution. In order to obtain quantitative information from impedance spectra in Fig. 5, two electric circuits were employed (Fig. 7). The electric circuit in Fig. 7a was used to simulate the impedance response of those spectra with OCP and E < 0.7 V, while the electric circuit in Fig. 7b was used for those with E ≥ 0.7 V.

Using the equivalent circuits shown in Fig. 7, a constant phase element (CPE) is suggested instead of pure capacitance (C), due to the non-homogeneous surface of the electrodes. The impedance of CPE is defined as [Y_o(jω)ⁿ]^-1, where Y_o is a constant with dimension (S.sⁿ), while the exponent n denotes the correction factor pertaining to the roughness of the electrode and has values that range from 0 to 1. A pure capacitance yields n=1, a pure resistance yields n=0, while n=0.5 represents the ideal Warburg impedance. The true capacitance values can be calculated using the following equation [38]:

C=[Y_o × R ^(1-n)]^1/n (14)

According to equivalent circuits shown in Fig. 7, R_s is associated with the resistance of the solution, connectors, leads and wires. R₁ is the charge transfer resistance of the reduction process from O₂ to H₂O₂, R₂ is the resistance of adsorbed species with H₂O₂ as the main intermediate or O₂ adsorption into the GDE pores in the figure 7.a_. The R₁ and R₂ circuit components in figure 7b are the charge transfer resistance of the reduction process of O^º_ads to OH_ads and the reduction process of OH_ads to OH^–, respectively. Parameters calculated from the equivalent circuits (Fig. 7) of O₂ reduction on the GDEs are listed in Table 3. According to table 3, the R_s values change as a function of the potential, indicating that the contribution of the apparatus (connectors, leads and wires) from the total impedance of system shouldn’t be dismissed. So, the R_s values have a contribution function of both electrolyte and the apparatus resistance, i.e., connectors, leads and wires.

R₁ and R₂ also show a dependence on the applied dc potentials (Fig. 8). By increasing the positive potential, the R₁ values decrease. In the potential region of lower than 0.7 V, the adsorption of free O₂ molecules happens on the metal catalyst and then O_2,ads reduces to OH_ads (Eq.8). In the applied dc potential of E≥0.7 V, the adsorption of O^º_ads free radicals happens. By increasing the positive applied potentials, the adsorbed amount of O_2,ads and O^º_ads increases and the further reduction process occurs more easily in the catalyst layer. So the R₁ values will be decreased. The dependence of true capacitance values of the applied potentials is shown in Fig. 10. The adsorbed species enhancement by increasing the more positive potentials causes to decrease of C₁ values. The R₂ values for EË‚0.7 V are related to the adsorption of molecules like H₂O_2,ads into the GDE pores. When the potential is increased to more positive potentials, the adsorption of H₂O_2,ads species by the oxygen atom orientation to the metal catalysts will be increased and R₂ values became higher. In the potential region of E≥0.7 V, the OH,_ads species which are reduced to OH^–_aq, increase and charge transfer happens more easily. So the R₂ values will be decreased. The C₂ values also decrease because of more species adsorption (Fig. 10). Comparison of R values for GDEs shows that the Pt.Ru/C electrocatalyst has the lowest resistance in the whole range of applied dc potentials because of charge transferring occurring more easily and so ORR happens more rapidly at this electrocatalyst type. This behavior can be observed due to the synergistic effects of Pt and Ru catalyst species. The true capacitances of GDEs also show that the calculated of Pt.Ru/C and Ru/C electrocatalyst are higher than Pt.C ones. It is because the Ru species act as a protonic capacitor in the Pt.Ru/C and Ru/C GDEs.

3.5. Chronoamperometery

The oxygen diffusion coefficients of GDEs were determined by chronoamperometry technique. Chronoamperograms were obtained by holding the potential of the electrodes at +1.2 V for 10 s and then holding it at +0.4 V relative to the Ag/AgCl electrode for 500 s with oxygen flowing along the electrolyte.

With plotting i vs. t^−1/2, the linear dependence relationship was obtained for different electrodes [42]:

(16)

Where I is the limited current, A the surface area of the electrode, D the diffusion coefficient, C the concentration of oxygen, n the number of electrons in the overall reaction of ORR, F the Faraday’s constant, t the time, and is equal to 3.14. Fig. 11 shows the chronoamperograms of GDEs at +0.4V in relative to the Ag/AgCl in alkaline media. . Cottrell parameters are listed in Table 4, also. The results confirmed the higher Cottrell slope and D values for Pt.Ru/C electrode. So, the Pt.Ru/C cathode has more permeability and activity towards oxygen reduction reaction.

Conclusion

In this investigation, Pt/C, Ru/C and Pt.Ru/C bimetallic electrocatalysts were prepared by chemical reduction process. Then the resulted inks were coated on carbon paper and used as gas diffusion electrodes for oxygen reduction reaction in alkaline media. The surface structure of oxygen depolarized cathodes was studied by SEM and EDX analysis.

The SEM results showed that all the cathodes consist of rough and porous structures. And Pt, Ru nanclusters were deposited quite uniformly onto/into Vulcan carbon supports with the average particle size of about 30-45 nm. The ORR activity of cathodes was evaluated in 0.1 M O₂ saturated NaOH media. Comparison of cyclic voltamograms of Pt/C, Ru/C and Pt.Ru/C electrodes in O₂ saturated solutions show that for Pt.Ru/C catalyst current increase induced by ORR is shifted towards more negative electrode potentials and only higher current values have been obtained within the ORR region. It can be concluded that the second metal addition has influenced the catalytic activity of electrocatalyst toward ORR. This matter can be related to synergistic effect, which is playing a critical role in ORR activity. The Pt.Ru/C cathodes showed lower Tafel slops and high current densities. An EIS study was carried out at different dc potentials in order to study the mechanism of ORR in O₂ saturated alkaline solutions on prepared GDEs. The Nyquist plots of GDEs under different polarization potentials show two different behaviors, suggesting different associative and dissociative electrochemical processes occurring on the electrode.